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PT Callaghan

Publications and source records attributed to PT Callaghan.

11 recordsLinked to original sources
PubMed1999

Spin Echo Analysis of Restricted Diffusion under Generalized Gradient Waveforms: Planar, Cylindrical, and Spherical Pores with Wall Relaxivity.

A simple matrix formalism presented by Callaghan [J. Magn. Reson. 129, 74-84 (1997)], and based on the multiple propagator approach of Caprihan et al. [J. Magn. Reson. A 118, 94-102 (1996)], allows for the calculation of the echo attenuation, E(q), in spin echo diffusion experiments, for practically all gradient waveforms. We have extended the method to the treatment of restricted diffusion in parallel plate, cylindrical, and spherical geometries, including the effects of fluid-surface interactions. In particular, the q-space coherence curves are presented for the finite-width gradient pulse PGSE experiment and the results of the matrix calculations compare precisely with published computer simulations. It is shown that the use of long gradient pulses (delta approximately a2/D) create the illusion of smaller pores if a narrow pulse approximation is assumed, while ignoring the presence of significant wall relaxation can lead to both an underestimation of the pore dimensions and a misidentification of the pore geometry. Copyright 1999 Academic Press.

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PubMed1998

Earth's field NMR in antarctica: A pulsed gradient spin echo NMR study of restricted diffusion in sea Ice

We report on the use in Antarctica of a nuclear magnetic resonance spectrometer which utilizes the terrestrial magnetic field. Free induction decay data are used to obtain estimates of brine content, in samples of sea ice extracted from the annual ice of McMurdo Sound, in the vicinity of Cape Evans, Ross Island. Pulsed gradient spin echo experiments were performed on these samples in which the orientation of the gradient with respect to the ice growth axis was varied and the separation time between the gradient pulses was varied. Anisotropic restricted diffusion effects are apparent and we use these to tentatively model the brine pocket morphology. Copyright 1998 Academic Press.

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PubMed1998

High magnetic field gradient PGSE NMR in the presence of a large polarizing field

A description is given of pulsed gradient spin echo (PGSE) NMR experiments in which large pulsed magnetic field gradients may be required. The design contraints are discussed and, in particular, the problem of the use of large pulsed magnetic field gradients in conjunction with large polarizing fields is considered. Issues addressed concern probe mechanical assembly, current supply requirements, and pulse shape design. We describe a quadrupolar coil with a gradient amplitude of 1.65 T m-1 A-1 which has been used successfully up to a maximum gradient of around 40 T m-1. A diffusion coefficent of 7.5 x 10(-16) m2 s-1 has been measured using this system, the lowest yet achieved by PGSE NMR methods. Copyright 1998 Academic Press.

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PubMed1997

A simple matrix formalism for spin echo analysis of restricted diffusion under generalized gradient waveforms

A simple mathematical formalism is presented which allows closed form expressions for the echo attenuation, E(q), in spin echo diffusion experiments, for practically all gradient waveforms and for the case of restricted diffusion in enclosing pores, with or without wall relaxation. The method, which derives from the multiple propagator approach of A. Caprihan et al. (1996, J. Magn. Reson. A 118, 94), depends on the representation of the gradient waveform by a succession of sharp gradient impulses. It leads to E(q) being expressed as a product of matrix operators corresponding quite naturally to the successive sandwich of phase evolution and Brownian migration events. Simple expressions are given for the case of the finite width gradient pulse PGSE experiment, the CPMG pulse train used in frequency-domain modulated gradient spin echo NMR, and the case of a sinusoidal waveform. The finite width gradient pulse PGSE and CPMG pulse trains are evaluated for the case of restricted diffusion between parallel reflecting planes. The former results agree precisely with published computer simulations while the latter calculation provides useful insight regarding the spectral density approach to impeded Brownian motion. Copyright 1997 Academic Press. Copyright 1997Academic Press

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