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Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging: Monte Carlo simulation study
Accepted Manuscript Title: Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging: Monte Carlo simulation study Author: Seungwan Lee Youngjin Lee Ph.D PII: DOI: Reference:
S0030-4026(16)31109-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.09.090 IJLEO 58233
To appear in: Received date: Accepted date:
25-7-2016 21-9-2016
Please cite this article as: Seungwan Lee, Youngjin Lee, Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging: Monte Carlo simulation study, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.09.090 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging : Monte Carlo simulation study Seungwan Leea and Youngjin Leeb,* a
Department of Radiological Science, Konyang University, 158 Gwanjeodong-ro, Daejeon, South Korea
b
Department of Radiological Science, Eulji University, 553, Sanseong-daero, Seongnam-si, Gyeonggi-do, South Korea
*Corresponding author. Prof. Youngjin Lee (Ph.D.), Department of Radiological Science, Eulji University, 553, Sanseong-daero, Seongnam-si, Gyeonggi-do, +82-31-740-7264 E-mail information: [email protected]
ABSTRACT Yttrium-90 (Y-90) has attracted great attention as a therapeutic radionuclide because of higher emitted particle energy and pure beta emitter. In Y-90 bremsstrahlung imaging, NaI(Tl) and GSO detectors are widely used in detecting photon but these detectors cannot be easily applied to this imaging because of relatively low sensitivity. Among detector candidates, cadmium tungstate (CdWO 4) can acquire high detection efficiency due to the high density and good light yield. The purpose of this study was to evaluate the feasibility of gamma camera system with CdWO 4 detector for quantitation of Y-90 bremsstrahlung imaging using a Geant4 Application for Tomographic Emission (GATE). To evaluate performance of the imaging system, a Y-90 point source was designed in GATE simulation. These detectors using CdWO4, NaI(Tl), and GSO were equipped with low energy general purpose (LEGP), medium energy general purpose (MEGP), and high energy general purpose (HEGP) parallelhole collimators. According to the results, the average sensitivities of the CdWO4 detector with LEGP, MEGP, and HEGP were 43.76, 37.16, and 28.22 cps/MBq, respectively. In addition, the sensitivity was higher for CdWO4 detector system than for NaI(Tl) or GSO detector systems for all parallel-hole collimator system with 55-285 keV energy window. However, differences of scatter fraction for three detectors were very similar. Our results showed that the effect and feasibility of CdWO 4 detector for Y90 bremsstrahlung imaging can be investigated by a GATE simulation. In conclusion, the results of this study suggest that CdWO4 detector can be achieved improved performance of Y-90 bremsstrahlung imaging.
Keywords: Yttrium-90 (Y-90); Bremsstrahlung radiation; X-ray and gamma ray photon detection; CdWO4 detector
1. Introduction Radionuclides are typically occurring unstable atoms due to the radioactive decay. Radionuclide imaging techniques have been widely researched for the medical imaging both pre-clinically and clinically [1-3]. Compared with optical imaging technique, advantages of radionuclide imaging techniques are high sensitivity and no tissue penetration quantitative limit. Among radionuclide imaging techniques, radionuclide radiation therapy internally delivers therapeutic doses to the targeted tissue. One of the most commonly used therapeutic radionuclide is iodine-131 (I-131) [4-6]. I-131 has been commonly used for treatment of both benign and malignant conditions. However, I-131 has two major disadvantages: (a) the patient with I-131 should be isolated due to high energy gamma ray except for beta ray and (b) relatively low emitted particle energy. To address these problems, Yttrium-90 (Y-90) is frequently used in the radionuclide radiation therapy. Y-90 sends radiation directly into the blood vessels and has higher emitted particle energy than I-131. Especially, Y-90 radiotherapy is the best option to treat tumor in liver [7]. Recent studies demonstrated the tumor response and the hepatic toxicity correlated to their respective Y-90 doses in liver [8, 9]. Thus, a way to quantitatively assess the Y-90 distribution in the patient body is highly valuable. By using a Y-90 with and Anger camera, the bremsstrahlung X-rays are spread along a continuous and complex spectrum, including scatter and un-scattered photon, extending up to the maximum emitted beta energy (approximately 2.3 MeV). This energy usable by a camera with a collimator being approximately 0.5 MeV and acquisitions are contaminated by high energy X-rays scattered down into the used energy window. There are three major problematic effects: (a) the scattering in the patient body and the collimator scattering, (b) the lead fluorescence X-ray around 80 keV, and (c) the backscatter from the photo multiplier tubes (PMTs), light guide and lead housing of the camera. Because of these effects, bremsstrahlung X-ray imaging represents less than 15% of the recorded events. Thus, quantitative evaluation of gamma camera using Y-90 is one of the most interesting topics in medical imaging. The studies of quantitative imaging of the bremsstrahlung from Y-90 radionuclide for choice of appropriate energy window and feasibility of NaI(Tl), bismuth germinate (BGO), lutetium oxyorthosilicate (LSO), and gadolinium oxyorthosilicate (GSO) detector materials have been previous reported [10-14]. Among these detector candidates, the best crystal for a quantitation of Y-90 bremsstrahlung imaging gamma camera is cadmium tungstate (CdWO 4) that has high density and high atomic number and good relative light yield. However, few studies have been conducted CdWO4 detector materials for quantitation of Y-90 bremsstrahlung imaging. The purpose of this study was to determine the feasibility of gamma camera system with cadmium tungstate (CdWO4) detector for quantitation of Y-90 bremsstrahlung imaging using Monte Carlo simulation. To compare with performance of the system, we also designed NaI(Tl) and GSO detectors in Geant4 Application for Tomographic Emission (GATE) simulation. In this study, three types of parallel-hole collimator, such as low energy general purpose (LEGP), medium energy general purpose (MEGP), and high energy general purpose (HEGP), were simulated. The Monte Carlo
simulation was performed in the GATE developed by the international OpenGATE collaboration in 2001 [15]. The accuracy and usefulness of the GATE simulation have been demonstrated in several studies [15-18]. The GATE is a well-validated for gamma camera, computed tomography (CT) and radiotherapy applications. For that purpose, we evaluated sensitivity and scatter fraction with respect to the detector materials and parallel-hole collimators. The simulation results obtained for various parallel-hole collimator designs with Y-90 bremsstrahlung imaging system are presented and discussed.
2. Materials and methods 2.1. Optimal crystal choice for Y-90 bremsstrahlung imaging
The performance of the imaging system is strongly related to both the physical and scintillation properties of the crystals. The main physical properties of commonly used scintillator materials are presented in Table 1 [19, 20]. NaI(Tl) is widely used in gamma camera imaging because of excellent light yield and efficient conversion of deposited X-ray or gamma ray energy to scintillation photons. However, major disadvantages of NaI(Tl) were relatively low density and highly hygroscopic. Because NaI(Tl) has highly hygroscopic performance, a great deal of effort has gone into the development of hermetic packaging to protect the material from moisture in the atmosphere. In contrast, BGO, LSO, and GSO have non-hygroscopic performance, thus allowing relatively simple fabrication of detectors. The major advantage of BGO and LSO is high density and of LSO and GSO is short decay time. Among scintillator detector candidates, CdWO4 has excellent physical properties. The detector based on CdWO4 has both a little better detection efficiency than BGO and a slightly better light yield than BGO and GSO. Also, CdWO4 has non-hygroscopic performance and good scatter rejection due to the energy resolution of CdWO4 is higher than other scintillators. Although, many studies have been conducted NaI(Tl), BGO, LSO, and GSO detector materials for Y-90 bremsstrahlung imaging, few studies have been conducted CdWO4 detector materials.
2.2. Simulation modelling: CdWO4 detector and parallel-hole collimator
The linear attenuation coefficient (Β΅) indicates how effective a given material is, per unit thickness, at promoting photon interactions. Attenuation is often described by the mass attenuation coefficient. GATE simulation was applied to estimate mass attenuation coefficients of different detector materials as a function of photon energy. Fig. 1 shows the mass attenuation coefficients for NaI(Tl), GSO, and CdWO4 detector materials. As shown in Fig. 1, CdWO4 has a high mass attenuation coefficient. The detection efficiencies of NaI(Tl), GSO, and CdWO4 detector as a function of detector thickness at photon energy of 60, 100, 200, and 300 keV are shown in Fig. 2. Fig. 3 presents the efficiency of the 10-mm thick above-mentioned detectors. As shown in Fig. 3, a detector with a thickness of 10 mm provides approximately 97% detection efficiency at photon energy of 100 keV. The GATE simulation was used to model gamma camera systems using NaI(Tl), GSO, and CdWO4
detectors. These gamma camera components are illustrated in Fig. 4. All photon interactions were simulated in GATE simulation for accurate evaluation. All detectors were simulated with 10 mm detector thickness and crystal size of 45 β Ή 25 cm2. Backscatter by photomultipliers and electronics was also simulated. Lead shielding surrounded the detector and backscatter compartment. The collimator is a very important role in the gamma camera system. Especially, collimators determine both the sensitivity and the spatial resolution. Nearly all gamma camera uses parallel-hole collimator as the image-forming aperture. For all parallel-hole collimators, the spatial resolution is best at the collimator face and linearly deteriorates with increasing distance between the source and collimator. Also, efficiency of a parallel-hole collimator is nearly constant over the source-to-collimator distances used for clinical and pre-clinical imaging. In parallel-hole collimator system, the efficiency (πππππππππββπππ ) and resolution (π ππππππππββπππ ) of the collimator was defined as follows [21]:
πππππππππββπππ = πΎ 2 (
π
πππππππ‘ππ£π
π ππππππππββπππ = π
πππππππ‘ππ£π +π πππππππ‘ππ£π
2
)
π2 (π+π‘)2
(πππππππ‘ππ£π = π β 2π β1 )
(1)
(2)
where K is the constant that depends on the collimator hole shape, d is the hole diameter, πππππππ‘ππ£π is the effective septal height, t is the septal thickness, l is the septal height, and b is the source-tocollimator distance. In this study, we designed LEGP, MEGP, and HEGP parallel-hole collimators in GATE simulation because the choice of collimator for Y-90 bremsstrahlung imaging with CdWO4 detector is unclear. The specifications of these parallel-hole collimators are shown in Table 2. A previous paper demonstrated that the reconstructed hot-rod phantom images using most frequently used collimator materials (lead, tungsten, gold, and depleted uranium) were difficult to distinguish accurately [22]. Based on this result, tungsten offers spatial resolution similar to those of the much more expensive gold and depleted uranium. Thus, we considered as tungsten collimator material in this study.
2.3. Performance evaluation of the imaging system for quantitation anlysis
To evaluate the performance of the system, sensitivity and scatter fraction were evaluated. The source-to-collimator distance was 10 cm. The sensitivity was represented in counts per second per MBq (cps/MBq). Moreover, we calculated the scatter fraction to compare the amount of scattered photons. For the measurement of scatter fraction, a phantom was designed using GATE simulation. This phantom consists of water. The size of the phantom was 20 cm in diameter and 10 cm in height. The scatter fraction (%) is calculated as follows:
Scatter fraction (%) =
ππ πππ‘π‘ππππ ππππππππ¦ +ππ πππ‘π‘ππππ
Γ 100
(3)
where ππ πππ‘π‘ππππ is the number of scattered photons and ππ πππ‘π‘ππππ is the number of primary photons. The point sources of Y-90 were simulated and the sources were positioned in front of LEGP, MEGP, and HEGP parallel-hole collimator. Several research groups have used experimental study to select the appropriate energy window settings for Y-90 bremsstrahlung imaging. According to S. Shen et al. [13], the energy window of 55-285 keV is best and most practical selections for Y-90 bremsstrahlung imaging. Based on this result, we were applied to the energy window of 55-285 keV for all systems. A spatial blur module was created to model the energy resolution caused by scattering effects and by the electronic readout. Ten simulations were performed and the standard deviation (Ο) was calculated as follows:
Ο=β
Μ 2 βπ π=1(ππ βπ) (πβ1)
(4)
Μ is the where n is the number of simulations taken (n = 10), ππ is each simulation datum and π average of the data.
3. Results and discussion Y-90 beta emitter has been widely used as a therapeutic radionuclide. By using a Y-90 beta radionuclide, the bremsstrahlung quantitatively imaging with a gamma camera is one of the most important topics in the field of medical imaging. In this study, we evaluated feasibility of gamma camera system with CdWO4 detector for quantitation of Y-90 bremsstrahlung imaging. We used previous reported optimal energy window (with reported window of 55-285 keV [13]) and NaI(Tl), GSO, and CdWO4 detector materials using three parallel-hole collimators to compare with performance of the imaging system. The evaluated averages of sensitivity for each detector material and parallel-hole collimator are shown in Fig. 5. A comparison of the sensitivities with respect to the three detector materials is shown in Fig. 6. The sensitivity goes from NaI(Tl), via GSO, to CdWO 4 detector in increasing order. In LEGP parallel-hole collimator system, the average sensitivity using the CdWO 4 detector was 1.60 and 1.12 times higher than that obtained with NaI(Tl) and GSO detector, respectively. When we compared sensitivity differences between LEGP and MEGP or HEGP parallel-hole collimator, they had the same tendency. In addition, a comparison of the sensitivities with respect to the three parallel-hole collimators with CdWO4 detector is shown in Fig. 7. In CdWO4 detector system, the average sensitivity using the LEGP parallel-hole collimator was 1.18 and 1.55 times higher than that obtained with MEGP and HEGP parallel-hole collimator, respectively.
The evaluated averages of scatter fraction for each detector material and parallel-hole collimator are shown in Fig. 8. A comparison of the scatter fractions with respect to the three detector materials is shown in Fig. 9. The scatter fraction goes from NaI(Tl), via GSO, to CdWO 4 detector in increasing order. In LEGP parallel-hole collimator system, the average scatter fraction using the NaI(Tl) detector was 1.08 and 1.10 times better than that obtained with GSO and CdWO 4 detector, respectively. When we compared sensitivity differences between LEGP and MEGP or HEGP parallel-hole collimator, they had the same tendency. In addition, a comparison of the sensitivities with respect to the three parallelhole collimators with CdWO4 detector is shown in Fig. 10. In CdWO4 detector system, the average sensitivity using the HEGP parallel-hole collimator was 1.04 and 1.12 times better than that obtained with MEGP and LEGP parallel-hole collimator, respectively. Although the scatter fraction of CdWO4 detector was slightly lower than other detectors, the CdWO 4 detector system acquired superb sensitivity in this study. The sensitivity difference between CdWO4 and NaI(Tl) detector was maximum 1.60 times. However, the scatter fraction difference between CdWO4 and NaI(Tl) detector was maximum 1.12 times. Thus, these results demonstrated the CdWO 4 detector system can acquire appropriate performance for Y-90 bremsstrahlung imaging. In addition, the higher sensitivity of CdWO4 detector system can achieve low patient dose and low scan time. For all detector systems, the sensitivity was higher for LEGP than for MEGP or HEGP parallel-hole collimator and the scatter fraction was better for HEGP than for MEGP or LEGP parallel-hole collimator. For a given total time, high sensitivity collimators achieve more counts, because they allow a wider range of photon paths through their collimator holes. However, it is increasingly becoming recognized that the quality of the counts is also important; thus, high sensitivity collimators are generally not recommended. Thus, trade-off between sensitivity and quality of the counts is very important in gamma camera system.
4. Conclusion This Monte Carlo simulation study evaluated the feasibility of gamma camera system with CdWO 4 detector for quantitation of Y-90 bremsstrahlung imaging. Based on our results, the higher sensitivity achieved with CdWO4 detector will be of particular importance for various medical imaging techniques. In addition, appropriate energy resolution allows a clear identification in energy spectrum and better scatter fraction. In conclusion, our results demonstrate that our proposed parallel-hole collimator, used in combination with a CZT pixelated semiconductor gamma camera system, could be useful for quantitation of Y-90 bremsstrahlung imaging. One of the disadvantages of a gamma camera for Y-90 bremsstrahlung imaging is the backscatter by the backscatter components such as light guide or PMTs. Especially, the photons are significantly contaminated by backscatter components in below 90 keV. Thus, further work is planned which will investigate the variation in performance of the imaging system with energy window applied to backscatter components.
Acknowledgments This work was supported by the Korea Foundation for the Advancement of Science & Creativity (KOFAC) grant funded by Ministry of Education (MOE).
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Fig. 1. Calculated mass attenuation coefficients for NaI(Tl), GSO, and CdWO4. Calculations and estimates were carried out by the GATE simulation code.
Fig. 2. The detection efficiency for NaI(Tl), GSO, and CdWO4 detector as a function of detecto r thickness at various photon energy: (a) 60 keV, (b) 100 keV, (c) 200 keV, and (d) 300 keV.
Fig. 3. The detection efficiency of a 10 mm-thick different detectors at photon energy of 100 k eV. The efficiency of the CdWO4 at a 10 mm-thick was 1.59 and 1.06 times higher than that o f NaI(Tl) and GSO, respectively.
Fig. 4. Schematic diagram for gamma camera system with source, collimator, aluminium layer, detector, backscatter compartment, and lead shielding.
Fig. 5. Simulation results for the sensitivity with respect to the detector material for various parallelhole collimators.
Fig. 6. Comparison of the simulation results for the sensitivity as a function of detector. The results are normalized with respect to the sensitivity obtained with a CdWO 4 detector using (a) LEGP, (b) MEGP, and (c) HEGP parallel-hole collimator. In all cases, CdWO4 detector showed improved sensitivities compared with the other detectors.
Fig. 7. Comparison of the simulation results for the sensitivity as a function of parallel-hole collimator. The results are normalized with respect to the sensitivity obtained with a LEGP parallel-hole collimator using CdWO4 detector.
Fig. 8. Simulation results for the sensitivity with respect to the detector material for various parallelhole collimators.
Fig. 9. Comparison of the simulation results for the scatter fraction as a function of detector. The results are normalized with respect to the scatter fraction obtained with a CdWO 4 detector using (a) LEGP, (b) MEGP, and (c) HEGP parallel-hole collimator. Scatter fraction difference showed little difference among the three types of detector materials tested.
Fig. 10. Comparison of the simulation results for the scatter fraction as a function of parallel-hole collimator. The results are normalized with respect to the scatter fraction obtained with a LEGP parallel-hole collimator using CdWO4 detector.
Table 1. Physical properties of the principal detector materials. Material
NaI(Tl)
BGO
LSO
GSO
CdWO4
Density (g/cm3)
3.67
7.13
7.40
6.71
7.90
Effective atomic number
50
73
66
59
64
Light yield (relative)
100
7 β 12
40 β 75
20
30 - 40
Decay time (nsec)
230
300
40
60
5,000
Energy resolution (at 511 keV, %)
6.6
10.2
10.0
8.5
8.8
Hygroscopic
Yes
No
No
No
No
Table 2. Specifications of parallel-hole collimators. LEGP
MEGP
HEGP
Length (mm)
36.0
42.0
44.0
Hole diameter (mm)
2.5
3.4
4.0
Septal thickness (mm)
0.3
1.4
3.2
S0030-4026(16)31109-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.09.090 IJLEO 58233
To appear in: Received date: Accepted date:
25-7-2016 21-9-2016
Please cite this article as: Seungwan Lee, Youngjin Lee, Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging: Monte Carlo simulation study, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.09.090 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Feasibility of gamma camera system with CdWO4 detector for quantitation of yttrium-90 bremsstrahlung imaging : Monte Carlo simulation study Seungwan Leea and Youngjin Leeb,* a
Department of Radiological Science, Konyang University, 158 Gwanjeodong-ro, Daejeon, South Korea
b
Department of Radiological Science, Eulji University, 553, Sanseong-daero, Seongnam-si, Gyeonggi-do, South Korea
*Corresponding author. Prof. Youngjin Lee (Ph.D.), Department of Radiological Science, Eulji University, 553, Sanseong-daero, Seongnam-si, Gyeonggi-do, +82-31-740-7264 E-mail information: [email protected]
ABSTRACT Yttrium-90 (Y-90) has attracted great attention as a therapeutic radionuclide because of higher emitted particle energy and pure beta emitter. In Y-90 bremsstrahlung imaging, NaI(Tl) and GSO detectors are widely used in detecting photon but these detectors cannot be easily applied to this imaging because of relatively low sensitivity. Among detector candidates, cadmium tungstate (CdWO 4) can acquire high detection efficiency due to the high density and good light yield. The purpose of this study was to evaluate the feasibility of gamma camera system with CdWO 4 detector for quantitation of Y-90 bremsstrahlung imaging using a Geant4 Application for Tomographic Emission (GATE). To evaluate performance of the imaging system, a Y-90 point source was designed in GATE simulation. These detectors using CdWO4, NaI(Tl), and GSO were equipped with low energy general purpose (LEGP), medium energy general purpose (MEGP), and high energy general purpose (HEGP) parallelhole collimators. According to the results, the average sensitivities of the CdWO4 detector with LEGP, MEGP, and HEGP were 43.76, 37.16, and 28.22 cps/MBq, respectively. In addition, the sensitivity was higher for CdWO4 detector system than for NaI(Tl) or GSO detector systems for all parallel-hole collimator system with 55-285 keV energy window. However, differences of scatter fraction for three detectors were very similar. Our results showed that the effect and feasibility of CdWO 4 detector for Y90 bremsstrahlung imaging can be investigated by a GATE simulation. In conclusion, the results of this study suggest that CdWO4 detector can be achieved improved performance of Y-90 bremsstrahlung imaging.
Keywords: Yttrium-90 (Y-90); Bremsstrahlung radiation; X-ray and gamma ray photon detection; CdWO4 detector
1. Introduction Radionuclides are typically occurring unstable atoms due to the radioactive decay. Radionuclide imaging techniques have been widely researched for the medical imaging both pre-clinically and clinically [1-3]. Compared with optical imaging technique, advantages of radionuclide imaging techniques are high sensitivity and no tissue penetration quantitative limit. Among radionuclide imaging techniques, radionuclide radiation therapy internally delivers therapeutic doses to the targeted tissue. One of the most commonly used therapeutic radionuclide is iodine-131 (I-131) [4-6]. I-131 has been commonly used for treatment of both benign and malignant conditions. However, I-131 has two major disadvantages: (a) the patient with I-131 should be isolated due to high energy gamma ray except for beta ray and (b) relatively low emitted particle energy. To address these problems, Yttrium-90 (Y-90) is frequently used in the radionuclide radiation therapy. Y-90 sends radiation directly into the blood vessels and has higher emitted particle energy than I-131. Especially, Y-90 radiotherapy is the best option to treat tumor in liver [7]. Recent studies demonstrated the tumor response and the hepatic toxicity correlated to their respective Y-90 doses in liver [8, 9]. Thus, a way to quantitatively assess the Y-90 distribution in the patient body is highly valuable. By using a Y-90 with and Anger camera, the bremsstrahlung X-rays are spread along a continuous and complex spectrum, including scatter and un-scattered photon, extending up to the maximum emitted beta energy (approximately 2.3 MeV). This energy usable by a camera with a collimator being approximately 0.5 MeV and acquisitions are contaminated by high energy X-rays scattered down into the used energy window. There are three major problematic effects: (a) the scattering in the patient body and the collimator scattering, (b) the lead fluorescence X-ray around 80 keV, and (c) the backscatter from the photo multiplier tubes (PMTs), light guide and lead housing of the camera. Because of these effects, bremsstrahlung X-ray imaging represents less than 15% of the recorded events. Thus, quantitative evaluation of gamma camera using Y-90 is one of the most interesting topics in medical imaging. The studies of quantitative imaging of the bremsstrahlung from Y-90 radionuclide for choice of appropriate energy window and feasibility of NaI(Tl), bismuth germinate (BGO), lutetium oxyorthosilicate (LSO), and gadolinium oxyorthosilicate (GSO) detector materials have been previous reported [10-14]. Among these detector candidates, the best crystal for a quantitation of Y-90 bremsstrahlung imaging gamma camera is cadmium tungstate (CdWO 4) that has high density and high atomic number and good relative light yield. However, few studies have been conducted CdWO4 detector materials for quantitation of Y-90 bremsstrahlung imaging. The purpose of this study was to determine the feasibility of gamma camera system with cadmium tungstate (CdWO4) detector for quantitation of Y-90 bremsstrahlung imaging using Monte Carlo simulation. To compare with performance of the system, we also designed NaI(Tl) and GSO detectors in Geant4 Application for Tomographic Emission (GATE) simulation. In this study, three types of parallel-hole collimator, such as low energy general purpose (LEGP), medium energy general purpose (MEGP), and high energy general purpose (HEGP), were simulated. The Monte Carlo
simulation was performed in the GATE developed by the international OpenGATE collaboration in 2001 [15]. The accuracy and usefulness of the GATE simulation have been demonstrated in several studies [15-18]. The GATE is a well-validated for gamma camera, computed tomography (CT) and radiotherapy applications. For that purpose, we evaluated sensitivity and scatter fraction with respect to the detector materials and parallel-hole collimators. The simulation results obtained for various parallel-hole collimator designs with Y-90 bremsstrahlung imaging system are presented and discussed.
2. Materials and methods 2.1. Optimal crystal choice for Y-90 bremsstrahlung imaging
The performance of the imaging system is strongly related to both the physical and scintillation properties of the crystals. The main physical properties of commonly used scintillator materials are presented in Table 1 [19, 20]. NaI(Tl) is widely used in gamma camera imaging because of excellent light yield and efficient conversion of deposited X-ray or gamma ray energy to scintillation photons. However, major disadvantages of NaI(Tl) were relatively low density and highly hygroscopic. Because NaI(Tl) has highly hygroscopic performance, a great deal of effort has gone into the development of hermetic packaging to protect the material from moisture in the atmosphere. In contrast, BGO, LSO, and GSO have non-hygroscopic performance, thus allowing relatively simple fabrication of detectors. The major advantage of BGO and LSO is high density and of LSO and GSO is short decay time. Among scintillator detector candidates, CdWO4 has excellent physical properties. The detector based on CdWO4 has both a little better detection efficiency than BGO and a slightly better light yield than BGO and GSO. Also, CdWO4 has non-hygroscopic performance and good scatter rejection due to the energy resolution of CdWO4 is higher than other scintillators. Although, many studies have been conducted NaI(Tl), BGO, LSO, and GSO detector materials for Y-90 bremsstrahlung imaging, few studies have been conducted CdWO4 detector materials.
2.2. Simulation modelling: CdWO4 detector and parallel-hole collimator
The linear attenuation coefficient (Β΅) indicates how effective a given material is, per unit thickness, at promoting photon interactions. Attenuation is often described by the mass attenuation coefficient. GATE simulation was applied to estimate mass attenuation coefficients of different detector materials as a function of photon energy. Fig. 1 shows the mass attenuation coefficients for NaI(Tl), GSO, and CdWO4 detector materials. As shown in Fig. 1, CdWO4 has a high mass attenuation coefficient. The detection efficiencies of NaI(Tl), GSO, and CdWO4 detector as a function of detector thickness at photon energy of 60, 100, 200, and 300 keV are shown in Fig. 2. Fig. 3 presents the efficiency of the 10-mm thick above-mentioned detectors. As shown in Fig. 3, a detector with a thickness of 10 mm provides approximately 97% detection efficiency at photon energy of 100 keV. The GATE simulation was used to model gamma camera systems using NaI(Tl), GSO, and CdWO4
detectors. These gamma camera components are illustrated in Fig. 4. All photon interactions were simulated in GATE simulation for accurate evaluation. All detectors were simulated with 10 mm detector thickness and crystal size of 45 β Ή 25 cm2. Backscatter by photomultipliers and electronics was also simulated. Lead shielding surrounded the detector and backscatter compartment. The collimator is a very important role in the gamma camera system. Especially, collimators determine both the sensitivity and the spatial resolution. Nearly all gamma camera uses parallel-hole collimator as the image-forming aperture. For all parallel-hole collimators, the spatial resolution is best at the collimator face and linearly deteriorates with increasing distance between the source and collimator. Also, efficiency of a parallel-hole collimator is nearly constant over the source-to-collimator distances used for clinical and pre-clinical imaging. In parallel-hole collimator system, the efficiency (πππππππππββπππ ) and resolution (π ππππππππββπππ ) of the collimator was defined as follows [21]:
πππππππππββπππ = πΎ 2 (
π
πππππππ‘ππ£π
π ππππππππββπππ = π
πππππππ‘ππ£π +π πππππππ‘ππ£π
2
)
π2 (π+π‘)2
(πππππππ‘ππ£π = π β 2π β1 )
(1)
(2)
where K is the constant that depends on the collimator hole shape, d is the hole diameter, πππππππ‘ππ£π is the effective septal height, t is the septal thickness, l is the septal height, and b is the source-tocollimator distance. In this study, we designed LEGP, MEGP, and HEGP parallel-hole collimators in GATE simulation because the choice of collimator for Y-90 bremsstrahlung imaging with CdWO4 detector is unclear. The specifications of these parallel-hole collimators are shown in Table 2. A previous paper demonstrated that the reconstructed hot-rod phantom images using most frequently used collimator materials (lead, tungsten, gold, and depleted uranium) were difficult to distinguish accurately [22]. Based on this result, tungsten offers spatial resolution similar to those of the much more expensive gold and depleted uranium. Thus, we considered as tungsten collimator material in this study.
2.3. Performance evaluation of the imaging system for quantitation anlysis
To evaluate the performance of the system, sensitivity and scatter fraction were evaluated. The source-to-collimator distance was 10 cm. The sensitivity was represented in counts per second per MBq (cps/MBq). Moreover, we calculated the scatter fraction to compare the amount of scattered photons. For the measurement of scatter fraction, a phantom was designed using GATE simulation. This phantom consists of water. The size of the phantom was 20 cm in diameter and 10 cm in height. The scatter fraction (%) is calculated as follows:
Scatter fraction (%) =
ππ πππ‘π‘ππππ ππππππππ¦ +ππ πππ‘π‘ππππ
Γ 100
(3)
where ππ πππ‘π‘ππππ is the number of scattered photons and ππ πππ‘π‘ππππ is the number of primary photons. The point sources of Y-90 were simulated and the sources were positioned in front of LEGP, MEGP, and HEGP parallel-hole collimator. Several research groups have used experimental study to select the appropriate energy window settings for Y-90 bremsstrahlung imaging. According to S. Shen et al. [13], the energy window of 55-285 keV is best and most practical selections for Y-90 bremsstrahlung imaging. Based on this result, we were applied to the energy window of 55-285 keV for all systems. A spatial blur module was created to model the energy resolution caused by scattering effects and by the electronic readout. Ten simulations were performed and the standard deviation (Ο) was calculated as follows:
Ο=β
Μ 2 βπ π=1(ππ βπ) (πβ1)
(4)
Μ is the where n is the number of simulations taken (n = 10), ππ is each simulation datum and π average of the data.
3. Results and discussion Y-90 beta emitter has been widely used as a therapeutic radionuclide. By using a Y-90 beta radionuclide, the bremsstrahlung quantitatively imaging with a gamma camera is one of the most important topics in the field of medical imaging. In this study, we evaluated feasibility of gamma camera system with CdWO4 detector for quantitation of Y-90 bremsstrahlung imaging. We used previous reported optimal energy window (with reported window of 55-285 keV [13]) and NaI(Tl), GSO, and CdWO4 detector materials using three parallel-hole collimators to compare with performance of the imaging system. The evaluated averages of sensitivity for each detector material and parallel-hole collimator are shown in Fig. 5. A comparison of the sensitivities with respect to the three detector materials is shown in Fig. 6. The sensitivity goes from NaI(Tl), via GSO, to CdWO 4 detector in increasing order. In LEGP parallel-hole collimator system, the average sensitivity using the CdWO 4 detector was 1.60 and 1.12 times higher than that obtained with NaI(Tl) and GSO detector, respectively. When we compared sensitivity differences between LEGP and MEGP or HEGP parallel-hole collimator, they had the same tendency. In addition, a comparison of the sensitivities with respect to the three parallel-hole collimators with CdWO4 detector is shown in Fig. 7. In CdWO4 detector system, the average sensitivity using the LEGP parallel-hole collimator was 1.18 and 1.55 times higher than that obtained with MEGP and HEGP parallel-hole collimator, respectively.
The evaluated averages of scatter fraction for each detector material and parallel-hole collimator are shown in Fig. 8. A comparison of the scatter fractions with respect to the three detector materials is shown in Fig. 9. The scatter fraction goes from NaI(Tl), via GSO, to CdWO 4 detector in increasing order. In LEGP parallel-hole collimator system, the average scatter fraction using the NaI(Tl) detector was 1.08 and 1.10 times better than that obtained with GSO and CdWO 4 detector, respectively. When we compared sensitivity differences between LEGP and MEGP or HEGP parallel-hole collimator, they had the same tendency. In addition, a comparison of the sensitivities with respect to the three parallelhole collimators with CdWO4 detector is shown in Fig. 10. In CdWO4 detector system, the average sensitivity using the HEGP parallel-hole collimator was 1.04 and 1.12 times better than that obtained with MEGP and LEGP parallel-hole collimator, respectively. Although the scatter fraction of CdWO4 detector was slightly lower than other detectors, the CdWO 4 detector system acquired superb sensitivity in this study. The sensitivity difference between CdWO4 and NaI(Tl) detector was maximum 1.60 times. However, the scatter fraction difference between CdWO4 and NaI(Tl) detector was maximum 1.12 times. Thus, these results demonstrated the CdWO 4 detector system can acquire appropriate performance for Y-90 bremsstrahlung imaging. In addition, the higher sensitivity of CdWO4 detector system can achieve low patient dose and low scan time. For all detector systems, the sensitivity was higher for LEGP than for MEGP or HEGP parallel-hole collimator and the scatter fraction was better for HEGP than for MEGP or LEGP parallel-hole collimator. For a given total time, high sensitivity collimators achieve more counts, because they allow a wider range of photon paths through their collimator holes. However, it is increasingly becoming recognized that the quality of the counts is also important; thus, high sensitivity collimators are generally not recommended. Thus, trade-off between sensitivity and quality of the counts is very important in gamma camera system.
4. Conclusion This Monte Carlo simulation study evaluated the feasibility of gamma camera system with CdWO 4 detector for quantitation of Y-90 bremsstrahlung imaging. Based on our results, the higher sensitivity achieved with CdWO4 detector will be of particular importance for various medical imaging techniques. In addition, appropriate energy resolution allows a clear identification in energy spectrum and better scatter fraction. In conclusion, our results demonstrate that our proposed parallel-hole collimator, used in combination with a CZT pixelated semiconductor gamma camera system, could be useful for quantitation of Y-90 bremsstrahlung imaging. One of the disadvantages of a gamma camera for Y-90 bremsstrahlung imaging is the backscatter by the backscatter components such as light guide or PMTs. Especially, the photons are significantly contaminated by backscatter components in below 90 keV. Thus, further work is planned which will investigate the variation in performance of the imaging system with energy window applied to backscatter components.
Acknowledgments This work was supported by the Korea Foundation for the Advancement of Science & Creativity (KOFAC) grant funded by Ministry of Education (MOE).
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Fig. 1. Calculated mass attenuation coefficients for NaI(Tl), GSO, and CdWO4. Calculations and estimates were carried out by the GATE simulation code.
Fig. 2. The detection efficiency for NaI(Tl), GSO, and CdWO4 detector as a function of detecto r thickness at various photon energy: (a) 60 keV, (b) 100 keV, (c) 200 keV, and (d) 300 keV.
Fig. 3. The detection efficiency of a 10 mm-thick different detectors at photon energy of 100 k eV. The efficiency of the CdWO4 at a 10 mm-thick was 1.59 and 1.06 times higher than that o f NaI(Tl) and GSO, respectively.
Fig. 4. Schematic diagram for gamma camera system with source, collimator, aluminium layer, detector, backscatter compartment, and lead shielding.
Fig. 5. Simulation results for the sensitivity with respect to the detector material for various parallelhole collimators.
Fig. 6. Comparison of the simulation results for the sensitivity as a function of detector. The results are normalized with respect to the sensitivity obtained with a CdWO 4 detector using (a) LEGP, (b) MEGP, and (c) HEGP parallel-hole collimator. In all cases, CdWO4 detector showed improved sensitivities compared with the other detectors.
Fig. 7. Comparison of the simulation results for the sensitivity as a function of parallel-hole collimator. The results are normalized with respect to the sensitivity obtained with a LEGP parallel-hole collimator using CdWO4 detector.
Fig. 8. Simulation results for the sensitivity with respect to the detector material for various parallelhole collimators.
Fig. 9. Comparison of the simulation results for the scatter fraction as a function of detector. The results are normalized with respect to the scatter fraction obtained with a CdWO 4 detector using (a) LEGP, (b) MEGP, and (c) HEGP parallel-hole collimator. Scatter fraction difference showed little difference among the three types of detector materials tested.
Fig. 10. Comparison of the simulation results for the scatter fraction as a function of parallel-hole collimator. The results are normalized with respect to the scatter fraction obtained with a LEGP parallel-hole collimator using CdWO4 detector.
Table 1. Physical properties of the principal detector materials. Material
NaI(Tl)
BGO
LSO
GSO
CdWO4
Density (g/cm3)
3.67
7.13
7.40
6.71
7.90
Effective atomic number
50
73
66
59
64
Light yield (relative)
100
7 β 12
40 β 75
20
30 - 40
Decay time (nsec)
230
300
40
60
5,000
Energy resolution (at 511 keV, %)
6.6
10.2
10.0
8.5
8.8
Hygroscopic
Yes
No
No
No
No
Table 2. Specifications of parallel-hole collimators. LEGP
MEGP
HEGP
Length (mm)
36.0
42.0
44.0
Hole diameter (mm)
2.5
3.4
4.0
Septal thickness (mm)
0.3
1.4
3.2