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.
References
1 Wenzel,A.
6)
7)
: Sensor noise in direct imaging (the
RadioVisioGraphy, Sens-A- Ray, and Visualix/Vixa
systems) evaluated by subtraction radiography. Oral
Surg. Oral Med. Oral Pathol. 77 : 70 74, 1994
Welander,U., Nelvig,P., Tronje, G., McDavid,W.D,
Dove,S.B., M6rner,A.-C., Cederlund,T. : Basic techni-
cal properties of a system for direct acquisition of
digital intraoral radiographs. Oral Surg. Oral Med.
Oral Pathol. 75 : 506-516, 1993 Molteni,R. : Direct digital dental x-ray imaging with
Visualix/VIXA. Oral Surg. Oral Med. Oral Pathol. 76
: 235-43, 1993 Benz,C., Mouyen,F.: Evaluation of the new Radio-
VisioGraphy system image quality. Oral Surg. Oral
Med. Oral Pathol. 72 : 627 631, 1991 Welander, U., McDavid, W. C., Sanderink, G. C. H.,
Tronje, G., M6rner, A. C., Dove,S.B. : Resolution as
defined by line spread and modulation transfer func-
tions for four digital intraoral radiographic systems.
Oral Surg. Oral Med. Oral Pathol. 78 : 109 115, 1994 Nelvig, P., Wing, K., Welander, U. : Sens-A-Ray : A
new system for direct digital intraoral radiography.
Oral Surg. Oral Med. Oral Pathol. 74 : 818 823, 1992 Harada, T, Nishikawa, K., Shibuya, H., Hayakawa,
Y., Kuroyanagi, K. : Sens A Ray characteristics with
variations in beam quality. Oral Surg. Oral Med.
Oral Pathol. Oral Radiol. Endodont., 80 : 120-123, 1995
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)