Wiktionary
nitrogen-13
n. (context isotope English) A radioactive isotope of nitrogen, (nuclide 13 7 N), having seven protons and six neutrons, and a half-life of less than ten minutes; it is used in positron emission tomography
Wikipedia
Nitrogen-13
Nitrogen-13 is a radioisotope of nitrogen used in positron emission tomography (PET). It has a half life of a little under ten minutes, so it must be made at the PET site. A cyclotron may be used for this purpose.
Nitrogen-13 is used to tag ammonia molecules for PET myocardial perfusion imaging.