PET (positron emission tomography) scans are a type of nuclear medicine imaging, which uses a small amount of weakly radioactive materials to diagnose diseases. The weakly radioactive substance called a radiotracer, is injected, swallowed, or inhaled into the body and allowed to travel throughout the body. The substance is tracked as it moves throughout the body so that physicians can understand how various organs are functioning. Many medical centres combine these scans with images taken using CT scans, (also known as CAT scans).