Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1990-09-17
1992-06-30
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01R 3320
Patent
active
051266725
DESCRIPTION:
BRIEF SUMMARY
An object of the present invention is a method for the measurement, in a given period of time, of the effects of eddy currents. This method is chiefly designed to be implemented in medicine, in nuclear magnetic resonance (NMR) experimentation. It is useful, there, for measuring the effects resulting from the application of magnetic field gradient pulses, by means of magnetic field gradient coils, in an NMR imaging machine.
Imaging by nuclear magnetic resonance is known. In imaging experiments according to methods such as this, a body, the internal parts of which are sought to be imaged, is placed in a region where an intense homogeneous magnetic field B.sub.0 is created by a magnet. Under the effect of this intense field, the magnetic moments of the body's particles get oriented in the direction of the homogeneous magnetic field. These magnetic moments are then subjected to an electromagnetic radio-frequency excitation tending to make them flip from this orientation. At the end of the excitation, it is possible to measure a de-excitation resonance signal, called an NMR signal, representing the internal parts of this body. This signal corresponds to the return of the orientation of the magnetic moments, in a precessional motion, towards their initial orientation. However, during experimentation such as this, all the particles resonate at the same time. They make their contribution simultaneously to the measured signal.
To enable the portions relating to each of these parts in this signal to be distinguished, in order to reconstruct the image therefrom, there is a known way to magnetically encode the space in which the body to be imaged is placed during the excitation, between the excitation and the measurement of the de-excitation, or even during the measurement of this de-excitation signal. These encodings are applied by coils producing a particular encoding magnetic field. These particular magnetic fields have an essential component, measured in parallel to the orientation of the homogeneous magnetic field, the value of which changes in space as a function of the coordinates of a point of this space where this component acts.
There is a known standard way to identify the examination space with an orthonormal reference XYZ. Generally, the direction Z is the direction attributed to the homogeneous magnetic field. The gradient coils then produce magnetic fields, the essential component of which, measured along Z, is a function of X for a coil said to have a gradient X, a function of Y for a coil said to have a gradient Y and a function of Z for a coil said to have a gradient Z. Then, knowing a sequence during which an excitation and magnetic space encodings have been applied, there is a known way to extract, from the measured de-excitation signal, information relating to the distribution of the different particles in the body. In fact, this distribution can be determined only after a reiteration of these acquisition sequences. During this reiteration, the encodings, the field gradients, are modified from one sequence to another.
One of the particular aspects of the field gradients created, therefore, is that they are pulsed. They are set up, they persist for a pre-determined period, then they are cut off. Now, their being set up or their being cut off are essentially the cause of the eddy current. In effect, an NMR machine includes the magnet to produce the homogoneous field, an antenna to apply the electromagnetic radio-frequency excitation, and the gradient coils proper. When the magnet is of the superconductor type, it further includes a screen capable of damping the magnetic energy created if, by accident, the phenomenon of superconductivity, which enables the field to be sustained, should happen to fail (by a fault in the magnet cooling system, for example). All these devices are mechanically supported by metallic structures, if not metallic themselves, and are therefore liable to let eddy currents develop when the field gradient pulses are set up and cut off. And eddy currents such as this are themselv
REFERENCES:
patent: 4623843 (1986-11-01), Macovski
patent: 4698591 (1987-10-01), Glover et al.
patent: 4978919 (1990-12-01), Hinks
General Electric CGR S.A.
Tokar Michael J.
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