O15

Oxygen-15 (15O or O-15) is a radioactive isotope of oxygen with mass number 15. It has a half-life of about 122 seconds and decays by positron emission to nitrogen-15. Because of its short half-life, 15O is produced on site for use in positron emission tomography (PET). Its natural abundance is zero.

Production and handling: 15O is typically produced in a cyclotron by proton bombardment of nitrogen-14 via the reaction 14N(p,γ)15O. The produced 15O, often in gaseous or aqueous form, is rapidly separated and formulated for use in radiopharmaceuticals. The short half-life requires rapid synthesis, immediate quality control, and prompt administration, with appropriate shielding and regulatory controls due to radioactivity.

Medical imaging applications: In PET imaging, 15O is used to study cerebral and myocardial blood flow and tissue oxygen metabolism. Common 15O-labeled tracers include H2O-15 for perfusion, C15O for blood volume, and O15O2 for oxygen metabolism. Dynamic PET studies with these tracers yield quantitative measures of perfusion and metabolic rate, but the very short half-life imposes stringent logistical requirements, including on-site production and rapid data acquisition.

Scientific significance: In astrophysics, 14N(p,γ)15O is the slowest step in the CNO cycle that powers many stars. The transient production of 15O and its subsequent decay to 15N influence stellar energy generation and neutrino emission, linking laboratory isotope physics to astrophysical processes.

See also: Positron emission tomography, CNO cycle, Oxygen isotope.