Evaluation of a 16:3 Signal Multiplexor to Acquire Signals From a SPM Array With Dual and Single Layer LYSO Crystal Blocks

For many years most PET scanners have used a large number of crystals with some form of light sharing technique to couple them to four photo-multiplier tubes (PMT). While the resolution of the scanners was improved by using a larger number of 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 3.3 mm × 3.3 mm LYSO crystals in the lower layer and an upper offset layer of 3 × 3 crystals coupled to a SensL SPMArray4 4 × 4 array of SPMs. A single layer array of 1.68 mm × 1.68 mm crystals is also studied. The standard SensL pre-amplifier and evaluation board were used to process the signals. The 16 outputs of the evaluation board 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 introduce the concept of “resolvability index” (RI) to compare the size of each crystal´s foot-print and distance to its neighbour. Our results suggest that the RI with the multiplexed SPM readout is comparable to that of a conventional PS-PMT readout, and superior to that of conventional PET detectors with four PMTs. We anticipate that even smaller crystals could be used on the SPM making this a good choice for small animal PET scanners.