Room: Room 202
Purpose: To determine the feasibility of using a solid-state photomultiplier based on amorphous Selenium (a-Se) in avalanche mode with unipolar time differential (UTD) charge sensing to achieve time resolution sufficient for time-of-flight (TOF) PET imaging in simultaneous PET/MRI.
Methods: A multi-well Se-SSPM, which we named the Nano-Electrode multi-Well High-gain Avalanche Rushing Photoconductor (NEW-HARP), was constructed using lithography to achieve UTD charge sensing and avalanche multiplication gain. Fabrication quality was checked using electron microscopy. Using Monte Carlo techniques, we simulated the timing jitter/spread of our NEW-HARP detector and compared it to that of a conventional planar a-Se detector to determine and compare the single-photon timing resolution (SPTR) of each. To experimentally test the SPTR, we repeatedly exposed our NEW-HARP detector to picosecond laser pulses and determined the FWHM of the output signalâ€™s timing jitter. We tested the SPTR at different high voltage (HV) bias potentials to determine optimal performance.
Results: Simulations revealed our NEW-HARP detector could achieve an SPTR=50ps, which is 2 orders of magnitude better than that of a conventional planar a-Se detector (SPTR=5ns). Experimental tests showed we were able to achieve SPTR=85ps at HV=2000V, SPTR=61ps at HV=2200V, and SPTR=52ps at HV=2400V, making HV=2400V the optimal bias potential for NEW-HARP. Based on Cramer-Rao lower bound calculations on coincidence time resolution (CTR), this value of SPTR corresponds to a theoretical CTR of 70ps with an optimal photon detection efficiency (PDE) of 75% at blue wavelengths.
Conclusion: We demonstrate the potential of our proposed NEW-HARP detector for TOF PET imaging. Currently, TOF PET systems only achieve a CTR of ~200ps due to poor system-level PDE (SiPM PDE<20%), whereas the CTR required to reduce patient scan time is <100ps. Our next step is to fabricate and test a full ring of PET detectors using NEW-HARP as the photomultiplier coupled to LYSO scintillators.