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Dynamic positron emission tomography (PET) scans of short-time frames are required to quantitatively estimate the uptake of PET ligands. Because such short-frame scans tend to be noisy, we propose kinetics-induced block matching and 5-D transform domain filtering (KIBM5D) specialized for dynamic PET image denoising. In the proposed algorithm, kinetics-induced block matching (KIBM) and 5-D transform domain filtering are alternately repeated in two cascading stages. In each stage of KIBM5D, all time frames are included in a patch of the KIBM to collect similar patch-wise time activity curves. These similar 4-D patches are then five-dimensionally grouped and transformed to the 5-D spectrum. In the 5-D transform domain, the 5-D spectrum is shrunk by hard thresholding and Wiener filtering in the first and second stage of KIBM5D, respectively. The sparsity of the 5-D spectrum is improved because signals of similar 4-D patches are correlated, while noises of these are uncorrelated. To evaluate the performance of KIBM5D, we used both computer simulation data of a dynamic digital brain phantom using [18F]FDG kinetics, and experimental data of a normal healthy volunteer using [11C]MeQAA, and compared the results of KIBM5D, Gaussian filter (GF), bilateral filter, nonlocal means, block matching and 4-D filtering, and 4-D Gaussian filtering. For simulation data, KIBM5D performed superiorly to the other methods in terms of the peak signal to noise ratio and structural similarity measures, in all time frames. Additionally, KIBM5D generated the best image quality not only in simulations but also with human data. Accordingly, KIBM5D enables the efficient denoising of dynamic PET images.

Various motion correction (MC) algorithms for positron emission tomography (PET) have been proposed to accelerate the diagnostic performance and research in brain activity and neurology. We have incorporated MC system-based optical motion tracking into the brain-dedicated time-of-flight PET scanner. In this study, we evaluate the performance characteristics of the developed PET scanner when performing MC in accordance with the standards and guidelines for the brain PET scanner.We evaluate the spatial resolution, scatter fraction, count rate characteristics, sensitivity, and image quality of PET images. The MC evaluation is measured in terms of the spatial resolution and image quality that affect movement.In the basic performance evaluation, the average spatial resolution by iterative reconstruction was 2.2 mm at 10 mm offset position. The measured peak noise equivalent count rate was 38.0 kcps at 16.7 kBq/mL. The scatter fraction and system sensitivity were 43.9% and 22.4 cps/(Bq/mL), respectively. The image contrast recovery was between 43.2% (10 mm sphere) and 72.0% (37 mm sphere). In the MC performance evaluation, the average spatial resolution was 2.7 mm at 10 mm offset position, when the phantom stage with the point source translates to ± 15 mm along the y-axis. The image contrast recovery was between 34.2 % (10 mm sphere) and 66.8 % (37 mm sphere).The reconstructed images using MC were restored to their nearly identical state as those at rest. Therefore, it is concluded that this scanner can observe more natural brain activity.

Objective. The aim of this study is to evaluate the performance characteristics of a brain positron emission tomography (PET) scanner composed of four-layer independent read-out time-of-flight depth-of-interaction (TOF-DOI) detectors capable of first interaction position (FIP) detection, using Geant4 application for tomographic emission(GATE). This includes the spatial resolution, sensitivity, count rate capability, and reconstructed image quality. Approach. The proposed TOF-DOI PET detector comprises four layers of a 50 × 50 cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) scintillator array with 1 mm pitch size, coupled to a 16 × 16 multi-pixel photon counter array with 3.0 mm × 3.0 mm photosensitive segments. Along the direction distant from the center field-of-view (FOV), the scintillator thickness of the four layers is 2.5, 3, 4, and 6 mm. The four layers were simulated with a 150 ps coincidence time resolution and the independent readout make the FIP detection capable. The spatial resolution and imaging performance were compared among the true-FIP, winner-takes-all (WTA) and front-layer FIP (FL-FIP) methods (FL-FIP selects the interaction position located on the front-most interaction layer in all the interaction layers). The National Electrical Manufacturers Association NU 2-2018 procedure was referred and modified to evaluate the performance of proposed scanner. Main results. In detector evaluation, the intrinsic spatial resolutions were 0.52 and 0.76 mm full width at half-maximum (FWHM) at 0° and 30° incident γ-rays in the first layer pair, respectively. The reconstructed spatial resolution by the filter backprojection (FBP) achieved sub-millimeter FWHM on average over the whole FOV. The maximum true count rate was 207.6 kcps at 15 kBq ml−1 and the noise equivalent count rate (NECR_2R) was 54.7 kcps at 6.0 kBq ml−1. Total sensitivity was 45.2 cps kBq−1 and 48.4 cps kBq−1 at the center and 10 cm off-center FOV, respectively. The TOF and DOI reconstructions significantly improved the image quality in the phantom studies. Moreover, the FL-FIP outperformed the conventional WTA method in terms of the spatial resolution and image quality. Significance. The proposed brain PET scanner could achieve sub-millimeter spatial resolution and high image quality with TOF and DOI reconstruction, which is meaningful to the clinical oncology research. Meanwhile, the comparison among the three positioning methods indicated that the FL-FIP decreased the image degradation caused by Compton scatter more than WTA.

X-ray and gamma-ray photons are widely used for imaging but require a mathematical reconstruction step, known as tomography, to produce cross-sectional images from the measured data. Theoretically, the back-to-back annihilation photons produced by positron–electron annihilation can be directly localized in three-dimensional space using time-of-flight information without tomographic reconstruction; however, this has not yet been demonstrated due to the insufficient timing performance of available radiation detectors. Here we develop techniques based on detecting prompt Cherenkov photons, which, when combined with a convolutional neural network for timing estimation, resulted in an average timing precision of 32 ps, corresponding to a spatial precision of 4.8 mm. We show this is sufficient to produce cross-sectional images of a positron-emitting radionuclide directly from the detected coincident annihilation photons, without using any tomographic reconstruction algorithm. The reconstruction-free imaging demonstrated here directly localizes positron emission and frees the design of an imaging system from the geometric and sampling constraints that are normally present for tomographic reconstruction. Positron emission imaging without tomographic reconstruction is demonstrated. A Cherenkov radiation detector detects gamma rays produced by positron–electron annihilation. The position of a positron source is determined with a precision of 4.8 mm.

A multilayer depth-of-interaction positron emission tomography (DOI-PET) detector with an independent readout structure has a potential advantage as a time-of-flight (TOF)-PET detector. To evaluate the potential TOF capabilities of a multilayer DOI-PET detector, which consists of thin layers of a cerium-doped lutetium-yttrium oxyorthosilicate (LYSO:Ce) scintillator coupled to a multi-pixel photon counter (MPPC) array, we examined the detector's CTR performance via Monte Carlo simulations. We used several types of scintillator structures: a monolithic plate, laser-processing array with 3.2-mm pitch, fine laser-processing array with 1.6-mm pitch, and pixelated array with 3.2-mm pitch, with 2-, 4-, 6-, and 8-mm thickness values. Here, we note that the CTR performance also significantly depends on the timing-detection method, which generates a timing trigger signal for coincidence detection. Thus, we evaluated the CTRs for each scintillator structure by adopting four timing-detection methods: using the total sum signal of MPPC chips (T_sum), the maximum signal in the MPPC chips (Max), the sum signal of a partial number of MPPC chips located at and in the vicinity of the -ray interaction position (P_sum), and the average of the timestamps generated at several MPPC chips (Ave). When using the T_sum for timing detection, the CTR full width at half-maximum (FWHM) values were ~100 ps regardless of the scintillator structure. However, when using the Max signal approach, the CTRs of the monolithic plates, laser-processing arrays, and fine-pitch laser-processing arrays were drastically degraded with increasing thickness. On the other hand, the CTRs of the pixelated arrays exhibited almost no degradation. To improve the CTRs of the monolithic plate and the (fine pitch) laser-processing array that exhibit a large light spread in the scintillator block, we applied the P_sum and Ave methods. The resulting CTRs significantly improved upon using P_sum; however, the Ave approach only worked for thicknesses of >6 mm.

A high-resolution positron emission tomography (PET) scanner, dedicated to brain studies, was developed and its performance was evaluated. A four-layer depth of interaction detector was designed containing five detector units axially lined up per layer board. Each of the detector units consists of a finely segmented (1.2 mm) LYSO scintillator array and an 8 × 8 array of multi-pixel photon counters. Each detector layer has independent front-end and signal processing circuits, and the four detector layers are assembled as a detector module. The new scanner was designed to form a detector ring of 430 mm diameter with 32 detector modules and 168 detector rings with a 1.2 mm pitch. The total crystal number is 655 360. The transaxial and axial field of views (FOVs) are 330 mm in diameter and 201.6 mm, respectively, which are sufficient to measure a whole human brain. The single-event data generated at each detector module were transferred to the data acquisition servers through optical fiber cables. The single-event data from all detector modules were merged and processed to create coincidence event data in on-the-fly software in the data acquisition servers. For image reconstruction, the high-resolution mode (HR-mode) used a 1.2 mm2 crystal segment size and the high-speed mode (HS-mode) used a 4.8 mm2 size by collecting 16 crystal segments of 1.2 mm each to reduce the computational cost. The performance of the brain PET scanner was evaluated. For the intrinsic spatial resolution of the detector module, coincidence response functions of the detector module pair, which faced each other at various angles, were measured by scanning a 0.25 mm diameter 22Na point source. The intrinsic resolutions were obtained with 1.08 mm full width at half-maximum (FWHM) and 1.25 mm FWHM on average at 0 and 22.5 degrees in the first layer pair, respectively. The system spatial resolutions were less than 1.0 mm FWHM throughout the whole FOV, using a list-mode dynamic RAMLA (LM-DRAMA). The system sensitivity was 21.4 cps kBq-1 as measured using an 18F line source aligned with the center of the transaxial FOV. High count rate capability was evaluated using a cylindrical phantom (20 cm diameter × 70 cm length), resulting in 249 kcps in true and 27.9 kcps at 11.9 kBq ml-1 at the peak count in a noise equivalent count rate (NECR_2R). Single-event data acquisition and on-the-fly software coincidence detection performed well, exceeding 25 Mcps and 2.3 Mcps for single and coincidence count rates, respectively. Using phantom studies, we also demonstrated its imaging capabilities by means of a 3D Hoffman brain phantom and an ultra-micro hot-spot phantom. The images obtained were of acceptable quality for high-resolution determination. As clinical and pre-clinical studies, we imaged brains of a human and of small animals.

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 new detector module for positron emission tomography using multi-pixel photon counters (MPPCs) was developed. The detection area is 40 mm × 40 mm using an 8 × 8 MPPC array. The active area of each MPPC is 3 mm × 3 mm. The detector module contains preamplifiers, digital-to-analog converters for setting the bias offset, a 2D resistor network for position encoding, a high-voltage module, a temperature sensor, and embedded microprocessor units. Since the MPPC gain depends on temperature, compensation was performed by varying the bias voltage. Two kinds of flood images of an 8 × 8 array (5-mm pitch) and a 32 × 32 (1.25-mm pitch) array were acquired and examined. The timing resolution and the effect of dark counts were evaluated. With a reference detector consisting of a BaF2 crystal coupled to a PMT, the timing resolution was 354 ps when a single 5 mm × 5 mm × 20 mm LYSO scintillator was coupled to one MPPC channel. However, the time resolution of the array deteriorated because of the increase in dark counts.

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.

A small animal PET scanner using 256-channel PS-PMTs has been designed, constructed, and evaluated. The scanner has twelve detector modules, each of which consists of a double-layer array of LYSO crystals and three PS-PMTs (Hama- matsu R8400-00-M256). In the LYSO crystal block, 32 times 53 crystal elements are optically coupled to 32 times 54 crystal elements with a shift of half the element pitch in the axial direction. The dimension of each crystal element is 1.275 times 2.675 mm2 in cross section and 7 mm in depth. The twelve detector modules are positioned on a 182 mm diameter ring to form 107 detector rings with 1.4 mm pitch. The transaxial FOV is 100 mm in diameter and the axial FOV is 151 mm, which is sufficient to cover the whole body of a mouse. In order to compensate for non-uniform outputs from the multi-anodes of the PS-PMTs, ASICs having 64-channel variable gain amplifiers and summing amplifiers are used in the front-end circuits. The preliminary experimental results are the transaxial resolution of 2.0 mm FWHM in the CFOV, and the axial resolution of 2.8 mm FWHM on the axis of the ring. The absolute coincidence sensitivity is 8.1% for a point source at the CFOV with setting an energy window of 350-750 keV and a timing window of 10 ns. The applicable imaging capability of the scanner was demonstrated by animal studies with a rat.

For developing a small animal Positron Emission Tomography (PET) scanner for whole-body imaging of mice, both structures of detector blocks and signal readout scheme are important factors to achieve sub-millimeter spatial resolution. These specifications are crucial for intrinsic spatial resolution. A sub-millimeter resolution PET detector composed of a 1.2 mm pitch MPPC array with TSV structure one-to-one coupled to LFS scintillator crystals has been constructed and evaluated. To evaluate intrinsic spatial resolution, coincidence response functions (CRFs) of the detector pair located parallel to each other at a distance of 80 mm were measured by scanning a 0.25 mm diameter 22Na point source with 0.1 mm steps. The data were acquired by the CAMAC system. The average FWHM of the CRFs was 0.751 mm without corrections of the point source size. Additionally, to improve detector efficiency by suppressing the positioning errors, an interaction-order defining algorithm based on Compton kinematics was adapted to the same data, and we consider inter-crystal scatter effects comparing errors arising from the interaction-order defining algorithm and the centroid calculation.

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.

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.

A high-resolution positron emission tomography (PET) scanner dedicated to brain studies has been developed and its physical performance was evaluated. The block detector consists of a new compact position-sensitive photomultiplier tube (PS-PMT, Hamamatsu R7600-C12) and an 8/spl times/4 bismuth germanate (BGO) array. The size of each crystal is 2.8 mm/spl times/6.55 mm/spl times/30 mm. The system has a total of 11 520 crystals arranged in 24 detector rings 508 mm in diameter (480 per ring). The field of view (FOV) is 330 mm in diameter/spl times/163 mm, which is sufficient to measure the entire human brain. The diameter of the scanner's opening is equal to the transaxial FOV (330 mm). The system can be operated in three-dimensional (3-D) data acquisition mode, when the slice septa are retracted. The mechanical motions of the gantry and bed are specially designed to measure the patient in various postures; lying, sitting, and even standing postures. The spatial resolution of 2.9 mm in both the transaxial and axial directions is obtained at the center of the FOV. The total system sensitivity is 6.4 kc/s/kBq/ml in two-dimensional (2-D) mode, with a 20-cm-diameter cylindrical phantom. The imaging capabilities of the scanner were studied with the Hoffman brain phantom and with a normal volunteer.

A new method for a four-stage depth of interaction (DOI) detector is proposed. The four-stage DOI detector is constructed with two kinds of Gd/sub 2/SiO/sub 5/:Ce (GSO) crystals doped with different amounts of Ce, 0.5 mol% and 1.5 mol%. The amount of Ce in GSO determines the scintillation decay time constant, and it is 60 ns for the 0.5 mol% GSO and 35 ns for 1.5 mol% GSO. This difference led to the introduction of pulse-shape discrimination, which would distinguish between two kinds of event data from respective GSOs and sort them into two groups. By independently applying Anger-type position arithmetic to the data of each group, two two-dimensional (2-D) histograms are obtained. The crystal of interaction can be identified on these histograms in which only 0.5 mol% or 1.5 mol% GSO crystal elements are expressed. To evaluate this method, we constructed the four-stage DOI detector by alternately stacking 1.5 mol% GSO crystal stages and 0.5 mol% stages. The result of a scanning measurement with a /sup 137/Cs gamma-ray beam proved that the DOI detector had enough accuracy in crystal identification.

A depth of interaction (DOI) detector for γ-rays has been developed for a high-resolution positron emission tomography (PET) system dedicated to small animal studies. The DOI detector consists of a double-layer array of lutetium oxyorthosilicate (LSO) crystals and a position-sensitive photomultiplier tube (PS-PMT), Hamamatsu R7600-C12, where the element size of the LSO array is 1.8×1.8×10 mm. In the LSO crystal block, the upper LSO array is placed on the lower LSO array with a shift of half the element pitch in both X and Y directions. Since the centroid of the output light distribution from each crystal element locates at a different position on the PS-PMT, the crystal element can be identified on the image plane derived from the centroid calculation. The performance of the DOI detector was measured in terms of its application to PET. Each crystal element is clearly separated on the position map so that a single bit of DOI information can be derived, as well as the position information. The timing resolution for the upper array was 0.98 ns in FWHM, and was 0.83 ns in FWHM for the lower array. The spatial resolution in coincidence was measured with a pair of DOI detectors at different incident angles of γ-rays. The averaged FWHM values were 1.4 mm at 0° and 3.4 mm at 30°, while those of the detector pair without DOI were 1.5 mm at 0° and 4.5 mm at 30°. It is confirmed that the DOI detector provides better spatial resolution for the oblique incidence angle of γ-rays, which results in improved PET resolution in the peripheral field of view (FOV). The experimental results show that the new DOI detector is applicable for high-resolution PET systems.

A detector unit having a depth encoding scheme was designed and an experimental study has been performed to test the new idea for a positron camera detector. The detector unit consists of a multistage rectangular block of scintillation crystals optically coupled at the bottom face to four photomultiplier tubes (PMTs) in a 2/spl times/2 array or to a position sensitive photomultiplier tube. Each stage of the crystal block consists of four elements in a 2/spl times/2 array. Each scintillation event is mapped in a two dimensional (2D) distribution through the relative ratio of the output signals of the PMTs. The three dimensional location in the detector unit projects into the 2D map so that four top crystals correspond to four innermost regions in the map, and the four bottom crystals facing the PMTs correspond to four outermost regions. With appropriate light control in such a crystal block of Gd/sub 2/SiO/sub 5/:Ce coupled to four PMTs, good positioning performance to encode the depth-of-interaction was obtained for Cs-137 gamma-rays incidence.

A new high resolution PET scanner dedicated to animal studies has been designed, built and tested. The system utilizes 240 block detectors, each of which consists of a new compact position-sensitive photomultiplier tube (PS-PMT) and an 8/spl times/4 BGO array. A total number of 7,680 crystals (480 per ring) are positioned to form a 508 mm diameter of 16 detector rings with 7.2 mm pitch and 114 mm axial field of view (FOV). The system is designed to perform activation studies using a monkey in a sitting position. The data can be acquired in either 2D or 3D mode, where the slice collimators are retracted in 3D mode. The transaxial resolution is 2.6 mm FWHM at the center of the FOV, and the average axial resolution on the axis of the ring is 3.3 mm FWHM in the direct slice and 3.2 mm FWHM in the cross slice. The scatter fraction, sensitivity and count rate performance were evaluated for a 10 cm diameter cylindrical phantom. The total system sensitivity is 2.3 kcps/kBq/ml in 2D mode and 22.8 kcps/kBq/ml in 3D mode. The noise equivalent count rate with 3D mode is equivalent to that with 2D mode at five times higher radioactivity level. The applicable imaging capabilities of the scanner was demonstrated by animal studies with a monkey.

Multi-Pixel Photon Counter (MPPC) is a solid state photon counting device consisting of Geiger-mode APD pixels and self-quenching resistors. The MPPC has attractive features, such as low bias voltage operation, high gain, compact size, robustness and insensitivity to magnetic fields. Therefore, MPPC applications are now being expanded, especially for very low light level measurements. We have developed a new MPPC based PET detector module dedicated to TOF-PET systems. The module consists of a 16 × 16 Lutetium Fine Silicate (LFS) scintillator array with 3.2 mm pitch and a 16 × 16 Through-Silicone-Via (TSV) MPPC array, included analyzing circuits for gamma-ray detection, The performance of the module has been evaluated. The coincidence timing resolution was obtained 440 ps FWHM with a pair of the modules. The energy resolution value was 12.3% at 511 keV. All crystal segments were also clearly separated with uniform irradiation of 511 keV gamma-rays.

A new two-dimensional angular correlation of annihilation radiation apparatus is described. Position-sensitive photomultiplier tubes coupled with two-dimensional arrays of small BGO scintillator blocks make simple and compact position-sensitive γ-ray detectors. With a sample-detector distance of 5 m, an angular resolution of 1.1 mrad FWHM and a coincidence count rate of ∼2.4 cps per mCi are obtained. Its performance is demonstrated by the result of a test measurement for KI crystal in which non-localized positronium exists at low temperatures.

A new high-resolution four-layer DOl detector using MPPCs for brain PET scanner has been developed. The new depth of interaction (DOl) detector was designed to compose of four layers of detector units, which were lined up five axially. Each of the detector units consists of a LYSO scintillator array finely segmented of 1.2 mm and an 8 × 8 array of multi-pixel photon counters (MPPCs), which are one of the products of silicon photomultiplier family. The MPPC is so compact and insensitive to gamma-ray that the detector units can be piled up with a small gap between each scintillator array in the depth direction. In order to have the detector in every layer equally sensitive to gamma-ray, the scintillator thickness was designed at 3 mm, 4 mm, 5 mm and 8 mm toward the bottom respectively, and the total thickness was 20 mm. We adopted an internal focused laser processing technique to a monolithic LYSO scintillator and fabricated a 2D segmented array of 32 × 32 with 1.2 mm pitch in 38.4 mm square cross-section. Each detector layer has independently front end circuits including ASICs for MPPCs and signal processing circuits for crystal identification, energy and timing detection. Each data set of four layers are fed into data interface circuits placed behind detector layers and transferred to a data acquisition unit as formatted list-mode data. The performance of the four-layer DOl detector has been evaluated. The coincidence timing resolution of the detector, with a reference BaF2 detector, was obtained 850 ps FWHM. The average energy resolution value was 24.5% at 511 keV. The crystal separation with finely segmented LYSO scintillator was also good enough at each layer.

A new brain PET scanner based on single event data acquisition with on-the-fly coincidence detection has been developed. The new scanner was designed to form a detector ring of 430 mm in diameter with 32 detector modules. The single event data generated at each detector module were transferred to the data acquisition system through a fiber cable. The single event data from all detector modules were merged and processed to create coincidence event data in software with on-the-fly. The validation results indicated that the single count rate capability was expected over 220 Mcps. At experiments using 18F radioisotopes, the maximum single count rate was approximately 20 Mcps, and the maximum coincidence count rate with prompt was obtained 3.2 Mcps. The system timing resolution was 2.6 ns FWHM, and the optimum coincidence time window was 4.3 ns, obtained by adjusting the timing delay variations between each detector module.

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 the six sides of a crystal block. The X'tal cube is composed of the 3D crystal block with isotropic resolution and arrays of multi-pixel photon counters (MPPCs). In this study, to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving (SSLE) technique to a monolithic crystal block instead of gluing segmented small crystals. The SSLE technique provided micro-crack walls which carve a groove into a monolithic scintillator block. Using the fabricated X'tal cube, we evaluated its intrinsic spatial resolution to show a proof of concept of isotropic resolution. The 3D grids of 2 mm pitch were fabricated into an 18 × 18 × 18 mm(3) monolithic lutetium yttrium orthosilicate (LYSO) crystal by the SSLE technique. 4 × 4 MPPCs were optically coupled to each surface of the crystal block. The X'tal cube was uniformly irradiated by (22)Na gamma rays, and all of the 3D grids on the 3D position histogram were separated clearly by an Anger-type calculation from the 96-channel MPPC signals. Response functions of the X'tal cube were measured by scanning with a (22)Na point source. The gamma-ray beam with a 1.0 mm slit was scanned in 0.25 mm steps by positioning of the X'tal cube at vertical and 45° incident angles. The average FWHM resolution at both incident angles was 2.1 mm. Therefore, we confirmed the isotropic spatial resolution performance of the X'tal cube.

We developed a new detector module for PET using multi-pixel photon counters (MPPCs). The detection area of the detector is 40 mm × 40 mm with an 8 × 8 MPPC array. The active area of each MPPC is 3 mm × 3 mm. The detector module contains preamplifiers, DACs for setting the bias offset, a 2D resistor network for position encoding, a high voltage module, a temperature sensor, and embedded MPUs. The MPPC gain depends on the atmospheric temperature. Temperature compensation was performed by changing the bias voltage. The timing resolution and the effect of dark counts were evaluated. The ability to identify small crystals was also examined.

A position-sensitive detector (PS-detector) for a time-of-flight (TOF) PET system has been developed and evaluated. The PS-detector consists of a flat panel position-sensitive photomultiplier tube (PS-PMT; Hamamatsu R8400-00-M64 MOD) and 2.9 mm times 2.9 mm times 20 mm LYSO crystals arranged in a 16 times 16 array. The PS-PMT has eight stages of metal channel dynode and 8 times 8 multiple anodes in a 52.0 mm square times 14.8 mm high metal can package. In order to compensate for the non-uniform amplitude from the multiple anodes of the PS-PMT, an application specific integrated circuit (ASIC) having 64-channel variable gain amplifiers is attached to the front-end circuits. The ASIC also has a readout circuit to reduce 64-channel outputs down to four position signals and an energy signal. The performance of PS-detector was evaluated for TOF-PET application. The coincidence response functions (CRFs) were measured by scanning a 22Na point source with 0.4 mm steps across the center of a pair of the PS-detectors. The average FWHM value of the CRFs is 2.3 mm. The energy resolution corresponding to each crystal element at 511 keV is 10.9% on the average. The timing resolutions of the PS-detector at FWHM, with a BaF2 reference detector, range from 421 ps to 787 ps, with an average of 505 ps. A new PS-PMT having high QE is now under development. The timing resolutions of the new PS-PMT coupled to a LaBr3 and a LYSO single crystal are 236 ps FWHM and 366 ps FWHM, respectively, with a BaF2 reference detector. We are developing a new TOF-PET system with PS-detectors.

A new positron emission tomography (PET) scanner for whole-body studies has been developed. The scanner has 720 block detectors, each of which consists of a flat panel position-sensitive photomultiplier and a 16/spl times/8 BGO crystal array. The detector system is composed of 12 layers of block detector rings stacked axially, and each ring consists of a circular array of 60 block detectors. Since each block detector ring contains eight crystal rings, the total number of detector rings is 96 and the axial field of view (FOV) is 685 mm, which is sufficient to measure a whole-body with two steps of scan. The detector ring diameter is 1 020 mm and the transaxial FOV is 600 mm in diameter. Coarse slice septa are placed between the block detector rings, which are effective to reduce scattered coincidence events while keeping a high detection sensitivity. Parallel 16 personal computers are used for the data acquisition and processing to deal with a large amount of event data. The acquired data in 3-D manner are converted to 2-D data set with Fourier rebinning algorithm and reconstructed with a fast iterative algorithm "DRAMA". The reconstructed images for the whole body are obtained within 5 min after the scan. By using the coarse septa, the scatter fraction measured with NEMA NU2-2001 standard was 31.4% whose value was significantly lower than that of 3-D PET without septa. The system sensitivity was 9.72 cps/kBq, and the peak counts of the noise equivalent count rate used with direct random-subtraction was 113.6 kcps at 10.5 kBq/ml.

A 256ch flat panel position sensitive photomultiplier tube (FP-PMT) is a promising device for a PET detector because of its large opening area, 52 mm /spl times/ 52 mm, and small dead space. The useful area of the 256ch FP-PMT is 89% to the opening area. The 256ch FP-PMT contains 16 /spl times/ 16 matrix anodes with 3.04 mm intervals between them so that the 256ch FP-PMT affords optical coupling with a 16 /spl times/ 16 array of scintillation crystals having 3 mm /spl times/ 3 mm bottom area. Its 14.7 mm thickness will also ensure a compact volume and less weight for the PET apparatus. Using a prototype 256ch FP-PMT, we measured light spread function of a central anode and crystal identification ability with 2.9 mm /spl times/ 2.9 mm /spl times/ 7.5 mm Gd/sub 2/SiO/sub 5/ (GSO) crystals. The full width at half maximum of the light spread function was found to be 4.6 mm. As regards positioning performance, the resultant positioning image map assures its capability for crystal identification in a 16 /spl times/ 16 array of the GSO crystals. For the use of a 256ch FP-PMT having a large useful area, we made a new proposal for easier construction of an array with many crystal elements. The way utilizing multilayer polymer mirrors, a reflector, properly processed by laser can be applied to other various shaped detectors.

As part of the next generation PET project, uniformity of full energy pulse height for all crystal elements was improved in the depth of interaction (DOI) detector constructed of three-dimensional crystal arrays. In our previous report, we found that the DOI detector constructed of four stages of a 2 /spl times/ 2 Gd/sub 2/SiO/sub 5/:Ce (GSO) crystal array provides good crystal identification performance but poor uniformity of the energy pulse height distribution. The upper stage crystal elements which stay further from the photocathode of a PMT have a tendency to show lower energy pulse height. For example, the ratio of the full energy peak of the top stage crystal to the bottom stage one was about 0.3. We designed a new DOI detector improved in the uniformity. By optimizing crystal surface finishes, reflector configurations, and optical coupling between crystal elements, we got comparable energy pulse height from the upper stage crystals to the bottom stage crystals. Despite this change of detector conditions, good separation between each area corresponding to crystal elements is maintained on two-dimensional histograms obtained by Anger-type position calculation. The uniform full energy pulse height of every stage crystal allows a narrower dynamic range of the electrical circuits, and may give a great advantage in getting an accurate scatter correction. It also improves energy and timing resolution.

A detector proposed by Murayama et al. (IEEE Trans. Nucl. Sci., vol. 45, no.3, p. 1152-7, 1998) for detection of depth-of-interaction (DOI) in PET consists of three-dimensionally arranged crystal blocks with proper optical reflectors and is coupled to an array of photomultiplier tubes. This detector has a great advantage in easiness and cost in fabrication. The authors built a simulator of such a detector which allows them to find appropriate values of parameters such as optical properties of crystal or detector unit geometry before making detectors actually. The simulator is based on the Monte Carlo method which traces the migration of optical photons generated by interaction of a gamma ray with crystal. First, the simulator performance was validated by comparing with the experimental data obtained with some prototype detectors. Then, on some parameters including refractive index of crystal, detector geometry, and light yield per interaction, appropriate values were investigated for accurate block discrimination from PMT output signals.

A detector unit using a depth encoding scheme was designed and evaluated. The detector unit consists of four crystal blocks in a 2/spl times/2 array coupled to a position-sensitive photomultiplier tube (PS-PMT) having metal channel dynodes and 4/spl times/4 multianodes. Each crystal block consists of three stages of 2/spl times/2 GSO arrays of which element size is 3.8 mm/spl times/3.8 mm/spl times/10 mm. Each scintillation event is mapped on a two dimensional positioning histogram according to the relative ratio of the output signals of the PS-PMT. With appropriate light control in the crystal block, good positioning performance for Cs-137 photons was obtained.

A high resolution BGO block detector has been developed for a positron emission tomography (PET) system. The detector consists of a newly developed compact position-sensitive photomultiplier tube (PS-PMT) and an 8(X)/spl times/4(Y) BGO array of which element size is 2.8 mm/spl times/6.8 mm/spl times/30 mm. The PS-PMT has ten stages of metal channel dynodes and crossed-wire anodes in a 28 mm square/spl times/20 mm high metal package. The performance of the PS-PMT and the block detector was evaluated in terms of applicability to PET. The coincidence timing resolution was 4.4 ns FWHM and 9.5 ns FWTM, and the energy resolution was about 30% for 511 keV /spl gamma/-rays, with the PS-PMTs coupled to single BGO segments. The spatial resolution of the block detector was evaluated by measuring the detector response functions (DRFs) for each BGO segment. The average FWHM values of the DRFs were 3.0 mm in the X direction, and 6.8 mm in the Y direction. Count rate performance of the block detector was measured at different activity levels, when no distinct mispositioning in crystal coding was seen at count rates up to 100 kcps. A compact SD-PET system dedicated to animal studies was designed by adopting the position-sensitive detectors. >

A detector unit having a depth encoding scheme was designed and an experimental study has been performed to test the new idea for a positron camera detector. The detector unit consists of a multistage rectangular block of scintillation crystals optically coupled at the bottom face to four photomultiplier tubes (PMTs) in a 2/spl times/2 array or to a position sensitive photomultiplier tube. Each stage of the crystal block consists of four elements in a 2/spl times/2 array. Each scintillation event is mapped in a two dimensional (2D) distribution through the relative ratio of the output signals of the PMTs. Three dimensional location in the detector unit projects into the 2D map so that four top crystals correspond to four innermost regions in the map, and the four bottom crystals facing the PMTs correspond to four outermost regions. With appropriate light control in such a crystal block of Gd/sub 2/SiO/sub 5/:Ce coupled to four PMTs, good positioning performance to encode the depth-of-interaction was obtained for Cs-137 gamma-rays incidence.

As part of the next generation PET development project, performances of one-dimensional GSO crystal arrays were measured and analyzed for the development of the depth-of-interaction (DOI) detector constructed of three-dimensional GSO crystal arrays. We considered two structures of one-dimensional crystal arrays, one is a "straight line type" which has the crystals of the array stacked in series, and the other is a "U shaped type" which is a crystal block of stacked crystals bent at the center. By scanning collimated gamma rays along the crystals, we measured the pulse height distributions from one end or both ends of the crystal array for different conditions of the surface of each crystal (rough or chemical etching) and arrangement of the reflector. The analysis of these data showed how these conditions affected the performances of the DOI detector. We also measured two-dimensional pulse height distributions for the "U shaped type". The analysis of the data on the combination of crystal surface treatment and reflector arrangement provided a method to optimize performances of the DOI detector.