Iodine-124

Iodine-124 decays with a 4.18-day half-life by a mix of electron capture and positron emission (positron abundance about 23%), giving it PET-imageable annihilation photons over a timescale of days rather than the minutes-to-hours typical of most PET isotopes. Because it shares iodine's biochemistry with the therapeutic isotope 131I, serial 124I PET/CT scans, typically at roughly 24 and 96-120 hours after a tracer dose, let physicists build a time-activity curve for each lesion and project the absorbed dose per unit of a subsequent 131I therapy activity, rather than relying on a fixed empiric dose.

Its decay scheme is complex: only about 23% of decays produce a positron (mean energy 819 keV, giving relatively poor spatial resolution), while high-energy prompt gammas (603 keV and others) contaminate the coincidence signal, so scanners need isotope-specific calibration rather than the F-18 calibration used by default; published work has found this calibration factor can differ from F-18 by roughly ten percent.

It is cyclotron-produced by proton bombardment of enriched tellurium-124, 124Te(p,n)124I, chosen for high radionuclidic purity at proton energies below about 14 MeV, though yields from this reaction are comparatively low, and by the older 124Te(d,2n)124I route.