Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three dimensional image or map of functional processes in the body. PET is both a medical and research tool. It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), for clinical diagnosis of brain diseases such as dementias. PET is also an important research tool to map human brain and heart function. To conduct the scan, a short-lived radioactive tracer isotope which decays by emitting a positron, and which has been chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (most commonly fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour) becomes concentrated in tissues of interest; then the research subject or patient is placed in the imaging scanner.