Diffusion NMR methods applied to xenon gas for materials study.
ABSTRACT We report initial NMR studies of (i) xenon gas diffusion in model heterogeneous porous media and (ii) continuous flow laser-polarized xenon gas. Both areas utilize the pulsed gradient spin-echo (PGSE) techniques in the gas phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients--a brief overview of this area is provided in the Introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t) (an indicator of mean squared displacement), to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. We find that D(t) of imbibed xenon gas at short diffusion times is similar for the mixed bead pack and a pack of the smaller sized beads alone, hence reflecting the pore surface area to volume ratio of the smaller bead sample. The approach of D(t) to the long-time limit follows that of a pack of the larger sized beads alone, although the limiting D(t) for the mixed bead pack is lower, reflecting the lower porosity of the sample compared to that of a pack of mono-sized glass beads. The Pade approximation is used to interpolate D(t) data between the short- and long-time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20-200 mm s-1). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm s-1 owing to the high diffusivity of gases compared with liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data, namely flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.
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ABSTRACT: To quantitatively evaluate hyperpolarized helium 3 ((3)He) diffusion magnetic resonance (MR) images of the lung in patients with emphysema and to determine whether apparent diffusion coefficients (ADCs) measured with MR imaging correlate with spirometric indexes. Hyperpolarized (3)He diffusion MR imaging was performed in 16 healthy volunteers and 11 patients. Coronal diffusion-sensitized MR images were obtained during suspended respiration after inhalation of laser-polarized (3)He gas, and images of the ADC were calculated. Spirometry was performed immediately before imaging. The mean and SD of the ADCs were compared between subject groups and were correlated with spirometric indexes. ADC images were homogeneous in volunteers, but demonstrated regional variations in patients. The mean and SD of the ADCs for patients were significantly larger (P <.002) than those for volunteers. The mean ADCs for all subjects correlated with the percentage of predicted forced expiratory volume in 1 second, or FEV(1), (r = -0.797, P <.001) and the ratio of FEV(1) to forced vital capacity, or FVC, (r = -0.930, P <.001). ADC images in patients demonstrated a significant increase (P <.001) in the ADCs in the upper regions compared with the lower regions of the lung. Hyperpolarized (3)He diffusion MR imaging demonstrated potential for use in evaluating the global and regional severity of emphysema.Radiology 01/2002; 222(1):252-60. · 6.34 Impact Factor
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ABSTRACT: Methods for T1 relaxation and diffusion measurements based on magnetic resonance signals from laser-hyperpolarized 129Xe nuclei are introduced. The methods involve optimum use of the perishable hyperpolarized magnetization of 129Xe. The necessary theoretical framework for the methods is developed, and then the methods are applied to measure the longitudinal relaxation constant, T1, and the self-diffusion constant, D, of hyperpolarized 129Xe. In a cell containing natural abundance 129Xe at 790 Torr, the T1 value was determined to be 155 +/- 5 min at 20 degrees C and at 2.0 T field. For a second cell at 896 Torr, at the same field and temperature, the T1 value was determined to be 66 +/- 2 min. At a higher field of 7.05 T, the T1 values for the two cells were found to be 185 +/- 10 and 88 +/- 5 min, respectively. The 129Xe self-diffusion constant for the first cell was measured to be 0.057 cm2/ s and for the second cell it was 0.044 cm2/s. The methods were applied to 129Xe in the gas phase, in vitro; however, they are, in principle, applicable for in vivo or ex vivo studies. The potential role of these methods in the development of newly emerging hyper-polarized 129Xe MRI applications is discussed.Journal of Magnetic Resonance 06/1997; 126(1):58-65. · 2.30 Impact Factor
- Physical review. B, Condensed matter 02/1994; 49(1):215-225.