Glenn P. Wong

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 milliseconds in most NMR experiments. In restricted environments, such as the lung, this rapid gas diffusion can lead to violations of the narrow pulse approximation, a basic assumption of the standard Stejskal–Tanner NMR method of diffusion measurement. We therefore investigated the effect of a common, biologically inert buffer gas, sulfur hexafluoride (SF6), on 129Xe NMR and diffusion. We found that the contribution of SF6 to 129Xe T1 relaxation in a 1:1 xenon/oxygen mixture is negligible up to 2 bar of SF6 at standard temperature. We also measured the contribution of SF6 gas to 129Xe T2 relaxation, and found it to scale inversely with pressure, with this contribution approximately equal to 1 s for 1 bar SF6 pressure and standard temperature. Finally, we found the coefficient of 129Xe diffusion through SF6 to be approximately 4.6 × 10−6 m2s−1 for 1 bar pressure of SF6 and standard temperature, which is only 1.2 times smaller than the 129Xe self diffusion coefficient for 1 bar 129Xe pressure and standard temperature. From these measurements we conclude that SF6 will not sufficiently reduce 129Xe diffusion to allow accurate surface‐area/volume ratio measurements in human alveoli using time‐dependent gas diffusion NMR.

Characterization and stabilization of fiber-coupled laser diode arrays

We have characterized the spectra and performance of an ensemble of 11 fiber-coupled laser diode arrays (LDAs) manufactured by Opto Power Inc. These high-power LDAs operate near 795 nm and are of a type commonly used for spin-exchange optical pumping of noble gases. We find the Opto Power LDAs to vary significantly in output power, spectral width, and other important characteristics, in a manner not correlated with age, operating lifetime, or information supplied by the manufacturer. In addition we have developed a two-loop feedback technique for use with LDAs that stabilizes the Rb magnetization in an optical pumping cell to better than one part in a thousand.