Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2000-11-20
2001-06-12
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C505S893000, C062S051100, C324S319000, C335S300000
Reexamination Certificate
active
06246308
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to superconductive magnets, and more particularly to a superconductive magnet having a cryocooler coldhead.
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 constant 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 open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.
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 imparted to the coil assemblies by the presence of a cryocooler coldhead. What is needed is a design for a superconductive magnet having a cryocooler coldhead which reduces vibrations imparted to the coil assemblies by the cryocooler coldhead 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 magnet includes a first assembly, a first cryocooler coldhead, and a first bellows. The first assembly has a longitudinally-extending first axis, at least one superconductive main coil, and a first vacuum enclosure. The at least one superconductive main coil is generally coaxially aligned with the first axis, and the first vacuum enclosure encloses the at least one superconductive main coil of the first assembly. The first cryocooler coldhead has a first central axis and a rigid first housing, wherein the first central axis is generally vertically aligned. The first bellows has a first bellows axis and first and second ends. The first bellows axis is generally vertically aligned, and the first bellows is flexible along the first bellows axis. The first end of the first bellows is hermetically attached to the first housing of the first cryocooler coldhead, and the second end of the first bellows is hermetically attached to the first vacuum enclosure of the first assembly.
In a second expression of an embodiment of the invention, an open magnet includes first and second assemblies, at least one nonmagnetizable support member, a first cryocooler coldhead, and a first bellows. The first assembly has a longitudinally-extending and generally-vertically-aligned first axis, at least one superconductive main coil, a first vacuum enclosure, and a first magnet pole piece. The at least one superconductive main coil is generally coaxially aligned with the first axis. The first vacuum enclosure encloses the at least one superconductive main coil of the first assembly and surrounds a first bore. The first magnet pole piece is generally coaxially aligned with the first axis and is positioned inside the first bore and outside the first vacuum enclosure. The first cryocooler coldhead has a first central axis and a rigid first housing, wherein the first central axis is generally vertically aligned. The first bellows has a first bellows axis and first and second ends. The first bellows axis is generally vertically aligned, and the first bellows is flexible along the first bellows axis. The first end of the first bellows is hermetically attached to the first housing of the first cryocooler coldhead, and the second end of the first bellows is hermetically attached to the first vacuum enclosure of the first assembly. The second assembly is longitudinally spaced apart from, and is positioned vertically below, the first assembly. The second assembly includes a longitudinally-extending second axis which is generally coaxially aligned with the first axis, at least one superconductive main coil, a second vacuum enclosure, and a second magnet pole piece. The at least one superconductive main coil is generally coaxially aligned with the second axis. The second vacuum enclosure encloses the at least one superconductive main coil of the second assembly and surrounds a second bore. The second magnet pole piece is generally coaxially aligned with the second axis and is positioned inside the second bore and outside the second vacuum enclosure. The at least one nonmagnetizable support member has a first end attached to the first magnet pole piece and has a second end attached to the second magnet pole piece.
Several benefits and advantages are derived from the invention. Applicants conducted an experiment wherein the cryocooler coldhead was vertically aligned, was flexibly attached by a flexible bellows to the vacuum enclosure of a superconductive magnet, and was rigidly attached to the support ceiling of the room by a rigid coldhead support. Applicants found more than a hundred times reduction in vibration of the vacuum enclosure caused by the cryocooler coldhead compared to having the cryocooler coldhead rigidly mounted to the vacuum enclosure. Engineering calculations show improved vibration reduction by having the central axis of the cryocooler coldhead, the first bellows axis, and the lengthwise axis of the rigid coldhead support (which attaches the cryocooler coldhead to the support ceiling or floor) all be generally coincident (and all be generally vertically aligned). Engineering analysis shows improved vibration reduction over a horizontally-aligned cryocooler coldhead. 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 an open magnet, when the support member(s) provide a “clam-shell” support for the assemblies, the superconductive coils of such assemblies are subject to a significant “clam-shell” vibration from the cryocooler coldhead(s) which is likened to the partial closing and opening of a clam shell. It is 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
Laskaris Evangelos Trifon
Thompson Paul Shadforth
Wang Yu
Barrera Ramon M.
General Electric Company
Ingraham Donald S.
Stoner Douglas E.
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