Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1988-11-21
1990-04-03
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3320
Patent
active
049143913
DESCRIPTION:
BRIEF SUMMARY
The present invention has for its object a method of fast imaging of part of a body which is subjected to an intense magnetic field known as an orienting field during a nuclear magnetic resonance (NMR) experiment. This type of experiment is at present meeting with increasing success in the medical field in which the images produced serve as aids to diagnosis, in particular to diagnosis of cancer. The application of the method according to the invention is not restricted to this field, however, and can in fact also be carried out, for example, in the field of physical measurements which also involve the use of spectrometers.
In order to obtain an image of a cross-section of a body to be examined, said body and in particular the part in which the cross-section is located, is subjected to a steady, intense and uniform magnetic field. Under the action of this field, the magnetic moments of the body particles are oriented after a few moments (in a few seconds) in the direction of the magnetic field : hence the name of orienting field which is given to said field. If the magnetic moments of the body particles are then excited with a radio-frequency magnetic excitation which oscillates at a suitable frequency, this causes angular shift of orientation of the excited magnetic moments. At the end of excitation, these magnetic moments tend to realign themselves with the orienting field in a natural movement of precession known as a free movement of precession. During this movement of precession, the particles radiate de-excitation electromagnetic energy which can be measured. The frequency of the de-excitation signal also known as the NMR signal is characteristic of the excited particles (specifically, in the medical field, the hydrogen atom contained in the water molecules disseminated throughout the human body) and of the force of the orienting field. The characteristics of the body are deduced from processing of the measured signal.
Processing of the measured signal in order to extract an image is complicated by the fact that all the particles of the body throughout the excited region reemit a de-excitation signal on completion of the excitation. It is therefore important to discriminate the contributions, in the global NMR signal, of all the elementary regions (known as voxels) of the excited volume in order to reconstruct their distribution and in fact in order to form the image. This discrimnation is possible only by carrying out a series of excitation-measurement sequences. During each sequence, the NMR signals to be measured are coded differently from one sequence to another. When the applied coding is known, the image can be reconstructed by pure imaging techniques.
Measurement of the NMR signal is in fact concerned with the amplitude of the signal. Taking account of a demodulation frequency about which the NMR signal is examined, the sole result of measurement that it can in fact be hoped to obtain is a measurement of the density, in the structures under study, of the specific particles (hydrogen), only one of the resonance frequencies of which is accordingly studied. Broadly speaking, at the end of a given time interval after excitation, the NMR signal is stronger as said density is higher. In fact, this density does not produce action solely on the original amplitude of the NMR signal. In the medical field, it is even assumed in practice that all the regions of the body make the same contribution to the NMR signal from this point of view. On the other hand, the density produces a fairly strong action on the damping, the relaxation, of said NMR signal. This damping is a complex damping : it is representative of a so-called spin-lattice interaction of the particles (the proton of the hydrogen atom) which are excited with surrounding matter and of a so-called spin-spin interaction of the protons with each other. In a known modelization of the physical phenomena which take place, it has been possible to determine that the spin-lattice relaxation time, also known as the time T.sub.1, corresponds to the time co
REFERENCES:
patent: 4684892 (1987-08-01), Graumann
patent: 4733186 (1988-03-01), Oppelt
IEEE Transactions on Nuclear Science, vol. NS-33, No. 1, Feb. 1986, IEEE, (New York, U.S.), T. C. Mills et al.: "Investigation of Partial Flip Angle Magnetic Resonance Imaging", pp. 496-600 voir chapitres NMR Intensity Model et Discussion.
General Electric CGR S.A.
Tokar Michael J.
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