The Experts below are selected from a list of 2322 Experts worldwide ranked by ideXlab platform
Volkmar Schulz - One of the best experts on this subject based on the ideXlab platform.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
EJNMMI Physics, 2020Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association’s (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard’s goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard.For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. If the PET ring has detectors which are perpendicular to a Cartesian axis, then the resolution along this axis will typically improve with larger projection windows.We show that the standard’s equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. However, a modified version can still be used to determine an approximation of the random rates under the assumption of negligible random rates for small activities and a constant scatter fraction. We compare the resulting estimated random rates to random rates obtained using a delayed coincidence window and two methods based on the singles rates. While these methods give similar estimates, the estimation method based on the NEMA equations overestimates the random rates.In the NEMA standard’s protocol for the evaluation of the sensitivity, the standard specifies to axially step a point source through the scanner and to take a different scan for each source position. Later, in the data analysis section, the standard does not specify clearly how the different scans have to be incorporated into the analysis, which can lead to unclear interpretations of publicized results.The standard’s definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e., noise), and recovery.With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
arXiv: Medical Physics, 2019Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association's (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard's goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard. For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. We show that the standard's equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. We compare random rates determined by a method based on the standard to to random rates obtained using a delayed coincidence window and two methods based on the singles rates. We discuss to what extend the NEMA standard's protocol for the evaluation of the sensitivity is unclear and ambiguous, which can lead to unclear interpretations of publicizeded results. The standard's definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e. noise), and recovery. With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
Yamaya Taiga - One of the best experts on this subject based on the ideXlab platform.
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Design study of a brain-dedicated time-of-flight PET system with a hemispherical detector arrangement
2020Co-Authors: Takyu Sodai, Yoshida Eiji, Tashima Hideaki, Ahmed Abdella, Kumagai Masaaki, Yamashita Taichi, Yamaya TaigaAbstract:Time-of-flight (TOF) is now a standard technology for positron emission tomography (PET), but its effective use for small diameter PET systems has not been studied well. In this paper, we simulated a brain-dedicated TOF-PET system with a hemispherical detector arrangement. We modeled a Hamamatsu TOF-PET module (C13500-4075LC-12) with 280 ps coincidence resolving time (CRT), in which a 12 × 12 array of multi pixel photon counters (MPPCs) is connected to a lutetium fine silicate (LFS) crystal array of 4.1 × 4.1 mm2 cross section each, based on one-to-one coupling. On the other hand, spatial resolution degradation due to the Parallax Error should be carefully addressed for the small diameter PET systems. The ideal PET detector would have both depth-of-interaction (DOI) and TOF capabilities, but typical DOI detectors that are based on light sharing tend to degrade TOF performance. Therefore, in this work, we investigated non-DOI detectors with an appropriate crystal length, which was a compromise between suppressed Parallax Error and decreased sensitivity. Using GEANT4, we compared two TOF detectors, a 20 mm long non-DOI and a 10 mm long non-DOI, with a non-TOF, 4-layer DOI detector with a total length of 20 mm (i.e. 5 × 4 mm). We simulated a contrast phantom and evaluated the relationship between the contrast recovery coefficient (CRC) and the noise level (the coefficient of variation, COV) for reconstructed images. The 10 mm long non-DOI, which reduces the Parallax Error at the cost of sensitivity loss, showed better imaging quality than the 20 mm long non-DOI. For example, the CRC value of a 10 mm hot sphere at COV = 20% was 72% for the 10 mm long non-DOI, which was 1.2 times higher than that of the 20 mm long non-DOI. The converged CRC values for the 10 mm long non-DOI were almost equivalent to those of the non-TOF 4-layer DOI, and the 10 mm long non-DOI converged faster than the non-TOF 4-layer DOI did. Based on the simulation results, we evaluated a one-pair prototype system of the TOF-PET detectors with 10 mm crystal length, which yielded the CRT of 250 ± 8 ps. In summary, we demonstrated support for feasibility of the brain-dedicated TOF-PET system with the hemispherical detector arrangement
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Helmet-type brain TOF-PET prototype: improved timing resolution and count rate performance by optimizing energy window
2020Co-Authors: Yoshida Eiji, Tashima Hideaki, Yamashita Taichi, Iwao Yuma, Wakizaka Hidekatsu, Maeda Takamasa, Takahashi Miwako, Yamaya TaigaAbstract:Objectives: Brain PET is a powerful imaging tool to visualize various functions and metabolism of the human brain. To make a high-sensitivity brain PET system with a limited number of detectors, we have proposed a hemispherical detector arrangement. In addition, we have developed a prototype system with a time-of-flight (TOF) measurement capability. To maximize the effective sensitivity of the prototype system, the energy window should be optimized in consideration of coincidence count detection sensitivity, timing resolution, and scatter fraction. In this work, we investigated optimal energy window of the helmet-type TOF-PET prototype so as to obtain better count rate performance.Methods: The helmet-type TOF-PET prototype consisted of 54 block detectors. The 45 detectors were arranged to form a hemisphere and the other 9 detectors were placed to form a half-ring behind the neck. The detector block was composed of 10-mm-thick lutetium fine silicate (LFS) scintillators and silicon photomultiplier (SiPM) with one-to-one coupling. The scintillator thickness was optimized through simulation so as to reduce the Parallax Error while maintaining the TOF gain in image quality. The system average energy resolution was 12.6% at 511 keV. First, we set a wide energy window of 400–700 keV. After singles data acquisition, 12 patterns of additional energy window were applied. We used the lower energy window of 400, 420 and 440 keV and the upper energy window of 570, 590, 620 and 700 keV. The timing resolution was measured with singles data of a 22Na point source at the center of the bottom detector ring. The count rate performance was measured using the brain-size (20-cm diameter and 17.5-cm length) cylindrical scatter phantom* and the scatter fraction was measured according to the procedure of the NEMA NU2 standards publication. Based on these data, we calculated a noise-equivalent count rate (NECR) with TOF sensitivity gain for each energy window. Finally, the image quality was evaluated with the brain-size image quality phantom*.Results: In all energy window settings, the system timing resolutions were 235.9–245.6 ps, the scatter fractions were 23.2–36.1% and the NECRs with the TOF gain at the 20 MBq were 117.5–128.9 kcps/MBq. The scatter fraction considerably depended on the lower energy window setting. Using the energy window of 440–590 keV, the highest NECR with TOF gain was achieved with the 236.1-ps timing resolution. For the image quality assessment, the higher contrast and lower noise were obtained with the energy window of 440–590 keV. Conclusions: We investigated optimal energy window for the helmet-type TOF-PET prototype. On the current setup, the highest count rate performance and better image quality were obtained with the energy window of 440–590 keV. Also, the timing resolution was improved to be 236.1 ps.[*: Akamatsu G, et al. Biomed Phys Eng Express. 2020;6:015012]SNMMI 2020 Annual Meetin
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Head-to-head comparison of performance characteristics between two helmet-type PET prototypes
2020Co-Authors: Yoshida Eiji, Tashima Hideaki, Yamashita Taichi, Iwao Yuma, Wakizaka Hidekatsu, Maeda Takamasa, Takahashi Miwako, Yamaya TaigaAbstract:[Purpose] Brain PET imaging is a powerful tool to visualize various functions and metabolism of the human brain. We proposed a helmet-shape detector arrangement for increasing sensitivity. For proof-of-concept, we developed the first prototype, which had depth-of-interaction (DOI) detectors without time-of-flight (TOF) capability. Based on the experience of the first prototyping, we have developed the second prototype which had TOF capability of a 245-ps timing resolution without DOI capability. The detector was composed of 12×12 lutetium fine silicate (LFS) scintillation crystals (4×4×10 mm3) connected to a 12×12 silicon photomultiplier (SiPM) array with one-to-one coupling. The crystal thickness was optimized through simulation so as to reduce the Parallax Error while keeping the gain in image quality. In this work, we evaluated and compared the performance characteristics of the two helmet-type PET prototypes based on standard procedures.[Methods] We evaluated spatial resolution, sensitivity, count rate characteristics and image quality based on the NEMA NU-2 standards. Some measurements were partially modified to be applicable for the helmet-type PET. [Results] The values measured with the first (DOI) and second (TOF) prototypes were: spatial resolution of 2.8 and 3.1 mm at the central position (1-cm offset) by the FBP reconstruction; sensitivities of 13.4 and 4.1 kcps/MBq; scatter fraction at the low activity of 31% and 31%; and peak noise-equivalent count rates (NECR) of 20.0 kcps at 2.6 kBq/mL and 21.1 kcps at 9.0 kBq/mL. With a TOF sensitivity gain of 5.4 (= 20-cm-diameter / 3.675 cm TOF localization), effective sensitivity and effective peak NECR were 22.3 kcps/MBq and 115 kcps at 9.0 kBq/mL for the second prototype. In the image quality assessment, the second prototype outperformed the first prototype. [Conclusion] Although the first prototype had better performance for intrinsic sensitivity, the second prototype showed higher effective sensitivity, higher effective count rate performance and superior image quality. The fast timing properties of the detector configuration (10-mm-thick LFS and SiPM with one-to-one coupling) and its TOF capability significantly improved the imaging performance.第119回日本医学物理学会学術大
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Estimating the spatial resolution limits for isotropic-3D PET detector
2019Co-Authors: Yoshida Eiji, Tashima Hideaki, Nishikido Fumihiko, Hirano Yoshiyuki, Inadama Naoko, Murayama Hideo, Yamaya TaigaAbstract:The detection of depth-of-interaction (DOI) is essential to achieve both high spatial resolution and high sensitivity for positron emission tomography (PET), since DOI information will reduce the Parallax Error due to the crystal penetration. In particular, performance of a small bore PET scanner is improved for dedicated human brain, human breast, and small animal imaging. We developed a novel, general purpose isotropic-3D PET detector X\u27tal cube by effective readout of scintillation photons from six sides of the crystal block. The X\u27tal cube is composed of the 3D crystal block with isotropic resolution and arrays of multi pixel photon counters (MPPCs). We have shown that the X\u27tal cube can achieve 1.9 mm uniform spatial resolution using the one pair prototype of the X\u27tal cubes with 3D grids of 2 mm pitch. Fig. 1 (a) shows the illustration of the X\u27tal cube with 3D grids of 2 mm pitch. In this work, we investigate spatial resolution of a PET scanner based on the X\u27tal cube using Monte Carlo simulations for predicting resolution performance in smaller 3D grid. The PET scanner of 15.6 cm in diameter was simulated. This PET scanner consisted of 24 X\u27tal cube detectors. For spatial resolution evaluation, a point source emitting 511 keV photons was simulated with all the physical processes involved during emission and interaction of positrons using the GATE. The interacted 3D grid is determined by finding the centroid which is weighted by each interaction intensity. These simulations were repeated, varying the radial offset of the point source to demonstrate the spatial resolution at different locations across the field-of-view (FOV). The simulated data were projected to the sinogram and reconstructed using the 2D filtered backprojection on each axial plane. Fig. 1 (b) shows average spatial resolutions for the X\u27tal cubes with 3D grids of several pitches. For all types of the X\u27tal cube, these detectors obtained uniform spatial resolution over the FOV. Also, as the 3D grid pitch was small, spatial resolution was improved adequately. The average spatial resolution of the X\u27tal cube with grids of 2 mm pitch was equal to the experimental data. Also, the average spatial resolution of the X\u27tal cube with 3D grids of 0.5 mm pitch was 0.7 mm. The X\u27tal cube along with excellent spatial resolution would lead to PET scanners with uniform spatial resolution across the FOV.2012 World Molecular Imaging Congres
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Influence of TOF information in OpenPET image reconstruction
2019Co-Authors: Yamaya Taiga, Yoshida Eiji, Nishikido Fumihiko, Inadama Naoko, Shibuya Kengo, Murayama HideoAbstract:We have proposed an "OpenPET" geometry, which consists of two axially separated detector rings. The central point of the field of view, where the highest sensitivity is obtained, is opened. The OpenPET mainly has three applications, namely, simultaneous PET/CT, extension of the axial FOV, and in-beam PET. In our previous report, we showed that axial spatial resolution, which is degraded with the extended gap due to the Parallax Error, can be recovered by use of depth-of-interaction (DOI) detectors. On the other hand, image reconstruction of the OpenPET is an incomplete problem because projection data do not satisfy Orlov\u27s condition. Low-frequency components are missing in oblique LORs, i.e., LORs with large ring differences. The gap would suffer from loss of low-frequency components because the gap, where direct LORs (i.e., LORs in the same ring) do not exist, is imaged only from the oblique LORs. In this paper, we focused on time-of-flight (TOF) information, which is expected to compensate for the loss of low-frequency components in the gap. We investigated influence of TOF information in the OpenPET image reconstruction through numerical simulations. Simulated OpenPET scanner had dual detector rings (827.0 mm in diameter and axial length of 153.6 mm each) separated by a gap of 153.6 mm. We supposed that the detectors had a DOI capability of 8 layers and had a TOF capability of 400 ps FWHM resolution. For the non-TOF case, hot-spot objects, which are commonly seen in cancer diagnosis with use of FDG, were imaged without any artifacts, but objects containing more low-frequency components suffered from strong distortion. However, these artifacts were effectively reduced by using TOF information. These results showed that TOF information can compensate for low-frequency components missing in the gap.2009 IEEE Nuclear Science Symposium and Medical Imaging Conferenc
Patrick Hallen - One of the best experts on this subject based on the ideXlab platform.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
EJNMMI Physics, 2020Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association’s (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard’s goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard.For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. If the PET ring has detectors which are perpendicular to a Cartesian axis, then the resolution along this axis will typically improve with larger projection windows.We show that the standard’s equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. However, a modified version can still be used to determine an approximation of the random rates under the assumption of negligible random rates for small activities and a constant scatter fraction. We compare the resulting estimated random rates to random rates obtained using a delayed coincidence window and two methods based on the singles rates. While these methods give similar estimates, the estimation method based on the NEMA equations overestimates the random rates.In the NEMA standard’s protocol for the evaluation of the sensitivity, the standard specifies to axially step a point source through the scanner and to take a different scan for each source position. Later, in the data analysis section, the standard does not specify clearly how the different scans have to be incorporated into the analysis, which can lead to unclear interpretations of publicized results.The standard’s definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e., noise), and recovery.With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
arXiv: Medical Physics, 2019Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association's (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard's goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard. For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. We show that the standard's equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. We compare random rates determined by a method based on the standard to to random rates obtained using a delayed coincidence window and two methods based on the singles rates. We discuss to what extend the NEMA standard's protocol for the evaluation of the sensitivity is unclear and ambiguous, which can lead to unclear interpretations of publicizeded results. The standard's definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e. noise), and recovery. With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
Yoshida Eiji - One of the best experts on this subject based on the ideXlab platform.
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Design study of a brain-dedicated time-of-flight PET system with a hemispherical detector arrangement
2020Co-Authors: Takyu Sodai, Yoshida Eiji, Tashima Hideaki, Ahmed Abdella, Kumagai Masaaki, Yamashita Taichi, Yamaya TaigaAbstract:Time-of-flight (TOF) is now a standard technology for positron emission tomography (PET), but its effective use for small diameter PET systems has not been studied well. In this paper, we simulated a brain-dedicated TOF-PET system with a hemispherical detector arrangement. We modeled a Hamamatsu TOF-PET module (C13500-4075LC-12) with 280 ps coincidence resolving time (CRT), in which a 12 × 12 array of multi pixel photon counters (MPPCs) is connected to a lutetium fine silicate (LFS) crystal array of 4.1 × 4.1 mm2 cross section each, based on one-to-one coupling. On the other hand, spatial resolution degradation due to the Parallax Error should be carefully addressed for the small diameter PET systems. The ideal PET detector would have both depth-of-interaction (DOI) and TOF capabilities, but typical DOI detectors that are based on light sharing tend to degrade TOF performance. Therefore, in this work, we investigated non-DOI detectors with an appropriate crystal length, which was a compromise between suppressed Parallax Error and decreased sensitivity. Using GEANT4, we compared two TOF detectors, a 20 mm long non-DOI and a 10 mm long non-DOI, with a non-TOF, 4-layer DOI detector with a total length of 20 mm (i.e. 5 × 4 mm). We simulated a contrast phantom and evaluated the relationship between the contrast recovery coefficient (CRC) and the noise level (the coefficient of variation, COV) for reconstructed images. The 10 mm long non-DOI, which reduces the Parallax Error at the cost of sensitivity loss, showed better imaging quality than the 20 mm long non-DOI. For example, the CRC value of a 10 mm hot sphere at COV = 20% was 72% for the 10 mm long non-DOI, which was 1.2 times higher than that of the 20 mm long non-DOI. The converged CRC values for the 10 mm long non-DOI were almost equivalent to those of the non-TOF 4-layer DOI, and the 10 mm long non-DOI converged faster than the non-TOF 4-layer DOI did. Based on the simulation results, we evaluated a one-pair prototype system of the TOF-PET detectors with 10 mm crystal length, which yielded the CRT of 250 ± 8 ps. In summary, we demonstrated support for feasibility of the brain-dedicated TOF-PET system with the hemispherical detector arrangement
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Head-to-head comparison of performance characteristics between two helmet-type PET prototypes
2020Co-Authors: Yoshida Eiji, Tashima Hideaki, Yamashita Taichi, Iwao Yuma, Wakizaka Hidekatsu, Maeda Takamasa, Takahashi Miwako, Yamaya TaigaAbstract:[Purpose] Brain PET imaging is a powerful tool to visualize various functions and metabolism of the human brain. We proposed a helmet-shape detector arrangement for increasing sensitivity. For proof-of-concept, we developed the first prototype, which had depth-of-interaction (DOI) detectors without time-of-flight (TOF) capability. Based on the experience of the first prototyping, we have developed the second prototype which had TOF capability of a 245-ps timing resolution without DOI capability. The detector was composed of 12×12 lutetium fine silicate (LFS) scintillation crystals (4×4×10 mm3) connected to a 12×12 silicon photomultiplier (SiPM) array with one-to-one coupling. The crystal thickness was optimized through simulation so as to reduce the Parallax Error while keeping the gain in image quality. In this work, we evaluated and compared the performance characteristics of the two helmet-type PET prototypes based on standard procedures.[Methods] We evaluated spatial resolution, sensitivity, count rate characteristics and image quality based on the NEMA NU-2 standards. Some measurements were partially modified to be applicable for the helmet-type PET. [Results] The values measured with the first (DOI) and second (TOF) prototypes were: spatial resolution of 2.8 and 3.1 mm at the central position (1-cm offset) by the FBP reconstruction; sensitivities of 13.4 and 4.1 kcps/MBq; scatter fraction at the low activity of 31% and 31%; and peak noise-equivalent count rates (NECR) of 20.0 kcps at 2.6 kBq/mL and 21.1 kcps at 9.0 kBq/mL. With a TOF sensitivity gain of 5.4 (= 20-cm-diameter / 3.675 cm TOF localization), effective sensitivity and effective peak NECR were 22.3 kcps/MBq and 115 kcps at 9.0 kBq/mL for the second prototype. In the image quality assessment, the second prototype outperformed the first prototype. [Conclusion] Although the first prototype had better performance for intrinsic sensitivity, the second prototype showed higher effective sensitivity, higher effective count rate performance and superior image quality. The fast timing properties of the detector configuration (10-mm-thick LFS and SiPM with one-to-one coupling) and its TOF capability significantly improved the imaging performance.第119回日本医学物理学会学術大
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Helmet-type brain TOF-PET prototype: improved timing resolution and count rate performance by optimizing energy window
2020Co-Authors: Yoshida Eiji, Tashima Hideaki, Yamashita Taichi, Iwao Yuma, Wakizaka Hidekatsu, Maeda Takamasa, Takahashi Miwako, Yamaya TaigaAbstract:Objectives: Brain PET is a powerful imaging tool to visualize various functions and metabolism of the human brain. To make a high-sensitivity brain PET system with a limited number of detectors, we have proposed a hemispherical detector arrangement. In addition, we have developed a prototype system with a time-of-flight (TOF) measurement capability. To maximize the effective sensitivity of the prototype system, the energy window should be optimized in consideration of coincidence count detection sensitivity, timing resolution, and scatter fraction. In this work, we investigated optimal energy window of the helmet-type TOF-PET prototype so as to obtain better count rate performance.Methods: The helmet-type TOF-PET prototype consisted of 54 block detectors. The 45 detectors were arranged to form a hemisphere and the other 9 detectors were placed to form a half-ring behind the neck. The detector block was composed of 10-mm-thick lutetium fine silicate (LFS) scintillators and silicon photomultiplier (SiPM) with one-to-one coupling. The scintillator thickness was optimized through simulation so as to reduce the Parallax Error while maintaining the TOF gain in image quality. The system average energy resolution was 12.6% at 511 keV. First, we set a wide energy window of 400–700 keV. After singles data acquisition, 12 patterns of additional energy window were applied. We used the lower energy window of 400, 420 and 440 keV and the upper energy window of 570, 590, 620 and 700 keV. The timing resolution was measured with singles data of a 22Na point source at the center of the bottom detector ring. The count rate performance was measured using the brain-size (20-cm diameter and 17.5-cm length) cylindrical scatter phantom* and the scatter fraction was measured according to the procedure of the NEMA NU2 standards publication. Based on these data, we calculated a noise-equivalent count rate (NECR) with TOF sensitivity gain for each energy window. Finally, the image quality was evaluated with the brain-size image quality phantom*.Results: In all energy window settings, the system timing resolutions were 235.9–245.6 ps, the scatter fractions were 23.2–36.1% and the NECRs with the TOF gain at the 20 MBq were 117.5–128.9 kcps/MBq. The scatter fraction considerably depended on the lower energy window setting. Using the energy window of 440–590 keV, the highest NECR with TOF gain was achieved with the 236.1-ps timing resolution. For the image quality assessment, the higher contrast and lower noise were obtained with the energy window of 440–590 keV. Conclusions: We investigated optimal energy window for the helmet-type TOF-PET prototype. On the current setup, the highest count rate performance and better image quality were obtained with the energy window of 440–590 keV. Also, the timing resolution was improved to be 236.1 ps.[*: Akamatsu G, et al. Biomed Phys Eng Express. 2020;6:015012]SNMMI 2020 Annual Meetin
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Estimating the spatial resolution limits for isotropic-3D PET detector
2019Co-Authors: Yoshida Eiji, Tashima Hideaki, Nishikido Fumihiko, Hirano Yoshiyuki, Inadama Naoko, Murayama Hideo, Yamaya TaigaAbstract:The detection of depth-of-interaction (DOI) is essential to achieve both high spatial resolution and high sensitivity for positron emission tomography (PET), since DOI information will reduce the Parallax Error due to the crystal penetration. In particular, performance of a small bore PET scanner is improved for dedicated human brain, human breast, and small animal imaging. We developed a novel, general purpose isotropic-3D PET detector X\u27tal cube by effective readout of scintillation photons from six sides of the crystal block. The X\u27tal cube is composed of the 3D crystal block with isotropic resolution and arrays of multi pixel photon counters (MPPCs). We have shown that the X\u27tal cube can achieve 1.9 mm uniform spatial resolution using the one pair prototype of the X\u27tal cubes with 3D grids of 2 mm pitch. Fig. 1 (a) shows the illustration of the X\u27tal cube with 3D grids of 2 mm pitch. In this work, we investigate spatial resolution of a PET scanner based on the X\u27tal cube using Monte Carlo simulations for predicting resolution performance in smaller 3D grid. The PET scanner of 15.6 cm in diameter was simulated. This PET scanner consisted of 24 X\u27tal cube detectors. For spatial resolution evaluation, a point source emitting 511 keV photons was simulated with all the physical processes involved during emission and interaction of positrons using the GATE. The interacted 3D grid is determined by finding the centroid which is weighted by each interaction intensity. These simulations were repeated, varying the radial offset of the point source to demonstrate the spatial resolution at different locations across the field-of-view (FOV). The simulated data were projected to the sinogram and reconstructed using the 2D filtered backprojection on each axial plane. Fig. 1 (b) shows average spatial resolutions for the X\u27tal cubes with 3D grids of several pitches. For all types of the X\u27tal cube, these detectors obtained uniform spatial resolution over the FOV. Also, as the 3D grid pitch was small, spatial resolution was improved adequately. The average spatial resolution of the X\u27tal cube with grids of 2 mm pitch was equal to the experimental data. Also, the average spatial resolution of the X\u27tal cube with 3D grids of 0.5 mm pitch was 0.7 mm. The X\u27tal cube along with excellent spatial resolution would lead to PET scanners with uniform spatial resolution across the FOV.2012 World Molecular Imaging Congres
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Influence of TOF information in OpenPET image reconstruction
2019Co-Authors: Yamaya Taiga, Yoshida Eiji, Nishikido Fumihiko, Inadama Naoko, Shibuya Kengo, Murayama HideoAbstract:We have proposed an "OpenPET" geometry, which consists of two axially separated detector rings. The central point of the field of view, where the highest sensitivity is obtained, is opened. The OpenPET mainly has three applications, namely, simultaneous PET/CT, extension of the axial FOV, and in-beam PET. In our previous report, we showed that axial spatial resolution, which is degraded with the extended gap due to the Parallax Error, can be recovered by use of depth-of-interaction (DOI) detectors. On the other hand, image reconstruction of the OpenPET is an incomplete problem because projection data do not satisfy Orlov\u27s condition. Low-frequency components are missing in oblique LORs, i.e., LORs with large ring differences. The gap would suffer from loss of low-frequency components because the gap, where direct LORs (i.e., LORs in the same ring) do not exist, is imaged only from the oblique LORs. In this paper, we focused on time-of-flight (TOF) information, which is expected to compensate for the loss of low-frequency components in the gap. We investigated influence of TOF information in the OpenPET image reconstruction through numerical simulations. Simulated OpenPET scanner had dual detector rings (827.0 mm in diameter and axial length of 153.6 mm each) separated by a gap of 153.6 mm. We supposed that the detectors had a DOI capability of 8 layers and had a TOF capability of 400 ps FWHM resolution. For the non-TOF case, hot-spot objects, which are commonly seen in cancer diagnosis with use of FDG, were imaged without any artifacts, but objects containing more low-frequency components suffered from strong distortion. However, these artifacts were effectively reduced by using TOF information. These results showed that TOF information can compensate for low-frequency components missing in the gap.2009 IEEE Nuclear Science Symposium and Medical Imaging Conferenc
David Schug - One of the best experts on this subject based on the ideXlab platform.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
EJNMMI Physics, 2020Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association’s (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard’s goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard.For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. If the PET ring has detectors which are perpendicular to a Cartesian axis, then the resolution along this axis will typically improve with larger projection windows.We show that the standard’s equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. However, a modified version can still be used to determine an approximation of the random rates under the assumption of negligible random rates for small activities and a constant scatter fraction. We compare the resulting estimated random rates to random rates obtained using a delayed coincidence window and two methods based on the singles rates. While these methods give similar estimates, the estimation method based on the NEMA equations overestimates the random rates.In the NEMA standard’s protocol for the evaluation of the sensitivity, the standard specifies to axially step a point source through the scanner and to take a different scan for each source position. Later, in the data analysis section, the standard does not specify clearly how the different scans have to be incorporated into the analysis, which can lead to unclear interpretations of publicized results.The standard’s definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e., noise), and recovery.With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
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comments on the nema nu 4 2008 standard on performance measurement of small animal positron emission tomographs
arXiv: Medical Physics, 2019Co-Authors: Patrick Hallen, David Schug, Volkmar SchulzAbstract:The National Electrical Manufacturers Association's (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard's goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard. For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by Parallax Error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. We show that the standard's equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. We compare random rates determined by a method based on the standard to to random rates obtained using a delayed coincidence window and two methods based on the singles rates. We discuss to what extend the NEMA standard's protocol for the evaluation of the sensitivity is unclear and ambiguous, which can lead to unclear interpretations of publicizeded results. The standard's definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e. noise), and recovery. With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.
