An element is defined by its atomic number (number of protons), but may have multiple different isotopes:  versions of that element with different numbers of neutrons and, therefore, different atomic weights.  For example, carbon (six protons) has multiple isotopes:  carbon-11, carbon-12, carbon-14.  Some isotopes may be stable and some are radioactive.  Some elements have no stable isotopes.
Radioactive elements undergo decay events and emit energy.  Energy can be emitted in multiple forms, either as energetic particles (e.g., alpha and beta particles) or as photons (having no mass; e.g., x-rays and gamma rays).  Only photons are used to create images.  Most studies are performed with gamma rays.

The four most commonly used isotopes in clinical nuclear medicine are:

1. Technetium-99m - used for a wide variety of study types
2. Indium-111 - e.g. for tagged white blood cell scans
3. Iodine-131/123 - thyroid imaging
4. Fluorine-18 - PET imaging