Determinants of Physiologic 18F-FDG Uptake in Brown Adipose Tissue in Sequential PET/CT Examinations

The aim of this study was to assess independent predictors of 2-deoxy-2-[F-18]fluoro-d-glucose (F-18-FDG) uptake in brown adipose tissue (BAT) in patients undergoing repeated positron emission tomography (PET)/computed tomography (CT) scans. Eight hundred forty-eight (mean age 50.9 +/- 16 years) patients in whom PET/CT scan was repeated (mean interval 5 +/- 1.5 months) constituted the study group. F-18-FDG uptake in characteristic areas of BAT, with CT density of adipose tissue, greater than background soft-tissue activity was considered as evidence of BAT uptake. Both distribution and maximum standardized uptake values (SUVmax) were registered. Clinical and anamnestic data were collected for each patient. F-18-FDG uptake in BAT was present in 8.6% patients at first scan. Independent predictors of presence of uptake were age (younger), gender (female), body mass index (lower), and maximum outdoor temperature (lower). Age was the only independent predictor of BAT F-18-FDG uptake distribution, while SUVmax was related to both age and outdoor temperature. Independent determinants of persistence of BAT F-18-FDG uptake at second PET/CT were outdoor temperature at time of second scan and extension of metabolically active BAT at first scan. Age, body mass index, and outdoor temperature are significant determinants of BAT evidence at F-18-FDG PET/CT. Moreover, extension of BAT and outdoor temperature are the strongest determinants of persistence of BAT evidence on F-18-FDG PET/CT in repeated scan.