Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-10-15
2001-04-10
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000, C335S296000
Reexamination Certificate
active
06215383
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a magnet, and more particularly to a magnet having shielding to protect the area around the magnet from stray magnetic fields originating from the magnet.
Magnets are used in diverse applications such as MRI (magnetic resonance imaging) systems. MRI systems include those employing superconductive magnets for medical diagnostics and procedures. Known superconductive MRI magnet designs include those having superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils create a static magnetic field within an MRI imaging volume which typically has the shape of a sphere centered within the magnet's bore where the object to be imaged is placed.
Superconductive magnets having shielding include those having superconductive shielding coils and those having a cylindrical iron shield. The superconductive shielding coils carry electric currents of generally equal amperage, but in an opposite direction, to the electric current carried in the superconductive main coils. The superconductive shielding coils are positioned radially outward from the superconductive main coils to prevent the high magnetic field created by and surrounding the superconductive main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Likewise, the cylindrical iron shield is positioned radially outward from the superconductive main coils to prevent the high magnetic field created by and surrounding the superconductive main coils from adversely interacting with electronic equipment in the vicinity of the magnet.
Short magnets with a uniform field, as would be used for MRI, may achieve field uniformity by use of superconductive bucking coils, permanent magnet rings, and iron rings placed radially inward of the main coils. Iron rings are low in cost, but, when positioned in the magnet bore, they must be temperature controlled to maintain a highly uniform field. In addition, the iron rings may interact with other MRI imaging components. Shielding short magnets with a thick iron shield adversely affects field uniformity.
Superconductive magnets having superconductive shielding coils use nearly twice as much superconductor as unshielded magnets. Short magnets with superconductive bucking coils for field correction use more superconductor than longer magnets without the bucking coils. What is needed is a short magnet having shielding which is not as expensive as known designs and which is without the problems associated with iron rings in the warm magnet bore.
BRIEF SUMMARY OF THE INVENTION
In a first expression of the invention, a magnet includes a longitudinally-extending axis, at least one main coil, at least one magnetizable article, at least one shielding coil, and at least two magnetizable members. The at least one main coil is generally coaxially aligned with the axis and carries an electric current in a first direction. The at least one magnetizable article is longitudinally spaced apart from the at least one main coil and does not carry an electric current. The at least one shielding coil is generally coaxially aligned with the axis, is positioned radially outward from the at least one main coil and magnetizable article, and carries an electric current in an opposite direction to the first direction. The at least two magnetizable members are not carrying an electric current, are longitudinally spaced apart from each other, are positioned radially outward from the at least one main coil and magnetizable article, and are positioned radially inward from the at least one shielding coil.
In a second expression of the invention, a closed, superconductive, magnetic-resonance-imaging (MRI) magnet includes a longitudinally-extending axis, longitudinally-outermost superconductive first and second main coils, first and second magnetizable rings, superconductive first and second shielding coils, and annularly-cylindrical first and second magnetizable members. The first and second main coils are generally coaxially aligned with the axis and carry an electric current in a first direction. The first and second correction magnetizable rings are generally coaxially aligned with the axis, are positioned longitudinally between the first and second main coils, and do not carry an electric current. The first and second shielding coils are generally coaxially aligned with the axis, are positioned radially outward from the first and second main coils and magnetizable rings, and carry an electric current in an opposite direction to the first direction. The first and second magnetizable members are not carrying an electric current, are positioned radially outward from the first and second main and coils and magnetizable rings, and are positioned radially inward from the first and second shielding coils. The first magnetizable member at least partially longitudinally overlaps the first main coil, and the second magnetizable member at least partially longitudinally overlaps the second main coil.
Several benefits and advantages are derived from the invention. The at least two (or the first and second) magnetizable members, being located radially inward from the at least one (or the first and second) shielding coil, supplement the strength, and increase the homogeneity of, the magnetic field created, at least in part, by the at least one (or the first and second) main coil and magnetizable article (or rings). The at least two (or the first and second) magnetizable members, being located radially outward from the at least one (or the first and second) main coil and magnetizable article (or rings), provide extra shielding and thus supplement the shielding provided, at least in part, by the at least one (or the first and second) shielding coil. The at least one magnetizable article (or the first and second magnetizable rings) will shorten the magnet (typically by about ten percent) and should result in a net longitudinally-inward electromagnetic (em) force experienced by the first and second (i.e., the longitudinally-outermost) main coils. For a 1.5 Tesla superconductive magnet, wherein the at least two (or the first and second) magnetizable members are annularly-cylindrical iron members, engineering calculations show superconductor use is reduced by at least 25% while magnet weight is increased by about 40% to generally 12,000 pounds. It is noted that a magnet weighing 12,000 pounds easily can be placed in medical buildings without expensive structural reinforcement. The projected cost savings (mainly in saved superconductor costs) over a similar magnet which relies only on its bigger superconductive shielding coils for shielding is about 10,000 US dollars.
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Laskaris Evangelos Trifon
Ogle Michele Dollar
Palmo Michael Anthony
Donovan Lincoln
General Electric Company
Nguyen Tuyen T.
Snyder Marvin
Stoner Douglas E.
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