PET identifies the presence of a metabolically active tumor within the body after injection of a radioactive tracer. CT uses x-rays to image the anatomy of patients. PET/CT systems combine the modalities to produce an image that shows the functional information from PET and the anatomical information from CT simultaneously. The resultant data are displayed as a fused PET/CT image.

The most common radioactive PET tracer is the glucose analog fluorine-18 FDG. F-18 FDG is injected intravenously and is taken up by normal and tumor cells in much the same way as glucose. Information on tracer uptake can be used to gather functional information.

Cardiac muscle, for example, preferentially uses free fatty acids as an energy source, but it can also use glucose, lipids, or amino acids if required. Glucose uptake within the heart varies among people and can change considerably within an individual over a short period in relation to his or her blood glucose level. Brain cells are fueled solely by glucose, and glucose activity within the brain is always high. The metabolic rate of many tumors is much higher than that of normal cells, and tumors consequently use considerably more glucose.

F-18 FDG has a half-life of approximately 109 minutes. Patients scheduled for a PET/CT examination involving injection of F-18 FDG should fast for at least four hours beforehand. This ensures that most tissues are using free fatty acids as their energy source. Diabetic patients are advised to take their normal insulin or medication.

Patients are advised to lie down and relax for approximately 45 minutes after injection to allow time for the radiotracer to accumulate in metabolically active cells. Any unnecessary patient movement during this period may result in uptake by muscle. Patients who are tense or shiver during this time often have FDG uptake in their neck muscles. Brown fat activation can cause confusion. Brown fat is seen more commonly in thin individuals during the winter months, and generally in young women with anxiety and stress.

The PET/CT examination is normally carried out from the base of the skull to the midthigh, the so-called partial body scan. Some patients, those with melanoma, for example, may have a whole-body scan, from skull vertex to feet, depending on the location of the primary tumor. This is because of the widespread and unpredictable lymphatic dissemination that characterizes this disease. Patients with head and neck disease may have scans that include the entire skull, and patients with softtissue sarcomas may also require additional views.

A semiquantitative method called the standardized uptake value (SUV) is often used as a parameter for measuring the uptake of FDG by tissue or lesions.

ONCOLOGY APPLICATIONS

The hybrid modality has many applications in oncology:

• Lung cancer. FDG-PET/CT is used to assess solitary pulmonary nodules and to stage non-small cell lung cancer (Figure 1).¹ It is additionally used to assess mediastinal lymphadenopathy, identify distant metastatic disease, and detect recurrent disease. FDG-PET/CT can assist with radiotherapy planning and therapy monitoring, while also acting as a prognostic indicator of time to progression.