Characterization of a high aspect ratio detector with lateral sides readout for Compton PET

Although positron emission tomography is probably the most specific molecular imaging modality, it still lacks a high detection sensitivity. One way of improving this is by implementing larger axial scanners and, therefore, increasing the solid angle coverage. Alternatively, it is possible to increase the sensitivity gain by improving the timing capabilities of the detectors. However, from the most fundamental nature of particle interactions with matter, the 511-keV gamma rays suffer, in most of the cases, from scatter collisions either in the patient or within the detector block, before a photoelectric event eventually occurs. Recovering all scattered (Compton) events would improve scanner sensitivity. In this work, we show the performance of a detector block geometry suitable for the development of PET scanners based on several detector layers. A geometry using multiple layers favor the process of scattered events, at the time that allows one for their proper identification. The detector block consists of a LYSO crystal with $51.5\times 51.5$ mm2 surface and 3 mm thickness, resulting in a very high aspect ratio above 17. Four custom-made SiPM arrays of $1\times 16$ elements with $3\times 3$ mm2 area each are coupled to the lateral sides of the crystal. Four different methods to estimate the gamma-ray interaction position using the information collected by the four SiPM arrays have been implemented and compared in order to assess the most suitable one for this detector configuration and aspect ratio. A novel calibration method based on Voronoi diagrams has been successfully implemented, allowing us to recover data for the entire detector block. We have reached an intrinsic spatial resolution for the whole block of less than 1.6 mm FWHM, combined with an energy resolution of 12.1%. We have also compared the performance results with detector blocks using the same crystal but employing the standard backreading approach. Similar results were obtained but making use of four times less SiPM active area in the case of the lateral reading compared to the backreading method, and with the possibility to minimize the undesirable scatter in the photosensor layers.