Development of a high-density position decoder circuit for GAPD-based PET with a large number of readout channels

Channel reduction techniques are useful for reducing the number of channels being digitized and for constructing cost effective DAQ systems. The purpose of this study was to develop a channel reduction circuit which could be used to identify the interacted channel with a coincidence event from the 256 signals transferred from PET detectors. The channel reduction circuit would then output a channel address and analog pulse corresponding to the valid channel. The circuit, called a “position decoder circuit (PDC)”, consisted of a 256 channel gain adjust circuit, eight 32:1 PDCs, and one 8:1 PDC. Two PDC boards were constructed to evaluate the performance and usefulness of the developed PDC for PET applications. The proof-of-principle PET detector block composed of a 4×4 matrix of detector modules, each of which consisted of a LYSO scintillator coupled to a 4×4 array GAPD, was also constructed. A valid channel was successfully identified by the developed PDC, and the channel address and analog pulse outputs of the interacted channel were accurately measured. The variation of the gains for all detector channels was decreased by about 78% using the gain adjustment circuit implemented in the PDC. Moreover, average energy resolution was improved by approximately 8%. The discrepancy of timing resolutions acquired with and without the PDC was negligibly small. Single/coincidence count rates were 209.3/3.2 cps with the PDC and 232.7/3.8 cps without the PDC. Tomographic image of the hot-rod phantom was successfully acquired using the PDC. Experimental results indicate that the PDC is useful to identify a valid channel among a large number of readout channels of PET detectors. The results also indicate that the PDC is feasible to provide high-quality PET image.