Aim: To create a regional body map for FDG-avid (activated) brown adipose tissue (BAT) and to assess the magnitude of BAT activation concerning the time of the year the study was conducted.
Material and Methods: PET/CT studies with FDG-avid BAT were included. BAT was classified based on its site as typical (frequent and predominantly superficial) or atypical (infrequent and predominantly deep). A score was generated for the overall brown fat (BFS-O), typical brown fat (BFS-T), and atypical brown fat (BFS-A) by summation of activated sites in each scan. The presence of brown fat was examined against time of the year the scan was conducted. The year was divided into two seasons (cold/hot), based on the average temperature in Riyadh, Saudi Arabia.
Results: We included 291 studies for 215 patients (mean age 27.6 ± 13.0 years; 56.9% females). Active BAT was found predominantly in supraclavicular, cervical, axillary, and paravertebral areas. These locations were considered typical sites and all patients had at least one of them involved. Atypical BAT sites were deeper in location and less frequently activated (50.5%). BFS-O and BFS-A were higher in the cold season. BFS-O >4 was the best cutoff for identifying studies performed during the cold season (p=0.004). More BFS-O of >4 and more frequent atypical BAT sites were detected during the cold season. BFS-T and BFS-A were found to be positively correlated (rho=0.369; p<0.001). Age was negatively correlated with BFS-O, BFS-T, and BFS-A (rho=-0.184; p=0.002), (rho=-0.195; p<0.001), and (rho=-0.116; p=0.049) respectively. Patients with BFS-O of >4 or those with atypical BAT sites were younger.
Conclusion: When present, active BAT is virtually always observed at one or more typical sites, but atypical sites are detected in a patient's subset with more extensive BAT activation. Atypical BAT activation was more frequent in younger patients and during colder months.