Preserving preclinical PET quality during intratherapeutic imaging in radionuclide therapy with Rose metal shielding reducing photon flux.

Performing Positron Emission Tomography (PET) imaging intratherapeutically during ongoing radionuclide therapy, can be a promising method to follow tumor response in vivo. However, the high therapeutic activity can interfere with the PET camera performance and degrade both image quality and quantitative capabilities. As a solution, low energy photon emissions from the therapeutic radionuclide can be highly attenuated, still allowing sufficient detection of annihilation photons in coincidence. Methods: Hollow Rose metal cylinders with 2-4 mm walls were used to shield a 22 Na point source and a uniform phantom filled with 18 F as they were imaged on a pre-clinical PET camera with increasing activities of 177 Lu. A mouse with a subcutaneous tumor was injected with 18 F-FDG and imaged with an additional 120 MBq of 177 Lu and repeated with shields surrounding the animal. Results: The addition of 177 Lu to the volume imaged continuously degraded the image quality with increasing activity. The image quality was improved when shielding was introduced. The shields showed a high ability to produce stable and reproducible results of both spatial resolution and activity quantification up to 120 MBq of 177 Lu activity (maximum activity tested). Conclusion: Without shielding, the activity quantification will be inaccurate at time points where therapeutic activities are high. The suggested method shows that the shields reduce the noise induced by the 177 Lu and therefore enables longitudinal quantitative intratherapeutic imaging studies.