Technetium-99m 'pseudogas' for diagnostic studies in the lung.

Combustion of a flammable aerosol of 99Tcm generates an ultrafine aerosol of approximately 0.06 micron particle radius with a narrow log-normal distribution (omega g typically 1.25), close to monodisperse. Particles with this dimension do not settle out, but diffuse in a gas-like manner without aggregation. The name 'Pseudogas' has been given to this agent, to identify its unique characteristics. The technique for Pseudogas production is described and its physical properties analysed. Predicted behaviour is verified and illustrated by a clinical example. Small-airways retention of Pseudogas in normal lungs has an estimated half-time of about ten minutes, clearing to the systemic blood stream, but considerably extended regional and total times have been observed (up to 100 min) in some patients.