A synopsis of brown adipose tissue imaging modalities for clinical research

Body weight gain results from a chronic excess of energy intake over energy expenditure. Accentuating endogenous energy expenditure has been accorded considerable attention ever since the presence of brown adipose tissue (BAT) in adult humans was recognized, given that BAT is known to increase energy expenditure via thermogenesis. Besides classic BAT, significant strides in our understanding of inducible brown adipocytes have been made regarding its development and function. While it is ideal to study BAT histologically, its relatively inaccessible anatomical locations and the inherent risks associated with biopsy preclude invasive techniques to evaluate BAT on a routine basis. Thus, there has been a surge in interest to employ non-invasive methods to examine BAT. The gold standard of non-invasive detection of BAT activation is ¹⁸F-fluorodeoxyglucose positron emission tomography (PET) with computed tomography (CT). However, a major limitation of PET/CT as a tool for human BAT studies is the clinically significant doses of ionizing radiation. More recently, several other imaging methods, including single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), infrared thermography (IRT)/thermal imaging and contrast ultrasonography (US) have been developed in hopes that they would allow non-invasive, quantitative measures of BAT mass and activity with lower costs. This review focuses on such methods to detect human BAT activation and white adipose tissue (WAT) browning to prompt the establishment of BAT-centric strategies for augmenting energy expenditure and combatting obesity. Clinical validation of these methods will most likely expand the scope and flexibility of future BAT studies.