Conventional 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 that use either segmented scintillator detectors with a large crystal-length-to-width aspect ratio or monolithic scintillator detectors with a large crystal-thickness-to-spatial-resolution ratio. Semi-monolithic scintillator detectors maintain the intrinsic DOI-encoding capability of monolithic detectors, but with a substantially smaller edge effect. The objective of this study was 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 \({0.96}~\text {mm}\times {56}~\text {mm}\times {10}~\text {mm}\) and \({0.81}~\text {mm}\times {56}~\text {mm}\times {10}~\text {mm}\) , with and without black paint on both the end and front surfaces, were measured. In addition, semi-monolithic detectors using either barium sulfate (BaSO \(_4\) ) or an enhanced specular reflector (ESR) as the inter-slab reflector were compared for the first time. The semi-monolithic detectors were read out by a \(4\times 16\) silicon photomultiplier array with a row and column summing readout circuit, and the signals were processed using electronics developed in our laboratory. Black paint treatment of the two end and front surfaces degraded the energy resolution but improved both the spatial resolution in the monolithic direction and DOI resolution, thereby improving the overall detector performance. The detector using an ESR reflector provided clearer individual slab identification in the flood histogram and a similar spatial resolution in the monolithic direction, DOI resolution, and energy resolution. The squared centroid of gravity (COG) method improved the spatial resolution in the monolithic direction by ~30% compared with the COG method. The long semi-monolithic scintillator detectors optimized in this work provide 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 ~ \(2.1 \pm {0.7}~\text {mm}\) , and an energy resolution of ~17.5 ± 2.0%. These detectors can be used to develop high-performance small-animal and organ-specific PET scanners.
