Do Gadolinium-Based Contrast Agents Affect 18F-FDG PET/CT Uptake in the Dentate Nucleus and the Globus Pallidus? A Pilot Study.

Gadolinium is toxic and to avoid its deposition in tissues, it must be chemically bonded with nonmetal ions to facilitate its excretion by the kidneys. High signal intensity in the dentate nucleus (DN) and globus pallidus (GP) on unenhanced T1-weighted MR images has been both morphologically and pathologically linked to gadolinium-based contrast agent (GBCA) retention in the brain. The purpose of this study was to determine whether repeated administrations of GBCA would affect the uptake of 18F-FDG in the DN and GP on PET/CT. Methods: Three hundred seventy-six patients who underwent both contrast-enhanced MR (CE MR) of the brain and PET/CT from January 2004 to October 2015 were identified. Patients with a history of brain irradiation or hepatic or renal disease were excluded. The SUVmax was measured in the DN and GP on the PET/CT scan in patients who had 3-6 successive CE MR brain studies. The SUVmax of the corresponding areas in the control group of patients who had not undergone previous CE MR and who had a normal, unenhanced MR finding of the brain was also measured. A Wilcoxon 2-sample test was used for statistical analysis. Results: Fifteen of 376 (4%) patients (mean age ± SD, 54 ± 18 y; 10 men and 5 women) were included in the subject group, and 15 patients (mean age ± SD, 36 ± 9 y; 11 men and 4 women) were included in the control group. The median DN SUVmax was significantly lower in the subject group than in the control group (5.4 vs. 6.4, respectively; P = 0.021). Similarly, the median GP SUVmax was significantly lower in the subject group than in the control group (8.8 vs. 12.1, respectively; P = 0.003). Conclusion: The median SUVmax in the DN and GP was 16% and 27% lower, respectively, in patients who received GBCAs than in those who had not received GBCAs, possibly related to gadolinium deposition in these areas.

Attenuation Effects of MR Headphones During Brain PET/MR Studies.

UNLABELLED: PET/MR offers potential advantages over PET/CT that are currently under investigation. One of the challenges of PET/MR is attenuation correction, as there is no simple correlation between MR signal intensity and the attenuation of 511-keV photons detected in PET. Currently, dedicated MR sequences are used to segment voxels into categories that are then assigned a predetermined attenuation coefficient. MR hardware such as the imaging table, coils, and headphones are also sources of attenuation. The purpose of this study was to evaluate the effect of MR-compatible headphones on average activity concentration measured with PET/MR. We also present a practical approach to correct for the attenuation effect of headphones using a CT-derived attenuation map. METHODS: Phantom studies were performed using a 3-L cylindric phantom containing 55 MBq of (18)F-FDG and water. Images were acquired on a PET/MR device in 2 settings-one with the PET/MR headphones on and one with the headphones off. Phantom images were analyzed to compare activity concentration with headphones on and off. A high-resolution CT and (57)Co transmission scan was obtained to construct a PET attenuation map of the headphones. The resulting attenuation map was registered to the phantom data to evaluate the ability to correct for headphone attenuation. One human subject was scanned to evaluate the clinical impact of headphone attenuation and the accuracy of the proposed correction. RESULTS: Activity concentrations measured in the phantom were reduced by as much as 13.2% with headphones on compared with headphones off. Using the modified attenuation maps that account for attenuation from the headphones resulted in a decrease in the headphone attenuation effect from a maximum of 13.2% to 1.9%. Comparable attenuation effects were observed in the human brain and were similarly reduced with correction using the modified attenuation maps. CONCLUSION: MR-safe headphones were a source of attenuation on our PET/MR phantom and human studies. Attenuation effects of headphones should be considered and can be corrected during quantitative brain PET/MR studies.