Inexpensive position sensitive detector block for 40 mm diameter PMT using quadrant sharing configuration

Recent approval by CMS (former HCFA) for reimbursement of PET scans used in certain cases of diagnostic oncology, and the rapid grow of the PET market that this approval has generated (100% increase in year 2000) creates the need for more affordable dedicated PET scanners. The objective of the work presented here is the development of a position sensitive block with the same detector area (40mm x 40mm) and number of crystals, (M) of the block of a commercial camera using the photomultiplier quadrant sharing technique (PQS). This block is coupled to four single-anode 40 mm diameter PMTs and each PMT is sheared by four block detectors. Significant savings come from the number of PMTs required which are only 25% of what otherwise would be necessary to build a comparable commercial camera. Due to unused photocatode area left by a PQS array of square blocks in a detector module, a rectangular (extended) block was developed. This block maximizes use of the PMT sensitive area and minimizes gap size between modules. It was only necessary to develop the extended block because a symmetric square block uses same type of partitions like those controlling light distribution along the short side of the rectangular one. White-paint masks applied with accurate templates and airbrush were fine-tuned for every pair of adjacent crystals. Crystal decoding presents good separation uniformly distributed over the two-dimensional decoding map of the block. Composite energy spectrum of all 64 crystals shows a prominent photopeak (39% energy resolution) and relatively small Compton edge. It indicates that the block has a very uniform light collection for all crystals. It is expected that the image resolution using this type of block be comparable to the resolution of commercial cameras, because we clearly decode the same number and size of crystals.