radionuclide /-no̅o̅″klīd/ [L, radiare + nucleus, nut kernel] , an isotope that undergoes radioactive decay. Any element with an excess of either neutrons or protons in the nucleus is unstable and tends toward radioactive decay, with the emission of energy that may be measurable with a detector. The processes of radioactive decay include beta particle emission, electron capture, isomeric transition, and positron emission. Positron-emitting radionuclides are important in positron emission tomography and in medical research. Radionuclides used in scintigraphy include 123I, 131I, 111In, 75Se, 99mTc, and 201Tl. Radionuclides of cobalt, iodine, phosphorus, strontium, and other elements are used for treatment of tumors and cancers and for nuclear imaging of internal parts of the body. See also nuclear scanning.
