Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-10-26
2001-08-14
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S303000, C335S306000, C324S320000
Reexamination Certificate
active
06275129
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnets, and more particularly to a shim assembly and to a method for making a shim assembly for reducing magnetic field inhomogeneity in a magnet, such as, but not limited to, a magnet having a pole piece.
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. 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). 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.
U.S. Pat. No. 5,923,235 issued Jul. 13, 1999 and entitled “Shim Assembly for a Pole Piece of a Magnet” was invented by the same inventor as that of the present patent application and discloses an easily installed shim holder. The shim holder is inserted and rotated to attach it to a shim tray without the need for waiting for an adhesive bond to set. Each shim holder has holes into which shims are placed, as needed for shimming, typically by a press fit.
What is needed is a shim assembly for a magnet wherein the shim assembly provides for easier shimming of the magnet.
BRIEF SUMMARY OF THE INVENTION
In a first expression of an embodiment of the invention, a shim assembly for a magnet includes a first spool and a first magnetizable tape. The first spool has a longitudinal axis, a generally-longitudinally-extending shaft, and generally-radially-extending first and second rims. The shaft has generally-opposing first and second longitudinal ends, wherein the first rim is attached to the first longitudinal end of the shaft, and wherein the second rim is attached to the second longitudinal end of the shaft. The first magnetizable tape has a first predetermined length and is wound around the shaft longitudinally between the first and second rims.
In a second expression of an embodiment of the invention, a shim assembly for a pole face of a magnet includes a non-magnetizable shim tray, a first non-magnetizable spool, and a first magnetizable tape. The pole face of the magnet has a longitudinal axis. The shim tray is positioned near the pole face. The shim tray has at least two generally-circular surface holes each having an axis generally parallel to the longitudinal axis, each having an opening facing away from the pole face, and each having a circumferential flat portion located only longitudinally near the opening. The first non-magnetizable spool is positioned in a first one of the surface holes, has an axis generally parallel to the longitudinal axis, has a generally-longitudinally-extending and generally-circular-cylindrical-shaped shaft, and has generally-identical, generally-radially-extending, and generally-circular-shaped first and second rims. The shaft has generally-opposing first and second longitudinal ends, wherein the first rim is attached to the first longitudinal end of the shaft and has a first circumferential flat region generally matching in shape the circumferential flat portion, and wherein the second rim is attached to the second longitudinal end of the shaft and has a second circumferential flat region generally aligned with the first circumferential flat region and generally matching in shape the circumferential flat portion. The first magnetizable tape has a first predetermined length and is wound around the shaft longitudinally between the first and second rims.
In a first expression of a method of the invention for making a shim assembly for a magnet, steps include obtaining a first spool, obtaining a first magnetizable tape, and determining a first length of the first magnetizable tape that provides a predetermined shim strength. Other steps include winding the first magnetizable tape on the first spool and cutting the wound first magnetizable tape to the determined first length.
In a second expression of a method of the invention for making a shim assembly for a magnetic resonance imaging (MRI) magnet, steps include obtaining a first non-magnetizable plastic spool, obtaining a first magnetizable tape including a magnetizable foil layer and a pressure-sensitive-adhesive-tape layer attached to the magnetizable foil layer, and determining a first length of the first magnetic tape that provides a predetermined shim strength. Other steps include winding the first magnetizable tape on the first spool and cutting the wound first magnetizable tape to the determined first length.
Several benefits and advantages are derived from the invention. Cutting the magnetizable tape to an exact determined length provides a spool (shim holder) which more accurately achieves a desired predetermined shim strength than do prior techniques which place a number of shims of limited discrete sizes in a shim holder. Predetermining a shim strength for a shim holder is more easily done when the shim is a magnetizable tape wound as a cylinder on a shim holder having the shape of a spool. This is so because determining the shim strength of a cylinder is more easily done than determining the shim strengths of a number of shims in a prior shim holder due to the magnetic interaction of the shims in the prior shim holder. Winding a magnetic tape on a spool is less labor intensive than the prior technique of manually press-fitting shims of different sizes into particular ones of available holes in a shim holder.
REFERENCES:
patent: 1306815 (1919-06-01), Houchin et al.
patent: 3943391 (1976-03-01), Fehr
patent: 4127933 (1978-12-01), Hansen et al.
patent: 5297675 (1994-03-01), Martucci
patent: 5923325 (1999-07-01), van Oort
Barrera Ramon M.
General Electric Company
Ingraham Donald S.
Stoner Douglas E.
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