PET scanning. Side view of a patient undergoing a Positron Emission Tomography (PET) brain scan. The patient's masked head is passing into the detector of the scanner. Red lights help to align the head. 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. Here the brain of a sleeping patient is being scanned, to be compared with the brain of an awake patient."