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Comparison of ultra-high-resolution parallel-hole collimator materials based on the CdTe pixelated semiconductor SPECT system
Nuclear Instruments and Methods in Physics Research A 713 (2013) 33–39
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Comparison of ultra-high-resolution parallel-hole collimator materials based on the CdTe pixelated semiconductor SPECT system Young-Jin Lee, Hyun-Ju Ryu, Seung-Wan Lee, Su-Jin Park, Hee-Joung Kim n Department of Radiological Science, College of Health Science, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon-do 220-710, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 4 October 2012 Received in revised form 26 December 2012 Accepted 6 March 2013 Available online 18 March 2013
Recently, many studies have sought to improve the sensitivity and spatial resolution of pixelated semiconductor detectors. Spatial resolution can be improved by using a pinhole or pixelated parallel-hole collimator with equal hole and pixel sizes. We compared a pinhole to a pixelated parallel-hole collimator and found that the pixelated parallel-hole collimator had higher sensitivity. Additionally, collimator materials with high absorption efficiency are often used because of their high spatial resolution. The purpose of this study was to compare the quality of images generated using a pixelated semiconductor single photon emission computed tomography (SPECT) system simulated with pixelated parallel-hole collimators of lead, tungsten, gold, and depleted uranium. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe pixelated semiconductor detector, which consists of small pixels (0.35 0.35 mm2), using a Geant4 Application for Tomographic Emission (GATE) simulation. Sensitivities and spatial resolutions were measured for the four collimator materials. To evaluate overall image performance, a hot-rod phantom was designed using GATE simulation. The results showed that with lead, sensitivity was 4.25%, 6.53%, and 10.28% higher than with tungsten, gold, and depleted uranium, respectively. Spatial resolution using depleted uranium was 3.19%, 4.19%, and 8.01% better than that of gold, tungsten, and lead, respectively. Sensitivity and spatial resolution showed little difference among the four types of collimator materials tested. It was difficult to visually distinguish between the reconstructed images of the hot-rod phantom for different collimator materials. The results are promising for notable cost reductions in collimator manufacturing while avoiding impractical and rare materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: SPECT Cadmium telluride (CdTe) pixelated semiconductor detector Ultra-high-resolution parallel-hole collimator materials Monte Carlo simulation
1. Introduction Recently, there has been an increasing interest in the development of technologies for in vivo nuclear medicine imaging. Single photon emission computed tomography (SPECT), including SPECT/ CT and SPECT/MRI, is a useful imaging modality in the growing field of nuclear medicine imaging and fusion imaging systems [1–3]. NaI(Tl) scintillation detectors are commonly used in a conventional SPECT systems due to their low cost and large field of view (FOV). However, this system has low intrinsic spatial resolution and low energy resolution. Ultra-high-resolution CT systems can reconstruct objects with a resolution of several tens to hundreds of microns, while conventional SPECT systems generally have low spatial resolutions. For example, the intrinsic spatial resolution of a typical NaI(Tl) scintillation detector used in SPECT systems is approximately 4.0 mm full width at half maximum (FWHM) [1]. This value is primarily due to the statistical fluctuation in detecting gamma rays using a scintillator and photomultiplier tube. Most SPECT systems are manufactured using the same detector technologies. However,
n
Corresponding author. Tel.: þ82 33 760 2475; fax: þ 82 33 760 2562. E-mail address: [email protected] (H.-J. Kim).
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higher spatial resolution is necessary to achieve excellent image performance. This limitation can be overcome using geometric magnification to improve the quality of images generated by pinhole collimators with a SPECT system [4,5]. Conventional pinhole systems typically demonstrate poor sensitivity due to the inferior geometric efficiency of ultra-highresolution collimators. Many researchers have proposed the utilization of pixelated semiconductor detectors made from cadmium telluride (CdTe) or cadmium zinc telluride (CZT) as a strategy to improve the spatial resolution of SPECT systems [6–12]. Recently, there has been increased demand for semiconductor detectors in the field of nuclear medicine imaging. A major advance was made in this field with the development of pixelated semiconductor detectors, which have excellent spatial resolution. Semiconductor materials have much lower carrier creation energy than scintillators, as well as high effective atomic numbers and density. The primary advantage of these detectors is superior spatial resolution for nuclear medicine applications. Each photon signal is collected individually for each pixel, thus the intrinsic resolution of the detector is almost the same as the pixel size. Comparison of a pixelated semiconductor to a conventional scintillation detector of the same size and thickness demonstrated that the pixelated semiconductor detector had superior sensitivity [13].
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Many have investigated ultra-high-resolution SPECT systems using pinhole apertures [14–16]. However, given the small collimator hole size, the system loses sensitivity when using a pinhole collimator with a pixelated semiconductor detector to acquire high spatial resolution. To address this problem, we utilized a pixelated parallel-hole collimator with equal hole and pixel sizes [17]. We achieved spatial resolution similar to that of the pinhole collimator as well as excellent sensitivity using small pixel size. Use of a pixelated parallel-hole collimator is recommended to improve sensitivity and spatial resolution when using small pixel size and the SPECT system with pixelated semiconductor detector materials. When we used the higher absorption collimator materials, capture more gamma rays naturally induced sensitivity decrement. Because these captured photons include the information of emitted gamma rays and decreasing collimator attenuation coefficient is due to gamma rays penetrating the septa. However, to obtain a high sensitivity, it is necessary degrading of spatial resolution because of spatial blurring. In this study, four materials were considered as collimators, and the sensitivity and spatial resolution of each collimator were compared. The collimator was made of lead (atomic number¼82, density¼ 11.53 g/cm3) or other material with high absorption efficiency. The most frequently used collimator is made of lead and tungsten (atomic number¼ 74, density¼19.3 g/cm3) materials. Gold (atomic number¼ 79, density¼ 19.32 g/cm3) and depleted uranium (atomic number¼ 92, density¼18.95 g/cm3) have higher gamma ray absorption than lead and tungsten. Gold is sometimes used to decrease radiation leakage, and some groups have also tried using depleted uranium [18,19]. Depleted uranium material has an even higher stopping power than gold, but it is hard to manufacture and is not commonly available. Toxic uranium oxide may be released, which requires wipe tests and plating. Uranium is also radioactive and causes a radiation background on the detector, which can be a significant problem in low count situations. The purpose of this paper is to compare the image performance of lead, tungsten, gold, and depleted uranium pixelated parallelhole collimators. Sensitivity and spatial resolution were evaluated using a GEANT4 Application for Tomographic Emission (GATE) simulation. To evaluate the overall image performance, a hot-rod phantom was designed on the GATE simulation. Pixelated parallelhole collimators and a PID 350 (Ajat Oy Ltd., Finland) CdTe pixelated semiconductor detector with small pixels (0.35 0.35 mm2) were also simulated. The simulated results of lead, tungsten, gold, and depleted uranium are presented in this paper.
include wide band gap, high atomic number (Cd and Te: 48 and 52, respectively) and good charge transport properties [25–29]. The wide band gap and charge transport properties allow high energy resolution detection at room temperature. Good sensitivity can be achieved by a high absorption coefficient due to the high atomic number. Fig. 1 shows the PID 350 CdTe pixelated semiconductor detector. This detector was capable of operating at room temperature because of the crystal’s wide band gap. The detector was subdivided into eight CdTe-CMOS hybrids, each with dimensions of 11.2 22.4 mm2. Pixel size was 0.35 0.35 mm2, and the number of pixels was 128 128. Each pixel was capable of storing energy information for every recorded event. Histograms were generated for event counts distributed in energy windows. The detector thickness was 1 mm with 16,384 total pixels, and the physical gap between hybrids was 1 pixel. The maximum data acquisition speed was 125 frames per second (fps), and the minimum data acquisition speed was 5 fps. Among the range of semiconductor detectors available for gamma ray detection, CdTe is preferable because of its high density, wide bandgap, and the high atomic numbers of its components. As shown in Fig. 2, CdTe has a high photoelectric attenuation coefficient. The efficiency of CdTe pixelated semiconductor detectors and NaI(Tl) scintillation detectors as a function of
Fig. 1. PID 350 CdTe pixelated semiconductor detector system. The detector was 44.8 44.8 mm2 in size and was subdivided into eight CdTe-CMOS hybrids of 11.2 22.4 mm2.
2. Materials and methods 2.1. GATE simulation Monte Carlo simulations are used extensively to address issues related to nuclear medicine imaging, including system design, protocol optimization, collimator development, correction method estimates, and tomographic reconstruction algorithms to improve image quantitation [20,21]. GATE is a Monte Carlo simulation platform based on GEANT4 that is dedicated to nuclear medicine imaging. The accuracy and usefulness of GATE simulations have been demonstrated by several studies [20–24]. In this study, simulations were conducted with GATE version 6 to evaluate the image quality of the proposed system. 2.2. PID 350 CdTe pixelated semiconductor detector CdTe (density ¼ 5.85 g/cm3) semiconductors have been studied for their applications in nuclear medicine imaging, especially for use in high energy gamma ray radiation. Useful properties of CdTe
Fig. 2. Linear attenuation coefficients for CdTe, NaI(Tl), Si, and Ge. Results for mass attenuation coefficient (drawn lines) and mass energy-absorption coefficient (dashed lines).
Y.-J. Lee et al. / Nuclear Instruments and Methods in Physics Research A 713 (2013) 33–39
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according to [31] εparallel-hole ¼ K2
Rparallel-hole ¼ d
Fig. 3. Efficiency for 140 keV gamma ray in various thicknesses of CdTe and NaI: results for total (drawn lines) and photoelectric (dashed lines). Efficiency of CdTe was an average of 1.5 times higher than that of NaI.
lef f ective
2
d
2
ðd þ tÞ2
lef f ective þ b ðlef f ective ¼ l−2μ−1 Þ lef f ective
2dp l−p
ð2Þ
ð3Þ
where p is the shortest path length for gamma rays to travel from one hole to the next. The limit of 5% penetration for gamma rays along the shortest path length can be applied by considering transmission in terms of
detector thickness is reported in Fig. 3. Even the CdTe pixelated semiconductor detector with thickness of 1 mm provides good detection efficiency for gamma rays. Fig. 4 presents the efficiency of a CdTe pixelated semiconductor detector with thickness of 1 mm. This detector has 32% efficiency at photon energy of 140 keV. 2.3. Pixelated parallel-hole collimator Collimators play a vital role in nuclear medicine imaging, particularly for modalities in which images are formed by selective absorption of emitted radiation. This is largely because unlike optical photons, gamma rays cannot be refracted or focused. Collimators are generally characterized by image performance, such as sensitivity and spatial resolution. Parallel-hole collimators were designed as planar parallel grids with uniform hole density and holes of equal diameter and length. A notable advantage of this geometric configuration is that the sensitivity and spatial resolution of the camera remain uniform over the entire FOV [30]. The efficiency (εparallel-hole) and resolution (Rparallel-hole) of a parallel-hole collimator are extrinsic parameters for the nuclear medicine detector, and both influence image performance. The εparallel-hole and Rparallel-hole were calculated
ð1Þ
where K is the constant that depends on hole shape, d is the hole diameter, t is the septal thickness, l is the length of the parallelhole collimator, m is the linear attenuation coefficient of the collimator materials, leffective is the effective length of the parallel-hole collimator, and b is the distance from the source to the collimator. Collimator efficiency is simply the ratio of isotropically emitted gamma rays that are appropriately collimated. By definition, collimator efficiency is the ratio of photons that pass through the collimator and reach the image plane to the total number of photons emitted from the source [32]. Fig. 5 shows a pixelated parallel-hole collimator in which hole size is equal to pixel size. Since the intrinsic resolution was equal to pixel size, excellent spatial resolution was achieved. The collimator hole was square with dimensions of 0.3 0.3 mm2. Septal thickness was 0.05 mm and septal height was 20 mm. Lead, tungsten, gold, and depleted uranium were considered as collimator materials. Minimum septal thickness is determined by the energy of the gamma ray used. Only a small ratio of the incident gamma ray can penetrate to cross from one hole to another, or else an inaccurate image will be projected onto the detector. The septa cannot be thick enough to absorb all off-axis gamma rays. Approximately 5% penetration along the shortest septal path is considered acceptable given the exponential process of gamma ray attenuation. Septal thickness (t) and shortest path length (p) are geometrically related by the equation [33] t¼
Fig. 4. Efficiency of CdTe pixelated semiconductor detector at different energies for a thickness of 1 mm: results for total (drawn lines) and photoelectric (dashed lines).
d
Fig. 5. Schematic diagram of pixelated parallel-hole collimator.
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the linear attenuation coefficient of the collimator materials. Thus, [33]
was calculated according to pffiffiffiffiffiffiffiffiffiffiffiffiffiffi FWHM ¼ 2 2log 2s
6d=μ : t≥ l−ð3=μÞ
where s is the standard deviation of the Gaussian fitting. Estimated sensitivity was represented in counts per second per kBq (cps/kBq). The distances between the point source and the collimator were 2, 4, 6, 8, and 10 cm. All analytical results of the collimator efficiency and spatial resolution are shown in Table 2. Ten simulations were performed for each source-to-collimator distance. The standard deviation was calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðN −NÞ2 Standard deviation ¼ Σ ni¼ 1 i 100 ð6Þ ðn−1Þ
ð4Þ
99m Tc (140 keV), which has a half-life of 6 h, is the most widely used radioisotope in nuclear medicine imaging and was used in this study. Table 1 shows the linear attenuation coefficient (cm−1) and acceptable septal thickness for various collimator materials at 140 keV energy of 99mTc.
2.4. Evaluation of image performance To evaluate the performance of this system, sensitivity and spatial resolution were estimated. A99mTc point source with an activity of 1 MBq (placed in air) was measured using a scan time of 900 s. Gamma rays were emitted from the source in the direction of a 4π steradian. The number of projections was 90 over 3601, and data acquisition time was 10 s/view. A 20% symmetrical energy window was applied, and images corresponding to the 126– 154 keV energy windows were generated using image reconstruction via the ordered subsets-expectation maximization (OSEM) method. Four subsets were used with five iterations. A spatial blurrer module was created to model the intrinsic spatial resolution. The extrinsic resolution from all other parts of the detector was also simulated. Spatial resolution was characterized by a point spread function (PSF) in air. The FWHM was obtained by PSF evaluation of the source image using Gaussian fitting. The FWHM
ð5Þ
where n is the number of measurements taken (n¼ 10), Ni is each measurement datum and N is the measured average of the data.
Table 1 Linear attenuation coefficient (cm−1) and acceptable septal thickness of collimator materials (mm) at 140 keV energy. Collimator materials Lead
Tungsten
Density (g/cm3) 11.53 19.30 Linear attenuation coefficient (cm−1) 27.51 36.21 Septal thickness (mm) 0.0346 0.0259
Gold
Depleted uranium
19.32 18.95 42.60 58.00 0.0219 0.00159
Fig. 6. Hot-rod phantom diagram for evaluating overall image performance. Six areas with rods were located in a water phantom: 0.5, 0.85, 1.2, 1.5, 1.8, and 2.1 mm. Activities were 9000, 15,500, 30,000, 45,000, 60,000, and 90,000 Bq, respectively.
Table 2 Analytical results of the collimator efficiency and spatial resolution for collimator materials. Collimator materials
Source-to-collimator distance (cm)
Lead
2 4 6 8 10
Tungsten
2 4 6 8 10
Gold
Depleted uranium
2 4 6 8 10 2 4 6 8 10
Analytical collimator efficiency (photons transmitted/photons emitted)
Analytical spatial resolution (mm, FWHM)
1.307 10−5
0.696 0.968 1.254 1.545 1.840
1.303 10−5
0.695 0.967 1.252 1.543 1.838
1.302 10−5
1.300 10−5
0.695 0.967 1.252 1.543 1.837 0.695 0.967 1.251 1.542 1.836
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The hot-rod phantom consisted of six areas with rods of varying diameters that can be filled with activity (Fig. 6). This phantom is filled with a water solution of 99mTc.
3. Results 3.1. Sensitivity The measured averages of sensitivity for each source-tocollimator distance and collimator material are shown in Fig. 7. A comparison of the sensitivities with respect to the linear attenuation coefficients for each collimator material is shown in Fig. 8(a). The results show that the measured averages of sensitivity using tungsten, gold, and depleted uranium were 4.25%, 6.53% and 10.28% lower than that of lead, respectively. 3.2. Spatial resolution The measured averages of spatial resolution for each source-tocollimator distance and collimator material are shown in Fig. 9. A comparison of the spatial resolution with respect to the linear attenuation coefficients for each collimator material is shown in Fig. 8(b). The results show that the measured averages of spatial resolution using depleted uranium were 8.01%, 4.19% and 3.19% better than those of lead, tungsten, and gold, respectively. 3.3. Hot-rod phantom images Reconstructed images of simulated hot-rod phantoms for each source-to-collimator distance and collimator material are shown in Fig. 10.
4. Discussion The development of pixelated semiconductor detectors such as CdTe and CZT, which work at room temperature, has facilitated use of the an ultra-high-resolution SPECT system. The primary advantages of these detectors include excellent spatial resolution and high sensitivity due to high effective atomic number and density. These features allow decreased detector thickness relative to a scintillator and improved spatial resolution for nuclear medicine imaging applications. The intrinsic resolution of CdTe or CZT semiconductor detectors is similar to the size of a pixel, largely
Fig. 8. Comparison of simulation results for (a) sensitivity and (b) spatial resolution at 2 cm from the collimator.
Fig. 9. Simulation results for spatial resolution of various collimator materials with respect to source-to-collimator distances. Symbols indicate different materials. □: lead, ○: tungsten, △: gold, and ☆: depleted uranium.
Fig. 7. Simulation results for sensitivity of various collimator materials with respect to source-to-collimator distances. Symbols indicate different materials. ■: lead, ●: tungsten, ▲: gold, and ★: depleted uranium.
because electrons generated by the interaction of gamma rays are collected individually for each pixel using a small electrode. While pinhole SPECT systems can acquire excellent spatial resolution, one notable disadvantage is a loss of sensitivity due to small pixel
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Fig. 10. Reconstructed images of hot-rod phantom for (a) lead, (b) tungsten, (c) gold, and (d) depleted uranium for each source-to-collimator distance.
size on the detector. To improve both sensitivity and spatial resolution, we recommend applying a pixelated parallel-hole collimator to the pinhole SPECT system using CdTe or CZT pixelated semiconductor detectors. To overcome the disadvantage of small pixel size, we implemented a high resolution SPECT system with a pixelated parallelhole collimator attached to the detector. This system achieved approximately 0.70 mm spatial resolution when the source was located 2 cm from the collimator. The increased sensitivity is attributed to the collection of photon signals by each individual pixel. We investigated the most frequently used collimator materials such as lead and tungsten. Additionally, we investigated gold and depleted uranium because of their high absorption efficiency. Four materials were considered as collimators, and the sensitivity and spatial resolution of each collimator was compared. The measured averages of sensitivity for each source-to-collimator distance were 0.00457, 0.00438, 0.00429 and 0.00414 cps/kBq for lead, tungsten, gold and depleted uranium, respectively. Sensitivity goes from depleted uranium, via gold, and tungsten, to lead in increasing order, as follows from Figs. 7 and 8(a). Sensitivity with lead was 4.25%, 6.53%, and 10.28% higher than that of tungsten, gold, and depleted
uranium, respectively. The difference between tungsten and gold was 2.19%. The spatial resolution goes from depleted uranium, via gold, and tungsten, to lead in increasing order, as follows from Figs. 8(b) and 9. Spatial resolution using depleted uranium was 3.19%, 4.19%, and 8.01% better than that of gold, tungsten, and lead, respectively. The difference between tungsten and gold was 0.97%. Although the analytical and simulation results showed identical tendency when we compared both results quantitatively, the difference in simulation results were more significant due to the various interactions of photons, detector characteristics and collimator design parameters which influence on the image performance. Reconstructed images of the hot-rod phantom were compared for each source-to-collimator distance. The 0.85 mm rods were clearly resolved for all four collimator materials at 2 cm from the collimator. This resolution is higher than that of today’s clinical PET system, and so the importance of the SPECT system may increase and these detectors will be used in several applications in nuclear medicine imaging in which the evaluation of physiological function is desired with ultra-high-resolution. Furthermore, 1.8 mm rods were resolved for all four collimator materials at a distance of 10 cm from the collimator.
Y.-J. Lee et al. / Nuclear Instruments and Methods in Physics Research A 713 (2013) 33–39
5. Conclusion We evaluated an ultra-high-resolution SPECT system with a pixelated semiconductor detector and pixelated parallel-hole collimators of lead, tungsten, gold, and depleted uranium. Although the results have the maximum 10% difference value among collimator materials, a distinction between the reconstructed images of the hot-rod phantom is hard to make visually. Thus, the results imply that lead and tungsten offer image performances similar to those of the much more expensive gold and depleted uranium. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0004846). References [1] M.T. Madsen, Journal of Nuclear Medicine 48 (2007) 661. [2] D.W. Townsend, Journal of Nuclear Medicine 49 (2008) 938. [3] M.J. Hamamura, S.H. Ha, W.W. Roeck, I.T. Muftuler, D.J. Wagennar, D. Meier, B.E. Patt, O. Nalcioglu, Physics in Medicine and Biology 55 (2010) 1563. [4] D.P. McElroy, L.R. MacDonald, F.J. Beekman, Y. Wang, B.E. Patt, J.S. Iwanczyk, B.M.W. Tsui, E.J. Hoffman, IEEE Transactions on Nuclear Science NS-49 (2002) 2139. [5] F.J. Beekman, F. van der Have, B. Vastenhouw, A.J.A. van der Linden, P.P. van Rijk, J.P.H. Burbach, M.P. Smidt, Journal of Nuclear Medicine 46 (2005) 1194. [6] G.A. Kastis, H.B. Barber, H.H. Barrett, J.J. Balzer, D. Lu, D.G. Marks, G. Stevenson, J.M. Woolfenden, M. Appleby, J. Tueller, IEEE Transactions on Nuclear Science NS-47 (2000) 1923. [7] G.A. Kastis, M.C. Wu, S.J. Balzer, D.W. Wilson, L.R. Furenlid, G. Stevenson, H.B. Barber, H.H. Barrett, J.M. Woolfenden, P. Kelly, et al., IEEE Transactions on Nuclear Science NS-49 (2002) 172. [8] H.K. Kim, L.R. Furenlid, M.J. Crawford, D.W. Wilson, H.B. Barber, T.E. Peterson, W.C.J. Hunter, Z. Liu, J.M. Woolfenden, H.H. Barrett, Medical Physics 33 (2006) 465. [9] L. Verger, M.C. Gentet, L. Gerfault, R. Guillemaud, C. Mestais, O. Monnet, G. Montemont, G. Petroz, J.P. Rostaing, J. Rustique, IEEE Transactions on Nuclear Science NS-51 (2004) 3111. [10] C.N. Brzymialkiewicz, M.P. Tornai, R.L. McKinley, J.E. Bowsher, IEEE Transactions on Nuclear Science NS-24 (2005) 868.
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Comparison of ultra-high-resolution parallel-hole collimator materials based on the CdTe pixelated semiconductor SPECT system Young-Jin Lee, Hyun-Ju Ryu, Seung-Wan Lee, Su-Jin Park, Hee-Joung Kim n Department of Radiological Science, College of Health Science, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon-do 220-710, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 4 October 2012 Received in revised form 26 December 2012 Accepted 6 March 2013 Available online 18 March 2013
Recently, many studies have sought to improve the sensitivity and spatial resolution of pixelated semiconductor detectors. Spatial resolution can be improved by using a pinhole or pixelated parallel-hole collimator with equal hole and pixel sizes. We compared a pinhole to a pixelated parallel-hole collimator and found that the pixelated parallel-hole collimator had higher sensitivity. Additionally, collimator materials with high absorption efficiency are often used because of their high spatial resolution. The purpose of this study was to compare the quality of images generated using a pixelated semiconductor single photon emission computed tomography (SPECT) system simulated with pixelated parallel-hole collimators of lead, tungsten, gold, and depleted uranium. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe pixelated semiconductor detector, which consists of small pixels (0.35 0.35 mm2), using a Geant4 Application for Tomographic Emission (GATE) simulation. Sensitivities and spatial resolutions were measured for the four collimator materials. To evaluate overall image performance, a hot-rod phantom was designed using GATE simulation. The results showed that with lead, sensitivity was 4.25%, 6.53%, and 10.28% higher than with tungsten, gold, and depleted uranium, respectively. Spatial resolution using depleted uranium was 3.19%, 4.19%, and 8.01% better than that of gold, tungsten, and lead, respectively. Sensitivity and spatial resolution showed little difference among the four types of collimator materials tested. It was difficult to visually distinguish between the reconstructed images of the hot-rod phantom for different collimator materials. The results are promising for notable cost reductions in collimator manufacturing while avoiding impractical and rare materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: SPECT Cadmium telluride (CdTe) pixelated semiconductor detector Ultra-high-resolution parallel-hole collimator materials Monte Carlo simulation
1. Introduction Recently, there has been an increasing interest in the development of technologies for in vivo nuclear medicine imaging. Single photon emission computed tomography (SPECT), including SPECT/ CT and SPECT/MRI, is a useful imaging modality in the growing field of nuclear medicine imaging and fusion imaging systems [1–3]. NaI(Tl) scintillation detectors are commonly used in a conventional SPECT systems due to their low cost and large field of view (FOV). However, this system has low intrinsic spatial resolution and low energy resolution. Ultra-high-resolution CT systems can reconstruct objects with a resolution of several tens to hundreds of microns, while conventional SPECT systems generally have low spatial resolutions. For example, the intrinsic spatial resolution of a typical NaI(Tl) scintillation detector used in SPECT systems is approximately 4.0 mm full width at half maximum (FWHM) [1]. This value is primarily due to the statistical fluctuation in detecting gamma rays using a scintillator and photomultiplier tube. Most SPECT systems are manufactured using the same detector technologies. However,
n
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higher spatial resolution is necessary to achieve excellent image performance. This limitation can be overcome using geometric magnification to improve the quality of images generated by pinhole collimators with a SPECT system [4,5]. Conventional pinhole systems typically demonstrate poor sensitivity due to the inferior geometric efficiency of ultra-highresolution collimators. Many researchers have proposed the utilization of pixelated semiconductor detectors made from cadmium telluride (CdTe) or cadmium zinc telluride (CZT) as a strategy to improve the spatial resolution of SPECT systems [6–12]. Recently, there has been increased demand for semiconductor detectors in the field of nuclear medicine imaging. A major advance was made in this field with the development of pixelated semiconductor detectors, which have excellent spatial resolution. Semiconductor materials have much lower carrier creation energy than scintillators, as well as high effective atomic numbers and density. The primary advantage of these detectors is superior spatial resolution for nuclear medicine applications. Each photon signal is collected individually for each pixel, thus the intrinsic resolution of the detector is almost the same as the pixel size. Comparison of a pixelated semiconductor to a conventional scintillation detector of the same size and thickness demonstrated that the pixelated semiconductor detector had superior sensitivity [13].
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Many have investigated ultra-high-resolution SPECT systems using pinhole apertures [14–16]. However, given the small collimator hole size, the system loses sensitivity when using a pinhole collimator with a pixelated semiconductor detector to acquire high spatial resolution. To address this problem, we utilized a pixelated parallel-hole collimator with equal hole and pixel sizes [17]. We achieved spatial resolution similar to that of the pinhole collimator as well as excellent sensitivity using small pixel size. Use of a pixelated parallel-hole collimator is recommended to improve sensitivity and spatial resolution when using small pixel size and the SPECT system with pixelated semiconductor detector materials. When we used the higher absorption collimator materials, capture more gamma rays naturally induced sensitivity decrement. Because these captured photons include the information of emitted gamma rays and decreasing collimator attenuation coefficient is due to gamma rays penetrating the septa. However, to obtain a high sensitivity, it is necessary degrading of spatial resolution because of spatial blurring. In this study, four materials were considered as collimators, and the sensitivity and spatial resolution of each collimator were compared. The collimator was made of lead (atomic number¼82, density¼ 11.53 g/cm3) or other material with high absorption efficiency. The most frequently used collimator is made of lead and tungsten (atomic number¼ 74, density¼19.3 g/cm3) materials. Gold (atomic number¼ 79, density¼ 19.32 g/cm3) and depleted uranium (atomic number¼ 92, density¼18.95 g/cm3) have higher gamma ray absorption than lead and tungsten. Gold is sometimes used to decrease radiation leakage, and some groups have also tried using depleted uranium [18,19]. Depleted uranium material has an even higher stopping power than gold, but it is hard to manufacture and is not commonly available. Toxic uranium oxide may be released, which requires wipe tests and plating. Uranium is also radioactive and causes a radiation background on the detector, which can be a significant problem in low count situations. The purpose of this paper is to compare the image performance of lead, tungsten, gold, and depleted uranium pixelated parallelhole collimators. Sensitivity and spatial resolution were evaluated using a GEANT4 Application for Tomographic Emission (GATE) simulation. To evaluate the overall image performance, a hot-rod phantom was designed on the GATE simulation. Pixelated parallelhole collimators and a PID 350 (Ajat Oy Ltd., Finland) CdTe pixelated semiconductor detector with small pixels (0.35 0.35 mm2) were also simulated. The simulated results of lead, tungsten, gold, and depleted uranium are presented in this paper.
include wide band gap, high atomic number (Cd and Te: 48 and 52, respectively) and good charge transport properties [25–29]. The wide band gap and charge transport properties allow high energy resolution detection at room temperature. Good sensitivity can be achieved by a high absorption coefficient due to the high atomic number. Fig. 1 shows the PID 350 CdTe pixelated semiconductor detector. This detector was capable of operating at room temperature because of the crystal’s wide band gap. The detector was subdivided into eight CdTe-CMOS hybrids, each with dimensions of 11.2 22.4 mm2. Pixel size was 0.35 0.35 mm2, and the number of pixels was 128 128. Each pixel was capable of storing energy information for every recorded event. Histograms were generated for event counts distributed in energy windows. The detector thickness was 1 mm with 16,384 total pixels, and the physical gap between hybrids was 1 pixel. The maximum data acquisition speed was 125 frames per second (fps), and the minimum data acquisition speed was 5 fps. Among the range of semiconductor detectors available for gamma ray detection, CdTe is preferable because of its high density, wide bandgap, and the high atomic numbers of its components. As shown in Fig. 2, CdTe has a high photoelectric attenuation coefficient. The efficiency of CdTe pixelated semiconductor detectors and NaI(Tl) scintillation detectors as a function of
Fig. 1. PID 350 CdTe pixelated semiconductor detector system. The detector was 44.8 44.8 mm2 in size and was subdivided into eight CdTe-CMOS hybrids of 11.2 22.4 mm2.
2. Materials and methods 2.1. GATE simulation Monte Carlo simulations are used extensively to address issues related to nuclear medicine imaging, including system design, protocol optimization, collimator development, correction method estimates, and tomographic reconstruction algorithms to improve image quantitation [20,21]. GATE is a Monte Carlo simulation platform based on GEANT4 that is dedicated to nuclear medicine imaging. The accuracy and usefulness of GATE simulations have been demonstrated by several studies [20–24]. In this study, simulations were conducted with GATE version 6 to evaluate the image quality of the proposed system. 2.2. PID 350 CdTe pixelated semiconductor detector CdTe (density ¼ 5.85 g/cm3) semiconductors have been studied for their applications in nuclear medicine imaging, especially for use in high energy gamma ray radiation. Useful properties of CdTe
Fig. 2. Linear attenuation coefficients for CdTe, NaI(Tl), Si, and Ge. Results for mass attenuation coefficient (drawn lines) and mass energy-absorption coefficient (dashed lines).
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according to [31] εparallel-hole ¼ K2
Rparallel-hole ¼ d
Fig. 3. Efficiency for 140 keV gamma ray in various thicknesses of CdTe and NaI: results for total (drawn lines) and photoelectric (dashed lines). Efficiency of CdTe was an average of 1.5 times higher than that of NaI.
lef f ective
2
d
2
ðd þ tÞ2
lef f ective þ b ðlef f ective ¼ l−2μ−1 Þ lef f ective
2dp l−p
ð2Þ
ð3Þ
where p is the shortest path length for gamma rays to travel from one hole to the next. The limit of 5% penetration for gamma rays along the shortest path length can be applied by considering transmission in terms of
detector thickness is reported in Fig. 3. Even the CdTe pixelated semiconductor detector with thickness of 1 mm provides good detection efficiency for gamma rays. Fig. 4 presents the efficiency of a CdTe pixelated semiconductor detector with thickness of 1 mm. This detector has 32% efficiency at photon energy of 140 keV. 2.3. Pixelated parallel-hole collimator Collimators play a vital role in nuclear medicine imaging, particularly for modalities in which images are formed by selective absorption of emitted radiation. This is largely because unlike optical photons, gamma rays cannot be refracted or focused. Collimators are generally characterized by image performance, such as sensitivity and spatial resolution. Parallel-hole collimators were designed as planar parallel grids with uniform hole density and holes of equal diameter and length. A notable advantage of this geometric configuration is that the sensitivity and spatial resolution of the camera remain uniform over the entire FOV [30]. The efficiency (εparallel-hole) and resolution (Rparallel-hole) of a parallel-hole collimator are extrinsic parameters for the nuclear medicine detector, and both influence image performance. The εparallel-hole and Rparallel-hole were calculated
ð1Þ
where K is the constant that depends on hole shape, d is the hole diameter, t is the septal thickness, l is the length of the parallelhole collimator, m is the linear attenuation coefficient of the collimator materials, leffective is the effective length of the parallel-hole collimator, and b is the distance from the source to the collimator. Collimator efficiency is simply the ratio of isotropically emitted gamma rays that are appropriately collimated. By definition, collimator efficiency is the ratio of photons that pass through the collimator and reach the image plane to the total number of photons emitted from the source [32]. Fig. 5 shows a pixelated parallel-hole collimator in which hole size is equal to pixel size. Since the intrinsic resolution was equal to pixel size, excellent spatial resolution was achieved. The collimator hole was square with dimensions of 0.3 0.3 mm2. Septal thickness was 0.05 mm and septal height was 20 mm. Lead, tungsten, gold, and depleted uranium were considered as collimator materials. Minimum septal thickness is determined by the energy of the gamma ray used. Only a small ratio of the incident gamma ray can penetrate to cross from one hole to another, or else an inaccurate image will be projected onto the detector. The septa cannot be thick enough to absorb all off-axis gamma rays. Approximately 5% penetration along the shortest septal path is considered acceptable given the exponential process of gamma ray attenuation. Septal thickness (t) and shortest path length (p) are geometrically related by the equation [33] t¼
Fig. 4. Efficiency of CdTe pixelated semiconductor detector at different energies for a thickness of 1 mm: results for total (drawn lines) and photoelectric (dashed lines).
d
Fig. 5. Schematic diagram of pixelated parallel-hole collimator.
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the linear attenuation coefficient of the collimator materials. Thus, [33]
was calculated according to pffiffiffiffiffiffiffiffiffiffiffiffiffiffi FWHM ¼ 2 2log 2s
6d=μ : t≥ l−ð3=μÞ
where s is the standard deviation of the Gaussian fitting. Estimated sensitivity was represented in counts per second per kBq (cps/kBq). The distances between the point source and the collimator were 2, 4, 6, 8, and 10 cm. All analytical results of the collimator efficiency and spatial resolution are shown in Table 2. Ten simulations were performed for each source-to-collimator distance. The standard deviation was calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðN −NÞ2 Standard deviation ¼ Σ ni¼ 1 i 100 ð6Þ ðn−1Þ
ð4Þ
99m Tc (140 keV), which has a half-life of 6 h, is the most widely used radioisotope in nuclear medicine imaging and was used in this study. Table 1 shows the linear attenuation coefficient (cm−1) and acceptable septal thickness for various collimator materials at 140 keV energy of 99mTc.
2.4. Evaluation of image performance To evaluate the performance of this system, sensitivity and spatial resolution were estimated. A99mTc point source with an activity of 1 MBq (placed in air) was measured using a scan time of 900 s. Gamma rays were emitted from the source in the direction of a 4π steradian. The number of projections was 90 over 3601, and data acquisition time was 10 s/view. A 20% symmetrical energy window was applied, and images corresponding to the 126– 154 keV energy windows were generated using image reconstruction via the ordered subsets-expectation maximization (OSEM) method. Four subsets were used with five iterations. A spatial blurrer module was created to model the intrinsic spatial resolution. The extrinsic resolution from all other parts of the detector was also simulated. Spatial resolution was characterized by a point spread function (PSF) in air. The FWHM was obtained by PSF evaluation of the source image using Gaussian fitting. The FWHM
ð5Þ
where n is the number of measurements taken (n¼ 10), Ni is each measurement datum and N is the measured average of the data.
Table 1 Linear attenuation coefficient (cm−1) and acceptable septal thickness of collimator materials (mm) at 140 keV energy. Collimator materials Lead
Tungsten
Density (g/cm3) 11.53 19.30 Linear attenuation coefficient (cm−1) 27.51 36.21 Septal thickness (mm) 0.0346 0.0259
Gold
Depleted uranium
19.32 18.95 42.60 58.00 0.0219 0.00159
Fig. 6. Hot-rod phantom diagram for evaluating overall image performance. Six areas with rods were located in a water phantom: 0.5, 0.85, 1.2, 1.5, 1.8, and 2.1 mm. Activities were 9000, 15,500, 30,000, 45,000, 60,000, and 90,000 Bq, respectively.
Table 2 Analytical results of the collimator efficiency and spatial resolution for collimator materials. Collimator materials
Source-to-collimator distance (cm)
Lead
2 4 6 8 10
Tungsten
2 4 6 8 10
Gold
Depleted uranium
2 4 6 8 10 2 4 6 8 10
Analytical collimator efficiency (photons transmitted/photons emitted)
Analytical spatial resolution (mm, FWHM)
1.307 10−5
0.696 0.968 1.254 1.545 1.840
1.303 10−5
0.695 0.967 1.252 1.543 1.838
1.302 10−5
1.300 10−5
0.695 0.967 1.252 1.543 1.837 0.695 0.967 1.251 1.542 1.836
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The hot-rod phantom consisted of six areas with rods of varying diameters that can be filled with activity (Fig. 6). This phantom is filled with a water solution of 99mTc.
3. Results 3.1. Sensitivity The measured averages of sensitivity for each source-tocollimator distance and collimator material are shown in Fig. 7. A comparison of the sensitivities with respect to the linear attenuation coefficients for each collimator material is shown in Fig. 8(a). The results show that the measured averages of sensitivity using tungsten, gold, and depleted uranium were 4.25%, 6.53% and 10.28% lower than that of lead, respectively. 3.2. Spatial resolution The measured averages of spatial resolution for each source-tocollimator distance and collimator material are shown in Fig. 9. A comparison of the spatial resolution with respect to the linear attenuation coefficients for each collimator material is shown in Fig. 8(b). The results show that the measured averages of spatial resolution using depleted uranium were 8.01%, 4.19% and 3.19% better than those of lead, tungsten, and gold, respectively. 3.3. Hot-rod phantom images Reconstructed images of simulated hot-rod phantoms for each source-to-collimator distance and collimator material are shown in Fig. 10.
4. Discussion The development of pixelated semiconductor detectors such as CdTe and CZT, which work at room temperature, has facilitated use of the an ultra-high-resolution SPECT system. The primary advantages of these detectors include excellent spatial resolution and high sensitivity due to high effective atomic number and density. These features allow decreased detector thickness relative to a scintillator and improved spatial resolution for nuclear medicine imaging applications. The intrinsic resolution of CdTe or CZT semiconductor detectors is similar to the size of a pixel, largely
Fig. 8. Comparison of simulation results for (a) sensitivity and (b) spatial resolution at 2 cm from the collimator.
Fig. 9. Simulation results for spatial resolution of various collimator materials with respect to source-to-collimator distances. Symbols indicate different materials. □: lead, ○: tungsten, △: gold, and ☆: depleted uranium.
Fig. 7. Simulation results for sensitivity of various collimator materials with respect to source-to-collimator distances. Symbols indicate different materials. ■: lead, ●: tungsten, ▲: gold, and ★: depleted uranium.
because electrons generated by the interaction of gamma rays are collected individually for each pixel using a small electrode. While pinhole SPECT systems can acquire excellent spatial resolution, one notable disadvantage is a loss of sensitivity due to small pixel
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Fig. 10. Reconstructed images of hot-rod phantom for (a) lead, (b) tungsten, (c) gold, and (d) depleted uranium for each source-to-collimator distance.
size on the detector. To improve both sensitivity and spatial resolution, we recommend applying a pixelated parallel-hole collimator to the pinhole SPECT system using CdTe or CZT pixelated semiconductor detectors. To overcome the disadvantage of small pixel size, we implemented a high resolution SPECT system with a pixelated parallelhole collimator attached to the detector. This system achieved approximately 0.70 mm spatial resolution when the source was located 2 cm from the collimator. The increased sensitivity is attributed to the collection of photon signals by each individual pixel. We investigated the most frequently used collimator materials such as lead and tungsten. Additionally, we investigated gold and depleted uranium because of their high absorption efficiency. Four materials were considered as collimators, and the sensitivity and spatial resolution of each collimator was compared. The measured averages of sensitivity for each source-to-collimator distance were 0.00457, 0.00438, 0.00429 and 0.00414 cps/kBq for lead, tungsten, gold and depleted uranium, respectively. Sensitivity goes from depleted uranium, via gold, and tungsten, to lead in increasing order, as follows from Figs. 7 and 8(a). Sensitivity with lead was 4.25%, 6.53%, and 10.28% higher than that of tungsten, gold, and depleted
uranium, respectively. The difference between tungsten and gold was 2.19%. The spatial resolution goes from depleted uranium, via gold, and tungsten, to lead in increasing order, as follows from Figs. 8(b) and 9. Spatial resolution using depleted uranium was 3.19%, 4.19%, and 8.01% better than that of gold, tungsten, and lead, respectively. The difference between tungsten and gold was 0.97%. Although the analytical and simulation results showed identical tendency when we compared both results quantitatively, the difference in simulation results were more significant due to the various interactions of photons, detector characteristics and collimator design parameters which influence on the image performance. Reconstructed images of the hot-rod phantom were compared for each source-to-collimator distance. The 0.85 mm rods were clearly resolved for all four collimator materials at 2 cm from the collimator. This resolution is higher than that of today’s clinical PET system, and so the importance of the SPECT system may increase and these detectors will be used in several applications in nuclear medicine imaging in which the evaluation of physiological function is desired with ultra-high-resolution. Furthermore, 1.8 mm rods were resolved for all four collimator materials at a distance of 10 cm from the collimator.
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5. Conclusion We evaluated an ultra-high-resolution SPECT system with a pixelated semiconductor detector and pixelated parallel-hole collimators of lead, tungsten, gold, and depleted uranium. Although the results have the maximum 10% difference value among collimator materials, a distinction between the reconstructed images of the hot-rod phantom is hard to make visually. Thus, the results imply that lead and tungsten offer image performances similar to those of the much more expensive gold and depleted uranium. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0004846). References [1] M.T. Madsen, Journal of Nuclear Medicine 48 (2007) 661. [2] D.W. Townsend, Journal of Nuclear Medicine 49 (2008) 938. [3] M.J. Hamamura, S.H. Ha, W.W. Roeck, I.T. Muftuler, D.J. Wagennar, D. Meier, B.E. Patt, O. Nalcioglu, Physics in Medicine and Biology 55 (2010) 1563. [4] D.P. McElroy, L.R. MacDonald, F.J. Beekman, Y. Wang, B.E. Patt, J.S. Iwanczyk, B.M.W. Tsui, E.J. Hoffman, IEEE Transactions on Nuclear Science NS-49 (2002) 2139. [5] F.J. Beekman, F. van der Have, B. Vastenhouw, A.J.A. van der Linden, P.P. van Rijk, J.P.H. Burbach, M.P. Smidt, Journal of Nuclear Medicine 46 (2005) 1194. [6] G.A. Kastis, H.B. Barber, H.H. Barrett, J.J. Balzer, D. Lu, D.G. Marks, G. Stevenson, J.M. Woolfenden, M. Appleby, J. Tueller, IEEE Transactions on Nuclear Science NS-47 (2000) 1923. [7] G.A. Kastis, M.C. Wu, S.J. Balzer, D.W. Wilson, L.R. Furenlid, G. Stevenson, H.B. Barber, H.H. Barrett, J.M. Woolfenden, P. Kelly, et al., IEEE Transactions on Nuclear Science NS-49 (2002) 172. [8] H.K. Kim, L.R. Furenlid, M.J. Crawford, D.W. Wilson, H.B. Barber, T.E. Peterson, W.C.J. Hunter, Z. Liu, J.M. Woolfenden, H.H. Barrett, Medical Physics 33 (2006) 465. [9] L. Verger, M.C. Gentet, L. Gerfault, R. Guillemaud, C. Mestais, O. Monnet, G. Montemont, G. Petroz, J.P. Rostaing, J. Rustique, IEEE Transactions on Nuclear Science NS-51 (2004) 3111. [10] C.N. Brzymialkiewicz, M.P. Tornai, R.L. McKinley, J.E. Bowsher, IEEE Transactions on Nuclear Science NS-24 (2005) 868.
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