Positron emission tomography (PET) is an imaging technique producing several cross-section images (the ancient Greek tome means “slice”, and Greek graphein means “to write”). This technique uses a weakly radioactive substance.that is injected into an arm vein and then spreads through the body in the bloodstream. The PET device can trace the path the substance takes in the body and measure the radiation it sends out. PET helps to find out how active metabolism is in a tissue, especially in one with high energy demand – for example, brain or heart muscle cells, but also inflamed tissue or tumors. People who have a PET examination are exposed to radiation.