Method of and apparatus for obtaining spatial NMR information

Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system

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324307, G01V 314

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active

055700187

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BRIEF SUMMARY
The present invention relates to a method of and apparatus for obtaining spatial nuclear magnetic resonance (NMR) information from a sample, and particularly from a sample having a short (e.g. less than 500 .mu.s) spin-spin relaxation time (T.sub.2). In general, solids exhibit such a short T.sub.2.


BACKGROUND OF THE INVENTION

The possibility of magnetic imaging in solids has attracted considerable interest recently. NMR imaging in solids is difficult because of the broad NMR lines. One possible approach to the problem is to artificially narrow the line. Such an approach is disclosed in UK Patent Application No. 2234594.
An alternative to line narrowing has been described by Cottrell et al. in their paper entitled "NMR imaging of solids using large oscillating field gradients" (Meas. Sci. Technol. 1, 1990, 624). This paper discloses the use of large oscillating field gradients to dominate the homogeneous dipolar broadening. A single 90.degree. excitation pulse is applied at the time when the field gradient passes through zero and a gradient echo is produced after one complete gradient cycle. Because the 90.degree.pulse is applied at the moment of zero gradient, the RF power requirements are quite modest. The imaging scheme disclosed in this paper used back projection.
Although back projection methods have been conventionally used in solid state NMR imaging, it would be advantageous to use two dimensional Fourier transform (2DFT) techniques since these might be expected to lead to an improvement in image quality.


SUMMARY OF THE INVENTION

According to the present invention, there is provided a method of obtaining spatial NMR information from a sample, comprising: having first and second gradient field components in first and second directions respectively; production of the magnetic field so as to produce a plurality of NMR sampling responses from the sample, the first and second components being so arranged that each sampling response provides a desired sampling path in k-space, the relative amplitudes of such components being so varied over the plurality of the sampling responses as to provide a desired distribution of sampling paths in k-space over said plurality of the sampling responses; sampling response; and
The invention can thus afford a technique which can provide NMR images using 2DFT methods, as is explained below. Further, unlIKe conventional 2DFT methods, there is no requirement that one gradient component (e.g. a phase encode component) be switched off before the other gradient component (e.g. a frequency encode component) is switched on. Both such components can be run continuously and simultaneously, thus avoiding any problems with spurious eddy current fields due to switching on or off such components.
As is shown in the the paper entitled "A simple graphical representation of Fourier-based imaging methods" by S. Ljunggren (J. Magn. Res. 54 (1983) 338), k-space (the spatial frequency domain) is defined by the following equations: ##EQU1## where G.sub.y is the gradient magnetic field component in the y-direction and G.sub.z is the gradient magnetic field component in the z-direction. For a sample which is uniform along x (no slice selection) and for which p (y,z) is the projection of the nuclear spin density onto the y, z plane, the above paper shows that the (t)z}]exp(-t/T.sub.2)dydz (3)
In order to reconstruct an NMR image from k-space using 2DFT techniques, information In k-space needs to be sampled on a rectilinear grid. In the present invention, by judicious choice of the waveforms of the first and second gradient components and of the way their relative amplitudes are varied over the plurality of responses, k-space can be sampled as a desired distribution of sampling paths (excursions). This distribution can be discretised into any desired set of distributed sampling points (such as an approximately square or rectangular grid) that is suitable for conversion into a rectilinear grid (e.g. by linearisation and interpolation).
Varying the relative amplitudes of the first and second c

REFERENCES:
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patent: 4691162 (1987-09-01), Van Uijen
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patent: 4983917 (1991-01-01), Le Roux
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D. G. Cory et al, "Communications NMR images . . . " Journal of Magnetic Resonance, 76, (1988), pp. 543-547.
S. P. Cottrell et al, "NMR imaging of solids using large . . . " Measurement Science & Technology, vol. 1, 1990, pp. 624-629.

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