Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1989-04-14
1991-05-07
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
324318, H01F 702
Patent
active
050140329
DESCRIPTION:
BRIEF SUMMARY
The present invention is due to Mr. Guy AUBERT, Director of the Service National des Champs Intenses, and it has, as an object, a cylindrical permanent magnet with longitudinal induced field. It finds application particularly in the medical field where magnets of this type are used in experiments of imaging by nuclear magnetic resonance for purposes of assistance in diagnosis. It can, nonetheless, find application in other fields where uniform induction fields are sought.
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. Permanent magnets have 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 a drift of their supplies, or possibly of the system for discharging the dissipated heat. They call for no cooling means, in particular 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 producing a transversal main field, namely a field perpendicular to a direction in which objects, patients, are introduced into the magnet. Permanent cylindrical magnet structures, producing a uniform magnetic field, but one transversal to the cylinder, in a relatively high volume, have been described in the state of the art. In particular, in an international patent application No. WO 84/01226 filed on 23rd Sept. 1983 and published on 29th Mar. 1984, D. LEE et al. have described a magnet of this type.
Now, for historal and technical reasons (insufficiency of the intensity of the induced field produced by a permanent magnet), the first systems of imaging by magnetic resonance were designed for superconductive magnets or resistive magnets, the fields of which are longitudinal. The adoption of permanent magnet structures with a transversal field then leads to the advantages of these magnets only on condition that the know-how acquired for the rest of the pieces of equipment of these systems is called into question. These items of ancillary equipment are, for example, a radio frequency antenna, gradient coils as well as field homogeneity correction devices. It is an object of the present invention to propose a cylindrical magnet, with permanent magnetization and a longitudinal induced field which enables the use of already existing items of equipment of systems for imaging by magnetic resonance. Under comparable conditions of use, in particular with equal effective volume of examination, the invention proposes a magnet for which the field intensity and, hence, the weight in magnetized magnetic materials is comparable to the new structures of permanent magnets with transversal fields.
It is known by the Patent Abstract of JAPAN vol 10, N.degree. 58 (E-386) (2115) Mar. 7, 1986, JP-A-60 210 804 and by the UK Patent Application GB-A-2 112 645 structures with a longitudinal induced field. But the homogeneity of these field is insufficient, or, if sufficient, the entrance is not along the axis of this field.
The present invention concerns a cylindrical permanent magnet with cylindrical entrance and with a longitudinal induced field comprising two annular structures, placed on either side of the center of a zone of interest of the magnet, magnetized radially with respect to the cylinder axis with a magnetization which is divergent in one structure and convergent in the other, the structures each comprising several rings adjacent along said axis of said cylinder and, in each ring, several magnetic blocks, distributed on the of said rings, characterized in
REFERENCES:
patent: 3237059 (1966-02-01), Meyerer
patent: 3768054 (1973-10-01), Neugebauer
patent: 4658228 (1987-04-01), Leupold
patent: 4701737 (1987-10-01), Leupold
Patent Abstracts of Japan, vol. 10, No. 58, Mar. 7, 1986 & JP, A, 60210804.
Control Engineering, vol. 24, No. 11, Nov. 1977, A. S. Rashidi: "Rare Earth Magnets Improve Response in Linear Actuators", pp. 56-58.
Centre National de la Recherche Scientifique
Harris George
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