Abstract
BackgroundMulti-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 18F and 99mTc to explore the challenges posed by dual tracer PET/SPECT imaging, and investigate potential practical ways to overcome them.MethodsMixed-radionuclide 18F/99mTc 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.ResultsPET scan image quality and quantification were adversely affected by 99mTc activities higher than 100 MBq due to a high singles rate increasing dead-time of the detectors. Below 100 MBq 99mTc, PET scanner quantification accuracy was preserved. SPECT scan image quality and quantification were adversely affected by the presence of 18F due to Compton scattering of 511 keV photons leading to over-estimation of 99mTc activity and increased noise. However, 99mTc:18F 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.ConclusionSuitable 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.
Highlights
PET/SPECT Imaging fluorine-18 (PET) and SPECT tracers allow us to probe the underlying molecular characteristics of physiological processes, one mechanism at a time
The effect of the presence of SPECT radionuclide 99mTc on 18F PET scans was assessed by the use of phantoms
The PET scanner was calibrated prior to the start of the study following manufacturer procedures; 18F-only phantoms measured in the dose calibrator and PET scanner showed good agreement
Summary
PET and SPECT tracers allow us to probe the underlying molecular characteristics of physiological processes, one mechanism at a time. Photons emitted from decaying 99mTc nuclei are not coincident, and their energy of 140 keV is much lower than the 511 keV PET scanner energy window, so do not contribute to the image data acquired They do interact with the PET detectors, and at high enough flux, can potentially prevent true coincidence events from the positron emitter being recorded. Results: PET scan image quality and quantification were adversely affected by 99mTc activities higher than 100 MBq due to a high singles rate increasing dead-time of the detectors. SPECT scan image quality and quantification were adversely affected by the presence of 18F due to Compton scattering of 511 keV photons leading to over-estimation of 99mTc activity and increased noise.
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