Investigation of timing performance of scintillation detectors for time of flight pet

The incorporation of TOF information into PET image reconstruction results in an improvement in image signal to noise, corresponding to improvement in image quality and clinical diagnostic utility. Detector timing performance has been a key area of improvement in PET in recent years. While partial models of detector timing performance have been developed, key aspects which are at the frontier of detector design are not fully elucidated. In order to understand the physical processes affecting timing performance of a full scale PET scanner, we investigated the contributions of the scintillator, photodetector, detector geometry and data acquisition separately, using a combination of analytical tools and experiments supplemented with Monte-Carlo simulation models. We investigate the intrinsic timing performance of multiple scintillator materials, with a focus on LaBr3:Ce based detectors, a novel class of scintillators exhibiting high light yields and short decay constants. Energy resolution and coincidence timing resolution values of 4% FWHM at 511KeV and \u3c100ps FWHM are measured using LaBr3:Ce based detectors, an improvement of over x2 in both parameters as compared with LYSO based detectors used commercially. We measure the effect of cerium concentration on scintillation properties of LaBr3:Ce. Inclusion of scintillator rise time in simulation model of detector timing resolution improves accuracy of the model. For LaBr3[5%Ce], bi-exponential timing model predicts timing resolution of 126±3ps, which is in good agreement with measured timing resolution of 127±5ps, in comparison to the single exponential model prediction of 82±2ps. We evaluate the effect of photodetector timing characteristics on detector timing performance using fast PMTs and SiPMs. In collaboration with the Silicon Radiation Sensors group at FBK, we evaluated the timing performance of SiPMs customized for TOF PET readout. Energy resolution and coincidence timing resolution values of 10% FWHM at 511KeV and 230ps FWHM, respectively, were measured using 4x4x5mm3 LYSO crystals, comparable to those achieved with a high performance PMT. Devices with improved Near-IR sensitivity were custom designed for improved detection efficiency of emission wavelength of LaBr 3:Ce scintillators, resulting in coincidence timing resolution of 95ps FWHM for a 4x4x5mm3, comparable to that achieved with high performance PMTs. Timing performance losses in thick crystals were investigated using crystal of different thickness and controlled DOI measurements. Degraded timing resolution was shown to be partially caused by the systematic shift in signal detection time with DOI, resulting from the slower propagation velocity of scintillation photons in the dense crystal. We show that by measuring other DOI dependent signal features such as light collection, we can correct for DOI dispersion and improve timing performance to \u3c200ps FWHM in a 30mm thick LaBr 3[30%Ce] crystal. In order to achieve both direct DOI measurement and good timing performance, we designed a mutli-layer detector comprising of optically contiguous layers of scintillator crystals of different rise time. Using signal rise time to encode each layer, we achieve excellent DOI discrimination while maintaining timing performance achieved with a standard, single-layer detector. Finally, we describe the design and performance of a large area light sharing detector, comprising of arrayed 4x4x30mm3 LaBr3:Ce crystals optically coupled to an array of PMTs via a waveguide. We show that position-dependent variation in light collection as well as differences in gain and transit time among PMTs lead to the observed losses in timing performance, and examine calibration techniques to mitigate these effects. We describe the design of a LaBr3:Ce based prototype whole-body TOF PET scanner, with particular emphasis on the design considerations of the data acquisition electronics used for tagging event timing. Measured average coincidence timing resolution of 375ps is superior to that achieved in commercial TOF-PET scanners (550-600ps). In addition, the energy resolution of 7% FWHM at 511Kev is better than that of commercial LYSO or LSO scanners (~12%), allowing for a tighter energy window with a reduced scatter contamination. We describe the imaging performance of the scanner, demonstrating the improvement in signal to noise for phantom and human studies