Effects of multiple photon interactions in a high resolution PET system that uses 3-D positioning detectors

A dual-panel, breast-dedicated PET system is being developed by our lab. The system’s detector panels are built from dual LSO-PSAPD modules - units holding two 8×8 arrays of 1 mm
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LSO crystals, where each array is coupled to a Position Sensitive Avalanche Photo Diode (PSAPD). These modules are capable of recording the 3-D coordinates of multiple-photon interactions in the LSO. The small size of the scintillation crystal arrays increases the likelihood of photon scattering among the arrays, hence the system was designed to acquire and include Multiple Photon Interaction Events (MPIEs) in image reconstruction to maintain high photon sensitivity. The study noted a singles (coincidence) photon sensitivity gain of 46.9% (115.7% for coincidence) when MPIEs are included in image reconstruction compared to the case of only accepting events with single interactions occurring within the photopeak. Further, this work demonstrated the infrequency with which incoming photons interacted with both crystal arrays within a single Dual-LSO-PSAPD module - verified by both experimental and Monte Carlo simulation data to be ≪0.4% of the events whose total energy deposition summed over all interactions falls within ±12% of 511 keV. This result establishes the feasibility of a proposed multiplexing of the analog signals of the two PSAPDs within a dual LSO-PSAPD module. Lastly, using a new method to estimate the location of the first photon interaction in MPIEs, phantom images reconstructed from Monte Carlo list-mode PET data exhibited a contrast to noise ratio (CNR) of 21.42±0.37 when MPIEs are included versus a CNR of 11.72±0.45 for images based on only events within 12%of the 511 keV photo peak. Overall the study’s results demonstrate that the system can competently acquire and analyze MPIEs towards producing high resolution PET images.