Reduced xenon diffusion for quantitative lung study - the role of SF6

The large diffusion coefficients of gases result in significant spin motion during the application of gradient pulses that typically last a few millis econds in most NMR experiments. In restricted environments, such as the lun g, this rapid gas diffusion can lead to violations of the narrow pulse appr oximation, a basic assumption of the standard Stejskal-Tanner NMR method of diffusion measurement. We therefore investigated the effect of a common, b iologically inert buffer gas, sulfur hexafluoride (SF6), on Xe-129 NMR and diffusion. We found that the contribution of SF6 to Xe-129 T-1 relaxation i n a 1:1 xenon/oxygen mixture is negligible up to 2 bar of SF6 at standard t emperature. We also measured the contribution of SF6 gas to Xe-129 T-2 rela xation, and found it to scale inversely with pressure, with this contributi on approximately equal to 1 s for 1 bar SF6 pressure and standard temperatu re. Finally, we found the coefficient of Xe-129 diffusion through SF6 to be approximately 4.6 x 10(-6) m(2)s(-1) for 1 bar pressure of SF6 and standar d temperature, which is only 1.2 times smaller than the Xe-129 self diffusi on coefficient for 1 bar Xe-129 pressure and standard temperature. From the se measurements we conclude that SF6 will not sufficiently reduce Xe-129 di ffusion to allow accurate surface-area/volume ratio measurements in human a lveoli using time-dependent gas diffusion NMR.