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

PET may provide important information on the response during radiopharmaceutical 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 intratherapeutic PET on 3 preclinical PET systems.

Using ²²Na point sources and phantoms filled with ¹⁸F, as well as a phantom filled with either ^99mTc or ¹⁷⁷Lu, we evaluated the coincidence counting rate and spatial resolution when both a PET and a therapeutic radionuclide were in the PET system. Because ^99mTc has a suitable half-life and is easy obtainable, we used it as a substitute for a generic therapeutic radionuclide.

High activities of ^99mTc deteriorated the coincidence counting rate from the ¹⁸F-filled phantom and the ²²Na 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 ^99mTc activities for all systems. On one of the systems, ¹⁷⁷Lu increased the coincidence counting rate and slightly affected the spatial resolution. The results for high ¹⁷⁷Lu activities were similar to those for ^99mTc.

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.