A study of coincidence line spread function (CLSF) estimation for small scintillators using quadrant sharing technique

Inter-crystal scatter and scintillation light spread are the primary factors for mispositioning of coincidence events in positron emission tomography (PET) for light sharing detectors. The probability of Compton scatter of a 511 keV photon within the crystal is more than 50% of the interactions. Inter-crystal scatter has been studied before and it was concluded that the coincidence line spread function (CLSF) was not significantly affected. Unlike one-to-one scintillator-photomultiplier tube (PMT) coupled (discrete) detectors, detector blocks utilizing the light sharing technique have additional degradation of their spatial resolution due to shared scintillation photons within the block. This study focuses on an estimation of CLSF for 7 * 7 BGO (bismuth germanate) detector blocks by comparing the simulation results for discrete and light shared detectors. The simulation model included inter-crystal scatter but the effect of scintillation light spread is the primary emphasis of our investigation. Based on experimental measurements, deterministic position profiles that calculate a spread (uncertainty) in positioning due to light sharing between neighboring crystals, were estimated from simple photoelectron statistics. The results were embedded in the Monte Carlo simulation to estimate the CLSF more accurately. Two 7 * 7 BGO detector blocks (2.68 mm * 2.68 mm *18 mm) with different surface treatments coupled to 4 PMTs were used to estimate the positioning spread values. For discrete detectors, the FWHM values of CLSF for 7 * 7 BGO block with crystal dimensions of 2.68 mm * 2.68 mm * 18 mm were found to be approximately 1.47 mm for discrete detectors and 1.51 mm for light shared detectors. The intrinsic spatial resolutions of 7 * 7 light shared BGO block with crystal dimensions of 2.68 mm * 2.68 mm * 18 mm for 20 cm, 40 cm and 80 cm detector ring diameters were estimated as 1.57 mm, 1.75 mm and 2.30 mm, respectively.