A pentagon photomultiplier-quadrant-sharing BGO detector for a rodent research PET (RRPET)

Using the photomultiplier-quadrant-sharing (PQS) technique, we designed pentagonal detector blocks with high resolution (2.0 mm) and high sensitivity for a rodent research PET (RRPET). In order to reduce the cost, 8 × 8 Bismuth germinate (BGO) crystal array and 19-mm regular round PMTs were used in this research. The average detector pitch was about 2.0 × 2.0 mm and 10 mm deep. To minimize the depth-of-interaction effect due to the "panelization" of the detectors, we modified the original cubical block for panelized PQS detector into a pentagon shape with the ends tapered, so that each pentagonal block acted as a facet of the polygonal detector ring. This tapered-pentagon-PQS design provides a very high packing-fraction (96%) for a ring-type PET. We fully exploited the flexibility of our latest PQS fabrication method, i.e., using partially painted reflective windows between crystals, in the pentagonal block development, this fabrication method allowed the fine tuning of the scintillation light distribution inside the block for optimal decoding. The effect of block geometry on PQS design was examined by developing cubical and pentagonal block directly coupled to four PMTs and decoding by pileup-recovery electronics. Two-dimensional crystal decoding maps and pulse-height spectra of the individual crystals with their corresponding energy resolutions were acquired. Each crystal was clearly decoded and the average individual crystal energy resolution was found to be 21% and 22.3% for cubical and pentagon block, respectively. The pentagon-PQS design is an alternative to the prevailing small animal PET detector assembly of lutetium oxyorthosilicate (LSO) or gadolinium oxyorthosilicate (GSO), optical-fiber or light-guide with PS-PMT.