Radioactivity is the emission of radiation when the nuclei of certain atoms become unstable.
Nuclear medicine is a specialized area of radiology that uses very small amounts of radioactive materials, or radiopharmaceuticals, to examine organ function and structure. Nuclear medicine imaging is a combination of many different disciplines, including chemistry, physics, mathematics, computer technology, and medicine. This branch of radiology is often used to help diagnose and treat abnormalities very early in the progression of a disease, such as thyroid cancer. Since x-rays pass through soft tissue, such as intestines, muscles, and blood vessels, contrast agents are used in nuclear imaging. Nuclear imaging examines organ function and structure, whereas diagnostic radiology is based on anatomy.