nuclear medicine, medical specialty that involves the use of radioactive isotopes in the diagnosis and treatment of disease. Nuclear medicine enables clinicians to noninvasively and precisely identify specific molecular activity within tissues and organs of the body, facilitating the early detection of disease and the immediate monitoring of therapeutic responses.
Nuclear medicine began only after the discovery by Enrico Fermi in 1935 that stable elements could be made radioactive by bombarding them with neutrons. The atoms of the elements so bombarded capture these neutrons, thus assuming a different nuclear form while remaining the same elements. These radioisotopes have unstable nuclei, however, and dissipate excess energy by emitting radiation in the form of gamma and other rays.
In radioisotope scanning (also called radionuclide scanning), a radioisotope is introduced into the body, usually by means of intravenous injection. The isotope is then taken up in different amounts by different organs. Its distribution can be determined by recording the radiation it emits, and through charting its concentration it is often possible to recognize the presence, size, and shape of various abnormalities in body organs. The radiation emitted is detected by a scintillation counter, which is moved back and forth over the organ being scanned; these messages can then be electronically recorded and studied by clinicians. Positron emission tomography (PET) scans commonly are used to detect radioactive signals. The radioisotope usually has a short half-life and thus decays completely before its radioactivity can cause any damage to the patient’s body.
Different isotopes tend to concentrate in particular organs: for example, iodine-131 settles in the thyroid gland and can reveal a variety of defects in thyroid functioning. Another isotope, carbon-14, is useful in studying abnormalities of metabolism that underlie diabetes mellitus, gout, anemia, and acromegaly.
Various scanning devices and techniques have been developed and combined with radioisotope scanning to produce specialized views of tissues. These technologies include computed tomography and magnetic resonance imaging. In some instances, nuclear medicine technologies and other techniques are used simultaneously, such as when computed tomography is combined with PET or single photon emission computed tomography (SPECT). See also radiology.