Inter-crystal scatter in a dual layer, high resolution LSO-APD positron emission tomograph.

Improving system efficiency without jeopardizing spatial resolution is one of the main problems of small animal PET scanners. In pursuit of this goal, the future LSO-APD-PET prototype MADPET-II will combine highly granulated detector modules with a dual layer structure. The individual readout of the LSO crystals allows separately handling multiple signals related to those photons scattering between different crystal units (inter-crystal scatter, ICS). The contribution of ICS events can significantly increase the system efficiency. Such coincidences are not characterized by a unique LOR. However, in order to minimize 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 recovery algorithms to select the primary crystal were implemented and developed, and applied to Monte Carlo simulated data. Some of these algorithms were based on the properties of Compton kinematics. For a centred point source and a 100 keV lower energy threshold, the absolute system efficiency was found to increase by 35% when including ICS events: from 1.8% without ICS events to 2.8% with ICS. Similarly, for a threshold of 200 keV, the contribution of ICS coincidences still represented approximately 20% of the total detected coincidences, leading to an absolute system efficiency of almost 2%. The mispositioning introduced by processing ICS coincidences only led to a moderate broadening of the axial line spread function (LSF), especially at the tails of the profile (FWTM). This effect was also noticeable in the transaxial plane. In presence of scattering media (water-filled cylinder), the resolution degradation was dominated by the contribution of object scatter. The reconstructed images from a simulated homogeneous cylinder filled with activity with a non-active rod at its centre were employed to estimate the impact of ICS on the image quality. In general, the use of ICS coincidences increased the signal-to-noise ratio (SNR) but worsened contrast. The effects of ICS on resolution could be reduced by employing a new identification scheme based on the maximum signal and the Compton kinematics. This method yielded the highest identification rate for the correct photon trajectory, even for a finite energy resolution of 15% (511 keV). This technique also increased the SNR by 17% to 30% and preserved the image contrast. In conclusion, by combining individual crystal readout, a low energy threshold and an appropriate recovery scheme, the processing of ICS coincidences significantly increases the system efficiency without any substantial deterioration of the image quality.