Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1990-05-11
1992-12-01
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
335306, G01V 300, H01F 702
Patent
active
051682311
DESCRIPTION:
BRIEF SUMMARY
The present invention is due to the collaboration of the Service National des Champs Intenses (Director: Mr. Guy AUBERT), and its object is a device for the correction, by magnetic elements, of the inhomogeneities of the magnetic field produced by a magnet. It particularly concerns an NMR imaging device, called an MRI device, a method for correcting inhomogeneity and a method for making magnets used in the device. The invention finds application particularly in medicine where magnets are used in methods of imaging by nuclear magnetic resonance. However, it can be applied in other fields, notably in the field of scientific research where intense fields are generated with magnets.
Magnetic resonance is a phenomenon of oscillation of the magnetic moment of the nuclei of the atoms or molecules of a body, at a frequency that depends on the intensity of a magnetic field in which this body is bathed. This means that if the intensity of the magnetic field varies, the frequency of the resonance phenomenon varies too. Hence, for technological and technical reasons, it is of the utmost importance that the field produced by the magnet should be very homogeneous in its zone of interest. The homogeneity required is commonly in the range of some parts per million in medicine, or even some parts per billion (1,000,000,000) in the scientific field. To achieve this goal, it is sought to build magnets that give a field that is as perfectly homogeneous as possible.
Unfortunately, however much care is taken in building magnets, their construction is never as perfect as the theory that has led to their design. Besides, even if this fault can be removed, the magnet, in order to be used, must be placed physically in a given place. Now, none of the regions of the space in an industrial or urban environment is wholly free of disturbing magnetic elements. The result thereof is that once the magnet is installed on the site, the field that it produces in a zone of interest has inhomogeneities that have to be corrected.
The principle of the correction of the inhomogeneities of fields is that of superimposition: namely the addition of coils, magnetic parts or any other means enabling the imperfections of the main field to be corrected and a total homogeneous field to be obtained in the zone of interest. A known method for correcting the inhomogeneities of the magnetic field produced by a magnet consists in using magnetic elements, such as magnetizable bars, made of soft iron for example, that are placed in the environment of the magnet and exert their influence in a zone of interest of the magnet so as to correct inhomogeneities of the field in this zone.
Such a method is commented upon by D. I. HOULT and D. LEE in "Shimming A Superconducting Nuclear Magnetic Imaging Magnet With Steel", Sci. Instrum., January 1985, pp. 131 to 135. The article deals particularly with a magnet for nuclear magnetic resonance imaging instruments. The magnet has the shape of a circular cylinder, within which is located the zone of interest, the centre of which is placed on the axis of the magnet, the magnetic field produced by the magnet being within this magnet, substantially parallel to the axis of the magnet. This document gives a detailed description of a method of calculation which can be used to determine the dimensions and the position of one or more magnetic bars made of soft steel around the axis of the magnet, as a function of a point of the volume of interest where an inhomogeneity of the field has to be corrected, the length of these corrective magnetic bars being parallel to the axis of the magnet.
Despite its relative complexity, such a method provides for the efficient correction of the possible inhomogeneities of the magnetic field in the volume of interest through the positioning of one or more corrective magnetic bars, of which the section, length and position with respect to the volume of interest are determined on the basis of known calculations, these bars being of the type indicated notably in the above-mentioned article. However, as is
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patent: 4639673 (1987-01-01), Zijlstra
patent: 4758813 (1988-07-01), Holsinger et al.
patent: 4771244 (1988-09-01), Vermilyea
Review of Scientific Insutruments, vol. 56, No. 1, Jan. 1985, D. I. Hoult et al.: "Shimming a Superconducting Nuclear-Magnetic-Resonance Imaging Magnet With Steel", pp. 131-135.
Arana Louis
Centre National de la Recherche Scientifique
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