Development of a 256-to-4 signal multiplexing readout for high resolution PET detectors

We developed a small animal PET named S-PET, which features LYSO/BGO phoswich detector (crystal size, LYSO: $0.79 \times 0.79 \times 5 \mathrm{~mm}^{3}$; BGO: $1.22 \times 1.22 \times 7.5 \mathrm{~mm}^{3}$). The system utilized ASIC that enables the individual reading of each MPPC pixel, providing more detailed information for event detection. Nevertheless, this readout approach comes with a relatively high cost. In this work, a multiplexing readout circuit was developed capable of reading 256 signals from 4 MPPCs and converting them into 4 position encoded signals. This approach only needs 40 electronic channels to read the 40 detector blocks in the S-PET system, leading to significant savings on hardware costs. Our preliminary tests demonstrated the feasibility of using this readout method to interpret data from the LYSO/BGO phoswich detector, and delayed charge integration (DCI) technique is used to discriminate between the LYSO and BGO events. Both the LYSO and the BGO flood images demonstrated high-quality position decoding in the central region, but a slight degradation was observed in the edge areas, indicating the need for further optimization in our next step. The average energy resolution was 14.3±4.6% and 15.2±4.7% for the LYSO and BGO crystals, respectively. In conclusion, preliminary results support the feasibility of the developed signal multiplexing method in reading LYSO/BGO phoswich detector, which introduces an alternative, cost effective approach for data acquisition in the S-PET system.