Preliminary PET performance of the detectors for OPET: a combined optical and PET imaging system

The dual modality imaging system for small animal optical and positron emission tomography imaging (OPET) is currently being developed at the Crump Institute for Molecular Imaging. The OPET system is capable of detecting and simultaneously imaging both high-energy gamma-rays from PET tracers and single photons in the optical wavelength from bioluminescence. The PET performance characteristics for a two-detector prototype of the OPET system are described here. Each detector consists of an 8 × 8 array of GSO crystals cut to different lengths providing a detector surface with a circular profile and a 3.4 cm ring diameter. The crystals were inserted into a laser-cut grid assembly constructed from 0.067 mm thick reflector film. Crystals were coupled to a 64-channel photomultiplier tube (PMT) and a charge-division readout circuit was implemented to decode the signals from the 64 outputs of the PMT into event location data. Flood images were taken to obtain look-up tables as well as energy spectra for each crystal. The timing resolution was determined to be 6.6 nsec. A 0.3 mm Na-22 point source was stepped across the field of view (FOV) and the intrinsic detector spatial resolution was 1.23 mm FWHM. The estimated peak absolute system sensitivity at the center of FOV for the two detector system was 0.33%. Data from a line phantom was acquired and reconstructed with an expectation maximization 3D reconstruction algorithm.