A pulse width modulation readout method for densely packed solid state photodetectors

The field of PET is moving toward systems comprising thousands of crystal elements coupled to thousands of semiconductor photodetector channels. Because of the large number of channels, developing methods for compact signal readout, while avoiding the need for new integrated circuit development is important. In this work a compact readout system for arrays of 0.5 mm × 0.5 mm × 1.0 mm LYSO crystal elements is presented. An 18×18 element crystal array was coupled to a 2×2 array of 5 mm × 5 mm position sensitive solid-state photomultipliers (PS-SSPM). Each PS-SSPM has 4 position channels. The 16 position channels of the array were multiplexed into a total of 6 readout channels. A novel version of pulse width modulation capable of high multiplexing ratios is used to readout the electronics instead of traditional analog-to-digital conversion. Each detector pulse was transformed into a digital pulse that was then readout by an FPGA-based TDC. The system successfully identified the 0.5 mm × 0.5 mm crystals at room temperature with a ratio of distance between crystal peaks to standard deviation of the peaks of greater than 3.2. The success of this simple compact readout strategy points to a practical readout strategy for PET systems comprising thousands of semiconductor photodetectors.