Gamma radiation from technetium

Most radioactive nuclides employed in radiopharmaceuticals have a short half-life. This is beneficial to the patient as the total number of radioactive atoms given to the patient to produce an image is small when the half-life of the radioactive nuelide is short, as compared to longer half-life radioactive nuclides. Fewer total atoms reduce the radiation dose to the patient and thus the risk from a nuclear medi-eine procedure. However, the short half-life of the radioactive nuclide results in a short shelf-life for the radiopharmaeeutical. As a result, most radiopharmaceuticals are eompounded on a daily basis. The most common radioactive nuclide used for this purpose is technetium-99m (Te-99m) with a half-life of 6 hr, emiting only gamma radiation with an energy almost ideal for detection. 

The radionuclide needs to be a gamma emitter (long-range ionizing radiation) that can be monitored from outside the patient s body. It should have a relatively short half-life to limit the radiation dose to the patient. The radionuclide most commonly used is technetium-99m, Tc. The m indicates that this is a metastable isotope, that is, that its half-life is considerably longer than most isotopes that undergo gamma decay. 

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