Purpose: Detectors for positron emission tomography (PET) typically use two types of scintillation crystals, pixelated or monolithic. A variant of these types of scintillators are the so-called semi-monolithic crystals. They consist of a monolithic crystal segmented in one direction in pieces called slabs. These scintillators have the potential to successfully combine the benefits of pixelated and monolithic configurations, providing good timing and spatial resolutions as well as the capacity to decode the depth of interaction (DOI) information. In this work, the timing performance of a detector based on semi-monolithic crystals was studied in depth. The energy response was also evaluated. Methods: The semi-monolithic detector consists of 1 × 24 LYSO slabs of 25.4 × 12 × 0.95 mm 3 each. The bottom surface of the slabs is coupled to an array of 8 × 8 silicon photomultipliers (SiPMs) of 3 × 3 mm 2 active area, 50 μm cell size and 3.2 mm pitch. The 64 output signals were independently readout by the TOFPET2 ASIC. In order to achieve the best coincidence time resolution (CTR), four different time walk corrections were tested. Additional work investigated the best method of combining the timestamps belonging to the same event.Results: The resolvability of the slabs in the measured flood maps improves with the thickness of a light guide placed in between the scintillators and photosensors. The energy resolution does not change significantly with values as good as 13.7%. Regarding the CTR, values of 335.8, 363, 369.8, and 402.5 ps have been obtained for the whole detector for no light guide, 0.5, 1.0, and 1.5 mm thickness light guide cases, respectively. These values further improve to 276.1, 302.6, 305.6 and 336.2 ps, respectively, when energy-weighted averaging of timestamps is applied. Conclusions: We have shown both an excellent timing resolution and good energy resolution for a PET detector based on semi-monolithic crystals. The use of light guides of different thicknesses does not significantly affect the energy resolution of the whole detector, but the timing capabilities slightly worsen with the increasing thickness of the light guide.
Objective: The goal of this work is to experimentally compare the 3D spatial and energy resolution of a semi-monolithic detector suitable for Total-Body Positron Emission Tomography (TB-PET) scanners using different surface crystal treatments and Silicon photomultiplier (SiPM) models. Approach: An array of 1×8 Lutetium Yttrium Oxyorthosilicate (LYSO) slabs of 25.8×3.1×20 mm3 separated with Enhanced Specular Reflector (ESR) was coupled to an array of 8×8 SiPMs. Three different treatments for the crystal were evaluated: ESR+RR+B, with lateral faces black (B) painted and a Retroreflector (RR) layer added to the top face; ESR+RR, with lateral faces covered with ESR and a RR layer on the top face and; All ESR, with lateral and top sides with ESR. Additionally, two SiPM array models from Hamamatsu Photonics belonging to the series S13361-3050AE-08 (S13) and S14161-3050AS-08 (S14) have been compared. Coincidence data was experimentally acquired using a 22Na point source, a pinhole collimator, a reference detector and moving the detector under study in 1 mm steps in the x- and DOI- directions. The spatial performance was evaluated by implementing a Neural Network (NN) technique for the impact position estimation in the x- (monolithic) and DOI directions. Results: Energy resolution values of 16±1%, 11±1%, 16±1%, 15±1%, and 13±1% were obtained for the S13-ESR+B+RR, S13-All ESR, S14-ESR+B+RR, S14-ESR+RR, and S14-All ESR, respectively. Regarding positioning performance, Mean Average Error (MAE) of 1.1±0.5, 1.3±0.5 and 1.3±0.5 were estimated for the x- direction and 1.7±0.8, 2.0±0.9 and 2.2±1.0 for the DOI- direction, for the ESR+B+RR, ESR+RR and All ESR cases, respectively, and regardless of the SiPM model. Significance: Overall, the obtained results show that the proposed semi-monolithic detectors are good candidates for building TB-PET scanners.
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