Oxygen-15

Oxygen-15 has the shortest half-life of any radionuclide used in PET, 122 seconds (about two minutes), and decays by pure positron emission. Labelled as [15O]water, it diffuses freely across cell membranes and its uptake is not confounded by metabolism, which makes it the physics gold standard for quantifying regional cerebral or myocardial blood flow; the very short half-life also allows repeat rest and stress or activation scans in the same session only 10-15 minutes apart, with a very low cumulative radiation dose.

Production requires an on-site cyclotron: nitrogen gas is bombarded with deuterons, 14N(d,n)15O, to give [15O]oxygen gas, which is then reacted over a heated palladium catalyst to form [15O]water vapour that is dissolved into saline for injection. Because a scan can only happen within a couple of minutes' walk of the cyclotron, [15O]water PET is confined to a small number of specialised academic centres worldwide.

[15O]water PET myocardial and cerebral blood flow imaging is clinically approved and reimbursed in a number of European countries but, as of the mid-2020s, has not received FDA approval for routine US clinical use, so its US role remains research and investigational protocols rather than everyday practice.