Time Resolution Studies for a 1-mm Resolution Clinical PET System With a Charge Sharing Readout and Leading Edge Discrimination

A 1-mm
³
resolution clinical locoregional positron emission tomography scanner is being built in our lab. A subsystem has been built comprising 98304 lutetium-yttrium oxyorthosilicate scintillation crystals coupled to position-sensitive avalanche photodiodes (PSAPDs). In the system, one leading edge discriminator (LED) threshold is used to trigger all 64 individual crystal elements on a PSAPD. In this paper, we study the effects of applying a single LED threshold across the different crystals on a PSAPD, and then look at the system-wide coincident timing resolution as a function of the LED threshold energy in the system. In addition, we also study the timing resolution behavior as a function of activity. The results show that an “effective” LED photon energy threshold across a PSAPD results in a distribution of crystal LED energy thresholds, with the interior crystal elements of the PSAPD having lower thresholds than the exterior crystal elements. The optimal LED energy threshold for individual crystal elements for coincident timing resolution was found to be between 80 and 180 keV. For an F-18 line source placed at the center, the coincidence timing resolution and randoms fraction degrade, respectively, from 13.8 ± 0.1 ns to 40.0 ± 0.4 ns, and from 2% to 38%, as the activity is increased from 44 to 1350 μCi.