Block Detector Research Articles

定量的ＰＥＴ測定に用いる測定器の信頼性‐電離箱式放射能測定装置，ウェル型シンチレーションカウンタ，持続動脈血中放射能濃度測定器およびＰＥＴ装置の評価‐

Positron emission tomography (PET) is a powerful tool for measuring in vivo functions such as blood flow, metabolism, enzyme activity, receptors, and transporters. However, plural measuring instruments (i.e., the dose-calibrator, the auto well gamma counter, the continuous blood sampling system) are necessary for the quantitative PET measurement as well as the PET scanner. The purpose of this study was to investigate the reliability of plural measuring instruments from the maintenance data for 6 years. Four kinds of measuring instrument were evaluated: a dose-calibrator (CAPINTEC, CRC-15R), an auto well gamma counter (ALOKA, ARC-400), a continuous blood sampling system (ESPEC Techno, PH type), and a dedicated PET scanner (Siemens, ECAT EXACT HR+). We examined whether the initial performance for system sensitivity is maintained. The reliability of the PET scanner was evaluated from the value of mean time between failures (MTBF) for each part of the system obtained from the maintenance data for 6 years. The sensitivity of a dose-calibrator and an auto well gamma counter were maintained virtually constant during the 6 years, but the sensitivity of a continuous blood sampling system was 0.1+/-3.2%. The sensitivity of a PET scanner was decreased to 92.3% of the initial value. Fifty-one percent of the problems with the PET scanner were for detector block (DB) and analog processor (AP) board. The MTBF of DB and AP board module were 199 and 244 days, respectively. The MTBF of the PET scanner was 56 days. The performance of three measuring instruments, excepting the PET scanner, was relatively stable. The reliability of the PET scanner strongly depends on the MTBF of the DB and AP board. For quantitative PET measurement, it is effective to evaluate the reliability of the system and to make it known to the users.

A Further Study of Timing With LSO on XP20D0 for TOF PET

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In our previous work, a very good time resolution recorded with a 4<formula formulatype="inline"><tex>$\,\times\,$</tex></formula>4<formula formulatype="inline"> <tex>$\,\times\,$</tex></formula>20 mm<formula formulatype="inline"><tex>$^{3}$</tex> </formula> LSO crystal coupled to a Photonis XP20D0 photomultiplier was shown, which suggests that TOF-PET based on the LSO is a realistic proposition. A further improvement of the time resolution, due to the common light readout by the cluster of PMTs in the block detector, proposed by Kuhn <etal/>, triggered our further study of the time resolution obtainable with the LSO crystals. In the present work, the optimization of timing using a leading-edge fast discriminator and a constant fraction discriminator was done and the results of the leading-edge timing were discussed in terms of the Hyman theory. In the second part of the study, the time resolution measurements and an optimization of the system realizing the light readout by two XP20D0 PMTs coupled to a common light diffuser were carried out for the 4<formula formulatype="inline"> <tex>$\,\times\,$</tex></formula>4<formula formulatype="inline"><tex>$\,\times\,$</tex> </formula>20 mm<formula formulatype="inline"><tex>$^{3}$</tex></formula> LSO crystal placed in various positions with respect to the PMTs. The results show that using a sum of signals and, in consequence, a better light collection from several PMTs improves timing and should allow good homogeneity of the time resolution over a whole surface of the detectors to be achieved. </para>

Development and Initial Results of a Tomographic Dual-Modality Positron/Optical Small Animal Imager

A novel concept for integrating PET and optical detection devices in a single instrument is presented. It is intended for simultaneous tomographic imaging of positron-labeled and optical probes in vivo. The optical imaging units consist of a microlens array (MLA) defining the field-of-view, a large area CMOS chip for light detection, and a transferable filter for excitation light blocking. The CMOS sensor array is positioned at the focal plane of the MLA. In a final design proposal optimized for fully tomographic mouse imaging the optical detector size is chosen to be 8.2 mm (transaxially) × 82.0 mm (axially) and has an effective thickness of 3.0 mm. A multitude of such ultra-thin detectors is allocated orthogonally with respect to the imaged object?s long axis for tomographic whole body data acquisition of small animals (mice). PET detector blocks are located in radial extension to the optical detectors and mounted on a common rotatable gantry. Laser(s) used for probe excitation are positioned at radial extension behind the PET rings and are translatable along the axis of rotation allowing for arbitrary probe excitation and improved spatial sampling, particularly of the optical system. Both, the optical detector arrangement as well as the PET detectors are mounted so that excitation light can be introduced through the gaps between individual blocks. An experimental setup has been assembled in our laboratory and phantom data were acquired.

Performance Results of BGO Block Detectors Based on Flat Panel PS-PMT for PET

We have developed a new detector composed of a 20times20 array of 2times2times20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (BGO) scintillators and a novel flat panel position sensitive photomultiplier tube(FP-PS-PMT) - Hamamatsu R8400-00-M256. The performance of the block detector was evaluated in terms of applicability to positron emission tomography (PET). The energy resolution was about 23% FWHM for 511 keV gamma-rays. The spatial resolution of the detector was evaluated by measuring the detector response functions (DRFs) for each BGO segment and by measuring the coincidence response functions (CRFs). The average FWHM values of the DRFs were 2.0 mm. For the CRFs performance, the average FWHM values were 2.82 mm at 0deg and 5.20 mm at 30deg.

Performance of the new generation of whole-body PET/CT scanners: Discovery STE and Discovery VCT

The new GE Discovery STE and Discovery VCT respectively combine 16-slice and 64-slice CT with PET. The PET scanner has a new BGO detector block of 8 x 6 matrix (6.3 x 4.7 x 30 mm(3)). The aim of this study was to test the performance of the new scanner. The PET performance evaluation was done using NEMA methodology. Owing to improved front-end electronics, the system was tested with different energy window and coincidence timing settings. Transaxial resolution FWHM for 2D(3D) mode at 1 cm offset from the centre of the field of view (R1) was 4.87 mm (5.12 mm) and at 10 cm off centre (R10) radially 5.70 mm (5.89 mm) and tangentially 5.84 mm (5.47 mm). The axial resolutions were 4.4 mm (5.18 mm) (R1) and 5.99 mm (5.86 mm) (R10). The sensitivities were 2.3 cps/kBq (8.8 cps/kBq) (R0, centre of field of view) and 2.3 cps/kBq (8.9 cps/kBq) (R10). The system scatter fraction was 21.4% in 2D at an energy of 375 keV (33.9% in 3D mode at a higher energy of 425 keV). Peak noise equivalent count rates (k=1) were 84.9 kcps at 43.9 kBq/ml (2D) and 67.6 kcps at 12.1 kBq/ml (3D). In image quality measurement the hot sphere to background contrast with 10- to 22-mm diameter spheres varied from 14% to 68%, being slightly better in 3D than in 2D mode. Cold sphere contrast was 67% in 2D and 59% in 3D mode. GE's new STE and VCT PET/CT systems have improved spatial resolution without loss in sensitivity. When compared with the LYSO crystal-based GE Discovery RX, the resolution and scatter fraction are comparable, the count rate capability is lower but the sensitivity is higher.

An investigation of sensitivity limits in PET scanners

Current systems for positron emission tomography (PET) generally cover a small solid angle which implies low sensitivity and therefore patient studies are relatively lengthy with acquisition comprising multiple bed positions. For cylindrical geometry, the axial field-of-view (FOV) may be increased by incorporating additional rings of block detectors in order to increase the solid angle coverage and hence the overall sensitivity. In this study we have taken that approach to the limit and studied an ultimate configuration with an axial extent up to 1 m or more. We have estimated the point source sensitivity and the absolute sensitivity (NEMA NU-2 2001). These sensitivity values can then be converted into count rates, for a particular phantom. A system comprising three rings of blocks based on the HIREZ block detector (Siemens Molecular Imaging) with 48 blocks/ring is taken as the starting point. Additional rings of blocks are then added. The diameter of the system for this study is 85.5 cm and the axial extent ranged from 16.4 cm, that of the current HIREZ system, up to over 3 m in order to obtain data points with a solid angle close to 4 π. In all calculations, the detectors were assumed to be lutetium oxyorthosilicate (LSO) with a crystal thickness of 2 cm. The calculated count rate values are based on actual experimental data from the Siemens HIREZ scanner and then scaled based on the ratio of the calculated absolute sensitivity to the measured HIREZ absolute sensitivity. The point source sensitivity is given by the solid angle, the square of the crystal sensitivity and the square of the detector packing fraction. The point source sensitivity as a function of the axial extent shows an exponential increase reaching a limiting value as the solid angle approaches 4 π. A system with 100 cm axial extent has a solid angle of ∼75% of 4 π.

Effects of system geometry and other physical factors on photon sensitivity of high-resolution positron emission tomography

We are studying two new detector technologies that directly measure the three-dimensional coordinates of 511 keV photon interactions for high-resolution positron emission tomography (PET) systems designed for small animal and breast imaging. These detectors are based on (1) lutetium oxyorthosilicate (LSO) scintillation crystal arrays coupled to position-sensitive avalanche photodiodes (PSAPD) and (2) cadmium zinc telluride (CZT). The detectors have excellent measured 511 keV photon energy resolutions (⩽12% FWHM for LSO-PSAPD and ⩽3% for CZT) and good coincidence time resolutions (2 ns FWHM for LSO-PSAPD and 8 ns for CZT). The goal is to incorporate the detectors into systems that will achieve 1 mm3 spatial resolution (∼1 mm3, uniform throughout the field of view (FOV)), with excellent contrast resolution as well. In order to realize 1 mm3 spatial resolution with high signal-to-noise ratio (SNR), it is necessary to significantly boost coincidence photon detection efficiency (referred to as photon sensitivity). To facilitate high photon sensitivity in the proposed PET system designs, the detector arrays are oriented ‘edge-on’ with respect to incoming 511 keV annihilation photons and arranged to form a compact FOV with detectors very close to, or in contact with, the subject tissues. In this paper, we used Monte Carlo simulation to study various factors that limit the photon sensitivity of a high-resolution PET system dedicated to small animal imaging. To optimize the photon sensitivity, we studied several possible system geometries for a fixed 8 cm transaxial and 8 cm axial FOV. We found that using rectangular-shaped detectors arranged into a cylindrical geometry does not yield the best photon sensitivity. This is due to the fact that forming rectangular-shaped detectors into a ring produces significant wedge-shaped inter-module gaps, through which Compton-scattered photons in the detector can escape. This effect limits the center point source photon sensitivity to <6% for a cylindrical system with rectangular-shaped blocks, 8 cm diameter and 8 cm axial FOV, and a 350–650 keV energy window setting. On the other hand, if the proposed rectangular-shaped detectors are arranged into an 8 × 8 × 8 cm3 FOV box configuration (four detector panels), there are only four inter-module gaps and the favorable distribution of these gaps yields >8% photon sensitivity for the LSO-PSAPD box configuration and >15% for CZT box geometry, using a 350–650 keV energy window setting. These simulation results compare well with analytical estimations. The trend is different for a clinical whole-body PET system that uses conventional LSO-PMT block detectors with larger crystal elements. Simulations predict roughly the same sensitivity for both box and cylindrical detector configurations. This results from the fact that a large system diameter (>80 cm) results in relatively small inter-module gaps in clinical whole-body PET. In addition, the relatively large block detectors (typically >5 × 5 cm2 cross-sectional area) and large crystals (>4 × 4 × 20 mm3) enable a higher fraction of detector scatter photons to be absorbed compared to a small animal system. However, if the four detector sides (panels) of a box-shaped system geometry are configured to move with respect to each other, to better fit the transaxial FOV to the actual size of the object to be imaged, a significant increase in photon sensitivity is possible. Simulation results predict a 60–100% relative increase of photon sensitivity for the proposed small animal PET box configurations and >60% increase for a clinical whole-body system geometry. Thus, simulation results indicate that for a PET system built from rectangular-shaped detector modules, arranging them into a box-shaped system geometry may help us to significantly boost photon sensitivity for both small animal and clinical PET systems.

Multi-CFD Timing Estimators for PET Block Detectors

In a conventional PET system with block detectors, a timing estimator is created by generating the analog sum of the signals from the four photomultiplier tubes (PMT) in a module and discriminating the sum with a single constant fraction discriminator (CFD). The differences in the propagation time between the PMTs in the module can potentially degrade the timing resolution of the module. While this degradation is probably too small to affect performance in conventional PET imaging, it may impact the timing inaccuracy for time-of-flight PET systems (which have higher timing resolution requirements). Using a separate CFD for each PMT might allow for propagation time differences to be compensated through calibration and correction in software. In this paper we investigate and quantify the timing resolution achievable when the signal from each of the 4 PMTs is digitized by a separate CFD. Several methods are explored for both obtaining values for the propagation time differences between the PMTs and combining the four arrival times to form a single timing estimator. We find that the propagation time correction offsets are best derived through an exhaustive search, and that the "weighted average" method provides the best timing estimator. Using these methods, the timing resolution achieved with 4 CFDs (1052plusmn82 ps) is equivalent to the timing resolution with the conventional single CFD setup (1064plusmn216 ps)

Expanding SimSET to include block detectors: performance with pseudo-blocks and a true block model.

We present a study that introduces two approaches to implementing block detectors into SimSET and compares their performance. SimSET is a photon tracking simulation package, which currently incorporates only detectors made of a solid annulus of scinitillator material. A pseudo-block approximation has been imposed on the solid annulus of conventional SimSET by discarding interactions in annulus segments that span the angular block gap. This yields blocks that are annulus segments, not rectangles. This is a quick and easy approximation of block structure, which brings SimSET results closer to actual scanner measurements. Even better agreement is expected with a deeper modification of the SimSET code that implements true rectangular blocks in the detector module (to be released late 2007/early 2008). This approach enables the greatest amount of variability and trueness to detail.We compare results from both block structure implementations to the conventional SimSET results and to measurements from a GE DSTE PET/CT scanner. Differences are evaluated in terms of sensitivities, crystal maps, and energy spectra, as well as in benchmark time tests of the simulation runs and their ease of use.Either implementation of block structure can aid in improving simulation accuracy by ameliorating one known cause of discrepancies, the geometric nature of the block detectors.

Analysis of different detector and electronics defects on F18-FDG images

The effect of malfunctioning detector blocks on accuracy of PET reconstructed images was evaluated qualitatively and quantitatively, by using the ECAT EXACT HR+ scanner. A procedure to simulate defective block was carried out after analyzing and comparing blank scans obtained with detection system in good operation with those having some faulty block detectors. Different levels of detector damages were simulated and tested with phantom studies. The obtained results allowed to better identify type and entity of the artifact expected on reconstructed data and then to know cases of malfunctioning blocks that really need to stop clinical activity.

Detector normalization and scatter correction for the jPET-D4: A 4-layer depth-of-interaction PET scanner

The jPET-D4 is a brain positron emission tomography (PET) scanner composed of 4-layer depth-of-interaction (DOI) detectors with a large number of GSO crystals, which achieves both high spatial resolution and high scanner sensitivity. Since the sensitivity of each crystal element is highly dependent on DOI layer depth and incidental γ ray energy, it is difficult to estimate normalization factors and scatter components with high statistical accuracy. In this work, we implemented a hybrid scatter correction method combined with component-based normalization, which estimates scatter components with a dual energy acquisition using a convolution subtraction-method for an estimation of trues from an upper energy window. In order to reduce statistical noise in sinograms, the implemented scheme uses the DOI compression (DOIC) method, that combines deep pairs of DOI layers into the nearest shallow pairs of DOI layers with natural detector samplings. Since the compressed data preserve the block detector configuration, as if the data are acquired using ‘virtual’ detectors with high γ-ray stopping power, these correction methods can be applied directly to DOIC sinograms. The proposed method provides high-quality corrected images with low statistical noise, even for a multi-layer DOI-PET.

Preliminary studies of a simultaneous PET/MRI scanner based on the RatCAP small animal tomograph

We are developing a scanner that will allow simultaneous acquisition of high resolution anatomical data using magnetic resonance imaging (MRI) and quantitative physiological data using positron emission tomography (PET). The approach is based on the technology used for the RatCAP conscious small animal PET tomograph which utilizes block detectors consisting of pixelated arrays of LSO crystals read out with matching arrays of avalanche photodiodes and a custom-designed ASIC. The version of this detector used for simultaneous PET/MRI imaging will be constructed out of all nonmagnetic materials and will be situated inside the MRI field. We have demonstrated that the PET detector and its electronics can be operated inside the MRI, and have obtained MRI images with various detector components located inside the MRI field. The MRI images show minimal distortion in this configuration even where some components still contain traces of certain magnetic materials. We plan to improve on the image quality in the future using completely non-magnetic components and by tuning the MRI pulse sequences. The combined result will be a highly compact, low mass PET scanner that can operate inside an MRI magnet without distorting the MRI image, and can be retrofitted into existing MRI instruments.

Initial studies using the RatCAP conscious animal PET tomograph

The RatCAP is a small, head-mounted PET tomograph designed to image the brain of a conscious rat without the use of anesthesia. The detector is a complete, high-performance 3D tomograph consisting of a 3.8 cm inside-diameter ring containing 12 block detectors, each of which is comprised of a 4×8 array of 2.2×2.2×5 mm 3 LSO crystals readout with a matching APD array and custom ASIC, and has a 1.8 cm axial field of view. Construction of the first working prototype detector has been completed and its performance characteristics have been measured. The results show an intrinsic spatial resolution of 2.1 mm, a time resolution of ∼14 ns FWHM, and a sensitivity of 0.7% at an energy threshold of 150 keV. First preliminary images have been obtained using 18F-FDG and 11C-methamphetamine, which show comparable image quality to those obtained from a commercial MicroPET R4 scanner. Initial studies have also been carried out to study stress levels in rats wearing the RatCAP.

A Simulation Study on Optically Decoding Reflecting Windows for PMT Quadrant Sharing Scintillation Detector Block

A large number of decodable crystals per photomultiplier tube (PMT) can be achieved by using the PMT-quadrant-sharing (PQS) technique with proper optically reflecting windows to channel light distribution in scintillation detector block. However, to develop brand new optically decoding reflecting windows for a detector block with different crystal material, PMT size or decoding resolution, is still very time-consuming and also requires much experience. This study is to develop a computer software tool that can simulate an expected two-dimensional (2-D) crystal decoding map before implementing a real detector block with a new set of decoding reflectors. After comparing the experimental decoding data to the simulated results with the same reflector set on a block, data are feed to adjust the software parameters. More accurate decoding reflectors will then be created by the adjusted parameters. Our result shows the decoding simulation of a 10times10 BGO block can be finished within a few minutes, which is much faster than using the Monte Carlo simulation with "DETECT". This software has been evaluated by five different PQS blocks. Our preliminary study shows this simulation tool is very promising which can significantly reduce a new product developing time; only about two-three development cycles are needed to get to the final optimized decoding reflectors

New Prospects for Time-of-Flight PET With LSO Scintillators

The growing interest in time-of-flight PET triggered the study of the time resolution obtainable with a 4times4times20 mm3 LSO crystal coupled directly to the center of a 52 mm in diameter Photonis XP20D0 photomultiplier as well as the time resolution obtainable with the use of an 11 mm thick lucite light diffuser that simulates the conditions in typical PET block detectors. The LSO crystal directly coupled to the PMT yielded a time resolution of 166plusmn5 ps, while in the case of light readout with the use of the light diffuser it degraded to 196plusmn5 ps and 277plusmn6 ps in the center and at the edge of the PMT, respectively. The light diffuser was coated on the sides with black tape to absorb light and to approximate in this way the realistic performance of a future block detector. Similar time resolution was obtained by coupling the LSO crystal either to the Photonis XP20D0 PMT or to a very fast 25 mm diameter Hamamatsu R5320 PMT. These results illustrate the advantages of the very low time jitter of the Hamamatsu PMT on one side, and high quantum efficiency and a screening grid at the anode of the Photonis PMT, on the other. This study strongly suggests that time-of-flight PET based on LSO crystals is a realistic proposition for the further development

Initial Results of a Positron Tomograph for Prostate Imaging

We present the status and initial images of a positron tomograph for imaging that centers a patient between a pair of external curved detector banks (ellipse: 45 cm minor, 70 cm major axis). The distance between detector banks adjusts to allow patient access and to position the detectors as closely as possible for maximum sensitivity with patients of various sizes. Each bank is composed of two axial rows of 20 CTI PET Systems HR+ block detectors for a total of 80 detectors in the camera. Compared to an ECAT HR PET system operating in 3D mode, our camera uses about one-quarter the number of detectors and has approximately the same sensitivity for a central point source, because our detectors are close to the patient. The individual detectors are angled in the plane to point towards the to reduce resolution degradation in that region. The detectors are read out by modified CTI data acquisition electronics. We have completed construction of the gantry and electronics, have developed detector calibration and data acquisition software, and are taking coincidence data. We demonstrate that we can clearly visualize a prostate in a simple phantom. Reconstructed images of two phantoms are shown

RF Transformer Coupled Multiplexing Circuits for APD PET Detectors

Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (~70% for APD vs. ~20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (~200 for APD vs. ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for PMT), and a smaller photoactive area (~ 5 mmtimes5 mm for APD vs. 10-52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2times2 APD array reading out an 8times8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2 mmtimes2 mmtimes20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmission through long twisted-pair cables

Design considerations and construction of a small animal PET prototype

We are developing a small animal PET scanner consisting of two block detectors, each made of 216 BGO crystals of dimensions 3.75 mm×3.75 mm×20 mm, cylindrically arranged and coupled to a position-sensitive photomultiplier tube (R2486 PSPMT). Our design was based on a very detailed Monte Carlo, that simulates the function of a PET scanner from the system level down to the individual γ-ray detectors. We have made laboratory measurements of the individual detector performance as well as measurements of characteristics of the PSPMTs. The two detector blocks which will form the basic tomographic unit have been assembled. We are developing electronics to individually process (amplify and digitize) anode signals, and use field programmable gate arrays (FPGAs) in the position determination and energy measurement of the γ-rays. At present, as an intermediate step, we are using the electronics supplied from Hamamatsu to study various aspects of the system and produce initial images.

Image reconstruction for the ClearPET™ Neuro

ClearPET™ is a family of small-animal PET scanners which are currently under development within the Crystal Clear Collaboration (CERN). All scanners are based on the same detector block design using individual LSO and LuYAP crystals in phoswich configuration, coupled to multi-anode photomultiplier tubes. One of the scanners, the ClearPET™ Neuro is designed for applications in neuroscience. Four detector blocks with 64 2×2×10 mm LSO and LuYAP crystals, arranged in line, build a module. Twenty modules are arranged in a ring with a ring diameter of 13.8 cm and an axial size of 11.2 cm. An insensitive region at the border of the detector heads results in gaps between the detectors axially and tangentially. The detectors are rotating by 360° in step and shoot mode during data acquisition. Every second module is shifted axially to compensate partly for the gaps between the detector blocks in a module. This unconventional scanner geometry requires dedicated image reconstruction procedures. Data acquisition acquires single events that are stored with a time mark in a dedicated list mode format. Coincidences are associated off line by software. After sorting the data into 3D sinograms, image reconstruction is performed using the Ordered Subset Maximum A Posteriori One-Step Late (OSMAPOSL) iterative algorithm implemented in the Software for Tomographic Image Reconstruction (STIR) library. Due to the non-conventional scanner design, careful estimation of the sensitivity matrix is needed to obtain artifact-free images from the ClearPET™ Neuro.

Preliminary resolution performance of the prototype system for a 4-Layer DOI-PET scanner: jPET-D4

We are developing a high-performance brain PET scanner, jPET-D4, which provides 4-layer depth-of-interaction (DOI) information. The scanner is designed to achieve not only high spatial resolution but also high scanner sensitivity with the DOI information obtained from multi-layered thin crystals. The scanner has 5 rings of 24 detector blocks each, and each block consists of 1024 GSO crystals of 2.9 mm/spl times/2.9 mm/spl times/7.5 mm, which are arranged in 4 layers of 16/spl times/16 arrays. At this stage, a pair of detector blocks and a coincidence circuit have been assembled into an experimental prototype gantry. In this paper, as a preliminary experiment, we investigated the performance of the jPET-D4's spatial resolution using the prototype system. First, spatial resolution was measured from a filtered backprojection reconstructed image. To avoid systematic error and reduce computational cost in image reconstruction, we applied the DOI compression (DOIC) method followed by maximum likelihood expectation maximization that we had previously proposed. Trade-off characteristics between background noise and resolution were investigated because improved spatial resolution is possible only when enhanced noise is avoided. Experimental results showed that the jPET-D4 achieves better than 3 mm spatial resolution over the field-of-view.