Intercrystal Scatter Studies for a Clinical PET System with 1mm~3 Resolution 3D Position Sensitive Scintillation Detectors

摘要

We are constructing the world's first 1 mm~3 resolution clinical PET system dedicated to locoregional imaging (e.g. breast, head/neck), which, when completed, will comprise two panels, each with sensitive area of 16×10 cm~2. The edge-on layout of densely-packed 8×8 arrays of 0.9×0.9×1 mm~3 LYSO crystal elements [two arrays in depth coupled to two position sensitive avalanche photodiodes (PSAPDs)] provides high sensitivity and the capability to position the individual 3-D coordinates of intercrystal scatter (ICS) interactions, which are events where the annihilation photon scatters from one scintillation crystal element to another, depositing a portion of its energy in each. In this work, we evaluate the capability of our system to detect ICS interaction pairs, which are defined as two ICS interactions originating from the same annihilation photon, and measure the contributions from these ICS events to coincidence timing resolution (CTR) and effective photon sensitivity. A 100 uCi ~(22)Na line source was scanned for 20 minutes, and a 200 uCi ~(18)F-FDG micro-hollow spheres phantom with an activity to background ratio of 5:1 was scanned for 120 minutes with a global hardware energy threshold of 50 keV (over 1536 PSAPDs/98,304 crystal elements). Prior to coincidence sorting, ICS interaction pairs can be seen in the energy histograms, while the measured time difference and median distance between ICS interactions over a subset of 672k events are 58.2 ns and 9.0 mm, respectively. Coincidence sorting was then performed with a timing window of +/- 100 ns, first limited only to photoelectric (PE) events (one interaction in one crystal element in one LYSO array that falls in the energy window), and then expanded to include ICS interaction pairs. The measured CTR for PE-PE/PE-ICS/ICS-ICS coincident events are 15.2/23.3/32.0 ns FWHM, respectively. By expanding coincidence sorting to include ICS events, the number of coincidence LORs increased from 19.2M to 30.1M, corresponding to a 56% increase in effective photon sensitivity.