Comparison of image uniformity with photon counting and conventional scintillation single photon emission computed tomography system: A monte carlo simulation study
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Comparison of image uniformity with photon counting and conventional scintillation single photon emission computed tomography system: A monte carlo simulation study. In this study, we compared a PCD and conventional scintillation detector with respect to the energy windows (5%, 10%, 15%, and 20%) using a 99mTc gamma source with a Geant4 Application for Tomography Emission simulation tool. The gamma camera systems used in this work are a CZT PCD and NaI(Tl) conventional scintillation detector with a 1-mm thickness. According to the results, although the IU and DU results were improved with the energy window, the SF and CNR results deteriorated with the energy window.
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Comparison of image uniformity with photon counting and conventional scintillation single photon emission computed tomography system: A monte carlo simulation study. In this study, we compared a PCD and conventional scintillation detector with respect to the energy windows (5%, 10%, 15%, and 20%) using a 99mTc gamma source with a Geant4 Application for Tomography Emission simulation tool. The gamma camera systems used in this work are a CZT PCD and NaI(Tl) conventional scintillation detector with a 1-mm thickness. According to the results, although the IU and DU results were improved with the energy window, the SF and CNR results deteriorated with the energy window..
N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 7 7 6 e7 8 0 777
radioisotope that emits gamma rays. In this technique, the SPECT using CZT PCDs and conventional NaI(Tl) scintillation
conventional scintillation SPECT system using an NaI(Tl) or detectors. The geometries of and information on these de-
CsI(Tl) detector is most frequently used [4e6]. However, a tectors are presented in Table 1. In consideration of the
limitation of this system is the inadequate quantitative accu- intrinsic resolution, we separately designed the size and
racy of several nonuniformity problems due to difficulties in number of pixels. Moreover, charge sharing was not simulated
obtaining a proper energy window setting and scatter radiation due to the clear and acceptable image quality of the CZT PCD.
. In particular, the common sources of imaging artifacts
were nonuniformity in SPECT. To overcome this limitation, a 2.2. Evaluation of image uniformity
photon counting detector (PCD) using cadmium zinc telluride
(CZT) or cadmium telluride (CdTe) has been developed for To evaluate image uniformity, we calculated IU, DU, SF, and
SPECT [8e10]. Using this, the quantitative accuracy and spec- CNR. We simulated a 99mTc (140 keV energy peak) point source
troscopic performance are improved due to the excellent en- with an activity of 1 MBq and used a 900-second scan time to
ergy resolution and direct conversion of gamma ray energy evaluate IU and DU. In addition, we designed a hot-rod
into electrons [2,4]. Lee and Kim  have acquired a high en- phantom using GATE with different diameters to estimate
ergy resolution (approximately 6.3%) from a 3-mm thick CZT SF and CNR (Fig. 2). The number of projections was 90 over 360
PCD using a 57Co source. The energy resolution of CZT PCD can degrees (acquisition time of 1 view: 10 seconds); image
be seen to be significantly improved when compared with that reconstruction was carried out using an ordered subset-
of a conventional scintillation detector . expectation maximization method with five iterations and
The frequently used measurement parameters for unifor- five subsets. The 5%, 10%, 15%, and 20% symmetrical energy
mity are integral uniformity (IU), differential uniformity (DU), windows were applied. Table 2 shows the range of the energy
scatter fraction (SF), and contrast-to-noise ratio (CNR) . IU window for each detector system.
and DU are calculated in both useful field of view (UFOV) with The values of IU and DU were calculated as follows :
a medial region of 95% of FOV and central field of view (CFOV)
with a medial region of 75% of UFOV. Fig. 1 shows both UFOV Mpixel mpixel
IUð%Þ ¼ 100 (1)
and CFOV descriptions. The standardization level and auto- Mpixel þ mpixel
mation of these parameters are not reached in the field of
nuclear medicine imaging. In addition, to the best of our Mlocal mlocal
DUð%Þ ¼ 100 (2)
knowledge, only a few studies have performed uniformity Mlocal þ mlocal
tests using a CZT PCD. Thus, herein, we investigated and where Mpixel is the maximum pixel count, mpixel is the mini-
compared the uniformity performances for different detector mum pixel count, Mlocal is the maximum percentage pixel
materials with respect to the energy window. For this purpose, count for all rows and columns in a localized line of pixels, and
we evaluated IU, DU, SF, and CNR using the Geant4 Applica- mlocal is the maximum percentage pixel count for all rows and
tion for Tomographic Emission (GATE) simulation (developed columns in a localized line of pixels.
by International OpenGATE collaboration). The values of SF and CNR, which indicate the value of en-
ergy distortion, were calculated as follows [12, 17]:
2. Materials and methods Pscattered
SFð%Þ ¼ 100 (3)
Pprimary þ Pscattered
2.1. Simulation set up
CNR ¼ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ (4)
Simulation tools of the gamma camera or SPECT imaging are s2ROI:rod þ s2ROI:background
applicable in the field of nuclear medicine [13e15]. These tools
are useful in assessing imaging characteristics and quantita- where Pscattered is the number of scattered gamma rays, Pprimary
tive accuracy . In this study, among various simulation is the number of primary gamma rays, PROI:rod is the count of
tools, we used the GATE simulation tool, which is very hot-rod regions in the phantom, PROI:background is the count of
powerful and widely used in the field of nuclear medicine, and background regions in the phantom, and sROI:rod and
which is based on the Monte Carlo platform. We simulated sROI:background are the standard deviation of the hot-rod region
and the background in the phantom, respectively.
Table 1 e Specifications of two detector materials for
evaluation of image uniformity.
Materials CZT NaI(Tl) scintillator
Thickness (mm) 1 1
Detector size (mm2) 44.8 44.8 44.8 44.8
Pixel size (mm2) 0.35 0.35 1.4 1.4
No. of pixels 128 128 32 32
Energy resolution (%) 6 9
Fig. 1 e Schematic description of field of view (FOV), central
CZT, cadmium zinc telluride.
field of view (CFOV), and useful field of view (UFOV).
778 N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 7 7 6 e7 8 0
Fig. 2 e Hot-rod phantom diagram. The phantom consisted
of six areas with rods of varying diameters (0.5 mm,
0.85 mm, 1.2 mm, 1.5 mm, 1.8 mm, and 2.1 mm) that can
be filled with activity. Activities were 9 kBq, 15.5 kBq, 30
kBq, 45 kBq, 60 kBq, and 90 kBq, respectively.
Ten sets of simulation results were acquired for each de-
tector system; error range (serror ) was calculated as follows:
2ﬃ Fig. 3 e Plot of the IU and DU in UFOV and CFOV. (A) With
i ¼ 1 Ni N CZT detector systems and (B) with NaI(Tl) detector systems.
serror ¼ (5)
ðn 1Þ CFOV, central field of view; CZT, cadmium zinc telluride;
DU, differential uniformity; IU, integral uniformity; UFOV,
where n is the number of measurements taken (n ¼ 10), Ni is useful field of view.
the datum from each measurement, and N is the measured
average of the data.
3. Results and discussion
One of the main defects in the SPECT system is its relatively
low image uniformity. The uniformity is affected by a scatter
Table 2 e Specifications of eight detector systems with
different energy windows. radiation due to spectral distortion in the energy spectrum.
Therefore, in the field of nuclear medicine, the scatter rejec-
Detector system Energy window Energy range
tion method is very important for the improvement in uni-
(keV) formity. The conventional scintillation detectors using NaI(Tl)
material are generally used in this field, but these detectors
CZT-1 5 136.5e143.5
have a limitation of lower energy resolution. To cope with this
CZT-2 10 133e147
CZT-3 15 129.5e150.5 problem, a PCD using CZT material has been developed that
CZT-4 20 126e154 efficiently generates electrons and holes. In our previous
NaI(Tl)-1 5 136.5e143.5 study, the measured energy resolution of a CZT PCD (eValu-
NaI(Tl)-2 10 133e147 ator-2500; eV Products, Arizona, USA) was approximately 6.3%
NaI(Tl)-3 15 129.5e150.5 full width at half maximum . Compared with the spectra
NaI(Tl)-4 20 126e154
obtained on a conventional scintillation detector, one can
CZT, cadmium zinc telluride. notice a markedly better energy resolution with CZT PCD [the
N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 7 7 6 e7 8 0 779
energy resolution of NaI(Tl) scintillation detector can be ac- NaI(Tl)-3, and NaI(Tl)-4 detector systems, respectively (for
quired by approximately 10% full width at half maximum] [18, identical detector thickness and energy window conditions).
19]. We compared the CZT-1 detector system with the NaI(Tl)-1
The uniformity can be divided into two types: (1) intrinsic detector system, which had been demonstrated to have a
uniformity and (2) system uniformity. The intrinsic unifor- 1.33 times lower average of SF and a 1.21 times higher average
mity is generally measured using 99mTc source with smooth- of CNR (for 1 mm and 5% energy window conditions).
ing filter and is calculated by IU and DU in both UFOV and In CZT PCDs, the average SF when using a 5% energy
CFOV. Moreover, as aforementioned, the scatter radiation and window was 1.05 times, 1.11 times, and 1.22 times better than
energy window setting affect image uniformity. The purpose those obtained with 10%, 15%, and 20% energy windows,
of this study was to evaluate and compare the uniformities respectively; and in the NaI(Tl) detector systems, the average
with CZT PCD and NaI(Tl) scintillation detectors using a CNR when using a 5% energy window was 1.03 times, 1.04
calculated IU, DU, SF, and CNR. times, and 1.09 times higher than those obtained with 10%,
The evaluated IU and DU in UFOV and CFOV for each de- 15%, and 20% energy windows, respectively. Moreover, in CZT
tector system are shown in Fig. 3 and Table 3. The results from PCDs, the average SF when using a 5% energy window was
IU and DU demonstrate that the CZT-1, CZT-2, CZT-3, and 1.05 times, 1.11 times, and 1.23 times better than those ob-
CZT-4 detector systems were lower than NaI(Tl)-1, NaI(Tl)-2, tained with 10%, 15%, and 20% energy windows, respectively;
NaI(Tl)-3, and NaI(Tl)-4 detector systems, respectively (for the in the NaI(Tl) detector systems, the average CNR using a 5%
same detector thickness and energy window conditions). In energy window was 1.04 times, 1.10 times, and 1.15 times
particular, we compared the CZT-4 detector system with an
NaI(Tl)-4 detector system that had been demonstrated to have
a 1.27-times higher average of IU and a 1.17-times higher
average of DU (for 1 mm and 20% energy window conditions).
In addition, the evaluated IU and DU results go from 5%, via
10% and 15%, to the 20% energy window in increasing order
for all cases (Fig. 3). In CZT PCDs, the average IU in UFOV and
CFOV using a 20% energy window was 1.01 times, 1.03 times,
and 1.09 times better than those obtained with 15%, 10%, and
5% energy windows, respectively; and in the NaI(Tl) detector
systems, the average IU in UFOV and CFOV using 20% energy
window was 1.09 times, 1.18 times, and 1.33 times better than
those obtained with 15%, 10%, and 5% energy windows,
respectively. Moreover, in CZT PCDs, the average IU in UFOV
and CFOV using a 20% energy window was 1.01 times, 1.02
times, and 1.04 times better than that obtained with 15%, 10%,
and 5% energy windows, respectively; in the NaI(Tl) detector
systems, the average IU in UFOV and CFOV using a 20% energy
window was 1.07 times, 1.12 times, and 1.17 times better than
those obtained with 15%, 10%, and 5% energy windows,
Fig. 4 e Plot of scatter fraction with respect to the detector
system. CZT, cadmium zinc telluride.
The evaluated SF and CNR for each detector system are
shown in Figs. 4 and 5, and Table 4. The results from SF and
CNR demonstrate that the CZT-1, CZT-2, CZT-3, and CZT-4
detector systems were better than the NaI(Tl)-1, NaI(Tl)-2,
Table 3 e IU and DU in UFOV and CFOV with respect to the
Detector IU IU DU DU
system (%, UFOV) (%, CFOV) (%, UFOV) (%, CFOV)
CZT-1 2.48 2.21 1.88 1.58
CZT-2 2.31 2.48 1.84 1.53
CZT-3 2.23 2.13 1.82 1.51
CZT-4 2.19 2.11 1.81 1.51
NaI(Tl)-1 3.30 2.70 2.11 1.95
NaI(Tl)-2 2.91 2.41 2.08 1.81
NaI(Tl)-3 2.61 2.34 1.93 1.78
NaI(Tl)-4 2.38 2.31 1.88 1.58
CFOV, central field of view; CZT, cadmium zinc telluride; DU, dif-
ferential uniformity; IU, integral uniformity; UFOV, useful field of
Fig. 5 e Plot of contrast-to-noise ratio with respect to the
detector system. CZT, cadmium zinc telluride.
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