Original A r t i c l e

Pixel Value Modification Using RVG-4 Automatic

Exposure Compensation for Instant High-Contrast Images

Yoshihiko H A Y A K A W A , Ph.D., Al lan G.FARMAN, B.D.S, Ph.D., D.Sc.,

Wil l iam C.SCARFE, B.D.S, M.S. and Kinya KUROYANAGI, D.D.S, Ph.D.*

Division of Radiology and Imaging Sciences, School of Dentistry, The University of Louisville, Kentucky, USA

*Dept. of Oral and Maxillofacial Radiology, Tokyo Dental College, Chiba, Japan

(Received : Oct. 25, 1995, Revison received : Jan. 17, 1996, Accepted: Jan. 31, 1996)

Key Words : Dental Radiography, Digital image processing, CCD--based intraoral radiographic system

The RVG 4 permits automatic exposure compensation (AEC). The purpose of this investigation was

to determine the effects of AEC on image contrast. Images were made either with or without a dental

QA jaw phantom using a fixed image projection geometry. Exposures were 6.3 through 27.3/zC/kg using

an X ray generator operated at 70 kVp. Region of interest pixel value distributions were measured at

tissue thicknesses in this phantom, and the average pixei values and signal-to-noise ratios (SNR) were

calculated. The use of AEC without an object in place resulted in a disproportionate relationship

between pixel value and exposure with a marked reduction in SNR. The use of AEC on under- and over

-exposed images of the phantom simultaneously enhanced image contrast and reduced SNR. Thus, AEC

provides a convenient and quick method for achieving high contrast images with sub optimal exposures,

however, this could lead to inappropriate patient dosages if the function is used for over-exposed images.

AEC reduces the SNR and produces disproportionate pixel values relative to exposure.

Oral Radiol. Vol.12 No.1 1996 (11~17)

I n t r o d u c t i o n

Digital image data can be t rea ted using a

number of a lgori thms to enhance image quaf-

ity. While this is usual ly accomplished after

display of the original image, specific pre-

display processing may also be used. Cur-

rent ly avai lable in t raora l CCD based and

storage phosphor digital radiographic sys-

tems provide var ious facilities for digital

image processing. 1-3)

Linear cont ras t enhancement occurs

when the relat ionship between the exposure

and gray level representa t ion is altered pro-

port ionately. As an example, the "X func-

t ion" provided by the RVG-32000 (Trophy

Radiologie, Vincennes, France) performed a

11

specific window operation to increase the

visual differences between gray levels, there-

by increasing contrast? )

Non-linear or logarithmic contrast en-

hancement involves disproportionate manipu-

lation of pixel values, particularly in low

pixel value ranges to highlight small differ-

ences and achieve images with optimal den-

sity and contrast? ) The VIXA 2 (Gendex

Dental Systems srl, Milan, Italy) display pro-

vides the observer with images pre-processed

using this function. 6) Gamma correction is a

complicated non-linear enhancement affect-

ing the overall histogram of an image prefer-

entially acting on low pixel values, spreading

them evenly over the entire range of values

(usually 256) to provide optimal visual

discernment. 7) Computed Dental Radiogra-

phy (Schick Technologies Inc., Long Island

City, NY, USA) refers to this function as

"equalization". However, the effect of this

function on image contrast has not been

previously described, s)

The RVG-4 is a CCD-based intraoral

radiographic system recently developed by

Trophy Radiologie. The proprietary soft-

ware provided with the RVG-4 has an auto-

matic exposure compensation (AEC) function

algorithm which produces a high-contrast

image irrespective of exposure. This func-

tion is found in the setup program in the

WINDOWS version software, 9) and the

default may be set to "on" or "off". Features

of this function on image contrast, though

probably non linear, have not been previous-

ly described. Ideally the function should

change not only image contrast but also the

signal to noise ratio (SNR). SNR is, how-

ever, usually exposure-dependent.

The aim of this study was to investigate

the effect of AEC on image contrast and

SNR.

12

Materials and Methods

CCD-based intraoral imaging system The RVG-4, incorporating the RVG-

STV PC WINDOWS Version 1.1a software

was used. The sensor active surface is larger

than for previous RVG generations ; the sen-

sor has a 29.8 mm x 19.8 mm sensitive area

and a 768 x 512 pixel matrix. Each pixel size

is 39 • 39/zm. The image is captured by the

sensor and its distributed electrostatic inten-

sities are converted to digital data. The size

of the RVG-4 image file is 418,752 bytes

without image compression.

X-ray generator The X ray generator used was an Irix 70

(Trophy Radiologie). The tube voltage was

70 kVp and the tube current was 8 mA.

Beam filtration was 2.5 mm Al-equivalent.

The exposure time was set at intervals in the

range of 0.02 to 0.24 s. The distance from

the focal spot to the cone tip was 20 cm. The

intraoraI sensor was set at 5 cm and 30 cm

from the cone tip.

Image contrast and SNR measurement

Images at various exposures were made

either with no object present or using a dental

QA jaw phantom (Model 501 jaw phantom,

Radiation Measurements Inc., Middleton, WI,

USA).

SNR was determined by selecting the full-

image area as a region of interest with no

object present and measuring the average

pixel value. Standard deviation was deter-

mined as a measure of the noise of each

image. The SNR was calculated as the ratio

of the image forming signal (average pixel

value) to the noise (standard deviation). 1~

The phantom consisted of a portion of a

human maxilla enclosed in tissue-equivalent

plastic. The same plastic (2 cm in thickness)

was located in front of the phantom as a soft-

tissue equivalent attenuator. The total thick-

Fig. 1 Images made with no object present were taken either with AEC ON or OFF. Exposure time increases from left to right. The seven upper images were taken with the AEC OFF ; exposure times ranged from 0.02 to 0.16 s. The ten lower images were taken with the AEC ON ; exposure times ranged from 0.02 t o 0.24 s.

ness became 4.5 cm. As in the descr ip t ion by

Kapa et al., n) denta l car ies and r e s to ra t ions

were s imula t ed in the tee th ; a bone s tepwed-

ge and severa l sizes of wire mesh were also

embedded in the phantom. Th icknesses of

the bone s t epwedge were 1.20 ram, 2.45 ram,

4.75 mm, and 9.50 mm, respect ive ly . The

pixel value d is t r ibut ion at four bone step

images in this phan tom was m e a s u r e d and

the ave rage pixel value, s t a n d a r d deviat ion,

and S N R were ca lcula ted .

The se lec ted RVG 4 images were t rans-

fe r red to a personal compute r (PowerBook

]80C, Apple Japan, Inc., Tokyo , Japan) and

displayed using the Photoshop s o f t w a r e (Ver-

sion 2.5, Adobe Sys tems, Inc., U.S.A.). Th{s

so f tware was used to r ead the pixel value in

the range of 0 (white) to 255 (black). The file

f o rma t of RVG-4 images is the T I F F (Tag-

ged Image Fi le Format ) . The file was t rans-

fe r red f rom a DOS disk to the P o w e r b o o k

using the Apple Fi le Exchange r (Apple Japan,

Inc.).

Radiation exposure

Radia t ion exposu re was measured wi th a

bery l l ium - windowed ioniza t ion chamber ,

D o s i m e t e r / E l e c t r o m e t e r Model 11 (CNMC

Corp., Nashvi l le , TN, USA) with a 3 cm 3

probe. Ca l ib ra t ion of this chamber could be

t r aced to the N I S T (Nat iona l Ins t i tu te of

S t a n d a r d s and Technology, Gai thersburg ,

MD, USA). The probe was p laced at the

same posi t ion as the sensor to measure expo-

sures in mR, which were then conver ted to

~ C / k g .

Resul ts

The resul t s f rom images with no object

being present a re shown in ~ Figs. 1-3. F igure 1

shows images bo th wi th the AEC O F F (upper

images) and ON (lower i m a g e s ) f o r v a r i o u s

increas ing exposures f r o m left t o right.

13

250

200

>

100 x

50

/ r

' w ' , , /

i i I ,

0 10 30

T i I i ~ i

20

Exposure C/k 9)

Fig. 2 The relationship between exposure and the average pixel value calculated from the full imaging area. 0 : AEC ON, �9 : AEC OFF. Curve fitting of AEC ON data was provided with a 3 polynominal formula. Fitting curve of AEC OFF data was provided with a linear formula : (Pixel value(0-255)) = 2.18• 101 x (Exposure (/~C/kg)) - 1.15 x 101, coeffi- cient of determinance = 0.99

When no object was present , the AEC over-

compensated , resu l t ing in a "ch ickenwi re -

l ike" image.

F igure 2 shows the re la t ionsh ip be tween

exposure and the ave r age pixel value calcu-

la ted f rom the full image a r ea when the

sensor was set 5 cm f rom the cone tip. Wi th

the AEC OFF, the pixel va lues were propor-

t ional to the exposure . Inc reases in pixel

values fo l lowed a curve wi th increases in

exposure wi th the AEC ON.

F igure 3 shows the S N R re la ted to the

ave rage pixel value. Da ta at the lowest

exposure was ob ta ined when the sensor was

set 30 cm f rom the cone tip. Wi th the AEC

OFF, the S N R increased wi th increas ing

pixel values. The SNR, however , was con-

s tan t and decreased s l ight ly at higher p ixel

40

O

30 I D

0 e- 20 I

0 I

r r

0

Fig. 3

A J D

7

50 100 150 200 250

Pixel value /?

The SNR related to the average pixel value calculated from the full imaging area. 0 : AEC ON, �9 : AEC OFF.

values (more than 80) with the AEC ON.

Also, the S N R was much lower wi th the AEC

ON than with it O F F even with low expo-

sures.

The resul ts f rom Q A - p h a n t o m ' s images

are shown in Figs. 4-6. F igure 4 shows

images which were t a k e n both with the AEC

O F F {upper images) and ON (lower images)

a t increas ing exposu res f rom left to right.

When this pha n tom was imaged, the AEC

c lear ly p roduced h igh -c on t r a s t images, irre-

spect ive of the level of exposure. When

under - (far left) and ove r - e xpose d (far right)

images of the p h a n t o m were obtained, the

AEC was ef fec t ive for both under and over-

exposures and c r e a t e d higher con t ras t by

changing re l a t ive p ixe l values.

F igure 5 (a,b) shows the re la t ionship

be tween the exposu re and the ave rage pixel

values ca lcu la t ed f rom four bone - s t ep thick-

nesses when the sensor was set a t a 5 cm in

d i s tance f rom cone tip. Both with the AEC

O F F {Fig. 5 a) and wi th the AEC ON (Fig. 5

14

Fig. 4 Images of QA phantom which were taken either with the AEC ON or OFF. Exposure ts increases from left to right. The ten upper images were taken with the AEC OFF. The ten lower images were taken with the AEC ON, Exposure times ranged from 0.02 to 0.24 s.

250

200

150

>

'~ I00 X r~

SO

S /f2

i l l l i l l

10

250

200

150

>

o , i

50

" ' q P' O ' ' '

0 20 30 0 30 Exposure (/~ C/kg)

i i ,~ , E i

10 20

Exposure (/a. C/kg)

a b

Fig'. 5(a, b) T h e re la t ionship be tween the exposure and the ave rage pixel va lues ca lcula ted f rom the four

bone-s teps , a : AEC OFF : filled symbol, b : AEC ON : b l ank symbol . Th icknesses of the bone s tepwedge were 1.2 m m (O, 0 ) , 2.45 m m ( A , A , ) , 4.75 m m ( [ ] , i , ) , and 9.5 m m (0,0), respectively. Fi t t ing curves were prepared wi th the l inear equat ion. Coeff icients of de t e rminance for f i t t ing curves were in the range of 0.97 to 0.99.

I 5

b), the pixel value increased in proportion to 50

the exposure, except for the darkest step

with the higher exposure. In particular, with .O 40

the AEC ON (Fig. 5 b), the difference in pixel

values between the four steps was increased.

Figure 6 shows the SNR related to the "~ 30

average pixel value. Data at the lower aver- r O

age pixel value were obtained when the sen- -~ 20

sor was set at a 30 cm in distance from the o~ l- cone tip. Both with the AEC ON and OFF, I:~ ~ 1 0 the SNR increased with increased exposure.

The SNR was lower with the AEC ON than

with the AEC OFF.

Discussion

The AEC algorithm provides compensa-

tion for slight under- or over exposure by

stretching the pixel value range to increase

image contrast. The details of this function

are not specified ; however, it is probable that

the software detects both the minimum and

maximum pixel values of the image and

shifts these values to increase the displayed

image contrast, resulting in a discontinuous

pixel value distribution.

Images which are obtained by digital

intraoral radiographic imaging systems are

generally captured in the 8 bit mode. 2'3~ RVG-

4 captures 10-bit images ; however, as per-

sonal computer monitors only actually have

the capacity to display 6 bit black/white

images (64 possible gray levels) 1'~'12~, the con-

trast possible is not fully realized. Consider:

ing this limitation, the discrete pixel value

distribution by the AEC function is a reason-

able method to obtain high-contrast images.

In the case of images made without an

object in place, the AEC function over reacts

(Fig. 1). The narrow pixel value distribution

which is enhanced by the AEC function

resulted in the disproportionate pixel values

(Fig. 2) and a low SNR (Fig. 3). Hence, the

16

~!, A D

0 50 1 O0 150 200 25(

Average pixel value at each step

Fig. 6 The SNR related to the average pixel value calculated from the four bone-steps. Sym- bols are the same as in Fig. 5.

method of Welander 1~ cannot be used to

assess SNR fairly when the AEC is in use.

The results from the OA phantom's

images (Figs. 4-6) show that the AEC func-

tion increases the image contrast and at the

same time reduces the SNR. Figs. 4 and 5

show that slightly under-exposed images are

darkened and slightly over-exposed images

are lightened by using the AEC function.

The RVG-4 provides the AEC function

at the setup program which is activated by

the manufacturer default? ) The AEC func-

tion is a convenient and quick method to

produce visually more acceptable images

with high contrast regardless of the level of

exposure. The usefulness of the AEC option

for general practitioners is that it provides

diagnostic images immediately over a wide

exposure range. A greater exposure latitude

could lead to inappropriate patient dosage if

compensation was used for over exposed

images.

The biggest potential problem with this

feature is that users may "pre-set" exposure

at a higher level than necessary and rely on

the AEC function to compensate disparate

images. Consequently, the patient might

receive a higher radiation dose than other-

wise necessary.

By comparison, the equalization of the

Computed Dental Radiography system per-

mits compensation only for low exposures by

stretching the pixel value range, and this

results in increased image contrast of under-

exposed images, s) It does not, however, com-

pensate for over exposure.

Similar functions are incorporated in the

software of other digital intraoral radiogra-

phic systems. The Dental Link system

(EScan Inc., Santa Rosa, CA, USA) provides

an equalize function to increase the register

number of a pixel to produce a brighter

image, TM and this operates optimally on over-

exposed images, functioning in reverse of the

CDR's equalization. The DIGORA system

(Orion Corp. Soredex, Helsinki, Finland) pro-

vides an automatic grayscale adjustment set-

up which sets the brightness and contrast to

"ideal" values? 4) If images are consistently

too dark or too bright, they can be adjusted

by a correction factor ; namely, a positive

percentage increases the brightness, and a

negative percentage decreases it. This is a

two-way function similar to that installed as

the AEC in the RVG-4.

The purpose of the AEC function of the

RVG-4 is to provide users a means of imme-

diate compensation of the displayed image

for under- and over-exposed images rather

than to simply increase the exposure latitude

of the system. In conclusion, both under and

over-exposed images are compensated by

using the AEC function. The AEC, however,

reduces the SNR and the pixel value changes

are not proportional to exposure.

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Reprint requests to :

Dr. Yoshihiko HAYAKAWA Division of Radiology and Imaging Sciences, School of Dentistry, The University of Louisvill

Louisville, Kentucky, 40292 USA