Position estimation using neural networks in semi-monolithic PET detectors

Objective. The goal of this work is to experimentally compare the 3D spatial and energy resolution of a semi-monolithic detector suitable for total-body positron emission tomography (TB-PET) scanners using different surface crystal treatments and silicon photomultiplier (SiPM) models. Approach. An array of 1 x 8 lutetium yttrium oxyorthosilicate (LYSO) slabs of 25.8 x 3.1 x 20 mm(3) separated with Enhanced Specular Reflector (ESR) was coupled to an array of 8 x 8 SiPMs. Three different treatments for the crystal were evaluated: ESR + RR + B, with lateral faces black (B) painted and a retroreflector (RR) layer added to the top face; ESR + RR, with lateral faces covered with ESR and a RR layer on the top face and; All ESR, with lateral and top sides with ESR. Additionally, two SiPM array models from Hamamatsu Photonics belonging to the series S13361-3050AE-08 (S13) and S14161-3050AS-08 (S14) have been compared. Coincidence data was experimentally acquired using a Na-22 point source, a pinhole collimator, a reference detector and moving the detector under study in 1 mm steps in the x- and DOI- directions. The spatial performance was evaluated by implementing a neural network (NN) technique for the impact position estimation in the x- (monolithic) and DOI directions. Results. Energy resolution values of 16 +/- 1%, 11 +/- 1%, 16 +/- 1%, 15 +/- 1%, and 13 +/- 1% were obtained for the S1 3-ESR + B + RR, S1 3-All ESR, S14-ESR + B + RR, S14-ESR + RR, and S14-All ESR, respectively. Regarding positioning accuracy, mean average error of 1.1 +/- 0.5, 1.3 +/- 0.5 and 1.3 +/- 0.5 were estimated for the x- direction and 1.7 +/- 0.8, 2.0 +/- 0.9 and 2.2 +/- 1.0 for the DOI- direction, for the ESR + B + RR, ESR + RR and All ESR cases, respectively, regardless of the SiPM model. Significance. Overall, the obtained results show that the proposed semi-monolithic detectors are good candidates for building TB-PET scanners.