Takahiro Moriya

Development of PET Detectors Using Monolithic Scintillation Crystals Processed With Sub-Surface Laser Engraving Technique

New monolithic scintillation detectors for PET have been developed, where the crystals are processed using internal focused laser processing technique, which is called subsurface laser engraving (SSLE) technique. When high intensity light pulses of short duration from a laser are focused into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG laser. The processed crystal array segmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the laser processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser processed crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. These results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.

Development of a high-resolution four-layer DOI detector using MPPCs for brain PET

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.

Development of a Small Animal PET Scanner Using DOI Detectors

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.

Fabrication of finely pitched LYSO arrays using sub-surface laser engraving technique with picosecond and nanosecond pulse lasers

We propose to adopt the sub-surface laser engraving (SSLE) technique for efficient and precise fabrication of finely pitched scintillation crystal arrays. However, its application to thicker crystals is still challenging. It is hard to focus the laser beam tightly at a point far from the crystals surface because of the large refractive index of the scintillator. Therefore, a higher laser energy is needed to create microcracks at deep positions in the crystal. Because this would cause excessive damage to the scintillation crystal during laser scans, the process yield of SSLE is reduced. We found that this issue could be overcome by a novel SSLE technique using both picosecond (ps) and nanosecond (ns) pulse lasers. The experimental results indicated that the total laser energy required for creating microcrack walls in a LYSO crystal can be reduced compared to that of conventional SSLE using only a ns pulse laser. The SSLE technique using both ps and ns pulse lasers would enable the fabrication of finely pitched LYSO arrays with a higher process yield.

Development of a new brain PET scanner based on single event data acquisition

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.

Development of PET detectors using monolithic scintillation crystals processed with sub-surface laser engraving technique

New monolithic scintillation detectors for PET have been developed, where the crystals are processed using internal focused laser processing technique, which is called subsurface laser engraving (SSLE) technique. When high intensity light pulses of short duration from a laser are focused into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG laser. The processed crystal array segmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the laser processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser processed crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. These results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.

Development of a position-sensitive detector for TOF-PET

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 8times8 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.

Development of a Position-Sensitive Detector for TOF-PET

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.

Performance evaluation of a high-resolution brain PET scanner using four-layer MPPC DOI detectors

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.

Cherenkov radiation‐based three‐dimensional position‐sensitive PET detector: A Monte Carlo study

PURPOSE Cherenkov radiation has recently received attention due to its prompt emission phenomenon, which has the potential to improve the timing performance of radiation detectors dedicated to positron emission tomography (PET). In this study, a Cherenkov-based three-dimensional (3D) position-sensitive radiation detector was proposed, which is composed of a monolithic lead fluoride (PbF2 ) crystal and a photodetector array of which the signals can be readout independently. METHODS Monte Carlo simulations were performed to estimate the performance of the proposed detector. The position- and time resolution were evaluated under various practical conditions. The radiator size and various properties of the photodetector, e.g., readout pitch and single photon timing resolution (SPTR), were parameterized. The single photon time response of the photodetector was assumed to be a single Gaussian for the simplification. The photo detection efficiency of the photodetector was ideally 100% for all wavelengths. Compton scattering was included in simulations, but partly analyzed. To estimate the position at which a γ-ray interacted in the Cherenkov radiator, the center-of-gravity (COG) method was employed. In addition, to estimate the depth-of-interaction (DOI) principal component analysis (PCA), which is a multivariate analysis method and has been used to identify the patterns in data, was employed. The time-space distribution of Cherenkov photons was quantified to perform PCA. To evaluate coincidence time resolution (CTR), the time difference of two independent γ-ray events was calculated. The detection time was defined as the first photon time after the SPTR of the photodetector was taken into account. RESULTS The position resolution on the photodetector plane could be estimated with high accuracy, by using a small number of Cherenkov photons. Moreover, PCA showed an ability to estimate the DOI. The position resolution heavily depends on the pitch of the photodetector array and the radiator thickness. If the readout pitch were ideally 0 and practically 3 mm, a full-width at half-maximum (FWHM) of 0.348 and 1.92 mm was achievable with a 10-mm-thick PbF2 crystal, respectively. Furthermore, first-order correlation could be observed between the primary principal component and the true DOI. To obtain a coincidence timing resolution better than 100-ps FWHM with a 20-mm-thick PbF2 crystal, a photodetector with SPTR of better than σ = 30 ps was necessary. CONCLUSIONS From these results, the improvement of SPTR allows us to achieve CTR better than 100-ps FWHM, even in the case where a 20-mm-thick radiator is used. Our proposed detector has the potential to estimate the 3D interaction position of γ-rays in the radiator, using only time and space information of Cherenkov photons.