Ultrashort cylindrical shielded electromagnet for magnetic resonance imaging

• US 5,359,310 A
• Filed: 04/15/1992
• Issued: 10/25/1994
• Est. Priority Date: 04/15/1992
• Status: Expired due to Term

• Current Assignee: Houston Advanced Research Center
• Original Assignee: Houston Advanced Research Center
• Inventors: Pissanetzky, Sergio
• Primary Examiner(s): Picard, Leo P.
• Assistant Examiner(s): BARRERA, RAMON M
• Application Number:
  US07/869,544
• Time in Patent Office: 923 Days
• Field of Search: 335/216, 335/296-301, 324/318, 324/319, 324/320
• US Class Current: 335/301
• 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 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/421 of main or gradient magnetic field

Abstract

A superconducting magnet suitable for use in magnetic resonance imaging applications is disclosed. A plurality of driving coils are located around a cylindrical bore, for generating an axial magnetic field therein. At the ends of the magnet, within the same cryostat as the driving coils, first and second shielding coils are located, each carrying a current of opposite polarity relative to the driving coils. The location, size, and current in the coils are determined by a methodology in which the error between the desired field and the simulated field is minimized at target locations within the bore and outside of the bore. A flux return is disposed in the gap between the first and second shielding coils, so that the shielding coils inject return magnetic flux into the flux return. The flux return may be of iron, or may consist of a superconducting coil. The resulting magnet has relatively low superconductor cost as well as relatively low weight, due to the efficient use of both superconductor and iron.

Claims

1. A superconducting magnet, comprising:

a plurality of driving coils surrounding a cylindrical bore, each comprises of superconducting cable, for conducting current in a first direction to generate magnetic flux in said bore having substantially an axial orientation therein, said plurality of coils disposed at varying axial distance surrounding said bore;

first and second shielding coils, each comprised of superconducting cable, each having a radius greater than that of said driving coil, and disposed at first and second ends of said electromagnet, respectively, said first and second shielding coils conducting current in a second direction opposing said first direction; and

a flux return, comprising ferromagnetic material and having an innermost radius greater than an outermost radius of said driving coils so as to radially surround at least one of said plurality of driving coils, said flux return axially disposed between said first and second shielding coils, for providing a return path for magnetic flux generated by said driving coil;

wherein the axial length of said flux return is less than the distance between axially outermost ones of said plurality of driving coils, so that magnetic flux is injected into the axial ends of said flux return by said first and second shielding coils.

2. The magnet of claim 1, wherein first and second ones of said plurality of driving coils are disposed radially inwardly from said first and second shielding coils, respectively.

3. The magnet of claim 2, wherein said first and second ones of said plurality of driving coils each conduct a higher magnitude of current than the remainder of said plurality of driving coils.

4. The magnet of claim 1, wherein at least one of said plurality of driving coils has a cross-section of irregular shape.

5. The magnet of claim 1, wherein said first and second shielding coils each have a cross-section of irregular shape.

6. The magnet of claim 1, further comprising:

a cryostat, surrounding said plurality of driving coils and said first and second shielding coils, for maintaining a cryogenic temperature therewithin.

7. The magnet of claim 6, wherein said flux return is disposed outside of said cryostat.

8. A superconducting magnet comprising:

a driving coil surrounding a cylindrical bore, comprised of superconducting cable, for conducting current in a first direction to generate magnetic flux in said bore having substantially an axial orientation therein;

first and second shielding coils, each comprised of superconducting cable, each having a radius greater than that of said driving coil, and disposed at first and second ends of said electromagnet, respectively, said first and second shielding coils conducting current in a second direction opposing said first direction; and

a flux return surrounding said driving coil and disposed between said first and second shielding coils, for providing a return path for magnetic flux generated by said driving coil, and injected thereinto by said first and second shielding coils, said flux return comprising a plurality of plates of ferromagnetic material arranged in segments around said driving coil, each of said segments comprising a group of said plurality of plates, disposed adjacent one another, and arranged according to increasing width.

9. The magnet of claim 8, wherein said flux return further comprises:

first and second ferromagnetic rings, disposed at first and second ends of said flux return, respectively, for supporting said plurality of plates thereupon.

10. The magnet of claim 9, wherein said flux return further comprises:

a plurality of inner plates, each disposed between said first and second rings, and each corresponding to one of said segments of said plurality of plates.

11. An apparatus for generating a high magnetic field suitable for nuclear magnetic resonance, comprising:

a superconducting driving solenoid comprising a plurality of driving coils, and having an inner and an outer radius so as to define a cylindrical bore, each of said driving coils conducting current of a first polarity to generate a magnetic field in said bore;

a flux return, comprising ferromagnetic material and having an inner radius greater than the outer radius of said driving solenoid so as to surround at least one of said plurality of driving coils in said driving solenoid, and having an axial length less than the distance between axially outermost ones of said plurality of driving coils; and

first and second superconducting shielding coils, disposed at first and second axial ends of said flux return, each of said first and second shielding coils surrounding said bore and conducting current of a second polarity opposing said first polarity to direct the path of return magnetic flux into and out of the first and second ends of said flux return, respectively.

12. The apparatus of claim 11, wherein said flux return comprises a plurality of segments of ferromagnetic material, each of said segments corresponding to a cross-sectional arc of said apparatus.

13. The apparatus of claim 11, wherein first and second ones of said plurality of driving coils are disposed radially inwardly from said first and second shielding coils, respectively.

14. The apparatus of claim 13, wherein said first and second ones of said plurality of driving coils each conduct a higher magnitude of current than the remainder of said plurality of driving coils.

15. The apparatus of claim 11, wherein at least one of said plurality of driving coils has a cross-section of irregular shape.

16. The apparatus of claim 11, further comprising:

a cryostat, surrounding said plurality of driving coils and said first and second shielding coils, for maintaining a cryogenic temperature therewithin;

wherein said flux return is disposed outside of said cryostat.

17. The apparatus of claim 11, wherein said first and second shielding coils each have a cross-section of irregular shape.

18. An apparatus for generating a high magnetic filed suitable for nuclear magnetic resonance, comprising:

a superconducting driving solenoid, surrounding a cylindrical bore, for conducting current of a first polarity to generate a magnetic field in said bore;

a flux return, surrounding said driving solenoid and comprising a plurality of segments of ferromagnetic material, each of said segments corresponding to a cross-sectional arc of said apparatus and comprising a plurality of plates of said ferromagnetic material arranged adjacent one another in increasing width to substantially fill the cross-section arc; and

first and second superconducting shielding coils, disposed at first and second axial ends of said flux return and surrounding said bore thereat, conducting current of a second polarity opposing said first polarity, and for injecting return magnetic flux into said flux return.

19. The apparatus of claim 18, wherein said flux return further comprises:

first and second ferromagnetic rings, disposed at first and second ends of said flux return, respectively, for supporting said plurality of plates in each of said segments thereupon; and

for each of said segments, an inner plate disposed between said first and second rings.