TEIE REPRODUCIBILITY AND ACCURACY OF CEPHALOMETRIC

ANALYSIS USING DIFFERENT DIGITAL IMAGING MODALITIES AND

IMAGE COMPRESSION

SUZANNE CZIRAKI B.Sc., M.Sc., B.Ed., D.D.S.

A thesis submitted in confomity with the requirements

for the degree of Master of Science

Graduate Department of Dentistry

University of Toronto

8 Copyright by Suzanne Elizabeth Cziraki 200 1

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THE REPRODUCIBILITY AND ACCURACY OF CEPHALOMETRlC ANALYSIS

USING DIFFERENT DIGITAL IMAGING MODALITIES AND IMAGE

COMPRESSION, Master of Science, 2001, Suzanne E. Cziraki, BSc, MSc, BEd, DDS,

Graduate Department of Dentistry, University of Toronto.

ABSTRACT

This study evaluated and cornpared the reproducibility and accuracy of cephdometric

landmarks and measurernents between conventional radiographie film and the

corresponding indirect, JPEG compressed or direct digital images (DI).

Twentysne conventional films were indirectly digitized using a scanner, and TIFF

images were converted to JPEG files using 2 levels of compression (12:l and 251).

Simultaneous acquisition of conventional and direct DI (stimulable-storage phosphor

(SPP)) was obtained for thirty patients.

Three observers for the indirect and one observer for the direct DI identified 17 and 22

landmarks respectively on the cornputer monitor. X and y-coordinates were recorded and

cephalomeûic measurernents calculated.

Statistical andysis involved use of the paired samples t-tests and ANOVA models.

The results showed that indirect DI have equivalent landmark reproducibility with

conventional film. JPEG compression of indirect DI at 251 level results in loss in

accuracy of cephaiornetric measurement when comparai to non-compressed DI, whereas,

JPEG at 12: 1 did not. Direct DI were less accurate for 8 out of 2 1 landmarks but clinical

accuracy of cephalometric measurement was equivalent to conventional film.

. . . I l l

ACKNOWLEDGEMENTS

The following individuals have played an integrai part in the completion of this M.Sc. thesis. It is with heartfelt acknowledgement that 1 thank you all.

This thesis is dedicated to my husband John Duncan, who stood by me and encourageci me as I pursued yet another academic degree. A special thank you to my beloved children, Madeleine and Elizabeth who were both bom during the course of this degree and reminded me dail y of the tme meaning of life. 1 wish to thank my parents, Gladys and John Cziraki, who supporteci me in every way possible in order that 1 rnight attain this goal.

Dr. MJ Pharoah, my thesis supervisor, who was my mentor and fnend over the past three years providing encouragement and enthusiasm for this project. He is a gified teacher and a talented academic who provided first-rate support in al1 phases of this research.

Drs. HP Lawrence, CG Petrikowski, P Birek, PE Rossouw, members of my research cornmittee, for their guidance and advice during the course of this research.

Dr. H Holmes, for his help with the cephalometric software.

Dr. R Walker, for providing the inspiration to pursue research in the area of digital radiology.

Drs. J Posluns and D Raveli and Mrs. G Jorgenson, my observers for the research project, who gave unselfishly of their time.

Patricia Shuter and the radiology support staff, for their technical assistance in the acquisition of the digital radiographs.

Mrs. J Hoffmeister, for her help with the Burlington Growth Centre radiographs.

Mrs. Lily Young, for her clerical help.

DenOptidGendex Industries for providing the equipment for the direct digital imaging project.

This research was funded by a Faculty of Dentistry Research Grant.

PREFACE

THESIS FORMAT

This M.Sc. thesis is presented in the 'Publishable Sîyle'. Chapters 2 and 3 will be

submitted for publication and they are presented in their publishable f o m except for

minor changes and additions that were incorporated to improve and enhance readability.

A general introduction and Concluding Discussion are included to put the experimental

work in context with the cwrent knowledge.

PUBLICATIONS that wili arise from the thesis chapters include:

PAPER 1 FROM CEIAPTER 2.

Cziraki SE, Pharoah MJ, Lawrence HP, Petrikowski CG, Birek P and Rossouw PE.

impact of lossy Joint Photographic Experts Group (JPEG) compression on the

reproducibili ty and accuracy of indirect digit al cephalometric radiographic anal ysis.

University of Toronto, Faculty of Dentistry, Discipline of Orthodontics, Toronto, Canada

PAPER 2 FROM CHAPTER 3.

Cziraki SE, Pharoah MJ, Lawrence HP, Petrikowski CG, Birek P and Rossouw PE. The

reproducibility and accuracy of cephalometric analysis using conventional film and direct

digital images obtained by the storage phosphor technique.

University of Toronto, Faculty of Dentistry, Discipline of Orthodontics, Toronto, Canada

ABSTRACT 1 FROM THESIS ABSTRACTO

Cziraki SE, Pharoah MJ, Lawrence HP, Petrikowski CG, Birek P and Rossouw PE.

The reproducibility and accuracy of cephalornetric analysis using different digital

imaging modalities and image compression.

University of Toronto, Faculty of Dentistry, Discipline of Orthodontics, Toronto, Canada

AWARDS AND GRANTS

Graduate Research Day Award Second Place, Best Poster Presentation Faculty of Dentistry, University of Toronto

2000 Faculty of Dentistry Research Grant

1999-2000 Ontario Graduate Scholarship Faculty of Dentistry, University of Toronto

SCIENTIFIC POSTER BOARD PRESENTATIONS

TITLE: The reproducibüity and accuracy of cephalometric analysis using different

imaging modaüties and image compression.

September 200 1 Canadian Association of Orthodontists Annuai Meeting, Quebec City

May 200 1 Arnerican Association of Orthodontists 1 0 1 Annual Meeting, Toronto

March 2001 International Association for Dental Research (IADR) Annual Meeting, Chicago, Ill.

February 200 1 Graduate Research Day, Faculty of Dentistry Annual Meeting, Toronto

TABLE OF CONTENTS

...................................................................... ACKNOWLEDGEMENTS iv

............................................................................................. PREFACE v ................................................................................. Thesis Format v

.............................................. Publications that will arise fiom this thesis v .......................................................................... Awards and Grants vi

.................................................... Scientific Poster Board Presentations vi . . ........................................................................... Table of Contents VII ................................................................................. List of Tables x . . ............................................................................. List of Figures .XII

........................................................................ List of Appendices xiv

...................................................... CHAPTER 1: LITERATURE REVIEW 1

................................................................................. Introduction -2 Direct Digital Imaging Techniques ....................................................... 3

............................................................ Charged Couple Device 3 Storage Phosphor Technique ..................................................... -3

.................................................... Indirect Digital Imaging Techniques 4 ............................................................................ Scanners -4

Video Cameras .................................................................... -4 ............................................................ Understanding Digital Imaging 6

.................................................................... Optical Density -6 ....................................................................... Image Display 7

.................................................................. Spatial Resolution 8 Digital Imaging in Medicine and Dentistry ............................................. 8 Digital Image Archiving and Compression .............................................. 9

............................................................... IPEG Compression 12 ............................................... Orthodontic Cephalometric Radiography 14

........................................ Digital Imaging in Otthodontic Cephalometry 16 ................................................................... Review of the Literature 17 .................................................................. Statement of the Problem 25

....................................................................... Purpose of the Study 26 ........................................................................... Nul1 Hypotheses -26

vii

CHAPTER 2: INDIRECT DIGITAL IMAGLNG AND FILE COMPRESSION .... 30

Impact of irreversible Joint Photographie Experts Group (JPEG) compression on the reproducibility and accuracy of indirect digital cephalomehic radiographs

................................................................................... ABSTRACT -31 INTRODUCTION ........................................................................... -33

........................................................... MATERIALS AND METHODS 38 ............................................................................. The Equipment 38

Part 1 . ....................... Radiographic Sample: lndirect DI versus Conventional Film 39

......... Landmark identification protocol: Indirect DI versus Conventional Film 39 Part 2 .

Radiographie Sample: JPEG compression of indirect digital image ............... 41 ............................... Landmark identification protocol (JPEG compression) 41

............................................................................. Data Treatment 42 Part 1 . Indirect digital image versus conventional film ....................... 42 Part 2 . JPEG compressed (Q2 and 47) versus non-compressed (TIFF) ... 43 ....................................................................................... RESCILTS 44

IntraClass Correlation Coefficient for Indirect DI versus conventional images.4 ........................................................ Intra-examiner Reliability 44 ........................................................ inter-examiner Reliability 44

.................................................. Intra-modality Reproducibility 44

.................................................. Inter-modality Reproducibility 45 ........................................................ Inter-modality Accuracy -46

Compression of Indirect Digital Image Files Using JPEC Q2 and 47 .............. 47 ...................................................... Intra-Image Reproducibility 47 ...................................................... Inter-image Reproducibili ty 48

............................................................ Inter-Image Accuracy 49 .................................................................................. DISCUSSION 51

.................................................................... IntraClass Correlation S l ................................................................................ Hard Tissues 52

Dental Tissues ............................................................................. -53 X versus y-coordinates ..................................................................... 53

............................................ Surnmary of Landmark Identification Error 54 Cephalometric Measurements ............................................................ 54

........................ Patteni of Distribution of Error for Landmark Identification 55 ............................................................................. CONCLUSIONS -57

CHAPTER 3: DIRECT DIGITAL IMAGING .............................................. 72

The reproducibility and accuracy of cephalometric analysis using conventional radiographie film and direct digital images obtained by the storage phosphor technique .

ABSTRACT ................................................................................... -73 INTRODUCTION ........................................................................... -74 MATERIALS AND METHODS ............................................................ 77

............................................................................. The equipment -77

............................................................................. Patient Sarnple -77 . . . ........................................................................ Image Acquisition -78 Landmark Definition and Sampling ...................................................... 78

....................................................................... Statistical Analysis -80 . . .................................................................... Reproducibility 80 .......................................................................... Accuracy -80

...................................................................................... RESULTS -82 Intra-modality reproducibility ............................................................ 82 Inter-modali ty reproducibili ty ............................................................ 83 Inter-modality accuracy ................................................................... 83

DISCUSSION ................................................................................. -85 ............................................................................. CONCLUSIONS -90

C W F R 4 . DISCUSSION AND CONCLUSIONS .................................... 105

............................................................................. DISCUSSION 106 . . Direct Digital Imaging .......................................................... 107 Image Compression .............................................................. 110

......................................................... Future Research Trends 113 CONCLUSIONS ......................................................................... 114

................................................................................... APPENDICES -116 Appendix 1 . Lateral Cephalometric Landmark Location and Definitions ........... 117

............................................................ Appendix 2 . Glossary of Terms 122 Appendix 3 . Indirect digital imaging and file compression data ....................... 124 Appendix 4 . Direct digital imaging data ................................................... 134

LIST OF TABLES

CHAPTER 2.

Table 2.1. Definitions of cephalometric landmarks. ......................................... .58

Table 2.2. Cephalometric measurements calculated by the DigiPlan software.. .......... 59

Table 2.3. Mean and standard deviation (SD) in millimeters (mm) for the absolute difference between replicate readings (reading 1 minus reading 2) for cephalometnc landmarks by image modality, x and y coordinate and tissue type, N=36. ................. 60

Table 2.4. Mean and standard deviation (SD) in millimeters (mm) for the absolute difference between image modalities (conventional images minus indirect digital images (DI)) for cephalometic landmarks by x and y-coordinate and tissue type,N=36.. ...... ..6 1

Table 2.5. Mean and standard deviation (SD) in millimeters (mm) of the absolute difference between replicate readings (reading 1 minus reading 2) for each of the cephalometric landmarks by image file type, N=2 1. ......................................... .62

Table 2.6. Mean and standard deviation (SD) in millimeters (mm) and degrees of the absolute difference between replicate readings (reading 1 minus reading 2) for each of the cephalometric measurements, by image file type, N=2 1 ...................................... 63

Table 2.7. Mean and standard deviation (SD) in millimeters (mm) of the absolute difference between image file types (non-compressed minus compressed images (42 or 47)) for each of the cephalometric landmarks, by x or y-coordinate, N=2 1.. .............. 64

Table 2.8. Mean and standard deviation (SD) in millimeters (mm) and degrees of the absolute difference between image file types (non-compressai minus compressed images (42 or 47)) for each of the cephalometric measurements, N=2 1. .......................... .65

CHAPTER 3.

Table 3.1. Definitions of cephalometric landmarks .......................................... .9 1

Table 3.2. Cephalometric measurements calculated by the DigiPlan software.. ......... .92

Table 3.3. Mean and standard deviation (SD) in millimeters (mm) of the absolute differences between replicate readings (reading 1 minus reading 2) for each landmark by

.......................................................................... image modality. (N=30). .93

Table 3.4. Mean and standard deviation (SD) in millimeters (mm) or degrees of the absolute differences between replicate measurements (measurement 1 minus

...... measurment 2) for each cephalometric measurement by image modality (N=30). .94

Table 3.5. Landmarks and measurements that are statisticall y and clinicall y si gni ficantl y different in reproducibility represented by the mean and standard deviation (SD) in millimeters (mm) for the absolute difference between replicate readings (reading 1 minus reading 2) by image modaiity (N=30). .......................................................... .95

Table 3.6. Mean and standard deviation (SD) in millimeters (mm) of the absolute differences between image modalities (conventional minus direct digital image) represented b y landmark (N=30). ................................................................ -96

Table 3.7. Mean and standard deviation (SD) in millimeters (mm) or degrees of the absolute differences between image modalities (conventional minus direct digital image) represented for each of the cephalometric measurements. (N=30). ......................... .97

Table 3.8. Landmarks that are statistically and clinically significantly diffaent in accmcy represented by the mean and the standard deviation (SD) in millimeters (mm) for the absolute difference between image modalities (conventional minus direct digital images) by x and y coordinate (N=30). ........................................................ ..98

LIST OF FIGURES

CHAPTER 1.

Figure 1.1. Flow diagram representing the direct digital imaging technique nomWhite and Pharoah, 2000.. ................................................................................. .27

Figure 1 3 . H and D curve fiom White and Pharoah, 2000 .................................. .28

Figure 1.3. Photograph of transparency flat-bed scanner showing placement of lateral cephalographi c radiograph for indirect digi ta1 imaging ...................................... .29

CHAPTER 2.

Figure 2.1. Bar graph illustrating x-coordinate landmark error in millimeters (mm) for JPEG Q2-TIFF compared to Q7-TIFF.. ........................................................ ..66

Figure 2.2. Bar graph illustrating y-coordinate landmark error in millimeters (mm) for JPEG Q2-TIFF compared to Q7-TIFF. ......................................................... $6

Figure 2.3. Bar graph illustrating cephalometric measurement mors in millimeters ................ (mm) or degrees for JPEG compressions Q2-TIFF compared to Q7-TIFF.. 67

Figure 2.4 and 2.5. Scattergrams representing the distribution of error fiom the mean in millimeters (mm) for cephalometric landmark Nasion for JPEG 4 2 (25: 1) and TIFF (non- compressed) digital images. N= 1 26. ............................................................. .69

Figure 2.6 and 2.7. Scattergrams representing the distribution of error fiom the mean in millimeters (mm) for cephalometric landmark ANS for JPEG 42 (25: 1 ) and TIFF (non- compressed) digital images. N= 1 26. ............................................................ -70

Figure 2.8. Cornputer display of lateral cephalographic digital images. A - JPEG 47 (12: 1) and B - JPEG 42 (25: 1). ................................................................ ...y 1

CHAPTER 3.

Figure 3.1. Photograph of DenOptixGendex digital, reusable storage phosphor plate (A) and conventional x-ray film cassette (B). ...................................................... .99

Figure 3.2. Traditional method of locating landmarks on cephalometric radiographs using a light box for illumination (A) to create an acetate tracing (B). ..................... 100

Figure 33. Adobe Photoshop superimposition technique for the layering of the acetate tracing over the digital image as viewed on the cornputer monitor. A- acetate tracing; B- initial layer; C-acetate made translucent with approximate superimposition; D-precise superimposition of acetate over digital image.. ............................................... i 0 1

xii

Fipre 3.4. Photograph of GendexlDentOptix Equipment. A - Storage phosphor plate, PSP, attached to drum. £3- DenOptix scanner showing placement of the d m with the PSP attached.. ..................................................................................... - 1 02

Figure 3.5. Digiplan software outlining the landmarks and cephalometric planes .... superirnposed on the direct digital PSP image as viewed on the computer monitor.. 103

Figure 3.6. Direct digital PSP cepahlometric images viewed within Digiplan and showing the difference in contrast and brightness that can be achieved. (A - low brightness, high contrast; B - high brightness, low contrast). .................... 104

xiii

LIST OF APPENDICES

APPENDICES.. ................................................................................. - 1 16

APPENDIX 1. LATERAL CEPEIALOGRAPHIC LANDMARK LOCATION

AND DEFINITIONS.. .......................................................................... 1 17

Figure 2.1. Lateral cephalographic radiograph acetate tracing with landmark numbers marked. See attached Appendix 1 for definitions of landmarks.. .......................... 1 18

Table 2.9. Landmark Definitions Needed for Leagan, McNamara and Steiner Analyses Using Digiplan (Orthovision) Software.. ....................................................... 1 19

APPENDIX 2. GLOSSARY OF TERMS. ................................................. .122

GLOSSARY.. .................................................................................... -123

APPENDIX 3. INDIRECT DIGITAL IMAGING AND FILE COMPRESSION DATA. ............................................................................................. - 1 24

Ail data represented in the following tables is descriptive and is not directly referred to in the text.

Table 2.9. Descriptive statistics representing the mean for al1 3 examinen and standard error of the mean for each of the HARD tissue cephalometric landrnarks by file type

.................................................................................... (TIFF, 47, 42). 125

Table 2.10. Descriptive statistics representing the mean for al1 3 examiners and standard error of the mean for each of the DENTAL tissue cephalometric landrnarks by file type

.................................................................................... (TIFF, 47, 42). 126

Table 2.1 1. Descriptive statistics representing the mean and standard m r of the mean for each of the cephalometric measurements for al! observers b y file type (TIFF, 47,

.................................................................................................. 42). 127

Table 2.12. Intra-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating SOFT tissue lateral cephalometric landmarks by file type (TIFF,

........................................................................................... 47, 42). -128

xiv

Table 2.13. Intra-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating HARD tissue lateral cephalometric landmarks by file type (TIFF, Q7,Q2). ............................................................................................. 129

Table 2.14. Intra-examiner variabili ty measured by htra Class Correlation Coefficient (ICC), for evaluating DENTAL tissue lateral cephalometric landmarks by file type (TIFF, 47 , Q2). ............................................................................................. 130

Table 2.15. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating SOFT tissue lateral cephalometric landmarks by file type (TIFF, 47, 42). ............................................................................................. 1 3 1

Table 2.16. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating HARD tissue lateral cephalometric landmarks by file type (TIFF, 47, 42). ............................................................................................. 1 32

Table 2.17. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating DENTAL tissue lateral cephalometric landmarks by file type (TIFF, 47, 42). ............................................................................................ 133

APPENDIX 4. DIRECT DIGITAL IMAGING DATA. ................................ .134

AM data represented in the following tables is descriptive and is not directly referred to in the text.

Table 3.9. Descriptive statistics representing the mean and standard error of the mean in rnillimeters (mm) for each SOFT tissue cephalometric landmarks by image modality (conventional and direct digital- storage phosphor technique (PSP)). ..................... 135

Table 3.10. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each HARD tissue cephalomeûic landmarks by image modality (conventional and direct digital- storage phosphor technique (PSP)). ..................... .136

Table 3.1 1. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each DENTAL tissue cephalometric landmarks by image modality (conventional and direct digital- storage phosphor technique (PSP)). .......... 137

Table 3.12. Descriptive statistics representing the mean and standard error of the mean for each of the cephalometric measurements by image modality (conventional and direct digital- storage phosphor technique (PSP)) .................................................... .13 8

Table 3.13. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each SOFT tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (P S P)) ............................................................................................... -1 39

Table 3.14. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each HARD tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). ............................................................................................... 140

Table 3.15. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each DENTAL tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). .............................................................................................. -14 1

Table 3.16. Descriptive statistics representing the mean and standard error of the mean for each of the cephalometric measurernents for reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). ...................... 142

xvi

CHAPTER 1

LITERATURE REVIEW

INTRODUCTION

During the last two decades digital imaging has become widely used as a medical

imaging format, which will ultimately replace conventional radiographic film. At the

present time both conventional film and digitally based radiographic imaging methods

are being ernployed in the dental profession both at the institutional level and the pnvate

clinical level. Digital imaging implies that the diagnostic image is in a digital format

instead of a conventional film. The acquisition, storage, enhancement, retrieval and

display of a digital image has been made possible with advances in computer technology.

With regard to image capture, the process may be direct or indirect. The direct

conversion of energy fiom the residual x-ray beam to a digital form for display is defined

as direct digital imaging. Conversely, the conversion of a conventional radiographic film

into digital fonn is called indirect digital imaging.

Only a few studies have dealt with the accuracy and reproducibility of identiwng

cephalometric landmarks on a digital image (Hagemann et al, 2000; Geelen et al, 1998,

Lim and Foong, 1997, Nimkam and Miles, 1995 and Macri and Wenzel, 1993) and even

fewer have evaluated cephalometic measurernents (Eppley and Sadove, 1991). The

application of digital imaging to orthodontie cephalometry will depend on whether these

images yield as much diagnostic information as is currently available on conventional

radiographic films. Also, in order to facilitate image storage, transmission and

manipulation, it is necessary to consider using image compression to reduce the size of

the image files ( K m , 1997). There are no published studies that examine the impact of

image compression on diagnostic accuracy or reproducibility of cephalometric analysis.

Direct Digital Imaging Techniques

Currently, two methods are used to obtain a direct digital radiographie image. They

include: 1) a charged couple device (CCD) and 2) photo-stimulable storage phosphor

plate (PSP) systems. These digitally acquired images can either be viewed directly on the

computer monitor or a hard copy of the image using a laser printer can be produced.

Charged Couple Device

The CCD device captures the energy fiom the residual x-ray beam directly or indirectly

fiom a scintillation device and the resulting analog electrical signal is digitized and

converted to an image by computer software. The entire process is continuous and the

image can be viewed within seconds after exposure of the CCD.

Storage Phosphor Technique

Sonada et al, 1983, first reported the second method, known as photo-stimulable storage

phosphor (PSP) technique. The storage phosphor plate is a thin (less than 1 mm), flexible

re-usable plate of polymer material coated with a photo-stimulable phosphor compound.

When these barium fluorohalide crystals are exposed they store the energy of the incident

x-ray photons and release this energy after the crystals are stimulated with a passing

scanning helium neon laser. The energy is released by each crystal in the f o m of

fluorescent blue light and is proportional to the amount of energy absorbed from the x-ray

beam. The process is termed photostimulated luminescence. The resulting fluorescent

blue light is amplified, converted to a time series electrical signal, which is digitized, and

processed into an image by computer software. A second passing laser scan removes al1

traces of residual energy within the crystal lattice and the imaging plate is then re-usable

(Figure 1.1). Unlike conventional film or the CCD devices the storage phosphor plate

has a wide dynarnic range, which in practice means that there is a linear relationship

between exposure dose and image density (Figure 1.2). In practical tenns this means that

there is wider latitude of exposure that can produce a diagnostic image unlike film or

CCD devices.

Indirect Digital Imaging Technique

Two methods are available to convert a conventional radiographic film to a digital image.

It can either be scanned into the computer or captured with the use of a digital video

camera and fiame grabber. The indirect digital image is then displayed on the computer

monitor for processing and interpretation.

Scanners

With the improved resolution and relative affordability of flat bed scanners many

orthodontists are able to digitize their cephalometric films thus converting the

conventional radiograph to a digital image. This method creates an indirect digital

image. In other words the conventional film (analog) image is converted to digital pixels

representing the gray scale of the original image. The resolution of the indirect digital

image is dependent on many factors including, the quality of the original film and the

resolution of both the scanner and the monitor (Forsyth, 1994). With a flatbed scanner

the radiograph is placed on a flat transparent surface and a light source is passed over it

(Figure 1.3). The amount of transmitted light is detected by a photocell and is converted

into a digital representation of position and intensity (Walker, 1994). A scan is only a

digital approximation of the original picture. Once the image is scanned into the

computer memory, it is graphically displayed on a computer monitor or cathode-ray tube

(CRT).

Video Cameras

Altematively, digital radiographic images c m be acquired with the aid of a video camera.

The video image is converted to a digital representation using a device called a frame

grabber and displayed on the computer monitor. The fiame grabber literally grabs an

image fiame fiom a continuous video signal. Video cameras employ at least one CCD to

detect the light that enters the camera through its lens. The resolution of the vîdeo image

is limited by the pixel count in the CCD and by the resolution inherent in the video

signals. Each pixel in the image represents a region of the film that is 0.5 mm by 0.5 mm

in size. This resolution is considerably lower than that which is possible with

transparency scanning. An additional limitation associated with video acquisition of

radiographs is related to image distortion. A video camera uses a lens to focus the image

ont0 the CCD. image distortion is introduced by the optics associated with any lens,

especially in the penpheral regions of the image.

Emerging technology will provide higher resolution video signals in the fuhire. Until

such time the combination of video carnera and fiame grabber rernains the least accurate

means of cephalometric digitization (Walker, 1994).

Understanding Digital Imaging

Optical Density

The quality of digital images is related to the number of gray levels and how the range of

gray level values is related to the optical density of the region of interest on the

conventional radiograph. The image space on a CRT is made up of tiny rectangular or

square pictiire elements termed pixels. Pixels are arranged in a series of horizontal lines

on the CRT called raster lines. Display resolution refets to the total number of pixels

displayed on the CRT and is commonly reported as the display width in pixels by the

height in pixels. The higher the resolution, the more refined the displayed images appear.

The entire displayed matrix of pixels is terrned a bitmap. The number of different

intensities or colours that each pixel can assume is termed wlour depth and is measured

in bits per pixel. Each pixel in a display of n bits pet pixel is capable of 2n different

colours or intensities. Gray-scale images may need 256 (28) (i.e., 8-bit) different

intensities to be perceived as having continuous shades of gray. in 1996, Wenzel assessed

the requirements of 3x4 cm intraoral periapical radiographs and found that diagnostic

accuracy of caries analysis was maintained when the gray levels were reduced to 6 bits,

(64 gray levels) (Wenzel et al, 1996). In medical imaging, 12-bit gray levels are required

to optimize diagnostic tasks. In order to achieve this the scanner must be equipped with

the appropnate hardware (cable connector) and software to allow 12-bit transfer to the

computer (Baker, 1997). The TWAIN connector found on most commercial scanners

intemally downsamples to 8-bits. In addition, the computer monitor can only display 8

bits or 256 shades of gray. Specific software must be used in order to Mew an image at

12 bits (Walker, 1994).

Image Display

The limiting factor in the quality of digital images may be the spatial resolution of the

display monitor, which is dictated by the nurnber of raster lines. Monitors displaying up

to 635 lines are routinely used for viewing of digital images. Where image quality is

particularly important, a 2048-line monitor should be used to give comparable resolution

to a radiographic film (Forsyth, 1994). Resolution is configured as pixels x lines, so that

a 1024 x 768 pixel monitor would have a 768-lines. The refiesh rate is also important to

prevent flicker and eye-strain. A video controller of 7SHz (refieshed every 1/75 second)

is recommended (Walker, 1994). Screen image sharpness is sometimes expressed as dots

per inch (dpi). In this usage the tenn dot means pixel not as in dot pitch. Dots per inch is

detemiineà by both the physical screen size and the resolution setting which is a fùnction

of the graphics board. Fewer dpi on a screen creates an image which has less sharpness,

and l e s resolution. If for example the resolution of the monitor is 75 dpi and images are

scanned at 150 dpi, then images will be shown on the monitor at 2 times the original size,

but nothing will be lost due to the resolution mismatch. If the monitor is large enough,

the entire image can be displayed. In the case of a 8 x 10 inch lateral cephalographic

radiograph, scanned at 150 dpi, the image size would be 1200 x 1500 pixels, which will

not fit entirely on a 1280 x 1024 screen. In this case, the user must scroll back and forth

to click on the landmarks, or the image can be resized. Resizing of the image will change

the effective resolution and should be avoided if possible.

Spatial Resolution

Spatial resolution is the ability to record separate images of small objects that are placed

closely together; it is measured in line pairs per mm (Iplmm). The smaller the pixel size,

the more detail in the image and therefore the greater the resolution. The smallest detail

detectable by the human eye is 0.1 mm by 0.1 mm. To provide digital images of

radiographs with at least as much infonnation as is available in the original conventional

radiograph, pixels no large than 0.1 mm are required, giving a spatial resolution of 5

I p / m (Walker, 1994). The direct digital radiograph created using the PSP technique

employs a neon scanner with a spatial resolution limited to 0.2 m. A light box and

computer digitizing wand provides 0.1 mm spatial resolution in both axes. However, in

1996 Conover reported the accuracy of linear measurement on digital images with a

maximum pixel size of 0.2 mm was equivalent to measurernents obtained fkom digitized

radiographs (Conover, 1996). They concludeci that PSP imaging was a viable alternative

to conventionai film.

Digital Imaging In Medicine and Dentishy

Today, most medical diagnostic image centers rely on or are planning to present, al1

diagnostic images in digital format. Direct digital imaging is employed in computed

tomography, ultrasound, nuclear medicine, magnetic resonance imaging, plain film

radiography and digital subtraction angiography. Many hospitals are in the process of

discarding conventional film radiography for a completeiy digital environment. These

institutions have replaced the conventional interpretation of "hard copy" films with the

analysis of the digital image on a computer monitor. These digital radiographie images

are stored as digital data. The image contrast and brightness c m be adjusted and other

image enhancernent tools (image processing) are available with various cornputer

software programs. On rare occasions, if a hardcopy is required, a set of images may be

selected for laser printing on paper or film. Most images are now transferable via the

Internet or on compact discs.

In the Iast ten years, research into the use of direct digital imaging for medical

radiography has progressed considerably (Forsyth et al, 1994). However, in the dental

field, its use has not been widely implemented (Hildebolt et al, 2000). Digital

radiography is less cornmon in the practice of orthodontics than in other areas of the

dental profession. in part this may be due to the lack of scientific research available to

purport its use in radiographic cephalometry, as well as the financial burden of

implementing a new radiographic system into a pnvate clinical practice.

in medicine, DICOM, (Digital Image and Communications in Medicine) is setting the

regulations regarding image processing and compression for archiving and teleradiology.

This task force is also responsible for implementing research whose data will ultimately

create regulations for al1 aspects of medical digital imaging (Wang et. al., 1988).

Digital Image Archiving and Compression

Depending on national laws and regulations the digital images have to be safely archived

for 10-30 years (Kamm, 1997). At the institutional level, storage requirernents for such

massive archiving require several Terabytes of space. This c m be accomplished by using

optical disks or magnetic tapes, which offer virtually unlimited mernory. The long-terni

stability of these systems has not been validated and the future may lie in remote storage

facili ties provided online through the Intemet.

In the private clinical setting, digital images can be stored on external hard drives

(>16Gb), DVD (17GB), Jazz drives (2GB), CD-ROM (700Mb) or ZIP drives (100-

250Mb). A floppy disc with 1.4 Megabytes (Mb) of space does not have sufficient

storage capacity for most digital images. On average, four digital intraoral images

require 1 .O Mb of space to store and an 8 x 10 inch lateral cephalograph requires about

1.5 Mb. Compression of digital images for the individual in private clinical practice

would be beneficial based on reduced cost of data storage. Also reduction in file sizes

would reduce the need for large RAM and faster computers.

Electronic archiving requires only one twentieth to one hundredth the space necessary for

archiving film (Barenghi, 1995). The immediate single or multiple accesses to the

images and the elirnination of lost or misplaced films are also advantages to the digital

fom. The transmission of images locally via a network and rernotely via the Intemet and

the access to digital archives throughout the World Wide Web (WWW), combined with

the fact that datasets of digital images may be duplicated without loss of information

(Wenzel, 1993) highlight the advantages of the digital format.

The pervasiveness of the WWW in modem society has made its impact in both the

medical and dental fields. Online radiographie consulting systems have been introduced

for use as remote conference systerns and as teleconference or network-based image

conference systems. Digital transmission of dental radiographs has previously been used

in the fields of telediagnostics and remote education (Wenzel et al, 1996). In digital

radiography, picture archiving and communication systerns (PACS) pennits links with

other communication networks, the so-called technology of Integrated Services Digital

Networks (ISDN). Image transmission speed depends on various factors and differs from

country to country. For example, dental practitioners in many countries can generally use

the WWW only through telephone lines. The transmission time for an image whose file

size is greater than 30 kilobytes may exceed 10 seconds on a telephone line even with the

fastest modem available at 56 Kilobytes per second (Kbps). With the presently available

infiastructure, an image with a file size of 30 kilobytes or greater cannot always be

handled with sufficient speed worldwide (Yuasa et. al., 1999). The introduction of high-

speed transmission networks with downstrearn rates such as ISDN (64Kbps), cable

(ZMbps), ADSL (Asymmetric Digital Subscriber Line) and satellite (8Mbps), permit

larger files sizes to be transmitted with ease. However, the upstream transfer rate is

limited to approximately 128Kbps for both cable and DSL systems. Even with these

capabilities, it would be advantageous to reduce the image file size for transmission

across networks. Thus, teleradiography necessitates image compression (Yassa et. al.,

1999). However, there exists a controversy as to whether the digital images should be

compressed solely for archiving or even to facilitate electronic transmission ( K m ,

1997).

JPEG Compression

Compression of digital images requires software to change the file format fiom that of a

standard biûnap (bmp) or tagged image file (TIFF) to an altemate format such as Joint

Photographers Expert Group (JPEG). With this fonnat, di@ ta1 information is pmanentl y

lost when the image is decompressed and it is thus refmed to as lossy compression. It

has been recently s h o w that not al1 software compresses and decompresses JPEG file

formats equivalently, which may lead to changes in image quality (Gurdal et al, 2001).

This loss of data may have an impact on diagnostic accuracy and may have medico-legal

implications (McDonnell, 1995).

The JPEG standard has become widely accepteci in nonmedical imaging applications, and

has been applied recently to images in certain areas of medicine, (Rigolin, 1996) and is

the format of choice by DICOM. JPEG is designed for compressing either full-colour or

gray scale still images of natural, real world scenes. It works well on photographs and

similar material; but not as well on lettering and simple line drawings. JPEG is designed

to exploit known limitations of the human eye, notably the fact that small color changes

are perceived less accurately than small changes in brightness. Thus, JPEG is intended

for compressing images that will be looked at by humans. A useful property of JPEG is

that adjusting compression parameters can vary the degree of lossiness. This means that

the image-maker can trade off file size against output image quality. The output quality

c m be increased to acceptable levels by changing the Q (Quantization) or (Quality)

setting, which controls the amount of information lost which c m be expressed as a ratio

of non-compressed to compressed image file size.

This Q factor algorithm has a non-linear relationship, which means that doubling the Q

factor does not double the compression. A high Q factor such as Q10 means that very

little compression has occwed and the file size rernains fairly large. In addition to

JPEG, a variety of other file formats exist. Other compression formats include; GIF

(Graphic Image File), LZW (Lempel, Zif and Welsh), Unisys and TIF (Tagged Image

File). The Apple file format is the PICT or picture file. PNG or Portable Network

Graphics is good for Iossless compression for 12 bit gray scale.

When considering image quality, lossy JPEG compression/decompression attempts to

throw away information that may not normally be visible to the human visual system.

Noisy or more random images tend to cornpress without noticeable image artifacts,

whereas less noisy images are more susceptible to more noticeable image artifacts. Great

care must be exercised in applying a lossy compression method to radiographic images.

A gray scale image such as a dental radiograph unlike colour images, cannot be

compressed by large factors, because the human eye is much more sensitive to brightness

variations than to hue variations. Lossy JPEG eliminates the high fiequency information

(ultra fine details) in the image. This type of compression systern works well when

viewing non-processed images. The effects of image processing before compression,

such as edge enhancement or digital zoom on highly compressed images can add

structure artifacts to the images making thern virtually unusable.

In surnmary, lossy compression is a category of digital image compression types that

possess the following characteristics:

*The original digital images cannot be exactly reproduced as originally acquired, but the

changes may be visually non apparent.

*The compression algorithms c m introduce artifacts into the images and affect p s t -

decompression digital image processing

*The prevalence of artifacts increases as the amount of compression increases or as the

image detail content increases

*Much higher degrees of compression are possible than with lossless compression.

Orthodontie Cephalometric Radiography

At the present time conventional film based methods of radiology rernain the standard in

the dental profession both at the institutional level and the private clinical level.

Cephalometric analysis is an essential aspect of the orthodontic diagnosis and treatment

planning. Traditionally, acetate tracings are made and analyzecl according to one or more

of the many cephalometric analyses that have gained common clinical acceptance. The

cephalometric tracing is also used as the starting point during the formulation of so-called

visualized treatment objectives (VTO) or surgical treatment objectives (STO).

Cephalometric treatment planning may include estimates of growth, contemplatecl

orthopedic intervention, orthodontic tooth movements, and orthognathic surgery.

The quantitative analysis of a cephalometric radiograph is performed by first producing

an acetate tracing of the film. The process of cephalometry involves the identification of

numerous anatomical landmarks on the image that are tramferreci to the tracing and used

to manually calculate linear and angular values, which are considered to be of diagnostic

importance. The errors of identification of anatomic landmarks on conventional

cephalographs have been thoroughly investigated (Baumrind and Frantz, 197 la and b,

Midgard et al., 1974, Houston et al, 1986; Sandler, 1988). Baumrind and Frantz reportecl

large variation in the magnitude and configuration of the envelope of error for the 16

anatomical landmarks studied. They recomrnended routine replication of landmark

estimates to d u c e these errors.

The use of a light box and computer linked digitizing wand to locate and transfer a small

subset of meaningful landmarks fiom the conventional cephalometrk radiograph into the

computer was introduced over thirty years ago, as an alternative to acetate tracings of the

film and manual measurements. In the literature, Uiis technique has been refmed to as

"digitizing the x-ray", which should not be confused with an indirectly digitized image,

since "digitizing the x-ray" merely creates a line drawing representation of the x-ray

image. In this thesis, this technique will be r e f d to as "computer aided analysis". This

method is currently used for both orthodontic and orthognathic surgical cephalometric

analyses, but has not entirely replaced the manual acetate tracing method. Indeed, some

clinicians still trace the film on acetate first and then transfer the landmarks to the

computer using the tracing on the light box instead of the radiograph. Houston showed

that this gave no advantage to improved diagnosis, and is more time consuming

(Houston, 1982).

Digital imaging in orthodontie cephalometric radiography has many advantages over

traditional film based cephalometric analysis. With the aid of computer software,

landmark identification can be perfomed on the computer monitor using the mouse

cursor to identify the landmarks, and rneasurements, both linear and angular, can be made

automatically with the software program. The VTO and STO, which use the

pretreatment radiograph as a tempiate for growth, treatment and surgical predictions, are

simplified with the computer-aided software. Some of the more cornmonly used

cephalometric software packages available on the commercial market include: DFP,

(Dentofacial Planner), OTP, (Orhodontic Treatment Planner), Quick Ceph, PorDios, and

Digigraph (http://www.novicom.com~op~readyreE/imaging.asp).

Cohen and Limey first developed automated landmark identification software (Cohen

and Linney, 1984). The artificial intelligence (AI) software enables the computer

(without the aid of the clinician), to automatically locate specific points on the digital

image and then calculates linear and angular measurements using these landmarks. To

date these systems have shown only varying degrees of success. Parathasarathy and

colleagues showed that 71 percent of cephalometric landmarks could be accurately

located with resulting error in landmark reproducibility similar to those of two expert

observers (Parathasarathy et.al., 1984). At present the computer is unable to match the

ski11 of the trained human observer in accuracy of landmark identification (Forsyth et al,

1994). However, since most of landmark error is due to inter- and intra-observer

differences, AI could eliminate observer related manual error in cephalometric analysis

(Liu and Cheng, 2000).

REVIEW OF THE LITERATURE

A review of the literature indicates that there is no agreement as to which imaging

technique yields an image that is superior for accurate and reproducible cephalometric

analysis. As each new technique is developed it must be rigorously investigated to be

sure that it yields as much diagnostic information as is currently available on

conventional film. Since the images derivecl fkom various forms of digital technology

have different image characteristics such as spatial resolution they cannot be compared

with one another. Therefore, each type of digital image must be considered as a unique

entity and guidelines should be developed that specifically relate to the characteristics of

that image format. Research studies looking at the use of direct digitization (Hagemann

et al, 2000; Geelen et al, 1998, Lim and Foong, 1997), indirect digitization (Nimkam

Miles, 1995 and Macn and Wenzel, 1993), and digitized tracings (Houston, 1982 and

Richardson, 1981) do not provide clear guidelines for their use. Many factors must be

considered before implementing new techniques. For instance since cornputer aided

analysis of digital cephalometric images is more efficient than rnanual methods, there

will be the time for replicate landmarking, which increases precision, reducing the

obsener random error in the reproducibili ty of landmarks (Baumrind and Frantz, 1 97 1 a).

Many studies have found the main source of error in cephalometry to be the visual

identification of the landmarks (Richardson, 1966; Midtgard et al., 1974; Houston et al.,

1986) and thus one of the efforts to improve the precision in landmark identification

should be directed towards improvement in the image quality (Eppley, 199 1 ; Macri and

Wenzel, 1993). The ability to enhance the digital images after they have been acquired

rnay have advantages over conventional film radiography. Jackson, Dickson and Birnie,

subjectively assessed cephalometric image processing and its effect on landmark error

(Jackson, et. al., 1985). Contrast and brightness manipulations were found to improve

certain landmark localizations, but were limited by the amount of information contained

within the original image.

Several studies have been published comparing indirectly digitized radiographs to

conventional film images. Macri and Wenzel compared conventional cephalometric film

to digital images acquired through use of a video camera and fiame grabber (Macri and

Wenzel, 1993). The results showed that the reliabiiity of landmark location using low-

cost/quality digital equipment was inferior compared to that obtained with conventional

equipment. The main problem with the digital system arose fiom its insufficient spatial

resolution of 0.5 mm in the x-axis and 0.3 mm in the y-axis, which affected both the

image quality and measuring accuracy.

Nimkarn and Miles captured radiographic films and their tracings to the computer using a

RGB composite video carnera OiJimkarn and Miles, 1995). Using Quick Ceph

cephalometric software, linear and angular measurements were calculated fiom the

landmarks identi fied on the computer monitor. So-called vertical and horizontal values

were obtained for each landmark which could be related to the x and y coordinate systern.

They found that differences in reproducibility existed only in the horizontal plane, not in

the vertical plane when cornparing the digital to the hand-measured methods. They

concluded that distortion was probably o c c h n g in the horizontal plane during video

capture and that the screen rnay also contribute to distortion especially along the right

side of the screen.

Transparency scanners are the prefmed method over video carnera for cephalometric

image acquisition transforming the analog film to a digital image. No studies have been

published looking at the difference in cephalometric landmark identification between

scanned and conventional radiographic images.

There are many published studies testing the diagnostic yield of intraoral dental

radiographic digital images using CCD as comparecl to conventional intraoral film.

Specifically these studies have investigated the accuracy of detection of caries,

penodontal and apical diseases (Nair et al, 1998; Hildebolt et al, 1994). However, many

of these study designs are flawed by ernploying artificial disease or in vitro imaging

procedures, which do not replicate parameters that influence image geometry and image

quality in the clinicai setting. Further technological development of CCD devices is

needed to produce an image that approaches the spatial resolution of conventional film.

Unfortunately, the results and conclusions of these intraoral imaging studies cannot be

used as guidelines for digital imaging devices in cephalometry, since intraoral

radiography has little bearing on the applications used in conventional film-screen

techniques such as cephalometric radiography. Few studies have dealt with the accuracy

and reproducibility of identifjring cephalometric landrnarks on a direct (SPP) digital

image (Hagemann et al 2000, Geelen et al, 1998 and Lim and Foong, 1997). Only one

study examined cephalometric measurements on digital versus conventional radiographs

(Eppley and Sadove, 199 1 ).

When comparing the reproducibility of cephalometnc landmark identification,

Hagemann reported that the reproducibility of 5 out of 21 cephalometric landmarks was

significantly higher on the digitally obtained hardcopy images (using storage phosphor

technology), than on conventional radiographs (Hagemann et al, 2000). For 16 out of 2 1

landrnarks, no significant difference in reproducibility was found between the two

modalities. In contrast a study by Lim and Foong, comparing digital image hardcopies

produced by storage phosphor systern found no significant difference in landmark

identification compared to conventional radiographs for 17 landmarks using differences

in both x-y coordinates (Lim and Foong, 1997). Although digital hard copy may produce

acceptable images, laser printers capable of this resolution are very expensive. These

two studies do not assess the reproducibility of landmark identification using monitor-

displayed images, which is the modality of choice for the future of digital radiography.

Although Hagemann et al, and Lim and Foong both used a paired design for their

experiments, the conventional and digital radiographs were not obtained during the same

x-ray exposure, but randomly assigned to one technique (either conventional or digital)

for the first set of images and the altemate modality used on the follow-up image. Since

the images are not identical then a comparison of landmark identification has

questionable validity. In both studies landmarks were evaluated but Hagemann did not

represent their corresponding x-y coordinates (Hagernann et al, 2000). Identification of

landmark error in the horizontal and vertical direction is important since this error is

known to have a systernatic envelope of error (Baumrind and Frantz, 197 1 a). Cornparison

of the documented envelope of error between conventional film and digital images would

be of value to determine if this error will be consistent in al1 image modalities.

Only one study that compares images displayed as a digital hard copy image, computer

monitor display and conkentional film (Geelen et al, 1998). in this paired samples

experiment, the digital image and conventional film were obtained simultaneously using

a single radiation exposure. However, they did not perfom replicate landmark

identification, which has been shown to be essential for reducing random error

(Baumrind and Frantz, 1971a). The reproducibility of landmarks was not assessed in

terms of x-y CO-ordinates but by the mean difference in millimeters as measured on a

Cartesian grid for each of the 21 landmarks among image moddities and among

observers. One of the advantages of the direct digital image is its ability to capture both

the hard and soA tissue elements within the sarne image. The digital image c m be

enhanced on the computer monitor in order to highlight various osseous structures and/or

the soft tissue. It would be interesting to look at the reproducibility of sofi tissue

landmarks between the two image modalities (conventional and digital); unfortunately,

soit tissues were not included in the landmarks for this study.

They found that digital images acquired by the storage phosphor system displayed on a

computer monitor had a lower precision than conventional radiographie film. Overall

reproducibility for a full cephalometric recording (sum of 21 landmarks) was lower for

the monitor-displayed image than both film and a hardcopy of the digital image, between

which there was no significant difference. No clinical significance could be attributed to

the differences since the effect of the error on cephalometric measurements was not

evaluated (Geelen et al, 1998).

The reason for differing results for these studies lies in the different digital image

modalities used (hardcopy versus monitor for digital image display) and the

methodology. For instance the use of x-y coordinates to measure reproducibility

differences in landmark identification; the choice and number of landmarks evaluated, the

number of observers and whether the landmarks measurernents were repeated. The

statistical analyses also differ between studies and the manner in which the error of

landmark identification is reported can make cornparisons between studies difficult.

Battage1 reviewed the literature and found no single accepted procedure for calculating

method error (Battagel, 1993). Multiple t-tests tend to overestimate the error and

increase the Type 1 error. The absolute difference between replicate readings, and

between methods, must be used since it is the magnitude of error that is important and not

the direction. If the integer value is used it will underestimate the emor of landmark

identification and measurement. Additionally, an ability to demonstrate statistical

significance does not necessarily have any clinical meaning (Battagel, 1993). Therefore,

statistical results should be related to clinical significance whenever possible.

Although these studies evaluated reproducibility of landmark identification, the clinical

significance of this difference, or the effect on the final angular and linear cephaiometnc

measurements, was not evaluated in these studies. Although the reproducibility of

landmark identification is an important deteminant, it is also critical to know if the

digital image is producing accurate landmark identifications, since this impacts on the

clinical accuracy of cephalometric measurements.

Eppley and Sadove in 199 1, published the on1 y study that deals with the clinical accuracy

of cephalometric measurements (Eppley and Sadove, 1991). In this paired samples

study (in which the conventional and digital radiograph were obtained in a single

radiographic exposure), both digital and conventional radiographic images exhibited

comparable accuracy at identifjmg bony landmarks, however, digital hardcopy images

were consistently supenor at delineating soft tissue relationships. This study did not

evaluate the digital image on the computer monitor. Although angular cephalometric

measurernents were recorded, no linear measurements were recorded. Furthermore, the

cephalometric landmarks' x and y-coordinates were not examineà and replicate

measurements were not performed. Additionally, the sample size was small with only

one obswver and the clinical significance limited since the six angular measurements are

derived from landmarks not distributed widely over the cephalometric radiograph.

There are no published studies that examine the impact of image compression on

diagnostic accuracy of cephalometric analysis. However, a study on the effects of image

compression on subjective image quality and accuracy of caries detection by Wenzel and

colleagues dernonstrated that an image compression of 12: 1 could be applied before there

was a significant loss in accuracy and image quality (Wenzel, et. al., 1996). In the

medical field, the lossy JPEG ratio of 15: 1 does not affect clinical decision-making for

digital coronary angiograms (Baker et. al, 1997). Unfortunately, the results of these

studies cannot be extrapolated to cephalometnc radiography because of the differences in

the diagnostic task and different film types. In the medical arena, DICOM has been

developing new rules and standards for digital storage, retrieval, transmission,

duplication and quality assurance in the changing field of radiology (Benn et. al., 1993).

DICOM is recomending JPEG compression as its standard.

Caution must be used when attempting to determine a compression ratio that could be

applied universally to al1 radiographs. The degree of compression applied may vary

depending on the quality of the original radiograph being obsened and the type of digital

image, e.g., indirect compared to direct image. It would be unredistic to insist that one

level of compression be used for al1 types of display, transfer, and storage. For

applications that do not have an immediate impact on patient care, small amounts of

image distortion and diminution in diagnostic accuracy may be tolerable. High levels of

compression may enable the clinician at a referral centre to send images fiom a patient's

file on a floppy disk to the pnmary provider at the time of discharge for illusirative

purposes, to complement the written discharge and sumrnary notes. Radiographs that are

relatively old with a low probability of being used for clinical purposes can be stored

inexpensively in a highly compressed format. In contrast, when immediate decisions

regarding patient care depend on the results of image interpretation, a rigorous standard

must be met before a compression mode can be broadly applied (Baker, 1997).

Ultimately, lossy compression can degrade image quality, therefore, in order to use it in

orthodontie cephalometric radiography we must determine what the minimum

compression ratio at which diagnostic information will be lost. The clinically acceptable

compression ratio is unknown at this time.

STATEMENT OF THE PROBLEM

There is no consensus in the published research to date on the reproducibility or accuracy

of the diagnostic information contained in digital cephalometric films compareci to

conventional radiographs and there is no study examining the loss of diagnostic

information with image compression of digital cephalometric images. A clinically

signifiant question is not just the evaluation of the identification of landmark mor but

the significance of the differences in linear and angular measurements in the

cephalometric analysis. It is clear that in the h r e digital imaging will replace

conventional film based radiographie methods. The establishment of standards for digital

images, especially as it applies to the cephalomebic radiography will be of interest to the

orthodontist.

PURPOSE OF THE STUDY

The purpose of the study is to compare the reproducibility and accuracy of landrnark

identification and cephalometric measurements between conventional radiographic film

and the corresponding indirect or direct digital image. Additionally, indirect digital

images will be compressed using JPEG lossy compression and compared to the aon-

compressed image.

NULL HYPOTHESES

Hypothesis 1

1. There are no significant differences in the reproducibility and accuracy of landmark identification between the indirect digital image and the conventional radiographic film image.

Hypothesis 2

2. There are no significant differences in the reproducibility and accuracy of landmark identification and cephalometric measurement between lossy JPEG compressed indirect digital images and the non-compressed indirect digital image.

CHAPTER 2

Hypothesis 1

1. There are no significant differences in the reproducibility and accuracy of landrnark identification and cephalometric measurement between the direct digital image and the conventional radiographic film image.

FIC. 12-12 ~ i ru !h i#9 iq wlth Psi! Mer the PSP has ben u t p d to r-ray kanu, it iJminthcreab.outunitandmdbyabkun.LaKiwnnHig-the- of energy from the PSP. This cntrgy, the interisrty of which is di- pqmthul to the quantity of x-nys abrorkd by the PSP, is in the fonn of visible Igtit, which is detectd by a photomultiplier &nd dignirrd to fonn an imagc. After the PSP has kcn nad, it is erased by illumination with vishie light and expod again.

Figure 1.1. Flow diagram representing the direct digital imaging technique fkom White and Pharoah, 2000.

FIG. 1 2- 1 3 Exporum latitude. The range of x-ray enpo- sures w e r which PSPs respond with densities in the diagnos- tically useful range is wider than that of radiographie film. This gives PSPs a much wider exposure latitude than film.

Figure 1.2. H and D curve f?om White and Pharoah, 2000.

Figure 1.3. Photograph o f transparency flat-bed scanner showing placement of lateral cephalographic radiograph for indirect digital irnaging.

CHAPTER 2

INDIRECT DIGITAL IMAGING AND COMPRESSION

Impact of irreversible Joint Photographie Experts Croup (JPEG) compression on

the reproducibiiity and accuracy of indirect digital cephalometric radiographs

ABSTRACT

The present study evaluated and compar5d the reproducibility and accuracy of

cephalometric analysis for: 1) conventional film and indirect digital image (DI), and 2)

JPEG compression of indirect digital images.

Four observers recorded 24 landmarks fkom nine lateral cephalometric radiographs and

from the indirectly digitized image (using a transparency flat bed scanner). X and y-

coordinates were recorded for each landmark identified on the computer monitor for both

image types within DigiPlan software.

SimilarIy, indirectly digitized images of twenty-one films selected fiom the Burlington

Growth Centre, were compressed to JPEG 4 2 (25:l) and 4 7 (12: 1). Three observers

identified 17 landmarks for each image file type directly on the computer monitor within

DigiPlan. X and y-coordinates were recorded and 1 2 cephalometric measurements

calculated.

Intraclass Correlation Coefficient (ICC) showed good overail examiner reliability for

both conventional and indirect digital images. One-way ANOVA (p<0.05), showed

JPEG 4 2 to have lower accuracy of landmark identification when compared to non-

compressed DI for A point-x, condylion-y, nasion-x, and measurements FMIA, lower

incisor to A-Pg and Upper incisor to NA. JPEG 4 7 did not decrease the accuracy of any

of the 12 measurements when compared to non-compressed DI.

Indirect digital radiographie images interpreted on the compter monitor have equivalent

landmark reproducibility as compared with conventional film. For indirectly digitized

radiographs, JPEG compression rates of 12: 1 had equivalent accuracy of cephalometrk

measurernent compared to non-compressed images, but 25: 1 compression did not.

INTRODUCTION

At the present time both conventional film and digitally based radiographic imaging

methods are being employed in the dental profession both at the institutional level and the

private clinical level. Digital imaging has been available for application in dental

radiology for more than two decades and may ultimately replace conventional

radiographic film. The advantages of the digital image format (digital imaging) are many.

Digital radiographic images can be processed (enhanced), easily stored and retrieved and

transmitted by teleradiology (Forsyth et al, 1996). National laws and regulations may

require digital images to be archived for 10-30 years (Kamrn, 1997). At botli the

institutional and private level, storage requirernents could be enormous for a digital

image archive. Additionally, the speed of transmission of digital images depends on

various factors, one being the file size of the image. Reducing the size of image files by

image compression has potential benefits for both storage and teleradiology. The

adoption of digital imaging and image file compression to the practice of orthodontic

cephalometry will depend on whether these modalities can yield images equivalent to

conventional radiographic films. Equivalence can be measured by the reproducibility and

accuracy of landmark identification and cephalometric measurements.

The conversion of a conventional radiographic film into digital form is called indirect

digital imaging. This can be achieved with a transparency flatbed scanner. The resolution

of the indirect digital image is dependent on many factors including, the quality of the

original film and the resolution of both the scanner and the monitor (Forsyth, 1994,

Walker, 1994).

There are no apparent published studies examining the difference in the reproducibility or

accuracy of cephalometric landmark identification between conventional radiographic

film and indirect1 y digi tized images acquired with transparent y flatbed scanners.

However, there are two published studies that evaluated cephalometric landmark

reproducibility for indirect digital images acquired by video carnera and displayed on

computer monitors compared to the conventional films (Macri and Wenzel, 1993;

Nimkam and Miles, 1995).

Macn and Wenzel in1993 compared conventionai cephalometric film to digital images

acquired through use of a v i d a camera (Macri and Wenzel, 1993). The reliability of

landmark location using low-cost digital equipment was inferior compared to that

obtaind witb conventional equipment. The digital system produced images with

insufficient spatial resolution of 0.5 mm in the x-axis and 0.3 mm in the y-axis, which

affected both the image quality and measurement reproducibility.

Nimkam and Miles, digitized radiographic films and their tracings using a RGB

composite video camera (Nimkarn and Miles, 1995). Using Quick Ceph cephalometric

software, linear and angular measurernents were calculated from the landmarks identified

from the computer monitor. Vertical and horizontal values were obtained for each

landmark which could be related to the x and y coordinate system. Differences in

reproducibility existed only in the horizontal plane, not in the vertical plane when

comparing the digital to the conventional methods. They concluded that distortion was

probably occumng in the horizontal plane during video caphue and that the screen may

also contribute to distortion especially along the right side of the screen.

Transparency scanners are the prefmed method over video camera for transforrning the

conventional cephalometric film to a digital image (Walker, 1994). The digitally

acquired image c m either be viewed on a computer monitor or a hard copy of the image

can be printed on paper. However, the reasonable and accepted practice is to interpret the

digital image on the computer monitor. With the aid of computer software, landmark

identification c m be performed on the computer monitor using the mouse cursor.

JPEG (Joint Photographie Experts Group) is a standardized image compression

mechanism. The JPEG standard has become widely accepted in nonmedical imaging

applications, and it has been applied to images in certain areas of medicine (Rigolin,

1996). It is the format of choice of DICOM (Digital Image and Communications in

Medicine). JPEG is "lossy", meaning that the decompressed image is not the same as the

original. When considenng image quality, lossy JPEG compression attempts to throw

away information that may not nomally be visible to the hurnan visual system. Care

must be exercised in applying a lossy compression method to radiographie images. A

gray scale image such as a dental radiograph, unlike colour images, cannot be

compressed by large factors, because the hurnan eye is much more sensitive to brightness

variations than to hue variations.

The arnount of information lost when applying JPEG compression to digital images can

be varied according to the Q (Quantization or Quality) setting. This Q factor algorithm

has a non-linear relationship and therefore a high Q factor such as QI O means that very

little compression has occurred and the file size rernains fairly large. in addition Q factors

are not standardized among software products that utilize JPEG compression and the

method of compression/ decompression may also Vary with the software. Gurdal and

colleagues found that lossy JPEG decompression was perfomed di fferentl y on two of the

software prograrns under investigation (Gurdal et. al., 2001). Specifically, variations in

JPEG compression/ decompression was shown to introduce potentially deleterious

variations to radiodensity data, and the resulting gray-scale values were not concordant

with tmth.

There are no published studies that examine the impact of image compression on

diagnostic accuracy of cephalometric analysis. However, a study on the effects image

compression on subjective image quaiity and accuracy of caries detection by Wenzel et

al, 1996, dernonstratecl that an image compression of 12: 1 could be applied before there

was a significant loss in accuracy and image quality. In the medical field, the lossy JPEG

ratio of 1 5: 1 does not affect clinical decision-making for digital coronary angiograms

(Baker et. al, 1997). Unfomuiately, the results of these studies cannot be extrapolated to

cephalometnc radiography because of the differences in the diagnostic task and different

film types.

The degree of compression that can be applied to any diagnostic image will depend the

native quality of image as well as the diagnostic task. When irnmediate decisions

regarding patient care depend on the results of image interpretation, a rigorous standard

must be met before a compression mode can be broadly applied (Baker, 1997).

The aim o f the present study was to evaluate and compare the reproducibility and

accuracy of cephalometric landmark identification and measurements for conventional

film and the corresponding indirect digital images (part 1) and detennine an acceptable

JPEG compression ratio for these indirect digital cephalometric images (part 2).

MATERIALS AND METHODS

The equipment

1) Computer: A Del1 Computer with a Pentium 3 microprocessor with 220 MHz and

128 MB RAM

2 ) Operating System: Windows 95

3) Hitachi SuperScan Supreme 803, 1 2 8 0 ~ 1024 pixels 2 1 -inch monitor was used to

view the digital images.

4) An AGFA -ARCUS II professional flat bed scanner with a transparency adaptor.

http://www.image-acquire.codscanner/htm1/agfaarc2.sh~~

5) Software:

a. Adobe Photoshop 4.0

b. DigiPlan (Orthovision) cephalometric software

c. Microsofi Excel; Office 2000

d. Statistical Software Package for Social Sciences (SPSS, Inc, Chicago, 11,

US)

6) Light box and man finish acetate tracing paper (0.003 inch) and HB pencil

Part 1

Radiographie Sample: Indirect DI versus conventional film

The sample consisted of 9 randomly selecteû lateral cephalographic radiographs fiom the

files of patients attending the Graduate Orthodontie Department at the University of

Toronto, for treatment. The radiographs were selected so that 3 fiom each of Angle Class

1, II and III were represented. The head films were of acceptable quality and

representative of those used in the department of orthodonties. The subjects were aged

between 10 and 1 7 years.

Landmark identification protocol: Indirect DI versus conventional film

Twenty-four hard, dental and soft tissue cephalometric landmarks (Table 2.1 and Table

2.3) were selected according to the following criteria: 1) common usage in cephalornetnc

analysis, 2) widely distributed over the total lateral cephalograph and 3) documented

level of reproduci bil i ty (Baurnrind and Frantz, 1 97 1 a). Four observers identi fied these

landmarks on nine conventional films and on the corresponding indirect digital image.

The observers included two qualified orthodontists, one postgraduate orthodontie resident

and a technician with thirty years of expertise tracing lateral cephalornetric radiographs.

Landmarks were ide~itified on the conventional radiograph as viewed on a light box and

recorded on acetate tracing paper. Replicate tracing and landmarking was done for each

of the nine radiographs in random order, with at least a one-week interval between

tracings, by each observer independently. The tracings and the conventional radiographs

were then scanned at 150 dpi (dots per inch). The digitized image of the line tracing of

the conventional film with the landmarks clearly visible, was precisely superimposed

over the indirect digital image of the corresponding radiograph using Adobe Photoshop

4.0 and saved as a TIFF. In this way the Cartesian coordinate systern for both the tracing

and the digital image were identical and no adjustrnent had to be made for resolution

errors, since both tracing and conventional film were scanned at the same resolution of

150 dpi. The 8 x 10 inch image scanned at 150 dpi and viewed on the computer monitor

(1 280x 1024 dpi) is equivalent to a resolution of 4 l p / m and 0.17 mdpixel. The TIFF

images of the tracings of the conventional films were randomly opened in the DigiPlan

program. Landmark sampling was perfomied within the DigiPlan program by selecting

the previously identified landmarks on the scanned tracings of the conventional film.

Similady, for each of the indirect digital images, landmarking was done on the monitor

using DigiPlan program in replicate with at least one week separating events. Obseners

were blinded to patient identification for the radiographic images.

Landmark identification on both the conventional films and the computer monitor was

done in a dimmed room. The observers were allowed to manipulate the digital image by

changing the contrast and brightness. Additionally, a zoom fùnction allowed enlargement

of the image. The x and y-coordinates for each of the twenty-four landmarks were

automatically transferred to an Excel spreadsheet within the DigiPlan program.

Part 2

Radiographie Sample: JPEG compression of indirect digital image

The sample consisted of twenty-one randomly selected lateral cephalographic

radiographs fiom the Burlington Growth Centre Archives, University of Toronto,

Canada. The radiographs were selected so that 7 fiom each of Angle Class 1, II and III

were represented. The subjects were aged between 12 and 14 years. Selection criteria

inciuded, permanent dentition only and radiographic quality deemed acceptable in t m s

of contrast and resolution.

Landmark identification protocol: (JPEG compression)

Seventeen commonl y used skeletal and dental cephalometric landmarks (Table 2.1 ) were

selected (same criteria as in Part 1) and identified by three observers on indirect digital

radiographic images as viewed on the cornputer monitor. The observers were the sarne as

in Part 1 excluding the technician. Each of the twenty-one radiographs was scanned and

digitized as in Part 1 and the resulting non-compressed TIFF images represented the

baseline, 'gold standard' image resolution. These images were then opened in Adobe

Photoshop 4.0 and saved as Quality factor 2 (Q2), (25:l) and Quality factor 7 (Q7),

(12:1) JPEG compressed image files. Al1 images were then opened in the DigiPlan

cephalometnc program and were coded and read blind in a varying randomized sequence,

thus ensuring examiner impartiality. Landmark sarnpling was perfomed within the

DigiPlan program in replicate with at least one week separating events and under the

same conditions as in Part 1. The x and y-coordinates for each landmark and six angular

and six linear measurements (Table 2.2) were transferred automatically to an Excel

spreadsheet.

Data Treatment

Al1 statistical analyses were performed using the software Statistical Package for the

Social Sciences (SPSS inc, Chicago, USA).

Part 1

Indirect digital image versus conventional film

Repeated measures ANOVA was used to calculate intraclass correlation coefficients,

(ICC) (Norman and Streiner, 1998) which measures the reliability among the observers

and within each observer for replicate readings. Each image modality (conventional and

indirect digital) was dealt with independently for the calculations. However, the image

type was factored into a four-way ANOVA model, which also examined the impact of

landmark tissue type (soft, hard or dental), patient and observer.

Descriptive statistics representing the mean and standard deviation of the absolute

diflerence between replicate readings (reading 1 minus reading 2) of the x and y-

coordinates for each of the 24 landmarks by image modality were calculated using the

pooled data fkom al1 four examiners. Additionally, the sum of al1 landmark errors by

tissue type for x and y-coordinate values were also calculated.

intra-modality reproducibility was assessed using the paired t-test, p<0.01 and one-way

ANOVA models at pc0.05. The paired t-test was also employed to evaluate inter-method

reproducibility, using the absolute differences between image modalities (conventional

minus indirect digital image) with the conventional image considered the gold standard,

( ~ ~ 0 . 0 5 ) .

Inter-modality accuracy was tested using one-way ANOVA for within coordinate

comparisons, pc0.05 and the paired t-test for the between coordinate comparisons,

@<O.OS).

Part 2

JPEG compressed (42 and Q7) versus non-compressed (TIFF)

Accuracy of landmark identification was statistically analyzed using the non-compressed

digital image as the gold standard. The pooled average values fiom replicate readings

fiom al1 three observers were used in the analysis stratified by x and y-coordinates and

landmarks. The absolute difference in landmark identification measurement was

calculated separately between the non-compressed image minus the compressed images,

either Q2 or Q7. Statistical analyses involved paired samples t-tests and ANOVA models

(p<0.05) to evaluate both intra- and inter-image file type reproducibility and accuracy.

For the intra-image file type reproducibility, the statistical level of probability was set at

p<O.01 because of multiple testing and the chance of Type 1 mor, while for inter-image

file type reproducibility the level of probability was set at ~'0.05. The analyses

perforrned to evaluate accuracy used pcO.05 for between image file types and x and y-

coordinate calculations.

Additionally, the sum of al1 landmark and measurement errors by image file type, tissue

type or measurement type, for x and y-coordinates were calculated.

RESULTS

IntraClass Correlation Coeffkieat for indirect digital versus conventional images

Intra-Examiner Reiiability

The range of ICC values pooled together from a11 four examiners indicated a high degree

of reproducibility of landmark identification (ICC > 0.90). The only exceptions were

Condylion-y, which had the lowest ICC score of 0.59 along with sofi tissue Glabella-y

with a score of 0.70. Examiner 4 had the lowest ICC scores for the conventional film

images compareci to al1 other examiners (Appendix Table 2 .W. 1 1 ). However, overall,

there were no differences among examiners, tissue type or patients using an ANOVA

mode1 (Appendix Tables 2.12- 2.14).

Inter-Examiner ReIiabüity

The calculated ICC values by x or y-coordinate between image modalities for al1 4

observas cornbined revealed that inter-examiner reliability was high, with a minimum

ICC of 0.86 regardless of image modality or tissue type, (Appendix Tables 2.15-2.17)

Intra-Modality Reproducibility

Al1 landmarks showed statistically significant differences in reproducibility between

replicate readings for both the wnventional film and the indirect digital images at the 1 %

level using the paired sarnples t-test (Table 2.3). However, clinically a threshold of 1 mm

is the tolerable limit for landmark errors (Liu et. al., 2000) thereby reducing the nurnber

of landmarks deemed both statistically and clinically significant. Specifically,

conventional images were found to be statistically and clinically significantly dieerent in

reproducibility of replicate readings in 13 of 24 landmarks. Classified by tissue type the

conventional film images showed lower reproducibility for 4 out of 5 sofi tissue, 8 out of

13 hard tissue and 1 out of 5 dental tissue landmarks, (Table 2.3). Digital images were

found to be statisticall y and clinicall y si gni ficantl y di fferent in reproducibility of replicate

readings in 12 of 24 landmarks, 4 of the 5 soft tissues, 6 of the 13 hard and 2 of the 5

dental tissue landmarks. The y-coordinates had an overall sum total for landmark m o t

that exhibited lower reproducibility than the corresponding landmark measured in the x-

coordinate for both image modalities. The sum of the error measurernents by tissue type

revealed that reproducibility was statistically significantly (paired t-test, p<0.05) lower in

the y-coordinate for both soft and hard tissue, but the dental tissue was not significantly

different between x and y-coordinate within each of the image modalities. The sum total

for al1 landmarks showed that within both conventional and digital image modalities, the

y-coordinate (25.66 and 24.46 mm) is less reproducible compared to the x-coordinate

(1 8.80 and 1 8.23 mm), respective1 y, p4.05, (Table 2.3)

Inter-Modality Reproducibility

Conventional and indirect digital images were found to be statistically significantly

different from each other (paired t-test @<O.OS or p<0.01)) in t m s of reproducibility of

landmark identification for 3 of 24 landmarks. That is, the digital image was more

reproducible for sofi tissue (nasion-y) and hard tissue (B point-y and menton-x)

landmarks, (Table 2.3). The sum total of al1 the errors for each landmark tissue type

revealed that for so fi tissue landmark, y-coordinate onl y, reproducibility is statisticall y

signi ficantl y better for the indirect digi ta1 image (7.64 mm) compared to conventional

images (9.43 mm). However, no statistically significant difference exists between the

imaging modalities for the sum total of al1 the errors when cornparhg x-coordinates

( I 8.80 vs. 1 8.23 mm) and y-coordinates (25.66 vs.24.46 mm), conventional and indirect

images respectively (Table 2.3).

Inter-Modality Accuracy

Table 2.4 shows the error of landmark identification between image modalities

conventional ('gold standard') minus indirect digital image. Statistically and ciinically

significant differences in accuracy were found between the two image modalities for 9 of

the 24 landrnarks. Specifically, a total of 3 out of 5 soft, 4 out of 13 hard and 2 out of 5

dental tissue landrnarks were less accurateiy identifieci for digital imaging. Table 2.4 also

shows the sum total of landmark identification errors in the x and y-coordinate and the

subtotals by tissue type. There was a statistically significant difference. which showed

that digital images were statistically significantly less accurate than conventional film

image in landmark identification in the y-coordinate (32.38 versus 27.59, respectively).

Compression of Indirect Digital Image Fies Using JPEG 4 2 and 4 7

Intra-Image Reproducibility

Statistically significant differences in reproducibility between replicate readings for each

image file type at the 1% level using the paired samples t-test are presented in Table 2.5.

Landmarks that also exhibited clinically significant loss in reproducibility (landmark

error > 1 mm) within each image file type are presented in terms of x and y-coordinates as

follows. In the x-coordinate, the TIFF images showed less reproducibility in 7 out of 17

landmarks, 5 out of 13 hard and 2 out of 4 dental tissue landmarks. The 4 2 images

showed lowa reproducibility in landmark identification for 8 out of 17 landmarks, which

included 6 hard and 2 dental tissues. Interestingly, the 47 image had only 1 hard tissue

landmark with lower reproducibility out of the total 17 identified landmarks. In the y-

coordinate, the TIFF images had a total of 6 out of 17 landrnarks with lower

reproducibility, including 4 hard and 2 dental tissue landrnarks. The 4 2 images had a

total of 10 out of 17 landmarks, including 8 hard and 2 of the dental tissues with lower

reproducibility, while 47 had a total of 7 out of 17, with 5 hard and 2 dental tissues

exhibiting Iowa reproducibility within the image file type. Additionally, al1 the images

had lower reproducibility determineci in the y-coordinate by the sum total of mors than

the corresponding landmark in the x-coordinate for within image comparisons, (Table

2.5).

Cephalometric angular measurements FMA, FMIA, SNA and Upper incisor to NA and

linear measurement Upper face height, were less reproducible within the 42 image, as

were FMA and FMIA for both TIF and 47 images (Table 2.6).

Inter-Image Reproducibility

Statistically significant differences were found between image file types for the surn total

of al1 landmark errors with the 4 2 image having a lower reproducibility in landmark

identification than either the TIFF or the 47 images in both the x-coordinate (1 7.82 mm

vs. 15.01 and 13.30 mm) and y-coordinate (22.34 mm vs. 16.34 and 15.97 mm),

respectively (Table 2.5). However, the 47 image is not statistically different fiom the

TIFF image in terms of reproducibility in the y-coordinate, but showed higher

reproducibility in the identification of dental landmarks (Table 2.5).

Looking at the individual landmarks, 4 2 images were less reproducible in 6 out of 17

landmarks, 4 (ANS-x, Nasion-y, Orbitale-x and y and PTM-y) out of 13 hard, and 2

(lower incisor apex-x and y and upper incisor apex-x and y) out of 4 dental tissue

landmarks compared to TIFF images (Table 2.5). The 4 7 images had lower

reproducibility in only 2 out of the 17 landmarks, both in hard tissues (Nasion-y and

PTM-y) (Table 2.5).

The sum total of the mors for angular and linear measurements revealed that the 4 2

images are statistically significantly less reproducible than TIFF or 47 images, and 47 is

not different fkom TIFF. No difference was found between image file types for the linear

measurements (Table 2.6). However, 4 2 images were statistically significantly different

for the sum total emor of angular measurements compared to TIFF and 47, while 4 7 was

not different fiom TIFF (Table 2.6). TIFF, Q2 and 47 images exhibited equivalently low

reproducibility for the angular measurements, FMA and FMI& but only Q2 images were

less reproducible for Upper incisor to NA angle and the linear measurement Upper face

height (Table 2.6).

IntefiIrnage Accuracy

Tables 2.7 and 2.8 present the results for the cephalomeûic landmarks and measurement

mors, respectively, represented by the absolute difference between non-compressed

('gold standard') and compressed images, i.e., TIFF-Q2 and TIFF-Q7. The statistical

analysis (paired t-test, p<0.05) revealed 4 hard and 1 dental tissue landmarks to be

significantly less accurate for 4 2 than 4 7 images when compared to TIFF images. The

landmarks included; A point-x, Nasion-x, ANS-y, Condylion-y and upper incisor tip-y.

For the y-coordinate sum of al1 mors, the 4 2 images (12.18 mm) were statistically

significantly (p~O.05) less reproducible than the 47 images (1 1.48 mm) in landmark

identification when compared to the gold standard TFF image (Table 2.7).

The sum total of al1 measurernents, both angular and linear, revealed 4 2 ( 1 1.34 mm) to

have lower accuracy than 47 (8.33 mm), but only for the angular cephalometric

measurements (7.09 versus 5.1 0 mm, respective1 y) (Table 2.8). The angular

measurements - FMIA, Upper incisor to NA, and the linear measurement - Lower incisor

to A-Pg, were statistically less accurate for 4 2 than 47 images when compared to non-

compressed TiFF images (Table 2.8).

Interestingly, although 42 images exhibited some statistically significant decrease in both

reproducibility and accuracy compared to 47 images in both landmark identification and

measurement these errors were below the clinically significant tolerance level (i.e.,

>2mm or 2 degrees (Cohen, 1984)). However, a trend was s h o w for 4 2 images to be

different fkom non-compressed images for 13 of the 17 landmarks in the x-coordinate and

9 of the 17 in the y-coordinate and for 1 1 of the 12 measurements (Figures 2.1 through

2.3).

To illustrate the error of Iandmark identification between TIFF and 4 2 images,

scattergram plots of landmark error for Nasion and ANS are depicted for each image file

type in Figures 2.4 through 2.7. Nasion and ANS have statistically significantly iower

reproducibility and accuracy of landmark identification for 42 images compared to non-

compressed images. Each scattergram displays 126 data points, which represented

landmark identification error, obtained ffom 3 examiners in replicate on 2 1 radiographic

images. The spread of error around the mean for a given landmark for each of the image

modalities could then be assessed visually. These figures show a similar pattern of

distribution for both non-cornpresseci and 4 2 images.

DISCUSSION

When comparing the analysis of conventional films versus indirect digital images (Part 1)

the key findings were: a) inter and intra-examiner reliability was excellent between the

indirect digital and conventional image, b) conventional and indirect digital images

showed similar variability in landmark identification with the y-coordinate exhibiting a

lower reproducibility than the x-coordinate for both images. When comparing the

analysis of non-compressed versus 4 2 (25: 1) or 47 (12: 1) digital images (Part 2) the key

findings were: a) 4 2 compressed images were 19% less reproducible in the x-coordinate

and 37% less reproducible in the y-coordinate than non-compressed images, while 4 7

images were actually 1 1% more reproducible in the x-coordinate, but showed quivalent

reproducibility in the y-coordinate to non-compressed images for landmark identification;

b) reproducibility of angular and linear measurement was lower for the 4 2 image

compared to either TIFF or 4 7 images with the error of anguiar measurements greater

than the linear measurements for 4 2 images; c) 4 2 images were less accurate than 4 7

images in 5 out of 17 landmarks when both were compared to non-compressed images

and; d) 4 2 images were less accurate than 4 7 for 3 of 12 cephalometric measurements

when both were compared to TIFF images.

IntraCIass Correlation

Inter and intra-observer variability is known to be the greatest source of error in landmark

identification (Richardson, 1950). In this study, the intraclass correlation coefficient

statistics showed that both inter and intra examiner reliability was excellent. This is very

important when evaluating a new image format, since any differences in reproducibility

and accuracy could then be attributed to the image and not the examiner. Therefore, in

this study a lower reproducibility and accuracy in landmark identification on the JPEG

Q2 image is attributed to the loss in image quality caused by lossy compression.

Hard Tissues

ANS and A Point are anatomical landmarks with low radiodensity and are reported in the

literature as having very high landmark identification error on conventional film

(Salzmann, 1964, Bjork and Solow, 1962). The 4 2 image was shown to have lower

reproducibility and accuracy for both these landmarks than the 4 7 image. B point y-

coordinate was found to have lower reproducibility in the Q2 image, which may be a

function of how distinctly the contour appeared in the region of the point of interest and

image degradation due to the loss of representative pixels. Landmarks which lie along the

hard tissue edge such as Nasion, B point, ANS, and A point, will al1 be subject to image

degradation by JPEG compression aigorithrns since image brightness is usually quite low

along the edge for low density bony tissues and edges tend to become fuzzy with high

levels of compression. Landmarks, which lie in the middle of the radiograph, such as

Condylion, Orbitale and PTM tend to be difficult to locate due to superimposition of

cranial anatomical structures. Since JPEG compression eliminates data redundancy it

may be discarding pixel information, which would nonnally help to distinguish

supenmposed structures. This could account for the lower reproducibility of landmark

identification of Condylion, and PTM-x coordinate, for the 4 2 image. Sella had

equivalent reproducibility between 4 2 and non-compresseci images. This can be

attributed to the technique for locating this landmark, which involves visually finding the

centre point of the bony contour, which would not be affected by compression.

Dental Tissues

A radiographie image is a 2-dimensional representation of a 3-D object, which creates

overlap of the incisal roots, making the localization of a given root apex difficult. in Part

2 of this study, the identification of crown tips was more reproducible than root apices in

al1 image file formats. Additionally, the identification of incisal root apices was

statistically and clinically significantly less accurate and less reproducible in the 4 2

image çompared to non-compressed image. These regions on the radiograph cm be

difficult to analyze since there is an overlapping of incisor roots, and with subsequent file

compression, small image details may be lost which may be important for accurate

landmark identification.

X versus y-coordinates

Many landmarks in the y-coordinate lie along a smooth contour, such as A point and B

point. Thus, the random error would be greater in the y-coordinate because of the

inability to locate a precise point of identification, resulting in a dispersion of

identification points along the smooth contour, irrespective of image quality (Baumrind

and Frantz, 197 1 a). This may explain the results of this study that found that al1 image

file types showed lower reproducibility in the y-axis compared to the x-mis, for both

hard and dental tissues. The fact that Q2 images were found to be statistically and

clinically significantly less accurate and less reproducible in the y-coordinate compared

to TIFF and 47 images cannot be explained at this time.

Summary of Landmark identification error

The landmarks on 42 images that were found to be least accurate included Nasion, A

point and Condylion and those with a lower reproducibility included ANS, Nasion,

Orbitale, PTM, lower and upper incisor apex as compared to non-compressed images. It

is not possible to determine how the compression algorithm caused these particular

landrnarks to have greater error of identification compared to the other 17. However,

these landmarks tend to be less reliable even on the conventional film (Baumrind and

Frantz, 1971), thus the loss of digital information which occurs with file compression,

increased the error for difficult anatomical points.

Cephalometric Measurements

The accuracy of cephalornetric measurement was greater for the 47 images than the 42

images when both are compared to TIFF images (8.33 versus 1 1.34 mm). Baumrind and

Frantz, reported that angular measurements are less reliable than linear in a full

cephalometric analysis which is consistent with the findings in this study (Baumrind and

Frantz, 197 1 a). Angular measurernents are calculated from a minimum of 3 landrnarks,

whereas linear measurements use only 2 landmarks therefore they are more likely to have

greater errors. The measurernents that were found to be less accurate (FMiA, lower

incisor to A-Pg and upper incisor to NA), al1 involved the landmark A point in the

calculation. Since A point was found to be both statistically significantly less

reproducible and less accurate, it follows that the measurements which rely on this

landmark would show similar inaccuracies.

Pattern of Distribution of Error for Landmark Identification

Baumrind and Frantz described the distribution of errors for landmarks as systematic and

following a typical pattern (non-circular envelope) making certain landmarks more

reliable in either the horizontal (x) or vertical (y) plane depending on the topographie

orientation of the anatomical structures along which they are defined (Baumrind and

Frantz, 197 la). in this study Nasion, A point, Condylion and ANS al1 follow similar

distribution errors around the x-y Cartesian coordinate system irrespective of image file

type. The pattern of distribution was also found to be similar to those reportecl by others

(Liu, Chen and Cheng, 2000). Therefore, the loss of digital information in the

compressed image does not appear to alter the typical pattern of error distribution in the x

or y coordinate. For instance the pattern for the distribution moi. for the identification of

landmark ANS on both the non-compressed and 4 2 images was dong the x-axis,

consistent with previous observations in the literature for this landmark (Liu, Chen and

Cheng, 2000 and Baumrind and Frantz, 197 1 a).

The results of this study indicate that JPEG (47) compression at a ratio of 12: 1 can be

useà to perform routine cephalometric analysis, without loss of clinical orthodontie

diagnostic information. This recommendation is based on the fact that reproducibility and

accuracy of cephalometric measurement, botb linear and angular, are not statistically

significantly different on the 12:l compressed image when compareci to the non-

compressed image. However, this does not indicate that this amount of compression is

acceptable for other functions such as the detection of pathological changes.

interestingly, al1 examiners agreed subjectively that the 4 2 images had visibly lower

resolution and did not feel confident in the amount of information contained within the

radiographic image at that level of compression. Even though the examiners were

blinded to the type of image file under interpretation, they were al1 able to distinguish

between the 42 and TIFF files, while the 4 7 and TIFF images were not as readably

visibly distinguishable (Figure 8). Although this is a subjective judgment, clinicians

would likely reject such images and deem hem unsuitable for diagnostic interpretation.

The present study was not designed to look at a large number of compression levels to

determine precisely the level at which diagnostic accuracy begins to suffer. For

cephalometric analysis, a specified compression level may yield sufficient information

for diagnosing craniofacial proportions, but may not be adequate for longitudinal growth

studies or for the cornparison of different treatment modalities. Further studies involving

a multicentre investigation with more examiners and a variety of film qualities and

compression levels would provide further information regarding the effects of image

compression on cephalometric analyses. With additional information precise

recornrnendations regarding acceptable levels of compression for cephalometric analysis

can be made.

JPEG compression algorithms may not be the best approach for compression of gray

scale radiographs. Other mathematical compression models may in the fùture be better

able to limit the amount of compression selectively in areas of diagnostic interest.

The next generation of compression techniques will likely move away from pixel-based

methods like JPEG (Discrete Cosine Transform), toward mathematical methods similar

to wavelets and fiactals, which can achieve much higher levels of compression with a

minimum impact on image quality (Rigolin, et al., 1996). Therefore there will be a

continua1 need for research concerned with the digital format of diagnostic images.

CONCLUSIONS

Reproducibility of landmark identification was equivalent on conventional films and their

corresponding indirect digital images recorded on a computer monitor. Similarly

equivalent reproducibility of landmark identification on non-compressed and JPEG

compressai images at the 12:l level were found. However, there was a statistically

significant decrease in the reproducibility and accuracy of landmark identification and

measurements when a compression level of 25: 1 was employed when compared to the

non-compressed image.

Table 2.1. Definitions of cephalometric landmarks. Landmark Definition Soft Tissues Embrasure

Soft tissue Nasion

Soft Tissue Pogonion

Soft tissue Subnasaie

Upper Lip Nasal Tip

Hard Tissues Anterior Nasal Spine (ANS)

Basion

Condylion

Gnathion

Menton

Nasion Orbitale

Point A

Point 8

Porion

Posterior Nasal Spine (PNS)

Pterygomaxillary Fissure (posterior) (PTM)

Sella

Dental Tissues Upper Incisor Apex

Lower lncisor Apex

Lower l ncisor Tip

Upper Incisor Tip

Tip Maxillary Molar

Deepest point on the contour of the upper lip as it meets the lower iip Point where the S-N plane interçects with soft tissue nose Most prominent point on the contour of the soft tissue covering the chin Point on the contour of the inferior aspect of the nose as it joins the filtrum of the lip Vermilion border of the upper lip Most prominent aspect of the soft tissue contour of the nose

Anterior tip of the anterior nasal spine of maxilia Lowest median point of the anterior margin of the foramen magnum Estimated mid-point between the most superior and the most posterior points on the head of the condyle Mid-point between pogonion and menton atong the contour of the chin Lowermost point on the mandibular symphyseal shadow Most anterior point of the nasio-frontal suture Lowermost point on the infraorbital (inferior) margin of the orbit Deepest concavity on the anterior border of the maxilla Deepest concavity on the contour of the alveolus of the anterior mandible Machine porion-centre of the ear roc! attachment of the cephalostat Most posterior aspect of the palatal process of the maxilla Most posterior-superior point on the contour of the Pterygomaxillary Fissure Centre of the hypophyseal fossa (sella tursica)

Most superior point on the root of the most prominent upper incisor Most inferior point on the root of the most prominent lower incisor Most superior point on the incisal edge of the most prom inent lower incisor Most infenor point on the incisal edge of the most prominent upper incisor The tip of the most mesial cusp of the upper first maxillary molar

Table 2.2. Cephalomeûic measurements calculated by the DigiPlan software. Measurement Definition Angular Facial angle lnterior angle formed by the intersection of the

line nasion-pogonion with the tine porion- orbitale

FMA Angle determined by the intersection of the line drawn through points porion, orbitale with the line drawn through gonion, menton

FMlA Frankfurt-mandibular incisor angle- the acute angle formed by the intersection between the line porion-orbitale and the line lower incisor edge to the Iine nasion- Point B, measured perpendicular to that line.

SNA Angle determined by the point's sella, nasion and Point A.

SN6 Angle determined by the point's sella, nasion and Point B.

Upper 1 to AP (angle) Acute angle formed by the intersection of the line Point A-pogonion and the line upper incisor edge-upper incisor apex.

Linear Lower 1 to A-Pg Distance in millimeters (mm) from the point on

the edge of the moat prominent lower incisor to a vertical line drawn from Point A to pogonion

Lower 1 to NB (mm) Distance (mm) from the point lower incisor edge to the line nasion-Point B, measured perpendicular to that line.

Lower anterior face height Distance (mm) from the point Anterior nasal spine to menton

Mandibular unit length Distance (mm) from the point condyiion to gnathion

Maxillary unit length Distance (mm) from the point condyiion to Point A

Upper face height Distance (mm) from the point anterior nasal spine to nasion

Table 2.3. Mean and standard deviation (SD) in millimeters (mm) for the absolute difference between replicate readings (reading 1 minus reading 2) for cephalomeîric landmarks by image modality, x and y-coordinate and tissue type, N=36.

CONVENTIONAL INDIRECT DlGlTAL IMAGE 1 Reading 1 - Reading 2 1 1 Reading 1 - Reading 2 1

Mean Mean Mean Mean ANATOMlC 2 SD (mm) ltSD (mm) ISD (mm) +SD (mm)

TISSUE TYPE LANDMARK X abordinate Y cosrdinate X mordinale Y coiirdinate s o n Embrassure 1 .O1 I0.99 0.61I0.83 1.12 M.91 0.59f0.4t

Glabella 0.66M.59 3.06 e . 9 1 0.47 10.42 2.79 k3.81

Nasal Tip 0.50I0.33 0.90I0.62 0.32I0.26 0.70I0.56

Nasion 0.86I0.87 2-92bej995 0.73I0.72 1.47bcf1.40 Subnasale 0.90îO.94 0.71I0.52 0.7910.66 0.86M.69

Upper lip 0.6139.52 1.23 M.97 0.5810.49 1.23 I0.84

TOTAL 4.54 9.43 4.01 ' 7.64 HARD ANS l.ooC* 1-01 0.64 f 0.53 1.20' I1.13 0.47t 0.48

A point 0.471 0.31 0.81 I 0.57 0.74 I 0.84 1.07Cs1.02

Basion 0.90 f 0.77 0.88 f 0.64 0.58 I 0.56 0.78 i 0.86

~ondylion 0.95 c 1.35 1.69 +_ 1.73 1.53 I 1-48 2.78 ' k 4.79

Menton 1.37~+1.41 0.77-10.65 0.71bi0.72 0.66t0.66

Nasion 0.38 10.32 0.59 î 0.45 0.48 f 0.55 0.93 + 1.34

Orbitale 1.28Ck1.54 0.9711.13 1.49C+2.47 1.41Ct1.69 PNS 0.87 1: 0.84 1.04' * 0.86 0.82 * 0.82 0.63I 0.66

Pogonion 0.64 -c 0.50 1.36 s 0.92 0.67 î 0.58 1.48 t1.26 Porion 0.57 10.60 0.49 k 0.58 0.41 f 1.28 0.39 f 1.41

PTM 0.88 10.88 1.70Ci 1.81 0.591 0.51 1.0OCk 0.84

Sella 0.49 f 0.36 0.51 f 0.37 0.25 10.27 0.32 i 0.26

TOTAL 10.39 12.86 9.91 12.77' DENTAL Lower 1 ~ p e ~ 0.96 i 0.73 0.79 * 0.75 1.26 f 1.23 1.34 k 1 .O5

Lower 1 Tip 0.62k1.30 0.4920.63 0.5611.24 0.54I0.79

. .

Upper 1 Tip 0.51 +. 0.49 0.52 i 0.46 0.39 î 0.43 0.41 i 0.42

TOTAL 3.87 3.37 4.31 4.05 ALL SUM TOTAL 18.80 25.66 ' 18.23 " 24.46 '

- -

Paired t-test; statistically significant difference between replicate readings when pc0.01, within images.

b N.B. All values listed in table reached this level.

Paired t-test; statistically significant difference between replicate readings when ~ ~ 0 . 0 5 , between x or y coordinates within each image modality.. Clinically significant difference deterrnined if mean difference >l mm. Paired t-test; statistically signifcant difference between replicate readings when pc0.05,

between image rnodalities by x or y-coordinates.

Table 2.4. Mean and standard deviation (SD) in millimeters (mm) for the absolute difference between image modalities (conventional images minus indirect digital images (DI)) for cephalometrk landmarks by x and y-coordinate and tissue type, N=36.

1 Conventional-Indirect DI 1 1 Conventional-Indirect DI 1 X-CO-ORDINATE Y-CO-ORDINATE

TISSUE TYPE LANDMARK Mean + SD (mm) Mean I SD (mm) SOFT Embrassure 2.08 i 1.78 1 .O7 ' i 0.78

Glabella 0.87+ 0.71 3.23 ac I 3.35

Nasal Tip 0.40I 0.31 0.92 î 0.77

Nasion 1.19' I 1.08 2.25 ac I 1.79

Subnasale 1.20'1 1.34 1.24 ' + 0.88

Upper lip 0.82 k 0.61 1.27 î 1.10 TOTAL 7.62 9.98

HARO ANS 1.49 ' f 1.12 0.64 I 0.66

A point Basion 6 point Condylion Menton Nasion Orbitale PNS Pogonion Porion PTM Sella 0.48 1ç, 0.47 0.47 i 0.43

TOTAL 14.41 16.28

DENTAL Lower 1 Apex 1.61 ac I 1 .24 1 .67~11.21

Lower 1 Tip 0.69 i 1.25 0.66 i 0.75 Tip Upper Molar 0.98 î 1.14 1 .O6 ' I 0.88

Upper 1 Apex 1.78 I 1.49 1 . 5 6 ~ f 1.39 . .

Upper 1 Tip 0.50 I 0.51 1.17 ' k0.82 TOTAL 5.56 6.12 - - - - --

ALL SUM TOTAL 27.59 32.38

Paired t - k t ; statistically significant difference belween replicate readings when p<O.Oi, within x or y coordinate. N.B. All values listed in table reached this level.

One-way ANOVA; statistically significant difference arnong landmarks when ~ ~ 0 . 0 5 , within x or coordinate.

Y Paired t-test; statistically signifcant difference between image modalities whan pe0.05, betmwn x and y coordinate. Clinically significant difference determined if mean difference > h m .

Table 2.5. Mean and standard deviation (SD) in millimeters (mm) of the absolute difference between replicate readings (reading 1 minus reading 2) for each of the cephalometric landrnarks by image file type, N=2 1 . TIFF refers to no compression. 47 refers to JPEG compression Quality factor of 7 ( 1 2: 1 ). 4 2 refers to JPEG compression Quality factor of 2 (25: 1).

X-COORDINATE

A POINT ANS B POINT BASION CONDYLE GNATHlON MENTON NASION ORBITALE POGONION PORION PTM SELLA 0.46I0.42 0.39I0.32 0.48k1.24 TOTAL 11.53~ 13.38 ' 10.98

Lower 1 apex 1 .6obC*i .48 2.23% -73 0.93 b ~ . 9 5 Lower 1 tip 0.4310.37 0.4710.67 0.27I0.30 Upper i apex 1 .00bC~.80 1 .M% .25 0.8810.92 Upper 1 tip 0.45I0.44 0.4010.44 0.2410.25 TOTAL 3.48 ' 4.44 2.32' SUM TOTAL 15.01d 17.82' 13.30

Paired t-test; statistically significant difference between reac

Y-COORDINATE 1 Reading 1-Reading 2 1

Tl FF Q2 Q7 MEAN f SD MEAN f SD MEAN f SD

16.34 22.wd 15.97 7gs when p<O.Ol, within image file

types. N.B. All valuesin table reached significance for withinimage file types. b~ne-way ANOVA; statistically significant difference among landmarks when p<0.05. between image file types. C Clinically significant difference when mean is >l mm. d~aired t-test; statistically significant difference between readings when p<0.05. behnreen image file type totals using TlFF as the 'gold standard', TlFF vs. Q2 and TlFF vs. Q7.

Tabk 2.6. Mean and standard deviation (SD) in millimeters (mm) and degrees of the absolute difference between replicate readings (reading 1 minus reading 2) for each of the cephalometric measurments, by image file type, N=2 1 . TIFF refers to no compression. 47 refers to JPEG compression Quality factor of 7 ( 1 2: 1). 42 refers to JPEG compression Quality factor of 2 (25: 1).

( ~ ~ ~ , . j i ~ l-Rmding 2 1 1 Reading 1-Reading 2 1 1 Reading 1 -Reading 2 1 MEASUREMENT TIFF Q2 a7

ANGULAR ~ e a n + S D MeankSD ~eanIsD (DEGREES) (mm) (mm) (mm)

Facial Axis 1.72f1.38 1.6611 -1 3 1.3511.24 FMA 2.30 bCtl .67 3.35 bCkZ.32 2 . 5 0 ~ kl.98 FMlA 1 -97 "=&1.20 3.20 k ~ . 8 1 1 SbC 21.68 SNA 1.7611.48 2.15' 32-16 1.65f 1.61 SNB 1.62k1.64 1.70+1.76 1.49k1.4 1 Upper 1 to NA 1 -82 bil -21 2.99 "e.39 1 .63b~1 .60

TOTAL 11.19' 15.05~~ 10.57~

LINEAR (mm) Lower Face Ht 1.44k1.14 1.38I1.15 1.07M.99 Lower 1 to A-Pg 1.06M.95 1.0*1 .O2 1.06I0.69 Lower 1 to NB 0.74a.60 0.72k.84 0.56M.56 Mandibular Length 1.8451.64 1.89k1.54 1.8ôf 1.63 Maxillary Length 1.752 1.55 1 -621 -34 1 .83t1 -34 Upper Face Ht 1.89rt1.5 1 2.08- 1.73 1.62k1.38 TOTAL 8.72 8.78 7.99 SUM OF ALL MEASUREMENTS 19.91 23.83de 18.56*

a Paired t-test; statistically significant difference between replicate readings when P<O.O?, within image file types. N.B. Alt values in table reached this level. b~ne-way ANOVA; statistically significant difference among landmarks when P<O.OS. between image file types. C Clinically significant difference when mean is >2mm for distances or >2 degrees for angles. d~aired t-test; statistically significant difference between replicate readings when Pc0.05. between image file type totals using TlFF as the 'gold standard', TlFF vs. Q2 and TlFF vs. Q7. e Paired t-test; statis~cally significant difference between replicate readings when P<O.OS. between image file type totals Q2 vs. Q7.

Table 2.7. Mean and standard deviation (SD) in millimeters (mm) of the absolute difference between image file types (non-compressed minus compressed images (42 or 47)) for each of the cephalometric landmarks, by x or y-coordinate, N=2 1 . TIFF refers to no compression. 47 refers to JPEG compression Quality factor of 7 ( 1 2: 1 ). 42 refers to .if EG compression Quality factor of 2 (25: 1).

TISSUE ~NATOMICA~ TYPE NDM MARKS HARO point

ANS B point Basion Cond Sion Gnathion Menton Nasion Orbitale Pogonion Porion PTM Sella 'TOTAL

DENTAL Lower 1 apex ower 1 tip

AL1 LANOMARKS SUM TOTAL

(mm) (mm)

0.66~10.35 0 .38~f 0.35

t-test; statistically significant difference between

MeanfSD MeanfSD (mm) (mm)

0 . 7 a 1 .Z3 0.7%1-25 0.91a*1 -42 0.71~11.34 0.84fl.84 0.7411 .O1 0.58fl.68 0.59i-û.69 0 .94~s .83 0.67~f0.89 0.4333.73 0.44I0.70 0.54a.66 0.5310.73 0.9&1 .O3 1 -03a-99 O.87f 1.42 0.71 11.42 0.8539.87 0.7010-86 03833.75 0.35a.66 0.8=.72 1.0011.18 0.19kO.26 O-20I0.25

9.05 8.44 0.845-1.16 O.8Of 1 .O6 0521.25 0.55i1.29 1.12k1.80 1.2011.83 0.65~11.25 0.4gaf 1.31

3.13 3.04

12.18~ t 1.48= TlFF and Q2 or Q7 when

between image file types and within x or y-coordinates.

Table 2.8. Mean and standard deviation (SD) in millimeters (mm) and degrees o f the absolute difference between image file types (non-compressed minus compressed images (42 or 47)) for each of the cephalometric measurernents, N=2 1 . TIFF refers to no compression. 47 refers to JPEG compression Quality factor of 7 ( 1 2: 1 ). 4 2 refers to JPEG compression Quality factor of 2 (25: 1).

MEASUREMENT 1 TIFF-CU 1 1 TiFFU7 1 ANGULAR-

-

~eankSD ~eankSD (DEGREES) (mm) (mm)

Facial Axis 0.56S.71 0.6610.84 FMA 0.9=.81 0-5H.62 FMlA 1.57 ai0.47 0.78~k1.16 SNA 1 -07M.89 0.93A4.81 SN6 1.15+1.63 1.10+_1.30 Upper 1 to NA 1.82~k1.48 1 . 0 4 ~ f1.13 TOTAL 7.09 a 5.1 a

LINEAR (mm) Lower Face Ht 0.60I0.54 0.52I0.36 Lower 1 to A-Pg 0.59 a ~ . 4 9 0.40af0.49 Lower 1 to NB 0.51iO.64 0.26I0.19 Mandibular Length 0.6710.64 0.43M.42 Maxillary Length 0.98+ 0.92 0.7H.94 Upper Face Ht 0 . 9 ~ ~ .oo 0.83M.62 TOTAL 4.25 3.23

SUM OF ALL MEASUREMENTS Il 34 a 8.33 a

a Paired t-test; statistically significant difference between image cornparisons when Pe0.05, between image file types.

Figure 2.1. Bar graph illustrating X-coordinate landmark error in millimeters (mm) for JPEG Q2-TIFF compared to 47- TIFF.

Figure 2.2. Bar graph illustrating Y-coordinate landmark enor in millimeters (mm) for JPEG Q2-TiFF compared to 47- TIFF.

Landmarks (YIColordinats)

Figure 23. Bar graph illustrating cephalometric measurement errors in millimeters (mm) or degrees for JPEG compressions Q2-TIFF compared to Q7-TIFF.

LEGEND FOR BAR GRAPHS FIGURES 2.1 THROUGH 2.3.

I. Condylion x 2. Condylion y 3. A point x 4. A point y 5. Gnathion x 6. Gnathion y 7. menton x 8. Menton y 9. A N S x 10. ANS y 1 1. Porion x 12. Porion y 13. Orbitale x 14. Orbitale y 1 5. Basion x 16. Basion y 1 7. Nasion x 1 8. Nasion y 19. Pterygomaxillary Fissure x 20. Pterygomaxillary Fissure y 2 1 . Upper Incisor Tip x 22. Upper Incisor Tip y 23. Pogonion x

24. Pogonion y 25. Sella x 26. Sella y 27. B Point x 28. B Point y 29. Lower Incisor Tip x 30. Lower Incisor Tip y 3 1 . Lower incisor Apex x 32. Lower Incisor Apex y 3 3. Upper Incisor Apex x 34. Upper Incisor Apex y 35. Maxillary Unit Length 36. Mandibular Unit Length 37. Lower Face Height 38. Facial Axis 39. Lower Incisor to A-Pg 40. SNA 41. SNB 42. FMA 43. FMIA 44. Upper Incisor to NA 45. Lower Incisor to NB 46. Upper Face Height

Figure 2.4 and 2.5. Scattergram representing the distribution of error fkom the mean in millimeters (mm) for cephalometric landmark Nasion for JPEG 42 (25: 1) and TIFF (non-compressed) digital images. N= 126

Scattergram of error for Nasion

Scattergram of enor for Nasion

Figure 2.6 and 2.7. Scattergrarns representing the distribution of error fiom the mean in millimeters (mm) for cephalometric landmark ANS for JPEG 4 2 (25: 1 ) and TIFF (non- compressed) digital images. N= 1 26

Scattergram of ANS landmark emr for Q2 DI

Scattergram of ANS landmark enor for TIFF Dl

Figure 2.8. Cornputer display of lateral cephalographic digital images. A - JPEG 4 7 (12: 1) and B - JPEG 42 (25: 1)

CHAPTER 3

DIRECT DIGITAL IIMAGING

The reproducibiüty and accuracy of cephalometric anrlysis using conventional

radiographic fiim and direct digital images obtained by the storage phosphor

technique

ABSTRACT

This study evaluated the reproducibility and accuracy of cephalometric analysis for

conventional radiographs and direct digital images (DI) acquired simultaneously by

photo-stimulable storage phosphor plates (PSP).

One observer recorded 21 landmarks in replicate fiom 30 lateral cephalometric

radiographs ont0 acetate tracing paper using a light box. The acetate tracing was then

scanned into the computer and layered precisely over the direct (PSP) DI, creating a

digital representation of the acetate tracing with the same resolution as the PSP image.

Landmarks were identified from the PSP image and re-recorded from the digitized

acetate tracing, as viewed on the computer monitor, using DigiPlan software. The x and

y-coordinates were recorded from identified landmarks and used to calculate 12

cephalometric measurernents for both image modalities.

Conventional films and SPP images were found to be statistically significantly different

@<O.OS) in terms of reproducibility of landmark identification for 3 of 2 1 landmarks, but

not clinically significantly different (measurement error < hm). Overall reproducibility

was clinically significantly lower in the y-coordinate for 4 landmarks identified on PSP

images compared to conventional images. PSP images were statistically, (P<0.01) and

clinically significantly less accurate for 8 out of 21 landmarks, with accuracy lower

overall in the y-coordinate, but accuracy of cephalometric measurement was not

statistically or clinically different as compared to conventional film.

INTRODUCTION

Digital imaging has been available for application in dental radiology for the last two

decades and may ultimately replace conventional radiographic film. It is aiso likely that

orthodontic cephalometric images will be produced and analysed in the digital form in

the near future (Hildebolt, et al, 2000). The direct conversion of energy from the residual

x-ray beam to a digital fom for display is defined as direct digital imaging. Currently,

two methods are used to obtain a direct digital radiographic image. n iey include 1) a

charged couple device (CCD) and 2) photo-stimulable storage phosphor plate (PSP)

systerns. The resulting digital image (DI) can either be viewed on the cornputer monitor

or printed in the f o m of a hard copy.

The reproducibility of landmark identification on conventional lateral cephalographic

radiographs has been investigated (Sandfer, 1988; Houston et al, 1986; Mitgard et al.,

1 974 and Baumrind and Frantz, 197 1 gb). However, only a few studies have dealt with

the accuracy and reproducibility of identimng cephalometric landmarks on a direct DI

(Hagemann et al, 2000; Geelen et al, 1998, Lim and Foong, 1997, Nirnkarn and Miles,

1995 and Macri and Wenzel, 1993) and even fewer have evaluated cephaiometric

measurernents (Eppley and Sadove, 1991). The appiication of DI to orthodontic

cephalometry will depend on whether these images will yield quivalent reproducibility

and accuracy as is currentl y available on conventional radiographic films.

When comparing the reproducibility of cephaiometnc landmark identification,

Hagemann and colleagues, reported that the reproducibility of 5 out of 2 1 cephalometric

landmarks was significantly higher on the digitally obtained hardcopy images (using

storage phosphor technology), than on conventional radiographs (Hagemann et. al.,

2000). In wntrast another study comparïng DI hardcopies produced by storage phosphor

system found no significant difference in landmark identification compared to

conventional radiographs for 17 landmarks (Lim and Foong, 1997). These two studies do

not assess the reproducibility of landmark identification using monitor displayed images.

Although both these groups used a paired design for their experiments, the conventional

and digital radiographs were not obtained during the same x-ray exposure, but rather

randomly assigned to the first set of images or for the follow-up examination. In both

studies landmarks were evaluated but only Lim and Foong measured landmark enor

using x and y-coordinates (Hagerrnan et. al., 2000; Lim and Foong, 1997).

Geelen and CO-workers, is the only study that compares cornputer monitor displayed

images, digital hard copy images and conventional film images (Geelen et al, 1998). In

this paired samples experiment the digital image (using PSP systern) and conventional

film were obtained at the same time using a single exposure. The reproducibility of

landmark identification was investigated, but not in terms of the x and y-coordinates.

Overall reproducibility for a fiil1 cephalometric recording (sum of 21 landmarks) was

lower for the monitor-displayed digital image than both film and hardcopy of the DI.

There was no significant difference between film and hardcopy of the DI.

The reason for the diffenng results for these studies lies in the different image modalities

used (hardcopy versus monitor for digital image display) and the methodology. For

instance: the use of x and y-coordinates to measure error in landmark identification, the

choice and number of landmarks evaluated, the number of obsewers and whether the

landmarks were recorded in replicate. Although these studies evaluated reproducibility

of landmark identification, the clinical significance of landmark identification error on

the calculated angular and linear cephalometric measurements was not evaluated.

The accuracy of cephalometric measurements was examined by Eppley and Sadove in

199 1. In this paired sarnples study (the conventional and digital images were obtained in

a single radiographie exposure), the analysis of both image formats exhibited comparable

reproducibility for identifjing bony relationships, however, digital hardcopy images were

wnsistently superior at delineating soft tissue relationships. The digital image was not

evaluated on a cornputer monitor making data comparison difficult between other studies

(Eppley and Sadove, 1 99 1 ).

There is no consensus in the published research to date on accuracy or reproducibility of

the diagnostic information contained in digital cephalometric films compared to

conventional radiographs. The evaluation of cephalometric landmarks and subsequent

measurements both linear and angular is of great orthodontie clinical importance and will

help determine if digital images will eventually replace the conventional film.

It is the aim of this study to evaluate the reproducibility and accuracy of cephalometric

analysis using conventionai film and monitor-displayed digital images obtained by the

PSP technique by examining both landmark identification in the x and y-coordinate as

well as six linear and six angular cephalometric measwements.

MATERIALS AND METHODS

The equipment

Computer: A Del1 Computer with a Pentiurn 3 microprocessor with 220 MHz and

128 MB RAM

Operating S ystem: Windows 95

Monitor: a Hitachi SuperScan Supreme 803, 1280~ 1024 pixels 2 1 -inch monitor.

Scanner: An AGFA - ARCUS II professional flat bed scanner with a

transparency adaptor.

http://www.image-acquire.com/scanner/html/agfaarç2.shtml

Software:

a. Gendex software (Dentsply International)

b. Adobe Photoshop 4.0

c. DigiPlan (Orthovision technologies - Dolphin Imaging) cephalometric

software was used to analyze the images; landmark identification with x-y

coordinates, and angular and linear distance measurement calculations

d. Microsofi Excel; Office 2000

e. Statistical Software Package for the Social Sciences (SPSS, hc , Chicago,

Il, USA)

Light box and matt finish acetate tracing paper (0.003 inch) and HB pencil

DenOptix Scanner (Dentsply International/Gendex, Chicago, IL)-

http ://www .gendex-den ta1 . wm/denop.htm

High-resolution Fuji CR Storage Phosphor Plate, 8x 10 inches (Fuji Photo Co Ltd,

Tokyo, Japan for Dentsply International)

Patient Sample

The sample consisted of 30 randomly selected patients attending the Orhodontic

Department of the University of Toronto for routine treatment. One lateral cephalometric

film was exposed for each patient. The age, gender, type of occlusion, and the skeletal

pattern were not taken into consideration in the study design.

Image Acquisition

The simultaneous acquisition of a conventional film image and a DI in a single exposure

was obtained using a storage phosphor plate (SPP) positioned over the film cassette

containing the conventional film (Figure 3.1). A standard 24x30-cm cassette with

intensifjmg screen (Lanex regular) containing 8 x 10 inch Kodak T-MAT G (Kodak

code:TMG-1), Rochester, NY, film was exposed on a General Electric (GE) head and

generator at 100 rnA, 80 kvp for % second. The radiographs were taken with the patients

in a fixed natural head position in the cephalostat. The focus-to-film distance was 160

cm. The conventional films were developed in an automatic film processor (Kodak RPX-

OMAT, mode1 M7B, Rochester, NY). After exposure, the SPP plate was removed in a

darkened room and placed in the DenOptix scanner (Figure 3.2) for direct conversion to a

digital image, which was saved as an 8-bit Tagged Image File Format (TIFF) using the

Gendex software. The 8 x 10 inch image scanned at 150 dpi and viewed on the cornputer

monitor (1 280x 1024 dpi) is equivalent to a resolution of 4 I p / m and 0.1 7 d p i x e l

(Conover, 1 996).

Landmark defmition and sampling

Twenty-one standard cephalometric landmarks were rigorously defined and are presented

in Table 3.1. One observer, who was an orthodontie resident, identifie. a total of 4 sofi

tissue, thirteen hard tissue and 4 dental tissue landmarks. The landmarks were chosen

according to the foilowing criteria: 1) that they were commonly used in cephalometric

analysis, 2) that they were distributed widely over the total lateral cephalograph, and 3)

that they had been investigated previously and their reproducibility docurnented

(Baumrind and Frantz, 1 97 1 a).

Throughout the study, al1 radiographie images were coded and read blind in a varying

randomized sequence, thus, ensuring examiner impartiality. Landmark identification on

both the conventional films (using a light box and acetate tracing paper (Figure 3.3)) and

the computer monitor was done in a dimmeû room in duplicate with at least a one-week

interval between recordings. A technique for transfemng the information fiom the

conventional film acetate tracing to the computer was created. This technique involved

scanning the tracing of the conventional film with the landmarks clearly marked at 150

dpi (dots per inch). The digital image of the tracing was precisely superimposed over the

direct digital image using a layering technique with Adobe Photoshop 4.0 (Figure 3.4)

and saved as a TIFF file. In this way the resolution of both the conventional film and the

DI were identical. The landmarks previously identified on the tracing were rerecorded in

the DigiPlan program.

Landmark sampling for the direct DI was performed within the DigiPlan program using a

mouse-controlled cursor on the monitor-displayed image (Figure 3.5). The observer was

allowed to manipulate the image by changing the setting of the contrast and brightness

within the DigiPlan program (Figure 3.6). in addition a zoom function allowed

enlargement of the image. Sampling was repeated after a one-week interval and in a

different random order. DigiPlan then computed the 6 linear and 6 angular cephalometric

measurements based on the landmarks as defined in Table 3.2. The x and y-coordinates

for each landmark and the linear and angular measurements were automatically

transfmed to an Excel spreadsheet.

Statistical Analysis

Al1 statistical analyses were canied out with the software Statistical Package for the

Social Sciences (SPSS Inc, Chicago, 11, USA). The statistical power was calculated a

priori and was greater than 80%.

The results were anal ysed as follows:

Reproducibility

Descriptive statistics, such as means and standard deviations, were computed for the

absolute differences for each landmark (x and y-coordinate) and linear and angular

measurement values between replicate readings (reading 1 minus reading 2) for each

image modality. Also, the measurement mors from replicate readings of each landmark

were totalled by tissue type, x and y-coordinate and image modality.

Intra-modality reproducibility was evaluated using the paired t-test at p<0.01,

conseivatively chosen because of multiple tests increasing the Type 1 error rate. One-way

ANOVA @< 0.05) and Duncan multiple cornparison tests were performed to assess the

effect of landmark on identification error. Al1 cornparisons were done within each

modality. The reproducibility between image modalities was detemined using paired a f-

test at p<0.05.

Accuracy

Means and standard deviations of the absolute differences were calculated for each

landmark (x and y-coordinate) and linear and angular measurement values between

image modalities (conventional minus direct digital image), with the conventional image

providing the 'gold standard'. Also calculated were the landmark identification mors

beîween image modalities for each landmark totalled by tissue type and x and y-

coordinates. The paired t-test was used to compare the differences between the x and y-

coordinates.

One-way ANOVA models were pedbrmed to compare the absolute mean difference

between the two image modalities for x and then for y-coordinate with landrnark as the

factor, followed by Duncan multiple comparison tests performed to assess the effect of

landrnark on landmark identification emor.

RESULTS

Intra-modality reproducibility

Al1 cephalometric landmarks and measurements showed statistically significant

differences between replicate readings for both the conventional film and the direct

digital images at the 1% level using the paired samples f-test (Tables 3.3 and 3.4).

However, clinically, a threshold of 1 mm is the tolerable limit for landmark errors,

whereas 2 mm and 2 degrees are the tolerable limits for linear and angular measurement

mor, respectively (Cohen, 1984), which reduces the number of landmarks deemed both

statistically and clinically significant (Table 3.5). Specifically, conventional images were

found to be statistically and clinically significantly different in reproducibility of replicate

readings in 4 of 21 landmarks. They were as follows: Condylion x and y, Orbitale-x, sofi

tissue Nasion-y and sofi tissue Pogonion-y (Table 3.5).

Digital images were found to be statistically and clinically significantly different in

reproducibility of replicate readings in 8 of 21 landmarks and 3 cephalometric

measurernents. The landmarks included: 2 out of 5 of the soft tissue landmarks in the y-

coordinate (Pogonion, Nasion); 4 of 12 hard tissue landmarks in the y-coordinate and 2 of

12 hard tissue in the x-coordinate (B point-y, Basion-y, Condylion-x and y, Orbitale-x,

PTM-y), and 1 of 4 dental landmarks (Upper incisor apex-y), and the measurements

FMIA, mandibular unit length and maxillary unit length (Table 3.5).

When comparing the sum total of landmark identification m r s between x and y-

coordinates within image modality, the x-coordinate was statistically significantly,

pc0.0 1, more reproducible for both digital and conventional images than the y-wordinate

landmark identification (Table 3.3). Similarly, the sum total of measurement m r s

comparing angular versus linear measurements showed equivalent reproducibility within

each of the image modalities (Table 3.4).

Inter-modality reproducibility

Conventional and direct digital images were found to be statistically significantly

different in terms of reproducibility of landmark identification for 3 of 21 landmarks.

The digital image was more reproducible for 1 each of sofl tissue (nasal tip - y) and

dental tissue (lower incisor tip-x) landmarks, whereas the conventional image was more

reproducible for 1 hard tissue (PTM-x) landmark. However, these differences were

below clinical significance since the mean error for landmark identification was less than

one millimetre (Table 3.3).

The sum of al1 the mors for each tissue type revealed that soft tissue landmark in the y-

coordinate only had statistically significantly better reproducibility for the direct digital

image (3.7ûn-m) compared to conventional image (4.92mm). However, no difference was

seen for the sum of al1 landmarks in hard or dental tissues in either coordinate (Table

3.3). The sums of angular and linear measurements between image modalities were not

statistically significantly different in terms of reproducibility of cephalometric

measurrement (1 3.1 8 versus 14.86 for conventional and DI respectively), (Table 3.4).

Inter-modality accuracy

Landmark identification on conventional film was used as the 'gold standard' for

cornparison to direct digital images. The landmark identification error and measurement

m r (accuracy) represented by the absolute difference between the image modalities

(conventional minus digital) are shown in Tables 3.6 and 3.7, respectively. Statistically

and clinically significant differences in accuracy between the two image modalities were

noted for 8 of the 21 landmarks. @<0.01 ). One-way ANOVA (pcO.05) concurred with

these findings. The landmarks included: 5 out of 12 hard tissues (Condylion-x and y,

Gnathion-x, Nasion-y, Orbitale-x, PTM x and y), 1 out of 5 soft tissue, (Pogonion-y), and

2 out of 4 dental tissue (lower and upper incisor apex x and y) (Table 3.8). However,

there was no clinically significant difference in accuracy of cephalometric measurement,

either angular or linear between the two modalities (Table 3.7).

The sum total of the m o r s for al1 landmarks in the x and y-coordinate are s h o w in Table

3.6. There was a statistically significant difference, which showed that digitai images

were less accurate in landmark identification in the y-coordinate (1 7.76 mm) than in the

x-coordinate (14.06 mm) @<O.OS). Additionally, there was no difference in accwacy

between linear and angular measurements expressed as the sum total of mors in

measurements by measurernent type (Table 3.7).

DISCUSSION

In this study the reproducibility and accuracy of the cephalometric anaiysis of direct

digital cephalometric radiographic images acquired by the PSP technique viewed on the

computer monitor was compared to their simultaneously derived conventionai film

images viewed on a light box. The rationale for investigating monitor displayed direct

digital images is that this technology is likely to be adopted in the future practice of

orthodontics.

The key findings were 1) Conventional and direct digital images were statistically but not

clinically significantly difkent in t m s of landmark reproducibility for 3 of 21

landmarks, 2) Landmark identification was statistically significantly more reproducible

for soft tissue, y coordinate and similar in reproducibility of hard and dental tissue

landmark identification for digital images when compared with conventional images, 3)

Direct digital images were statistically and clinically significantly less reproducible in 3

hard, 1 dental tissue landmark and 3 measurements (2 angular and 1 linear) for within

image comparisons, while the conventional film image was less reproducible in only 4

landmark identification, 4) Direct digital images were statistically and clinically

significantly less accurate for 8 of 21 landmarks (1 soft, 5 hard and 2 dental tissues),

however, cephalometric measurement was equivalent in accuracy to conventional film.

It is well documented that the greatest errors in cephalometric measurement arise in

landmark identification (Richardson, 1966; Baurnrind and Frantz 197 1). Attempts to

minimize this error are the key to more reproducible and accurate cephalometric

interpretation. With the introduction of digital imaging in cephalometric radiology the

clinician and researcher must be sure that the new image modality yields as much

information as is currently available on conventional film and that reproducibility and

accuracy of landmark identification is no worse and ideally better.

Digital images were s h o w to have a statistically and clinically significantly lower

accuracy than conventional images for 5 of the 12 hard tissue landmarks investigated

while reproducibility was clinically significantly worse for 3 of the 12 hard tissues.

Interestingly, only PTM was found to be both inaccurate and to have lower

reproducibility between image modalities. Thus, it appears that it is not necessary that a

decrease in reproducibility result in less accurate landmark identification or vice versa.

Landmarks, which lie in the middle of the radiograph, tend to be difficult to locate due to

superimposition of cranial anatomical structures. This was the case for Condylion-x and

y, and PTM-x, which were found to have statistically and clinically significantly lower

accuracy of identification on the digital images and for Basion-y, which had clinically

significantly lower reproducibility compared to conventional film images. Additionally,

Condylion is often hard to locate due to superimposition of the ear rod of the cephdostat

and was found to have worse reproducibility than conventional images. Nasion can be

difficult to locate due to superimposition of the eyelid and was found to be statistically

and clinically significantly less accurately located on the digital image compared to the

conventional images.

Gnathion-x was statistically and clinically significantly less accurate for digital images

compared to conventional film. This may be due to the difficulty in interpreting images

on a cornputer monitor which does not provide the reference planes or construction lines

commonly us& in conventionai cephalomehic measurement. These references assist in

the location of certain 'midpoint between' landmarks such as Gnathion-x, which is the

midpoint between Pogonion and Menton along the bony contour of the chin.

Only Pogonion-y of the 5 sofi tissue landmarks investigated was found to have

statistically and clinically significantly lower accuracy in the digital image than

conventional image. Pogonion is identified based on the subjective definition described

as the most prominent aspect of the sofi tissue contour of the chin and does not provide

any concrete anatomical structures to orient the observer. Thus, error in landmark

identification may be a result of observer's interpretation of the landmark definition

rather than inability to visualize the landmark on the image.

The identification of crown tips was statistically significantly more reproducible than root

apices in both image modalities. As the image is a 2-dimensional representation of a 3-

dimensional object, the incisa1 roots overlap one another, making the localization of a

given root apex more difficult. For this reason, the position of upper and lower root

apices is ofien made based on the estimation of the usual length of a tooth, and on the

expected rate of taper perceived fiom the crown and visible portion of the root. Upper

incisor apex y-coordinate had statistically and clinically significantly lower

reproducibility in the digital image compared to al1 other dental landmarks, but was not

less reproducible than the conventional image. However, the upper and lower incisor

apices were found to be statistically and clinically significantly less accurately identified

on the digital image compared to the conventional image in contrast to the findings of

Hagemann and colleagues (Hagemann et. al., 2000). However, the digital image did not

improve the localization of root apices, which are known to be difficult to identie on the

conventional film (Geelan, et al, 1998).

When comparing the reproducibility of landmark identification in the x and y-coordinate,

both images showed statistically significantly lower reproducibility in the y-axis for soft

and hard tissues. For digital images, the y-coordinate landmarks were statistically and

clinically significantly less accurate than their corresponding x-coordinate. Many

landmarks lack a definite anatomic point but have a smooth contour orientated in the y-

coordinate, such as most of the soft tissue landmarks and the hard tissue landmarks

including A point, B point, and Pogonion. As such, the random error would be greater in

the y-coordinate because of the inability to define on a precise point of identification

resulting in a dispersion of identification points along the smooth contour, irrespective of

image quality (BaumRnd and Frantz, 197 1 a). This does not explain why in this study,

both A point and B point were found to have clinically significantly lower reproducibility

than conventional film when identified on the digital image. It would be easier to explain

if both image modalities had similar reproducibility for these landmarks. However, it

could be a tinction of how distinctly the contour appeared in the region of the point being

estimated on the digital image.

Baumrind and Frantz (1 97 1 a and b) described the distribution of mors for landmarks as

systernatic and following a typical pattern (non-circular envelope) making certain

landmarks more reliable in either the horizontal (x) or vertical (y) plane depending on the

topographie orientation of the anatomical structures along which they are defined. Ln this

study, Nasion-y, B Point-y, Condylion-x and y al1 follow similar distribution mors

around the x-y Cartesian coordinate system as reported previously (Baumrind and Frantz,

197 1 a).

Since error in landmark identification is often due to superimposition of anatomical

structures or poor quality of the film, then it is possible that the new digital format may

improve on these aspects of radiographie interpretation especially considering the

capability of post acquisition image processing and enhancement (Hildebolt et al, 2000).

This enhancement is made possible due to the inherent physical properties the SPP plate

system. Specifically, the storage phosphor plate has a greater dynamic range in response

to x-ray exposure than conventional film. This means that a greater range of exposure

values can be used to produce an acceptable image thus, reducing the need for repeat

exposures. Also the linear response of the storage phosphor plate pemits image

processing to correct exposure emors and to enhance the image. This enhances the display

of tissues with large differences in density such as compact bone of the cranium and thin

bone such as the ANS or soft tissue structures on one image (Hagemann et al, 2000).

The influence of image processing on the accuracy of cephalometric analysis was not

determined in this study. However, the digital images were enhanced on screen within the

cephalometric software using magnification, brighmess and contrast adjustments, which

gave the impression that the sofl tissue outline was more prominent than seen on the

conventional image. This subjective preference is supportai by studies on a phantom

skull with sofi tissue drape (Calderazzi, 1992). Indeed, the digital image showed

statistically significantly better reproducibility in the localization of soft tissue landmarks

as compared to conventional images. Unfortunately, Orbitale, an anatornical landmark

with low radiodensity, was shown to be statistically significantly less accurate in the

digital image, leading one to believe that image processing modifications are currently

limited in their ability to enhance certain anatomic landmarks.

CONCLUSIONS

The use of direct digital imaging (PSP technique) is comparable to conventional

radiographs for clinical cephalometric measurernent and would not alter the clinical

diagnoses of craniofacial proportions. The spatial resolution of film currently exceeds

that of PSP (Hildebolt et al, 2000). It is not clear, however, that the decreased spatial

resolution of PSP images compared to film images are the cause of the loss in

reproducibility and accuracy for the identification of certain landmarks in this study.

Additional investigations would be necessary to determine the impact of PSP DI on other

diagnostic and research tasks.

More speci ficall y this research dernonstrated that :

1. The reproducibility of landmark identification for direct digital images was

statistically significantly better for sofl tissues, but not clinically significantly

better than conventional film images. However, overall reproducibility of

landmarks for the digital image was clinically significantly worse in 4 other

landmarks and 3 cephalometric measurements compared to conventional film

images.

2. Direct digital images were less accurate for landmark identification but accuracy

of measurement was not statisticall y or clinicall y signi ficantl y di fferent compared

to conventional film.

91

Table 3.1. Definitions of cephalometric landmarks.

Landmark Definit ion Soft Tissues Soft tissue Nasion

Soft Tissue Pogonion

Soft tissue Subnasale

Upper Lip Nasal Tip

Hard Tissues Anterior Nasal Spine (ANS) Basion

Condyiion

Gnathion

Menton

Nasion Orbitale

Point A

Point B

Porion

Pterygomaxilla~ Fissure (posterior) (PTM)

Sella

Dental Tissues Upper lncisor Apex

Lower lncisor Apex

Lower l ncisor Tip

Upper lncisor Tip

Point where the S-N plane intersects with soft tissue nose Most prominent point on the contour of the soft tissue covering the chin Point on the contour of the inferior aspect of the nose as it joins the filtrum of the lip Vermilion border of the upper lip Most prominent aspect of the soft tissue contour of the nose

Anterior tip of the anterior nasal spine Lowest median point of the anterior margin of the foramen magnum Estimated mid-point between the most superior and the most posterior points on the head of the condyle Mid-point between pogonion and menton along the contour of the chin Lowermost point on the mandibular symphyseal shadow Most anterior point of the nasio-frontal suture Lowermost point on the infraorbital (inferior) margin of the orbit Deepest concavity on the anterior border of the maxilla Deepest concavity on the contour of the alveolus of the anterior mandible Machine porion-centre of the ear rod attachment of the cephalostat Most posterior-superior point on the contour of the Pterygomaxillary Fissure Centre of the hypophyseal fossa (sella tursica)

Most superior point on the root of the most prominent upper incisor Most inferior point on the rw t of the most prominent lower incisor Most superior point on the incisal edge of the most prominent lower incisor Most inferior point on the incisal edge of the most prominent upper incisor

Tabie 3.2. Cephalometric measurernents calculated by the DigiPlan software. Measurement Definition Angular Facial angle

SNA

SN6

Soft Tissue (ST) Angle of convexity

Upper 1 to A-Pg (angle)

Linear Lower 1 to NB (mm)

Lower anterior face height

Mandibular unit length

Maxillary unit length

Upper face height

Upper lip protrusion

lnterior angle fonned by the intersection of the line nasion-pogonion with the line porion- orbitale (Frankfurt horizontal) Frankfurt-mandibular incisor angle the acute angle forrned by the intersection between the line porion-orbitale and the line lower incisor edge to the line nasion- Point B. measured perpendicular to that line. Angle determined by the point's sella, nasion and Point A. Angle determined by the point's sella, nasion and Point B. Angle determined by the ST point's nasion, Point A and pogonion. Acute angle formed by the intersection of the line Point A-pogonion and the line upper incisor edge-upper incisor apex.

Distance in millimeters (mm), from the point lower incisor edge to the Iine nasion-Point B, measured perpendicular to that line. Distance (mm) from the point Anterior nasal spine to menton Distance (mm) from the point condyiion to gnathion Distance (mm) from the point wndyiion to Point A Distance (mm) from the point anterior nasal spine to nasion Line drawn from the tip of the nose to ST pogonion, upper lip is measured (mm) from this reference line

Tabb 33. Mean and standard deviation (SD) in millimeters (mm) of the absolute differences between replicate readings (reading 1 minus reading 2) for each landmark by image modality (N=30).

CONVENTIONAL IMAGE DIRECT DIGITAL IMAGE IReading 1 -2 ( IReading 1 -2 1 IReading 1 -2 1 IReading 1 -2 1

Mean Mean Mean Mean TISSUE TYPE ANATOMIC i SD (mm) I SD (mm) I SD (mm) I SD (mm)

LANDMARK X cwrdinate Y co-ordinate X CO-ordinate Y CO-ordinate SOFT Nasal tip 0.35 * 0.26 0.78'1 0.51 0.4110.39 0 .54~k 0.37

Nasion 0.39k0.28 1.47W~0.93 0.31i0.22 1.09W+1.36 Pogonion 0.65 10.59 1.58W~ 1.40 0.511 0.44 1.15 1.09 Subnasale 0.4810.55 0.5110.47 0.4810.39 0.4010.29 Upper lip 0.51I0.41 0.5810.53 0.4010.30 0.5210.51 TOTAL 2 . ~ ~ 4.92 C e 2.1 1 3.70'

HARO A point 0.72k0.77 0.7710.57 0.95k1.06 0.7810.64

ANS 0.82I0.68 0.61k0.51 0.9610.63 0.50î0.34 B point 0.34 I0.37 0.93 10.74 0.34 s0.26 1 . 1 6 ~ 10.74 Basion 0.74i0.57 0.93it0.82 0.971:1.30 1.09di1.07 Condylion l.llWi: 1.42 1.51"11.47 1.13"11.14 2.0fWf2.30 Gnathion 0.81 10.54 0.5 i 0.49 0.67 I 0.47 0.46 * 0.38 Menton 0.86I0.72 0.60k0.48 0.7510.66 0.53I0.58 Nasion 0.50 k 0.41 0.76 k 0.88 0.38 * 0.34 0.74 10.87 Orbitale 1.07~10.81 0.8310.85 1 . 1 0 ~ f 1.06 0.77f1.30 Porion 0.4210.36 0.5810.58 0.7311.11 0.8510.98 PTM 0.53~10.38 0.87Il.11 0.92'10.87 1.54W~1.82 Sella 0.35 * 0.27 0.46 I 0.38 0.25 I 0.24 0.39 I 0.27 TOTAL 8.27 9.35 9 .15~ 10.88'

DENTAL Lowerlapex 0.69k0.61 0.6310.57 0.88k0.76 0.84I0.74 Lower 1 tip 0.45% 0.27 0.6910.26 0.20~10.17 0.38 I 0.40 Upper 1 apex 0.68 i 0.57 0.84 t 0.79 0.92 I 0.71 1.24 0.92 Upper 1 tip 0.30k0.28 0.3210.33 0.2010.18 0.2610.26 TOTAL 2.12 2.48 2.20 2.72

SUM OF ALL LANDMARKS TOTAL 12.77 16.75 13.46 13 1 7.30

Paired t-test; statistically signifiant difference between replicate readings when p<O.Ol, within image and x or y coordinate.

N.B. All values listed in table reached this level b One-way ANOVA; statistically significant difference among landmarks when PcO.05, within image and x or y coordinate.

Paired t-test; statistically significant difference between image modalities m e n Pc0.05, within x or y coordinates.

Clinically significant difference determined if Mean Difference >l mm. Paired t-test; statistically significant difference between x and y coordinates M e n Pc0.05, within

images.

Table 3.4. Mean and standard deviation (SD) in rnillimeters (mm) or degrees of the absolute differences between replicate readings (reading 1 minus reading 2) for each cephalometric measurement by image modality (N=30).

CONVENTIONAL IMAGE DIRECT DIGITAL IMAGE

MEASUREMENT !Reading 1 - Reading 2 ( !Reading 1 -Reading 2 ( +YPE MEASUREMENT Mean 1+ SD Mean I SD ANGULAR (wlrees ) Facial Axis 0.78 1 0.66 1 .O7 i 0.73

FMlA 1.66 I 1.34 2.03~11.38 SNA 0.99 1 1.47 1.51 rt 1.27 SN0 0.69 * 0.83 0.88 10.69 Sofi Tissue Convexity 0.58 i 0.48 0.58 k0.43

SUM ALL ANGULARUpper lncisal Angle 1.89 d* 2.12 1.74 i 1-56 MEASUREMENTS TOTAL 6.59 7.81 LINEAR (Millirnetres) Lower Face Height 0.78 I 0.74 0.98 10.71

Lower lncisor to NB 1.66 + 0.25 0.30 î 0.26 Mandibular Length 0.99 * 1.87 2.02 *i 2.00 Maxillary Length 0.69 t 1.60 1.99~1 1-58 Upper Face Height 0.58 * 0.84 1 .O0 i 0.93

SUM ALL LlNEARUpper tip Protmsion 1.89 df 0.53 0.76 iO.72 MEASUREMENTS TOTAL 6.59 7.05

OVERALL TOTAL 13.18 14.86 Paired t-test; statistically significant difference between replicate readings when P<O.Ol, within image.

b N.B. All values listed in table reached this level;

One-way ANOVA; statistically significant difference among landmarks M e n P<0.05, within image. Paired t-test; statistically signifiant difference between image modalities when Pe0.05.

N.B. Cornparisons were not significant. Clinically significant difference determined if Mean Difference >2 mm or >2 degrees.

Table 3.5. Landmarks and measurements that are statistically and clinically significantly different in reproducibility represented by the mean and standard deviation (SD) in millirneters (mm) for the absolute difference between replicate readings (reading 1 minus reading 2) by image modality (N=30).

CONVENTIONAL IMAGE DIRECT DIGITAL IMAGE JReading 1 -2 1 1Reading 1 -2 1 IReading 1 -21 /Reading 1 -2 1

ANATOMIC Mean c SD Mean I SD Mean I SD Mean I SD TlSSUE LANDMARK TYPE

(mm) (mm) (mm) (mm) X coordinate Y coordinate X amdinate Y coordinat8

SOFT Nasion - 1.47 M.93 - 1.09" f1.36

HARD A point ANS B point - 0 .93dfi. 74 - 1 .16 Hl.74 Basion - 0.93 dî0.82 0.97 d i l -30 1 .O9 k1 .O7 Condylion 1.11 k11.42 1.51 bc+1.47 1 . 1 3 ~ ~ f1.14 2.07 "d2.30 Orbitale 1 .O7 a.81 - 1.10~11.06 - PTM - - 1.54 M.33

DENTAL Upper incisor apex - 1.24 bc f0.92

Measurements FM1A - 2.03 11 .38

Mandibular Length - 2.02 C a m -

Maxillary Length - 1.99' f1.58 Paired samples t-test; statistically significant difference between replicate readings when P<O.Ol, within image.

b N.B. NI values in the table reached this level.

One-Way ANOVA; statistically significant difference among landmarks M e n P<0.05, within image. CClinically significant difference determined if Mean Difference plmm for landmark error and > 2mm for linear and >2 degrees for angular measurement error. values are just below the clinically significant level for the Mean Difference.

Paired t-test; statistically significant difference between replicate readings when Pc0.05, between image modalities.

N.B. No comparisons were significant in this table.

Table 3.6. Mean and standard deviation (SD) in millirneters (mm) of the absolute differences between image modalities (wnventional minus direct digital image) represented b y landmark (N=30).

1 Conventional-Digital/ / Conventional -Digitall ANATOMIC Mean i S D (mm) Mean îSD (mm)

TISSUE TYPE LANOMARK X colordinate Y co-ordinate

SOFr Nasal tip Nasion Pogonion Subnasale Upper lip A point ANS B point Basion Condylion Gnathion Menton Nasion Orbitale Porion PTM Sella

DENTAL Lower 1 apex

Lower 1 tip Upper 1 apex Upper 1 tip 0.22 k 0.23 0.39 i 0.33

SUM OF AL1 LANDMARKS TOTAL 14.06 a 1 7.76a

Paired t-test; statistically signifiant difference between image modalities when p<O.Ol, within x or y-coordinate. N.B. All values listed in table reached this level. b One-Way ANOVA; statistically signifiant difference among landmarks when pe0.05, within x or y-coordinate. CClinically significant difference determined if Mean Difference >1 mm. d~aired t-test; statistically significant difference between x and ycoordinates when p<0.05.

Table 3.7. Mean and standard deviation (SD) in millimeters (mm) or degrees of the absolute differences between image modalities (conventional minus direct digital image) represented for each of the cephalometric measurernents (N=30).

IConventional-Digital1 1 Conventional -Digital( Mean f SD LlNEAR Mean ISD

ANGUlAR MEASUREMENT (degrees) MEASUREMENT (mm) Facial Axis 1.45 I 1.15 Lower Face Height 0.86 I0.69 FMlA 1.50 i 1.49 Lower Incisor to NB 0.33 10.26 SNA 1.01 k 1.21 Mandibular Length 1.28 I 1.22 SN8 0.86 -e 1 .O9 Maxillary Length 1.26 * 1.34 Soft Tissue Convexity 0.50 I 0.59 Upper Face Height 1.35 * 1.36 Upper lncisal Angle 1.69 * 1.35 Upper Lip Protnision 0.70 * 0.74 TOTAL 7.01 TOTAL 5.78

Paired t-test; statistically significant difference between image rnodalities when pc0.01, within measurement type.

N.B. All values Iisted in table reached this level. b~ne-Way ANOVA; statistically signifiant difference among landmarks when pc0.05.

N.B. None of the values listed in table reached this level. CClinically significant difference determined if Mean Difference >2mm or >2 degrees.

N.B. None of the values listed in table reached this level.

Table 3.8. Landmarks that are statistically and clinically significantly different in accuracy represented by the mean and the standard deviation (SD) in millimeters (mm) for the absolute difkence between image modalities (conventional minus direct digital images) by x and y-coordinate (N=30).

IConventional-Digital1 1 Conventional -Digital1 Mean i SD Mean f SD

TISSUE AfJATOMlC (mm) (mm) TYPE LANDMARK X co-ordinate Y CO-ordinate

SOFT Pognonion HARD Cond ylion

Gnathion 1.01s 0.82 - Nasion Orbitale PTM 1.21I 1.31 1.93 î 1 -93

DENTAL Lower incisor apex 1 .O6 k 0.14 1-04 i 0.13 Upper incisor apex 1-32 f 0.87 1.65 i 1.16

aPaired sarnples t-test; statistically significant difference between image modalities when p<O.Ol, within coordinate x or y.

b N.B. Ail values listed in table reached this level.

One-way ANOVA; statistically significant difference among landmarks when pc0.05, within coordinate x or y.

N.B. Ail values listed in table reached this level. CClinically significant difference determined if Mean Difference >1 mm for landmark error. N.B. Ail values listed in table reached this level.

Figure 3.1. Photograph of DenOptix/Gendex digital, reusable storage phosphor plate (A) and conventional x-ray film cassette (BI-

Figure 3.2. Traditional method of locating landmarks on cephalometric radiogaphs using a light box for illumination (A) to create an acetate tracing (B).

Figure 3.3. Adobe Photoshop superimposition technique for the layering of the acetate tracing over the digital image as viewed on the computer monitor. A- acetate tracing; B-initial layer; C-acetate made translucent with approximate superimposition; D-precise superimposition of acetate over digital image.

Figure 3.4. Photograph of DenOptidGendex Equipment. A - Storage phosphor plate, PSP, attached to drum. B - DenOptix scanner showing placement of the d m with the PSP attached.

Figure 3.5. Digiplan software outlining the landmarks and cephalometnc planes superimposed on the direct digital PSP image as viewed on the cornputer monitor.

Figure 3.6. Direct digital PSP cepahlometric images viewed within Digiplan and showing the difference in contrast and brightness that cm be achieved. A - low brightness, hi& contrast; B - high brightness, low contrast.

CHAPTER 4

DISCUSSION AND CONCLUSIONS

DISCUSSION

Digital imaging has been available for application in dental radiology for the last two

decades and is likely to eventually replace conventional radiographic film. It is also

likely that orthodontic cephalomeûic radiographic images will be produced and analysed

in the digital f o m in the near future (Miles and Razzano, 2000). This research project

was designed to evaluate the two modes of digital radiographic image capture, narnely

direct and indirect acquisition, dong with the impact of file compression on the indirect

digital image. in the second study, the aim was to evaluate the reproducibility and

accuracy of cephalometric analysis for wnventional films and direct digital images

acquired simultaneously by photo-stimulable storage phosphor plate (PSP) radiology. In

the first study, an additional two topics were explored. The aim was to evaluate and

compare the reproducibility and accuracy of cephalometric analysis for 1) conventional

film compared to its indirect digital image, and 2) JPEG compression of indirect digital

images (DI) compared to its non-compressed image.

This research differs fiom published studies in the following points. 1) Accuracy of

1 andmark identification of indirect digital lateral cephalographic images acquired with a

transparency flat bed scanner was investigated, 2) the effect of JPEG file compression of

indirect DI on cephalometric landmark and measurement error was explored, 3) the

reproducibility and accuracy of cephalometric analysis in terms of clinical significance

was quantifiai, 4) x and y-coordinates were used to identify landmark m r in both

vertical and horizontal planes, 5 ) angular and linear measurements w a e calculated and,

6) the evaluation of the image on the computer monitor was compared to conventional

film images. In addition, an unique technique was developed to superimpose the

conventional film acetate tracing on the digital image as viewed on the cornputer monitor

in order to establish a cornmon x and y-coordinate system that enabled accurate

comparison between image modalities.

It is well documented that the greatest mors in cephalometric measurement arise in

landmark identification (Richardson, 1 966; Baumind and Frantz 1 97 1 ). Wi th the

introduction of digital imaging in cephalometric radiology the clinician and researcher

mus2 be sure that the new image modality yields as much information as is currently

available on conventional film and that reproducibility and accuracy of landmark

identification is no worse and ideall y better. Therefore, accuracy of indirect and direct DI

was evaluated using the conventionai image as the 'gold stacdard' (Farman and Farman,

2000).

Direct Digital Imaging

The conventional and direct DI were statistically but not clinically significantly diffment

in ternis of landmark reproducibility for 3 of the 2 i landmarks. Landmark identification

was statistically significantly more reproducible for soft tissue and similar in

reproducibility of hard and dental tissue landmark identification for the direct DI

compared with conventional images. However, direct DI were statistically and clinically

significantly less accurate for 8 of 21 landmarks (1 sofl, 5 hard and 2 dental tissues).

Despite this, cephalometric measurement was equivalent in accuracy to conventional

film. Although comparison to 0 t h published studies using the SPP system is difficult

due to variations in method protocol, Eppley and Sadove also found sofi tissue to be more

reproducible on the direct digital image compared to the conventional images using hard

copy images (Eppley and Sadove, 199 1 ).

Since error in landmark identification is often due to superimposition of anatomical

structures or poor quality of the film, then it is possible that the new digital format may

improve on these aspects of radiographic interpretation especially considering the

capability of post acquisition image processing and enhancernent (Mol, 2000). This

enhancement is made possible due to the inherent physical properties the PSP plate

system. Specificall y, the storage phosphor plate has a greater dynamic range in response

to x-ray exposure than conventional film. This means that a greater range of exposure

values can be used to produce an acceptable image thus reducing the need for repeat

exposures. Also the linear response of the storage phosphor plate perrnits image

processing to correct exposure mors and to enhance the image. This enhances the display

of tissues with large differences in density such as compact bone of the cranium and thin

bone such as the ANS or soft tissue structures on one image (Hildebolt et al, 2000).

Hagemann and colleagues showed that PSP systern showed no polarization between hard

and sofi tissue with both having equivalent reproducibility on conventional and digital

images. They also showed that PSP radiography allows for the reduction in exposure due

to the physical properties of the systern combined with the ability to perform post

acquisition digital enhancernent (Hagemann et. al., 2000).

The influence of image processing on the accuracy of cephalometric analysis was not

determined in this research study. However, the DI were enhanced on screen within the

cephalometric software using magnification, brightness and contrast adjustments, which

gave the impression that the sofi tissue outline was more prominent than seen on the

conventional image. This subjective preference is supported by studies on a phantom

skull with soft tissue drape (Calderazzi, 1992). Indeed, the direct DI showed statistically

significantly better reproducibility in the locaiization of soft tissue landmarks as

compared to conventional images. Unfortunately, Orbitale, an anatomical landmark with

low radiodensity, was shown to be statistically significantly less accurate in the digital

image, leading one to believe that image processing modifications are limited in their

ability to enhance certain anatomic landmarks. This situation implies that the effect of

contrast enhancement cannot be easily predicted. The purpose of an enhancement

operation is to facilitate the detection of features that are relevant (signal) compared with

those that are not relevant (noise). If the image has only limited potential to be fùrther

enhanced or if the signal-to-noise ratio is not affected by the enhancernent operation, the

diagnostic value of the image cannot be expected to increase (Mol, 2000).

Digital image processing on an indirect DI is limited by the native quality of the original

radiograph being scanned. Some manipulation is possible with contrast and brightness

control, but it may not improve over what is possible using the traditional light box and

magnimng aids which are customarily ernployed in cephalometric landmark

identification (Mol, 2000). Further studies are required to compare the accuracy of

cephalometric analysis of a DI displayed on a monitor without any image processing

wmpared to analysis with image manipulation. This would help to determine whether

there is beneficial effect fiom image processing.

Image Compression

The effect of JPEG compression on the cephalometric task of identiwng landmarks was

examined by comparing indirect DI (non-compressed (TIFF)) to the corresponding

compressed image at two JPEG levels, 4 2 (25: 1) and Q7 (12: 1). For this study, the TIFF

image was considered the 'gold standard' for the evaluation of accuracy of landmark

identification. Our key findings were: a) 4 2 compressed images were 19% less

reproducible in the x coordinate and 37% less reproducible in the y-coordinate than non-

compressed images, while 4 7 images were actually 1 1 % more reproducible in the x-

coordinate but showed equivalent reproducibility in the y coordinate to non-compressed

images for landmark identification; b) Reproducibility of angular and linear measurernent

was lower for the 42 image compared to either TIFF or 4 7 images with the error of

angular measurernents greater than the linear for 4 2 images; c) 4 2 images were less

accurate than 4 7 images in 5 out of 17 landmarks when both are compared to non-

compressed images; d) Q2 images are less accurate than 4 7 for 3 of 12 cephalometric

measurements when both are compared to TIFF images.

Inter and intra-observer variability is known to be the greatest source of error in landmark

identification (Richardson, 1950). The intraclass correlation coefficient statistics showed

that both inter- intra-examiner reliability was excellent between indirect and

conventional film images. This is very important when evaluating a new image format,

since any differences in reproducibility and accuracy could than be attnbuted to the

image and not the examiner. Therefore, a lower reproducibility and accuracy in

landmark identification on the JPEG 42 (25: 1) image compared to the conventional film

image is attributed to the loss in image quality caused by lossy compression.

Since the JPEG 47 (12:l) compression did affect the reproducibility and accuracy of

identification for a few landmarks, but did not impact on the accuracy of cephalometric

measurement, it may be acceptable to use this compression level in individual cases to

assess dentofacial proportions. For applications that do not have an immediate impact

on patient care, small arnounts of image distortion and diminution in diagnostic accuracy

may be tolerable. For example, high levels of compression may expedite the transfer of

images fkom a patient's chart to the r e f h n g dentist or specialist for illustrative purposes

to complement the written report. images that are relatively old (>5 years) with a low

probability of being used for clinical purposes could be stored inexpensively in a highly

compressed format (Baker, 1997). In contrast, high accuracy is required, for exarnple in

monitoring growth or in longitudinal studies comparing different treatment modalities, or

when archival records are used for research such as the Burlington growth centre data.

Thus a rigorous standard must be met before a compression mode c m be broadly applied.

Therefore, it would be unrealistic to insist that one level or mode of compression be used

for al1 types of display, transfer and storage. Instead, the amount of compression must be

chosen for each unique diagnostic task and based on the native quality of the original

radiographie image as well as the usefblness of the image for future reference (Mol,

2000).

Interestingly, even though the examiners were blinded to the type of image file under

interpretation, they were usually able to distinguish between the 4 2 and TIFF files, while

the 47 and TIFF images were not as easily distinguishable. Al1 examiners agreed that the

42 image quality was subjectively worse than the other image file types. They felt that

the 4 2 images had visibly lower resolution and did not feel confident in the amount of

information contained within the radiographic image at that level of compression.

Although this is not a scientific reason to eliminate the use of a compression level, due to

hurnan instincts, clinicians and researchers alike may reject such radiographs and deem

them unsuitable for diagnostic interpretation.

From the limited number of studies, it seems clear that little is yet known about the effect

of compression on diagnostic quality. To îome degree, this dilemma exposes a general

lack of knowledge regarding the relationship between radiographic features of disease

and specific aspects of image quality (Mol, 2000). The diagnostic yield of radiographic

systems for various dental applications still leaves much to be desired, therefore, Mer

degradation of the images through lossy compression should be used with great caution

and only afier evaluating its effect for specific diagnostic tasks. Consquences of quality

of care as well as legal implications should be considered.

Future Research Trends

The information reported in this study is encouraging for the fùture of digital imaging in

cephalomeîric radiography. Indirect digitization of radiographs should be reserved for

archiving of historic records and friture research of digital radiographie imaging should

be concemed with various types of direct digital acquisition. Some dentists will use

hybrid systems in which they make conventional radiographs and then digitize them into

their patient's records. This scenario offers a gradua1 transition for the dentist, allowing

the high spatial resolution of film with the advantages of image display, manipulation and

transmission inherent with digital imaging.

The development of 'smart tools', which c m segment an image into distinct anatomic

regions and then apply image enhancernents specific to each region will facilitate

diagnostic tasks. The arena of digital imaging awaits higher resolution scanners,

monitors and direct digital CCD's, faster cornputer processors, more intelligent software

and more efficient compression aigorithms. The next generation of compression

techniques will likely move away from pixel-based methods like JPEG (Discrete Cosine

Transforrn), toward mathematical methods similar to wavelets and fiactals, which can

achieve much higher levels of compression with a minimum impact on image quality

(Miles et al, 2000; Rigolin, et al., 1996). It will challenge the orthodontic research

cornmunity to keep abreast of these changes. Continued research is required to evaluate

the impact of emerging hardware and software technologies on the reproducibility and

accwacy of cephalometric anal ysis.

CONCLUSIONS

The use of direct digital imaging using the PSP technique is positively evaluated for

clhical cephalometric measurement, however, due to a decrease in reproducibility and

accuracy for the identification of certain landmarks further investigation is warranted.

Reproducibility of landmark identification was equivalent on conventional films and their

corresponding indirect digital images recorded on a computer monitor. Similarly

equivalent reproducibility of landmark identification on non-compressecl and JPEG

compressed images at the 12:l level were found. However, there was a statistically

significant decrease in the reproducibility and accuracy of landmark identification and

measurements when a compression level of 25:l was employed when compared to the

non-compressed image.

The results of this study indicate that JPEG (47) compression at a ratio of 12: 1 c m be

used to perform routine cephalometric analysis, without loss of clinical orthodontie

diagnostic information. This recornrnendation is based on the fact that reproducibility and

accuracy of cephalornetric measurement, both linear and angular, are not statistically

significantly different on the 12: 1 compressed image when compared to the non-

compressed image. However, this does not indicate that this amount of compression is

acceptable for other fûnctions such as the detection of pathological changes. Likewise the

direct digital images were not evaluated for other diagnostic tasks other than

cephalometric analysis leaving room for continued investigations in the field of digital

radiography.

Furthmore, the research presented for both the direct and indirect experiments is in no

way comprehensive. Further studies involving a multicentre investigation with more

examiners and a variety of film qualities and compression levels would provide f.urther

information regarding the effects of digital radiographie imaging on cephalometric

analyses. With additional information general recommendations will be formalized

regarding 1 ) acceptable levels of compression for cephalometric anal ysis 2) teleradiology

3) archiving and storage and 4) p s t acquisition enhancement.

APPENDICES

APPENDIX 1

LATERAL CEPHALOGRAPHIC LANDMARK LOCATION AND DEFINITIONS

Figure 2.1. Lateral cephalographic radiograph acetate tracing with landmark numbers marked. See attached Appendix 1 for definitions of landmarks.

Landmark Defmitions Needed for Leagan, McNamara and Steiner Analyses Using Digiplan (Orthovision) Software

Hard and Dental Tissue Landmarks

23. A Point (A) -the innennost point on the contour of the premaxilla between anterior nasal spine and the incisor tooth

22. Anterior Nasal Spine (ANS) - anterior tip of the anterior nasal spine

35. Articulare - the point of intersection between the shadow of the zygomatic arch and the posterior border of the mandibular ramus

30. B Point (B) - most posterior point in the concavity between infiadentale and pognonion

(marked at right angle to the occlusal plane) 36. Basion (Ba)

-1owest median point of the anterior margin of the forarnen magnum, at the base of the clivus

39. Condylion (Co) - estimated mid-point between the most superior and the most posterior points

on the head of the condyle 38. Gnathion (Gn) - mid-point between Pognonion and Menton along the contour of the chin 32. Gonion (Go)

-the most inf&or and posterior point of the mandibular angle. Determined by bisecting the angle formed by the posterior ramus border and the inferior mandibular border

27. Lower incisor apex (LIA) - mid-point on the root apex of the Iowa central incisor

28. Lower incisor tip (LIT) - incisa1 tip of the lower central incisor crown (most prominent contour)

29. Menton (Me) - the junction of the anterior of the mandibular lower border with the symphysis

of the chin 2 1. Nasion (Na)

- most anterior point of the intersection between the nasal and frontal bones 1 7. Orbitale (Or)

- lowermost point on the inferior margin of the orbit 3 1. Pognonion (Pg)

- most anterior point on the bony contour of the chin 16. Porion (Po)

- centre of machine porion 25. Posterior Nasal Spine (PNS)

- tip of the posterior nasal spine of the palatine bone in the hard palate 37. Pterygomaxillary Fissure (PTM)

- most superior and posterior point of the pterygomaxillary fissure 34. R2 - posterior border of ramus at midpoint between gonion and condylion 20. Sella (S)

- centre of the hypophyseal fossa; mid-point of the cavity of sella tursica 33. Supragonion (SGo)

- point on the posterior border of the ramus where upward curvature begins 24. Upper central incisor root apex (UIA)

- mid-point on the root apex of the upper central incisor 18. Upper cental incisor tip (UT) - incisa1 tip of the upper central incisor crown (most prominent contour) 26. Upper molar crown tip (UMT)

- mesial crown tip of the first permanent maxillary molar (midplane if overlapping images)

Soft Tissue Landmarks for Legan Analysis

4. Anterior Columella (ST-AC) - soft tissue mid-point of columella 8. Embrasure of upper lip (ST-Em) - most distal aspect of upper lip before the infkrïor contour curves superiorly 10. Lower lip (ST- LL)

-vermillion (mucocutaneous) border of lower lip 3. Nasal tip (ST-NT) - most prominent point on the sofi tissue nose 9. Posterior Lower lip (ST-LL)

- most distal aspect of iower lip before the superior contour curves inferiorly 15. Reflex of the neck; (C) point

- soft tissue point of intersection of chin and neck 7. Upper lip {ST-UL)

-vermillion (mucocutaneous) border of upper lip 6. Sofi tissue A point (ST-A) - the most posterior point of the philtrum of the maxillary lip (marked at right

angle to the S-N plane) 1 1. Sofi tissue B point (ST- B) - the most posterior point on the contour between the most prominent point on

the vennillion of the mandibular lip and the sofi tissue pogonion 1. Soft tissue Glabella (ST-GI) - most anterior point on the soft tissue over the frontal bone - mid-sagittal 1 3. Soft tissue Gnathion (ST 4x1)

- midpoint between Pogonion and Menton dong the sofl tissue contour of the chin 14. Soft tissue Menton (ST- Me)

- sofi tissue point corresponding to Menton point. Point where a vertical (paralie1 to tme vertical) dropped from the hard tissue Menton intersects with the sofl tissue covering the lower chin

2. Sofi tissue Nasion (ST- Na)

- point where S-N plane intersects with sufi tissue nose -mid sagittal 12. Sofi tissue Pognonion (ST-Pg)

- most prominent point on the contour of the sofi tissue covering the chin 5. Sofl tissue Subnasale (ST-Sn) - soft tissue junction of colurnella of nose and philtrum 19. Throat (ST -Trt)

- point on the anterior border of the sofl tissue throat

APPENDIX 2

GLOSSARY OF TERMS

GLOSSARY

ACR-NEMA -American College of Radiology and the National Electrical

Manufacturers Association, çommittee founded in 1982 to develop a standard to:

promote a genenc digital image communication format; facilitate the development of

picture archiving and communications sytsems (PACS);l allow the creation of diagnostic

databases for remote access; and help assure the usability of new equipment with existing

systems.

bmp - Bitmap

CCD - Charged Couple Device

DI - Digital Imaging

DICOM - Digital Imaging and Communications in Medicine

dpi - dots per inch where dot equals a pixel

GIF - Graptiic Image File

ISDN- Integrated Systems Digital Network

JPEG - Joint Pfiotographic Experts Group

Kbps - Kilobytes per second

LZW - Lernpel, Ziv, and Welch

Mbps - Megabytes per second

PACS - Picture Archiving and Communication Systems

PICT - picture file

PNG- Portable Network Graphics

ppi - pixels per inch

Q factor - Quantization or Quality

PSP - Photostimulable Storage Phosphor Plate

TIFF - Tagged Image File Format

MWW - World Wide Web

APPENDIX 3

INDIRECT DIGITAL IMAGING AND FILE COMPRESSION DATA

Table 2.9. Descriptive statistics representing the mean for al1 3 examiners and standard error of the mean for each of the HARD tissue cephalometric landmarks by file type (TIFF, Q7,Q2). Method TIF refers to no compression. Method 47 refers to JPEG compression Quality factor o f 7 (ie., 1 : 12). Method 42 refers to JPEG compression Quality factor o f 2 (ie., 1 :25). i) X- Coordinates

iil Y- Coordinates I

METHOD Q7 I METHOD Q2 N = 63 N = 63

A point Y 91.25I 3.52 92.641 2.02 93.13e.01 92.96k1.98 ANS Y 95.73S3.63 96.8m.08 97.33U.06 97.65k2.06

B point Y

Pwonion Y Porion Y PTM Y Sella Y

, Basion Y 104.94~.19 i02.23+1.38 î02.03+1.40 1 02.32+t .41

-

53.46k3.65

37.0243.62 1 1 5.35S.66 125.82e.89 141 .54S.70

52.68I2.09

38.24e.W 11 5.m1.54 125.8Sk1.71 142.0211.52

52.93fl.07 52.2=.05

38.JoI2.03 1 14.Wk1 .56 125.2W1.85 141.75+,1 .Si

39.32I2.13 t 14.93f 1.56 1 25.70I1.68 142.m1.53

Table 2.10. Descriptive statistics representing the mean for al1 3 examiners and standard error of the mean for each of the DENTAL tissue cephalometic landmarks by file type (TIFF, 4 7 , Q2). Method TIF refers to no compression. Method 47 refers to JPEG compression Quality factor of 7 (ie., 1 : 12). Method 4 2 refers to JPEG compression Quality factor of 2 (ie., 1 :25).

i) X- Coordinates

ii) Y- Coordinates

METHOD TIF : N=63 N=63 N = 63

Meanfstd errortmm) ~ e a n i s t d errot~mrnlhteanhtd errortmmk

Table 2.11. Descriptive statistics representing the mean and standard error of the mean for each of the cephalometric measurements for al1 obsewers by file type (TIFF, Q7,Q2). Method TIF refers to no compression. Method 47 refers to JPEG compression Quality factor of 7 (ie., 1 : 12). Method 4 2 refers to JP EG compression Quality factor of 2 (ie., 1 :25).

MEASUREMENT

Table 2.12. Intra-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating SOFT tissue lateral cephalometic landmarks by file type (TIFF, Q7,Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmarking. ICC = Inter Class Correlation

1 1 EXAMINER 1 1 EXAMINER2 1 EXAMINER3 1 EXAMINER4 ( N = 9 ICC ICC [ I I i c c I i c c I ICC I ICC I ICC I ICC 1

Table 2.1 3. Intra-examiner variabili ty measured by Intra Class Correlation Coefficient (ICC), for evaluating HARD tissue lateral cephalometric landmarks by file type (TIFF, Q7,Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmarking. ICC = Inter Class Correlation

Menton x 0.98 0.99 0.99 1 .O0 0.99 1 .O0 0 -84 1 .O Menton y 1 .O0 0.99 0.99 1 .O0 0.98 0.99 0.80 0.9 Nasion x 1 1 .O0 1 .O0 1 .O0 0.98 1 .O0 0.99 0.93 1 .O

1

Nasion y 1 .O0 0.99 1 .O0 0.97 1 .O0 0.97 0.98 1 .O Orbitale x 0.99 0.99 0.99 0.98 0.98 0.88 0 -95 3 .O Orbitale y 0.97 0.94 0.99 0.98 0.99 , 0.96 0.99 0.9

PNS x 1 .O0 0.99 0.99 1 .O0 0.98 0.99 0.98 0.9 PNS y 0.97 0.97 0.98 1 .O0 0.98 1 .O0 0.91 0.9

Pogonion x 1 .O0 1 .O0 1 .O0 1 .O0 1 .O0 1 .O0 0.83 1 .O Pogonion y 0.97 0.97 0.96 0.99 0.96 0.94 0.91 0.9

Porion x 1 .O0 1 .O0 0.99 1 .O0 1 .O0 0.95 0.98 1 .O Porion y 1 .O0 1 .O0 0.99 1 .O0 0.99 0.92 1 .O0 1 .O

Table 2.14. Intra-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating DENTAL tissue lateral cephalometric landmarks by file type (TIFF, Q7,Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmarking. ICC = Inter Class Correlation

EXAMINER 1 q EXAMINER 4 N = 9 ICC icc ICC ICC cc ICC ICC ICC

LANDMARK METHOD 1 METHOD 2 METHOD 1 METHOD 2 METHOD 1 METHOD 2. METHOD 1 METH00 : Lower incisor

apex x 1 0.99 1 0.99 1 0.99 1 0.M 1 0.99 1 0.99 1 0.94 1 1-00 Lower incisor

apex y , 0.98 0.93 0.96 0.92 0.96 0 -97 0.92 0.98 Lower incisor

tip x 0.97 0.94 , 1.00 0.98 1 .O0 1 .O0 0.97 1 .O0 Lower incisor

tip y 0.97 1 .O0 0.99 0.98 1 .O0 1 .O0 0.99 0.96 Tip maxillary

molar x 0.99 0.99 0.99 0 33 0.99 1 .O0 0.93 1 .O0 Tip rnaxillary

rnolar y 0.99 1 .O0 1 .O0 0.97 0.99 1 .O0 0.74 1 .O0 Upper incisor

apex x 0.99 0.93 0.99 0.99 0.99 1 .O0 0.96 0.99 Upper incisor

apex y 0.95 0.96 0.98 0.98 0.96 0,93 0.96 0.91

Table 2.15. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating SOFT tissue lateral cephalometric landmarks by file type (TIFF, Q7,Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmarking. ICC = inter Class Correlation; 1 .O = perfect correlation, 0.00 = no correlation.

c l ICC ICC

Table 2.16. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating HARD tissue lateral cephalometic landmarks by file type (TIFF, 47, Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmarkkg. IC lation, 0.00 = no correlation.

Table 2.17. Inter-examiner variability measured by Intra Class Correlation Coefficient (ICC), for evaluating DENTAL tissue lateral cephalornetric landmarks by file type (TIFF, Q7,Q2). Method 1 refers to acetate tracing. Method 2 refers to cornputer monitor landmatking. ICC = Inter Class Correlation; 1 .O0 = perfect correlation, 0.00 = no correlation. 4 obsewers, 9 lateral cephalometric radiographs

# Lower incisor I 1 1 apex x 1 0.99 1 0.99 1 Lower incisor I 1 1 apex y 1 0.96 1 0.96

Lower incisor tip x 0.99 0.99

Lower incisor tip y 0.99 0.99

Tip maxillary rnolar x 0.99 0.99

Tip maxillary molar y 1 0.97 1 0.98

Upper incisor apex x 0.98 0.99

Upper incisor apex y 0.95 0.95

Upper lncisor tip x 1 0.99 1 1 .O0

APPENDIX 4

DIRECT DIGITAL IMAGING DATA

Table 3.9. Descriptive statistics representing the mean and standard error of the mean for each of the SOFT tissue cephalometric landmarks by image modality (conventional and direct digital- storage phosphor technique (PSP)). Method 1 refers to acetate tracing of film. Method 2 refers to cornputer rno&tor landmarking of digital image.

Table 3.10. Descriptive statistics representing the mean and standard emr of the mean for each of the HARD tissue cephalometric landmarks by image modality (conventional and direct digital- storage phosphor technique (PSP)).

Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarking of digital image.

Table 3.11. Descriptive statistics representing the mean and standard error of the mean for each of the DENTAL tissue cephalometric landmarks by image modality (conventional and direct digital- storage phosphor technique (psp)) - Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarking of digital image.

I METHOO 1 l METHOO 2 I METHOO 1 I METHOD 2 N = 30 N = 30 N = 30 N=30

LANDMARK

Jupper lncisor tip 1 183.4îk1.49 1 183.5111.49 1 69.81I1.44 1 70.07k1.41 1

iuieankstd erroflmrn) X m-ordinate

ower incisor apex' 165.3e1.52 f 66.31 11.53 179.61 11.45 t73.98k1 .42

ower incisor tip pper incisor apex,

Meanbtd erroqmm) X amdinate

179.57I1.43 1 72.8711.40

Meanfstd error(mm) eanfstd emr(mm) Y co-ordinate Y cûordinate

53.24i1.53 73.33k1.49 94.e1.42

!i4.01*1.47 73.25I1.50 93.42k1.37

Table 3.12. Descriptive statistics representing the rnean and standard error of the mean for each of the cephalometric measurements by image rnodality (conventional and direct digital- storage phosphor technique (PSP)). Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarkhg of digital image.

METHOD 1

Facial Axis 88.21fl.82

81.93HI.74

Convexitv 9.29I0.62 Upper lncisal

Angle 24.41i1.28

1 METHOD2 1 1 METHOD 1 1 METHOD2 -

N = 30

Meanhtd erroi (mm)

Lower Face Heig ht

Lower lncisor to

Mandibular Length

Maxillary Length Upper Face

Height U p ~ r Lip

MeanMd error ~eanfstd erroi 1

Table 3.13. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each SOFT tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarking of digital image.

i) X Co-ordinate

U rli 197.9tkl.SS 5 1 97.9011 .SS

ii) Y Co-ordinate METHOD 1 METHOD 2

READING 1 1 READING 2 READING 1 1 READING 2 N = 30 N = 30 N = 30 N = 30

Meanfstd error(mm) Meanhtd erroamm) Meanhtd erroqmm) ~ e a n b t d error(mm) NDMARK Y anxdinate Y coordinate Y co-ordinate Y coordinate

108.18+1.38 107.w1.37 107.85i1.43 107.79I1.41 148.21 11.54 148.17+i.51 148.85f 1.54 148.23tl.4û

ognion 39.66k1.74 38.73k1.66 38.695t1.77 . 38.67î1.79 u bnasale 96.2m1.32 96.08k1.31 96.W1.32 96.41 Il .30

U rli 1 9 7 7 83.41 k1.42 83.40fl.41 1

Table 3.14. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each HARD tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). Method 1 refers to acetate tracing of film. Method 2 refers to amputer mo&tor landrnarking of digital image.

METHOD 1 METHOD 2 READING 2 REAOING 1 REAûlNG 2

N=30 N = 30

m) Meanhtd error(mm) beamtstd enor(rnrn1 ~ e a n i s t d error(mm: X ~ r d i n a t e X ~oordinate X coordinate

A point 178.89&1.30 178.86k1.31 178.87k1.35 178.63st1.32 ANS 184.16k1.25 181.1?+1.25 183.74k1.30 183.72I1.25

8 point 170.8611.52 170.9311 .Si 170.721 1 .Si 170.7611.53 Basion 80.25k1.34 80.8Okl .31 80.49+1.34 80.32I1.30

Condyle 90.0Ok1.22 89.6S1.23 90.08+1.31 90.4Sk1.26 Gnathion 166.9&1.66 1 66.87k1.67 166.=1 .70 186.0a1.66 Menton 1 W.17k1.72 180.1311.71 1 59.92k1.75 159.90I1.72

I 1 METHOD 1 1 METHOD 2 READING 1 1 READING 2 READING 1 1 READING 2

N = 30 N=30 N=30 N = 30 Meanfstd error(mm) Meandstd error(rnm) ~ e a n î s t d emor(mm: Meanhtd emr(rnm

NDMARK Y CO-ordinate Y co-ordinate Y ~oordinate Y ceordinate

94.69k1.26 94.87k1.27

B point S3.44&1 .!i6 53.53+1.57 53.1 5k 1.59 53.45Il.51 Basion 1 06.32k1.42 1 06.6011.46 1 07.3h1.44 106.8311.46

Condyle 124.44k1.42 1 24.01k1.45 125.52I1.37 124.36&1 .41 Gnathion 33.71 k1.79 33.61 11 .77 33.5311.74 33.43k1.76 Menton 34.1 W1.78 3J.2311.76 34.5W1.77 34.5711.76 Nasion 1 54.83î1.40 1 54.6a1.43 1 S A 1 k1.41 154.90I1.39 Orbitale 125.05I1.33 125.05k1.32 124.87I1.35 124.5811.36 Porion 120.23kl .JO 120.2&1.45 1 20.0311.49 119=4=1 -42 PTM 129.77I1.41 1 29.88k1. 51 130.4W1.45 130-5411.45

Table 3.15. Descriptive statistics representing the mean and standard error of the mean in millimeters (mm) for each DENTAL tissue cephalometric landmarks for Reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (psp)). Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarking of digital image.

I 1 METHOD 1 METHOD 2 READING 1 1 READING 2 READING 1 1 READING 2 1

LANDMARK ower incisor apex ower incisor tip

ii) Y Co-ordinate METHOD 2

READING 2 READING i 1 READING 2

N = 30 Meadstd error(mrn)

x coordinate

Upper incisor apex Upper Incisor tip

j65.17k1.53 179.5Sk1.42

N=30 Mean+std error(mrn)

x coordinate

172.83+1.40 183.41 k1.50

165.5ôkl.52 1 16ô.33i1.50 179.55Il.44 1 179.59k1.45

N = 30 Meanhtd error(mm)

X coordinate 166.28I1.56 179.63Il.45

172.90I1.41 183.3W1 .48

N = 30 Meanbtd error(mm)

x ceordinate

174.08I1.45 183.51 k1.49

173.8Skl.41 183.51Il .48

Table 3.16. Descriptive statistics representing the mean and standard error of the mean for each of the cephalometic measurements for reading 1 and 2 by image modality (conventional and direct digital- storage phosphor technique (PSP)). Method 1 refers to acetate tracing of film. Method 2 refers to cornputer monitor landmarking of digital image.

FMlA S1.1S1.32 51.68I1.35 SNA 81.81M.73 82.0M78 SNB 77.88N.62 78.0710.65

Soft Tissue Convexity 9.2234.64 9.36I0.6 1 Upper Incisa! Angle 24.5Ek 1.27 24.31 f 1.34

METHOD 2 READING 1 READING 2

N = 30 N = 30 Mean*td error Meanfstd error

(de~rees) (degrees) 87.81 f l .76 87.34M.79

ii)

Upper Face Height 55.4ûM.64 55.23fl.73 54.54kO.66 55.14M.70

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