Positron emission tomography (PET) is an imaging technique in which very small quantities of a radioactive substance are used. Such a substance, e.g. radioactive glucose or water, is administered to a patient and sends out radiation as positrons. After annihilation, two 0.511 mega electron volts (MeV) photons are formed which can be detected by the PET scanner. In this way, the presence of (for instance) tumors can be determined with 18F-FDG (radioactive glucose) because cancer cells absorb more glucose than healthy cells.