Technical Report

Beam Quality and Image Contrast with VIXA-2

Yoshihiko HAYAKAWA, Ph.D., Allan G. Farman, B.D.S., Ph.D.,

William C. Scarfe, B.D.S., MS, Kinya KUROYANAGI, D.D.S., Ph.D.*

and Roberto Molteni, Ph.D.**

Division of RadioIogy and Imaging Science, School of Dentistry, The University of Louisville, Kentucky, USA

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

**Gendex Dental Systems srl, Milan, Italy

(Received : Dec. 25, 1994, Revision received : April 26, 1995, Accepted : May 17, 1995)

Key Words :Dental radiography, Digital image processing, CCD-based intraoraI radiographic system

Purpose : Current CCD-based in t r ao ra l radiographic systems permit the use of any dental X - r a y

genera tor . As a consequence, beam quality can be altered. This study was carr ied out to invest igate

studied the effects of va ry ing beam quality on the VIXA 2 image cont ras t (Gendex Dental Systems srl

: Milan, I taly).

Methods : Images were made of a s tandard aluminum stepwedge and the pixel value of each step was

measured. An optical bench was used to s tandardize geometr ic projection. Soft-t issue equivalent

a t t enua t ion was effected using 1.75 cm plexiglass. Exposures were made at 2-48 impulses using 50-90 kVp

set t ings a t 10 kVp intervals . Exposures (#C/kg) were determined using a beryll ium-windowed ioniza-

t ion chamber.

Results : The pixel values for each step decreased bo th with increased exposure (#C/kg) and with

increased kVp. The relat ionship between exposure and pixel value was not l inear. The longest scale of

con t ras t was obtained a t 17.3, 15.2, 13.5, 11.7, and 11.3/zC/kg respectively at 50, 60, 70, 80, and 90 kVp.

The grad ien t for pixel values a long the steps was steeper at lower kVp set t ings than at h igher kVp

settings.

Conclusions : The VIXA 2 can be operated at a wide range of kVp settings. Gamma conversion inhe ren t

in the VIXA 2 creates wide var ia t ions in the pixel values for d i f ferent stepwedge thicknesses.

Oral Radiol. VoLll No.1 1995 (31436

Introduction

Both analog dental film and charge cou-

pled device (CCD) -based intraoral radiogra-

phic systems have three functions, namely, X-

ray photon detection, image storage, and

display. While these functions are intrinsic

and closely related in the processed analog

radiographic film, they are attained indepen-

31 (31)

dently with CCD-based systems. X- r ay infor-

mation is captured by the image sensor.

Images are either displayed on a CRT moni-

tor or output as hardcopy. Images are preser-

ved on storage hardware such as a computer

hard drive, optical drive or magnetic tape.

Several CCD-based intraoral radiogra-

phic systems are commercial ly available 1-5).

The image sensor consists either of the com-

bination of a scintil lator-optical couple and

CCD 6-s), or a radiat ion-hardened CCD with

or without a scintillator applied to its

surfaceg,10).

VIXA is a digital X- ray imaging system

manufactured and distributed by Gendex

Dental Systems srl (Milan, Italy)1o,n~. It

is known as Visualix in Europe and VIXA in

the USA. The specifications of VIXA are

reported in detail elsewhere 3,~~ A radia-

t ion-hardened CCD with a scintillator applied

to its surface is used in the second generation

VIXA system, denoted as VIXA-2.

Most CCD-based intraoral radiographic

systems permit the use of any dental X ray

generator. The image contrast effect of

beam energy on the Sens-A-Ray (Regam

Medical Systems AB, Sundsvall, Sweden)

has been reported by Harada et a lY ~ and

Goshima et al. 14~, however, these studies were

published only as abstracts. Both investiga-

t ions descr ibed a p p r o x i m a t e l inear i ty

between radiation exposure and gray level at

each kVp setting. In a more limited study on

the effects of beam quality, McDonnell and

Price TM reported Sens -A-Ray image contrast

at 60 kVcp and 70 kVcp. Unlike the Sens-A-

Ray, the VIXA-2 default displays the image

using a logari thm-l ike (Log) gray scale l~

This scale has been proposed to provide

optimal contrast discernment, because it

approximates the characteristic curve of

radiographic film more closely than does a

32 (32)

linear scale.

The purpose of this study was to exam-

ine the effects of varying tube voltage on

image contrast using the VIXA-2.

Materials and Methods

Images were made of a standard alumi-

num stepwedge (Picker, Wood Dale, IL,

USA) with steps from 0-12.7mm(0-0.5")

thicknesses at 1.6mm (0.06") intervals. The X

ray generator was a GE-100 (General Elec-

tric, Milwaukee, WI, USA). Tube voltage

was varied f rom 50 to 90 kVp at 10 kVp

intervals. The tube current was set at 10 mA.

Beam filtration was 2.7mm Al-equivalent.

The exposure t ime was set at intervals in the

range of 2 to 48 impulses. The distance from

focal spot to cone tip was 36 cm. An optical

bench was used to standardize distance and

geometric projection.

A second generation VIXA receptor was

employed as the image sensor. The image

was captured by the sensor and its distributed

electrostatic intensities were digitized to 8-

bit data. An 8-bit image has 256 pixel levels

from 0 (black) to 255 (white). The sensor

was set 5 cm from the cone tip. The stepwed-

ge was placed immediately above the sensor.

A soft-t issue equivalent at tenuator (1.75 cm

thick plexiglass) was placed in front of the

stepwedge s,16,1n. Contrast resolution was

measured as gray level, or pixel value, differ-

ences in the logari thm-like (Log) gray scale

by default. The average gray level at five

random points was measured for each step.

The gray level differences between images of

adjoining steps were compared.

Radiation exposure was measured with a

be ry l l ium-windowed ionization chamber

(Dosimeter/Electrometer Model 11, CNMC

Corp., Nashville, TN, USA) with a 3 cm ~

probe. This chamber was calibrated by the

N I S T (Na t iona l Ins t i tu te of S t a n d a r d s and

Technology, Gai thersburg , MD, USA) . The

5

105

>,, E 155 (.9

Fig. 1

50 kVp

,.m

f f $5 J j . . J

205 < " ~ " ~

255 ~ . . . . . . . . . . . . 5 10 15 20 25 30

Exposure (~ C/kg) Contrast resolution plotted for each step wedge thickness at a tube voltage of 50 kVp.

: 0 mm(O inch), �9 : 1.6 mm(0.06"), �9 : 3.2 mm(O.13"), �9 : 4.8 mm (0.19"), �9 : 6.4 mm(0.25"), 0 : 7.9 mm(0.31"), A : 9.5 mm (0.38"), [] : 11.1 mm (0.44"), ~ : 12.7 mm(0.50").

probe was p laced a t the s ame posi t ion as the

sensor to m e a s u r e exposures in mR, which

were then conver t ed BC/kg .

Results

Figures 1 th rough 5 show the con t ras t

reso lu t ion for kVp se t t ings of 50, 60, 70, 80,

and 90, respect ive ly . The g r a y levels for each

s tep dec reased wi th e i ther increased expo-

sure ( /zC/kg) or inc reased kVp. The rela-

t ionship be tween exposu re and g r a y level was

not l inear wi th each s tep on the l oga r i t hm-

l ike (Log) scale. T h e g r a y level d i f ference

be tween images of adjoin ing s teps decreased

wi th inc reased kVp.

T h e longes t con t r a s t scale was ob ta ined

at 17.3, 15.2, 13.5, 11.7, and l l .3 ]~C/kg (31, 19,

13, 9, and 7 impulses) , r espec t ive ly at 50, 60,

70, 80, and 90 kVp. These exposure t imes

were the min imum necessa ry to ob ta in a g r ay

level of zero, or s a tu r a t i on a t the b l ack end of

the scale, for zero th ickness of a luminum.

5

55

-~ 105

155 (,9

205

255

Fig. 2

60 kVp

l | | | |JJ!

5 10 15 20 25 30 35 40 Exposure (~ C/kg)

Contrast resolution plotted for each step wedge thickness at a tube voltage of 60 kVp. Symbols are the same as in Fig. 1.

55

105

155 (_9

20S

255

70 kVp

: t 7" J

T Z < - " l - / . , l i l y

10 20 :30 40 50 60 Exposure(~C/kg)

Fig. 3 Contrast resolution plotted for each step wedge thickness at a tube voltage of 70 kVp. Symbols are the same as in Fig. 1.

33 (33)

80 kVp

_-->,~ 105

155

205 ~ J / 4

Fig. 4

P I

,//

, , 1 ~ l l l l l i l t i 1 , , , , , , 1 , , ,

10 20 30 40 50 60 70 Exposure (~ C/kg)

Contrast resolution plotted for each step wedge thickness at a tube voltage of 80 kVp. Symbols are the same as in Fig. 1.

55

"~ 105

155 (.9

205

255

Fig. 5

9 0 kVp

, tt !

f [ . . . . . . . , . . . . , ~ . . . . . . , . . . .

10 20 30 40 50 60 Exposure ( u C/kg)

70 80

Contrast resolution plotted for each step wedge thickness at a tube voltage of 90 kVp. Symbols are the same as in Fig. 1.

Figure 6 shows the longest contras t scale

plot ted for each s t e p w e d g e th i ckness

obtained at 17.3, 15.2, 13.5, 11.7, and 11.3#C/

kg (31, 19, 13, 9, and 7 impulses) respectively,

at 50, 60, 70, 80, and 90 kVp. The gradient for

pixel values along the steps was steeper at

lower kVp settings than at higher kVp set-

tings.

55

105

155 (.9

5 I

Discussion The finding tha t the contras t resolution

of CCD images is greates t with l ow-kVp 205

techniques agrees with the findings of

McDonnell and Price TM who reported a reduc- 255

t ion in cont ras t with high beam energies.

The g ray level differences between adjoining

steps in our study, however, were still distin-

guishable even with h igh -kVp settings. Fig. 6

H a r a d a et a lY ) repor ted that, with the

Sens -A-Ray , g ray level differences with alu-

minum more than 9 thick of were too small to

distinguish f rom each other at 50 kVp. This

34 (34)

~Q

0 3 6 9 12 AI-step thickness (mm)

15

Longest contrast scale plotted for each step- wedge thickness obtained at 17.3, 15.2, 13.5, 11.7, and 11.3 #C/kg (31, 19, 13, 9, and 7 impulses) respectively at 50, 60, 70, 80, and 90 kVp. �9 : 50 kVp, �9 : 60kVp, �9 : 70 kVp, �9 : 80 kVp, �9 : 90 kVp.

was not the case with the VIXA-2. Hence, it

seems that the VIXA-2 can be used at a

wider range of tissue thickness and kVp

settings than the Sens-A-Ray.

Our findings that exposures of 15.2 and

13.5 /zC/kg were necessary to obtain the

longest contrast scale at 60 kVp and 70 kVp

are in keeping with Molteni's 1~ report of 12.0

/zC/kg for the largest scale of VIXA when

the X-ray generator was operated at 65 kVp.

It is possible that exposures required for a

typical dental radiograph are similar to those

providing the longest contrast scales, as the

stepwedge used in each case was designed to

simulate radiodensities of the teeth and jaws.

The approximately straight relation

between gray level and stepwedge thickness

in the logarithm-like (Log) gray scale at

higher kVp (Fig. 6) was probably caused by

the exponential X - r a y penetrability with respect to the object's thickness TM. Since

beam-hardening occurred at low kVp, the

relation was curved.

WenzeF ) stated that the VIXA-1, at the

time of her writing, was the fastest available

CCD-based radiographic system but also

provided the noisiest images. While the loga-

rithm-like (Log) gray scale processing

default in the VIXA-2 improves image con-

trast, it also results in increased noise at

lower exposures. If this noise is clinically

acceptable for the observer, the VIXA-2 may

provide marked exposure reductions compar-

ed to other available systems. In conclusion,

the gamma conversion inherent in the VIXA

created wide variations in the image densities

of the different steps.

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35 (35)

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materials .

Reprint requests to : Yoshihiko HAYAKAWA, Ph.D.

Division of Radiology and Imaging Sciences, School of Dentistry, The University of Louisville. Louisville, Kentucky, 40292, USA

36 (36)