Positioning Annihilation Photon Interactions in a Thin LSO Crystal Sheet With a Position-Sensitive Avalanche Photodiode

Using scintillation crystal sheets instead of discrete crystal arrays in high-resolution PET has the immediate advantage of reduced complexity as well as a potential for increased sensitivity. In order to evaluate the positioning capability of a position sensitive avalanche photodioide (PSAPD) using a sheet Lutetium Oxyorthosilicate (LSO) crystal scintillator, we studied the dependence of detected event position versus the known source position. In particular, we studied positioning in a continuous 8 mmtimes8 mm LSO sheet coupled to a PSAPD with an 8 mmtimes8 mm active area experimentally, studied optical transport in the sheet crystal with Monte Carlo simulation, and used two positioning methods to evaluate PSAPD pincushioning effects. Both collimated ⁵⁷Co 122 keV and coincidence-triggered ²² Na 511 keV sources were used in the experiments performed. We analyze the energy resolution, sensitivity, photopeak position and spatial resolution as a function of source position. An average point spread function (PSF) resolution of 2.86 and 1.12 mm FWHM for ⁵⁷ Co and ²²Na respectively was observed. Within 1.2 mm from the edge of the LSO sheet, the average photopeak position change was 5%. Simulations using annihilation photon interactions from GATE and scintillation photon transport from DETECT2000, as well as utilizing a pincushion-reduction posioning algorithm, have confirmed that the positioning and energy response observed experimently at the edge of the crystal is due to optical transport near those edges