Protocols for Dual Tracer PET/SPECT Preclinical Imaging

Background: Multi-tracer PET/SPECT imaging enables different modality tracers to be present simultaneously, allowing multiple physiological processes to be imaged in the same subject, within a short time-frame. Fluorine-18 and technetium-99m, two commonly used PET and SPECT radionuclides, respectively, possess different emission profiles, offering the potential for imaging one in the presence of the other. However, the impact of the presence of each radionuclide on scanning the other could be significant and lead to confounding results. Here we use combinations of F-18 and Tc-99m to explore the challenges posed by dual tracer PET/SPECT imaging, and investigate potential practical ways to overcome them. Methods: Mixed-radionuclide F-18/Tc-99m phantom PET and SPECT imaging experiments were carried out to determine the crossover effects of each radionuclide on the scans using Mediso nanoScan PET/CT and SPECT/CT small animal scanners. Results: PET scan image quality and quantification were adversely affected by Tc-99m activities higher than 100 MBq due to a high singles rate increasing dead-time of the detectors. Below 100 MBq Tc-99m, PET scanner quantification accuracy was preserved. SPECT scan image quality and quantification were adversely affected by the presence of F-18 due to Compton scattering of 511 keV photons leading to over-estimation of Tc-99m activity and increased noise. However, Tc-99m:F-18 activity ratios of > 70:1 were found to mitigate this effect completely on the SPECT. A method for correcting for Compton scatter was also explored. Conclusion: Suitable combinations of injection sequence and imaging sequence can be devised to meet specific experimental multi-tracer imaging needs, with only minor or insignificant effects of each radionuclide on the scan of the other.