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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- 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 Available online at ScienceDirect Nuclear Engineering and Technology journal homepage: www.elsevier.com/locate/net Original Article Comparison of Image Uniformity with Photon Counting and Conventional Scintillation Single-Photon Emission Computed Tomography System: A Monte Carlo Simulation Study Ho Chul Kim a, Hee-Joung Kim b, Kyuseok Kim b, Min-Hee Lee c, and Youngjin Lee a,* a Department of Radiological Science, Eulji University, 553, Sanseong-daero, Seongnam-si, Gyeonggi-do, 13135, South Korea b Department of Radiological Science, Yonsei University, 1, Yonseidae-gil, Wonju-si, 26493, South Korea c Department of Biomedical Engineering, Yonsei University, 1, Yonseidae-gil, Wonju-si, 26493, South Korea article info abstract Article history: To avoid imaging artifacts and interpretation mistakes, an improvement of the uniformity Received 3 June 2016 in gamma camera systems is a very important point. We can expect excellent uniformity Received in revised form using cadmium zinc telluride (CZT) photon counting detector (PCD) because of the direct 11 October 2016 conversion of the gamma rays energy into electrons. In addition, the uniformity perfor- Accepted 5 December 2016 mance such as integral uniformity (IU), differential uniformity (DU), scatter fraction (SF), Available online 28 December 2016 and contrast-to-noise ratio (CNR) varies according to the energy window setting. In this study, we compared a PCD and conventional scintillation detector with respect to the 99m Keywords: energy windows (5%, 10%, 15%, and 20%) using a Tc gamma source with a Geant4 Medical Application Application for Tomography Emission simulation tool. The gamma camera systems used Monte Carlo Simulation in this work are a CZT PCD and NaI(Tl) conventional scintillation detector with a 1-mm Nuclear Medicine thickness. According to the results, although the IU and DU results were improved with Photon Counting Detector the energy window, the SF and CNR results deteriorated with the energy window. In Scintillation Detector particular, the uniformity for the PCD was higher than that of the conventional scintillation Single-Photon Emission detector in all cases. In conclusion, our results demonstrated that the uniformity of the Computed Tomography System CZT PCD was higher than that of the conventional scintillation detector. © 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/). 1. Introduction the field of diagnostic medicine or radiotherapy [1e3]. SPECT has become an essential device and is considered a valuable Nuclear medicine imaging devices using single-photon emis- functional imaging tool. In general, to acquire physiological sion computed tomography (SPECT) has many advantages in information using SPECT, patients are injected with a suitable * Corresponding author. E-mail address: radioyoungj@gmail.com (Y. Lee). http://dx.doi.org/10.1016/j.net.2016.12.002 1738-5733/© 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
- 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. [7]. 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 [11] 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 [12]. 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) [7]. 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 [7]: 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 PROI:rod PROI:background 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 [16]. 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: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi Fig. 3 e Plot of the IU and DU in UFOV and CFOV. (A) With Pn 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- (%) (symmetrical) (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 [11]. 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 respectively. 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 systems. 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 view. detector system. CZT, cadmium zinc telluride.
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