Design study of a gamma ray detector based on LSO:Ce,Ca and SiPM for PET applications

Positron Emission Tomography (PET) is a nuclear medicine imaging technique that produces images of functional processes in the patient by means of radiotracers injected in the body and metabolized by the organ under examination. A radiotracer is a biologically active molecule linked to a beta+ radionuclide: a positron annihilates with an electron in the body producing two opposite 511 keV gamma rays. The annihilation photons are then detected by a system composed by a ring of detectors in temporal coincidence in order to identify the Line Of Response (LOR) of the emitted photons. The intersection of several LORs allows to determine the gamma rays emission site. The informations obtained by PET can give feedback on blood flow, oxygen or glucose consumption and protein synthesis. One of the major limitation of PET is the low sensitivity. A way to improve it is to measure with high precision the arrival time difference between the two annihilation photons in order to constrain the emission point to a particular segment along the LOR. This technique is called Time Of Flight (TOF) PET: using TOF algorithm it would be possible to improve the PET performances in terms of noise variance, reduced random event rate, axial blurring and total scan time.\\The detector modules used in PET are usually composed by scintillating crystals like LSO:Ce coupled with PhotoMultiplier Tubes (PMT). To develop TOF PET it is necessary to study a fast detector system: several researches are being performed to improve the time performances of the PET detectors. Recent studies demonstrate that LSO:Ce codoped with Ca (LSO:Ce,Ca) shows reduced decay time, higher light output and better energy resolution compared to the standard LSO:Ce. For what concerns the photodetectors, a valid alternative to PMTs is represented by the Silicon PhotoMultipliers (SiPM). They are inherently fast (the single photon timing resolution is of 60 ps), show high gain at rather low operating bias (order of 50 V) and are insensitive to magnetic fields, making them attractive for new development of PET like TOF PET and hybrid PET-MRI imaging. For these reasons, a detector based on SiPM coupled to an LSO:Ce,Ca crystal could introduce several improvements with respect to the currently used PET modules, especially for what concerns their timing performances. To optimise the detector performances, a study of the shape of the current pulse produced in response to the incident photons is required. The aim of this thesis is to assess the overall performance of a PET detector composed by a SiPM coupled to LSO:Ce,Ca (with 0, 0.1, 0.2 and 0.3 % Ca concentration) and connected to a fast transimpedance amplifier. Initially, an overview of PET, TOF PET and PET-MRI imaging techniques is presented, focusing on their basic working principles, detection systems and figures of merit. The detection system composed by SiPM and LSO:Ce,Ca is then described. The SiPM working principles and the physics of the scintillating crystals are presented, mainly focusing on the suitability of the detector components for PET, TOF PET and PET-MRI applications. Then, a modeling of the detector is described: it considers a PET detector module composed by a scintillator, a SiPM and its amplification circuit. A suitable electric model for the SiPM and an analytic model for the scintillator have been considered to estimate the system characteristics, like the time jitter of the signals. Several measurements have been performed in order to fully characterize the detector composed by SiPM and LSO:Ce,Ca and the results have been compared with the simulations. The photodetector has been studied in terms of the current drawn as a function of the applied bias voltage (I-V curve) and in terms of dark noise. Subsequently, the detector linearity and energy resolution have been measured by using crystals with different Ca concentration. A timing study has been carried out, focusing on the measurements of the scintillators decay time and of the timing resolution achievable in coincidence evaluated for different Ca codoping. A spectral characterization of LSO:Ce,Ca has been performed in terms of absorption, emission and excitation spectra in order to study the Ca influence on the optical properties of the crystals