Embedded time of arrival estimation for digital silicon photomultipliers with in-pixel TDCs

Abstract

Including precise time-of-flight measurements in the reconstruction of positron emission tomography (PET) images reduces the required radiotracer dose, scan time and improves the contrast-to-noise ratio. However, major improvements of photodetectors and scintillators are still required to fully exploit the projected 10 ps FWHM coincidence timing resolution (CTR). On the photodetector side, single photon avalanche diode (SPAD) detectors based on digital architectures promise better CTR by timestamping multiple scintillation photons from a single PET event with an array of in-pixel time-to-digital converters (TDCs). The digital nature of these architectures allow the introduction of data processing schemes to optimally combine the produced timestamps for a scintillation event with a time of arrival estimator. This processing is typically performed on an external computing platform, which requires the transfer of large quantities of raw data and leads to a higher dead time owing to the large number of TDCs. To circumvent this issue, we propose a photodetector architecture with in-pixel TDCs that embeds the data processing scheme, reducing the dataset produced by the TDC array to a single timestamp per scintillation event. Hardware-in-the-loop simulations of the fabricated CMOS 65 nm post-processing circuit coupled with simulated SPADs and 1 × 1 × 10 mm3 LYSO scintillators demonstrate a CTR improvement from 160 ps to 126 ps, a bandwidth reduction from 34.7 Mbit/s to 0.5 Mbit/s and a serial transmission dead time reduction from 69 μs to 1μs.