An imaging technique, often used in brain imaging. For a PET scan of the brain, a radioactive “marker” that emits, or releases, positrons (parts of an atom that release gamma radiation) is injected into the bloodstream. Detectors outside of the head can sense these “positron emissions,” which are then reconstructed using sophisticated computer programs to create computer images. Since blood flow and metabolism increase in brain regions at work, those areas have higher concentrations of the marker, and researchers can see which brain regions activate during certain tasks or exposure to sensory stimuli. Ligands can be added to a PET scan to detect pathological entities such as amyloid or tau deposits.