Sunday, May 18, 2025
5/18/2025
Tl-Halide Scintillators for TOF-PET - Project Summary The current detector technology in PET requires scintillation that has fast response, excellent timing resolution, high detection sensitivity, good energy resolution, and last but not least acceptable cost. At present, most PET systems use crystals of LSO (Lu2SiO5:Ce) or its analog LYSO, which satisfy many of the listed requirements. But LSO, after years of development, has reached its performance limit, especially for the scanners with a long axial field of view (AFOV) that are currently being developed. The goal of these scanners is to increase the geometrical coverage and significantly increase detection sensitivity (by a factor of 30-40), thereby reducing the scanning times (30-40 times faster) or the patient’s radiation exposure. However, long AFOV scanners face two main challenges: greater depth-of-interaction (DOI) effects, which increase blurring and noise; and an increase in the volume required for the constituent crystals, which make up some 50% of the cost of the entire scanner. The use of shorter crystals can counteract both the DOI effects and the increased crystal volume (hence cost), but with a major loss of detection efficiency, defeating the original purpose. Another approach for reducing DOI effects is a double-ended read-out but this increases both cost and system complexity. Therefore, to achieve viable and affordable long AFOV scanners, a new scintillation material is required that would provide higher stopping power than LSO, with similar or better timing properties, and at a lower cost. These requirements can be met by a scintillator based on TlCl. This host has a comparable density (7.0 g/cm3 vs. 7.4 g/cm3 for LSO) and a much higher effective Z of 77 (vs. 65 to LSO.) When double doped with Be and I, it exhibits a fast donor-acceptor type luminescence with decay time <10 ns along with ultra-fast Cherenkov component (vs. 40 ns of LSO.) While the material’s light yield is low, its timing properties are excellent with better than 300 ps resolution FWHM when paired with LYSO, without any significant optimization. The only property where the material is deficient is its energy resolution (>15% at 511 keV, due to its low light yield). The key metric for TlCl is also its very low cost, in terms of raw materials as well as production – estimated 50% lower than LSO. In this project, we plan to optimize the doping content of TlCl to maximize its scintillation properties and achieve energy resolution of about 10% at 511 keV and timing resolution of 200 ps. In Phase II we will increase the volumes of produced material, develop cost reduction schemes, and produce and evaluate PET detection modules with the same performance goals.
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