Evaluation of high density pixilated 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. We tested individual dual-layer blocks of pixilated lutetium oxy-orthosilicate (LYSO) coupled to SensL 4×4 SiPM arrays in terms of their ability to resolve all elements using resolvability index (RI) defined by the FWHM of the crystal response function divided by the separation. Our crystal blocks had 49 1.67 × 1.67 × 6.0 mm³ crystals on the bottom layer and 36 1.67 × 1.67 × 4.0 mm³ crystals in the top layer which was offset by ½ of the crystal pitch. All 85 crystals were well resolved: RI=0.46 compared with RI=.41 for a conventional pre-clinical PET scanner´s block with 40% lower crystal density. A pair of crystal blocks mounted on translation stages scanned a 0.25 mm ²²Na source in 60 0.25 mm steps with the detectors angulated as if there were 16 blocks in a ring. The FWHM near the centre of the field of view was 1.31 mm and FWTM was 2.7 mm.