Evaluation of performance and stability of an MR compatible PET detector

We have developed an MR compatible PET insert prototype to enable simultaneous PET-MR imaging of small animals inside a 7T animal MR imaging system. The 16 detectors in this system are based on the SensL SPMArray4B silicon photomultipliers (SiPMs) and dual layer offset LYSO crystal blocks with crystal size of 1.2 × 1.2 × 4 mm³ in the top layer and 1.2 × 1.2 × 6 mm³ in the bottom layer, with 409 total crystals per detector. Each detector was evaluated by generating flood histogram images and measuring per-crystal photopeak amplitude and energy resolution. The system was tested for MR compatibility and no visible artefacts were seen in the MR images or in the PET detector flood images. The temperature of the detector modules was monitored and found to stabilize after approximately two hours of operation between 32°C and 34°C. Photopeak amplitude, energy resolution and count rates recorded per crystal were studied at a baseline temperature of 30°C and at temperatures ranging from 20°C to 40°C in increments of 2°C. When bias voltage to the detectors was fixed at the value corresponding to an overvoltage of 2.5V at 30°C, the photopeak amplitude changed by as much as 19% and 28% at 20°C and 40°C respectively. Energy resolution and count rates demonstrated similar degrees of change. Maintaining a fixed overvoltage at each temperature point reduced but did not completely eliminate the change in these output characteristics. A lookup table based on empirical measurements was employed for the adjustment of bias voltage at different temperatures to maximize the stability of detector modules. Measurements obtained by using this lookup table method demonstrate better stability when compared to results obtained by using a fixed overvoltage.