Combining 123I-Metaiodobenzylguanidine SPECT/CT and 18F-FDG PET/CT for the Assessment of Brown Adipose Tissue Activity in Humans During Cold Exposure

Brown adipose tissue (BAT) has become a focus of research in the hope of finding a new target to fight obesity. Metabolic BAT activity can be visualized with F-18-FDG PET/CT. Furthermore, the sympathetic innervation of BAT can be visualized with the radiolabeled norepinephrine analog I-123-metaiodobenzylguanidine (I-123-MIBG). We aimed to determine whether I-123-MIBG SPECT/CT and F-18-FDG PET/CT identify the same anatomic regions as active BAT in adult humans. Furthermore, we investigated whether the magnitude of BAT activity measured by these techniques correlated. Finally, we tried to establish the optimal time interval between I-123-MIBG administration and subsequent SPECT/CT acquisition to visualize sympathetic stimulation of BAT. Methods: Ten lean (body mass index, 19-25 kg/m(2)), healthy Caucasian men (age, 18-32 y) underwent one F-18-FDG PET/CT and two I-123-MIBG-SPECT/CT scans within a 2-wk interval. On 2 separate occasions, the subjects were exposed to mild cold (17 degrees C) for 2 h after an overnight fast. After 1 h of cold exposure, F-18-FDG (one occasion) or I-123-MIBG (other occasion) was administered. F-18-FDG PET/CT was performed at 1 h after F-18-FDG administration, and I-123-MIBG-SPECT/CT was performed at 4 and 24 h after I-123-MIBG injection. Results: F-18-FDG uptake in BAT was observed in 8 of 10 subjects, whereas I-123-MIBG uptake was observed in 7 of 10 subjects in both the SPECT/CT scans acquired at 4 h after I-123-MIBG administration and the SPECT/CT scans acquired at 24 h after I-123-MIBG administration. All subjects who showed I-123-MIBG uptake in BAT also showed F-18-FDG uptake in BAT. There was no statistically significant correlation between maximal standardized uptake value of F-18-FDG and semiquantitative uptake of I-123-MIBG at 4 h after administration. However, a positive correlation was found between the maximal standardized uptake value of F-18-FDG and semiquantitative uptake of I-123-MIBG at 24 h after administration (r = 0.64, P = 0.04). Conclusion: I-123-MIBG SPECT/CT, as a marker of sympathetic activity, and F-18-FDG PET/CT, as a marker of metabolic activity, identified the same anatomic regions as active BAT. Moreover, when I-123-MIBG SPECT/CT was performed at 24 h after I-123-MIBG administration, the magnitude of BAT activity measured with these techniques correlated strongly. This finding not only supports that BAT activity in humans is sympathetically influenced but also identifies I-123-MIBG SPECT/CT, when performed 24 h after I-123-MIBG injection, as a method to visualize and quantify sympathetic stimulation of BAT.