Narrowing Coincidence Timing Window Through Time-Walk Correction In Silicon Ring Enhanced Pet Scanner

We have constructed an enhanced PET scanner from a 1 mm thick silicon detectors as the inner detectors combined with a partial BGO ring. The inner detector trigger is provided by a leading edge discriminator on a CR-RC shaped signal induced by annihilation photon interactions. A shaping time of 150 ns implies a time-walk of similar duration, requiring comparably long hardware coincidence timing windows. The system was set to a planar mode and tested using a Jaszczak phantom with, among others, 1.6 mm diameter holes which match systematic resolution of the mixed mode silicon-BGO events.Here we present a strategy of time-walk compensation, based on the pulse height, sampled for each event, and the known shape of the pulse that enters the discriminator. Since the detector bias at 136 V is similar to the full depletion voltage of the sensors, the remaining, post-corrected timing spectrum still suffers from impact position related jitter, nearly masking the beneficial effect of the time-walk compensation.An off-line reduced coincidence window can be applied on the data collected by the long hardware window. We reconstructed images from such samples for window durations of 10 and 20 ns, and both time-walk corrected and raw data, and compared performance to data from unrestricted window of 200 ns. We found out that a window of 20 ns will capture 30 %, and 10 ns 15 % of all events, reduced number of events reflected in compromised image quality. The time-walk compensation yielded a 20 % larger data sample compared to raw, uncompensated data. Images reconstructed from 20 ns filtered events preserve systematic resolution, while 10 ns filtered events already show fidelity degradation. Samples obtained with time-walk compensation have consistently shown better image quality than un-corrected samples.The impact related jitter depends on detector bias. Combining higher bias and time-walk compensation, timing resolution is further improved, as we show for non-position sensitive setup.