Optimization and characterization of PET scanners for Medical Imaging

Positron emission tomography is an imaging technique that appeared to be a valid instrument for cancers detection and neuro-imaging studies. Since first models built during 1960s, an incredible effort has been done by researchers to develop scanners more and more advanced with higher specificity and efficiency. Monte Carlo simulations have shown to be a very important tool during design phase of PET prototypes thanks to their ability to simulate systems with many coupled degrees of freedom, as it happens when particles interact with matter. This Thesis work has started in the frame the Crystal Clear Collaboration when the EndoTOFPET scanner was already under development. This prototype is an high spatial resolution scanner for the study of pancreatic carcinoma and prostatic cancer, composed by a PET head mounted on an ultrasound endoscope and a PET plate to be placed outside the body. The Collaboration has chosen to use Monte Carlo simulations to support the design of this project and two simulations toolkits were available: Geant4 and SLitrani. In this work both the toolkits are studied and ray tracing in scintillator crystals are tested. In particular photon extraction efficiency is simulated under different surface treatments as coating and wrapping. Also the influence of the crystals geometry on light output is tested simulating different scintillators sections and lengths. Both Geant4 and SLitrani have shown to give similar results under these conditions. A main issue was observed regarding secondary particles since Geant4 is able to simulate their production while it is not possible with SLitrani. On other hand crystals anisotropy for optical photons can be activated in SLitrani. Light yield measurements were performed in laboratory on LYSO and PbWO 4 crystals to have a comparison with the results obtained by mean of simulations. Good agreements are obtained for what regards surfaces treatments while more tuning was required to simulate the effect of surface imperfections and diffusion inside crystals. For the Collaboration purposes, Geant4 results more reliable and it allows to use GATE, an open source software specifically developed for the simulation of medical imaging scanners. Due to the peculiar structure of the PET prototype it was necessary to develop a code to simulate the electronic chain, responsible for transforming the gammas detected in usable data for image reconstruction. Different coincidences sorting algorithms were studied and methods to introduce instrumental uncertainties in data were developed and reported in this work. Simulations of EndoTOFPET scanner with different scintillator dimensions, modules geometries and plate configurations were performed. Sensibility and spatial resolution were used as elements of comparison and results collected by simulations are reported and analysed in this work. Time of flight was tested applying different time resolutions while system response to DOI analysis was studied too. Thanks to these first simulations, valuable information for the developing of this prototype were collected.