Stray-field imaging

If the field gradient is applied along the z-direction, the Hamiltonian for the spin interaction A is given by 

This expression applies in the rotating frame where tDo = ft rf so the fictitious field (cf. Fig. 2.2.7) is zero. If the gradient strength is larger than the local fields corresponding to the spin interactions A, the total Hamiltonian is dominated by the Zeeman interaction with field gradient, i.e., 

This is the high-gradient approximation [Ran2]. It corresponds to a truncation of all spin interactions including moderate quadrupole interactions. 

The spatial distribution of the transverse magnetization after application of a hard 90° pulse of duration tp can be evaluated from the density operator 

Equation (8.1.4) is rewritten in a reference frame oriented along the local effective field, which changes direction with the resonance offset from the rf excitation frequency (cf. Fig. 2.2.7), 

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Under such circumstances, spin density diffusometry turned out to be more favorable. We are not discussing solvent intrusion experiments with solid-like materials, as they are typically carried out with the aid of stray field imaging [24]. 

ID, 2D and 3D experiments, all H, and Sn chemical shifts for four postulated Sn species could be resolved unambiguously. Variable temperature and variable stress NMR spectroscopic and imaging studies are reported for a sample of high impact polystyrene. NMR microscopy and solid state stray field imaging studies indicated that imposed stress affects the molecular chain dynamics only of the polybutadiene region and not of the polystyrene(PS) matrix.The principal values of the chemical shift tensors and the separated-local field pattern are reported for the all-... 

This section gives a brief outline of the basic principles of stray field imaging in sufficient detail to allow an understanding of the applications of the technique. For more comprehensive descriptions, the reader is referred to two excellent recent... 

T. G. Nunes, P. Bodart and E. W. Randall, The hardening of Portland cement studied by H stray field imaging influence of concentration and evaporation rate of water. Proc. Second Int. Conf. NMR Spect. of Cement Based Mater, (eds P. Colombat and H. Zanni), Springer Verlag, Weinheim, 1997, p. 411. 

R. Pires, T.G. Nunes, I. Abrahams, G.E. Hawkes, C.M. Morals, C. Fernandez, Stray-field imaging and multinuclear magnetic resonance spectroscopy studies on the setting of a commercial glass-ionomer cement, J. Mater. Sci. Mater. Med. 15 (2004) 201-208. 

Stray-field MRI was used to measure methanol ingress into 500 /xm thick PMMA pre-swollen with acetone [669]. With stray-field imaging the rigid and swollen polymer and the solvent are separately visualised with a resolution of the order of 20 /xm. The different components are distinguished on the basis of their differing spin-spin relaxation times. For a polymer partially swollen with solvent the spatial distributions of relaxation times reveal the interactions between solvent and polymer in the diffusion process. Proton NMR images of 1,4-dioxane in swollen polybutadiene rubber were reported [393]. 

Initially the acronym STRATI was used to denote stray field imaging, but more recently it has been used by Randall and colleagues more generally to denote simply the stray field itself. Studies are not confined simply to imaging. The three types of work in the stray field may thus be referred to as STRATI imaging (or STRATI-MRI), STRATI spectroscopy and STRATI diffusion studies. All of these are covered in this article, since the technique is relatively... 

Bodart P, Nunes T and Randall EW (1997) Stray-field imaging of quadrupolar nuclei of half integer spin in solids. Solid State NMR, 8 257-263. 

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