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"Diffusion-diffraction" experiments on water, yielding "q-space" plots, were conducted on suspensions of oxygenated (diamagnetic) human erythrocytes. (i) These suspensions displayed diffusion-diffraction of water; (ii) the shape of the q-space plots depended on the direction along which the diffusion was measured, thus implying alignment of the cells in the magnetic field of the NMR spectrometer; (iii) the diffusion anisotropy was altered in a predictable way by converting the hemoglobin to a paramagnetic form; (iv) the shapes of the q-space plots were altered in a predictable way by inhibiting water transport; (v) the pseudo-first order rate constant characterizing the covalent inhibition of water transport, by p-chloromercuribenzenesulfonate (p-CMBS), was measured; and (vi) the cell diameter and intercellular spacing were measured from the positions of the interference minima and maxima in the q-space plots. The relevance of these findings to NMR-based histological characterization of tissues, and the implications, for magnetic resonance imaging (MRI), of erythrocyte alignment in the small vessels of the brain in particular, are noted.

We report a combined experimental, theoretical, and simulation study of pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) for fluid saturated porous media. A simple pore hopping theory is developed on the basis of the assumption that diffusion within pores is very much faster than diffusion between pores. For suitable periodic media, the theoretical results are found to be in good agreement with random-walk simulations. The theory for glasslike media is then used to analyze experimental PGSE NMR data for a water-saturated random loose pack of nearly monodisperse polystyrene spheres. The structural parameters extracted by this method are consistent with the known geometry of such packings. An important observation from the simulations is that the long-time effective diffusion constant is already accessed at times so short that a single spin will only have diffused across one pore width.

Articles you may be interested inSelf-diffusion of linear and cyclic alkanes, measured with pulsed-gradient spin-echo nuclear magnetic resonance Water selfdiffusion in lyotropic systems by simulation of pulsed field gradientspin echo nuclear magnetic resonance experiments J. Chem. Phys. 97, 7781 (1992); 10.1063/1.463446 Diffusion in porous systems and the influence of pore morphology in pulsed gradient spinecho nuclear magnetic resonance studiesThe application of pulsed gradient spin echo nuclear magnetic resonance (NMR) to the case of molecules trapped between two plane parallel boundaries, has been examined theoretically, with computer simulation and by experiment. A new closed-form analytic expression is obtained for the averaged propagator and the echo attenuation when the walls have finite relaxivity and this expression is verified by computer simulations. It is shown that "diffraction" effects are still strongly apparent when wall relaxation is taken into account and that deviations in the barrier spacing parameter obtained from the position of the echo minimum, are weak. In particular we show that for the pulsed gradient spin echo (PGSE) pulse separation time on the order of a 2 /2D, the deviation is less than 10% provided that the relaxation is not so severe as to reduce the zero gradient signal amplitude below 10% of its unrelaxed value. We further examine the influence of finite gradient pulse and find, as with wall relaxation, that diffusion during the gradient pulse has the effect of shifting to higher q the position of the first minimum in the echo diffraction pattern. The diffusive diffraction effect is demonstrated experimentally using a stack of pentane-filled microcapillaries of wall spacing 100 ,urn, and fits to the data yield realistic values for the known experimental parameters.

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