Brown fat generates heat in response to cold exposure. It is sympathetically innervated. FDG uptake in brown fat can be very high, and may therefore be a cause for false positive findings in the assessment for malignancy. Similar to malignancy, FDG uptake in brown fat increases with time on dual-time-point imaging. Uptake is commoner in females, paediatic patients, and possibly in those with a low body mass index. Uptake is commoner in cold temperatures.

The common locations for brown fat uptake are: neck; supraclavicular regions (usually symmetric); axillae; around large vessels in mediastinum; interatrial septum; azygo-oesophageal recess; paraspinal; intercostal; perinephric; paracolic; and parahepatic. Uptake outside the neck and supraclavicular regions rarely occurs in isolation.

Methods for reducing brown fat uptake include: 1. patient minimization of cold exposure prior to the scan; and 2. pharmacologic interventions such as diazepam.