Experimental characterization of monolithic-crystal small animal PET detectors read out by APD arrays

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic scintillator crystals (e.g., 20 mm x 10 mm x 10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such crystals can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the crystal. The entry point of a gamma photon on the crystal surface can be estimated from the measured distribution of the scintillation light over the APD array(s). By estimating the entry point, correction for the depth-of-interaction (DOI) is automatically provided. We are studying the feasibility of such detector modules. To this end, a 64-channel test setup has been developed. Experiments to determine the effect on the spatial resolution of crystal surface finish and detector geometry have been carried out. The first results of these experiments are presented and compared to simulation results. The crystal surface finish has only a small influence on the spatial resolution. The spatial resolution of 20 mm x 10 nun x 10 mm detectors is significantly better when read out on the front side than when read out on the back side. With a 20 mm x 10 mm x 20 mm crystal coupled to two APD arrays, a very small resolution degradation of only similar to 0.2 min is observed for an incidence angle of 30 degrees compared to normal incidence.