A PET radiotracer (also known as PET tracer) is a positron-emitting radiopharmaceutical used in positron emission tomography (PET). Each tracer consists of a positron-emitting isotope (radioactive tag) bound to an organic ligand (targeting agent). The ligand component of each tracer interacts with a protein target, resulting in a characteristic distribution of the tracer throughout the tissues. The ideal PET radiopharmaceutical should only interact with the protein target and not give accumulation phenomena 2.
For example, the pre-eminent PET radiotracer fluorodeoxyglucose (FDG) is comprised of a fluorine-18 isotope bound to 2-deoxy-2-glucose, an analog to glucose. The 2-deoxy-2-glucose ligand is a substrate for the hexokinase/glucokinase enzymes involved in the early carbohydrate metabolism; thus, FDG is chemically linked to cellular metabolic activity 1. It serves as a particularly good tracer agent because it tends to stay "trapped" within metabolically active cells due to the absence of the hydroxide group ("deoxy-").
There is an increasing list of chemical compounds which are being used for PET imaging. A list of commonly used compounds, with their radioactive isotope in parentheses, includes:
- acetate (C-11)
- choline (C-11)
- fludeoxyglucose (F-18)
- sodium fluoride (F-18)
- fluoro-ethyl-spiperone (F-18)
- methionine (C-11)
- prostate-specific membrane antigen (PSMA) (Ga-68)
- DOTATOC, DOTANOC, DOTATATE (Ga-68)
- florbetaben, florbetapir (F-18)
- rubidium (Rb-82) chloride
- ammonia (N-13)
The synthesis of PET tracers begins in cyclotrons with the formation of small molecules called precursors. For example, the isotope carbon-11 can be produced both as carbon dioxide (C-11 CO2) and as methane (C-11 CH4) 3, respectively the most oxidized and the most reduced chemical form.
History and etymology
The "radioactive tracer theory" underlies nuclear medicine's imaging. George Charles de Hevesy (1885–1966), a Hungarian chemist, is considered the first scientist to have identified this theory 4.
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