Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1987-08-06
1989-04-11
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324309, G01R 3320
Patent
active
048209830
DESCRIPTION:
BRIEF SUMMARY
This invention relates to nuclear magnetic resonance (NMR) methods.
One NMR method makes use of the so called "surface coil" defined herein as a coil which provides an inhomogeneous radio frequency magnetic field. The surface coil has proven to be a useful device for irradiation and detection of tissue regions near to the surface of animal or human subjects, particularly for in vivo NMR spectroscopy. Unfortunately, the gross inhomogeneity of the rf field provided by a surface coil leads to complications. If the surface coil is used to transmit a simple square rf pulse, maximum positive signals will be excited from sample regions where the pulse angle .theta. is 90.degree., no signals can be excited in regions where .theta. is 180.degree. or 360.degree. and maximum negative signals will be generated from regions where .theta. is 270.degree. and so on, where signal excitation is given by sin .theta.. The sensitivity of the coil to a sample point is proportional to the pulse angle .theta. that can be induced at that point, and so the coil is more sensitive to surface regions closest to it. Thus overall, following a square rf pulse, detected signal is proportional to .theta. sin .theta.. A further complication is that various signal regions have a complicated shape. Thus, on the one hand the sample is excited in a grossly non-uniform manner, but on the other hand only crude localisation of the signal from the sample is achieved, so the surface coil is far from ideal from either point of view.
The shape of the rf field provided by a surface coil has been detailed (A. Haase, W. Hanicke and J. Frahm, J. Magn. Reson. 65 401 (1984) (Ref 1) and M. R. Bendall, J. Magn. Reson. 59 406 (1984) (Ref 2) and various methods for improving sample localisation have been reviewed (M. R. Bendall in "Magnetic Resonance (MR) Imaging", 2nd Edition (C. L. Partain, A. E. James, J. A. Patton, R. R. Price and J. P. Jones, Eds), W. B. Saunders, Philadelphia, 1986) (Ref 3).
In accordance with the first aspect of the invention, a method of obtaining a nuclear magnetic resonance signal from a sample using a radio frequency irradiation coil which provides an inhomogeneous radio frequency magnetic field, comprises causing the coil to apply a radio frequency pulse to the sample in the presence of an external magnetic field, performing an adiabatic passage, and obtaining the resultant NMR signal.
Conveniently, the adiabatic passage comprises a so called "half adiabatic rapid passage", as explained in greater detail hereinafter.
The methods in accordance with the invention may be utilised to obtain localisation of N.M.R. signal from regions in space, using switched field gradients.
In accordance with a second aspect of the invention, a method of obtaining a nuclear magnetic resonance signal from a sample, comprises applying a radio frequency magnetic field pulse to the sample in the presence of an external magnetic field, wherein the rf pulse is controlled such that: of the spectrum, and where v allows for variation of B.sub.1 throughout the sample, .DELTA.H is the resonance offset of the nuclear springs, and B.sub.1 is the rf field of a spin-lock pulse.
In accordance with a third aspect of the invention, a method of obtaining a nuclear magnetic resonance signal from a sample comprises applying a radio frequency magnetic field pulse to the sample in the presence of an external magnetic field, wherein the effective magnetic field applied to the sample is inverted after substantially half the duration of the pulse.
The methods according to the second and third aspects are particularly useful in combination with methods according to the first aspect of the invention. The NMR methods may, for example, form part of an imaging experiment, or a spectroscopy experiment.
Some examples of methods in accordance with the present invention and an example of apparatus for carrying out such methods will now be desribed with reference to the accompanying drawings, in which:
FIG. 1 is a schematic magnetisation diagram;
FIG. 2 is a diagram showing signal respons
REFERENCES:
patent: 4390840 (1983-06-01), Ganssen et al.
patent: 4528508 (1985-07-01), Vail III
patent: 4535290 (1985-08-01), Post et al.
patent: 4695799 (1987-09-01), Hardy
patent: 4774466 (1988-09-01), Saffin
Bendall Max R.
Pegg David T.
Berkowitz Edward H.
Cole Stanley Z.
Tokar Michael J.
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