A longitudinal slited-reflector light-distribution scheme for position-sensitive PET scintillation block detectors

Abstract

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Objectives: A higher spatial resolution of a position-sensitive PET block detector requires more accuracy in event positioning, a better scintillation light-output and a more uniform energy resolution. We have investigated a new light-distribution scheme for block detectors by applying reflectors with a long slit along the whole length of each crystal needle to determine if these important areas can be achieved.

Methods: Four major light-sharing methods that have been used in scintillation detector blocks: (1) sawing a block into pseudo-discrete elements following a certain light-sharing pattern and filled the gaps with a reflective material; (2) having varying degrees of roughness for inner discrete elements and coupled to each other by air gap or coupling compounds; (3) coupling plastic light guides to a block with completed optical isolated elements; (4) using internal step-shape light guides among the individual elements with optical reflectors. The drawback of method-(1) includes low production yield from breakage, small packing fraction and the event positioning shift with the interaction depths along the length of a crystal. Method-(2) has no effect of positioning shift but there is limited degree of freedom to allow the decoding of larger crystal matrix. Method-(3) has a major drawback of light loss by using the extra light guide. Method-(4) has a very small event positioning shift benefited by applying reflectors directly to individual elements, but step reflectors may cause block slant. We proposed a new light-sharing scheme by applying slit window along the length of discrete crystal covered by white-paint or mirror film reflector, the light-distribution is controlled by applying unique width of the optical slit for individual crystals. Light sharing occurs along the full length of discrete elements and light from an event hitting at different depth of an element has the same probability going to neighboring elements, thereby minimizing the positioning shift with depth.

Results: We compared the new slited-reflector light-distribution method to the method-(4) using step-reflectors by keeping the same light distribution ratios in 9 x 9 GSO block (2.04 mm x 2.04 mmx 10 mm) and 9 x 9 BGO block (4.3 mm x 4.3 mm x 25 mm). The blocks built with new method obtained better decoding results and uniformity of energy resolution, no block slanting was observed; 22% and 18% more light outputs were collected by the GSO and BGO block respectively.

Conclusions: The Longitudinal slited-reflector light-distribution scheme has been proposed; it is more suitable method for making better position-sensitive block detectors with narrower crystals used in higher resolution PET than the current commercial methods.