Half-life
All radioactive substances are by definition unstable. When unstable forms (= radioactive isotope) of an element (atom) decay into stable forms (= stable isotope), they do so by releasing energy (radiation) and/or particles. The rate at which this decay occurs is called the half-life - i.e. the time required for half of the atoms of a radioactive substance present to become disintegrated. The half-lives vary greatly. Some radionuclides have a half-life of only a few seconds, while others may have a half-life of several thousand years.
Radionuclides are widely used in medical imaging. Isotopes with a very short t1/2 are not very useful for Nuclear Medicine, because by the time they have been produced, dispensed in patient dosages and sent to a hospital, they will have decayed almost completely into stable nuclides. In order for such nuclides to be useful in medicine, they would have to be supplied in such large quantities that they would still retain sufficient radioactivity when it was time for them to be used in the patient. This would obviously be costly, because much material would go to waste during production and transportation, and it would also involve a higher risk of radiation exposure for the people engaged in producing and transporting the substance.
The other extreme is not desirable, either. For long-lived nuclides, the decay occurs so slowly that there is hardly any measurable radioactivity available to produce an image in the patient. Moreover, their use could also involve a certain risk for the patient. If the body has no natural mechanism for ridding itself of a given isotope, it may be retained within the body. It is for instance not uncommon that a radionuclide can accumulate in the liver, where tissue and cells can be damaged because it emits small amounts of radiation for many years.
For use in medical diagnostic procedures, therefore, a radionuclide should have neither too short nor too long a half-life. Most of the nuclides we supply have a t1/2 of from 12 hours to 2-3 months. However, one of our nuclides has a half-life of only 13 seconds (krypton gas, inhaled for lung scans), and perhaps the most important of all, technetium, has a half-life of only six hours.
GE Healthcare Glossary