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.
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Epidemiology
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 brown adipose tissue activity is still visible in up to 1.9% adult patients (cf. 15% children) 6.
A retrospective study of PETs from more than 52,000 patients found ~10% individuals had brown adipose tissue, most commonly in supraclavicular or cervical regions 7.
Gross anatomy
Compared to white fat, brown fat 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.
Location
The principal sites of brown adipose tissue are:
- neck
- supraclavicular fossa
- paravertebral tissue
- axilla
- mediastinum
- abdomen
- para-aortic
- perihepatic
- paracolic
- suprarenal
- perinephric
Radiographic features
MRI
Current studies have demonstrated novel MR sequences can differentiate between white and brown fat but this is yet to be validated 5.
Nuclear medicine
F-18-FDG PET-CT is the current modality of choice and demonstrates brown fat as areas of high uptake.
Clinical importance
A retrospective study of PET scans from more than 52,000 patients found that those with brown adipose tissue, were significantly less likely to have type 2 diabetes mellitus, dyslipidemia, hypertension and cardiovascular disease, including ischemic stroke, ischemic heart disease and heart failure 7.
Practical points
Brown adipose tissue is a dangerous potential source of false-positive FDG PET interpretations in oncologic imaging. F-18-FDG is highly concentrated in hypermetabolic brown adipose tissue 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 brown adipose tissue 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 brown adipose tissue. Keeping the injection and waiting rooms at warmer temperatures may help to reduce brown adipose tissue metabolism and therefore the FDG uptake. Premedication with IV diazepam 6 or the oral beta blocker, propranolol, has been proven to decrease brown fat activity.
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