Real time digital signal processing implementation for an APD-based PET scanner with phoswich detectors

Recent progress in advanced digital signal processing provides an opportunity to expand the computation power required for real time extraction of event characteristics in avalanche photodiode (APD)-based Positron Emission Tomography (PET) scanners. These developments are made possible by a highly parallel data acquisition (DAQ) system based on an integrated analog front-end and a high-speed fully digital signal processing section that directly samples the output of each preamplifier with a free-running, off-the-shelf, 45-MHz analog-to-digital converter that feeds the sampled data into a field programmable gate array (FPGA) VirtexII PRO from Xilinx. This FPGA features ∼ 31 000 logic cells and two PowerPC processors, which allows up to 64 channels to be processed simultaneously. Each channel has its own digital signal processing chain including a trigger, a baseline restorer and a timestamp algorithm. Various timestamp algorithms have been tested so far, achieving a coincidence timing resolution of 3.2-ns full-width at half-maximum (FWHM) for APD coupled to Lutetium Oxyorthosilicate (APD-LSO) and 11.4-ns FWHM for APD coupled to Bismuth Germanium Oxide (APD-BGO) detectors, respectively. Channels are then multiplexed into a DSP processor from Texas Instruments for crystal identification by an ARMAX recursive algorithm borrowed from identification and vector quantization theory. The system can sustain an event rate of 10 000 events/s/channel without electronic dead time.