Effects of inter-crystal scattering events on the performance of SIAT aPET

Objective. The probability of 511 keV γ-rays undergoing inter-crystal scattering (ICS) in current PET detectors is high. The objective of this research is to study the impact of the ICS events on the performance of the small animal SIAT aPET scanner. Approach. The SIAT aPET scanner is composed of 48 detectors organized in 4 rings with each ring containing 12 detectors. Each detector is composed of a 23×23 crystal array dual-ended read out by two 8×8 SiPM arrays with resistor network circuits. In this work, a specific method was proposed to identify the single interaction (non-ICS) and ICS events (both interactions are undergoing ICS). A point source, two hot rod phantoms and a NEMA image quality (IQ) phantom were scanned and images were reconstructed using different types of events. Main results. The average FWHM spatial resolutions of the point source obtained with all, non-ICS and ICS events are 0.62 mm, 0.52 mm, and 0.97 mm, respectively. 1.0 mm dimeter rods can be clearly resolved from the images of the ultra-micro hot rod phantom obtained with both all events and only the non-ICS events, but the image obtained with the non-ICS events show a 40% better peak-to-valley ratio for the 1.0 mm rods. The 1.35 mm rods can be clearly resolved from the image obtained with the ICS events. The images of the NEMA IQ phantom showed that the contrast recovery coefficients (CRC) for hot rods with 1-5 mm diameters were improved by 24% in average, but the signal-to-noise ratio (SNR) of the uniform region was decreased by 46% with the rejection of the ICS events (at least one γ-ray undergoing ICS). Significance. The results of this work provide useful guidance on whether it is necessary to use detectors that are capable of identifying the ICS events for PET scanners of different spatial resolution.