Performance of APD-based monolithic-crystal detectors for small animal PET

Recent years have seen an increased interest in high resolution, high sensitivity detectors for small animal positron emission tomography (PET). Detectors based on large (e.g. several cm³ of LSO) monolithic scintillators optically coupled to one or more avalanche photodiode (APD) array(s) minimize loss of sensitivity due to dead space. In such detectors, the primary event position and depth of interaction (DOI) correction are derived from the measured distribution of scintillation light over the APD array(s). The performance of detectors consisting of a monolithic LYSO scintillator read out by two Hamamatsu S8550 APD arrays is investigated. Monte Carlo simulations indicate that the scanner sensitivity and uniformity are improved when trapezoidal crystals are used instead of rectangular ones. The performance of both types of detectors was studied. Experiments show that the influence of the angle of incidence on the spatial resolution of both detector types is small, ranging from ∼2.0 mm FWHM at 0° to ∼2.4 mm FWHM at 30°. An energy resolution of 11.5% at 511 keV and a time resolution of 2.4 ns was measured. Monte Carlo simulations predict a sensitivity of ∼21% for a point source at the center of the field of view for a four-ring scanner based on trapezoidal monolithic detectors.