Sixty five years ago artificial radioactivity was discovered.1 Radioactive tracers such as ¹¹C,¹⁵O, and ¹⁸F, which decay by positron emission, were used to derive an image of the distribution in organs during the the mid 1970s. This became clinically important by the 1980s. The physics of position emission tomography (PET) and the logistics of its application are complex. Imaging is based on coincidence detection of annihalation radiation. The unstable radionuclides are neutron deficient and decay by emission of a postively charged electron (positron). The positron travels a few millimetres in the tissue before combining with an electron. Mass is lost and energy is emitted in the form of two gamma rays of energy 511 KeV, which travel in almost opposite directions from the site of annihalation. Multiple detectors on the scanner surrounding the patient identify each event, determine the site of origin, and reconstruct a map of distribution of the radionuclide. Because the path that the positron travels is unpredictable, this sets an irreducible limit to the resolution of the method, which with the currently available systems is in the order of 4–5 mm.

An approximation of the distribution of glucose metabolic rates can be derived by using ¹⁸F labelled flurodeoxyglucose (FDG)—a glucose analogue. After cell membrane transport, FDG is phosphorylated to FDG-6 phosphate (FDG-6P), which is not metabolised thereafter. Because the cell is impermeable to FDG-6P, and enzymes that reverse this reaction are low in concentration, FDG is trapped within cells sufficiently long enough for rates of glucose uptake to be estimated. FDG uptake takes place over a 45 minute period and reflects the metabolic activity of the brain during that time. Some degree of standardisation of normal brain activity can be imposed by ensuring that during the period of FDG the subject lies with …