Counting Rate Characteristics and Image Distortion in Preclinical PET Imaging During Radiopharmaceutical Therapy

PET may provide important information on the response during radio pharmaceutical therapy (RPT). Emission of radiation from the RPT radionuclide may disturb coincidence detection and impair image resolution. In this study, we tested the feasibility of performing intra-therapeutic PET on 3 preclinical PET systems. Methods: Using Na-22 point sources and phantoms filled with F-18, as well as a phantom filled with either Tc-99m or Lu-177, we evaluated the coincidence counting rate and spatial resolution when both a PET and a therapeutic radionuclide were in the PET system. Because Tc-99m has a suitable half-life and is easy obtainable, we used it as a substitute for a generic therapeutic radionuclide. Results: High activities of Tc-99m deteriorated the coincidence counting rate from the F-18-filled phantom and the Na-22 point source on all 3 systems. The counting rate could be corrected to a high degree on one of the systems by its dead-time correction. Spatial resolution was degraded at high Tc-99m activities for all systems. On one of the systems, Lu-177 increased the coincidence counting rate and slightly affected the spatial resolution. The results for high Lu-177 activities were similar to those for Tc-99m. Conclusion: Intratherapeutic imaging might be a feasible method of studying the response to RPT. However, some sensitive preclinical PET systems, unable to handle high counting rates, will have count losses and may also introduce image artifacts.