Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-11-09
2001-03-06
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000
Reexamination Certificate
active
06198371
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to open magnets, and more particularly to an open magnet having a floor mount.
Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense “boiled-off” helium.
Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet's bore where the object to be imaged is placed.
Open magnets, including “C” shape and support-post magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.
It is also known in open magnet designs to place an iron pole piece in the bore of a resistive or superconductive coil assembly. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving the homogeneity of the magnetic field. Nonmagnetizable support posts are connected to the face of the pole pieces. It is additionally known in horizontally-aligned open magnets to support the magnet on the floor using two spaced-apart feet attached to each assembly, such feet raising the assemblies to provide room underneath the assemblies for necessary wires, pipes, etc.
The sharpness of an MRI image depends, in part, on the magnetic field in the imaging volume being time-constant and highly uniform, such magnetic field suffering time and spatial deformation caused by vibrations, especially vibrations imparted to the coil assemblies of an open magnet by the presence of a cryocooler coldhead. What is needed is a design for a superconductive open magnet which reduces vibrations and hence which improves the sharpness of an MRI image.
BRIEF SUMMARY OF THE INVENTION
In a first expression of an embodiment of the invention, a open magnet includes first and second assemblies, at least one support beam, and a support skirt. Each assembly includes a longitudinally-extending axis, at least one superconductive main coil generally coaxially aligned with the axis, and a vacuum enclosure enclosing the assembly's at least one main coil. The first axis of the first assembly is generally vertically aligned, the second assembly is positioned generally vertically below the first assembly, and the second axis of the second assembly is generally coaxially aligned with the first axis. The at least one support beam has a first end attached to the first assembly and has a second end attached to the second assembly. The support skirt is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis, which has a first longitudinal end attached to the second assembly, and which has a second longitudinal end which can be supported by a floor. In one example, the open magnet also includes a first cryocooler coldhead having a first housing attached to one of the first and second vacuum enclosures.
In a second expression of an embodiment of the invention, an open magnet includes first and second assemblies, nonmagnetizable first and second support beams, and a support skirt. Each assembly includes a longitudinally-extending axis, at least one superconductive main coil generally coaxially aligned with the axis, a vacuum enclosure enclosing the assembly's at least one main coil and surrounding a bore, and a magnet pole piece. The magnet pole piece is located inside the bore and outside the vacuum enclosure and is attached to the vacuum enclosure. The first axis of the first assembly is generally vertically aligned, the second assembly is positioned generally vertically below the first assembly, and the second axis of the second assembly is generally coaxially aligned with the first axis. The first and second support beams each are generally vertically aligned, each have a first longitudinal end attached to the first magnet pole piece of the first assembly, and each have a second longitudinal end attached to the second magnet pole piece of the second assembly. The support skirt is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis, which has a first longitudinal end attached to the second assembly, and which has a second longitudinal end which can be supported by a floor. In one example, the open magnet also includes a first cryocooler coldhead having a first housing attached to the first vacuum enclosure.
Several benefits and advantages are derived from the invention. Engineering analysis shows that, compared to using conventional feet found on horizontally-aligned open magnets, the support-skirt design for a vertically-aligned open magnet stiffens the support of the magnet thereby shifting the natural frequency of the open magnet to a higher value which reduces the susceptibility of the open magnet to vibrate at the dominant low-excitation-frequencies imparted to the magnet by the presence of a cryocooler coldhead attached to an assembly (such as attached to a pole piece of an assembly). Applicants found that cryocooler vibrations cause vibration of the superconductive main coils, cause unwanted eddy-currents generated by vibrations of the thermal shields, and cause unwanted movement of the superconductive coils relative to the pole pieces all contributing to MRI image degradation. It is noted that, in a vertically-aligned open magnet, when the support member(s) provide a “clam-shell” support for the assemblies, the superconductive coils of such assemblies are subject to significant vibration from the cryocooler coldhead(s). It is further noted that a “clam-shell” support is provided by having only two support members, especially when the two support members are not diametrically aligned. Such clam-shell support is a very open support providing ease of patient table access to the imaging volume and providing ease of patient positioning within the imaging volume. Engineering analysis shows the support-skirt design of the invention reduces magnet vibrations in a vertically-aligned open magnet having a “clam-shell” support for the assemblies, especially f
Alexander James Pellegrino
Laskaris Evangelos Trifon
Wang Yu
Donovan Lincoln
General Electric Company
Nguyen Tuyen T.
Snyder Marvin
Stoner Douglas E.
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