RF Transformer Coupled Multiplexing Circuits for APD PET Detectors

Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (~70% for APD vs. ~20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (~200 for APD vs. ~10
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for PMT), and a smaller photoactive area (~ 5 mmtimes5 mm for APD vs. 10-52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2times2 APD array reading out an 8times8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2 mmtimes2 mmtimes20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmission through long twisted-pair cables