Abstract
BackgroundThe goal of this work was to determine the quantitative accuracy and optimal reconstruction parameters for 124I-PET imaging in the presence of therapeutic levels of 131I. In this effort, images were acquired on a GE D710 PET/CT scanner using a NEMA IEC phantom with spheres containing 124I and increasing amounts of 131I activity in the background. At each activity level, two scans were acquired, one with the phantom centered in the field of view (FOV) and one 11.2 cm off-center. Reconstructions used an ordered subset expectation maximization algorithm with up to 100 iterations of 16 subsets, with and without time-of-flight (TOF) information. Results were evaluated visually and by comparing the 124I activity relative to the scan performed in the absence of 131I.Results131I within the FOV added to the randoms rate, to dead time, and to pile-up within the detectors. Using our standard clinical reconstruction parameters, the image quality and quantitative accuracy suffered at 131I activities above 1.4 GBq. Convergence rates slowed progressively in the presence of increasing amounts of 131I for both TOF and nonTOF reconstructions. TOF reconstructions converged more quickly than nonTOF but often towards erroneous concentrations. Iterating nonTOF reconstructions to convergence produced quantitatively accurate images except for the off-center phantom at the very highest level of background 131I tested.ConclusionsThis study shows that quantitative PET is feasible in the presence of large amounts of 131I. The high randoms fractions resulted in slow reconstruction convergence and negatively impacted TOF corrections and/or the accuracy of TOF information. Therefore, increased iterations and nonTOF reconstructions are recommended.
Highlights
The goal of this work was to determine the quantitative accuracy and optimal reconstruction parameters for 124I-Positron emission tomography (PET) imaging in the presence of therapeutic levels of 131I
Activity quantification when using clinical reconstruction parameters The initial measurements of the 124I activity concentration were based on PET images reconstructed using our standard clinical reconstruction parameters, 2 iterations of the ordered subset expectation maximization (OSEM) algorithm utilizing 16 subsets (2 × 16) in TOF-mode with point-spread modeling (SharpIR)
The activity increase for the small spheres was attributable to the oversized Volume of interest (VOI) we drew to capture all the activity emanating from each sphere assuming that the true background was zero
Summary
The goal of this work was to determine the quantitative accuracy and optimal reconstruction parameters for 124I-PET imaging in the presence of therapeutic levels of 131I. For pre-therapy dosimetry, a diagnostic level of radiotracer is injected into the patient, followed by serial blood sampling and imaging [1, 2] This information is used to estimate the patient (2021) 8:50 dose distribution for the planned therapy with the assumption that the dose will scale linearly with the radioactivity injected. To validate this assumption in the context of radionuclide therapy with 131I-Iodide, we considered comparing pre-therapy PET images made using (the more quantitatively accurate) 124I-Iodide against similar PET images acquired following co-injection of 124I-Iodide along with the 131I-Iodide therapeutic dose. Knowing the patient-specific uptake of the radionuclide targeting agent allows for the customization of the therapeutic plan so that it is consistent with the dose constraints of the dose-limiting tissues [5]
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