A positron emission tomograph is a medical imaging device that allows for a functional rather than an anatomical analysis of the human body. The technique employs a glucose molecule that has been radioactively labeled by chemically binding a fluorine molecule to it. This fluorine molecule was first placed in a particle accelerator (a cyclotron) to bring it in an unstable state. Fluorine, when hit by an ion beam in such a cyclotron, becomes an unstable positron emitter with a half life of 2 hours. The chemical bond of fluorine and glucose is known as FDG (fluorodeoxyglucose).
After injection of FDG in the study object (e.g. a patient), the body starts to metabolise the FDG as if it were glucose. Cells with high metabolism have higher energy needs and thus take up more glucose, or FDG, from the bloodstream. Typically, cancer cells have high energetic needs and thus take up most of the FDG presented. At these sites of high FDG concentration, the concentration of positron particles emitted is consecutively high.
When such a positron collides with an electron, they annihilate (after briefly forming a positronium) into pure energy. Because of the law of conservation of energy, two photons (basically energy quanta) are formed, each having half of the original energy and they are emitted in exactly opposite directions. When a scanning device detects two such photons striking a detector ring at 180° simultaneously, their origin had to be a positron emitted from the FDG.
This way, a PET scanner is able to visualise high FDG concentrations in the body, such as those occuring in cancer cells.