Human Brown Fat Radiodensity Indicates Underlying Tissue Composition and Systemic Metabolic Health

Context: Metabolic imaging studying brown adipose tissue (BAT) physiology has increased, in which computed tomography (CT) is commonly used as an anatomical reference for metabolic positron emission tomography (PET) imaging. However, the capacity of CT to provide metabolic information has been underexploited. Objective: To evaluate whether CT radiodensity of BAT could noninvasively estimate underlying tissue morphology, regarding amount of stored triglycerides. Furthermore, could the alteration in tissue characteristics due to cold stimulus, as a marker for active BAT, be detected with radiodensity? Can BAT be differentiated from white adipose tissue (WAT) solely using CT-based measurements? Design, Setting, and Participants: A cross-sectional study evaluating 66 healthy human subjects with CT, PET, and H-1-magnetic resonance spectroscopy (H-1-MRS). Main Outcome Measures: BAT radiodensity was measured with CT. BAT-stored triglyceride content was measured with H-1-MRS. Arterial blood volume in BAT, as a marker of tissue vascularity, was measured with [O-15]H2O, along with glucose or fatty acid uptake using [F-18]2-fluoro-2-deoxy-D-glucose or 14(R,S)-[F-18]fluoro-6-thia-heptadecanoic acid PET imaging, respectively. Results: BAT radiodensity was found to be correlating with tissue-retained blood and triglyceride content. Cold stimulus induced an increase in BAT radiodensity. Active BAT depots had higher radiodensity than both nonactive BAT and WAT. BAT radiodensity associated with systemic metabolic health parameters. Conclusion: BAT radiodensity can be used as a marker of underlying tissue morphology. Active BAT can be identified using CT, exploiting tissue composition information. Moreover, BAT radiodensity provides an insight into whole-body systemic metabolic health.