Maximum likelihood estimation of inter-crystal events scattering events for light sharing PET detectors

PET detectors based on light sharing, which can obtain spatial resolution smaller than the photo-detector size with a simple calculation to estimate the center of gravity, have often been studied, but the difficulty in identifying inter-crystal scattering (ICS) has not been resolved yet. In particular, ICS identification is strongly required to make full use of a minutely segmented crystal block such as used in the X´tal cube. The maximum likelihood (ML) method that calculates the likelihood function with response functions of photo-detector signals is suitable for the X´tal cube. However, the method does not take ICS into account. Therefore, in this paper, we developed a new method taking ICS into account for the X´tal cube detector. The proposed method was based on the ML method, and unknown positions were expanded to two to estimate scattering position and absorbed position for the ICS event. We evaluated performance of the proposed method using simulated data. We simulated the X´tal cube composed of the 6 × 6 × 6 array of (3 mm)³ LGSO crystals. Each surface of the crystal block was covered with 4 × 4 MPPCs, each with a 3 × 3 mm2 active area. As the results, the proposed method promises high ICS event identification, but crystal identification performance of the proposed method is almost equal to previous methods.