Positron Emission Tomography (PET)
PET produces images of the body by detecting the radiation emitted from radioactive substances. These substances are injected into the body, and are usually tagged with a radioactive atom, such as Carbon-11, Fluorine-18, Oxygen-15, or Nitrogen-13, that has a short decay time. These radioactive atoms are formed by bombarding normal chemicals with neutrons to create short-lived radioactive isotopes. PET detects the gamma rays given off at the site where a positron emitted from the radioactive substance collides with an electron in the tissue (Figure 1).
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In a PET scan, the patient is injected with a radioactive substance and placed on a flat table that moves in increments through a "donut" shaped housing. This housing contains the circular gamma ray detector array (Figure 2), which has a series of scintillation crystals, each connected to a photomultiplier tube. The crystals convert the gamma rays, emitted from the patient, to photons of light, and the photomultiplier tubes convert and amplify the photons to electrical signals. These electrical signals are then processed by the computer to generate images. The table is then moved, and the process is repeated, resulting in a series of thin slice images of the body over the region of interest (e.g. brain, breast, liver). These thin slice images can be assembled into a three dimensional representation of the patient's body.
PET provides images of blood flow or other biochemical functions, depending upon the type of molecule that is radioactively tagged. For example, PET can show images of glucose metabolism in the brain, or rapid changes in activity in various areas of the body. However, there are few PET centers in the country because they must be located near a particle accelerator device that produces the short-lived radioisotopes used in the technique.