Around one third of the supply of medical isotopes used worldwide comes from the Chalk River Laboratories in Ontario, Canada and another third comes from the Petten nuclear reactor in the Netherlands.

In Canada, the National Research Universal reactor was shut down on November 18, 2007, by the Canadian Nuclear Safety Commission so that safety systems could be updated to modern standards. The upgrade took longer than planned, which led to a critical shortage of medical isotopes in December 2007. The Canadian government therefore passed emergency legislation to allow the reactor to be re-started on 16 December 2007 so that medical isotope production could continue.

Another commonly used radioisotope in PET F-18 is not produced in a nuclear reactor but in a circular accelerator referred to as a cyclotron. The cyclotron accelerates protons to bombard the stable heavy isotope of oxygen O-18. The F-18 is then generally used to make FDG.The Chalk River reactor irradiates materials with neutrons that are produced in large quantities during the fission of U-235. These neutrons cause nucleic changes in the irradiated material through addition of a neutron or they act by splitting it through nuclear fission. Within a reactor, one fission product of uranium is molybdenum-99 and this is transported to radiopharmaceutical houses across the whole of North America. The Mo-99 radioactive beta has a half-life of 2.7 days and turns into Tc-99m, which is then extracted. The Tc-99m further decays when taken by a patient and releases a gamma photon that can be picked up by a gamma camera. It decays to its ground state of Tc-99, which is less radioactive than Tc-99m.

Some examples of commonly used radionuclides in nuclear medicine include:

• Fluorine-18 (18F)
• Gallium-67 (67Ga)
• Krypton-81m (81mKr)
• Rubidium-82 (82Rb)
• Technetium-99m (99mTc)
• Indium-111 (111In)
• Iodine- 123 (123I)
• Xenon-133 (133Xe)
• Thallium-201(201Tl)
• Yttrium-90 (90Y)
• Iodine-131 (131I)