Figure 21.20 Brown adipose tissue is revealed on exposure to cold. The results of PET–CT (positron emission and computerized tomography) scanning show the uptake and distribution of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG), a nonmetabolizable glucose analog, in adipose tissue. The patterns of ¹⁸F-FDG uptake in the same subject are dramatically different under thermoneutral conditions (left) and after exposure to cold (right).