Positron emission tomography, also called a “PET scan,” is a diagnostic tool that can locate malignant cells in the body. PET is based on the detection of high-energy gamma rays created when positrons (subatomic particles of antimatter), produced during decay of radioactive mate-rials, are encountered by electrons. The procedure re-quires the injection of a radioactive tracer, most commonly 18-fuorine-2-fuoro-2-deoxy-D-glucose (18F-FDG). This radioactive glucose isotope is used in PET imaging because malignant cells metabolize glucose at a greater rate than normal cells. After injection of the isotope, a de-tector scans the entire body and records radiation emitted by the 18F-FDG tracer as it becomes incorporated within the body’s cells. A computer reassembles the signals into images, which are, in effect, biological maps of 18F-FDG distribution in the body. Recently, due to greater diagnostic accuracy and improved biopsy methods, combined positron emission tomography and computed tomography (PET/CT) scanners are utilized more frequently. One drawback to PET imaging is that many normal tissues and benign lesions also show increased glucose metabolism and can thus be misinterpreted as malignant. For example, brown adipose tissue, with its increased glucose uptake me-diated by increased activity of glucose transporters, can be a potential source of false-positive interpretation of PET scans. Since brown adipose tissue is present in the neck, supraclavicular regions, and mediastinum (see page 260) it is commonly observed on PET scans, especially in underweight patients and during winter months, when brown adipose tis-sue is more predominant. This 18F-FDG uptake most likely represents activated brown adipose tissue during increased sympathetic nerve activity related to cold stress. A typical PET image of brown fat is usually bilateral and symmetric; however, in the mediastinum, the image may be asymmetric or focal and can mimic malignancy. False-positive results from brown fat 18F-FDG uptake in these areas have been reported in young women undergoing scans for diagnosis and staging of breast cancer. Therefore, under-standing that brown fat can show that increased radioac-tive tracer uptake is crucial for establishing an accurate diagnosis and avoiding false-positive results (Fig. F9.3.1). FIGURE F9.3.1 • Coronal positron emission tomography/ computed tomography (PET/CT) image of a healthy young woman. This upper part of the coronal section of this whole-body PET/CT scan shows extensive bilateral increased 18F-FDG uptake (red color) in the neck, supraclavicular, and upper axillary regions. Note that moderate increased radioactive tracer uptake is also detectable in the myocardium (yellow color). Regions of extensive metabolic activity correlate with the distribution pattern of low-density brown adipose tissue. PET/CT imaging allows for precise localization of increased 18F-FDG uptake areas and differentiation between brown adipose tissue tracer uptake and malignant tumor findings. (Courtesy of Dr. Jolanta Durski.)
UCP-1 activity has been shown to increase during cold stress. In addition, cold stimulates glucose utilization in brown adipocytes by overexpression of glucose transporters (Glut-4).
Recent clinical studies using PET scans in adults have shown a direct relationship between outdoor temperature and the amount of brown fat accumulated in the body. An increase in the amount of brown adipose tissue has been reported on the neck and supraclavicular regions during the winter months especially in lean individuals. This is supported by autopsy findings of larger amounts of brown fat in outdoor workers exposed to cold. Modern molecular imaging techniques now allow clinicians to precisely locate where brown fat is distributed in the body, which is essential for proper diagnosis of cancerous lesions (see Folder 9.3).
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