Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-11-23
2001-09-11
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000
Reexamination Certificate
active
06288624
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the magnet arts. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with magnetic resonance spectroscopy and other applications which use strong, controlled magnetic fields.
Heretofore, open or vertical field magnetic resonance imaging systems have typically included a pair of circular pole pieces disposed parallel to each other on opposite sides of an examination region. The pole pieces included various ferrous, magnetic, and other structures for shaping the magnetic field in the examination region. The pole pieces also supported radio field and gradient magnetic field coils.
Typically, cylindrical ferrous elements extended from the back sides of the pole face away from the examination region. The cylindrical ferrous elements commonly connected with ferrous flux return paths reduce fringe fields and electromagnetic drive coil requirements. Of course, magnetic fields will return through the air and non-ferrous structures, with a greater expenditure of energy.
Magnetic coils were commonly placed around the cylindrical ferrous members and adjacent the pole pieces to generate a magnetic field that flows between the pole pieces through the examination region and back around through a ferrous or non-ferrous return path. Resistive and superconducting magnets have been utilized. Electromagnetic coils have also been placed along the return path, primarily when the magnets have concentrated return paths as are found in C-magnets or H-magnets.
Magnetic field uniformity in the region of interest is a significant concern in magnetic resonance imaging applications. Circular symmetry of the pole pieces and adjacent rose ring ferrous structure has been used extensively as a tool for achieving and promoting this uniformity.
The prior open magnetic resonance imaging systems have tended to have relatively small imaging regions. Placement of the electromagnetic coils for generating the magnetic field was usually close to the poles. This symmetrical circular arrangement supported a uniform and symmetrical magnetic field distribution in the imaging region. Power dissipation in resistive coils and saturation properties of the ferrous core were additional engineering considerations. Unfortunately, the electromagnetic coils often occupy valuable space close to the imaging region and introduce thermal management problems near the poles.
The present invention overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a magnet assembly is provided. A magnet coil surrounds a first cross-sectional area. A ferrous element of a second, cross-sectional area smaller than the first cross-sectional area is disposed adjacent an examination region. The ferrous element concentrates a magnetic field generated by the magnet coil through the examination region. A ferromagnetic path, preferably integral with the first ferrous element, extends between the ferrous element and an inner surface of the magnet coil. The ferromagnetic path spreads in one dimension and thins in another to steer and concentrate the magnetic field through the ferrous element. Preferably, the spreading and thinning are coordinated to maintain a substantially constant cross-section.
In accordance with a more limited aspect of the present invention, the magnet coil is a dipole magnet. The dipole magnet includes a ferrous member having a first winding of a first polarity on one side and a second winding of a second polarity on an opposite side. Arched elements extend from the ferrous member over each of the windings. Potting structures extend between the arched elements, the ferrous yoke, and the windings to hold the windings securely in place.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus is provided including a magnet assembly as described above.
In accordance with another aspect of the present invention, a magnetic field is generated peripherally in a generally radial direction. The magnetic field is channeled, turned, and converged. The converged magnetic field is passed through an examination region.
In accordance with another aspect of the present invention, a magnet construction is provided. First and second ferrous elements are disposed on opposite sides of a gap for focusing magnetic field across the gap. At least one of the ferrous elements has a maximum axial extent such that as it expands outward to larger peripheral dimensions, it also tapers toward a peripheral edge. First and second coil windings extend peripherally adjacent the peripheral edge and carry current of opposite polarity. In this manner, the first and second windings both generate magnetic fields in a common peripheral direction.
One advantage of the present invention is that it enables the examination region to be enlarged.
Another advantage of the present invention is that it opens the area around the examination region and improves patient access.
Another advantage of the present invention is that it permits higher magnet currents without saturating ferrous magnet structures.
Yet another advantage of the present invention is that it facilitates examination regions with elongated cross-sections.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.
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“The LHC Conceptual Design Report—The Yellow Book”, CERN/AC/95-05 (LHC), (no date).
European Organization for Nuclear Research Oct. 20, 1995 (latest edition available at: http://www.cern.ch/CERN/LHC/YellowBook95/LHC95/ ).
Donovan Lincoln
Fay Sharpe Fagan Minnich & McKee LLP
Picker Nordstar Corporation
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