Modern Medicine

How does a PET scan work?
Answered by Craig C. Freudenrich and Discovery Channel
  • Craig C. Freudenrich

    Craig C. Freudenrich

  • Discovery Channel

    Discovery Channel

  1. Positron emission tomography (PET) is a nuclear medicine imaging technology. In PET, a radioactive substance (such as fluorine-18, carbon-11, nitrogen-13 and oxygen-15) is combined with a biologically active molecule, such as glucose, water or ammonia; the resulting molecule is called a radiotracer. The radiotracer is then injected into your blood. The radiotracer goes to areas of high metabolic activity because they get more blood flow. In your body, the radioactive isotope emits a type of antimatter called a positron, which has the same mass as an electron, but a positive charge. The positron hits an electron of an atom in your tissue. The two particles annihilate each other and emit two gamma rays, each in opposite directions. The scanner can detect these gamma rays and determine the location of the radiotracer in your body, converting this to an image using a computer and special software.

    The radioisotopes used must be made in a type of particle accelerator called a cyclotron. The isotopes decay quickly with half-lives as little as two minutes and up to two hours; a half-life is the time it takes for the amount of radioactive material to decay by 50 percent. The short half-life of these radioisotopes is good for you because it limits your exposure to radiation. Unfortunately, it also requires that most hospitals and imaging centers with PET need to be located close to cyclotrons. Using cyclotrons to produce the radioisotopes also makes the technique expensive.

    The PET equipment looks like other medical scanners such as computed tomography or magnetic resonance imaging equipment. The patient lies on a table, which moves into a ring. The ring is composed of numerous detectors. Each one has a crystal and a photomultiplier tube. The gamma rays hit the crystal and emit light, which the photomultiplier tube converts to electrical signals. The scanner amplifies the electrical signals and feeds them to a computer. For each annihilation, gamma rays are emitted opposite each other. The computer can calculate the locations of the tracer and produce an image, which represents a slice of the body anatomy inside the ring. As the patient moves through the ring, the computer can compile images into a 3-D picture.

    PET is a sensitive imaging technique that is useful for measuring blood flow to various tissues such as the brain, liver or cancerous tumors.


    Doctors use many diagnostic tools to scan the brain and study human memory.
    Doctors use many diagnostic tools to scan the brain and study human memory. (Harry Sieplinga/HMS Images/Getty Images)

    More answers from Craig C. Freudenrich »

  2. Positron Emission Tomography (PET) scans operate using radiation. If you have to have a PET scan, you will have a radioactive substance injected into your body. The radioactivity, in the form of gamma rays that the substance will emit, is then picked up by the large machine you will be under. The rays are converted first to light, then to electrical signals and eventually to computer-generated images. The PET scanner will produce multiple images from the area of your body under investigation and compile a 3-D model.

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