Positron Emission Tomography (PET)
PET is a unique tomographic examination that, unlike computer or magnetic resonance imaging (CT or MRI), provides detailed information about body functions. High-energy radiation of two gamma quantums resulting from the annihilation of the positron electron pair leaves the site of generation simultaneously and in an anti-parallel manner.
This principle leads to a high local resolution that was unknown to nuclear medicine before. Typical positron emitters are the isotopes carbon-11, oxygen-15, nitrogen-13, fluorine-18. These elements commonly appear in almost all biological molecules. These positron emitters may be labeled with glucose, proteins, DNA-components and even with water and take part in the naturally running metabolism. Then, with the help of positron emission tomography the depiction of the natural cell functions can be received, for example in sugar metabolism or protein biosynthesis.
Desoxyglucose labeled with 18F (18FDG) is useful to identify malignant tissue, to study the glucose metabolism of the heart and the brain. FDG is metabolised by the body up to a certain stage of metabolism as regular glucose, which serves as “fuel” for almost all body cells. While using FDG in cancer, the uptake is based on the principle that cancer cells, due to their abnormal growth rate, need more glucose than normal cells. Desoxyglucose is thus accumulated in the deregulated cells affected by cancer. Therefore, in PET images the tumor shows an increased contrast as compared to healthy tissue. Even small metastases with a diameter less than one centimeter can be depicted.
Usually positron emitters are given to the patient intravenously; their half time varies between several minutes up to two hours.
