A pentagon Photomultiplier-Quadrant-Sharing BGO detector for a rodent research PET

Using the Photomultiplier-Quadrant-Sharing (PQS) design, BGO-detector blocks have been developed to show the feasibility of high resolution (~2mm) at much lower cost for small animal PET. It is an alternative to the prevailing animal PET detector assembly of LSO or GSO, optical-fiber or light-guide with PS-PMT. In order to reduce cost, BGO crystals and 19-mm regular round PMTs were used in this rodent research PET. The block was an 8 × 8 BGO crystal array, the average detector pitch was about 2 × 2 mm and 10 mm deep. To minimize the depth-of-interaction effect, the original cubical block for panelized PQS detector needs to be modified into pentagon shape with the ends tapered, each pentagon block is a facet of the polygonal detector ring. The tapered-pentagon-PQS design provides a very high packing-fraction (98%) for ring type PET. We fully exploited the flexibility of our latest PQS fabrication method of using partially painted reflective window between crystals into pentagon block development, which allowed fine tuning the light distribution by changing reflectant shape on each crystal for optimal decoding. In order to study the effect of block geometry and PQS design, cubical and pentagon block were developed to directly couple to four PMTs and decoded by pileup-recovery electronics. Two-dimensional crystal-decoding map, pulse-height spectrum for single crystal and block with their corresponding energy resolution 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.