Semi-Monolithic Detectors for TOF-DOI Brain PET: Optimization of Time, Energy, and Positioning Resolutions With Varying Surface Treatments

Semi-monolithic detectors, a hybrid configuration combining the benefits of pixelated arrays and monolithic blocks, present a compelling and cost-effective solution for positron emission tomography (PET) scanners with both time-of-flight (TOF) and depth-of-interaction (DOI) capabilities. In this work, we evaluate four LYSO-based semi-monolithic arrays with various surface treatments, read out with the PETsys TOFPET2 ASIC, to identify the optimal configuration for a novel brain PET scanner. The chosen array, featuring ESR on all surfaces except for the black-painted lateral pixelated ones, achieved 15.9 ± 0.6 % energy resolution and 253 ± 15 ps detector time resolution (DTR). Neural networks with multilayer perceptron architectures were used to estimate the annihilation photon impact position, yielding average accuracies of 3.7 ± 1.1 mm and 2.6± 0.7 mm (FWHM) along the DOI and monolithic directions, respectively. The comparative analysis of the four arrays also prompted an investigation into light sharing in semi-monolithic detectors, supported by a GATE-based simulation framework which was designed to complement the experimental results and confirm the observed trends in time resolution. By refining the detector design based on semi-monolithic geometry and optimized surface crystal treatment to enhance positioning accuracy, this study contributes to the development of a next-generation brain PET scanner, with competitive performance but at a moderate cost.