The effect of cascade and coincident gamma rays on PET I-124 imaging of non-uniform activity distributions

The positron emitting nuclide 1-124 is of interest as a surrogate for 1-131 prior to radioimmunotherapy. PET imaging with 1-124 can be used to view the expected biodistribution of the therapeutic agent and to provide high resolution data for dosimetry calculations prior to treatment. 1-124 imaging is complicated by gamma rays that accompany positron emission as well as by gamma ray cascades associated with decay by electron capture. Phantom imaging experiments were performed to explore whether asymmetric, non-uniform activity distributions of 1-124 significantly affect the quantitative accuracy of 1-124 measurements. The use of a scatter correction that fits single scatter simulation results to the observed sinogram tails with both a scale factor and an amplitude offset helps to provide accurate 1-124 imaging, with relative residual errors of less than about 1%. More sophisticated modeling of gamma ray emissions associated with decay of 1-124 may not be necessary for clinical applications.