Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1997-06-30
2001-05-01
Barrera, Ray (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S301000
Reexamination Certificate
active
06225887
ABSTRACT:
The invention is directed to apparatus for generating highly uniform magnetic fields using permanently magnetized wedge-shaped blocks of magnetic material as a major contributor of the uniform magnetic field in a desired region of interest.
BACKGROUND OF INVENTION
The related copending application, Ser. No. 08/754,916, whose contents, including the contents of the patents incorporated by reference in the said copending application, are herein incorporated by reference, describes apparatus for generating uniform magnetic fields in a region of interest employing wedge-shaped blocks as the primary generator of the main magnetic field. The copending application also describes supplying a high-permeability soft magnetic material member (referred to therein as a &mgr;=∞ material) positioned along one side of a magnetic structure comprising the wedge-shaped blocks, which has the effect of canceling the field on the back side of the high-permeability member thus concentrating the field on only one side of the structure. An important application for such apparatus is in magnetic resonance imaging (MRI) equipment for medical diagnosis, where a highly uniform magnetic field is essential to obtain high resolution images. As used herein, the term “uniform magnetic field” or “highly uniform magnetic field” means a magnetic field intensity H in a region of interest substantially satisfying the relationship (Hmax−Hmin)/Havg≦10
−4
, equivalent to the field intensity H over the region of interest varying less than 100 ppm. As is explained in the copending application, when the wedge-shaped blocks satisfy certain magnetization and geometrical conditions, the desired uniform magnetic field will be realized when the blocks extend over a very large distance in the y and z directions of the arbitrary x,y,z coordinate system used to define the configuration of the magnetic structure. This is impractical in a usual implementation, and thus the blocks have to be shortened or truncated in the y and z directions. This action introduces distortions in the uniformity of the magnetic field in the region of interest. The copending application also describes one scheme for compensating such distortions using concepts described in a commonly-owned patent U.S. Pat. No. 5,495,222. Another commonly owned patent U.S. Pat. No. 5,428,333 describes another scheme for compensating for distortions in a uniform magnetic field produced by a magnetic structure based on the use of rectangular permanently magnetized blocks, in which a series of slits are introduced into the rectangular permanently magnetized blocks as the compensation means. This latter scheme cannot be used in a magnetic structure based on the wedge-shaped blocks of the present invention.
SUMMARY OF INVENTION
A principal object of the invention is apparatus for generating a uniform magnetic field in a region of interest employing wedge-shaped blocks to generate the main magnetic field.
Another object of the invention is apparatus for generating a uniform magnetic field in an enclosed region of interest employing wedge-shaped blocks wherein the overall size of the apparatus is reduced.
Still another object of the invention is apparatus for generating a uniform magnetic field in a region of interest employing wedge-shaped blocks as a major contributor of the main magnetic field and further comprising compensation means to improve the field uniformity in the region of interest.
A further object of the invention is apparatus for generating a uniform field in a region of interest using wedge-shaped blocks and further comprising compensation means for field distortions wherein the compensation means are provided separately from the wedge-shaped blocks and together supply the main field in the region of interest.
These and other objects are achieved in accordance with one aspect of the invention by apparatus for generating a highly uniform magnetic field using permanently magnetized wedge-shaped blocks of magnetic material as the major contributor of a magnetic field generated in a desired region of interest. In a preferred embodiment, the permanently magnetized wedge-shaped blocks are provided in symmetrical pairs positioned on opposite sides of the xz plane of an arbitrary x,y,z coordinate system, and each of the blocks of a pair are substantially uniform magnetized in certain directions such that their respective y axis components of their magnetization J extend in the same direction but their respective x axis components of their magnetization J extend in opposite directions. In addition, the blocks are contacted on one side by a high-permeability member to cancel the field on the back side of the latter and concentrate it in the region of interest.
In accordance with another aspect of the invention, the parts of the structure below the xz plane are replaced by a high-permeability, soft magnetic plate to form a so-called mirror system wherein the resultant magnetic field in the region of interest is as if the plate were replaced by the second wedge of the full wedge structure; however, since only one-half of the amount of permanently magnetized material is actually present in this mirror system, the region of interest is reduced in size by about one-half.
In accordance with a further aspect of the invention, the magnetic structure using wedge-shaped blocks as a major contributor to the field generated employs permanently magnetized blocks positioned separately from the wedge-shaped blocks as the main compensation means for field distortions arising from the practical need to truncate the wedge-shaped blocks to a practical size.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described the preferred embodiments of the invention, like reference numerals or letters signifying the same or similar components.
REFERENCES:
patent: 5055812 (1991-10-01), Abele et al.
patent: 5107239 (1992-04-01), Abele
patent: 5278534 (1994-01-01), Abele et al.
patent: 5428333 (1995-06-01), Abele et al.
patent: 5438264 (1995-08-01), Takeshima et al.
patent: 5467769 (1995-11-01), Yoshino et al.
patent: 5475355 (1995-12-01), Abele et al.
patent: 5495222 (1996-02-01), Abele et al.
patent: 5790006 (1998-08-01), Abele et al.
Abele Manlio G.
Jensen Jens
Rusinek Henry
Barrera Ray
Darby & Darby
New York University
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