Development of a new position decoder circuit for PET consisting of GAPD arrays to recover inter-crystal scattered events

In a PET detector using pixelated scintillator, a 511 keV annihilation photon incident on a pixel might deposit part of its energy and could be almost simultaneously detected in several adjacent pixels. These inter-crystal scattered events are typically considered invalid events in PET if their energies are not within the energy window. The purpose of this study is to develop a new position decoder circuit (PDC) capable of multiplexing a number of channels and recovering the inter-crystal scattered events to improve the sensitivity of the PET detector consisting of Geiger-mode avalanche photodiode (GAPD) arrays. The PET detector module consisted of a 4 × 4 array of LYSO crystals and a 4 × 4 array of 3 × 3 mm2 GAPDs. The output signals from 2 detector modules were transmitted to the PDC which amplified the signals, selected the two fastest amplified signals among 32 signals and output their digital addresses and analog pulses. The PDC outputs were fed into a data acquisition system and arrival time, energy and address of the outputs were estimated and stored in list mode format. The recovery of inter-crystal scattered events was carried out by using the energy and time criteria in post-processing. The average energy resolution of a detector module at 511 keV was 17.1 ± 1.1% and 17.8 ± 1.3% before and after the recovery of inter-crystal scattered events, respectively. The increase in photopeak counts ranged from 5.3% to 26.7% depending on the pixel and source positions. The results of this study show that the new PDC design is useful to increase the sensitivity of PET detector through the recovery of inter-crystal scattered events while maintaining its high channel reduction ratio.