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Positron Emission Tomography
Positron Emission Tomography (PET) is a medical imaging technique which is commonly used to map out metabolic activity in the body. Typically a positron emitting radionuclide (such as 18F) attached to a glucose molecule is injected into the body where it is taken up by tissues in proportion to their metabolic activity. Positrons produced by the decay of the radionuclide usually travel less than 1mm in human tissue before they annihilate with an electron. Most of the positron/electron annihilations result in the emission of a back-to-back pair of 511keV photons. These photon pairs leave the body and can be detected. Areas of the body where there is high metabolic activity, such as cancer cells or active regions in the brain, will be more intense emitters of these 511keV (gamma-ray) photon pairs. In a PET scanner arrays of scintillator+PMT detectors are used to measure the intensity of this radiation along well defined planes (referred to as slices) passing through the patients body. These slices are then used to reconstruct a three dimensional image.
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