Faculty Bibliography

The standard method of diffusion tensor imaging (DTI) involves one diffusion-sensitizing gradient direction per acquired signal. This paper describes an alternative method in which the entire direction set required for calculating the diffusion tensor is captured in a few scans. In this method, a series of radiofrequency (RF) pulses are applied, resulting in a train of spin echoes. A pattern of applied magnetic field gradients between the RF pulses generates a different diffusion weighting in both magnitude and direction for each echo, resulting in a dataset sufficient to determine the tensor. This significantly reduces the time required for a full DTI scan and potentially allows a tradeoff of this time for image quality. In the present work, this method is demonstrated in an anisotropic diffusion phantom (asparagus)

We describe methods for the measurement of translational diffusion in very large static magnetic field gradients by NMR. The techniques use a 'hole-burning' sequence that, with the use of fringe field gradients of 42 T/m, can image diffusion along one dimension on a submicron scale. Two varieties of this method are demonstrated, including a particularly efficient mode called the 'hole-comb,' in which multiple diffusion times comprising an entire diffusive evolution can be measured within the span of a single detected slice. The advantages and disadvantages of these methods are discussed, as well as their potential for addressing non-Fickian diffusion, diffusion in restricted media, and spatially inhomogeneous diffusion

We report spatially resolved nuclear-magnetic-resonance measurements on a high-temperature superconductor that indicate the presence of correlated antiferromagnetic fluctuations in the vortex core. The nuclear-spin-lattice relaxation rate 1/T-17(1) of planar O-17, in near-optimally doped YB2Cu3O7-delta, was measured. Outside of the core, ((T1T)-T-17)(-1) is independent of temperature consistent with theoretical predictions for a d-wave superconductor. In the vortex core, ((T1T)-T-17)(-1) increases with decreasing temperature following an antiferromagnetic Curie-Weiss law.

NMR measurements of the linewidth of planar O-17 in near-optimally doped YBa2Cu3O7-delta in the vortex lattice state are reported. We find that the lineshape, i.e. magnetic field probability distribution, is much broader than what is expected based on the calculated distribution arising from the vortex supercurrents. We can qualitatively account for this anomalously large NMR linewidth measured at low temperatures by postulating that the broadening is caused by antiferromagnetism in the vortex core. (C) 2003 Elsevier Science B.V. All rights reserved.

Combined theoretical and experimental studies are reported on alkali ion-cryptand interactions which affect charge transport in liquid electrolytes. Encapsulation of the cation by a cryptand cage can significantly improve the ionic conductivity, by reducing the fraction of time in which the cation is bound to its counterion. Comparison between experimental conductivities and NMR measurements of diffusion rates of Li and F in the amorphous LiMPSA electrolyte suggest that in the presence of the C222 cryptand, Li is bound to MPSA about 75% of the time. A hybrid classical/quantum methodology is used to model dynamics and electronic structure of a number of cryptand-alkali ion-MPSA and cryptand-ion-cryptand systems in an effort to extract general features which may be useful in designing electrolytes with improved performance.