Technetium-99m

Technetium-99m is a metastable nuclear isomer used globally in medical diagnostics. A metastable isomer is a type of nuclide that exists in an excited state for a longer period than usual. This specific isotope is the most commonly used medical radioisotope in the world. It is used in tens of millions of medical diagnostic procedures every year. Doctors use it as a radioactive tracer to help visualize how the body is functioning.

Because it is so useful, it has become a cornerstone of modern medical imaging.

The isotope works by emitting gamma rays, which are high-energy forms of light. These rays have a photon energy of 140.5 keV. This specific energy level is very helpful because it falls within the range used by conventional X-ray equipment. Medical professionals use specialized machines called gamma cameras to detect these emissions. When the tracer is inside a patient, the camera picks up the rays to create an image. This process allows doctors to see internal organs and biological functions clearly.

Technetium-99m has a physical half-life of 6.0066 hours. The half-life is the time required for half of the radioactive atoms to decay. Within 24 hours, approximately 93.7% of the isotope decays into technetium-99. It also has a biological half-life of about one day based on human metabolism. This combination of short half-lives is vital for patient safety. It allows for rapid data collection during scans while keeping the total radiation exposure to the patient very low. However, these same traits make it unsuitable for therapy. It cannot deliver a strong, localized dose to destroy diseased tissue because it decays too quickly.

Most technetium-99m is produced from a parent isotope called molybdenum-99. This parent isotope has a longer half-life of 2.75 days. To create it, scientists use nuclear reactors to perform fission on highly enriched uranium targets. The molybdenum-99 is then shipped to medical facilities. Once it arrives, the technetium-99m is extracted from the sample as it is produced. In 1958, Walter Tucker and Margaret Greene developed the first technetium-99m generator to handle this extraction.

This system allows hospitals to have a steady supply of the short-lived isotope.

The history of this discovery began in 1938. Emilio Segrè and Glenn T. Seaborg isolated the metastable isotope for the first time. They used a cyclotron to bombard natural molybdenum with 8 MeV deuterons. Later, in 1940, Segrè and Chien-Shiung Wu analyzed uranium-235 fission products and detected the isomer. In the 1950s, Powell Richards recognized its medical potential. He worked at the Brookhaven National Laboratory. By 1960, Richards suggested using it as a medical tracer. In 1963, Sorensen and Archambault demonstrated that injected molybdenum-99 could concentrate in the liver. This allowed them to visualize the organ using gamma ray emissions.

Production has shifted across different countries over the decades. For many years, the NRU reactor in Canada was a major supplier. Between 1967 and 1984, molybdenum-99 was produced for the Mallinckrodt Nuclear Company at the Missouri University Research Reactor. In the United States, Cintichem, Inc. became a single major producer during the 1980s. However, an underground leak in 1989 led to the shutdown of their reactor. This ended commercial production of molybdenum-99 in the U.S. for a time. Other countries like Argentina, Belgium, and South Africa also maintain reactors to support isotope production.

The global supply chain can be sensitive to technical issues. In the late 2000s, the world faced shortages of technetium-99m. This happened because two aging reactors, the NRU and the HFR, provided about two-thirds of the global molybdenum-99 supply. Both reactors faced maintenance issues and shutdowns. The NRU reactor in Canada experienced a heavy water leak in 2009. The HFR reactor in the Netherlands also required safety investigations and repairs. These events highlight how important stable nuclear research facilities are for maintaining the global supply of medical isotopes.