Evaluation of depth of interaction measurements using discrete crystal arrays and digital silicon photomultipliers

Digital silicon photomultiplers (dSiPMs) show promise for the advancement of PET detectors; advantages include fast timing, reduction in dark counts via selective cell deactivation, and possible advancement in photon depth of interaction (DOI) estimation. Improved spatial resolution is especially desirable in smaller ring diameter PET systems, such as in whole body PET/MRI. This study explored timing and DOI resolution in multiple discrete crystal arrays and light coupling configurations with dSiPMs. Two dSiPM tiles and a ⁶⁸Ge source were placed in coincidence, with a ~ 2 mm diameter beam on the test crystal surface. Test crystals consisted of 6×6, 2×2, 6×1, or 6×2 arrays of 4×4×22 mm³ crystals with different combinations of reflective and transmitting surface treatments. In most cases timing FWHM was below 425 ps, with several configurations displaying sub-400 ps timing. In almost all configurations DOI separation into top, middle, and bottom regions was easily achievable. Crystals with greater light sharing showed improved DOI resolution, but degraded timing resolution and partial reflectors improved DOI estimation near the entrance surface. The best overall performance (412 ps timing, 4.9 mm DOI) was achieved with the 6×6 meltmount + mirror film array.