Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
1998-11-02
2001-07-03
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S299000, C335S301000
Reexamination Certificate
active
06255928
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnets, and more particularly to a magnet having a laminated pole piece and having a shim for reducing magnetic field inhomogeneity in the magnet.
Magnets include those having spaced-apart first and second pole pieces with generally opposing first and second pole faces, such as (but not limited to) “C” -shaped magnets. Typically, the pole piece has a laminated portion attached to a non-laminated iron base. The laminated portion includes alternating layers of magnetizable iron and non-magnetizable epoxy. Some magnet applications, such as magnetic resonance imaging (MRI) for medical diagnostics, employ superconducting magnets to generate a high strength magnetic field within an imaging volume located between the pole pieces. The imaging volume must have a low magnetic field inhomogeneity for high quality imaging. Known techniques for reducing the magnetic field inhomogeneity within the imaging volume include using shimming rings attached to each pole face to reduce axisymmetric magnetic field inhomogeneity and using shims adhesively bonded to a shim tray attached to the shimming rings to reduce 3D (three-dimensional) magnetic field inhomogeneity (including non-axisymmetric inhomogeneity and any remaining axisymmetric inhomogeneity). Typically, the laminated portion of the pole piece is configured as a plurality of concentric shimming rings, with a central circular ring surrounded by concentric annular rings, wherein the annular rings are formed from circumferentially-arrayed annular segments which are electrically insulated from each other. The number, mass, and position of the shims on the pole face are determined by shimming techniques known to those skilled in the art. Adhesively bonding a shim to the shim tray is time consuming as the shim must be manually held in place, against the magnetic field, until the adhesive sets. Reshimming the magnet requires debonding the attached shim and cleaning the removed shim and the attachment area on the pole face.
What is needed is a magnet design for a pole face magnet wherein the shims are more quickly and easily installed and removed.
BRIEF SUMMARY OF THE INVENTION
The magnet of the invention includes a magnetic resonance imaging volume, a pole piece, a non-magnetizable first insert, and a first shim. The pole piece has a laminated portion which includes alternating layers of magnetizable and non-magnetizable material and which includes a pole face facing generally toward the imaging volume. The pole piece also has a back face which is spaced apart from the pole face and which faces generally away from the imaging volume. The pole piece further has a first surface hole with a front opening which is located in the laminated portion at the pole face, wherein the first surface hole extends into the laminated portion of the pole piece. The first insert is positioned in the laminated portion of the first surface hole, is attached to the pole piece, and has an internally-threaded hole in communication with the front opening. The first shim has a shape of an externally-threaded bolt, wherein the first shim is threadably engageable in the hole of the first insert proximate the front opening. In one embodiment, the first shim is threadably engaged in the hole of the first insert, the pole piece includes additional surface holes, the magnet includes additional inserts positioned one each in a corresponding one of the additional surface holes, and the magnet includes additional shims threadably engaged one each with the hole of a corresponding one of only some of the additional inserts.
Several benefits and advantages are derived from the invention. Shims are quickly attached to the pole face by being threaded into holes in non-magnetizable inserts which are positioned in the laminated portion of the surface holes of the pole piece. The length of the shims can be chosen for a coarse shimming effect, and the threaded location of the shims (including how far each shim is threadably inserted into the hole and whether each shim has a portion left extending a particular distance outside the hole) can be chosen for a fine shimming effect, as can be appreciated by those skilled in the art. The non-magnetizable inserts prevent the creation of an electrical path between two magnetizable layers of the laminated portion through an installed shim, such a path creating unwanted eddy currents. Eddy currents increase the inhomogeneity of the magnetic field in the imaging volume which decreases the quality of the images. It is estimated that the time for shimming a magnet having opposing pole pieces may be reduced from about forty hours to about eight hours.
REFERENCES:
patent: 5061897 (1991-10-01), Danby et al.
patent: 5124651 (1992-06-01), Danby et al.
patent: 5317298 (1994-05-01), Dorri et al.
patent: 5345208 (1994-09-01), Dorri et al.
patent: 5446434 (1995-08-01), Dorri et al.
patent: 5627471 (1997-05-01), Kinanen
patent: 5677854 (1997-10-01), Dorri
Recently filed application 09/178320 van Oort, entitled, “Shim Assembly For a Pole Face of a Magnet”.
Laskaris Evangelos Trifon
van Oort Johannes Martinus
Barrera R.
Donovan Lincoln
Erickson Douglas E.
General Electric Company
Snyder Marvin
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