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

Inter-crystal scatter is one of the primary reasons to misplace the coincidence events in positron emission tomography (PET). The probability of Compton scattering of a 511 KeV photon within the crystal is more than 50% of all interactions. Unlike one-to-one scintillator-photomultiplier tube (PMT) coupled (discrete) detectors, detector blocks utilizing the light sharing technique must have an additional degradation on their spatial resolution due to shared scintillation photons within the block. This study focuses on an estimation of the coincidence line spread function (CLSF) for 7×7 BGO (bismuth germanate) detector blocks by comparing the simulation results for discrete and light shared detectors. Based on experimental measurements, decoding probabilities, which describe the uncertainty in positioning between neighboring crystals, were estimated from photoelectron statistics. The results were embedded into the Monte Carlo simulation to estimate the CLSF more accurately. In this study, two 7×7 BGO detector blocks (2.68mm × 2.68mm × 18mm) with different surface treatments coupled to 4 PMTs were used to estimate the decoding probability values. For discrete detectors, the FWHM values of CLSF for 7×7 BGO block with crystal dimensions of 2.68mm × 2.68mm × 18mm were approximately found 1.47mm for discrete detectors and 1.51mm for light shared detectors. The intrinsic spatial resolutions of 7×7 light shared BGO block with crystal dimensions of 2.68mm × 2.68mm × 18mm for 20cm, 40cm and 80cm detector ring diameters were projected to 1.57mm, 1.75 and 2.30mm, respectively. Making the assumption that LSO (lutetium orthosilicate) crystals have 5 times more light output than BGO, 14×14 LSO block with crystal dimensions of 1.32mm × 1.32mm × 20mm can be decoded and intrinsic spatial resolutions for 20cm, 40cm and 80cm detector ring diameters were expected to be 0.87mm, 1.15mm and 1.91mm, respectively.