Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-11-09
2001-03-13
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000
Reexamination Certificate
active
06201462
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to open superconductive magnets, and more particularly to an open 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 an open 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, an open magnet includes first and second assemblies, at least one support member, and at least one cryocooler coldhead. The first assembly has a longitudinally-extending and generally-vertically-aligned first axis and has at least one superconductive main coil generally coaxially aligned with the first axis. The second assembly is longitudinally spaced apart from, and positioned generally vertically below, the first assembly. The second assembly has a longitudinally-extending second axis generally coaxially aligned with the first axis, and has at least one superconductive main coil generally coaxially aligned with the second axis. The at least one support member is generally vertically aligned, has a first longitudinal end attached to the first assembly and has a second longitudinal end attached to the second assembly. The at least one cryocooler coldhead is aligned generally horizontally and has a housing attached to, and receiving weight-bearing support from, one of the first and second assemblies.
In a second expression of an embodiment of the invention, an open magnet includes first and second assemblies, nonmagnetizable first and second support members, and first and second cryocooler coldheads. The first assembly has a longitudinally-extending and generally-vertically-aligned first axis, at least one superconductive main coil generally coaxially aligned with the first axis, and a first magnet pole piece generally coaxially aligned with the first axis and spaced apart from the at least one superconductive main coil of the first assembly. The second assembly is longitudinally spaced apart from, and positioned generally vertically below, the first assembly. The second assembly has a longitudinally-extending second axis generally coaxially aligned with the first axis, at least one superconductive main coil generally coaxially aligned with the second axis, and a second magnet pole piece generally coaxially aligned with the second axis and spaced apart from the at least one superconductive main coil of the second assembly. The first and second support members each: are vertically aligned; have a first longitudinal end attached to the first magnet pole piece; and have a second longitudinal end attached to the second magnet pole piece. The first and second support members are not diametrically aligned and are the only support members of the open magnet which are attached to both the first and second magnet pole pieces. The first cryocooler coldhead is generally horizontally aligned and has a first housing attached to, and receiving weight-bearing support from, one of the first and second assemblies near the first support member. The second cryocooler coldhead is generally horizontally aligned and has a second housing attached to, and receiving weight-bearing support from, one of the first and second assemblies near the second support member.
Several benefits and advantages are derived from the invention. Applicants found that, when the cryocooler coldheads are attached to, and receive weight-bearing support from, the first and/or second assembly, having the cryocooler coldhead(s) be horizontally aligned reduces vibration imparted to the assemblies by the cryocooler coldhead(s). Such vibrations are also reduced, in two-cryocooler and two-support-member magnets, by positioning the two horizontal cryocoolers in opposing alignment and in the same vertical plane as the two support members. 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 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 a very open support providing ease of patient table access to the imaging volume and providing ease of patient positioning within the imaging volume.
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Havens Timothy John
Jiang Longzhi
Laskaris Evangelos Trifon
Wang Yu
Donovan Lincoln
General Electric Company
Snyder Marvin
Stoner Douglas E.
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