arXiv subjects

R. W. Mair

Publications and source records attributed to R. W. Mair.

14 recordsLinked to original sources

arXiv2009

Hyperpolarized Long-T1 Silicon Nanoparticles for Magnetic Resonance Imaging

Silicon nanoparticles are experimentally investigated as a potential hyperpolarized, targetable MRI imaging agent. Nuclear T_1 times at room temperature for a variety of Si nanoparticles are found to be remarkably long (10^2 to 10^4 s) - roughly consistent with predictions of a core-shell diffusion model - allowing them to be transported, administered and imaged on practical time scales without significant loss of polarization. We also report surface functionalization of Si nanoparticles, comparable to approaches used in other biologically targeted nanoparticle systems.

cond-mat.mtrl-sci↗

arXiv2007

Posture-Dependent Human 3He Lung Imaging in an Open Access MRI System: Initial Results

The human lung and its functions are extremely sensitive to orientation and posture, and debate continues as to the role of gravity and the surrounding anatomy in determining lung function and heterogeneity of perfusion and ventilation. However, study of these effects is difficult. The conventional high-field magnets used for most hyperpolarized 3He MRI of the human lung, and most other common radiological imaging modalities including PET and CT, restrict subjects to lying horizontally, minimizing most gravitational effects. In this paper, we briefly review the motivation for posture-dependent studies of human lung function, and present initial imaging results of human lungs in the supine and vertical body orientations using inhaled hyperpolarized 3He gas and an open-access MRI instrument. The open geometry of this MRI system features a "walk-in" capability that permits subjects to be imaged in vertical and horizontal positions, and potentially allows for complete rotation of the orientation of the imaging subject in a two-dimensional plane. Initial results include two-dimensional lung images acquired with ~ 4 mm in-plane resolution and three-dimensional images with ~ 1.5 cm slice thickness. Effects of posture variation are observed.

physics.med-ph↗

arXiv2005

Using NMR to Validate First-Principles Granular Flow Equations

Nuclear magnetic resonance (NMR) experiments are described for two granular-flow systems, the vibrofluidized bed and the gas-fluidized bed. Using pulsed field gradient, magnetic resonance imaging, and hyperpolarized gas NMR, detailed information is obtained for the density and motions of both grains and interstitial gas. For the vibrofluidized bed, the granular temperature profile is measured and compared with a first-principles formulation of granular hydrodynamics. For the gas-fluidized bed, dynamic correlations in the grain density are used to measure the bubble velocity and hyperpolarized xenon gas NMR is used to measure the bubble-emulsion exchange rate. A goal of these measurements is to verify in earth gravity first-principles theories of granular flows, which then can be used to make concrete predictions for granular flows in reduced gravity.

cond-mat.soft↗

arXiv2004

3He Lung Imaging in an Open Access, Very-Low-Field Human MRI System

The human lung and its functions are extremely sensitive to gravity, however the conventional high-field magnets used for most laser-polarized 3He MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser-polarized 3He gas is demonstrated in an open-access very-low-magnetic-field (< 5 mT) MRI instrument. This prototype device employs a simple, low-cost electromagnet, with an open geometry that allows variation of the orientation of the imaging subject in a two-dimensional plane. As a demonstration, two-dimensional lung images were acquired with 4 mm in-plane resolution from a subject in two orientations: lying supine, and sitting in a vertical position with one arm raised. Experience with this prototype device will guide optimization of a second-generation very-low-field imager to enable studies of human pulmonary physiology as a function of subject orientation.

physics.med-ph↗

arXiv2004

Study of Gas-Fluidization Dynamics with Laser-Polarized 129Xe

We report initial NMR studies of gas dynamics in a particle bed fluidized by laser-polarized xenon (129Xe) gas. We have made preliminary measurements of two important characteristics: gas exchange between the bubble and emulsion phases; and the gas velocity distribution in the bed. We used T2* contrast to differentiate the bubble and emulsion phases by choosing solid particles with large magnetic susceptibility, in order. Experimental tests demonstrated that this method was successful in eliminating 129Xe magnetization in the emulsion phase, which enabled us to observe the time-dependence of the bubble magnetization. By employing the pulsed field gradient method, we also measured the gas velocity distribution within the bed. These results clearly show the onset of bubbling and can be used to deduce information about gas and particle motion in the fluidized bed.

cond-mat.mtrl-sci↗

arXiv2004

Xenon NMR Measurements of Permeability and Tortuosity in Reservoir Rocks

In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.

cond-mat.mtrl-sci↗

arXiv2003

Simultaneous Measurement of Rock Permeability and Effective Porosity using Laser-Polarized Noble Gas NMR

We report simultaneous measurements of the permeability and effective porosity of oil-reservoir rock cores using one-dimensional NMR imaging of the penetrating flow of laser-polarized xenon gas. The permeability result agrees well with industry standard techniques, whereas effective porosity is not easily determined by other methods. This novel NMR technique may have applications to the characterization of fluid flow in a wide variety of porous and granular media.

cond-mat.mtrl-sci↗

arXiv2003

NMR Experiments on a Three-Dimensional Vibrofluidized Granular Medium

A three-dimensional granular system fluidized by vertical container vibrations was studied using pulsed field gradient (PFG) NMR coupled with one-dimensional magnetic resonance imaging (MRI). The system consisted of mustard seeds vibrated vertically at 50 Hz, and the number of layers N_ell <= 4 was sufficiently low to achieve a nearly time-independent granular fluid. Using NMR, the vertical profiles of density and granular temperature were directly measured, along with the distributions of vertical and horizontal grain velocities. The velocity distributions showed modest deviations from Maxwell-Boltzmann statistics, except for the vertical velocity distribution near the sample bottom which was highly skewed and non-Gaussian. Data taken for three values of N_ell and two dimensionless accelerations Gamma=15,18 were fit to a hydrodynamic theory, which successfully models the density and temperature profiles including a temperature inversion near the free upper surface.

cond-mat.soft↗

arXiv2002

Tortuosity Measurement and the Effects of Finite Pulse Widths on Xenon Gas Diffusion NMR Studies of Porous Media

We have extended the utility of NMR as a technique to probe porous media structure over length scales of ~ 100 - 2000 micron by using the spin 1/2 noble gas 129Xe imbibed into the system's pore space. Such length scales are much greater than can be probed with NMR diffusion studies of water-saturated porous media. We utilized Pulsed Gradient Spin Echo NMR measurements of the time-dependent diffusion coefficient, D(t) of the xenon gas filling the pore space to study further the measurements of both the surface area-pore volume ratio, S/Vp, and the tortuosity (pore connectivity) of the medium. In uniform-size glass bead packs, we observed D(t) decreasing with increasing t, reaching an observed asymptote of ~ 0.62 - 0.65D0, that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D0 at differing gas pressures showed this tortuosity limit was not affected by changing the characteristic diffusion length of the spins during the diffusion encoding gradient pulse. This was not the case at the short time limit, where D(t)/D0 was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D0 and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D0 from the S/Vp relation. The Pade approximation is used to interpolate between the long and short time limits in D(t). While the short time D(t) point lay above the interpolation line in the case of small beads, due to diffusion during the gradient pulse on the order of the pore size, it was also noted that the experimental D(t) data fell below the Pade line in the case of large beads, most likely due to finite size effects.

cond-mat.soft↗

arXiv2002

The Narrow Pulse Approximation and long length scale determination in xenon gas diffusion NMR studies of model porous media

We report a systematic study of xenon gas diffusion NMR in simple model porous media: random packs of mono-sized glass beads, and focus on three specific areas peculiar to gas-phase diffusion. These topics are: (i) diffusion of spins on the order of the pore dimensions during the application of the diffusion encoding gradient pulses in a PGSE experiment (breakdown of the 'narrow pulse approximation' and imperfect background gradient cancellation), (ii) the ability to derive long-length scale structural information, and (iii) effects of finite sample size. We find that the time-dependent diffusion coefficient, D(t), of the imbibed xenon gas at short diffusion times in small beads is significantly affected by the gas pressure. In particular, as expected, we find smaller deviations between measured D(t) and theoretical predictions as the gas pressure is increased, resulting from reduced diffusion during the application of the gradient pulse. The deviations are then completely removed when water D(t) is observed in the same samples. The use of gas also allows us to probe D(t) over a wide range of length scales, and observe the long-time asymptotic limit which is proportional to the inverse tortuosity of the sample, as well as the diffusion distance where this limit takes effect (~ 1 - 1.5 bead diameters). The Pade approximation can be used as a reference for expected xenon D(t) data between the short and long time limits, allowing us to explore deviations from the expected behaviour at intermediate times as a result of finite sample size effects. Finally, the application of the Pade interpolation between the long and short time asymptotic limits yields a fitted length scale (the "Pade length"), which is found to be ~ 0.13b for all bead packs, where b is the bead diameter.

cond-mat.soft↗

arXiv2002

Diffusion NMR Methods Applied to Xenon Gas for Materials Study

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 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. 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). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm/s due to the high diffusivity of gases compared to 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.

cond-mat.soft↗

arXiv2002

Applications of controlled-flow laser-polarized xenon gas to porous and granular media study

We report initial NMR studies of continuous flow laser-polarized xenon gas, both in unrestricted tubing, and in a model porous media. The study uses Pulsed Gradient Spin Echo-based techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients. Pulsed Gradient Echo studies of continuous flow laser-polarized xenon gas in unrestricted tubing indicate clear diffraction minima resulting from a wide distribution of velocities in the flow field. The maximum velocity experienced in the flow can be calculated from this minimum, and is seen to agree with the information from the complete velocity spectrum, or motion propagator, as well as previously published images. The susceptibility of gas flows to parameters such as gas mixture content, and hence viscosity, are observed in experiments aimed at identifying clear structural features from echo attenuation plots of gas flow in porous media. Gas-phase NMR scattering, or position correlation flow-diffraction, previously clearly seen in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack is not so clear in experiments using a different gas mixture. A propagator analysis shows most gas in the sample remains close to static, while a small portion moves through a presumably near-unimpeded path at high velocities.

cond-mat.soft↗

arXiv2001

Measurements of Grain Motion in a Dense, Three-Dimensional Granular Fluid

We have used an NMR technique to measure the short-time, three-dimensional displacement of grains in a system of mustard seeds vibrated vertically at 15g. The technique averages over a time interval in which the grains move ballistically, giving a direct measurement of the granular temperature profile. The dense, lower portion of the sample is well described by a recent hydrodynamic theory for inelastic hard spheres. Near the free upper surface the mean free path is longer than the particle diameter and the hydrodynamic description fails.

cond-mat.soft↗

arXiv2001

Measuring surface-area-to-volume ratios in soft porous materials using laser-polarized xenon interphase exchange NMR

We demonstrate a minimally invasive nuclear magnetic resonance (NMR) technique that enables determination of the surface-area-to-volume ratio (S/V) of soft porous materials from measurements of the diffusive exchange of laser-polarized 129Xe between gas in the pore space and 129Xe dissolved in the solid phase. We apply this NMR technique to porous polymer samples and find approximate agreement with destructive stereological measurements of S/V obtained with optical confocal microscopy. Potential applications of laser-polarized xenon interphase exchange NMR include measurements of in vivo lung function in humans and characterization of gas chromatography columns.

cond-mat.soft↗