PET scanning. Head on view of a female patient undergoing a Positron Emission Tomography (PET) brain scan. The patient's head has passed into the circular detector of the scanner. PET scanning is a method used to investigate the metabolic and chemical activity of the brain. A radioactive tracer (usually radiolabelled glucose) is injected into the patient's bloodstream, which is then absorbed by active tissues of the brain. The PET scanner detects photons emitted by the tracer, and produces slice" images of the brain. The technique may be used not only for researching healthy brain activity, but for detecting tumours, sites of mental illness and epileptic activity."