Performance evaluation of PET detectors based on monolithic scintillating crystals

This thesis aims to test a detector based on a monolithic crystal coupled to an 8$\times$8 SiPM matrix and to compare the performance between different scintillating materials (BGO and LYSO). Different time and position reconstruction algorithms such as k-Nearest Neighbor (k-NN) and Maximum Likelihood Estimation (MLE) are used. The experimental apparatus includes two detectors: the first has a monolithic crystal which can be moved in 2 directions to vary the radiation entrance position. The second features a single LYSO pixel crystal, used as a coincidence detector for the collimation of the annihilation photons emitted by a ^22Na radioactive source. While the pixel detector remains the same for all measurements, the monolithic crystals tested are a 51.8x51.8x12 mm^2 LYSO crystal and two 50.2x50.2 mm^2 BGO crystals of 2 different thicknesses: 16 mm and 20 mm. Regarding the spatial resolution, k-NN and MLE demonstrated comparable performance. Despite its higher light yield, the LYSO crystal exhibited the worst spatial performance, achieving a resolution between 3 mm and 3.5 mm (FWHM) in the two planar directions away from the crystal edges, while for both the BGO crystals a resolution between 1.8 mm and 2.5 mm was achieved. The low spatial resolution, especially for the LYSO crystal, can be explained by the fact that the DAQ system is optimized for timing resolution, therefore it doesn't permit to measure, with the best resolution, the energy, and so the light pattern on the SiPM matrix, which is crucial to achieve better performance. For timing resolution, the best performance was achieved with the Maximum Likelihood Interaction Time Estimation (MLITE): a coincidence time resolution (CTR) of 183 +- 7 ps (FWHM) has been obtained with the LYSO crystal, which is close to the state-of-art for ToF-PET applications, while a CTR of around 700 ps has been achieved for both the BGO crystals. To prove the ToF capability of this system, the ^22Na source was also moved along the axis connecting the two detectors and some measurements were taken to evaluate the displacement of the source from the calibration position. A linear dependence of the ToF on the source position was verified.