MRI characterization of brown adipose tissue in adult humans with validation by PET-CT

Abstract

Brown adipose tissue (BAT) known to function in a non-shivering thermogenic capacity, also performs as an endocrine organ, and assists with glucose homeostatsis and insulin sensitivity. Positron emission tomography (PET)-computed tomography (CT) imaging has identified depots of BAT in humans, igniting scientific interest. Our understanding of human BAT is limited because the primary technique for detecting BAT is through 18F-fluorodeoxyglucose (18F-FDG) PET-CT. Although useful for detecting active BAT glucose metabolism, it disregards the fatty acid substrate fuel, and is limited due to the use of ionizing radiation. Therefore to fully understand the function of BAT and its role in whole body energy metabolism, new imaging techniques need to be employed. The purpose of this dissertation 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-FDG PET-CT and magnetic resonance imaging (MRI) scans of healthy adults. Unlike 18F-FDG PET, which can only detect active BAT, we show MRI is detects 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.