Evaluation of very highly pixellated crystal blocks with SiPM readout as candidates for PET/MR detectors in a small animal PET insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxyorthosilicate (LYSO) crystals their energy resolution, and coincidence response function. Two SensL SPMarray-4 were used as light sensors. Two L YSO crystal blocks consisting of a arrays of 8 × 8 1.2 mm × 1.2 mm × 6.0 mm crystals on the lower layer and 7 × 7 1.2 mm × 1.2 mm × 4.0 mm crystals on the upper layer (which is offset by Yz the crystal width) were mounted on the SensL arrays. A second study was performed with single layer arrays of 8 × 8 1.2 mm × 1.2 mm × 10.0 mm crystals. All of the crystals in the dual layer block were easily identified with a peak to valley ratio of 7.2 while the single layer blocks had a peak to valley ratio of 11.2. The crystal dots were more uniformly spaced in the dual layer arrays suggesting that the inter-layer connection affords some useful light sharing. The average energy resolution for all 113 crystals was 16.3 ± 2.3%. The coincidence response to a 0.25 mm Na-22 source in plastic was 0.97 ± 0.12 mm for normally incident gamma rays with the detectors 120 mm apart. These results indicate that it should be possible to achieve a spatial resolution of about one millimetre near the center of the field of view.