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
Reexamination Certificate
active
06215384
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.
Superconductive magnets having superconductive shielding coils use nearly twice as much superconductor, and therefore are more expensive, than superconductive magnets having a cylindrical iron shield. Superconductive magnets having a cylindrical iron shield are too heavy to be placed in medical buildings without expensive structural reinforcement. In addition, the presence of a thick iron shield close to the magnet adversely affects the field homogeneity of a short magnet employing bucking coils or iron rings. What is needed is a magnet having shielding which is not as expensive or as heavy as known designs.
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 correction coil, 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 correction coil is generally coaxially aligned with the axis, longitudinally spaced apart from the at least one main coil, and carries an electric current in an opposite direction to the first direction. The at least one shielding coil is generally coaxially aligned with the axis, is positioned radially outward from the at least one main and correction coils, and carries an electric current in the previously-mentioned opposite 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 and correction coils, 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, superconductive first and second correction coils, 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 coils are generally coaxially aligned with the axis, are positioned longitudinally between the first and second main coils, and carry an electric current in an opposite direction to the first direction. The first and second shielding coils are generally coaxially aligned with the axis, are positioned radially outward from the first and second main and correction coils, and carry an electric current in the previously-mentioned opposite direction. The first and second magnetizable members are not carrying an electric current, are positioned radially outward from the first and second main and correction coils, 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 and correction coils. 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 and shielding coils, 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 combined positioning of the shielding coils, the magnetizable members, and the main and correction coils can be arranged, as is within the skill of the artisan, to reduce the longitudinally-outward EM (electromagnetic) forces experienced by the longitudinally-outermost main coils. The magnetizable members play a key role in that effect. The at least one (or the first and second) correction coil will shorten the magnet (typically by about ten percent). 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 20% while magnet weight is increased by about 30% to generally 11,000 pounds. It is noted that a magnet weighing 11,000 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. The calculated weight of a similar magnet which relies only on iron for its shielding is about 60,000 pounds which prevents it from being placed in medical buildings without expensive structural reinforcement.
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Laskaris Evangelos Trifon
Ogle Michele Dollar
Palmo Michael Anthony
Donovan Lincoln
General Electric Company
Snyder Marvin
Stoner Douglass E.
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