Performance of high-resolution PET detectors based on long semi-monolithic scintillator slabs

onventional positron emission tomography (PET) scanners use either highly-segmented or monolithic scintillator detectors. Depth of interaction (DOI) information is vital for high-resolution PET scanners using either segmented scintillator detectors with a large aspect ratio of crystal length to width or monolithic scintillator detectors with a large ratio of crystal thickness to spatial resolution. Semi-monolithic scintillator detectors maintain the intrinsic DOI encoding capability of monolithic detectors and meanwhile, the edge effect is much smaller. The objective of this study is to compare the performance of semi-monolithic scintillator detectors with different slab thicknesses, slab surface treatments, and reflector types. Four long semi-monolithic detectors consisting of lutetium yttrium oxyorthosilicate (LYSO) slabs of 0.96×56×10 mm3 and 0.81×56×10 mm3, with and without black paint at both end and front surfaces were measured. Additionally, semi-monolithic detectors using either barium sulfate (BaSO4) or enhanced specular reflector (ESR) as the inter-slab reflector were compared for the first time. The semi-monolithic detectors were read out by 4×16 silicon photomultiplier (SiPM) array with a row and column summing readout circuit and the signals were processed using electronics developed in our lab. Black paint treatment of the two end and front surfaces degrades the energy resolution but improves both the spatial resolution in the monolithic direction and DOI resolution, thus improving the overall performance of the detector. The detector using ESR reflector provides clearer individual slab identification in the flood histogram, similar spatial resolution in the monolithic direction, DOI resolution, and energy resolution. The squared centroid of gravity (squared COG) method improves the spatial resolution in the monolithic direction by ~30% as compared to the COG method. The long semi-monolithic scintillator detectors optimized in this work provide a clear identification of LYSO slabs of 0.96 and 0.81 mm thick, a spatial resolution in the monolithic direction of ~1.7±0.3 mm, a DOI resolution of ~2.1±0.7 mm, and an energy resolution of ~17.5±2.0%. The detectors can be used to develop high-performance small animal and organ-specific PET scanners in the future.