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 Lu2O3. This host has a very high density (9.4 g/cm3 vs. 7.4 g/cm3 for LSO) and an effective Z of 68 vs. 65. When doped with Yb3+, it exhibits an ultra-fast charge transfer (CT) luminescence with decay time on the order of 1 ns, substantially faster than the 40 ns of LSO. While the material's light yield is low, its timing properties are excellent with better than 250 ps resolution FWHM when paired with LaBr3:Ce. The only property where the material is deficient is its energy resolution (>15% at 511 keV, due to its low light yield). Fortunately, this shortcoming can be addressed by double doping, which increases its light yield to about 20,000 ph/MeV. In this project, we plan to optimize the doping content of Lu2O3 so as to maximize its scintillation properties and achieve energy resolution of about 8% 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.
The project aims at development of a new scintillator for a long axial field of view (AFOV) PET scanners. Such scanners currently suffer from blurring and noise that can be addressed by more efficiency detectors. Our material will compete with the current detector choices on the detection efficiency, overall performance, and cost.
