PET scanner. Patient in a positron emission tomography (PET) scanner. PET scans are used to detect abnormal metabolic or chemical activity within the body, most often the brain, heart or tumours. A radioactive tracer, usually radiolabelled glucose, is injected into the patient's bloodstream, from where it is absorbed into active tissues. The scanner detects photons emitted by the tracer. The results are combined with computed tomography (CT) scans to show them against slices through the body.