A dual-layer LYSO crystal PET detector using a SPM array and a 16:3 signal multiplexor

For many years most PET scanners had used a large number of crystals with some form of light sharing technique to couple them to four photo-multipliers (PMT). While the resolution of the scanners was improved by using more smaller crystals, the PMT size had remained about the same since it is not practical to make PMTs very small. Avalanche photo-diodes (APD) and silicon photo-multipliers (SPM) are now replacing PMTs in small animal PET scanners and in MR-PET scanners in which PMTs are not practical due to their sensitivity to magnetic fields. In this paper we study the performance of a dual layer PET detector consisting a 4×4 array of LYSO 3.3 × 3.3 mm crystals in the lower layer and an upper offset layer of 3×3 crystals coupled to a SensL SMA4 4 × 4 array of SPMs. The standard SensL pre-amplifier and evaluation board were used to process the signals. The 16 outputs were connected directly to 16 ADC channels or through a 16:3 multiplexor which uses simple summing amplifiers to provide bipolar X and Y signals as well as the sum of all inputs. In this case only 3 ADCs are required to encode the signals. In both cases a crystal identification map is produced in software. The use of the multiplexor has a negligible effect on the size of each crystal´s foot-print or its energy resolution. We anticipate that even smaller crystals could be used on the SPM making this a good choice for small animal PET scanners.