Feasibility study of a compact high resolution dual layer LSO-APD detector module for positron emission tomography

An existing high resolution scanner for positron emission tomography (PET) of small animals was modified to investigate a scheme of depth-of-interaction (DOI) detection. The detectors were based on the readout of small (3.7×3.7×12 mm³) lutetium oxyorthosilicate (LSO) crystals with avalanche photodiodes (APD). For DOI measurements, 4 detector modules (arrays of 2×8 LSO-APD detectors) were mounted on a rotatable gantry in a dual layer setup. The energy resolution at 511 keV of a detector in the first layer was 17.7% (FWHM), decreasing to 19.3% in the second layer. The intrinsic resolution using detectors in the front layer varied between 2.1 mm and 3.5 mm. Combinations of crystals in the second layer yielded intrinsic resolutions of 2.0 mm to 2.8 mm (FWHM). The systems sensitivity could be increased by 20% when data of the second layer was included. Reconstruction of a ¹⁸F line source yielded a tomographic image resolution of 2.5 mm to 2.6 mm, depending on the usage of data from different layers. Without DOI information, the spatial resolution was 3.0 mm (FWHM). Although a degradation in energy resolution was observed in the dual layer setup compared to only one layer, this study showed the feasibility of individual readout in a dual layer module for improved sensitivity and sampling