Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1989-04-14
1991-02-05
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
324318, H01F 702
Patent
active
049908790
DESCRIPTION:
BRIEF SUMMARY
The present invention is due to the Service National des Champs Intenses, Director, Mr. Guy AUBERT, and its object is a magnetic block with adjustable magnetization for producing a permanent magnetic field in a zone of interest. It finds application particularly in the medical field where magnets are used in nuclear magnetic resonance imaging experiments. It can also find application in all fields where particular distributions of a magnetic induction field are required.
In the field of imaging by magnetic resonance, it is necessary to place the objects to be imaged, the patients, in a high magnetic induction field (usually of 0.1 to 1.5 Tesla) which is homogenous and uniform (with a few parts per million of variation) in a large volume of interest (commonly a sphere of 50 cm diameter). Several classes of magnetic field generators have been developed until now. The main ones are: superconductive magnets, so-called resistive magnets and permanent magnets. Superconductive magnets, which enable very intense fields to be obtained, are very expensive to make and use because of their complicated technology. They are, furthermore, consumers of rare and dear cryogenic products: like liquid helium. The so-called resistive magnets, with or without associated magnetic material, consume a fairly large amount of electrical energy. They further raise thermal problems of discharge of the heat produced when it is sought to exceed a field of 0.2 Tesla. Permanent magnets have, on the other hand, many advantages.
In particular, they require no energy supply to produce the field. They therefore do not run the risk of drift in their field value due to the drift of these supplies or of the system for discharging the dissipated heat. They call for no particular cooling means with sophisticated regulation techniques for the flow of cryogenic fluids. Their working temperature is easily stabilized. They are furthermore particularly suited to the making of structures or systems that produce a transversal main field, namely a field perpendicular to the direction in which objects, patients, are introduced into the magnet. This arrangement is highly favorable to the making of antennas receiving high gain and highly uniform resonance signals. The main drawback of permanent magnets lies in their weight and cost once it is sought to produce a sufficient field for imaging by NMR.
It is, therefore, particularly important to make the best possible use of the capacities of a given material, namely, to use only the minimum quantity of materials needed to achieve the set objective. Besides, the conditions of homogeneity of the field in the zone of interest are extremely difficult to fulfil, and it is indispensable for the magnetization of the material, at any point of the structure, to be the one that it is desired to achieve in theory. The main difficulty encountered is due to the fact that the magnetized magnetic blocks, which produce the desired magnetic induction field in the zone of interest, produce a magnetic excitation H throughout the space and, therefore, also at any point within the material itself. As is well known those skilled in the art, this magnetic excitation H at a of the material where the magnetization is M is very generally "demagnetizing", namely it can be broken down into a component perpendicular to M, that is H.sub.I and a component parallel to M, that is, H.sub.//, which is directed in the reverse direction of M. The material should be chosen and used in such a way that, as far as possible, it preserves the magnetization conferred on it during the so-called magnetization process which corresponds to the application of a sufficient magnetic excitation having the direction that it is sought to give to M.
If the structure of the final magnet is such that, in certain zones occupied by the material, the component H.sub.I is not very small, it is necessary to use, at these places, a material said to have high anistropy, namely, a material, that tends very much to preserve its magnetization aligned in a preferred direction c
REFERENCES:
patent: 4614730 (1986-09-01), Hickey et al.
patent: 4692732 (1987-09-01), Leupold et al.
patent: 4706057 (1987-11-01), Schwab
patent: 4707663 (1987-11-01), Minkoff et al.
Centre National de la Recherche Scientifique
Harris George
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