Design study of a novel geometrical arrangement for an in-beam small animal PET scanner

This contribution describes a novel solution for a small animal in-beam positron emission tomography (PET) scanner. Our detector is meant to be used in the SIRMIO project (Small animal proton Irradiator for Research in Molecular Image-guided Radiation-Oncology). The goal of this study is to compare a more conventional dual ring scanner shape (as an in-beam modification of the standard concept of human PET systems) with our novel solution. The conventional one is basically based on a cylindrical shape around the body of the mouse, whereas our shape is based on a so-called spherical design and it takes into account also some additional constraints related to the other components of the SIRMIO project. The system is based on a maximum of 56 scintillator blocks of pixelated LSO (Lu2SiO5). The size of each crystal is 0.9×0.9×6.7 mm3. The pixels are arranged in three layers of 25, 23 and 19 crystals per side to obtain a pyramid-like shape useful for optimizing the geometrical coverage. With this set-up it is possible to reach a good trade-off between a solid angle coverage of more than 44% and sufficient available space for integration of additional imaging components (e.g., ultrasound) and the movement of the mouse holder inside the PET scanner. The detection efficiency is calculated to be between 7% and 12%. The image reconstruction to evaluate the geometrical spatial resolution is done using the MEGAlib software (Medium-Energy Gamma-ray Astronomy library). The physical simulations are based on the GEANT4 software. MEGAlib uses a machine-learning environment based on a ROOT Toolkit for Multivariate Analysis (TMVA) to retrieve the valid events of the PET signal. Image reconstruction is based on a Maximum-Likelihood Expectation Maximization algorithm (EM-LM). According to the preliminary results, the distribution of the crystals on a large coverage of different angles allows us to obtain a spatial resolution of 0.4 - 1 mm (FWHM), consistent with the specification. The thorough characterization of the expected performances of our prototype system for in-beam PET imaging of small animal proton irradiation is still ongoing and will be presented.2019 IEEE Nuclear Science Symposium and Medical Imaging Conferenc