Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1991-12-11
1993-10-19
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3320
Patent
active
052549496
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to NMR imaging methods and has particular application in methods of obtaining NMR imaging information from solid objects.
The NMR imaging of solid and other materials which have short spin-spin relaxation times has not been developed to the same level of sophistication as the NMR imaging of liquid and quasi-liquid materials. This is because of the inherently greater linewidth in solids compared to liquids which forces the use of excessively large magnetic field gradients in prohibitively short times if traditional techniques are to be adopted. One kind of method that has been proposed to overcome the difficulties associated with imaging in solids uses multipulse sequences to provide line narrowing. However, the biggest drawback with multipulse sequencing concerns the large radio-frequency pulse power which is required. In part at least this is needed to overcome the large spectral width in an imaging experiment resulting from the presence of magnetic field gradients during the time of application of rf pulses.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method in which the requirements of rf power are reduced.
According to the invention a method of obtaining NMR imaging information from a solid object comprising subjecting the object to a static magnetic field, applying a gradient magnetic field which varies sinusoidally in amplitude about a zero value, applying a repetitive sequence of radio frequency pulses at instants of zero value of the said gradient field each of such frequency, magnitude and duration and successive pulses of the sequence being of such relative rf phase, that selected nuclei in the object precess with accumulated phase in each sequence, and measuring the resulting NMR signal from the object.
Since the magnetic field gradients are at or close to zero during the time of application of rf pulses, considerable advantage is gained. Furthermore while rapid gradient switching is difficult to achieve on the timescale of multipulse experiments, it can nevertheless be achieved more easily when the coil windings generating the magnetic gradient fields are incorporated in a tuned circuit driven resonantly at a frequency with cycle time equal to twice the rf interpulse period .tau., thus generating a sinusoidally time varying magnetic gradient field.
In carrying out the invention the rf pulse sequence should preferably be such that the pulses of the sequence are separated by constant interpulse window lengths, equal to half the magnetic gradient field cycle time 2.tau.. In this arrangement the sign of the magnetic gradient field reverses in successive windows and the sequence therefore is such as to make the spins accumulate phase in the rotating frame from an oscillating magnetic field offset as opposed to a static field offset. The spins specifically do not accumulate phase from a static resonance offset. This prevents magnet inhomogeneities and mistuning introducing errors into the phase accumulated by the spins due to the field gradients. As an added bonus it also removes chemical shift broadening.
In some embodiments of the invention a sinusoidal offset magnetic field of spatially uniform amplitude is superimposed on the sinusoidally time-varying gradient magnetic field to shift spikes in the observed signal after Fourier transformation caused by rf phase errors away from areas of interest.
A sequence of 90.degree. pulses and windows of length .tau. which fulfills the above requirements is -.tau.-90.sub.-x -.tau.-90.sub.-y -.tau.-90.sub.-x -.tau.-).sub.n( 1) of a sequence relative to each other and n is an integer. The product of the six pulses of the repeated cycle is unity as can be easily verified using simple matrices. For the case of an oscillating magnetic field resonance offset of mean magnitude B.sub.g the mean Hamiltonian acting in odd numbered windows of the cycle is ##EQU1## and in even numbered windows is ##EQU2## In equations (2) and (3) the first term on the right-hand side is the Zeeman interacti
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McDonald Peter J.
Tokarczuk Pawel F.
British Technology Group Ltd.
Tokar Michael J.
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