Technetium-99m

Technetium-99m is a metastable nuclear isomer of technetium-99 (itself an isotope of technetium), symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope.we discovered an isotope of great scientific interest, because it decayed by means of an isomeric transition with emission of a line spectrum of electrons coming from an almost completely internally converted gamma ray transition. (...) This was a form of radioactive decay which had never been observed before this time. Segrè and I were able to show that this radioactive isotope of the element with the atomic number 43 decayed with a half-life of 6.6 h and that it was the daughter of a 67-h molybdenum parent radioactivity. This chain of decay was later shown to have the mass number 99, and (...) the 6.6-h activity acquired the designation ‘technetium-99m. Technetium-99m is a metastable nuclear isomer of technetium-99 (itself an isotope of technetium), symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope. Technetium-99m is used as a radioactive tracer and can be detected in the body by medical equipment (gamma cameras). It is well suited to the role, because it emits readily detectable gamma rays with a photon energy of 140 keV (these 8.8 pm photons are about the same wavelength as emitted by conventional X-ray diagnostic equipment) and its half-life for gamma emission is 6.0058 hours (meaning 93.7% of it decays to 99Tc in 24 hours). The relatively 'short' physical half-life of the isotope and its biological half-life of 1 day (in terms of human activity and metabolism) allows for scanning procedures which collect data rapidly but keep total patient radiation exposure low. The same characteristics make the isotope suitable only for diagnostic but never therapeutic use. Technetium-99m was discovered as a product of cyclotron bombardment of molybdenum. This procedure produced molybdenum-99, a radionuclide with a longer half-life (2.75 days), which decays to Tc-99m. At present, molybdenum-99 (Mo-99) is used commercially as the easily transportable source of medically used Tc-99m. In turn, this Mo-99 is usually created commercially by fission of highly enriched uranium in aging research and material testing nuclear reactors in several countries. In 1938, Emilio Segrè and Glenn T. Seaborg isolated for the first time the metastable isotope technetium-99m, after bombarding natural molybdenum with 8 MeV deuterons in the 37-inch (940 mm) cyclotron of Ernest Orlando Lawrence's Radiation laboratory. In 1970 Seaborg explained that: Later in 1940, Emilio Segrè and Chien-Shiung Wu published the experimental results of the analysis of fission products of uranium-235, among which was present molybdenum-99, and detected the 6-h activity of element 43, later labelled as technetium-99m. Tc-99m remained a scientific curiosity until the 1950s when Powell Richards realized the potential of technetium-99m as a medical radiotracer and promoted its use among the medical community. While Richards was in charge of the radioisotope production at the Hot Lab Division of the Brookhaven National Laboratory, Walter Tucker and Margaret Greene were working on how to improve the separation process purity of the short-lived eluted daughter product iodine-132 from tellurium-132, its 3.2-days parent, produced in the Brookhaven Graphite Research Reactor. They detected a trace contaminant which proved to be Tc-99m, which was coming from Mo-99 and was following tellurium in the chemistry of the separation process for other fission products. Based on the similarities between the chemistry of the tellurium-iodine parent-daughter pair, Tucker and Greene developed the first technetium-99m generator in 1958. It was not until 1960 that Richards became the first to suggest the idea of using technetium as a medical tracer. The first US publication to report on medical scanning of Tc-99m appeared in August 1963. Sorensen and Archambault demonstrated that intravenously injected carrier-free Mo-99 selectively and efficiently concentrated in the liver, becoming an internal generator of Tc-99m. After build-up of Tc-99m, they could visualize the liver using the 140 keV gamma ray emission. The production and medical use of Tc-99m rapidly expanded across the world in the 1960s, benefiting from the development and continuous improvements of the gamma cameras. Between 1963 and 1966, numerous scientific studies demonstrated the use of Tc-99m as radiotracer or diagnostic tool. As a consequence the demand for Tc-99m grew exponentially and by 1966, Brookhaven National Laboratory was unable to cope with the demand. Production and distribution of Tc-99m generators were transferred to private companies. 'TechneKow-CS generator', the first commercial Tc-99m generator, was produced by Nuclear Consultants, Inc. (St. Louis, Missouri) and Union Carbide Nuclear Corporation (Tuxedo, New York). From 1967 to 1984, Mo-99 was produced for Mallinckrodt Nuclear Company at the Missouri University Research Reactor (MURR).