Brown adipose tissue (BAT) (also known as brown fat) is one of two types of adipose tissue (the other one being white fat) important for producing thermal energy (heat, non-shivering thermogenesis), especially in the newborn. It constitutes ~5% of body mass in the newborn and tends to reduce markedly in volume in adulthood. It is important to know for the radiologist and/or nuclear medicine physician as a potential source of error on FDG-PET.
Women present a greater mass of brown adipose tissue than men. Extreme conditions such as cold temperature and stimulation of the sympathetic nervous system (i.e. higher levels of blood catecholamine in anxious patients) are known triggers. Studies have found BAT activity is still visible in up to 1.9% adult patients (cf. 15% children) 6.
Compared to white fat, it contains smaller fat vacuoles, a higher number of mitochondria and more vascularization. Moreover, it is richly innervated by sympathetic nerves. Thermogenin is a protein located on the mitochondrial membrane responsible for producing heat energy.
The principal sites of brown adipose tissue are:
- supraclavicular fossa
- paravertebral tissue
Current studies have demonstrated novel MR sequences can differentiate between white and brown fat but this is yet to be validated 5.
F-18-FDG PET-CT is the current modality of choice and demonstrates brown fat as areas of high uptake.
Brown adipose tissue (BAT) is a dangerous potential source of false-positive FDG PET interpretations in oncologic imaging. F-18-FDG is highly concentrated in hypermetabolic BAT because glucose, even if not as a direct source of heat production but as a source of adenosine triphosphate, is required for heat production.
Because hypermetabolic BAT is typically bilateral and symmetric and corresponds on PET-CT images to fat areas, the appearance is rarely confused with malignancy. The acronym “USA-fat” (uptake in supraclavicular area fat) has been proposed to portray this typical appearance.
However, when brown fat occurs in the mediastinum or the abdomen, or near lymph-nodes or masses, the correct interpretation of focal FDG uptake can result in a real challenge. This is especially true for Hodgkin lymphoma patients that are often young and where the disease is frequently located in the same regions as the main locations of BAT. Keeping the injection and waiting rooms at warmer temperatures may help to reduce BAT metabolism and therefore the FDG uptake. Premedication with IV diazepam 6 or the beta blocker, oral propranolol, has been proven to decrease brown fat activity.
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