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

Abstract

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/spl times/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/spl times/7 BGO detector blocks (2.68 mm/spl times/2.68 mm/spl times/18 mm) 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/spl times/7 BGO block with crystal dimensions of 2.68 mm/spl times/2.68 mm/spl times/18 mm were approximately found 1.47 mm for discrete detectors and 1.51 mm for light shared detectors. The intrinsic spatial resolutions of 7/spl times/7 light shared BGO block with crystal dimensions of 2.68 mm/spl times/2.68 mm/spl times/18 mm for 20 cm, 40 cm and 80 cm detector ring diameters were projected to 1.57 mm, 1.75 and 2.30 mm, respectively. Making the assumption that LSO (lutetium orthosilicate) crystals have 5 times more light output than BGO, 14/spl times/14 LSO block with crystal dimensions of 1.32 mm/spl times/1.32 mm/spl times/20 mm can be decoded and intrinsic spatial resolutions for 20 cm, 40 cm and 80 cm detector ring diameters were expected to be 0.87 mm, 1.15 mm and 1.91 mm, respectively.