For each element the number of neutrons in the nucleus can vary.
Isotopes :Nuclei of given element with different numbers of neutrons.If they are not radioactive they called stable isotopes, and if they are radioactive they are called radioisotopes, for example carbon has two stable isotopes (12C and 13C ) and several radioisotopes (e.g. 11C,14Cand 15C).
In a modern hospitals about one third of the patients have a nuclear medicine study of some type .Although many nuclear medicine tests are concerned with the detection of cancer ,others are used to detect problems of heart ,lungs, kidneys and joints.The most useful radionuclide for nuclear medicine are those that emit gamma rays