Techn ica l R e p o r t

Signa l - to -no i se

sus S e n s - A - R a y

ratio : Computed Dental Radiography ver-

Yoshihiko H A Y A K A W A , Ph.D., Al lan G. F A R M A N , BDS, Ph.D.Sc.,

Michael S. KELLY, MS* and Kinya KUROYANAGI, DDS, Ph.D.**

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

*Radiation Safety Office, The University of Louisville, Kentucky, USA

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

(Received : March 15, 1995, Revision received : June 20, 1995, Accepted : June 30, 1995)

Key Words : Dental Radiography, CCD-based intraoral imaging system, Signal-to-noise ratio

The signal-to-noise ratio (SNR) describes the ability of a detector to differentiate a signal from

random fluctuations in signal intensity or noise in an image. The dose-response curves and the SNRs

were measured and compared between Computed Dental Radiography (CDR) and Sens A Ray. The

dose-response curves at 60, 70 and 80 kVp of both systems indicated that the pixel values increased in

proportion to the radiation exposure. The pixel value gradient was slightly steeper for the CDR than

for the Sens A-Ray. Sensitivity increased slightly at the higher kVp setting with the CDR only. All

SNRs increased with increased exposure for both systems. The SNR for the CDR was superior to that

for the Sens A-Ray, even with low exposures. The SNR for CDR increased steeply as exposure and

average pixel values increased. On the other hand, the SNR for the Sens-A-Ray showed only a

relatively slight increase with exposure. In conclusion, due to the high SNR in the lower exposure

range, the CDR has the capability of substantially reducing the level of exposure in comparison with the

Sens A-Ray.

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

Computed Dental Radiography (CDR) is

a digital X- r ay imaging system manufac tur -

ed and distr ibuted by Schick Technologies,

Inc. (Long Island City, NY, USA). The

sensor comprises a scinti l lator, a fiber optic

couple and a charge-coupled device (CCD).

Oral Radiol. Vol.11 No.2 1995 (109~113)

The s i g n a l - t o - n o i s e rat io (SNR) desc-

ribes the abi l i ty of a detector to discern a

signal f rom random fluctuat ions in signal

intensi ty or noise for any imaging system. 1,2)

The SNR was calculated as the rat io between

the image- fo rming signal and the noise.

Noise l imits the visibili ty of low-cont ras t

59 (109)

structures, and is largely determined by quan-

tum noise from statistical fluctuations in

photon density. Diagnostic quality and expo-

sure reduction are influenced by this parame-

ter.

Wenzel 1) evaluated the random noise in

digital intraoral radiography by recording

two identically exposed images after which

these were subtracted. The homogeneity of

the subtraction image served as an expres-

sion of system noise for her study. The

fastest system, VIXA/Visualix (Gendex

Dental Systems, Milan, Italy)a) provided the

noisiest images. In spite of the low sensitivity

compared with RadioVisioGraphy (Trophy

Radiologie, Vincennes, France) 4), Sens A-

Ray (Regam Medical Systems, Sundsvall,

Sweden) 2) also produced relatively noisy

images. 1)

The SNR for Sens A Ray was reported

in detail by Welander et al. 2) SNRs were

calculated from the mean and the standard

deviation of the pixel value over most of the

detector surface. More recently, Welander et

al? ) found that the Sens-A-Ray was the best

available system in image resolution mea-

sured by the modulation transfer function

and the noise equivalent passband.

The purpose of the present study was to

measure the SNR of the CDR digital intraor-

al imaging system and compare it to that of

the Sens A-Ray.

Materials and Methods

CCD-based intraoral imaging system

The CDR and Sens A Ray receptor

were employed as the image sensors.

Three different-sized receptors are

available with the CDR. The sensor sizes are

respectively comparable to size 0, 1, and 2

dental films. For our study, the size 2 rece-

ptor (the largest receptor), with a 36.5 mm x

60 (Ho)

25.2 mm sensitive area and 760 x 524 pixel

matrix, was employed. Each pixel was 48 x

48 /Lm. The image was captured by the

sensor and its distributed electrostatic inten-

sities were converted to 8-bit digital data.

The file size of the CDR image was 399,033

bytes without image compression. The pixel

value measured using the CDR software was

expressed by the range of 0 (white) to 255

(black).

The digital X-ray imaging system, Sens-

A-Ray, was also used. The specifications of

Sens A - Ray were reported in detail

elsewhere. 2'6) An 8-bit image has 256 pixel

levels from 0 (for Sens-A Ray corresponding

to black) to 255 (white). For the purpose of

comparing the CDR and the Sens-A-Ray,

these settings are inverted in the Results and

Discussions sections.

X ray generator

The X-ray generator was a GE 100

(General Electric, Milwaukee, WI, USA).

The tube voltage was set at 60, 70, or 80 kVp.

The tube current was set at 10 mA. Beam

filtration was 2.7 mm A1 equivalent. The

exposure time was set at intervals in the

range of 1 to 8 impulses (1/60 to 8/60 sec).

The distance between the focal spot and the

cone tip was 36 cm. The CDR or Sens-A Ray

intraoral sensor was always set 5 cm from

the cone tip in this experiment.

Dose response curve

The dose-response curve was established

from pixel value changes at the three tube

voltage settings. The sensor was uniformly

exposed to X-rays without any interfering

object.

The average pixel value was measured

using the specific region in the sensor. In the

CDR system, each image was magnified 400

% and the average pixel value within the 190

x 130 pixel matrix at the center of the sensor

was measured. In the S e n s - A - R a y system,

the region measured was set near the center

of the sensor by inputting two (x, y) dimen-

sions, namely (100, 200) and (200, 400).

More than 20,000 pixel values were used to

calculate the average in both systems.

In the S e n s - A - R a y system, even in the

absence of X ray irradiat ion there is a spon-

taneous generat ion of charge within the CCD

detector. 2) This gives rise to a background

signal that is usually referred to as the dark

current. The dark current was subtracted

f rom the raw data of pixel values in the

previous report. 2'r) The raw data, however,

was used to evaluate the SNR in this experi-

ment. Because the contr ibut ion of the dark

current and its stat ist ical noise is thought to

be of minor impor tance at short exposure

times (less than 8/60 sec).2"7) On the other

hand, there was no significant increase in the

dark current in the CDR at any exposure

tested in the present study.

Signal-to noise ratio

The S N R was established f rom the dose-

response curve data. The s tandard deviation

is a measure of the noise. The SNR was

calculated by the rat io between the image-

forming signal (average pixel value) and the

noise (standard deviation). The raw pixel

value data was used to calculate the SNR.

Radiation exposure

Radiat ion exposure was measured with a

be ry l l ium- windowed ionization chamber,

Dos imeter /Elec t rometer Model 11 (CNMC

Corp., Nashville, T N, USA) with a 3 cm a

probe. Calibration of this chamber could be

t raced to the N I S T (Nat ional Institute of

Standards and Technology, Gaithersburg,

MD, USA). The probe was placed at the

same position as the sensor to measure expo-

sures in mR, and then conve r t ed /zCkg -1.

200

0 2 4 6 8

E x p o s u r e ( / , C / k g )

Fig. 1 Dose-response curve: Relation between expo- sure and pixel value. CDR ; �9 : 60kVp, �9 : 70kVp, �9 : 80kVp Sens-A Ray ; (~) : 60kVp, A : 70kVp, [ ] : 80kVp Linear regression expression (x: exposure, y: pixel value) and correlation coefficient: �9 :y=44.1x+11.7, 1.00 �9 : y-42.4x+28.7, 0.99 �9 : y-43.Tx+33.6, 0.98 C) : y=36.2x+ 1.0, 1.00 /x : y=35.0x+ 9.2, 9.99 [ ] : y=37.1x+ 1.2, 9.98

Results

Dose-response curve

Fig. 1 shows the dose-response curves at

three kVp settings, 60, 70 and 80 kVp of both

systems. The pixel value increased in propor-

tion to the exposure. The relation between

exposure and pixel value was approximated

by a s t ra ight line. The lines obtained by the

CDR data did not pass through the origin of

this figure, par t icular ly in 70 and 80 kVp.

The pixel value gradient as exposures in-

creased was slightly steeper for the CDR than

for the Sens -A-Ray .

Fig. 1 also shows the receptor sensitivity

at each applied tube voltage. While the dose-

response curve at each kVp sett ing showed a

linearity to the exposure, the sensitivity was

slightly increased at the higher kVp setting

61 (111)

with the CDR. The sensitivity with the Sens 120 -A-Ray showed no difference in the mea-

sured kVp setting range. 0 "~ 90 Signal-to-noise ratio

Fig. 2 shows the SNR related to the r

exposure at the three kVp settings of both "~ r 60

systems. All SNRs increased with increased 6 I exposure. The SNR for the CDR was supe- -~

rior to the SNR for the Sens-A-Ray, even at ~ 3o , m

the low exposures. Although the SNR for the

CDR increased steeply as exposure increased, o

the SNR for the Sens-A-Ray showed a rela-

tively slight increase. Concerning the kVp

dependence, a slight difference was observed Fig. 2

for the CDR. No kVp dependency was shown

for the Sens-A-Ray~

Discussion

Three sensor characteristics, low noise, a

large recording gradient, and a wide dynamic

range, are required to receive X ray informa-

tion optimally. Noise suppresses informa-

tion. A small gradient is detrimental to

contrast resolution. A narrow dynamic range

may cut off the contrast of intensely

radiolucent or radiopaque structures.

In principle, more sensitive sensors will

result in lower SNRs. The dose-response

curve in Fig. 1 indicates that the CDR was

more sensitive to X-ray exposure than the

Sens-A-Ray, but the SNR is also higher for

the CDR than for the Sens-A-Ray (Fig. 2).

This relation was extrapolated from a previ-

ous report 2). It clearly shows that SNR for

the CDR is larger than that for the Sens-A-

Ray. Furthermore, the SNR of the CDR

increased steeply as the average pixel value/

exposure increased. This result depends on

the difference in the sensor configuration,

especially the noise reduction technique.

Fig. 1 also shows that the CDR has both

a narrower dynamic range and larger gradi-

1 1 1

. . . . , , , , , , , ,

2 4 6 8

Exposure (/~ C/k9) Relation between exposure (l~Ckg -I) and sig -'~ hal-to-noise ratio. CDR ; �9 : 60kVp, A : 70kVp, �9 : 80kVp Sens A Ray ; (~) : 60kVp,/x : 70kVp, [ ] : 80kVp Each solid line is a three polynominal regres- sion expression.

ent in comparison with the Sens-A-Ray.

These characters for the CDR are an advan-

tage for contrast resolution.

Wenzel 1) stated that the VIXA-1, at the

time of her writing, was the fastest available

CCD-based intraoral imaging system but also

provided the noisiest images. The Sens-A-

Ray also provided a noisy image. The stan-

dard deviation values were reportedly 2.90, 3.

98 and 3.96, respectively. 1) On the contrary,

the minimum SNR for the Sens-A-Ray was

1.7 at the exposure of 0.75 izC/kg at 60 kVp.

Welander et al. 2) reported that the SNR for

the Sens A-Ray was in the range of 10 15

for exposures higher than about 2 /~Ckg -1.

At the same exposure range, the SNRs were

in the range of about 25 - 35 in the current

study (Fig. 2). Two reasons are considered,

(1) Currently available Sens-A-Ray sys-

tems use a sensor with a configura-

tion that is different from that used

in the prototype, hence, the improve-

ment in the measured SNR.

62 (112)

(2) Both papers written by Wenzel 1) and

Welander et al. 2) reported the SNR

which was calculated over the com-

plete detector rather than the central

area. Since the dark current in the

Sens A Ray is not evenly distribut-

ed over the area of the detector,

which was indicated by Welander et

al. 2), this inhomogeneity should

reduce the SNR.

In the Sens A Ray, the dark current was

not extracted from the raw data, but Welan-

der et al. 2) described that the addition of a

dark current and electronic noise has a lim-

ited effect on the SNR. Also, the short

exposure time used in this study makes this

effect negligible.

In conclusion, due to the high SNR with

low exposures, the CDR provides substantial

exposure reduction in comparison with the

Sens-A-Ray. The high SNR supports the

CDR's capability to provide a clinically

acceptable image.

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Reprint requests to :

Yoshihiko HAYAKAWA Ph.D.

Division of Radiology and Imaging Sciences,

School of Dentistry, The University of Louisville,

Louisville, Kentucky, 40292 USA

63 a13)