A nuclear medicine procedure is sometimes described as an "inside-out" x-ray because it records radiation emitting from the patient's body rather than radiation that is directed through the patient's body. Nuclear medicine procedures use small amounts of radioactive materials, called radiopharmaceuticals, to create images of anatomy. Radiopharmaceuticals are substances that are attracted to specific organs, bones or tissues. They are introduced into the patient's body by injection, swallowing or inhalation. As the radiopharmaceutical travels through the body, it produces radioactive emissions. A special type of camera detects these emissions in the organ, bone or tissue being imaged and then records the information on a computer screen or on film.

Nuclear medicine is unique because it documents function as well as structure. For example, nuclear medicine allows physicians to see how a kidney is functioning, not just what it looks like. Most other diagnostic imaging tests, in comparison, reveal only structure. Nuclear medicine procedures are performed to assess the function of nearly every organ. Common nuclear medicine procedures include thyroid studies, brain scans, bone scans, lung scans, cardiac stress tests, and liver and gallbladder procedures.