Krypton-81m

Krypton-81m is an inert gas with a 13-second physical half-life and a 190 keV gamma emission, eluted from an 81Rb/81mKr generator: rubidium-81, bound to a zirconium phosphate cation exchange resin, sits in equilibrium with its krypton daughter, and passing oxygen or air through the generator carries the gas to the patient. The patient inhales continuously through the acquisition, so the image is a steady-state map of where air is arriving, and because the activity vanishes between views the study can be repeated in as many projections as wanted with negligible dose.

Its distinctive advantage is the 190 keV photon, well separated from technetium-99m's 140 keV, which means ventilation and perfusion can be acquired simultaneously rather than sequentially. The 2019 EANM V/Q SPECT guideline puts krypton alongside Technegas as preferred over Tc-99m DTPA aerosol in patients with COPD, where a wet aerosol deposits centrally, and one comparative review argues krypton is the better ventilation tracer for its penetration and absence of hot spots, cautioning against replacing it with Technegas in severe COPD. The counterweight is that in badly obstructed lungs the time to reach steady state may be too long.

Availability is the whole problem. Rubidium-81 is cyclotron-made, principally by the 82Kr(p,2n)81Rb reaction, and has a half-life of about 4.6 hours, so a generator has to be delivered to the hospital daily from a nearby cyclotron. That economics confined krypton to a handful of countries: Kr-81m/Tc-99m V/Q imaging was once, by one account, the second most performed nuclear medicine procedure in the United Kingdom after bone scanning, and a generator remains licensed in Denmark by Rigshospitalet's own cyclotron. In the United States GE Healthcare's MPI Krypton 81m generator (NDA 018088, approved 1980) was withdrawn effective 7 December 2007, and US commercial availability had already ended years earlier.