Activated brown adipose tissue (BAT) plays an important role in thermogenesis and whole-body metabolism in mammals. Positron emission tomography (PET)-computed tomography (CT) imaging has identified depots of BAT in adult humans, igniting scientific interest. The purpose of this present study is to characterize both active and inactive supraclavicular BAT in adults, and compare the values to those of subcutaneous white adipose tissue (WAT). We obtained ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET-CT and magnetic resonance imaging (MRI) scans of 25 healthy adults. Unlike ¹⁸F-FDG PET, which can only detect active BAT, MRI is capable of detecting both active and inactive BAT. The MRI derived fat-signal fraction (FSF) of active BAT was significantly lower than inactive BAT (mean ± SD): 60.2 ± 7.6% vs. 62.4 ± 6.8%, respectively. This change in tissue morphology was also reflected as a significant increase in Hounsfield Units (HU): -69.4 ± 11.5 HU vs. -74.5 ± 9.7 HU, respectively. Additionally, the CT HU, MRI FSF and MRI R2* values are significantly different between BAT and WAT, regardless of the activation status of BAT. To the best of our knowledge this is the first study to quantify PET-CT and MRI measures of BAT in both active and inactive states in the same adult subjects. Our findings support the use of these metrics to characterize and distinguish between BAT and WAT, and lay the foundation for future MRI analysis with the hope that someday MRI-based delineation of BAT can stand on its own.