Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-06-20
2002-05-21
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S296000
Reexamination Certificate
active
06392518
ABSTRACT:
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. 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.
The parent application, Ser. No. 09/069,389, whose contents, including the contents of the patents and publications incorporated by reference in the said copending application, are herein incorporated by reference, describes a new open magnetic structure for MRI with two principal novel features. The first is what is referred to as a unipolar structure. Most permanent magnetic structures employed in this field possess two distinct opposed poles, and the main magnetic field through the region of interest in a cavity in the magnetic structure flows directly between the opposed pole faces. The unipolar structure of the parent application is unusual because it has only one distinct pole which faces the opening provided for accessing the region of interest, the opening normally being the place where the second pole would be located if the opening were not present. The main magnetic field still flows from the unipole through the region of interest but is now collected by the magnetic members adjacent the opening. As explained in that application, opening such a structure to provide access to the cavity provides field perturbations that are unacceptable for MRI applications. The second novel feature is to configure the structure to form at the location of the opening, i.e., where the cavity interfaces with the non-magnetic external medium, an equipotential surface. This is the equivalent of the equipotential surface formed by having a high-permeability membrane parallel to the single pole piece present which closes off the interface and thus the opening. This design constraint was described as a hypothetical high permeability membrane present where the open second side of the cavity will be made, the high permeability membrane forming the desired equipotential surface at the interface with the surrounding medium. The presence of the equipotential surface at the opening represented by the hypothetical high permeability membrane, in effect, extends the uniform field up to the opening. This minimizes the fringe field generated in the surrounding medium when the second side of the cavity, when this hypothetical high permeability membrane is removed, is opened.
A limitation of that open unipolar magnetic structure is that the region of interest is most uniform close to the bottom side or first side of the cavity which comprises a high permeability ferromagnetic member interfacing with the first side of the cavity and forming the single pole piece of the magnetic structure. There may be applications of an open unipolar magnetic structure where it is desirable to be able to arbitrarily locate the region of interest within the cavity. Usually, it would be preferred to move the region of interest closer to the opening at the top or second side of the cavity simplifying access by a surgeon to the part of a patient located in the region of interest.
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 “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, now U.S. Pat. No. 5,798,680, and a 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 region of interest within 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 opposed 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.
The term “substantially” appears from time to time in the specification and claims. It should be understood in a practical sense to mean small variati
Darby & Darby
Donovan Lincoln
New York University
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