Animated Physics - PET Scanners

PET Scanner

PET/CT-System; the ceiling mounted device is an injection pump for CT contrast agent.

PET scans of brain and body

How it works

The radioisotope in the contrast agent decays and through a series of events, cause gamma rays to be emit in opposite directions. It is the detection of these pairs of gamma rays that allow the computers to determine where the original decay took place.

Positron emission tomography (PET scans)
Nuclear medicine imaging

    Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.
    To conduct the scan, a short-lived radioactive tracer isotope is injected into the living subject (usually into blood circulation). The tracer is chemically incorporated into a biologically active molecule. There is a waiting period while the active molecule becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner.
    The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour. During the scan a record of tissue concentration is made as the tracer decays.

(Click for 3d view)

    Within 110 minutes, there is a 50% chance that one of the fluorine protons will decay into a neutron and positron.
As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, an antiparticle of the electron with opposite charge. The fluorine nucleon becomes a oxygen nucleon with extra neutrons. More..

    The emitted positron travels in tissue for a short distance (typically less than 1 mm, but dependent on the isotope), during which time it loses kinetic energy, until it decelerates to a point where it can interact with an electron. The positron (sometimes called an anti-electron or a left-handed electron) cannot have its spinning components lined up with an electron no matter their orientation. The encounter annihilates both electron and positron, producing a pair of annihilation (gamma) photons moving in approximately opposite directions. More..
    The most significant fraction of electron-positron decays (electron/positron mass = 511 keV) result in two 511 keV gamma photons being emitted at almost 180 degrees to each other. These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes (Si APD). Inorganic crystals are best suited as a scintillator for the detection of gamma rays through the photoelectric effect. More..

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