Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1989-04-14
1991-03-12
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
324318, H01F 702
Patent
active
049996002
DESCRIPTION:
BRIEF SUMMARY
The present invention is due to Mr. Guy Aubert, Director of the Service National des Champs Intenses, and its object is a cylindrical permanent magnet to produce a transversal and uniform magnetic field. 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 such distributions of a magnetic induction fields 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 homogeneous 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 the drift of their supplies, or possibly of the system for discharging the dissipated heat. They therefore call for no cooling systems 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 the direction in which objects, patients, are introduced into the magnet. This arrangement is highly favorable to the making of antennas receiving highly uniform and high gain resonance signals. A major difficulty in the use of permanent magnets is located, however, at the level of their industrial-scale manufacture.
Permanent magnet structures producing a transversal, uniform magnetic field in a relatively big volume have been described in the state of the art. In particular, in an international patent application No. WO 84/01226 filed on Sept. 23, 1983 and published on Mar. 29, 1984, D. Lee et al. have described a magnet of this type. In it, the cylindrical structure (theoretically of infinite length) is approximated by a stacking of a certain number of annular sections each provided with a certain number of magnetized blocks. The blocks are distributed on the rim of the rings in a polygonal architecture which reproduces, as far as possible, the circular appearance of a theoretical cylinder. To produce a field transversal to the axis of the cylinder, the magnetization in each of the blocks is constant as regards modulus and is oriented, with respect to the direction of the induction field to be produced, with an angle equal to or twice that which measures the positioning angle of the block in question with respect to the axis of this cylinder. The blocks described are, in a preferred way, prismatic volumes with trapezoidal sections.
The result of the distribution of magnetization thus recommended is that the magnetization of certain blocks has to be oriented, with respect to this block, in a direction which is parallel with none of the sides of the trapezoidal section. The making of magnetic blocks of this type therefore necessitates the use, industrially, of special magnetizers. While this use, albeit more expensive than the use of standard magnetizers, is still possible, the same is not the case for the forming of the blocks. In effect, the distribution of the magnetization imposed in the cylinder creates a demagnetizing excitation, the orientation of which is rarely parallel, in each block, to that of the magnetization. This implies, for the fabrication, the choice of so-called anisotropic magnetic materials. Now, anisotropic magnetic materials which, as it happens, have the best magnetic properties, have the drawback of being hard to machine in directions that are oblique with respect to the direction of their anisotropy. The above-mentioned patent application indicates, especially in its FI
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Centre National de la Recherche Scientifique
Harris George
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