Nuclear Medicine

A nuclear medicine procedure always requires the use of a radionuclide. Radionuclides, by virtue of their natural tendency to achieve stability, decay or disintegrate at a constant rate. Each radionuclide has its own distinct method of decay and rate of decay, or half-life. During disintegration, radionuclides emit electromagnetic radiation (photons), which can be detected, localized, and quantitated by sophisticated radiation detectors. Most frequently, the radionuclide is chemically bound to a stable molecule or compound chosen for its ability to localize in a specific organ system. The combination of the radionuclide bound to a molecule or compound is known as a radiopharmaceutical. The foundation of radionuclide or radiopharmaceutical use is based on the tracer principle, invented by the Hungarian chemist Georg von Hevesy (1885-1966) in 1912. Hevesy demonstrated that radioactive nuclides had chemical properties that were identical to those of their nonradioactive, or stable, form, and could therefore be used to "trace" various biochemical and physiological behaviors in the body and obtain diagnostic information.

Typically, the radiopharmaceutical is injected intravenously (in a vein), but some studies require inhalation (as a radioactive gas), or ingestion. The distribution of the radiopharmaceutical in the body or organ can reveal the normal or altered state of blood flow, capillary permeability, tissue metabolism, or specific function of an organ system. For example, if the physiology of an organ system or area of an organ is changed for reasons such as a tumor, absence of blood flow, duct blockage, or disease process, the way in which the radiopharmaceutical is incorporated will reflect any alteration. Nuclear medicine procedures can show structural as well as functional changes.

Radiopharmaceuticals are also chosen for their particular radioactive properties such as half-life, type of radiation emitted during decay, photon energy, cost, and availability. Today, ^99mTechnetium (^99mTc [half-life = 6.0 hours]), a daughter product of ⁹⁹Molybdenum (⁹⁹Mo), is the most commonly used radionuclide for nuclear medicine procedures and for making radiopharmaceuticals. Technetium is considered ideal because it gives a low radiation dose to the patient, has a low energy (140keV), most of its decay emissions are gamma-rays, it has a short half-life (six hours), is inexpensive and readily obtained, and combines easily with many compounds.