Characterization and processing of inter-crystal scatter in a dual layer, high resolution LSO-APD-PET

Improving sensitivity without jeopardizing resolution is one of the main problems of small animal PET scanners. Pursuing this goal, a future LSO-APD-PET prototype will combine highly granulated detector modules with a dual layer structure. The individual readout of the LSO crystals allows to separately handle multiple signals related to those photons scattering between different crystals (inter-crystal scatter, ICS). The contribution of ICS-events can be decisive to increase the sensitivity. However, such coincidences are not characterized by a unique LOR. In order to prevent potential resolution degradation, it would be desirable to identify the primary path of the ICS events. Since ICS is geometry dependent, this work was aimed at investigating the effects of ICS in the performance of the dual layer prototype. Different schemes to select the primary crystal were implemented and developed, with special emphasis on the properties of Compton scatter. The recovery algorithms were applied to Monte Carlo simulated data. For a centered point source, the sensitivity could be increased in 35% by including ICS-events and using a lower threshold of 100 keV. For a threshold of 200 keV, the contribution of ICS-coincidences still represented ≈ 20% of the total sensitivity, giving rise to an improvement from ≈ 1.7% without ICS events to almost 3%. The mispositioning introduced by processing ICS coincidences only led to a moderate broadening of the axial LSF, especially visible at the tails of the profile (FWTM). In the transaxial plane, a slight worsening of the resolution was also noticeable at the FWTM. The degradation effects could be reduced by employing a new identification scheme based on the maximum signal and the Compton kinematics.