Implementation and characterisation of radiation detectors based on SIPM for medical imaging

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In the last decades, medical imaging techniques have revolutionized medicine facilitating the work to the clinicians, favouring the earlier diagnosis of diseases such as cancer, and reducing the time required for surgical procedures. Among these techniques, one of the most promising is Positron Emission Tomography (PET) due to its constant evolution, the functional information it provides and the possibility of combining it with other structural techniques such as CT or MRI. Recently, new generations of PET detectors have been developed leaving behind the conventionally used photomultiplier tubes (PMTs) for the state-of-the-art digital silicone photomultipliers (d-SiPM). In this work, the last generation of radiation detectors, Philips Digital Photon Counting’s (PDPC) d-SiPMs, was studied and characterized. These detectors are used in the commercial Philips Vereos time-of-flight PET/CT scanner, as well as in the Hyperion-IID preclinical PET scanner. The main objective of this work was to learn how to operate this new system in optimum conditions for small-animal imaging, how to design a precise centre of gravity (COG) algorithm for the localization of the scintillator pixels in a scintillator array, and to characterize the energy and spatial resolutions obtained with this PDPC module. Different COG algorithms were tested, and the final one was designed in such a way that only valid events were considered. This algorithm focuses on the main pixel of each event and the eight pixels surrounding it, discarding scatter and noise as much as possible. The energy resolution was measured by studying the full width half maximum (FWHM) of the photopeak, whereas the spatial resolution was measured by computing the valley-to-peak ratio (V/P) and the resolvability index (RI) of a profile taken from the flood field images acquired. In this project, we used a 30×30 scintillator matrix of LYSO crystals of 1.3×1.3×12 mm3, coupled to a 50×50×2 mm3 light guide in order to spread the scintillation photons among 36 of the 64 die sensors integrated in the PDPC DPC 3200-22 module. A study of how the temperature affects the performance of the system and which acquisition parameters, light guide and time window gives better results was performed. As a final check, we compared the initial and final images obtained, considering their spatial and energy resolution.
Medical imaging techniques, Positron Emission Tomography (PET), Radiation detectors, Small-animal imaging, Centre of gravity algorithms, Algorithms
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