The most common radioisotope used in nuclear medicine procedures in the U.S. is technetium-99m (99mTc). 99mTc has found extensive use in nuclear cardiology (50% of procedures), nuclear oncology (25%), and in other imaging of the brain, endocrine system, lungs, gastrointestinal, genitourinary, and bones. There are currently 40 million nuclear medicine procedures performed annually in the U.S., with 80% using 99mTc-based radiopharmaceuticals. Although the radioisotope can be produced directly on a cyclotron or other type of particle accelerator, 99mTc is most conveniently obtained from the beta-decay of99Mo with a half-life of 66 hours. 99mTc and 99Mo cannot be stockpiled due to the short half-lives, and the radioisotopes must be produced in fresh batches and distributed to medical imaging centers almost daily.
99Mo is a fission fragment that is abundantly produced in the neutron-induced fission of uranim-235 (235U) (6% of all fissions). The 99mTc generators allow a quick and convenient chemical separation of 99mTc daughter nuclei from the 99Mo parent material. The 66-hour half-life enables 99Mo to be produced at large-capacity locations and then transported to centralized radiopharmacies in the U.S., which distribute the 99mTc generators to hospitals and other imaging facilities. 99Mo production is typically measured in“6-day curies” based on the material activity 6 days after being shipped (the actual value is roughly four times the 6-day value). The estimated worldwide demand for 99Mo is ~9,400 6-day Ci per week (NEA, 2018).
The last commercial production of 99Mo in the U.S. ended in 1989 and the process was just restarted in 2018 by NorthStar Medical Technologies, LLC. Domestic supply has and will continue to rely on international producers who employed the high efficiency of irradiating highly enriched uranium (HEU) targets, with much of the HEU exported from the U.S. Currently, six multipurpose research reactors, with five of these sites being over 45–55 years old, irradiate HEU or LEU targets for 99Mo production. As part of nuclear nonproliferation efforts, under the American Medical Isotope Production Act of 2012, the U.S. must stop exporting HEU by 2020, which is used for targets for isotope production and for fuel for reactors. The industry has largely converted to the use of LEU.