Magnetic apparatus for MRI

• US 6,600,401 B2
• Filed: 12/18/2000
• Issued: 07/29/2003
• Est. Priority Date: 09/25/1997
• Status: Expired due to Term

• Current Assignee: ODIN Technologies Limited
• Original Assignee: Odin Medical Technologies, Ltd. (Medtronic Plc)
• Inventors: Katz, Yoav, Rotem, Haim, Katznelson, Ehud, Zuk, Yuval
• Primary Examiner(s): Barrera, Ramon M.
• Application Number:
  US09/738,777
• Publication Number:
  US 20020050895A1
• Time in Patent Office: 953 Days
• Field of Search: 335/216, 335/296-306, 324/318-320, 600/410, 600/421, 600/422
• US Class Current: 335/299
• CPC Class Codes: G01R 33/34061 Helmholtz coils
• CPC Class Codes: G01R 33/341 comprising surface coils
• CPC Class Codes: G01R 33/3678 involving quadrature drive or detection, e.g. a circularly polarized RF magnetic field
• 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 33/383 using permanent magnets
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils
• CPC Class Codes: G01R 33/3858 Manufacture and installation of gradient coils, means for providing mechanical support to parts of the gradient-coil assembly manufacture of inductances or coils in general H01F41/00
• CPC Class Codes: G01R 33/422 of the radio frequency field
• CPC Class Codes: H01F 7/0278 for generating uniform fields, focusing, deflecting electrically charged particles for magnetic separation by Lorentz force B03C1/023; specially adapted for NMR applications G01R33/383

Abstract

Magnetic apparatus for MRI/MRT probes and methods for construction thereof are disclosed. One embodiment includes a pair of opposed magnet assemblies defining an open region therebetween, a transmitting RF coil having at least a portion thereof disposed within the open region, at least one receiving RF coil disposed within the open region and X, Y and Z gradient coils. At least one of the X, Y and Z gradient coils is disposed outside of the open region. Another embodiment of the apparatus includes a single magnet assembly having a first surface and a second surface opposing the first surface, a transmitting RF coil having at least a portion thereof opposing the first surface, at least one receiving RF coil and X, Y and Z gradient coils. At least one of the X, Y and Z gradient coils opposes the second surface. In another embodiment the magnet assembly generates a permanent z-gradient magnetic field and therefore includes only X and Y gradient coils, at least one of which opposes the second surface. The apparatuses may also include one or more shim coils.

Claims

1. Yokeless electromagnetic apparatus for use in an MRI device, the apparatus comprising:

a first permanent magnet assembly having a first surface and a second surface thereof;

a second permanent magnet assembly having a third surface and a fourth surface thereof, said second permanent magnet assembly opposing said first permanent magnet assembly such that said second surface and said third surface define an open region therebetween, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction parallel to a first axis, said volume is disposed within said open region;

an energizable transmitting RF coil for producing an RF electromagnetic field within said volume;

an energizable z-gradient coil for producing a magnetic field gradient extending within said open region in said first direction and parallel to said first axis;

an energizable x-gradient coil for producing a magnetic field gradient extending within said open region in parallel to a second axis orthogonal to said first axis; and

an energizable y-gradient coil for producing a magnetic field gradient extending within said open region in parallel to a third axis orthogonal to said first axis and said second axis,

wherein at least one of said x-gradient coil, y-gradient coil and z-gradient coil is positioned outside of said open region.

2. The apparatus according to claim 1 wherein said transmitting RF coil includes at least a first portion thereof positioned within said open region adjacent said second surface and at least a second portion thereof positioned within said open region adjacent said third surface, said first portion and said second portion of said transmitting RF coil are electrically connected in series.

3. The magnetic probe according to claim 2 wherein said transmitting RF oil further includes a third portion thereof comprising current return conductors positioned outside of said open region and adjacent said first surface and at least a fourth portion thereof comprising current return conductors positioned outside of said open region and adjacent said fourth surface to increase the efficiency of said transmitting RF coil, said first portion, second portion, third portion and fourth portion of said transmitting RF coil are electrically connected in series.

4. The apparatus according to claim 1 further including an energizable shim coil for improving the homogeneity of said substantially uniform magnetic field.

5. The apparatus according to claim 4 wherein said shim coil comprises:

a first shim coil portion positioned outside of said open region and opposed to said first surface of said first permanent magnet assembly; and

a second shim coil portion positioned outside of said open region and opposed to said fourth surface of said second permanent magnet assembly.

6. The apparatus according to claim 5 wherein said first shim coil portion and said second shim coil portion are electrically connected in series.

7. The apparatus according to claim 1 wherein said at least one of said x-gradient coil, y-gradient coil and z-gradient coil includes a first coil portion thereof opposed to said first surface of said first permanent magnet assembly and a second complementary coil portion thereof opposed to said fourth surface of said second permanent magnet assembly.

8. The apparatus according to claim 7 wherein said first coil portion and said second coil portion of at least one of said x-gradient coil, y-gradient coil and z-gradient coil are electrically connected in series.

9. The apparatus according to claim 7 wherein said first coil portion and said second coil portion of at least one of said x-gradient coil, y-gradient coil and z-gradient coil are substantially planar printed circuits, said first coil portion is assembled into a first multi-layer printed circuit assembly opposed to said first surface, and said second coil portion is assembled into a second multi-layer printed circuit assembly opposed to said fourth surface.

10. The apparatus according to claim 9 wherein each of said first multi-layer printed circuit assembly and second multi-layer printed circuit assembly further includes a portion of an energizable shim coil, said portion of said shim coil is a substantially planar printed circuit.

11. The apparatus according to claim 1 further comprising a mounting of low permeability material for mounting said first permanent magnet assembly and said second permanent magnet assembly in opposition to each other.

12. The apparatus according to claim 1 wherein said first permanent magnet assembly includes a first annular permanent magnet with a first and a second surface thereof, said first surface of said first annular permanent magnet being of a first magnetic polarity and said second surface of said first annular permanent magnet being of a second magnetic polarity, said first annular permanent magnet having an inside diameter, said first annular permanent magnet having at least a portion of said first surface of said first annular magnet lying in a first plane to provide a first magnetic field in said open region, said first magnetic field having a zero rate of change in a first direction at a first point in said open region;

said first magnet assembly also includes at least a second annular permanent magnet with a first and a second surface thereof, said first surface of said at least second annular magnet being of said first magnetic polarity and said second surface of said at least second annular permanent magnet being of said second magnetic polarity, said at least second annular permanent magnet having an outside diameter which is smaller than said inside diameter of said first annular permanent magnet, with at least a portion of said first surface of said at least second annular magnet lying in a second plane spaced from said first plane to provide a second magnetic field whereby said second magnetic field is superimposed upon said first magnetic field in said open region, having a zero rate of change in said first direction at a second point different from said first point;

said second permanent magnet assembly includes a third annular permanent magnet with a first and a second surface thereof, said first surface of said third annular permanent magnet being of said second magnetic polarity and said second surface of said third annular permanent magnet being of said first magnetic polarity, said third annular permanent magnet having an inside diameter, said third annular permanent magnet having at least a portion of said first surface of said third annular magnet lying in a third plane to provide a third magnetic field, whereby said third magnetic field is superimposed on said first and second magnetic fields in said open region, having a zero rate of change in said first direction at a third point different from said first and second points; and

said second magnet assembly also includes at least a fourth annular permanent magnet having a first and a second surface thereof, said first surface of said at least fourth annular magnet being of said second magnetic polarity and said second surface of said at least fourth annular permanent magnet being of said first magnetic polarity, said at least fourth annular permanent magnet having an outside diameter which is smaller than said inside diameter of said third annular permanent magnet, with at least a portion of said first surface of said at least fourth annular permanent magnet lying in a fourth plane spaced from said third plane to provide a fourth magnetic field, whereby said fourth magnetic field is superimposed upon said first, second and third magnetic fields, in said region, having a zero rate of change in said first direction at a fourth point different from said first, second and third points.

13. The apparatus according to claim 12 wherein said first axis passes through the centers of said first annular permanent magnet, said at least second annular permanent magnet, said third annular permanent magnet and said at least a fourth annular permanent magnet.

14. The apparatus according to claim 12 wherein said first annular permanent magnet, said at least second annular permanent magnet, said third annular permanent magnet and said at least fourth annular permanent magnet are rare-earth permanent magnets.

15. The apparatus according to claim 14 wherein said rare-earth permanent magnets are neodymium-iron-boron alloy permanent magnets.

16. The apparatus according to claim 12 wherein at least one of said first annular permanent magnet, said at least second annular permanent magnet said third annular permanent magnet and said at least fourth annular permanent comprises a plurality of segments attached to adjacent segments using an electrically non-conductive adhesive.

17. The apparatus according to claim 16 wherein said segments are equiangular segments.

18. The apparatus according to claim 17 wherein said segments have a trapezoidal cross-section in a plane orthogonal to said first direction.

19. The apparatus according to claim 12 wherein said z-gradient coil includes a first gradient coil portion concentrically disposed between said first annular permanent magnet and said at least second annular permanent magnet, and a second gradient coil portion concentrically disposed between said third annular permanent magnet and said at least fourth annular permanent magnet, said first and second gradient coil portions having their longitudinal axes coincident with said first axis.

20. The apparatus according to claim 1 further including at least one receiving RF coil placeable adjacent to an organ or body part disposed within aid open region.

21. The apparatus according to claim 1 wherein said transmitting RF coil is a linearly polarizing RF coil.

22. The apparatus according to claim 1 wherein said transmitting RF coil is a circularly polarizing RF coil.

23. The apparatus according to claim 22 wherein said circularly polarizing RF coil is a quadrature-hybrid RF coil.

24. The apparatus according to claim 1 wherein said first permanent magnet assembly includes a first plurality of nested polygonally or elliptically shaped annular permanent magnets, and said second permanent magnet assembly includes a second plurality of nested polygonally or elliptically shaped annular permanent magnets said first plurality being opposed to said second plurality such that said second plurality is configured as a mirror image of said first plurality.

25. The apparatus according to claim 1 wherein said at least one of said x-gradient coil, y-gradient coil and z-gradient coil is positioned below said first permanent magnet assembly and said second permanent magnet assembly.

26. The apparatus according to claim 1 wherein said x-gradient coil, said y-gradient coil and said z-gradient coil are planar printed circuit coil boards assembled within a single multi-layer printed circuit assembly positioned underneath said first permanent magnet assembly and said second permanent magnet assembly.

27. A method for constructing a yokeles, electromagnetic apparatus for use in an MRI device, the method comprising the steps of:

providing a first permanent magnet assembly having a first surface and a second surface thereof;

providing a second permanent magnet assembly having a third surface and a fourth surface thereof;

positioning said second permanent magnet assembly opposite said first permanent magnet assembly such that said second surface and said third surface define an open region therebetween, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction parallel to a first axis, said volume being disposed within said open region;

providing an energizable transmitting RF coil for producing an RF electromagnetic field within said volume;

providing an energizable z-gradient coil for producing a magnetic field gradient extending within said open region in said first direction and parallel to said first axis;

providing an energizable x-gradient coil for producing a magnetic field gradient extending within said open region in parallel to a second axis orthogonal to said first axis;

providing an energizable y-gradient coil for producing a magnetic field gradient extending within said open region in parallel to a third axis orthogonal to said first axis and said second axis;

providing at least one receiving RF coil placeable adjacent to an organ or body part to be imaged for receiving RF signals from said organ or body part; and

positioning at least one of said x-gradient coil, y-gradient coil and z-gradient coil outside of said open region for reducing the loading of said transmitting RF coil and said at least one receiving RF coil by said at least one of said x-gradient coil, y-gradient coil and z-gradient coil.