Active brown fat during ¹⁸FDG-PET/CT imaging defines a patient group with characteristic traits and an increased probability of brown fat redetection

Abstract

Brown adipose tissue (BAT) provides a means of non-shivering thermogenesis. In humans, active BAT can be visualized by ¹⁸F-fluoro-desoxyglucose (FDG) uptake as detected by positron emission tomography (PET) combined with computer tomography (CT). The retrospective analysis of clinical scans is a valuable source to identify anthropometric parameters that influence BAT mass and activity and thus the potential efficacy of envisioned drugs targeting this tissue to treat metabolic disease. We analyzed 2854 FDG-PET/CT scans from 1644 patients and identified 98 scans from 81 patients with active BAT. We quantified the volume of active BAT depots (mean values in ml ± standard deviation (n): total BAT 162 ± 183 (98), cervical 40 ± 37 (53), supraclavicular 66 ± 68 (71), paravertebral 51 ± 53 (69), mediastinal 43 ± 40 (51), subphrenic 21 ± 21 (29)). Since only active BAT is detectable by FDG uptake, these numbers underestimate the total amount of BAT. Considering only 32 scans of the highest activity as categorized by a visual scoring strategy, we determined a mean total BAT volume of 308 ± 208 ml. In 30 BAT positive patients with three or more repeated scans we calculated a much higher mean probability to re-detect active BAT (52 ± 25%) as compared to the overall prevalence of 4.9 %. We calculated a BAT activity index (BFI) based on volume and intensity of individual BAT depots. We detected higher total BFI in younger patients (P = 0.009), while sex, body mass index (BMI), height, mass, outdoor temperature and blood parameters did not affect total or depot specific BAT activity. Surprisingly, renal creatinine clearance as estimated from mass, age and plasma creatinine was a significant predictor of BFI on the total (P = 0.005) as well as on the level of several individual depots. In summary, we detected a high amount of more than 300 ml of BAT tissue. BAT-positive patients represent a group with a higher than usual probability to activate BAT during a scan. Estimated renal creatinine clearance correlated with the extent of activated BAT in a given scan. These data imply an efficacy of drugs targeting BAT to treat metabolic disease that is at the same time higher and subject to a larger individual variation than previously assumed.