Eddy current control in NMR imaging system

• US 5,592,089 A
• Filed: 06/01/1995
• Issued: 01/07/1997
• Est. Priority Date: 01/19/1993
• Status: Expired due to Term

• Current Assignee: Fonar Corporation
• Original Assignee: Fonar Corporation
• Inventors: Rimkunas, Joseph F., Brukl, Charles E., Gudimetta, Krishna M., Danby, Gordon T., Jackson, John W.
• Primary Examiner(s): Arana, Louis M.
• Application Number:
  US08/457,014
• Time in Patent Office: 586 Days
• Field of Search: 324/318, 324/319, 324/320, 324/322, 324/307, 324/309, 128/653.5
• US Class Current: 324/318
• CPC Class Codes: G01R 33/383 using permanent magnets
• CPC Class Codes: G01R 33/4215 of the gradient magnetic field, e.g. using passive or active shielding of the gradient magnetic field
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field

Abstract

A primary field magnet assembly, of the type used in magnetic resonance imaging systems, which has been designed with the capability to suppress eddy currents during magnetic resonance imaging through the use of eddy current suppressing material. In addition, this invention includes the use of high conductivity material to decouple the imaging system radio frequency antennas from their environment, which generally includes the primary field magnet assembly, and can be used to preserve the signal-to-noise performance of the scanner. Such highly conductive material is effective when employed in combination with the primary field magnet assembly described herein.

Claims

1. A primary field magnet assembly for use in medical NMR imaging systems, comprising:

a) a ferromagnetic structure which provides a return path for magnetic flux,

b) means for generating said magnetic flux, said means magnetically coupled to said ferromagnetic structure to establish magnetic flux flowing through said ferromagnetic structure,

c) two opposed ferromagnetic poles spaced apart from one another and forming a gap therebetween for receiving a portion of human anatomy therein, said ferromagnetic poles being magnetically coupled to said ferromagnetic structure to develop a magnetic field between said poles, and

d) a layer of eddy current suppressing material having an initial permeability greater than approximately 1000, and a resistivity less than 1000 micro ohm-cm, said material providing means to reduce eddy currents in said ferromagnetic poles, and said material placed proximate to a surface of each said pole which faces said gap.

2. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises permanent magnetic material.

3. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises resistive electromagnetic windings.

4. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises superconductive windings.

5. The primary field magnet assembly as described in claim 1, wherein said eddy current suppresing material has a homogeneous solid form.

6. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material is a metal alloy.

7. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material is a metal.

8. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material is a nickel-iron alloy comprising at least 35% nickel.

9. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material is a nickel-iron alloy comprising approximately 80% nickel, approximately 15% iron, and the remaining approximately 5% comprising at least one element of the group consisting of molybdenum, silicon, manganese, copper and carbon.

10. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material is a nickel-iron alloy comprising about 80% nickel, 14.93% iron, 4.2% molybdenum, 0.35% silicon, 0.5% manganese, and 0.02% carbon.

11. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material comprises a single layer.

12. The primary field magnet assembly as described in claim 1, wherein said eddy current suppressing material comprises multiple layers.

13. The primary field magnet assembly as described in claim 12, wherein each layer of said multiple layers of said eddy current suppressing material is electrically isolated from other layers of said multiple layers.

14. The primary field magnet assembly as described in claim 1, wherein said layer of eddy current suppressing material is in a substantially homogeneous solid form.

15. A method for upgrading a primary field magnet assembly of a medical NMR imaging system to improve the eddy current response characteristics of said primary field magnet assembly, said method comprising the following steps:

a) separating subassemblies from said primary field magnet assembly to allow placement of a layer of high permeability magnetic material,

b) placing said layer of high permeability magnetic material proximate to a ferromagnetic pole, and covering a substantial portion thereof, and

c) securing subassemblies into said primary field magnet assembly, thereby reassembling said primary field magnet assembly for magnetic resonance imaging.

16. The method as described in claim 15, further comprising the step of placing means for preventing radio frequency absorption by said high permeability magnetic material, said step following the step of placing said layer of high permeability magnetic material.

17. The method as described in claim 15, further comprising the step of placing a means for preventing radio frequency absorption by said high permeability magnetic material, said step following the step of securing said subassemblies.

18. A medical NMR imaging system comprising:

a) a primary field magnet assembly comprised of structural elements at least one of which is electrically conductive,

b) means for generating a magnetic flux through an interior portion of said magnet assembly, said interior portion comprising a plurality of interior surfaces, and

c) a layer of high permeability magnetic material placed proximate to at least one of said interior surfaces of said interior portion, said magnetic material providing means to reduce eddy currents along said at least one of said interior surfaces.

19. The medical NMR imaging system as described in claim 18, wherein said high permeability material comprises an eddy current suppressing material having an initial permeability greater than approximately 1000 and a resistivity less than 1000 micro ohm-cm.

20. The medical NMR imaging system as described in claim 18, wherein said high permeability magnetic material is a nickel-iron alloy comprising at least 35% nickel.

21. The medical NMR imaging system as described in claim 18, wherein said high permeability magnetic material is a nickel-iron alloy comprising approximately 80% nickel, approximately 15% iron, and the remaining approximately 5% comprising at least one element of the group consisting of molybdenum, silicon, manganese, copper and carbon.

22. The medical NMR imaging system as described in claim 18, wherein said high permeability magnetic material is a nickel-iron alloy comprising about 80% nickel, 14.93% iron, 4.2% molybdenum, 0.35% silicon, 0.5% manganese, and 0.02% carbon.

23. The medical NMR imaging system as described in claim 18, wherein said high permeability material is selected from a group consisting of ferrites, bonded particulate metals, unbonded particulate metals, composites incorporating particulate metals and combinations thereof.

24. The medical NMR imaging system as described in claim 18, wherein said high permeability magnetic material comprises a single layer.

25. The medical NMR imaging system as described in claim 18, wherein said high permeability magnet material comprises multiple layers.

26. The medical NMR imaging system as described in claim 25, wherein each layer of said multiple layers of said high permeability magnetic material is electrically isolated from other layers of said multiple layers.

27. The medical NMR imaging system as described in claim 18, further comprising means for preventing radio frequency absorption, said means placed proximate to and approximately co-extensive with said layer of high permeability magnetic material.

28. The medical NMR imaging system as described in claim 27, wherein said means for preventing radio frequency absorption comprises high electrical conductivity material.

29. The medical NMR imaging system as described in claim 27, wherein said means for preventing radio frequency absorption is comprises at least one element of the group consisting of aluminum, copper, silver and gold.

30. The medical NMR imaging system as described in claim 27, wherein said means for preventing radio frequency absorption has a thickness of approximately 0.0003 inches.

31. The medical NMR imaging system as described in claim 18, wherein said high permeability material is in a substantially homgeneous solid form.

32. The medical NMR imaging system as described in claim 18, further comprising at least one set of conductors for generating magnetic filed gradients for conducting magnetic resonance imaging, each of said set of conductors placed proximate to said layer of high permeability magnetic material along at least a portion of one of said interior surfaces.