Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1998-04-29
2001-07-24
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S296000
Reexamination Certificate
active
06265959
ABSTRACT:
The invention is directed to a magnetic structure for generating a highly uniform magnetic field using permanently magnetized blocks of magnetic material as the predominant generator of the uniform magnetic field in a desired region of interest.
BACKGROUND OF INVENTION
Commonly-owned U.S. Pat. Nos. 5,475,355 and 5,495,222, whose contents are herein incorporated by reference, describe, among other things, a generally C-shaped magnetic structure for generating uniform magnetic fields in a region of interest, for example, for use in magnetic resonance imaging (MRI) by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure. Among the features described in those two patents are various schemes for compensating for field distortions introduced by, among other things, the opening of the structure.
A commonly-owned, copending application, Ser. No. 08/754,916, filed Nov. 22, 1996, whose contents, including the contents of the patents incorporated by reference in the said copending application and the contents of a continuation-in-part application, Ser. No. 08/885,095, 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. In that magnetic structure, there is no necessary relationship between the orientation of the magnetic field in a cavity defined by the wedge-shaped blocks and the block surfaces interfacing with the cavity. A major thrust of this application is how to generate a field that is sufficiently uniform for use in MRI by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure.
While the magnetic structures described in these referenced patents and applications are suitable for their intended purpose, nevertheless a possible shortcoming is the generation of a substantial fringe field in the medium surrounding the magnetic structure as a result of opening up the structure for surgical intervention. The fringe field can undesirably interfere with a surgeon's use within or adjacent the magnetic structure of various instruments and equipment which may be of ferromagnetic material and may be subject to magnetic field forces.
DEFINITION OF TERMS
A “high permeability” member means a soft ferromagnetic body having a permeability &mgr; exceeding 10 and preferably as high as possible. In several of the referenced patents and publications, as well as in this application, it is sometimes referred to as a (&mgr;=∞) material, in other words, an ideal soft ferromagnetic body. Practically speaking, the differences in function of a (&mgr;=∞) material and a high permeability material are small. The behavior of infinite permeability ferromagnetic material can be approximated to a high degree of accuracy by soft iron as long as it is not magnetically saturated.
A “yoked magnetic system” means a permanent magnetic structure surrounded by a body of high permeability material—typically called a yoke—that functions to carry the return flux of the induction B from one pole of the magnetic structure to the other pole. To perform this function, typically the yoke requires a significant cross sectional area sufficient to convey the flux without saturating, and often has a varying cross section in accordance with the local flux-carrying requirement.
A “yokeless magnetic system” means a permanent magnetic structure in which the return flux of the induction B is carried by the magnetic material.
A “hybrid magnetic system” means a permanent magnetic structure which has the attributes of both a yoked and a yokeless magnetic system.
A “strap” means a high permeability member that functions to establish a unipotential or equipotential surface within or at the surface of a permanent magnet that may have inhomogenieties due to manufacturing tolerances and whose interior region or a surface thus deviates from what should have been an equipotential surface. In other words, the strap functions to create the equipotential condition that would have existed if the magnet were ideal with perfect materials and geometry. An external strap is somewhat equivalent to the yoke of a magnet, the basic difference between the yoke and the external strap being that the strap function is limited to closing the flux of the statistical fluctuation of the magnetic induction, i.e., to eliminate the components of the field parallel to the strap surface, while the yoke is designed to close the entire flux generated by the magnet. Since the strap only carries the flux of the statistical fluctuation of the magnetic induction, it requires only a small cross sectional area and typically is a very thin layer of uniform thickness. For a further description, reference is had to a copending application, Ser. No. 08/613,756, filed Feb. 26, 1996, and our paper entitled “Strapping Techniques For Permanent Magnets, IEE Transactions On Magnetics, 32/5, September 1996, Pgs. 5082-5084, whose contents are herein incorporated by reference.
“Unipolar magnetic structure” is used herein in the sense that a single magnetic pole piece of a magnetic structure functions as a platform adjacent a cavity for receiving an object to be analyzed, the single magnetic pole generating flux in the cavity, whereas the magnetic structure, rather than a second distinct pole piece, collects that magnetic flux. In a unipolar magnetic structure, there typically will not exist a single magnetic element that can perform the function of a second magnetic pole piece.
A “uniform magnetic field” means a magnetic field that over a region of interest has an intensity that varies by less than 100 ppm.
SUMMARY OF INVENTION
A principal object of the invention is a magnetic structure for generating a uniform magnetic field in a region of interest, the magnetic structure employing blocks of permanent magnetic material to generate the main magnetic field.
Another object of the invention is a magnetic structure comprising permanent magnetic material blocks for generating a uniform magnetic field in a cavity containing a region of interest, with the cavity being open to the surrounding medium to allow access to the region of interest.
Still another object of the invention is a magnetic structure for generating a uniform magnetic field in a cavity containing a region of interest, with the cavity being open to the surrounding medium, and in which a relatively small fringe field is generated by the magnetic structure in the surrounding medium via the cavity opening.
Still a further object of the invention is a magnetic structure for generating a uniform magnetic field in a cavity containing a region of interest, with the cavity being sized to accommodate at least a part of a human body for MRI, and with the cavity being open to allow surgical intervention from a surrounding medium to the body part, resulting in a minimum fringe field in the surrounding medium.
These and other objects are achieved in accordance with one aspect of the invention by a magnetic structure for generating a uniform magnetic field using permanently magnetized blocks of magnetic material as the principal generator of the magnetic field in a desired region of interest within a cavity of the structure. In a preferred embodiment of the invention, the cavity interfaces on one side with a plane surface of a high permeability material on the surface of one of the blocks and which serves as a platform for supporting an object to be analyzed in a region of interest adjacent the platform. The magnetic structure is configured as a unipolar magnetic structure with the high permeability material functioning as the single magnetic pole piece of the magnetic structure, and with the magnetic structure functioning to collect the magnetic flux flowing through the single pole piece and through the cavity.
In accordance with another aspect of the invention, the cavity at another side is open to the surrounding medium to allow access to the object from the surrounding medi
Abele Manlio G.
Jensen Jens
Rusinek Henry
Darby & Darby
Donovan Lincoln
New York University
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