Brown fat on PET/CT
50 year-old female referred for initial staging of a known rectosigmoid carcinoma.
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There is diffuse symmetric bilateral FDG uptake within the mediastinum, neck and upper chest, paraspinal muscles and retroperitoneum corresponding to brown fat activity. The soft tissue uptake is clearly localized within the fatty tissue as demonstrated by PET/CT co-registration.
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.
- Cohade C, Osman M, Pannu HK et-al. Uptake in supraclavicular area fat ("USA-Fat"): description on 18F-FDG PET/CT. J. Nucl. Med. 2003;44 (2): 170-6. Pubmed citation
- Cohade C, Mourtzikos KA, Wahl RL. "USA-Fat": prevalence is related to ambient outdoor temperature-evaluation with 18F-FDG PET/CT. J. Nucl. Med. 2003;44 (8): 1267-70. Pubmed citation
- Hany TF, Gharehpapagh E, Kamel EM et-al. Brown adipose tissue: a factor to consider in symmetrical tracer uptake in the neck and upper chest region. Eur. J. Nucl. Med. Mol. Imaging. 2002;29 (10): 1393-8. doi:10.1007/s00259-002-0902-6 - Pubmed citation
- Paidisetty S, Blodgett TM. Brown fat: atypical locations and appearances encountered in PET/CT. AJR Am J Roentgenol. 2009;193 (4): 359-66. doi:10.2214/AJR.09.3052 - Pubmed citation