Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1989-08-02
1991-09-10
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
G01R 3320
Patent
active
050477200
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 correction device using magnetic elements to correct unhomogeneities of the magnetic field produced by a magnet. It particularly concerns means for the assembly and positioning of one or more of these magnetic elements. The invention can be applied especially in the medical field where magnets are used in methods of nuclear magnetic resonance imaging. However, it can find application in other fields, notably in that of scientific research, where intense fields produced by magnets are used.
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. It follows that if the intensity of the magnetic field varies, the frequency of the phenomenon of resonance 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 requisite homogeneity is usually of the order of some parts per million in the medical field, or even some parts per billion (1000,000,000) in the scientific field. To achieve this end, it is sought to construct magnets with a field that is as perfectly homogeneous as possible.
Unfortunately, irrespectively of the care that might be taken in constructing the magnets, they are never as perfect as the theory that has led to their design. Besides, even if this fault can be eliminated, the magnet to be used should be placed physically in a given location. Now, none of the regions of space, in an industrial or urban environment, is totally free of magnetically disturbing 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 non-homogeneities which must be corrected.
The principle of the correction of non-homogeneities of the fields is that of superimposition: coils, magnetic pieces, or any other means enabling a correction of the imperfections of the main field and the obtaining of a homogeneous total field in the zone of interest are added on. A known method to correct the non-homogeneities of the magnetic field produced by a magnet consists in the use of magnetic elements such as magnetizable bars, for example made of soft iron, which are placed in the environment of the magnet and exert their influence in a zone of interest of the magnet so as to correct the non-homogeneities of the field in this zone.
A method of this type is commented on in an article by D. I. Hoult and D. Lee, "Shimming a Superconducting Nuclear Magnetic Resonance Imaging Magnet With Steel" in Revue Sci. Instrum., January 1985, pp. 131 to 135. This article concerns, particularly, a nuclear magnetic resonance imaging apparatus. The magnet has the shape of a circular cylinder, within which the zone of interest is located. The center of this zone of interest is placed on the axis of the magnet. The magnetic field produced by the magnet is within this magnet, substantially parallel to the axis of the magnet. This document gives a detailed description of a method of computation which can be used to determine the dimensions and 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 a non-homogeneity of the field has to be corrected. The length of these correction bars is parallel to the axis of the magnet.
A method such as this, despite its relative complexity, enables the efficient correction of the possible non-homogeneities of the magnetic field in the volume of interest, by the placing of one or more magnetic bars, for which the section, length and position with respect to the volume of interest are determined on the basis of computations that are known and are, for example, of the type indicated notably in the above-mentioned article. But, as kno
REFERENCES:
patent: 3905417 (1975-09-01), Delassus
patent: 4511870 (1985-04-01), Caillat
patent: 4743853 (1988-05-01), Frese
Patent Abstracts of Japan, vol. 9, No. 174 (P-374) (1987) Jul. 19, 1985, & JP, A, 60-50442 (Yokokawa Medical System K.K.) Mar. 20, 1985.
Centre National de la Recherche Scientifique
Tokar Michael J.
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