The most commonly used medical isotope is technetium-99m. It is not used to treat disease but to create images of patients to help in diagnosis. It is injected into a patient, and as it decays it emits energy. Energy in sunlight can be detected on photographic film to make an image. In the same way, energy emitted from the technetium-99m can be detected to create an internal image of the patient. This is a very powerful technique for diagnosing cancer.
The majority of the technetium-99m used in the world comes from the NRU reactor: more than 10,000 people treated every day.