Intense ¹⁸F-FDG uptake in brown fat can be reduced pharmacologically

Physiologic ¹⁸F-FDG uptake in areas of supraclavicular fat in humans ("USA-Fat") has recently been recognized as ¹⁸F-FDG uptake in apparent brown adipose tissue (BAT) using fused PET/CT technology. In this study, we evaluated ¹⁸F-FDG uptake in BAT of rats to determine whether pharmacologic or physiologic interventions affect the uptake, knowing that BAT has a high density of adrenergic innervation. Methods: Seven- to 8-wk-old female Lewis rats receiving intravenous ¹⁸F-FDG injections were examined under various conditions to evaluate ¹⁸F-FDG biodistribution into interscapular BAT and major organs. In series 1, rats were given ketamine-based anesthesia or were exposed to cold (4°C for 4 h) to determine whether these interventions increased ¹⁸F-FDG uptake in BAT. In series 2, anesthetized rats (ketamine-based anesthesia) were given propranolol, reserpine, or diazepam intraperitoneally before ¹⁸F-FDG injection to determine whether the drug reduced ¹⁸F-FDG uptake in BAT. The control and treated groups in series 2 were also evaluated with ¹⁸F-FDG PET/CT imaging. Results: In series 1, anesthesia or exposure to cold increased ¹⁸F-FDG uptake in BAT to levels 14-fold and 4.9-fold, respectively, greater than the control nonstimulated values. BAT uptake was high, comparable to that in the brain. In series 2, ¹⁸F-FDG uptake in BAT was significantly decreased to less than 30% of the control level after propranolol or reserpine (P < 0.05). Diazepam did not significantly decrease ¹⁸F-FDG uptake in BAT. ¹⁸F-FDG PET/CT findings reflected these biodistribution data: The control and diazepam groups exhibited intense ¹⁸F-FDG uptake in BAT, whereas the propranolol and reserpine groups showed only faint to mild ¹⁸F-FDG uptake in BAT. Among several organs whose ¹⁸F-FDG uptake was affected after predosing drugs, the heart exhibited considerable decreases in tracer uptake with propranolol or reserpine. Conclusion: This rodent study demonstrated that BAT can exhibit high ¹⁸F-FDG uptake under stimulated conditions including exposure to cold and that propranolol or reserpine treatment can remarkably reduce the high ¹⁸F-FDG uptake in BAT. The effect of these drugs on ¹⁸F-FDG uptake in human BAT, as well as on tracer accumulation in other organs, should carefully be evaluated clinically to minimize the USA-Fat artifact.