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 19mm PMT). 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 PMT, we find small difference in light collection between center and corner crystals.
A PET camera using the PQS technique consists of flat detector panels arranged in a polygon shape around the patient as shown by the first whole-body/brain prototype built by us (MDAPET). The detector panel is formed by a continuum of blocks extendable to any number of blocks rather than just the aggregate of optically and mechanically independent blocks with gaps between them. Thus, the PQS mode presents better detector packing fraction, improving the camera's sensitivity.
By using the same type of PMT as in commercial cameras, we were able to reduce the crystal’s dimensions to one-half, improving the resolution of the camera. On the other hand, if the crystal dimension is maintained, the PMT can be doubled in diameter with fewer PMT required to build a camera with equal number of crystals, reducing the cost of the machine (see 40mm PMT).
PQS Quadrant Sharing Design
Four Independent Blocks Share One PMT
Conventional Design (One Block for Four PMTs)
Detector Panel Using PQS