Imaging of BAT
BAT was rediscovered in adult humans due to chance findings on PET scan using 18F-fluorodeoxyglucose tracer combined with CT.
At present, PET-CT scans remain the gold standard of imaging BAT and recent data using this technique demonstrate that while lean subjects have more cold-induced thermogenesis, this response decreases in the elderly. Further studies using PET-CT show that BAT activity, in adult humans, is enhanced by cold acclimation and significant reduction in BMI following bariatric surgery.
Despite its widespread use, PET-CT using 18F-fluorodeoxyglucose may not be the ideal method for BAT imaging. The main substrate of BAT is fatty acids, so measurement of glucose uptake may present an inaccurate picture and the use of alternative tracers such as 15O-labeled water for perfusion, 18F-fluoro-6-thiaheptadecanoic acid for non-esterified fatty acid uptake and 11C-acetate for oxidative metabolism may be more appropriate. In addition, as the dose of radiation involved in PET-CT is high and does not lend itself to dynamic or repeated imaging in different physiological states, there is great interest in developing alternative imaging techniques.
Thermal imaging is one such modality and findings presented illustrated its role in demonstrating the effects of mild cold exposure, diet and BMI on the supraclavicular depot of BAT in humans. Despite the caveats that such superficial imaging may be confounded by blood flow and insulation from subcutaneous fat, thermal imaging has the potential to emerge as a noninvasive and cost-effective technique for dynamic imaging BAT in both humans and animals.
Other imaging modalities discussed included the use of MRI in humans and elegant experiments showing the delivery of lipophilic nutrients to brown and BRITE adipocytes in vivo, using lipoproteins engineered with superparamagnetic or fluorescent nanocrystals.
Expert Rev Endocrinol Metab. 2013;8(2):123-125. © 2013 Expert Reviews Ltd.