PMT Quadrant-Sharing Technology

The Photomultiplier-Quadrant-Sharing (PQS) technique improves PET camera resolution and reduces the production cost of the detector. The PQS technique is a position-sensitive block design where a gamma hit on the block is decoded using one quadrant of four separate photomultipliers (PMT). In turn, one PMT participates in the decoding of four independent blocks. This design establishes a ratio of just one PMT to one detector block, reducing the production cost of the detector array by 75% over the conventional position-sensitive detector in which four PMTs are coupled to only one block.

Conventional position-sensitive detector blocks used in some commercial PET cameras are optically coupled to four single - anode PMT and the entire photosensitive area of the PMT is coupled to the block. In order to minimize the dead space between blocks, the crystals located on the periphery of the block, and particularly the corner ones, are placed on the rim of the PMT where the photosensitivity is very low. Consequently, decoding these crystals is difficult due to the lower number of photons collected from them. In the PQS design these crystals sit in the middle of the PMT, making light collection optimum, and all crystals are well decoded (see Detector block-to-PMT coupling). Overall light collection throughout the PQS block is very uniform because every crystal is selectively optically coupled to the surrounding crystals. This converts the block into a fine-tuned light guide, bringing the appropriate number of photons to each PMT for every crystal. In spite of the light losses in the middle of the block above the space between the four cylindrical PMTs, we find small difference in light collection between center and corner crystals.