Nonpathologic supraclavicular uptake of 18F-FDG seen with PET was initially believed to be predominantly muscular uptake because it could be suppressed with muscle relaxants and sedatives. However, with the introduction of FDG PET integrated with CT (PET/CT), Cohade et al. in 2003 used CT to correlate focal FDG supraclavicular uptake to tissue fat density rather than solely muscle and termed the finding as uptake in supraclavicular area fat or “USA fat.”
Several studies suggested this tissue to be brown fat versus white fat because the tracer uptake could be well correlated with the physiology of brown fat, exemplified by the increased uptake in response to cold exposure.
FDG uptake in fatty tissue was observed in well-defined patterns consistent with the typical locations of brown fat.