Calibration of a PEM detector with depth of interaction measurement

We present an in situ calibration technique for the LBNL positron emission mammography (PEM) detector module that is capable of measuring depth of interaction (DOI). The detector module consists of 64 LSO crystals coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD). The PMT provides an accurate timing pulse, the PDs identify the crystal of interaction, the sum provides a total energy signal and the Γ=PD/(PD+PMT) ratio determines the depth of interaction. We calibrate using the ¹⁷⁶Lu natural background radiation of the LSO crystals. We determine the relative gain (K) of the PMT and PD by minimizing the asymmetry of the Γ distribution. We determine the depth dependence from the width of the Γ distribution with optimal K. The performance of calibrated detector modules is evaluated by averaging results from 12 modules. The energy resolution is a function of depth ranging from 24% FWHM at the PD end to 51% FWHM at the PMT end, and the DOI resolution ranges from 6 mm FWHM at the PD end to 11 mm FWHM at the PMT end.