How low is the dose of FDG in a PET/MRI scan?

A group at the Sorbonne in Paris simulated a PET/MRI scan with a 90% reduction in the dose of the injected radioactive F-18 FDG in patients with cognitive impairment. The images remained reliable in distinguishing between Alzheimer’s disease and frontotemporal dementia, according to the results.

“The ultra-low-dose protocol offers the potential for a significant reduction in exposure to ionizing radiation,” medical physicist Maren Sort, PhD, and colleagues write.

In the past five years, PET imaging technology has improved significantly, with PET/MRI scanners providing an increased focal field of view resulting in higher sensitivity. For brain F-18 FDG-PET/MRI, the reduction in the overall time to PET acquisition (and thus radiation exposure by F-18 FDG) is limited by MRI, which typically requires an image acquisition time of 20 min or longer.

Thus, the researchers suggest that increasing the time of PET acquisition to match that of the MRI allows to reduce the dose of the F-18 FDG radiotracer. In previous work, the group showed that the activity of the radio tracker could be reduced to 1 MB/kg versus the standard 2 MB/kg without significant image quality deterioration. In this feasibility study, they aimed to reduce the activity of the injected F-18 FDG by 10-fold.

The authors used a previously validated method to simulate a 0.2 MBq/kg dose of injected FDG. Essentially, they reconstructed a second set of images from data acquired during the first 2 minutes of a standard PET/MRI scan (Signa PET/MRI, GE Healthcare), with scanner parameters optimized by physicists to ensure a high signal-to-noise ratio. .

The researchers then compared imaging from a simulated ultra-low-dose PET/MRI protocol with imaging from the standard protocol in 50 patients with cognitive impairment. They compared visual analyzes of methods by three experienced clinicians.

Cross-sectional slides of two patients with suspected Alzheimer’s disease with mild hypometabolism (patient 1) and severe hypometabolism (patient 2) at standard dose (PET/STD, 2 MBq/kg) and very low dose (PET/ULD, 0.2 MBq/kg) protocol . The cross-sectional slices are extracted from the Advantage Workstation (AW 3.2 server, GE Healthcare) used to perform the interpretation. Image courtesy of Scientific Reports.

Physician readers categorized F-18 FDG PET patterns into seven categories: normal (N), non-valuable (NE), Alzheimer’s disease (AD), variable lupusic variable primary aphasia (lvPPA), posterior cortical atrophy (PCA), semantic progressive variable Primary aphasia (svPPA) and frontotemporal dementia (FTD).

Based on a consensus reading of images with standard F-18 FDG dose amount, tests were visually normal in 16 patients and suggestive of AD in 15 patients, svPPA in six, FTD in eight, and lvPPA or PCA in five.

According to the results, the intraoperator agreement between the standard dose FDG (2 MBq/kg) and the ultra-low dose FDG (0.2 MBq/kg) to visually classify patients in the seven categories was nearly perfect in one assessor and significant in the other two categories (≥ 0.71) over the entire group.

In addition, group comparisons of radioactive absorption between 15 patients with Alzheimer’s disease and eight patients with frontotemporal dementia in consecutive scans using both methods yielded very similar results, the authors wrote. As expected, uptake ratios were significantly decreased in Alzheimer’s disease compared with FTD in the parietal cortex, and lateral occipital cortex (p < 0.005).

The authors concluded that performing a brain PET/MRI for 20 min while reducing F-18 FDG activity to less than 0.2 Mb/kg is feasible without compromising the quantitative measurements.

“It does not alter imaging diagnosis in patients with obvious regional abnormalities,” the researchers wrote about the protocol.

Ultimately, this was a feasibility study showing that an ultra-low-dose protocol involving a reduction in F-18 FDG injection activity by 90% did not significantly modify evaluations of PET/MR brain images. This suggests that an activity as low as 0.2 MPa/kg may be sufficient to differentiate Alzheimer’s disease from FTD, the group wrote.

The authors wrote that reducing the injected dose of F-18 FDG could significantly reduce radiopharmaceutical costs in a department that performs a lot of FDG-PET scans, as well as improve radiation protection for young patients who undergo frequent screenings.

Based on these findings, they recommend injections of 1-2 MB/kg (70-140 MB) for early diagnosis of neurodegenerative disorders to ensure accurate diagnosis and to maintain a very low dose protocol for subsequent scans.

Soret and colleagues conclude that, “The ultra-low-dose protocol offers the potential for a significant reduction in exposure to ionizing radiation, but should be reserved for differential diagnosis or follow-up in patients with severe cognitive impairment or dementia.”

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