FPGA-Only MVT Digitizer for TOF PET

Abstract

The multi-voltage threshold (MVT) method, proposed by us, is a method for digitizing a PET event pulse by sampling with respect to certain reference voltages. Previously, we have implemented this method by using discrete voltage comparators together with discrete time-to-digital converters (TDCs) or TDCs implemented on a field-programmable-gate-array (FPGA), and shown that the method is useful for developing PET detectors capable of time-of-flight (TOF) measurement. In this paper, we investigate an FPGA-only implementation in which the differential I/Os of the FPGA operating in the low-voltage-differential-signaling (LVDS) receiver mode are configured to provide the function of the voltage comparators. Using an Altera Cyclone II Family FPGA (EP2C70F896C7), we built two MVT digitizing channels, each of which contains four comparators to yield eight samples for each event pulse. We describe the design, implementation and calibration of this FPGA-only MVT digitizer and report its performance properties. By connecting the channels to SensL's silicon photomultipliers (SiPMs) FM30035 coupled to 2 ×2×10 mm3 LYSO crystals, we measured an energy resolution of ~16% (at 511 keV) and a coincidence resolving time (CRT) of ~ 500 ps (FWHM). These resolutions are promising for developing PET and TOF PET detectors. On the other hand, based on digitized waveforms of pulses generated by the same SiPMs and LYSOs acquired by using a 50 giga-sample-per-second (Gsps) digital oscilloscope (Tektronics DPO71604B), we measured an energy resolution of ~9% and a CRT of ~390 ps. The comparison suggests that our current implementation of the FPGA-only MVT digitizer contributes an ~13% energy resolution and ~310 ps CRT.