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A whole-body positron emission tomography scanner must be equipped with many detectors so that they are often composed of low-cost photomultiplier tubes (PMTs), that is, large anode-type PMTs. The general detector structure has a scintillation crystal element array coupled on an array of the large anode PMTs, and for crystal element identification, the method is needed to get sufficient spread of scintillation light for distribution among distant PMT anodes. Besides the common method of using a light guide, some methods have been proposed for better element identification performance. In this paper, we introduce a new method, in which the scintillation light spread is promoted not only by removing reflectors between crystal elements but also by restricting light exit to the PMTs by placing additional reflectors at the bottom of the crystal element array. Because the additional reflectors are parallel to the PMT surface, we call them parallel reflectors. We verified our method with a detector consisting of the 2.45 mm $\times \,\, 5$ mm $\times \,\,15$ mm Lu2 x Gd2(1– x )SiO5:Ce crystal elements and two dual-photocathode PMTs. Each photocathode was 8 mm $\times \,\,18$ mm in size. We set a $9 \times 10$ crystal element array on the two PMTs and tried to identify the elements by the $2\times 2$ PMT signals. Detector performance was evaluated with 137Cs point sources (662-keV gamma rays). The results showed that despite a significant decrease in light output at the boundary of the two PMTs, the method made crystal element identification possible. We measured energy resolutions of 13.1% and 17.8% for the elements on the PMT photocathode area and the PMT boundary, respectively. Our method is applicable to the depth-of-interaction detector consisting of multilayer crystal element arrays. By inserting parallel reflectors between the layers, we would be able to control the path of scintillation light originating in the upper layer. The effect was first examined in a basic study with two layers of a $2 \times 5$ crystal element array. Then, the array of each layer was increased to $9\times 9$ for performance evaluation. It was difficult to identify all 162 crystal elements with four PMT signals; however, results suggest the possibility of scintillation light path control in each layer separately by the parallel reflectors and the potential for better performance by more precise adjustment of parallel reflectors.

Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomography (PET) scanners to improve the sensitivity of radiation detection and the uniformity of spatial resolution. We recently developed a series of dual-ended detectors using crystal bars segmented by applying the subsurface laser engraving (SSLE) technique to 7, 13 and 20 DOI segments. It is crucial to achieve the submillimeter level spatial resolution for our detector as the narrow crystal bars are highly fragile after applying the SSLE technique. In this work, we focused on optimizing the surface area of the SSLE-induced layer to the narrow crystal bars in order to resolve the issue of fragility while maintaining sufficient crystal segment separation. The SSLE layers were induced to the middle of the cross section of the five crystal bars with dimensions of 1.5 × 1 . 5 × 20 mm3 with a distance of 0.1, 0.2, 0.3, 0.4 or 0.5 mm from the two opposite lateral edges of each crystal bar. All crystals were segmented into four DOI segments and the performances of the five sets of dual-ended DOI detectors consisting of one crystal bar were evaluated from the viewpoints of optimum crystal segment identification and energy resolution while mitigating the fragility issue. The 3D position maps of the detectors were obtained by the Anger-type calculation, and the crystal identification performance was evaluated. Clear separation of the segments with a distance-to-width ratio (DWR) larger than 7 was obtained for the detectors in which the distances of the SSLE-induced layer from the two opposite lateral edges of the crystal were 0.1, 0.2 and 0.3 mm. The DWR for the other two detectors was less than 4.4. Average energy resolutions of 11.8 ± 0.2%, 10.6 ± 0.2% and 8.8 ± 0.2% were obtained for the detectors in which the SSLE layers were induced with 0.1, 0.3 and 0.5 mm distances from the two opposite lateral edges of the crystal, respectively. There was a compromise between energy resolution and crystal segment identification. The optimum surface area of the SSLE-induced layer was accepted as a trade-off between crystal segment identification and fragility. The crystal segmented by the SSLE-induced layer at the middle of the cross section with a distance of 0.3 mm from the two opposite lateral edges was considered to be a good candidate with acceptable performance and fragility.

We have developed a four-layer depth of interaction (DOI) detector with single-side photon readout, in which segmented crystals with the patterned reflector insertion are separately identified by the Anger-type calculation. Optical conditions between segmented crystals, where there is no reflector, affect crystal identification ability. Our objective of this work was to improve crystal identification performance of the four-layer DOI detector that uses crystals segmented with a recently developed laser processing technique to include laser processed boundaries (LPBs). The detector consisted of 2 × 2 × 4 mm 3 LYSO crystals and a 4 × 4 array multianode photomultiplier tube (PMT) with 4.5 mm anode pitch. The 2D position map of the detector was calculated by the Anger calculation method. At first, influence of optical condition on crystal identification was evaluated for a one-layer detector consisting of a 2 × 2 crystal array with three different optical conditions between the crystals: crystals stuck together using room temperature vulcanized (RTV) rubber, crystals with air coupling and segmented crystals with LPBs. The crystal array with LPBs gave the shortest distance between crystal responses in the 2D position map compared with the crystal array coupled with RTV rubber or air due to the great amount of cross-talk between segmented crystals with LPBs. These results were used to find optical conditions offering the optimum distance between crystal responses in the 2D position map for the four-layer DOI detector. Crystal identification performance for the four-layer DOI detector consisting of an 8 × 8 array of crystals segmented with LPBs was examined and it was not acceptable for the crystals in the first layer. The crystal identification was improved for the first layer by changing the optical conditions between all 2 × 2 crystal arrays of the first layer to RTV coupling. More improvement was observed by combining different optical conditions between all crystals of the first layer and some crystals of the second and the third layers of the segmented array.

X’tal cube is the cubic depth of interaction (DOI) PET detector which our research group developed. In this work, aiming to get higher sensitivity, we developed the long rectangular shape X’tal cube (long-XC) by extending the cubic X’tal cube structure in one direction. We verified performance of this long-XC and also studied detector parameters for optimization. The same as the X’tal cube, the crystal block of the long-XC is composed of a 3D array of cubic scintillation crystal elements. Reflectors are not inserted between these crystal elements. The scintillation light then spreads without being obstructed by reflectors and is detected by multiple numbers of the multi-pixel photon counters (MPPCs) coupled on all six sides of the crystal block. For crystal element identification, a simple Anger-type calculation is used. In this study, we arranged 3.0 mm $\times 3.0$ mm $\times 3.0$ mm LGSO crystal elements in a $6\times 6\times 14$ array for the long-XC. In a previous study, we had already confirmed that for the X’tal cube consisting of a $6\times 6\times 6$ array of the same crystal elements and 54 MPPCs, identification of all 216 crystal elements was possible and the average energy resolution for all the elements was about 11 %. The long-XC contains more than twice the number of the crystal elements but less than twice the number of the MPPCs compared to the previous X’tal cube. The detector parameters investigated with the long-XC were: the number of MPPCs on both sides in the extended direction (edge MPPCs); the MPPC type, the MPPCs of $25\,\,\mu \text {m}\,\,\times 25\,\,\mu \text {m}$ or $50\,\,\mu \text {m}\,\,\times 50\,\,\mu \text {m}$ pixel sizes; the material between the crystal elements, an air gap or an optical glue having a closer refractive index to that of LGSO than air has; and the MPPC signals used in the Anger-type calculation. Results of the crystal element identification performance showed that reducing the number of the edge MPPCs caused performance degradation only at the part near the edge. For the MPPC type, the $50~\mu \text {m}$ type was better than the $25~\mu \text {m}$ type, and for the material, air was much better than the optical glue. We found that the choice of MPPC signals for the Anger-type calculation was effective in the optical glue condition. For the long-XC in the air gap condition and using the $50~\mu \text {m}$ type MPPCs, we observed it had good performance and there was no significant degradation at the central part which is far from the edge MPPCs. For irradiation of 662 keV gamma-rays, we measured approximately 11 - 13 % energy resolution for each crystal element and there was only a small difference in light outputs between crystal elements at the central part and at the edges. These results at the central part suggested the possibility of further extension of the long-XC for higher sensitivity.

We are developing a PET-MRI system which consists of PET detectors integrated with the head coil of the MRI in order to realize high spatial resolution and high sensitivity in simultaneous measurements. In the PET-MRI system, the PET detectors which consist of a scintillator block, photo-detectors and front-end circuits with four-layer depth-of-interaction (DOI) encoding capability are placed close to the measured object. Therefore, the proposed system can achieve high sensitivity without degradation of spatial resolution at the edge of the field-of-view due to parallax error thanks to the four-layer DOI capability. In this paper, we fabricated a prototype system which consists of a prototype four-layer DOI-PET detector, a dummy PET detector and a prototype birdcage type head coil. Then we used the prototype system to evaluate the performance of the four-layer DOI-PET detector and the reciprocal influence between the PET detectors and MRI images. The prototype DOI-PET detector consists of six monolithic multi-pixel photon counter (MPPC) arrays (S11064-050P), a readout circuit board, two scintillator blocks and a copper shielding box. Each scintillator block consists of four layers of Lu1.8Gd0.2SiO5:Ce (LGSO) scintillators and reflectors are inserted between the scintillation crystals. The dummy detector has all these components except the two scintillator blocks. The head coil is dedicated to a 3.0 T MRI (MAGNETOM Verio, Siemens) and the two detectors are mounted in gaps between head coil elements. Energy resolution and crystal identification performance of the prototype four-layer DOI-PET detector were evaluated with and without MRI measurements by the gradient echo and spin echo methods. We identified crystal elements in all four layers from a 2D flood histogram and energy resolution of 15–18% was obtained for single crystal elements in simultaneous measurements. The difference between the average energy resolutions and photo-peak positions with and without MRI measurements was lower than 0.3 percentage points and 1.7% for all layers. The results indicated that these performances were sufficient as PET detectors for the proposed PET-MRI system and there was no influence from the MRI measurements on the PET imaging in the simultaneous measurements. The signal-to-noise ratio of the MRI image and static magnetic field of the MRI were also evaluated with and without measurements of the PET detectors. The maximum decrease of the static magnetic field due to the LGSO scintillators was approximately 1 ppm. The signal-to-noise ratio decreased from 242.80 without the PET detector to 44.948 in simultaneous measurements.

We have developed a three-dimensional (3-D) position-sensitive radiation detector named X'tal cube, which can be applied to a PET detector. The X'tal cube is composed of a scintillation crystal block and multi-pixel photon counters (MPPCs). The crystal block is segmented three-dimensionally into small cubes by optical discontinuity (3-D segmentation) and no reflector is inserted inside. Scintillation light originating in a segment then spreads three-dimensionally so that the MPPCs are set on all six surfaces of the crystal block to detect the light. Regarding the 3-D segmentation of the crystal block, we have already succeeded in getting the segmentation inside of a monolithic scintillator block by a laser processing technique instead of the general way, arranging small cubic scintillator elements into a 3-D array. We have confirmed that utilizing the laser processing technique not only eliminates the difficulty of handling the small scintillator elements but also improves detector performance. As a new trial, we considered fabrication of the crystal block by stacking the scintillator plates which were segmented two-dimensionally by the laser processing technique (2-D segmentation). Plates are also easier to handle and for the laser processing, 2-D segmentation is simpler than 3-D segmentation. In this study, we prepared the X'tal cube with the scintillator plates (Plate-XC) and evaluated its performance to confirm its technical feasibility. For the Plate-XC, we segmented 18 mm × 18 mm × 2.0 mm LYSO plates two-dimensionally by laser processing so as to make a 9 × 9 array of 2.0 mm × 2.0 mm segments. The crystal block was composed of 9 stacked LYSO plates without using coupling material but with air gaps. The Plate-XC showed sufficient crystal identification performance when 662 keV gamma-rays were irradiated. Furthermore, to understand the characteristics of the Plate-XC, we also analyzed the scintillation light distribution in the crystal block. Results indicated that light spread from outer segments was influenced by the segment boundary conditions, air gaps or laser-processed gaps, while the spread from the center segment did not seem to have such an influence. Regarding energy performance, we obtained around 10% energy resolution for the outer segments as well as for the center segment.

Positron emission tomography (PET) has become a popular imaging method in metabolism, neuroscience, and molecular imaging. For dedicated human brain and small animal PET scanners, high spatial resolution is needed to visualize small objects. To improve the spatial resolution, we are developing the X’tal cube, which is our new PET detector to achieve isotropic 3D positioning detectability. We have shown that the X’tal cube can achieve 1 mm 3 uniform crystal identification performance with the Anger-type calculation even at the block edges. We plan to develop the X’tal cube with even smaller 3D grids for sub-millimeter crystal identification. In this work, we investigate spatial resolution of a PET scanner based on the X’tal cube using Monte Carlo simulations for predicting resolution performance in smaller 3D grids. For spatial resolution evaluation, a point source emitting 511 keV photons was simulated by GATE for all physical processes involved in emission and interaction of positrons. We simulated two types of animal PET scanners. The first PET scanner had a detector ring 14.6 cm in diameter composed of 18 detectors. The second PET scanner had a detector ring 7.8 cm in diameter composed of 12 detectors. After the GATE simulations, we converted the interacting 3D position information to digitalized positions for realistic segmented crystals. We simulated several X’tal cubes with cubic crystals from (0.5 mm) 3 to (2 mm) 3 in size. Also, for evaluating the effect of DOI resolution, we simulated several X’tal cubes with crystal thickness from (0.5 mm) 3 to (9 mm) 3 . We showed that sub-millimeter spatial resolution was possible using cubic crystals smaller than (1.0 mm) 3 even with the assumed physical processes. Also, the weighted average spatial resolutions of both PET scanners with (0.5 mm) 3 cubic crystals were 0.53 mm (14.6 cm ring diameter) and 0.48 mm (7.8 cm ring diameter). For the 7.8 cm ring diameter, spatial resolution with 0.5×0.5×1.0 mm 3 crystals was improved 39% relative to the (1 mm) 3 cubic crystals. On the other hand, spatial resolution with (0.5 mm) 3 cubic crystals was improved 47% relative to the (1 mm) 3 cubic crystals. The X’tal cube promises better spatial resolution for the 3D crystal block with isotropic resolution.

Silicon photomultipliers (SiPMs) provide many advantages for PET detectors, such as their high internal gain, high photon detection efficiency and insensitivity to magnetic fields. The number of detectable scintillation photons of SiPMs, however, is limited by the number of microcells. Therefore, pulse height of PET detectors using SiPMs is saturated when large numbers of scintillation photons enter the SiPM pixels. On the other hand, we previously presented a depth-of-interaction (DOI) encoding method that is based on the light sharing method. Since our encoding method detects scintillation photons with multiple readout pixels, the saturation effect can be suppressed. We constructed two prototype four-layer DOI detectors using a SiPM array and evaluated their performances. The two prototype detectors consisted of four layers of a 6×6 array of Lu 2(1− x ) Y 2 x SiO 5 (LYSO) crystals and a SiPM (multi-pixel photon detector, MPPC, Hamamatsu Photonics K.K.) array of 4×4 pixels. The size of each LYSO crystal element was 1.46 mm×1.46 mm×4.5 mm and all surfaces of the crystal elements were chemically etched. We used two types of MPPCs. The first one had 3600 microcells and high photon detection efficiency (PDE). The other one had 14,400 microcells and lower PDE. In the evaluation experiment, all the crystals of the detector using the MPPC which had the high PDE were clearly identified. The respective energy and timing resolutions of lower than 15% and 1.0 ns were achieved for each crystal element. No saturation of output signals was observed in the 511 keV energy region due to suppression of the saturation effect by detecting scintillation photons with several MPPC pixels by the light sharing method.

The X'tal cube is our original PET detector, which is being developed to achieve isotropic 3D positioning detectability. The X'tal cube is based on a 3D segmented crystal block for which all surfaces are covered with photo-detectors. Previously, instead of our initial approach of gluing segmented pieces of crystals, we successfully constructed a crystal block segmented by laser processing, and we developed the X'tal cube with the laser-processed 3D square grids of 2 mm length. In this paper, we extend the laser processing to 3D square grids of 1 mm length. The volume of a 1-mm crystal segment is 1/8 of that of a 2-mm crystal segment. We also evaluate imaging resolution performance with a newly developed one-pair prototype system to simulate a ring-type scanner, while our previous reports were limited to development and evaluation of a single detector. In particular, we compare 1-mm X'tal cube detectors to 2-mm X'tal cube detectors. The one-pair prototype system consisted of two X'tal cubes, two rotating stages, and a 192-channel data acquisition system. Each X'tal cube consisted of the LYSO cubic crystal block of 18 × 18 × 18 mm3 in which the 3D grids of 1 mm pitch were fabricated by internal laser processing. The 4 × 4 arrays of multi pixel photon counters were optically coupled to each surface of the crystal block. The detector positions were automatically controlled to simulate a ring-type PET with a 14.6 cm diameter. Data were collected for all assumed detector positions and then a sinogram was obtained. The data were reconstructed using filtered backprojection. The average spatial resolution of the 2-mm X'tal cube was 1.9 mm full width at half maximum (FWHM) over the field-of-view (FOV). On the other hand, the average spatial resolution of the 1-mm X'tal cube was 1.3 mm FWHM over the FOV. By applying deconvolution with the assumption that the point source was a Gaussian function of 1.0 mm FWHM, we estimated the average spatial resolution of the 2-mm X'tal cube and the 1-mm X'tal cube as 1.6 mm FWHM and 0.83 mm FWHM, respectively. The average spatial resolution of the 1-mm X'tal cube was improved approximately 48% compared to the 2-mm X'tal cube. In conclusion, we confirmed the potential of the X'tal cube for uniform and high resolution imaging.

A conventional PET scanner has a 15–25 cm axial field-of-view (FOV) and images the whole body using about six bed positions. We proposed our original OpenPET geometry which can extend the axial FOV with a limited number of detectors. For an alternative approach, we then designed an axially extendable multiplex cylinder (AEMC) PET scanner to provide high versatility for clinical and research studies using a silicon-photomultiplier (Si-PM)-based DOI detector. Since Si-PMs have high gain like PMTs and a compact design, the Si-PM-based detector is expected to enable various new detector arrangements. The AEMC-PET scanner consists of four independent and laminated detector rings using a four-layer depth-of-interaction (DOI) detector. This PET scanner can extend the axial FOV as each stacked detector ring can be slid aside. When this scanner is used for the four-layer DOI detector, its minimum axial FOV is 24 cm and its maximum crystal thickness is 3 cm. On the other hand, the axial FOV can be extended to 96 cm while keeping a continuous axial FOV, but the crystal thickness must be 1/4 of 3 cm. In addition, using the OpenPET geometry, the axial FOV can be extended to 168 cm. In this work, we studied the feasibility of the PET scanner with a variable axial FOV using Monte Carlo simulation. For the 180-cm line source simulation, the 96-cm axial FOV was obtained with twofold better sensitivity compared to the 24-cm axial FOV. Also, we showed that the AEMC-PET scanner had a continuous axial FOV to 168 cm. We expect the AEMC-PET scanner will provide high versatility such as for measuring trace whole-body uptakes while keeping the continuous axial FOV and the scan time for static images will be reduced with a limited number of detectors., 2012IEEE NSS&MIC

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed an open-type PET scanner, OpenPET. The original OpenPET had a physically opened field-of-view (FOV) between two detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wide axial FOV including the gap. This geometry was not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we have proposed a new single-ring OpenPET (SROP) geometry which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the DROP. The proposed geometry was a cylinder shape with its ends cut at a slant, in which the shape of each cut end became an ellipse. In this work, we developed and evaluated a small prototype of the SROP geometry for proof-of-concept. The SROP prototype was designed with 2 ellipse-shaped detector rings of 16 depth-of-interaction (DOI) detectors each. The DOI detectors consisted of 1024 GSOZ scintillator crystals which were arranged in 4 layers of 16×16 arrays, coupled to a 64-channel FP-PMT. Each ellipse-shaped detector ring had a major axis of 281.6 mm and a minor axis of 207.5 mm. For the slant mode, the rings were placed at a 45-deg slant from the axial direction and for the non-slant mode (used as a reference) they were at 90 deg from the axial direction with no gap. The system sensitivity measured from a 22 Na point source was 5.0% for the slant mode. The average spatial resolutions of major and minor axis directions were calculated as 3.8 mm FWHM and 4.9 mm FWHM, respectively for the slant mode. This difference resulted from the ellipsoidal ring geometry and the spatial resolution of the minor axis direction degraded by the parallax error. Comparison between the slant mode and the non-slant mode showed no remarkable difference in terms of the sensitivity distribution and spatial resolution performance. Therefore, we concluded that the SROP geometry has a good potential as an open geometry especially suitable for in-beam imaging.

The X'tal cube is a depth-of-interaction (DOI)-PET detector which is aimed at obtaining isotropic resolution by effective readout of scintillation photons from six sides of the crystal block. The X'tal cube is composed of a 3D crystal block with isotropic segments. Each face of the 3D crystal block is covered with a 4×4 array of multi-pixel photon counters (MPPCs). Previously, in order to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving technique to a monolithic crystal block instead of gluing segmented small crystals. A dense arrangement of multiple micro-cracks carved by the laser beam works efficiently as a scattering wall for the scintillation photons. The X'tal cube with the laser-processed block showed excellent performance with respect to crystal identification and energy resolution. In this work, for characteristics comparison between the laser-processed block and the conventional segmented array block, we made the laser-processed block and two types of segmented array blocks, one with air gaps and the other with glued segmented small crystals. All crystal blocks had 3D grids of 2 mm pitch. The 4×4 MPPC arrays were optically coupled to each surface of the crystal block. When performance was evaluated using a uniform irradiation of 511 keV, we found that the X'tal cubes with the laser-processed block could easily achieve 2 mm 3 uniform crystal identification. Also, the average energy resolution of each 3D grid was 11.1±0.7%. On the other hand, the glued segmented array block had a pinched distribution and crystals could not be separated clearly. The segmented array block with air gaps had satisfactory crystal identification performance; however, the laser-processed block had higher crystal identification performance. Also, the energy resolution of the laser-processed block was better than for the segmented array blocks. In summary, we found the laser-processed X'tal cube had excellent crystal identification performance and high energy performance due to its effective dense arrangement of micro-cracks.

The accumulation of induced radioactivity within in-beam PET scanner scintillators is of concern for its long-term clinical usage in particle therapy. To estimate the effects on OpenPET which we are developing for in-beam PET based on GSOZ (Zi doped Gd2SiO5), we measured the induced radioactivity of GSO activated by secondary fragments in a water phantom irradiation by a (12)C beam with an energy of 290 MeV u(-1). Radioisotopes of Na, Ce, Eu, Gd, Nd, Pm and Tb including positron emitters were observed in the gamma ray spectra of the activated GSO with a high purity Ge detector and their absolute radioactivities were calculated. We used the Monte Carlo simulation platform, Geant4 in which the observed radioactivity was assigned to the scintillators of a precisely reproduced OpenPET and the single and coincidence rates immediately after one treatment and after one-year usage were estimated for the most severe conditions. Comparing the highest coincidence rate originating from the activated scintillators (background) and the expected coincidence rate from an imaging object (signal), we determined the expected signal-to-noise ratio to be more than 7 within 3 min and more than 10 within 1 min from the scan start time. We concluded the effects of scintillator activation and their accumulation on the OpenPET imaging were small and clinical long-term usage of the OpenPET was feasible.

We have developed a new three-dimensional (3D) position sensitive radiation detector, called the X'tal cube. The X'tal cube is composed of a scintillation crystal block and a number of multi-pixel photon counters (MPPCs) which are coupled on all six sides of the block. The block is segmented into cubes and no reflector is used between the segments. Scintillation light originating in a crystal segment accordingly propagates three-dimensionally along the alignment of the crystal segments and is efficiently detected by the MPPCs. The X'tal cube could be used as the detector element of a PET system, for instance. We constructed two prototypes of the X'tal cube and evaluated their performance using gamma-ray sources. The crystal block of each prototype is composed of a 3D array of Lu2xGd2(1-x)SiO5: Ce (LGSO, x = 0.9) crystal segments. Each crystal volume is 3.0 mm x 3.0 mm x 3.0 mm. MPPCs of a 3.0 mm x 3.0 mm active area are coupled to each surface of the crystal block. In this work, we show that all crystal segments are identified by a simple Anger-type calculation performed on the MPPC signals for both prototypes. The X'tal cube provides high spatial resolution in three dimensions regardless of the incident angle of the radiation.

The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings. In this paper, we developed the first small prototype to show a proof-of-concept of OpenPET imaging. Two detector rings of 110 mm diameter and 42 mm axial length were placed with a gap of 42 mm. The basic imaging performance was confirmed through phantom studies; the open imaging was realized at the cost of slight loss of axial resolution and 24% loss of sensitivity. For a proof-of-concept of PET image-guided radiation therapy, we carried out the in-beam tests with (11)C radioactive beam irradiation in the heavy ion medical accelerator in Chiba to visualize in situ distribution of primary particles stopped in a phantom. We showed that PET images corresponding to dose distribution were obtained. For an initial proof-of-concept of real-time multimodal imaging, we measured a tumor-inoculated mouse with (18)F-FDG, and an optical image of the mouse body surface was taken during the PET measurement by inserting a digital camera in the ring gap. We confirmed that the tumor in the gap was clearly visualized. The result also showed the extension effect of an axial field-of-view (FOV); a large axial FOV of 126 mm was obtained with the detectors that originally covered only an 84 mm axial FOV. In conclusion, our initial imaging studies showed promising performance of the OpenPET.

A conventional PET scanner has a 15–25 cm axial field-of-view (FOV) and images a whole body using about six bed positions. An OpenPET geometry can extend the axial FOV with a limited number of detectors. The entire whole-body PET scanner must be able to process a large amount of data effectively. In this work, we study feasibility of the fully 3D entire whole-body PET scanner using the GATE simulation. The OpenPET has 12 block detector rings with the ring diameter of 840 mm and each block detector ring consists of 48 depth-of-interaction (DOI) detectors. The OpenPET has the axial length of 895.95 mm with five parts of 58.95 mm open gaps. The OpenPET has higher single data loss than a conventional PET scanner at grouping circuits. NECR of the OpenPET decreases by single data loss. But single data loss is mitigated by separating the axially arranged detector into two parts. Also, multiple coincidences are found to be important for the entire whole-body PET scanner. The entire whole-body PET scanner with the OpenPET geometry promises to provide a large axial FOV with the open space and to have sufficient performance values. But single data loss at the grouping circuits and multiple coincidences are limited to the peak noise equivalent count rate (NECR) for the entire whole-body PET scanner.

Avalanche photodiodes (APDs) are being used as photodetectors in positron emission tomography (PET) because they have many advantages over photomultipliers (PMTs) typically used in PET detectors. We have developed a PET detector that consists of a multi-pixel APD and a 6×6×4 array of 1.46×1.46 mm 2 ×4.5 m LYSO crystals for a small animal PET scanner. The detector can identify four-layer depth of interaction (DOI) with a position-sensitive APD coupled to the backside of a crystal array by just an optimized reflector arrangement. Since scintillation lights are shared among many pixels by the method, weaker signals in APD pixels far from the interacting crystals are affected by noise. To evaluate the performance of the four-layer DOI detector with the APD and the influence of electrical noise on our method, we constructed a prototype DOI detector and tested its performance. We found, except for crystal elements on the edge of the crystal array, all crystal elements could be identified from the 2D position histogram. An energy resolution of 16.9% was obtained for the whole crystal array of the APD detector. The results of noise dependence of detector performances indicated that the DOI detector using the APD could achieve sufficient performance even when using application-specific integrated circuits.

We have proposed an ldquoOpenPETrdquo geometry, which consists of two axially separated detector rings, each of axial length W. A long and continuous field-of-view (FOV) including a 360-degree open gap G between two detector rings can be imaged through iterative image reconstruction. In addition to providing stress-less PET scanning and simultaneous PET/CT, the OpenPET is expected to lead to realization of in-beam PET. The OpenPET also extends the axial FOV with a limited number of detectors. However, the axial FOV is limited to 3 W because the maximum limit of G to obtain the axially continuous FOV is W. In this paper, therefore, we propose an alternative geometry to extend axial FOV even more without increasing the number of detectors. The proposed geometry consists of multiple detector rings separated by multiple gaps. By optimizing each width of the gaps based on a new concept of multiplex geometry of the OpenPET, the axial FOV can be theoretically increased to an unlimited extent without increasing the number of detectors. The multiplex OpenPET geometry was compared with the standard OpenPET and the conventional PET using numerical simulation data and experimental data. The results show that similar reconstructed images are obtained by three geometries. The proposed geometry is expected to help realize an affordable entire body PET scanner that enables whole body dynamic imaging.

In a conventional PET scanner, coincidence events are limited by the energy window for detection of photoelectric events. In contrast, Compton scatter events occur both in a patient and in detector crystals. Scatter events within the patient cause scatter coincidences, but scatter events within crystals have useful information for an activity distribution. The PET scanner with an extended energy window has higher sensitivity but higher scatter fraction than the PET scanner with a default energy window. In this work, we develop a scatter reduction method using a depth-of-interaction (DOI) detector. The DOI detector can make an upper layer act as an absorber of patient scatter, thus shielding a lower layer. Therefore, Compton scatter events with an interaction at the lower layer mainly have more crystal scatter events than patient scatter events. Our method proposes a different energy window for each layer. The energy window of the upper layer is limited to the region of photopeak events, the same as done for the conventional PET scanner. The energy window of the lower layer is extended to the region of crystal scatter events and photopeak events. We tested our proposed technique for a whole-body PET scanner using GATE simulations. Simulation results show the DOI-PET scanner can provide more true events while keeping a low scatter fraction using our scatter reduction method. We conclude this method is promising for obtaining high image quality using DOI information and energy information.

Depth-of-interaction (DOI) detectors are considered to be advantageous for time-of-flight positron emission tomography (TOF-PET) because they can correct timing errors arising in the scintillation crystals due to a propagation speed difference between annihilation radiation and scintillation photons. We experimentally measured this timing error, using our four-layer DOI encoding method. The upper layers exhibited the larger timing delays due to the longer path lengths after conversion from annihilation radiation into scintillation photons that traveled by zigzag paths at a speed decreased by a factor of the refractive index (n). The maximum timing delay between the uppermost and the lowermost layers was evaluated as 164 ps when n=1.47. A TOF error correction was demonstrated to improve the timing resolution of the four-layer DOI detector by 10.3%, which would increase the effective sensitivity of the scanner by about 12% comparison with a non-DOI TOF-PET scanner. This is the first step towards combining these two important fields in PET instrumentation, namely DOI and TOF, for the purpose of achieving a higher sensitivity as well as a more uniform spatial resolution.

The long patient port of a PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we proposed an 'open PET' geometry, which consists of two axially separated detector rings. A long and continuous field-of-view (FOV) including a 360 degrees opened gap between two detector rings can be imaged enabling a fully 3D image reconstruction of all the possible lines-of-response. The open PET will become practical if iterative image reconstruction methods are applied even though image reconstruction of the open PET is analytically an incomplete problem. First we implemented a 'masked' 3D ordered subset expectation maximization (OS-EM) in which the system matrix was obtained from a long 'gapless' scanner by applying a mask to detectors corresponding to the open space. Next, in order to evaluate imaging performance of the proposed open PET geometry, we simulated a dual HR+ scanner (ring diameter of D = 827 mm, axial length of W = 154 mm x 2) separated by a variable gap. The gap W was the maximum limit to have axially continuous FOV of 3W though the maximum diameter of FOV at the central slice was limited to D/2. Artifacts, observed on both sides of the open space when the gap exceeded W, were effectively reduced by inserting detectors partially into unnecessary open spaces. We also tested the open PET geometry using experimental data obtained by the jPET-D4. The jPET-D4 is a prototype brain scanner, which has 5 rings of 24 detector blocks. We simulated the open jPET-D4 with a gap of 66 mm by eliminating 1 block-ring from experimental data. Although some artifacts were seen at both ends of the opened gap, very similar images were obtained with and without the gap. The proposed open PET geometry is expected to lead to realization of in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting the beams pass through the gap. The proposed open PET geometry will also allow simultaneous PET/CT measurements of the same PET FOV as the CT FOV, in contrast to the conventional PET/CT where each FOV is separated by several tens of centimeters.

We are developing a small animal PET scanner, “jPET-RD” to achieve high sensitivity as well as high spatial resolution by using four-layer depth-of-interaction (DOI) detectors. The jPET-RD is designed with two detector rings. Each detector ring is composed of six DOI detectors arranged hexagonally. The diameter of the field-of-view (FOV) is 8.8 cm, which is smaller than typical small animal PET scanners on the market now. Each detector module consists of a crystal block and a 256-channel flat panel position-sensitive photomultiplier tube. The crystal block, consisting of 32×32×4 crystal (4096 crystals, each 1.46 mm×1.46 mm×4.5 mm) and a reflector, is mounted on the 256ch FP-PMT. In this study, we evaluated the spatial resolution of reconstructed images with the evaluation system of two four-layer DOI detectors which consist of 32×32×4 LYSO (Lu: 98%, Y: 2%) crystals coupled on the 256ch FP-PMT by using RTV rubber. The spatial resolution of 1.5 mm was obtained at the center of the FOV by the filtered back projection. The spatial resolution, better than 2 mm in the whole FOV, was also achieved with DOI while the spatial resolution without DOI was degraded to 3.3 mm.

Depth-of-interaction (DOI) measurement is essential for PET to reduce the parallax error, which results in improved spatial resolution while maintaining the sensitivity. One of the ultimate goals for improvement of DOI detectors is achieving their isotopic spatial resolution. For this purpose, we have developed an isotropic 3D position-sensitive PET detector named the X'tal cube detector. The X'tal cube detector is composed of a scintillation crystal block segmented into cubes and arrays of multi pixel photon detectors (MPPCs) which cover all surfaces of the crystal block. Segment responses are made as result of the 3D Anger-type calculation and are drawn in a 3D position histogram. In our previous study, although we achieved (0.8 mm)3 isotropic spatial resolution in most of the detector, we were not able to discriminate the outermost responses and the second outermost responses. This was because of a small mismatch of the size between the MPPC array (13 × 13 mm2) and crystal block (13.6 × 13.6 × 13.6 mm3), a dead space, 0.5 mm width gap, existed at the edge. In this study, we improved both the crystal identification performance at the edge and got 10% higher volume resolution than that of the previous detector without any degradation. To reduce the gap between the MPPC array and the scintillator crystal block, the scintillation crystal block used was a 13.1 × 13.1 × 13.1 mm3 monolithic LYSO having 3D segmentation in a 17 × 17 × 17 array of 0.77 × 0.77 × 0.77 mm3 grids fabricated by laser processing. Each face of the crystal block was coupled to a monolithic 4 × 4 array of MPPCs of 13 × 13 mm2. We irradiated 511 keV annihilation gamma rays from two 22Na sources to the detector and acquired the data with NIM and CAMAC. According to the profile of the response, 17 segments in a row were clearly identified. The peak-to-valley ratio was 4.4 ± 0.6 at the central.

Previously, we proposed a new depth of interaction (DOI) encoding method and proved that it worked successfully with four-layered Gd/sub 2/SiO/sub 5/ crystals for a small animal positron emission tomography (PET) detector. We are now planning to develop a small animal PET scanner, jPET-RD (for rodents with DOI detectors), which has both high resolution and high sensitivity by the use of a DOI detector with a 32/spl times/32/spl times/4 crystal array. The scintillator for the detector will be Lu/sub 2(1-x)/Y/sub 2x/SiO/sub 5/ (LYSO). In this work, we evaluated performance of a DOI detector composed of four layers of a 12/spl times/12 LYSO (Lu: 98%, Y: 2%) crystal array by irradiating 511 keV gamma rays uniformly. The new encoding method was used for crystal identification. The size of each crystal was 1.46 mm/spl times/1.46 mm/spl times/4.5 mm. The crystal block was coupled to a 256-channel flat panel position sensitive photomultiplier tube, which has 16/spl times/16 multi anodes at intervals of 3.04 mm. As we expected, all crystals are expressed on a single two-dimensional position histogram without overlapping. Energy resolution of all events is 21.8% and time resolution of all events is 0.69 ns in FWHM. When layers are counted from the top, the energy resolutions of the first, second, third, and fourth layer events are 11.6%, 12.3%, 13.3%, and 19.1% and the time resolutions are 0.60ns, 0.59ns, 0.60ns, and 0.66ns, respectively.

We found that Bi/sub 4/Ge/sub 3/ O/sub 12/ (BGO) scintillator can be elements of a four-layer depth of interaction (DOI) detector and it was proved with a 12/spl times/12/spl times/4 array of BGO crystals in dimensions of 2.9 mm/spl times/2.9 mm/spl times/7.5 mm coupled to a 256-channel flat panel position sensitive photomultiplier tube. Appropriate reflector insertion in the array makes all crystal identification possible on one position histogram. Despite the large refractivity and small light output of BGO, the four-layer BGO detector showed no significant variation in the full energy peaks among all crystal elements. When no optical grease was used in the construction of the BGO DOI-block and irradiated with gamma-rays from /sup 137/Cs, a top layer crystal has 80% of light output relative to the bottom layer. The obtained two-dimensional position histogram by the irradiation was clear enough to allow identification of the crystals of interaction. Profiles of the histogram show peak-to-valley ratio of 1.9:1 for the top layer crystals and larger ratio for other layer crystals in the experiment.

In a PET scanner, a line of response is drawn between the two scintillators detecting the pair of annihilation radiation. However, the non-collinearity of the two photons limits the special resolution. The slight deviation from 180 degrees is known to be 0.47 degrees in the full width half maximum (FWHM) for pure water; however, the value may be changed by chemical and physical conditions for the positrons in vivo. The aim of this study is an experimental determination of the angular correlation of the annihilation radiation from a human body. \nUsually, the angle is measured by using plural position sensitive detectors such as gamma-cameras; however, this method cannot be used when the radiation source is specially distributed and the acquisition time is limited. The authors overcame this problem by measuring the energy distribution instead of the angular distribution; the two distributions can be converted each other by the conservation laws of momentum and energy. \nThe photopeak due to in vivo [18F]-FDG was measured by using a germanium detector with the energy resolution of 1.17 keV at 511 keV (0.23%), and the Gaussian distribution was determined to be 2.68 keV in FWHM. Therefore, the energy deviation due to the momentum of electron-positron pair before annihilation was calculated to be 2.41 keV in FWHM. This value is corresponding to 0.54 degree in the angular distribution and about 15% larger than that had obtained for pure water. \nBy the convolution of the annihilation radiation non-collinearity into the positron range, the physical limit of spatial resolution of FDG-PET with ideal radiation detectors can be estimated: 0.5 mm for a small animal PET with a 10-cm diameter, 1.2 mm for a human brain PET with a 40-cm diameter, and 2.1 mm for a human whole-body PET with a 80-cm diameter., World Congress on Medical Physics and Biomedical Engineering 2006

PET detectors based on light sharing, which can obtain spatial resolution smaller than the photo-detector size with a simple calculation to estimate the center of gravity, have often been studied, but the difficulty in identifying inter-crystal scattering (ICS) has not been resolved yet. In particular, ICS identification is strongly required to make full use of a minutely segmented crystal block such as used in the X'tal cube. The maximum likelihood (ML) method that calculates the likelihood function with response functions of photo-detector signals is suitable for the X'tal cube. However, the method does not take ICS into account. Therefore, in this paper, we developed a new method taking ICS into account for the X'tal cube detector. The proposed method was based on the ML method, and unknown positions were expanded to two to estimate scattering position and absorbed position for the ICS event. We evaluated performance of the proposed method using simulated data. We simulated the X'tal cube composed of the 6 × 6 × 6 array of (3 mm)3 LGSO crystals. Each surface of the crystal block was covered with 4 × 4 MPPCs, each with a 3 × 3 mm2 active area. As the results, the proposed method promises high ICS event identification, but crystal identification performance of the proposed method is almost equal to previous methods.

The OpenPET is our original idea that realizes the world's first open-type 3D PET scanner for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Even with a full-ring geometry, the OpenPET has an open gap between its 2 detector rings through which the treatment beam passes. Following our initial 2008 proposal, we developed a small prototype in 2010 to show a proof-of-concept. Now, we report the development of a prototype whole-body OpenPET. The key technology which enabled the OpenPET realization is our original, 4-layered depth-of-interaction detector. In order to measure a radiation from the limited activity produced though fragmentation reactions, Zr-doped GSO (GSOZ), which contains less natural radioactivity, was chosen for the scintillators instead of Lu-based scintillators although timing performance was compromised. In order to compensate for the limited light yield, on the other hand, we used 64-channel flat-panel PMTs with a super-bialkali photocathode, which had a 30% higher quantum efficiency. In order to enable stable in-beam PET measurement even under high background radiations, voltage divider circuits were designed to provide 5 times higher linearity. Additionally, to avoid severe radiation damage, we did not use gain control ASICs in the front-end circuits, and position analyzer circuits were placed with a 15-m cable extension. The prototype consists of 2 detector rings, and each detector ring has 2 sub-rings of 40 detectors. Each detector consists of 16 × 16 × 4 array of GSOZ (2.8 × 2.8 × 7.5mm3). The portable gantry has a compact design; each detector ring has a 940 mm outer diameter and 171 mm thickness for the detector inner bore of 640 mm diameter and 113 mm thickness. The system was tested with a carbon beam irradiation at a clinical intensity. Phantom images were obtained by applying a GPGPU-based, list mode iterative reconstruction algorithm with geometrical detector response modeling.

Depth-of-interaction measurement is essential for PET to reduce the parallax error, resulting in improved spatial resolution with maintaining sensitivity. One of ultimate goals of improvement in DOI detectors is achieving isotopic spatial resolution. For this purpose, we have developed an isotropic 3D position sensitive PET detector named X'tal cube. The X'tal cube is composed of a scintillation crystal block segmented into cubes. Arrays of multi pixel photon detectors (MPPCs) cover all surfaces of the crystal block. In our previous study, we achieved 1.0 mm isotropic spatial resolution. In this study, we finally succeeded to develop the X'tal cube with (0.8 mm)3 isotropic segments. The scintillation crystal block was a 13.6 × 13.6 × 13.6 mm3 monolithic LYSO. 3D segmentation of 17 × 17 × 17 array of 0.8 mm × 0.8 mm × 0.8 mm grids was made by the laser processing. We used 96 MPPCs (S12642-0404PB-50(X)). For each face of the crystal block, monolithic 4 × 4 array of MPPCs with about 0.2 mm gaps was coupled. The position histograms were made by the 3D Anger-type calculation from uniform irradiation data, which showed good identification of each segments. We obtained 9.0% energy resolution for the central part of the crystal block. In conclusion, we succeeded to develop a prototype PET detector with the world's finest segments of (0.8 mm)3 grids.

A 256 channel flat panel position sensitive photomultiplier tube (256ch FP-PMT) was evaluated for application to a depth of interaction (DOI) PET detector composed of small crystal elements. The 256ch FP-PMT has 52 /spl times/ 52 mm opening area with 89% useful area. Its 256 anodes are placed at a 3.04 mm interval. To estimate the FP-PMT performance, light spread functions of anodes at central and peripheral regions of a prototype 256ch FP-PMT were measured with a Gd/sub 2/SiO/sub 5/ (GSO) crystal sized 1.42 /spl times/ 1.42 /spl times/ 4.5 mm. The crystal cross section area is about one quarter of one anode area of the FP-PMT. It was found that light spread functions of peripheral anodes were similar to the functions of central anodes and also the functions measured with a 2.9 /spl times/ 2.9 /spl times/ 7.5 mm GSO crystal which has almost the same dimensions of cross-section as the anode area. Transit time fluctuation among the 256ch FP-PMT anodes was estimated by measuring time resolutions of a LuYSiO/sub 5/ (LYSO) crystal at some central and peripheral positions. The LYSO crystal has the same dimensions as the smaller GSO crystal. BaF/sub 2/ was used for a reference detector. The obtained distributions show about the same time resolution between the central and peripheral positions and the full width at half maximum is (366 /spl plusmn/ 15) ps. The transit time fluctuation is /spl plusmn/ 106 ps. Two-dimensional position histograms of a 32 /spl times/ 32 GSO crystal array and two layers of 9 /spl times/ 9 GSO crystal arrays are obtained by uniform gamma-ray irradiation. The crystal dimensions are 1.42 /spl times/ 1.42 /spl times/ 4.5 mm so that the 32 /spl times/ 32 array covers all the useful area and indicates variation of crystal identification performance over the useful area. The 9 /spl times/ 9 /spl times/ 2 array is coupled to peripheral region. The resultant histograms confirm that a 256ch FP-PMT has enough capability for identifying crystals of this size even on the periphery and in DOI arrangement.

This paper describes a depth-of-interaction recognition method that can be applied to radiation detectors for positron emission tomography (PET). The proposed method uses optical filters to differentiate wavelength distribution of scintillation photons depending on crystal depth positions. Then the crystal of interaction can be identified with a multi-anode photo-detector whose sensitive surface is partly covered with optical filters. A validation experiment was performed with a position sensitive photomultiplier. As a result, it was proved that the crystal depth of interaction was clearly identified for photo-absorption events.

We are now planning to develop a positron emission tomograph dedicated to small animals, such as rats and mice, which meets the demand for higher sensitivity. For the positron emission tomography (PET), we newly designed a four-layer depth of interaction (DOI) detector, which is constructed of all the same crystal elements. In this DOI detector, appropriate insertion of the reflector controls the behavior of scintillation photons by inserting the reflectors between crystal elements so that the DOI information of four layers can be extracted from one two-dimensional (2-D) position histogram made by Anger-type calculation. As a preliminary experiment, we measured crystal identification performance of the DOI detector, which consists of four layers of a 16 /spl times/ 16 crystal array using Gd/sub 2/SiO/sub 5/ crystals with Ce concentration of 0.5 mol %. Each crystal is in dimensions of 1.42 mm /spl times/ 1.42 mm /spl times/ 4.5 mm. A crystal block is optically coupled to a 256-channel flat panel position sensitive photomultiplier tube whose opening area is 52.0 mm /spl times/ 52.0 mm. We obtained sufficient positioning performance for this four-layer DOI detector on the 2-D position histogram. We concluded that it would be a promising device to realize a small animal PET scanner with high sensitivity and high resolution.

Conventionally, positron emission tomograph (PET) scanners use scintillators which have a high effective atomic number. Recently, novel scintillators like LaBr 3 have been developed which have excellent timing and energy resolutions. LaBr 3 has a high performance for PET scanner use, but its effective atomic number is lower than that of lutetium oxyorthosilicate (LSO). As an alternative, we have developed a scatter reduction method using depth-of-interaction (DOI) information and energy information to increase the sensitivity. The sensitivity of the PET scanner with LaBr 3 can be improved using the DOI-dependent extended-energy window (DEEW) method. In this work, our method is applied to the whole-body LSO/LaBr 3 PET scanner using the GATE simulation toolkit. Simulation results show the number of true coincidences can be increased while minimizing the scatter and random coincidences by using the DEEW method. Noise equivalent count rate (NECR) can be improved by 20–70% for the whole-body DOI-PET scanner. Sensitivity of the PET scanner with a scintillator of low-effective atomic number can be improved by the DEEW method.

We are developing the world's first, open-type 3D PET scanner “OpenPET” for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Following our first idea of a dual-ring Open PET (DROP), we proposed our second-generation geometry, single-ring OpenPET (SROP), which is more efficient than DROP in terms of manufacturing cost and sensitivity. In this paper, we have developed a SROP prototype based on a novel detector arrangement, in which block detectors originally forming a conventional PET scanner were axially shifted little by little. Sixteen detector units each of which consists of two depth-of-interaction detectors are arranged to form a perfect circle, 25cm in diameter. Detector units have an axial shifting mechanism so that they can be transformed into the SROP; adding this mechanism to the units allows us to use the scanner as a conventional (i.e., non-open) PET when in-beam PET measurements are not required. After confirming its basic imaging performance using a phantom filled with 18F solution, we carried out in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC). In addition to the usual carbon (12C) beam, we applied RI beams of 11C and 10C. Stopping positions of primary particles were directly imaged with the RI beam irradiation, while the stopping position distribution of secondary particles was imaged with the 12C beam irradiation. Phantom study results with pencil beam irradiation of about 2.5Gy showed that beam stopping positions can be measured with the precision better than 2mm with the 11C beam irradiation followed by 20 min PET measurement. With the 10C beam, PET measurement time could be reduced to 1/10 while still maintaining the precision. For both 11C and 10C, there is room for further reduction of PET measurement time.

The X’tal cube is a next-generation DOI detector for PET that we are developing to offer higher resolution and higher sensitivity than is available with present detectors. It is constructed from a cubic monolithic scintillation crystal and silicon photomultipliers which are coupled on various positions of the six surfaces of the cube. A laser-processing technique is applied to produce 3D optical boundaries composed of micro-cracks inside the monolithic scintillator crystal. The current configuration is based on an empirical trial of a laser-processed boundary. There is room to improve the spatial resolution by optimizing the setting of the laser-processed boundary. In fact, the laser-processing technique has high freedom in setting the parameters of the boundary such as size, pitch, and angle. Computer simulation can effectively optimize such parameters. In this study, to design optical characteristics properly for the laser-processed crystal, we developed a Monte Carlo simulator which can model arbitrary arrangements of laser-processed optical boundaries (LPBs). The optical characteristics of the LPBs were measured by use of a setup with a laser and a photo-diode, and then modeled in the simulator. The accuracy of the simulator was confirmed by comparison of position histograms obtained from the simulation and from experiments with a prototype detector composed of a cubic LYSO monolithic crystal with 6 × 6 × 6 segments and multi-pixel photon counters. Furthermore, the simulator was accelerated by parallel computing with general-purpose computing on a graphics processing unit. The calculation speed was about 400 times faster than that with a CPU.

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed an open-type PET scanner, OpenPET. The original OpenPET had a physically opened field of view (FOV) between two detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wide axial FOV including the gap. This geometry was not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we have proposed a new single-ring OpenPET (SROP) geometry which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the DROP. The proposed geometry was a cylinder shape with its ends cut at a slant, in which the shape of each cut end became an ellipse. In this work, we developed and evaluated a small prototype of the SROP geometry for proof-of-concept. The SROP prototype was designed with 2 ellipse-shaped detector rings of 16 DOI detectors. The DOI detectors consisted of 1024 GSOZ crystals which were arranged in 4 layers of 16 X 16 arrays, coupled to a 64-ch FP-PMT. Each ellipse-shaped detector ring had a major axis of 281.6 mm and a minor axis of 207.5 mm. For the open mode, the rings were placed at a 45-deg slant from the axial direction and for the non-open mode (used as a reference) they were at 90 deg from the axial direction with no gap. The system sensitivity and average spatial resolution were measured from a 22Na point source and were 5.0 % and 2.7 mm, respectively, for the open mode. Comparison between the open mode and the non-open mode showed no remarkable difference in terms of the sensitivity distribution and spatial resolution performance. Therefore, we concluded that the SROP geometry has a good potential as an open geometry especially suitable for in-beam imaging., 2012IEEE NSS&MIC

The X'tal cube is our original PET detector, which is being developed to achieve isotropic 3D positioning detectability. The X'tal cube is based on a 3D segmented crystal block for which all surfaces are covered with photo-detectors. In the last year, instead of our initial approach of gluing segmented pieces of crystals, we successfully constructed a crystal block segmented by laser processing, and we developed the X'tal cube with the laser-processed 3D grids of 2 mm. In the present work, we developed the X'tal cube with 3D grids of 1 mm and a new one-pair prototype system which we used to simulate a ring-type scanner. The one-pair prototype system consisted of two X'tal cubes, two rotating stages, and a 192-channel data acquisition system. Each X'tal cube consisted of the LYSO cubic crystal block of (18 mm)3 in which the 3D grids of 1 mm pitch were fabricated by internal laser processing. The 4 X 4 arrays of multi pixel photon counters were optically coupled to each surface of the crystal block. The detector positions were automatically controlled to simulate a ring-type PET with a 14.6 cm diameter. Data were collected for all assumed detector positions and then a sinogram was obtained. The data were reconstructed using filtered backprojection. We successfully obtained uniform spatial resolution of 1.3 mm full width at half maximum (FWHM) averaged over the FOV. By applying deconvolution with the assumption that the source distribution can be represented as a Gaussian function with 1 mm FWHM, we estimated the averaged spatial resolution of the system as 0.86 mm. In conclusion, we confirmed the potential of the X'tal cube for uniform and high resolution imaging., 2012IEEE NSS&MIC

We are developing a novel PET detector with 3D isotropic resolution, the X'tal cube. Our original idea was to cover all 6 surfaces of a segmented crystal block with multi-pixel photon counters (MPPCs) for effective scintillation photon acquisition. On the other hand, modification toward practical use is important in terms of assembling efficiency and production costs. In addition, when the X'tal cubes are arranged to form a PET scanner, insensitive inter-detector gaps made by the MPPC arrays should not be too wide or better yet, they should be removed. Therefore, reducing the number of faces to be connected to the MPPCs has become our top priority. In this paper, we evaluated the effect of reducing the number of MPPCs on positioning accuracy through numerical simulations. We optimized the X'tal cube in terms of the gap material of crystal segments and the number of MPPC connection faces. We simulated the X'tal cube with (3.0 mm)3 crystal segments; the crystal block was composed of 6 × 6 × 6 array of (3.0 mm)3 LGSO crystals. Each surface of the crystal block was covered with a 4 × 4 array of MPPCs, each of which had a 3.0 × 3.0 mm2 active area. Outer surfaces of the crystal block, except for the partitions that the MPPCs were arranged on, were covered with reflectors. For material between crystal elements, we compared optical glue and air gap. The air gap showed better crystal identification performance than the optical glue, although good crystal identification performance was obtained even with optical glue for the 6-face MPPC connection. We showed that the number of MPPC connection faces could be reduced to two when the gap material was air.

DOI-PET detectors can provide good imaging performance even in the peripheral region of the PET FOV. In the development of DOI-PET detectors, one of the important steps toward their practical use is an evaluation of imaging performance such as the spatial resolution measured by using a point source and a one-pair system which simulates actual PET geometries. Because the DOI-PET detectors have an extended FOV compared with conventional detectors, evaluation including the region from the center to the periphery close to the detector ring is required to show their advanced performance for uniform spatial resolution. In this study, we proposed a one-pair system that can simulate an arbitrary ring diameter and acquire projection data efficiently by cutting unnecessary combinations according to the position of the point source. When we compared results by the full measurement scheme, the proposed system and our measuring scheme could significantly reduce the measurement time especially for a large ring size such as used in brain PET scanners. We used the system to evaluate the X'tal cube PET detector with a 2 mm crystal array arranged in a simulated PET geometry with the ring radius of 73 mm and 18 detector blocks. The results showed that uniform spatial resolution was achieved even in the peripheral region and measurements were obtained semi-automatically in a short time., 2012 Nuclear Science Symposium, Medical Imaging Conference & Workshop on Room-Temperature Semiconductor X-Ray and Gamma-Ray Detectors

X'tal cube is a next generation DOI detector for PET with high resolution and sensitivity that we are developing. It is constructed from a cubic scintillation crystal and silicon photomultipliers (SiPMs) coupled on various positions of six surfaces of the cube. A laser processing technique is applied to produce 3D optical boundaries composed of micro-cracks inside a monolithic scintillator crystal. In this paper, in order to design optical characteristics properly, we developed a Monte Carlo simulator which is able to arrange laser-processed optical boundaries (LPB) arbitrarily. Optical characteristics of LPB were measured using a set of laser and photo-diode and then modeled for simulator. Effectiveness of the simulator was confirmed from a comparison with the experiment using a cubic LYSO monolithic crystal with 6 × 6 × 6 segments. Furthermore, the simulator was accelerated by parallel computing with a general purpose GPU (GPGPU). Calculation speed with GPGPU was 350 to 430 times faster than that with only CPU.

We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap is suitable for in-beam PET. We have developed the small prototype of the OpenPET especially for a proof of concept of in-beam imaging. This paper presents an overview of the main features implemented in this prototype. We also evaluated the detector performance. This prototype was designed with 2 detector rings having 8 depth-of-interaction detectors. Each detector consisted of 784 Lu(2x)Gd(2(1-x))SiO₅:Ce (LGSO) which were arranged in a 4-layer design, coupled to a position-sensitive photomultiplier tube (PS-PMT). The size of the LGSO array was smaller than the sensitive area of the PS-PMT, so that we could obtain sufficient LGSO identification. Peripheral LGSOs near the open gap directly detect the gamma rays on the side face in the OpenPET geometry. Output signals of two detectors stacked axially were projected onto one 2-dimensional position histogram for reduction of the scale of a coincidence processor. Front-end circuits were separated from the detector head by 1.2-m coaxial cables for the protection of electronic circuits from radiation damage. The detectors had sufficient crystal identification capability. Cross talk between the combined two detectors could be ignored. The timing and energy resolutions were 3.0 ns and 14%, respectively. The coincidence window was set 20 ns, because the timing histogram showed that not only the main peak, but also two small shifted peaks were caused by the coaxial cable. However, the detector offers the promise of sufficient performance, because random coincidences are at a nearly undetectable level for in-beam PET experiments.

OpenPET which consists of two detector rings separated axially is suitable for in-beam PET in heavy ion therapy. Although the primary ion beam do not enter into the PET detector in the OpenPET, light fragment ions from a target can enter into the detectors and could affect the detector performance under the actual in-beam condition. Previously, we demonstrated that the OpenPET detector can be sufficiently operated in off-beam experiments after carbon beam irradiation. In this presentation, we reported the results of in-beam measurements with a prototype OpenPET detector optimized for the in-beam measurement. Experiments were performed in the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Sciences. The OpenPET detector consisted of a 8 × 8 × 4 LGSO crystal array and a 64-channel PS-PMT. The size of the crystals was 2.9mm × 2.9mm × 5.0mm. The energy and intensity of the 12C beam were 290MeV/u and 108 −109 particle per second (pps). This beam condition is similar to the actual treatment condition in HIMAC. Carbon ions entered to a water phantom and all of the primary carbon ions were stopped in the water. The OpenPET detector was positioned 30cm apart from the backside of the water phantom at an angle of 30 degree. A coincidence detector was positioned at the opposite side of the water phantom. As a result, the sufficient crystal identification performance was achieved with slight deterioration of the position histograms.

We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap would be suitable for in-beam PET, which is a method for in situ monitoring of charged particle therapy. In the present work, we developed the first prototype of the OpenPET, especially for in-beam experiments using phantoms. The OpenPET prototype was designed with 2 detector rings of 8 detector blocks. Ring diameter was 110 mm and the axial FOV was 126 mm. Distance between detector rings is 42 mm and could be controlled. The DOI detector consisted of 784 LGSO crystals which were arranged in a 4-layer design of 14 × 14 arrays, coupled to a 64-ch PS-PMT. Each crystal element was 2.9 × 2.9 × 5 mm3. After 64-channel gain correction of the PS-PMT, output signals of two DOI detectors stacked axially were projected on one 2-dimensional position histogram by an Anger calculation. The system sensitivities were measured from a 22Na point source. The system sensitivity was 5.5 %. A count rate performance test was done that utilized a cylinder phantom (4 cm diameter, 8 cm length). Maximum count rate of prompt was 28 kcps at 3.5 MBq. The OpenPET prototype has the promise of high sensitivity and sufficient performance for testing in in-beam experiments.

We have devised a standardization method for a sealed point source that can be applied to nuclides such as 22Na that emit a positron and at least one gamma ray. This method is based on coincidence counting of annihilation radiation and gamma rays using a scintillation detector array. In the present study, we performed Monte Carlo simulations to examine this method for various measurement conditions. EGS5 Monte Carlo simulation results confirm that this method is effective for measurements of a 22Na sealed point source using BGO scintillation detector arrays

We are investigating the influences of heavy ion irradiation on the detectors for in-beam OpenPET by using the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National institute of Radiological Sciences (NIRS). "OpenPET" which consists of two detector rings separated axially is suitable for in-beam PET in heavy ion therapy due to the open space between the separated detector rings. All of the annihilation gamma rays from irradiated volumes enter obliquely into the OpenPET detectors. Therefore, detectors which can measure depth of interaction (DOI) are required to reduce parallax error. Previously, we have succeeded in crystal identification using the DOI detector consisting of four-layer crystal blocks and a 256-channel position-sensitive photomultiplier tube by optimization of the reflector arrangement. We plan to adopt DOI detectors using this method as the OpenPET detectors; however, secondary particles entering into this type detector have a possibility of degradation of detector performances. In the experiment described here, the energy and intensity of the 12C beam were 290MeV/u and 1.0E+8 -1.0E+9 particle per second (pps). Carbon ions entered into a water phantom and the influences of irradiation, especially fragment particles, on DOI detectors were evaluated. In this presentation, we report the results of this experiment., 2009 IEEE Nuclear Science Symposium & Medical Imaging Conference

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's 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 Conference

We have proposed a depth of interaction (DOI) PET detector named X'tal cube, in which a number of Multi-Pixel Photon Counters (MPPCs) are coupled on various positions of six surfaces of a segmented scintillation crystal block. There are no reflectors within the block, and the areas among MPPCs on the surface are covered with reflector. The MPPC is thin and light solid-state photo-detector so that it is possible to set the MPPCs on the subject side in the detector ring of a PET scanner and the crystal blocks can be closely placed each other in the PET detector ring. To study the characteristics of the X'tal cube, we constructed a segmented crystal block consisting of six layers of a 6 × 6 crystal array with Lu 2x Gd 2(1−x) SiO 5 : Ce (LGSO, x=0.9) crystals. Each crystal is 3.0 × 3.0 × 3.0 mm3 in dimensions. We examined crystal identification performance for different arrangements of photo-detector elements on the block surfaces. The preliminary experiments showed the possibility of realizing a 3D detector having isotropic resolutions.

A conventional PET scanner has a 15-25 cm axial field-of-view (FOV) and images a whole body using about six bed positions. An OpenPET geometry can extend the axial FOV with a limited number of detectors. The entire whole-body PET scanner must be able to process a huge amount of data, effectively. In this work, we study feasibility of the full 3D entire whole-body PET scanner using the GATE simulation. The OpenPET has 12 block detector rings with the ring diameter of 840 mm and each block detector ring consists of 48 DOI (depth-of-interaction) detectors. Total axial length is 955 mm. Open gap setting of 318 mm provides the continuous axial FOV. The OpenPET has higher loss of single data than a conventional PET scanner at grouping circuits. NECR of the OpenPET decreases by loss of single data. But, loss of single data is improved by separating the axially arranged detector into two parts. Also, multiple coincidences are found to be important for the entire whole-body PET scanner. The entire whole-body PET scanner with the OpenPET geometry promises to provide a large axial FOV with the open space and to have sufficient performance values.

Previously, we developed a 4-layer depth of interaction (DOI) detector. It is composed of four layers of a scintillation crystal array and a position sensitive photomultiplier tube (PS-PMT) and by removing some reflectors between the crystal elements in the array, we control scintillation light distribution on the PS-PMT so that the responses of all crystal elements are discriminated. We also proposed a 2-layer DOI encoding method that uses the crystal elements cut as a triangular prism and proved its capability. Combining these two methods, here we propose an 8-layer DOI encoding method. The method makes it possible that the 8-layer DOI detector is consisted of same scintUlation crystal elements, that is to say, any scintillation crystals. The method was demonstrated with the Lu 2x G d2 (LGSO, x = 0.9) crystals. The obtained results indicated the possibility of the proposed method.

The "OpenPET" geometry, which consists of two axially separated detector rings, has been proposed to image the 360-degree open gap. The open gap is less stressful for patients and suitable for simultaneous PET/CT, entire whole-body PET and in-beam PET. It has been shown that OpenPET geometry can maintain high spatial resolution using a depth-of-interaction (DOI) detector. However, when the PET scanner is used for an extended axial field-of-view (FOV), several characteristics of oblique lines-of-response (LOR), such as decreased sensitivity due to smaller acceptance angle and increased scatter fraction (SF) and attenuation, should be carefully discussed. It is expected that these characteristics are dependent on the gap size and the target object size. In this work, we study sensitivity and count rate characteristics for OpenPET geometry using GATE simulation. We simulate three OpenPET geometries. (The dual scanner has 15-cm open gap and 45-cm axial FOV. The quad scanners have 30-cm open gap and 90-cm axial FOV.) When the gap is extended, the axial FOV is extended at the cost of the decreased sensitivity of the skirts of the main peak. Maximum sensitivity of the dual scanner with the open gap is almost the same as the scanner without the open gap. In the quad scanner, however, maximum sensitivity decreases by 25 % due to the decreased acceptance angles. Noise equivalent count rate (NECR) curves depend on not only the maximum sensitivity but also the target object size, because the extended gap results in the broadened sensitivity profile. For the 100-cm cylinder phantom with 100-cm line source, the loss of the peak NECR is slight for both scanners. For the 100-cm cylinder phantom with local activity, however, NECR of the quad scanner maintain approximately half of NECR without the open gap., 2008 Nuclear Science Symposium, Medical Imaging Conference