PET scan. View of a patient undergoing a positron emission tomography (PET) scan of the brain. This is a technique used to detect areas of abnormal activity in the body. Usually, radioactive glucose is injected into the bloodstream. The PET scanner detects the presence of the glucose in body tissues, and builds up a picture of glucose concentration, and thus metabolic activity, in the scanned area. This is particularly useful in detecting brain tumours, which show a marked difference in activity to healthy brain tissue. Here, the scan is being used to determine where in the brain to take a biopsy from. Biopsies are used to determine whether or not a tumour is malignant.