Real-time discreet SPAD array readout architecture for time of flight PET

Single photon avalanche diode (SPAD) arrays have proven themselves as serious candidates for time of flight positron emission tomography (PET). Discreet readout schemes mitigate the low-noise requirements of analog schemes and offer very fine control on threshold levels and timing pickup strategies. A high optical fill factor is paramount to timing performance in such detectors, and therefore space is limited for closely integrated electronics. On the other hand, a production, daily used PET scanner must minimize bandwidth usage, data volume, data analysis time and power consumption, and therefore requires a real-time readout and data processing architecture as close to the detector as possible. We propose a vertically integrated architecture that allows placement of the real-time data extraction directly under the detector while maintaining high optical fill factor for optimal timing performance. This paper focuses on the real-time data acquisition engine. It reduces transmitted data by a factor of 8 in standard operational mode. Combined with small local memory buffers, this significantly reduces acquisition dead time. Finally, auxiliary circuits accelerate channel self-diagnosis, essential for large PET systems. A prototype device featuring individual readout for 6 scintillator channels was fabricated. Timing readout is provided by a first photon discriminator driving a TDC, while energy reading and event packaging is done using standard logic. The dedicated serial output line supports a sustained rate of 170k counts per second (cps) in waveform mode, while the standard operational mode supports 2.2M cps. In normal PET acquisition conditions (up to 1250 cps/mm2), this provides