Medical imaging is a mainstay in the field of nuclear medicine. In nuclear medicine, radioactive elements (as isotopes) that are part of specific fluids are introduced into the body (usually by injection into the blood). As it circulates, a particular radioisotope tends to distribute throughout the body at points served by the blood flow and may even concentrate preferentially in certain organs (for example, radioactive iodine in the thyroid gland). As the isotope decays, it gives off radiation (most commonly, gamma rays) which can be intercepted by a gamma camera or other detector. Variations in radiation intensity and in spatial location at point sources in the body activate film or more usually a detector array that responds by mapping the radiation intensity in X-Y space to create an image. The radioisotopes in normal usage have relatively short half lifes, thus decaying rapidly, and minimizing the exposure to damaging radiation.