Magnetic resonance imaging of hyperpolarized nuclei provides high image contrast with little or no background signal. To date, in vivo applications of prehyperpolarized materials have been limited by relatively short nuclear spin relaxation times. Here, we investigate silicon nanoparticles as a new type of hyperpolarized magnetic resonance imaging agent. Nuclear spin relaxation times for a variety of Si nanoparticles are found to be remarkably long, ranging from many minutes to hours at room temperature, allowing hyperpolarized nanoparticles to be transported, administered, and imaged on practical time scales. Additionally, we demonstrate that Si nanopartides can be surface functionalized using techniques common to other biologically targeted nanoparticle systems. These results suggest that Si nanoparticles can be used as a targetable, hyperpolarized magnetic resonance imaging agent with a large range of potential applications.

Rationale and Objectives. 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 He-3 magnetic resonance imaging (MRJ) of the human lung, and most other common radiologic imaging modalities including positron emission tomography and computed tomography, restrict subjects to lying horizontally, minimizing most gravitational effects. Materials and Methods. In this article, we 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. Results. Initial results include two-dimensional lung images acquired with similar to 4 X 8 rum in-plane resolution and three-dimensional images with similar to 2-cm slice thickness. Conclusions. Effects of posture variation are observed, including posture-related effects of the diaphragm and distension of the lungs while vertical.

We describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in vivo hyperpolarized He-3 imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine, including asthma and obesity. The imager uses a bi-planar B-0 coil design that produces an optimized 65 G (6.5 mT) magnetic field for He-3 MRI at 210 kHz. Three sets of bi-planar coils produce the x, y. and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RE coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D He-3 images with up to 2.8 mm resolution, and present initial 2D and 3D He-3 images of human lungs in both supine and upright orientations. H-1 MRI can also be performed for diagnostic and calibration reasons. Published by Elsevier Inc.

Nuclear magnetic resonance (NMR) experiments are described for gas-fluidized granular beds, which are important systems for many materials-processing operations. 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. In particular, 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.

We present a novel nuclear magnetic resonance (NMR) technique that provides a non-invasive, direct measurement of gas exchange in a three-dimensional gas-fluidized bed of solid particles. The NMR spectrum of hyperpolarized Xe-129 gas in an Al2O3 particle bed displays three resolved peaks corresponding to xenon in bubbles, the interstitial spaces (emulsion), and adsorbed on particles. Modified NMR exchange and saturation recovery sequences, together with data analysis based on an exchange-coupled set of Bloch equations, yield gas exchange rate constants between the emulsion and adsorbed phases, and between the bubble and emulsion phases. The results are in approximate agreement with previously unverified predictions from well-known models of fluidized bed behavior. Incorporation of NMR imaging methodologies would straightforwardly allow similar measurements on a spatially resolved basis.

We describe a prototype system built to allow open-access very-low-field MRI of human lungs using laser-polarized He-3 gas. The system employs an open four-coil electromagnet with an operational B-o held of 4 mT, and planar gradient coils that generate gradient fields up to 0.18 G/cm in the x and y direction and 0.41 C/cm in the z direction. This system was used to obtain H-1 and He-3 phantom images and supine and upright He-3 images of human lungs. We include discussion on challenges unique to imaging at 50-200 kHz, including noise filtering and compensation for narrow-bandwidth coils. (C) 2006 Wiley Periodicals, Inc.

We report initial NMR studies of gas dynamics in a particle bed fluidized by laser-polarized xenon (Xe-129) 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 T-2{*} contrast to differentiate the bubble and emulsion phases by choosing solid particles with large magnetic susceptibility. Experimental tests demonstrated that this method was successful in eliminating Xe-129 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. (c) 2005 Elsevier Inc. All rights reserved.

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. (c) 2005 Elsevier Inc. All rights reserved.

The human lung and its functions are extremely sensitive to gravity; however, the conventional high-field magnets used for most laser-polarized He-3 MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser-polarized 3 He 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. Magn Reson Med 53:745-749, 2005. (c) 2005 WileyLiss, Inc.

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 NMR technique may have applications to the characterization of fluid flow in a wide variety of porous and granular media.

A three-dimensional granular system fluidized by vertical container vibrations was studied using pulsed field gradient NMR coupled with one-dimensional magnetic resonance imaging. The system consisted of mustard seeds vibrated vertically at 50 Hz, and the number of layers Nlless than or equal to4 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 Nl and two dimensionless accelerations Gamma=15,18 were fitted to a hydrodynamic theory, which successfully models the density and temperature profiles away from the vibrating container bottom. A temperature inversion near the free upper surface is observed, in agreement with predictions based on the hydrodynamic parameter mu which is nonzero only in inelastic systems.

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. (C) 2003 Elsevier Science Inc. All rights reserved.

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 (similar to1-1.5 bead diameters). The Pade approximation can be used as a reference for expected xenon D(t) data between the short and the long time limits, allowing us to explore deviations from the expected behavior at intermediate times as a result of finite sample size effects. Finally, the application of the Pade interpolation between the long and the short time asymptotic limits yields a fitted length scale (the Pade length), which is found to be similar to0.13b for all bead packs, where b is the bead diameter. (C) 2002 Elsevier Science (USA).

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.

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. Copyright (C) 2002 John Wiley Sons, Ltd.

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 Xe-129 between gas in the pore space and Xe-129 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.

Gas-phase nuclear magnetic resonance (NMR) has great potential as a probe for a variety of interesting physical and biomedical problems that are not amenable to study by water or similar liquid. However, NMR of gases was largely neglected due to the low signal obtained from the thermally polarized gases with very low sample density. The advent of optical pumping techniques for enhancing the polarization of the noble gases He-3 and Xe-129 has bought new life to this field, especially in medical imaging where He-3 lung inhalation imaging is approaching a clinical application. However, there are numerous applications in materials science that also benefit from the use of these gases. We review primarily nonmedical applications of laser-polarized noble gases for both NMR imaging and spectroscopy and highlight progress with examples from our laboratory including high-resolution imaging at millitesla applied field strength and velocity imaging of convective flow. Porous media microstucture has been probed with both thermal and laser-polarized xenon, as xenon is an ideal probe due to low surface interaction with the grains of the porous media.

We have extended the utility of NMR as a technique to probe porous media structure over length scales of similar to 100-2000 mum by using the spin 1/2 noble gas Xe-129 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 pore surface-area-to-volume ratio, S/V-p, 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 similar to0.62-0.65D(0) that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D-0 at differing gas pressures showed this tortuosity Emit 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)/D-0 was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D-0 and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D-0 from the S/V-p relation. The Fade approximation is used to interpolate between the long and short time limits in D(t). While the short time D(t) points 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 Fade line in the case of large beads, most likely due to finite size effects. (C) 2001 Elsevier Science Inc. All rights reserved.

Using a novel NMR scheme we observed persistence in 1D gas diffusion. Analytical approximations and numerical simulations have indicated that for an initially random array of spins undergoing diffusion, the probability p(t) that the average spin magnetization in a given region has not changed sign (i.e., ``persists{''}) up to time t follows a power law t(-theta), where theta, depends on the dimensionality of the system. Using laser-polarized Xe-129 gas, we prepared an initial ``quasirandom{''} 1D array of spin magnetization and then monitored the ensemble's evolution due to diffusion using real-time NMR imaging. Our measurements are consistent with analytical and numerical predictions of theta approximate to 0.12.

We demonstrate nuclear magnetic resonance (NMR) imaging of the flow and diffusion of laser-polarized xenon (Xe-129) gas undergoing convection above evaporating laser-polarized liquid xenon. The large xenon NMR signal provided by the laser-polarization technique allows more rapid imaging than one can achieve with thermally polarized gas-liquid systems, permitting shorter time-scale events such as rapid gas flow and gas-liquid dynamics to be observed. Two-dimensional velocity-encoded imaging shows convective gas flow above the evaporating liquid xenon, and also permits the measurement of enhanced gas diffusion near regions of large velocity variation.

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 Xe-129 NMR and diffusion. We found that the contribution of SF6 to Xe-129 T-1 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 Xe-129 T-2 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 Xe-129 diffusion through SF6 to be approximately 4.6 x 10(-6) m(2)s(-1) for 1 bar pressure of SF6 and standard temperature, which is only 1.2 times smaller than the Xe-129 self diffusion coefficient for 1 bar Xe-129 pressure and standard temperature. From these measurements we conclude that SF6 will not sufficiently reduce Xe-129 diffusion to allow accurate surface-area/volume ratio measurements in human alveoli using time-dependent gas diffusion NMR.

We show that gas diffusion nuclear magnetic resonance (GD-NMR) provides a powerful technique for probing the structure of porous media. In random packs of glass beads, using both laser-polarized and thermally polarized xenon gas, we find that GD-NMR can accurately measure the pore space surface-area-to-volume ratio, S/V-p, and the tortuosity, alpha (the latter quantity being directly related to the system's transport properties). We also show that GD-NMR provides a good measure of the tortuosity of sandstone and complex carbonate rocks.

A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized He-3 and Xe-129 gas. Laser polarization enhances the NMR sensitivity of these noble gases by >10(3), but creates an unstable, nonthermal polarization that is not readily renewable, A new method is presented which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. The NMR characterization of a sample's diffusion behavior can be accomplished in a single measurement, using only a single polarization step. As a demonstration, the diffusion coefficient of a sample of laser-polarized Xe-129 gas is measured via this method. (C) 1999 Academic Press.

We used both conventional rheometry and nuclear magnetic resonance (NMR) velocimetry to study shear banding in a solution of 200 mM cetylpyridinium chloride and 120 mM sodium salicylate in 0.5 M sodium chloride. The solution behaved as a Maxwell fluid up to frequencies of 10 Hz. Theoretical predictions of critical strain rate and shear stress were in good agreement with measurements obtained using controlled strain rate rheometry. Using NMR velocimetry, we observed convincing evidence of shear banding in capillary flow with a band of very high, approximately constant, shear rate next to the wall that grew in thickness with increasing apparent shear rate. We believe that the shear rate in this band ( similar to 600 s(-1)) marks the beginning of the hypothesized high shear rate limb of the flow curve. We also observed shear banding in both the cylindrical Couette and cone-and-plate geometries. Shear banding started at shear rates that were approximately the same as the critical shear rate measured with the mechanical rheometer. With increasing shear rate in the fluid, more than two shear bands were sometimes evident, although they exhibited dynamical instabilities. That is, the highest shear rate band was variable in both magnitude and position. (C) 1999 The Society of Rheology. {[}S0148-6055(99)00104-2].

We demonstrate magnetic resonance imaging (MRI) of laser polarized liquid xenon, and image exchange between the liquid and vapor phases. The exceptionally large magnetization density of this liquid should allow MRI with micron-scale spatial resolution without signal averaging. Applications may include imaging of density equilibration and convective flow near xenon's liquid-vapor critical point, tow-field imaging of porous media microstructure, and mapping of the dynamics of two-phase (liquid-gas) flows. {[}S1063-651X(99)05602-0].

We describe a device for performing MRI: with laser-polarized noble gas at low magnetic fields (<50 G). The system is robust, portable, inexpensive, and provides gas-phase imaging resolution comparable to that of high field clinical instruments. At 20.6 G, we have imaged laser-polarized He-3 (Larmor frequency of 67 kHz) in both sealed glass cells and excised rat lungs, using similar to 0.1 G/cm gradients to achieve similar to 1 mm(2) resolution. In addition, we measured He-3 T-2({*}) times greater than 100 ms in excised rat lungs, which is roughly 20 times longer than typical values observed at high (similar to 2 T) fields. We include a discussion of the practical considerations for working at low magnetic fields and conclude with evidence of radiation damping in this system. (C) 1999 academic Press.

NMR images of laser polarized He-3 gas were obtained at 21 G using a simple, homebuilt instrument. At such low fields magnetic resonance imaging (MRI) of thermally polarized samples (e.g., water) is not practical. Low-field noble gas MRI has novel scientific, engineering, and medical applications. Examples include portable systems for diagnosis of lung disease, as well as imaging of voids in porous media and within metallic systems. {[}S0031-9007(98)07442-0].

Pulsed-held-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O-2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 x 10(-6) m(2) s(-1) for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-held-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times, (C) 1998 Academic Press.

We describe several different rheometric devices for use within the nuclear-magnetic-resonance probe of a standard widebore microimaging system. These include bath vertical and horizontal Couette cells and the cone-and-plate cell, which produce shearing flows, and the four-roll mill and the opposed-jet (cross-flow junction) cells which produce extensional flow. We demonstrate that velocity images can be obtained for each and that detailed information about local shear and extension rates can be extracted. These systems have considerable potential for use in the study of non-Newtonian viscosity, and of molecular ordering under shear or extension.

The nonlinear viscosity of the wormlike surfactant system cetyl pyridinium chloride/sodium salicylate (60 mM/100 mM in water) has been investigated in both pipe and cylindrical Couette geometries, using nuclear magnetic resonance to image both velocity end diffusion. In pipe flow we observe transitions from Newtonian to non-Newtonian viscosity, to spurt, to unstable flow, and then to a regime where fluctuations an rapid on the timescale of a few milliseconds. In the Couette cell we observe apparent slip at the inner wall as well as a high shear rate band located away from the wall in the body of the fluid. The banding phenomenon, which has its counterpart in the pipe flow, is consistent with double valuedness in the stress versus rate of strain relationship for this fluid. (C) 1997 The Society of Rheology.

We report an observation of shear-banding, under Couette how, of the worm-like surfactant system cetyl pyridinium chloride/sodium salicylate (respective concentrations of 100 mM and 60 mM in pure water). The method, based on NMR velocity imaging, allows the direct measurement of velocity, and consequent calculation of shear rate, at a spatial resolution of around 10 mu m, sufficient to resolve apparent slip at the inner wall from shear banding within the bulk of the fluid. Above a critical shear rate (around 1 s(-1)) a high shear rate band is observed in the annulus between concentric rotating cylinders, the width of the band being of order 30 mu m with a corresponding shear rate of order 500 s(-1). The precise location of the band is sensitive to boundary conditions at the inner wall and is shown to be susceptible to significant broadening.