These are computer generated images called PET (positron emission tomography) of the brain. It consists of injecting a radioactive analogue of glucose, FDG (fluorodeoxyglucose) into the bloodstream; the three- dimensional images of tracer concentration within the brain are then constructed by computer analysis. The more metabolically active areas will retain more FDG, and consequently retain more radioation (orange color). It is an important tool for detecting malignant tumors such as metastasis mainly in other parts of the body. In the brain, it also has been used to detect areas of the brain that generate seizures.