The (13)C NMR signal of acetic acid 1-(13)C-AcH is enhanced by polarization transfer from hyperpolarized (129)Xe using a thermal mixing procedure. 1-(13)C-AcH acid and hyperpolarized (129)Xe are mixed as gases to disperse (129)Xe in the acetic acid. The mixture is frozen with liquid N(2) at 0.5 T. The magnetic field is then momentarily dropped to allow for exchange of spin polarization between (13)C and (129)Xe. After polarization exchange the magnetic field is raised to its original value and the mixture is thawed, resulting in a solution of polarization enhanced 1-(13)C-AcH.
The problem of minimizing the space needed to house two or three superconducting magnets was studied. Dipolar fields were used to approximate the stray fields. Field contours of a single dipole mu are reviewed. The three-dimensional contour surface is discussed in terms of its intersection with planes parallel to a base plane containing mu. The area within a field contour H in the base plane was numerically determined to be 142.55 m2[(mu/mu 0)(H0/H)]2/3 where mu 0 = 10(9) erg/G and H0 = 5 G.
This review of basic physics of nuclear magnetic resonance (NMR) discusses precession of magnetic nuclei in a static external field, introduces the concept of the rotating frame, and describes excitation of nuclei by an RF field. Treats subject of T1 and T2 relaxation from the dual viewpoints of (1) phenomena of relaxation times for both the longitudinal and transverse magnetization and (2) relaxation resulting from local field fluctuations.
To determine whether skeletal muscle hydrogen ion mediates ventilatory drive in humans during exercise, 12 healthy subjects performed three bouts of isotonic submaximal quadriceps exercise on each of 2 days in a 1.5-T magnet for 31P-magnetic resonance spectroscopy (31P-MRS). Bilateral lower extremity positive pressure cuffs were inflated to 45 Torr during exercise (BLPPex) or recovery (BLPPrec) in a randomized order to accentuate a muscle chemoreflex.
An NMR imaging technique sensitive to slow flow (approximately 1 mm/s) using a conventional imaging gradient strength (0.025 mT/cm) is described. Two projections with different spatial magnetic periodicity (determined by the SSFP pulse interval), and thus with different flow sensitivities, are subtracted to give signal from flows in a velocity window.
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 (SF(6)), on (129)Xe NMR and diffusion.
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 rho, 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.
We have extended the utility of NMR as a technique to probe porous media structure over length scales of approximately 100-2000 microm 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.
A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized 3He and 129Xe 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.
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.
