POLE PIECE FOR PERMANENT MAGNET MRI SYSTEMS

• US 20110234347A1
• Filed: 03/16/2011
• Published: 09/29/2011
• Est. Priority Date: 03/24/2010
• Status: Abandoned Application

• Current Assignee: Aspect Imaging Ltd.
• Original Assignee: Aspect Magnet Technologies, Ltd.
• Inventors: RAPOPORT, Uri
• Application Number:
  US13/049,119
• Publication Number:
  US 20110234347A1
• Time in Patent Office: Days
• US Class Current: 335/302
• CPC Class Codes: G01R 33/383 using permanent magnets
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field
• CPC Class Codes: H01F 1/24 the particles being insulated
• CPC Class Codes: H01F 3/08 made from powder powder coatings on sheets H01F3/02; on strips or ribbons H01F3/04; on wires H01F3/06
• CPC Class Codes: Y10T 29/49826 Assembling or joining

Abstract

A pole piece for a permanent magnet MRI system and a method for increasing the stability of a gradient field in an MRI system. The method includes: obtaining an MRI system comprising a magnet capable of providing a gradient magnetic field within an image volume in an air gap; and fixing a plurality of pole pieces within said MRI system, thereby defining the air gap, the raw material of construction of the pole piece being a material including a plurality of ferromagnetic particles coated with an electrically insulating substance. The fixing increases the stability of said gradient field by at least 10% relative to that of a gradient magnetic field in an MRI system identical except for the use of the material in the fabrication of the pole pieces.

Claims

1. A pole piece for a permanent magnet MRI system, wherein the raw material of construction of said pole piece is a material comprising a plurality of ferromagnetic particles coated with an electrically insulating substance.

2. The pole piece of claim 1, wherein said pole piece is substantially parallelepiped shaped.

3. The pole piece of claim 1, wherein said material comprising a plurality of ferromagnetic particles comprises substantially pure iron.

4. The pole piece of claim 1, wherein said ferromagnetic particles have a maximum dimension of less than about 0.1 mm.

5. The pole piece of claim 1, wherein said material comprising a plurality of ferromagnetic particles is characterized by at least one characteristic chosen from the group consisting of (a) maximum relative magnetic permeability of about 205;

(b) maximum differential magnetic permeability of about 280;

(c) initial permeability at 60 mT of about 130;

(d) saturation magnetization at 16 kA/m of about 1.5 T;

(e) coercive force of about 380 A/m; and

(f) absolute energy loss of one cycle of about 2100 J/m³.

6. The pole piece of claim 1, wherein said material comprising a plurality of ferromagnetic particles is characterized by a resistivity of at least 0.009 ohm-m.

7. The pole piece of claim 1, wherein the eddy current decay constant is less than about 50 μ

s when said pole piece is exposed to an external magnetic field of 1 T.

8. The pole piece of claim 1, wherein the residual magnetization of said pole piece is about 0.1%.

9. An MRI magnet providing a gradient magnetic field within an image volume in an air gap, said air gap defined by a plurality of pole pieces, wherein the raw material of construction of at least one of said plurality of pole pieces is a material comprising a plurality of ferromagnetic particles coated with an electrically insulating substance.

10. The MRI magnet of claim 9, wherein said pole pieces are substantially parallelepiped shaped.

11. The MRI magnet of claim 9, wherein said material comprising a plurality of ferromagnetic particles comprises substantially pure iron.

12. The MRI magnet of claim 9, wherein said ferromagnetic particles have a maximum dimension of less than about 0.1 mm.

13. The MRI magnet of claim 9, wherein said material comprising a plurality of ferromagnetic particles is characterized by at least one characteristic chosen from the group consisting of (a) maximum relative magnetic permeability of about 205;

(b) maximum differential magnetic permeability of about 280;

(c) initial permeability at 60 mT of about 130;

(d) saturation magnetization at 16 kA/m of about 1.5 T;

(e) coercive force of about 380 A/m; and

(f) absolute energy loss of one cycle of about 2100 J/m³.

14. The MRI magnet of claim 9, wherein said material comprising a plurality of ferromagnetic particles is characterized by a resistivity of at least 0.009 ohm-m.

15. The MRI magnet of claim 9, wherein the eddy current decay constant within said pole pieces is less than about 50 μ

s when said pole piece is exposed to an external magnetic field of 1 T.

16. The MRI magnet of claim 9, wherein the residual magnetization of each of said pole pieces is about 0.1%.

17. The MRI magnet of claim 9, wherein said gradient magnetic field is substantially free of B₀ and x² components.

18. The MRI magnet of claim 9, wherein the gradient efficiency is about 3 times greater than in an MRI magnet otherwise identical except for the use of shielded pole pieces in place of pole pieces constructed from said material comprising a plurality of ferromagnetic particles coated with an electrically insulating substance.

19. A method for increasing the stability of a gradient field in an MRI system, said method comprising steps of:

a. obtaining an MRI system comprising a magnet capable of providing a gradient magnetic field within an image volume in an air gap; and

,

b. fixing a plurality of pole pieces within said MRI system, thereby defining said air gap, the raw material of construction of said pole piece being a material comprising a plurality of ferromagnetic particles coated with an electrically insulating substance;

wherein said step of fixing said pole pieces within said MRI system increases the stability of said gradient field by at least 10% relative to that of a gradient magnetic field in an MRI system identical except for the use of said material in the fabrication of said pole pieces.