Digital SPAD scintillation detector simulation flow to evaluate and minimize real-time requirements

Radiation detection used in positron emission tomography (PET) exploit the timing information to remove background noise and refine the position measurement through time-of-flight (TOF) information. In PET, very fine time resolution (in the order of 10 ps FWHM) would not only improve contrast in the image, but would also enable real-time image reconstruction without iterative or back-projected algorithms. The current performance limitations will be pushed off through the optimization of faster light emission mechanisms (prompts photons), after which the burden of timing resolution will fall to the readout optoelectronics. Digital SPAD arrays offer compelling possibilities to minimize timing jitter in these future detector systems such per-cell timestamps granularity and per-cell configuration parameters, providing a highly flexible signal processing environment. However, processing hundreds of timestamps per detection event places a toll on the real-time processing, which increases rapidly with embedded channel count. Furthermore, if the processing is sent to an external device such as an FPGA, the bandwidth and related power requirements also increase. The simulation flow presented here offers perspectives on how many time to digital converters (TDC) would be required to reach the 10 ps FWHM CTR range for PET. Using this information, designers can estimate the compromises between timing performance, bandwidth requirements, data transmission, power consumption and real-time dataflow processing in the DAQ at the chip and system level. With a standard 1.1 × 1.1 × 3.0 mm3 LYSO scintillator, the coincidence timing resolution (CTR) changed by less than 3% within the range of 4 to 484 implemented TDCs for evaluated system conditions. On the other hand, an LYSO-based photonic crystal with 2.5% prompt emission rate needs a detector with at least 36 TDCs to reach within 3% CTR of an equivalent array with one TDC per SPAD. This gives significant insights on how this change of crystal material will affect system real time requirements for future detector design.