  Single photon avalanche diode (SPAD) arrays have proven themselves as serious
candidates for time of flight positron emission tomography (PET). Discrete
readout schemes mitigate the low-noise requirements of analog schemes and offer
very fine control over threshold levels and timing pickup strategies. A high
optical fill factor is paramount to timing performance in such detectors, and
consequently space is limited for closely integrated electronics. Nonetheless,
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 fully digital, embedded real-time readout architecture
for SPAD-based detector. The readout circuit is located directly under the SPAD
array instead of within or beside it to remove the fill factor versus circuit
capabilities tradeoff. The overall real-time engine reduces transmitted data by
a factor of 8 in standard operational mode. Combined with small local memory
buffers, this significantly reduces overall acquisition dead time. A prototype
device featuring individual readout for 6 scintillator channels was fabricated.
Timing readout is provided by a first photon discriminator and a 31 ps time to
digital discriminator, while energy reading and event packaging is done using
standard logic in real-time. The dedicated serial output line supports a
sustained rate of 170k counts per second (CPS) in waveform mode, while the
standard operational mode supports 2.2 MCPS.
