18F-FDG Uptake in Brown Adipose Tissue After Exposure to the Cold: From Possible Pitfall in Early PET Scans to Metabolic Biomarker

here are 3 types of fat in the human body: white, brown, and beige ( 1 ). White adipocytes deposit extra energy into triglycerides, whereas beige and brown adipocytes have the unique ability to convert mitochondrial energy into heat (rather than adenosine triphosphate) via uncoupling protein 1. Obesity, especially excess fat in tissue that is normally lean, increases the risk of cardiovascular disease ( 2 ). In addition to the amount of fat, the distribution of fat, especially increased abdominal fat, evaluated by the ratio of waist to hip circumfer-ences, predicts glucose intolerance, insulin resistance, hyperten-sion, and hypertriglyceridemia ( 3,4 ). PET/CT with 18 F-FDG provides a unique opportunity to view the metabolic activity of brown adipose tissue (BAT). However, even though visceral and subcutaneous fat are substantially less metabolically active than BAT, both are metabolically active tissues ( 5 ). Visceral adipose tissue is more metabolically active than subcutaneous fat. BAT is a thermoregulatory organ that consumes stored energy to produce heat through the expression of uncoupling protein 1. This phenomenon is called nonshivering thermogenesis and plays an important role in glucose and lipid metabolism ( 6 ). It is particularly intense in newborns, in whom it helps to maintain a