Effects of high photon fluence rate from therapeutic radionuclides on preclinical and clinical PET systems

Tumor response in radionuclide therapy can be monitored with PET/CT and/or PET/MR. A high background photon fluence from a therapy radionuclide may influence both image quality and quantification, when imaging is performed intra-therapeutically, i.e. with high activity of the therapeutic radionuclide present. Here, count losses and image distortion have been investigated for preclinical and clinical PET systems with different detector designs. The effect on the spatial resolution was studied with a point source of 22Na in a background of 99mTc, where 99mTc emulated the photon emission from a therapeutic radionuclide. An in-house made mouse phantom with silicon tubes filled with 99mTc with a centrally placed 22Na point source was used. For the clinical systems, a 70 cm long NEMA PET Scatter Phantom was used, with a 22Na point source placed at the center whereas the off-center silicon tube was filled with 99mTc. In addition, image quality was also evaluated in the presence of different levels of 99mTc with a 18F-filled NEMA image quality phantom on the preclinical systems and a 18F-filled Jaszczak phantom on the clinical system. Preclinical PET systems with different detector geometries showed that the addition of 99mTc affected the count rate capability considerably, especially those with a low number of read-out channels. The coincidence rate for was significantly reduced when high activities of 99mTc were present. The clinical PET system also showed an effect of reduced coincidence rate with increased photon fluence rate. At high 99mTc activities, the spatial resolution was degraded for both the preclinical and the clinical systems. The quantitative capability of PET systems used intra-therapeutically is significantly affected by the additional high photon fluence rate. The dead-time correction implemented on some of the investigated PET systems, was able to accurately compensate for the coincidence count losses. The reduced spatial resolution at high photon fluence rate, however, remains a potentially limiting factor.