Natural Radioactivity first appeared concomitantly to the creation of the Earth, and the radionuclides that can still be found in soil today are in fact long period Isotopes or Decay nuclides dating from original matter. As a consequence, man has always been exposed to environmental Radioactivity, whether from terrestrial or cosmic origin, and of course we continue to find these radionuclides in our food and indirectly in each of our body cells.

A Radionuclide is a substance that degrades in a very constant manner over time and emits one or several radiations. This degradation or Decay is defined by a constant, the period (or Half-life) corresponding to the time it takes for half of the remaining substance to disappear. This Half-life is specific for each Radionuclide.

The type of emitted Radiation is also specific for each Radionuclide. There are five types of Radiation which are of interest to Nuclear medicine: for diagnosis purposes, Gamma rays (γ) and beta plus (or Positron) emissions (β+) have led to the development of respectively the Imaging modalities SPECT (Single Photon Emission Computed Tomography) and PET (Positron Emission Tomography). Beta minus (β–), Alpha (α) radiations and soon conversion or Auger electrons are used in Metabolic Radiotherapy.

Among all the radionuclides with potential in Nuclear medicine, we shall remember those that are currently most widely used: Iodine-123 and Technetium-99m as γ emitters, Fluorine-18 and Gallium-68 as β+ emitters, Iodine-131 and Lutetium-177 as β– emitters. Newcomers will include Astatine-211 and Actinium-225 in the Alpha-emitting series and Tin-117m as Conversion electron emitter.