Positron Emission Tomography

The basic principle of PET imaging is the detection of coincidence photons at 511 keV that are generated as a result of the positron-electron annihilation after positron emission (Ak et al, 2000; Delbeke and Martin, 2001; Mankoff and Bellon, 2001). Currently, clinical PET has very high sensitivity in the detection of these coincidence photons. PET can detect radio-pharmaceuticals in the femtomolar to picomolar range, compared with the millimolar range of contrast materials used in CT or MRI. This sensitivity makes it possible to detect metabolic activity at the cellular and molecular level and overcomes the relatively low spatial resolution of 5 to 10 mm in clinical practice. Although several isotopes, such as 11C and 15O, have been used in clinical PET imaging, 18F is the most commonly used positron-emitting isotope because it has a half-life of 110 minutes, which makes it commercially accessible. 18F fluoro-2-deoxy-D-glucose (FDG) is the most commonly used radiopharmaceutical for PET imaging in clinical oncology.

The increased uptake of 18F-FDG in tumor cells is based on the upreg-ulation of glucose transporter and hexokinase activity of the tumor. 18F-FDG is transported into tumor cells and phosphorylated into 18F-FDG-6-P by hexokinase. However, 18F-FDG-6-P cannot be rapidly cleared from cells because of its low membrane permeability. Accumulation of 18F-FDG-6-P in tumor cells is therefore the basis for 18F-FDG PET imaging in clinical oncology (Reske and Kotzerke, 2001). In GI malignancies, 18F-FDG PET imaging is currently approved for diagnosis, staging, and restaging of colorectal and esophageal cancers (Figures 3-1 and 3-2).

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