A radioisotope is most useful diagnostically when it has a short half life and is taken up by more specific tissues. An isotope with a short duration of activity is desired, because it is preferred not to have radioisotopes persist in the patient's body any longer than necessary. Uptake by specific organs is desired because the radioactive decay is used to create an image, and it is better for the image to be specific to the tissues or organs of interest. Otherwise, the image would not be useful for diagnostic purposes.
