Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1988-10-28
1990-07-31
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01R 3320
Patent
active
049453084
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This Invention relates to improvements in volume-selected NMR spectroscopy.
(2) Prior Art
With the recent development of wholebody NMR spectroscopy, there is a need to find a technique to obtain high quality spectra from a particular volume in space. In this way, biochemical data may be obtained from one organ, or preferably from selected regions within such an organ, in a non-invasive manner. There have been a number of methods proposed to do this and these fall into three broad groups. They are:
(1) The use of a pulse train which exploits the pulse inhomogeneity to select a volume within a sample by selecting only the region with a particular pulse angle;
(2) The use of some form of field profile technique to produce a region of high magnetic field homogeneity within a larger region where the field changes sharply with position. (The result is that high resolution spectra are obtained from the region of interest whilst signals arising from the inhomogeneous region are so broad that they may be eliminated from spectra.)
In principle, methods (1) and (2) generally rely upon physical movement of the sample within the system to obtain signals from various regions.
(3) The third method is to use a combination of frequency selective radio-frequency pulses in the presence of magnetic field gradients to effect volume selection. It appears that this third approach may well be the most useful as it combines the versatility of complete software control of gradients and selective excitation pulses to produce volume-selection in all three directions. As well, the technique may be image encoded, the consequence being that the position of the selected volume is accurately known.
For effective use in practice, strong signal-to-noise must be obtained from any particular technique. To do this, we must keep three objects in mind:
(1) Maximum signal should be excited within the confines of the slice of choice:
(2) Magnetization of interest should remain in the transverse plane for as little time as possible particularly while applied field gradients are present. The resultant loss of coherence leads to a reduction in signal strength: and
(3) The spectra should be obtained after applied gradients have reached zero amplitude so that line widths are narrow and the chemical shifts displayed are those present independent of volume selection.
These last two points combined mean it is preferable to have the magnetization aligned with the field axis while gradients are being changed.
Some of the methods proposed previously for volume-selection include, DRESS, [Depth-resolved surface-coil in spectroscopy (DRESS) in vivo .sup.1 H, .sup.31 P, and .sup.31 C NMR, P. A. Bottomley, T. H. Foster and R. D. Darrow, J. Magn. Reson. 59, 338, (1984)]; VSE (H. Post, D. Ratzel and P. Brunner, West German Patent No. 3209263-6, Mar. 13, 1982, and also Volume-selected Excitation. A novel approach to topical NMR. W. P. Aue, S. Miller, T. A. Cross and J. Seeliq. J. Magn. Reson. 56, 350 (1984)]: and ISIS, [Image-selected in
vivo Spectroscopy (ISIS). A new technique for spatially selective NMR Spectrosscopy. R. J. Ordidge, A. Connelly and J. A. B. Lohman. J. Magn. Reson. 66, 283, (1986)]; These methods involve the application of narrow bandwidth radio-frequency excitation in the presence of a field gradient applied across the sample. The gradient is then switched off and the signal acquired. In principle, these methods can be extended to include slicing in all directions.
The known methods can be summarised as follows:
(1) DRESS uses a selective rf pulse in the presence of an applied field gradient to excite a slice of magnetization. Gradient reversal is then used to form a spin-echo which refocusses the off-resonance effects of the pulse. The major drawback of this method is that the magnetization of interest lies in the transverse plane during refocussing and so is T.sub.2 contrasted. Furthermore the acquired signals are either gradient broadened or reduced in intensity or both because of the nee
REFERENCES:
patent: 4480228 (1984-10-01), Bottomley
patent: 4486709 (1984-12-01), Bendall
patent: 4698592 (1987-10-01), Feinberg
patent: 4727324 (1988-02-01), Bendall et al.
patent: 4733185 (1988-03-01), Bottomley
patent: 4777439 (1988-10-01), Granot
patent: 4816764 (1989-03-01), Weber
Brooks William M.
Doddrell David M.
Tokar Michael J.
University of Queensland
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