Open superconductive magnet having shielding

• US 5,883,558 A
• Filed: 04/16/1998
• Issued: 03/16/1999
• Est. Priority Date: 02/19/1998
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Laskaris, Evangelos Trifon, Xu, Bu-Xin, Palmo, Michael Anthony
• Primary Examiner(s): DONOVAN, LINCOLN D
• Application Number:
  US09/061,415
• Time in Patent Office: 334 Days
• Field of Search: 335/216, 335/296, 335/297-301, 324/318, 324/319, 324/320, 324/321, 505/5
• US Class Current: 335/216
• CPC Class Codes: G01R 33/3806 Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
• CPC Class Codes: G01R 33/3815 with superconducting coils, e.g. power supply therefor
• CPC Class Codes: G01R 33/421 of main or gradient magnetic field

Abstract

An open superconductive magnet useful in magnetic resonance imaging (MRI) applications. The magnet has two spaced-apart assemblies and preferably has a "C" shape. Each assembly has: a vacuum enclosure having a longitudinal axis and surrounding a bore; a magnetizable pole piece located within the bore and outside the vacuum enclosure; and a superconductive main coil, a superconductive shielding coil, and a magnetizable ring positioned within the vacuum enclosure. There is no magnetizable solid path between the two pole pieces. Preferably, the ring is positioned longitudinally between the main and shielding coils.

Claims

1. An open superconductive magnet comprising:

a) a first assembly including;

(1) a generally toroidal-shaped first vacuum enclosure surrounding a first bore and having a generally longitudinally-extending first axis;

(2) a generally annular-shaped first superconductive main coil generally coaxially aligned with said first axis, disposed within said first vacuum enclosure, and carrying a first main electric current in a first direction;

(3) a generally annular-shaped first superconductive shielding coil generally coaxially aligned with said first axis, disposed within said first vacuum enclosure longitudinally outward from said first superconductive main coil, and carrying a first shielding electric current in a direction opposite to said first direction;

(4) a magnetizable and generally annular-shaped first ring generally coaxially aligned with said first axis, disposed within said first vacuum enclosure, and spaced apart from said first superconductive main and shielding coils; and

(5) a magnetizable and generally cylindrical-shaped first pole piece generally coaxially aligned with said first axis and disposed inside said first bore and outside said first vacuum enclosure; and

b) a second assembly including;

(1) a generally toroidal-shaped second vacuum enclosure surrounding a second bore, having a generally longitudinally-extending second axis generally coaxially aligned with said first axis, and longitudinally spaced apart from said first vacuum enclosure;

(2) a generally annular-shaped second superconductive main coil generally coaxially aligned with said second axis, disposed within said second vacuum enclosure, and carrying a second main electric current in said first direction;

(3) a generally annular-shaped second superconductive shielding coil generally coaxially aligned with said second axis, disposed within said second vacuum enclosure longitudinally outward from said second superconductive main coil, and carrying a second shielding electric current in said opposite direction, and wherein said first and second superconductive main coils are longitudinally closer to each other than are said first and second superconductive shielding coils;

(4) a magnetizable and generally annular-shaped second ring generally coaxially aligned with said second axis, disposed within said second vacuum enclosure, and spaced apart from said second superconductive main and shielding coils; and

(5) a magnetizable and generally cylindrical-shaped second pole piece generally coaxially aligned with said second axis, disposed inside said second bore and outside said second vacuum enclosure, and longitudinally spaced apart from, and without a magnetizable solid path to, said first pole piece.

2. The magnet of claim 1, wherein said first pole piece is attached to said first vacuum enclosure, wherein said second pole piece is attached to said second vacuum enclosure, wherein said magnet also includes only one support member connecting said first and second pole pieces, wherein said support member is a nonmagnetizable support member disposed longitudinally completely between said first and second pole pieces, and wherein said support member and said first and second assemblies together have a generally "C" shape when viewed in a cross section of the magnet created by a cutting plane, wherein said first axis lies completely in said cutting plane, and wherein said cutting plane generally bisects said support member.

3. The magnet of claim 1, wherein said second assembly is a general mirror image of said first assembly about a plane disposed longitudinally equidistant between said first and second assemblies and oriented generally perpendicular to said first axis.

4. The magnet of claim 3, wherein said magnet also includes a magnetic resonance imaging volume disposed generally longitudinally equidistant between said first and second assemblies.

5. The magnet of claim 4, wherein said imaging volume is a generally spherical imaging volume having a center lying generally on said first axis.

6. The magnet of claim 3, wherein said first superconductive main and shielding coils are the only superconductive coils disposed within said first vacuum enclosure.

7. The magnet of claim 3, wherein said first ring is disposed longitudinally between said first superconductive main and shielding coils.

8. The magnet of claim 7, wherein said first superconductive main coil is disposed longitudinally closer to said first ring than is said first superconductive shielding coil.

9. The magnet of claim 7, wherein a longitudinal projection of said first ring onto said plane completely covers a longitudinal projection of said first superconductive main coil onto said plane and completely covers a longitudinal projection of said first superconductive shielding coil onto said plane.

10. The magnet of claim 7, wherein a longitudinal projection of said first superconductive shielding coil onto said plane covers at least half of a longitudinal projection of said first superconductive main coil onto said plane.

11. The magnet of claim 3, wherein said first assembly also includes:

a main coil form disposed within said first vacuum enclosure, supporting said first superconductive main coil, and having a main flange abutting said first ring;

a shielding coil form disposed within said first vacuum enclosure, supporting said first superconductive shielding coil, and having a shielding flange abutting said first ring; and

a plurality of spaced-apart bolts each disposed within said first vacuum enclosure, together arrayed circumferentially about said first axis, each aligned generally parallel to said first axis, and each passing completely through said main flange, said first ring, and said shielding flange.

12. The magnet of claim 11,

wherein said bolts each have an externally-threaded longitudinally-outer end extending longitudinally outward and beyond said shielding flange;

wherein said bolts each have an externally-threaded longitudinally-inner end extending longitudinally inward and beyond said main flange; and

wherein said first assembly further includes a multiplicity of internally-threaded nuts threadably engaged on said longitudinally-inner and longitudinally-outer ends of said bolts.

13. The magnet of claim 3, wherein said first pole piece consists essentially of iron, and wherein said first ring consists essentially of carbon steel.

14. The magnet of claim 3, wherein said magnet also includes a first cryogenic vessel disposed within said first vacuum enclosure and surrounding said first superconductive main and shielding coils and said first ring, and wherein said first cryogenic vessel contains a liquid cryogen.

15. The magnet of claim 14,

wherein said first assembly also includes a first thermal shield, an inner support cylinder, and an outer support cylinder all disposed within said first vacuum enclosure;

wherein said first thermal shield is disposed between, and spaced apart from, said first cryogenic vessel and said first vacuum enclosure;

wherein said inner support cylinder is generally coaxially aligned with said first axis, has a first end secured to said first vacuum enclosure, and has a second end secured to said first thermal shield; and

wherein said outer support cylinder is generally coaxially aligned with said first axis, has a first end secured to said first thermal shield, and has a second end secured to said first cryogenic vessel.

16. The magnet of claim 15, wherein said inner and outer support cylinders each consist essentially of a fiber-reinforced composite, and wherein said liquid cryogen consists essentially of liquid helium.

17. The magnet of claim 3, wherein said magnet also includes a cryocooler coldhead having a cold stage, and wherein said cold stage is in solid thermal conduction contact with said second superconductive shielding coil.