Adjustable hybrid magnetic apparatus

• US 6,411,187 B1
• Filed: 11/27/2000
• Issued: 06/25/2002
• Est. Priority Date: 07/23/1997
• Status: Expired due to Term

• Current Assignee: Odin Medical Technologies, Ltd. (Medtronic Plc)
• Original Assignee: Odin Medical Technologies, Ltd. (Medtronic Plc)
• Inventors: Katznelson, Ehud, Rotem, Haim
• Primary Examiner(s): Barrera, Ramon M.
• Application Number:
  US09/721,767
• Time in Patent Office: 575 Days
• Field of Search: 335/216, 335/296-306, 324/318-320, 600/407, 600/410, 600/421, 600/422
• US Class Current: 335/296
• 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: 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

Adjustable hybrid magnetic apparatus for use in medical and other applications includes an electromagnet flux generator for generating a first magnetic field in an imaging volume, and permanent magnet assemblies for generating a second magnetic field superimposed on the first magnetic field for providing a substantially homogenous magnetic field having improved magnitude within the imaging volume. The permanent magnet assemblies may include a plurality of annular or disc like concentric magnets spaced-apart along their axis of symmetry. The electromagnet flux generator may include two electromagnet assemblies. The electromagnet assemblies and the permanent magnet assemblies may be static assemblies or may be movable and/or tiltable and/or movable and tiltable, for fine tuning of the magnetic field within the imaging volume. The hybrid magnetic apparatus may include a high magnetic permeability yoke for increasing the intensity of the magnetic field in the imaging volume of the hybrid magnetic apparatus.

Claims

1. An open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of said volume, the apparatus comprising:

a first electromagnet assembly disposed at a first position along said axis, said first electromagnet assembly includes at least a first electromagnet coil, said at least first electromagnet coil is radially symmetric with respect to said axis;

a second electromagnet assembly disposed at a second position spaced apart from said first position of said first electromagnet assembly along said axis, said second electromagnet assembly includes at least a second electromagnet coil, said at least second electromagnet coil is radially symmetric with respect to said axis, said at least first electromagnet coil and said at least second electromagnet coil are substantially equidistant from the center of said is volume, said first electromagnet assembly and said second electromagnet assembly are configured for generating a first magnetic field within said volume;

a first permanent magnet assembly positioned at a third position disposed between said first position and said second position along said axis, said first permanent magnet assembly has an inner surface facing said volume and an outer surface facing said first electromagnet assembly, said first permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets are radially symmetric with respect to said axis; and

a second permanent magnet assembly opposed to said first permanent magnet assembly, said second permanent magnet assembly is positioned at a fourth position spaced apart from said third position along said axis, said second permanent magnet assembly has an inner surface facing said volume and an outer surface facing said second electromagnet assembly, said second permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets of said second permanent magnet assembly are radially symmetric with respect to said axis,

said inner surface of said first permanent magnet assembly and said inner surface of said second permanent magnet assembly define an open region therebetween, said volume is disposed within said open region, said third position of said first permanent magnet assembly and said fourth position of second permanent magnet assembly are substantially equidistant from the center of said volume along said axis,

said first permanent magnet assembly and said second permanent magnet assembly are configured for generating a second magnetic field superimposed on said first magnetic field to provide said substantially uniform magnetic field within said volume.

2. The magnetic apparatus according to claim 1, wherein said at least first electromagnet coil and said at least second electromagnet coil are configured such that the radius of said at least first electromagnet coil is equal to the radius of said at least second electromagnet coil, and the distance between said at least first electromagnet coil and said at least second electromagnet coil is equal to the radius of said at least first electromagnet coil.

3. The magnetic apparatus according to claim 1, wherein said at least first electromagnet coil and said at least second electromagnet coil are configured such that the radius of said at least first electromagnet coil is equal to the radius of said at least second electromagnet coil, and the distance between said at least first electromagnet coil and said at least second electromagnet coil is greater than the radius of said at least first electromagnet coil.

4. The magnetic apparatus according to claim 1, wherein at least one electromagnet assembly of said first electromagnet assembly and said second electromagnet assembly is a movable electromagnet assembly.

5. The magnetic apparatus according to claim 1, wherein at least one electromagnet assembly of said first electromagnet assembly and said second electromagnet assembly is controllably movable relative to said volume.

6. The magnetic apparatus according to claim 5, wherein said open magnetic apparatus further comprises at least one moving mechanism configured for moving said at least one electromagnet assembly relative to said volume.

7. The magnetic apparatus according to claim 6, wherein said at least one moving mechanism is configured for moving said at least one electromagnet assembly towards or away from said volume in a direction parallel to said axis.

8. The magnetic apparatus according to claim 6, wherein said at least one moving mechanism is configured for tilting said at least one electromagnet assembly at an angle with respect to a plane orthogonal to said axis.

9. The magnetic apparatus according to claim 6, wherein said at least one moving mechanism is configured for moving said at least one electromagnet assembly towards or away from said volume in a direction parallel to said axis and for tilting said at least one electromagnet assembly at an angle with respect to a plane orthogonal to said axis.

10. The magnetic apparatus according to claim 6, wherein said at least one moving mechanism is a motorized moving mechanism.

11. The magnetic apparatus according to claim 6, wherein said motorized moving mechanism comprises at least one motor for controllably moving said at least one electromagnet assembly.

12. The magnetic apparatus according to claim 11, wherein said at least one motor is a magnetic resonance imaging compatible motor.

13. The magnetic apparatus according to claim 1, wherein at least one permanent magnet assembly of said first permanent magnet assembly and said second permanent magnet assembly is a movable permanent magnet assembly.

14. The magnetic apparatus according to claim 1, wherein at least one permanent magnet assembly of said first permanent magnet assembly and said second permanent magnet assembly is movable relative to said volume.

15. The magnetic apparatus according to claim 14, wherein said open magnetic apparatus further comprises at least one moving mechanism configured for moving said at least one permanent magnet assembly relative to said volume.

16. The magnetic apparatus according to claim 15, wherein said at least one moving mechanism is configured for moving said at least one permanent magnet assembly towards or away from said volume in a direction parallel to said axis.

17. The magnetic apparatus according to claim 15, wherein said at least one moving mechanism is configured for tilting said at least one permanent magnet assembly at an angle with respect to a plane orthogonal to said axis.

18. The magnetic apparatus according to claim 15, wherein said at least one moving mechanism is configured for moving said at least one permanent magnet assembly towards or away from said volume in a direction parallel to said axis and for tilting said at least one permanent magnet assembly at an angle with respect to a plane orthogonal to said axis.

19. The magnetic apparatus according to claim 15, wherein said at least one moving mechanism is a motorized moving mechanism.

20. The magnetic apparatus according to claim 15, wherein said motorized moving mechanism comprises at least one motor for controllably moving said at least one permanent magnet assembly.

21. The magnetic apparatus according to claim 20, wherein said at least one motor is a magnetic resonance imaging compatible motor.

22. The magnetic apparatus according to claim 1, wherein said at least first electromagnet coil and said at least second electromagnet coil are super-conducting electromagnet coils.

23. The magnetic apparatus according to claim 22 further including at least a first cooling device for cooling said at least first electromagnet coil and a second cooling device for cooling said at least second electromagnet coil.

24. The magnetic apparatus according to claim 22 wherein said at least first cooling device and said at least second cooling device comprise a Dewar container or cryostat.

25. The magnetic apparatus according to claim 22 wherein said at least first cooling device and said at least second cooling device are selected from a low temperature gas operated cooling device, a low temperature liquefied gas based cooling device, an active cryo-cooling device, and a cooling device based on low pressure evaporative cooling of a liquefied gas.

26. The magnetic apparatus according to claim 22, wherein said super-conducting electromagnet coils comprise a low temperature super-conducting material.

27. The magnetic apparatus according to claim 22, wherein said super-conducting electromagnet coils comprise a high temperature super-conducting material.

28. The magnetic apparatus according to claim 1, wherein said at least two coaxial permanent magnets of said first permanent magnet assembly comprise

a first permanent magnet having an outer diameter, said first permanent magnet has a first magnetization direction parallel to said axis, and

at least a second generally annular permanent magnet, said at least second permanent magnet has an inner diameter larger than said outer diameter of said first permanent magnet, said second permanent magnet has a second magnetization direction parallel to said axis, and

said at least two coaxial permanent magnets of said second permanent magnet assembly comprise

a third permanent magnet having an outer diameter, said third permanent magnet has a magnetization direction equal to said first magnetization direction of said first permanent magnet, and

at least a fourth generally annular permanent magnet, said at least fourth permanent magnet has an inner diameter larger than said outer diameter of said third permanent magnet, said at least fourth permanent magnet has a magnetization direction equal to said second magnetization direction of said at least second permanent magnet.

29. The magnetic apparatus according to claim 28, wherein said first permanent magnet has a first surface facing said volume, said at least second permanent magnet has a second surface facing said volume, said third permanent magnet has a third surface facing said volume, and said at least fourth permanent magnet has a fourth surface facing said volume, and wherein at least part of said second surface is offset from at least part of said first surface by a first distance along said axis, at least part of said fourth surface is offset from at least part of said third surface by a second distance along said axis, said first distance is substantially equal to said second distance.

30. The magnetic apparatus according to claim 28, wherein said first permanent magnet has a first surface facing said volume, said at least second permanent magnet has a second surface facing said volume, said third permanent magnet has a third surface facing said volume, and said at least fourth permanent magnet has a fourth surface facing said volume, at least part of said second surface is coplanar with at least part of said first surface, and at least part of said third surface is coplanar with at least part of said fourth surface.

31. The magnetic apparatus according to claim 28, wherein said first permanent magnet and said third permanent magnet are selected from a disc-like permanent magnet having a circular cross section in a plane perpendicular to said axis, a regular right polygonal prism-like permanent magnet having a regular polygonal cross-section in a plane perpendicular to said axis and having N sides, a ring-like annular permanent magnet and a annular right regular polygonal permanent magnet having N sides.

32. The magnetic apparatus according to claim 31, wherein N is equal to or larger than eight.

33. The magnetic apparatus according to claim 28, wherein said at least second and said at least fourth permanent magnets are selected from a ring-like annular permanent magnet and an annular right regular polygonal permanent magnet having N sides.

34. The magnetic apparatus according to claim 33, wherein N is equal to or larger than eight.

35. The magnetic apparatus according to claim 28, wherein said first permanent magnet and said third permanent magnet comprise a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.

36. The magnetic apparatus according to claim 35, wherein said segments are equi-angular segments.

37. The magnetic apparatus according to claim 28, wherein said at least second annular permanent magnet and said at least fourth annular permanent magnet comprise a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.

38. The magnetic apparatus according to claim 37, wherein said segments are equi-angular segments.

39. The magnetic apparatus according to claim 28, wherein said first permanent magnet assembly includes a first low magnetic permeability frame for supporting said first permanent magnet and said at least second permanent magnet, and said second permanent magnet assembly includes a second low magnetic permeability frame for supporting said third permanent magnet and said at least fourth permanent magnet.

40. The magnetic apparatus according to claim 39, wherein said first magnetization direction and said second magnetization direction are parallel to said axis and have the same polarity.

41. The magnetic apparatus according to claim 39, wherein said first magnetization direction and said second magnetization direction are parallel to said axis and have opposite polarities.

42. The magnetic apparatus according to claim 1, wherein said at least first electromagnet coil and said at least second electromagnet coil are selected from a circular coil and a regular polygonal shaped coil having N sides.

43. The magnetic apparatus according to claim 42, wherein N is equal to or larger than eight.

44. The magnetic apparatus according to claim 1, wherein said first permanent magnet assembly includes a first low magnetic permeability frame for supporting said at least two coaxial permanent magnets included in said first permanent magnet assembly, and said second permanent magnet assembly includes a second low magnetic permeability frame for supporting said at least two coaxial permanent magnets included in said second permanent magnet assembly.

45. The magnetic apparatus according to claim 1, wherein said first permanent magnet assembly comprises a first adjustment mechanism configured for moving at least one of said at least two coaxial permanent magnets of said first permanent magnet assembly with respect to said volume.

46. The magnetic apparatus according to claim 45, wherein said second permanent magnet assembly comprises a second adjustment mechanism configured for moving at least one of said at least two coaxial permanent magnets of said second permanent magnet assembly with respect to said volume.

47. The magnetic apparatus according to claim 1, further including a first gradient coil assembly disposed between said first permanent magnet assembly and said first electromagnet assembly, and a second gradient coil assembly disposed between said second permanent magnet assembly and said second electromagnet assembly.

48. The magnetic apparatus according to claim 47 wherein each of said first gradient coil assembly and said second gradient coil assembly are non-movably attached within said open magnetic apparatus.

49. The magnetic apparatus according to claim 47, wherein each of said first gradient coil assembly and said second gradient coil assembly includes one or more gradient coils selected from a x-gradient coil, a y-gradient coil, a z-gradient coil, and any combination thereof.

50. The magnetic apparatus according to claim 49, wherein each of said first gradient coil assembly and said second gradient coil assembly is a multi-layer printed circuit assembly, and wherein at least one of said x-gradient coil, y-gradient coil and z-gradient coil of each of said first gradient coil assembly and said second gradient coil assembly is a substantially planar printed circuit coil.

51. The magnetic apparatus according to claim 1, further including an open high magnetic permeability yoke configured for closing the magnetic field lines of said open magnetic apparatus.

52. The magnetic apparatus according to claim 51, wherein said open high magnetic permeability yoke comprises at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.

53. The magnetic apparatus according to claim 51, wherein said open high magnetic permeability yoke is selected from a generally c-shaped yoke, a generally U-shaped yoke, and a generally Y-shaped yoke.

105. The magnetic apparatus according to claim 1, further including one or more support members attached to said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly, for supporting said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly within said open magnetic apparatus.

107. The magnetic apparatus according to claim 47, wherein said first gradient coil assembly is attached to said first electromagnet assembly.

108. The magnetic apparatus according to claim 47, wherein said first gradient coil assembly is attached to said first permanent magnet assembly.

109. The magnetic apparatus according to claim 47, wherein said first gradient coil assembly is attached to a support member, said support member is suitably attached to said open magnetic apparatus.

110. The magnetic apparatus according to claim 47, wherein said second gradient coil assembly is attached to said second electromagnet assembly.

111. The magnetic apparatus according to claim 47, wherein said second gradient coil assembly is attached to said second permanent magnet assembly.

112. The magnetic apparatus according to claim 47, wherein said second gradient coil assembly is attached to a support member, said support member is suitably attached to said open magnetic apparatus.

54. An open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of said volume, the apparatus comprising:

a first electromagnet assembly disposed at a first position along said axis, said first electromagnet assembly includes at least a first electromagnet coil, said at least first electromagnet coil is radially symmetric with respect to said axis;

a second electromagnet assembly disposed at a second position spaced apart from said first position of said first electromagnet assembly along said axis, said second electromagnet assembly includes at least a second electromagnet coil, said at least second electromagnet coil is radially symmetric with respect to said axis, said at least first electromagnet coil and said at least second electromagnet coil are substantially equidistant from the center of said volume, said first electromagnet assembly and said second electromagnet assembly are configured for generating a first magnetic field within said volume;

a first permanent magnet assembly positioned at a third position disposed between said first position and said second position along said axis, said first permanent magnet assembly has an inner surface facing said volume and an outer surface facing said first electromagnet assembly, said first permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets are radially symmetric with respect to said axis; and

a second permanent magnet assembly opposed to said first permanent magnet assembly, said second permanent magnet assembly is positioned at a fourth position spaced apart from said third position along said axis, said second permanent magnet assembly has an inner surface facing said volume and an outer surface facing said second electromagnet assembly, said second permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets of said second permanent magnet assembly are radially symmetric with respect to said axis,

said inner surface of said first permanent magnet assembly and said inner surface of said second permanent magnet assembly define an open region therebetween, said volume is disposed within said open region, said third position of said first permanent magnet assembly and said fourth position of second permanent magnet assembly are substantially equidistant from the center of said volume along said axis,

said first permanent magnet assembly and said second permanent magnet assembly are configured for generating a second magnetic field superimposed on said first magnetic field to provide said substantially uniform magnetic field within said volume,

at least one of said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly and said second permanent magnet assembly is controllably movable relative to said volume.

55. The magnetic apparatus according to claim 54, wherein at least one electromagnet assembly of said first electromagnet assembly and said second electromagnet assembly is controllably movable relative to said volume.

56. The magnetic apparatus according to claim 55, wherein said open magnetic apparatus further comprises at least one moving mechanism configured for moving said at least one electromagnet assembly relative to said volume.

57. The magnetic apparatus according to claim 56, wherein said at least one moving mechanism is configured for moving said at least one electromagnet assembly towards or away from said volume in a direction parallel to said axis.

58. The magnetic apparatus according to claim 56, wherein said at least one moving mechanism is configured for tilting said at least one electromagnet assembly at an angle with respect to a plane orthogonal to said axis.

59. The magnetic apparatus according to claim 56, wherein said at least one moving mechanism is configured for moving said at least one electromagnet assembly towards or away from said volume in a direction parallel to said axis and for tilting said at least one electromagnet assembly at an angle with respect to a plane orthogonal to said axis.

60. The magnetic apparatus according to claim 56, wherein said at least one moving mechanism is a motorized moving mechanism.

61. The magnetic apparatus according to claim 60, wherein said motorized moving mechanism comprises at least one motor for controllably moving said at least one electromagnet assembly.

62. The magnetic apparatus according to claim 61, wherein said at least one motor is a magnetic resonance imaging compatible motor.

63. The magnetic apparatus according to claim 54, wherein at least one permanent magnet assembly of said first permanent magnet assembly and said second permanent magnet assembly is movable relative to said volume.

64. The magnetic apparatus according to claim 63, wherein said open magnetic apparatus further comprises at least one moving mechanism configured for moving said at least one permanent magnet assembly relative to said volume.

65. The magnetic apparatus according to claim 64, wherein said at least one moving mechanism is configured for moving said at least one permanent magnet assembly towards or away from said volume in a direction parallel to said axis.

66. The magnetic apparatus according to claim 64, wherein said at least one moving mechanism is configured for tilting said at least one permanent magnet assembly at an angle with respect to a plane orthogonal to said axis.

67. The magnetic apparatus according to claim 64, wherein said at least one moving mechanism is configured for moving said at least one permanent magnet assembly towards or away from said volume in a direction parallel to said axis and for tilting said at least one permanent magnet assembly at an angle with respect to a plane orthogonal to said axis.

68. The magnetic apparatus according to claim 64, wherein said at least one moving mechanism is a motorized moving mechanism.

69. The magnetic apparatus according to claim 68, wherein said motorized moving mechanism comprises at least one motor for controllably moving said at least one permanent magnet assembly.

70. The magnetic apparatus according to claim 69, wherein said at least one motor is a magnetic resonance imaging compatible motor.

71. The magnetic apparatus according to claim 54, wherein said at least first electromagnet coil and said at least second electromagnet coil are configured such that the radius of said at least first electromagnet coil is equal to the radius of said at least second electromagnet coil, and the distance between said at least first electromagnet coil and said at least second electromagnet coil is equal to the radius of said at least first electromagnet coil.

72. The magnetic apparatus according to claim 54, wherein said at least first electromagnet coil and said at least second electromagnet coil are configured such that the radius of said at least first electromagnet coil is equal to the radius of said at least second electromagnet coil, and the distance between said at least first electromagnet coil and said at least second electromagnet coil is greater than the radius of said at least first electromagnet coil.

73. The magnetic apparatus according to claim 54, wherein said at least first electromagnet coil and said at least second electromagnet coil are super-conducting electromagnet coils.

74. The magnetic apparatus according to claim 73 further including at least a first cooling device for cooling said at least first electromagnet coil and a second cooling device for cooling said at least second electromagnet coil.

75. The magnetic apparatus according to claim 73 wherein said at least first cooling device and said at least second cooling device comprise a Dewar container or cryostat.

76. The magnetic apparatus according to claim 74 wherein said at least first cooling device and said at least second cooling device are selected from a low temperature gas operated cooling device, a low temperature liquefied gas based cooling device, an active cryo-cooling device, and a cooling device based on low pressure evaporative cooling of a liquefied gas.

77. The magnetic apparatus according to claim 73, wherein said super-conducting electromagnet coils comprise a low-temperature super-conducting material.

78. The magnetic apparatus according to claim 73, wherein said super-conducting electromagnet coils comprise a high-temperature super-conducting material.

79. The magnetic apparatus according to claim 54, wherein said at least two coaxial permanent magnets of said first permanent magnet assembly comprise a first permanent magnet having an outer diameter, said first permanent magnet has a first magnetization direction parallel to said axis, and

at least a second generally annular permanent magnet, said at least second permanent magnet has an inner diameter larger than said outer diameter of said first permanent magnet, said second permanent magnet has a second magnetization direction parallel to said axis, and

said at least two coaxial permanent magnets of said second permanent magnet assembly comprise

a third permanent magnet having an outer diameter, said third permanent magnet has a magnetization direction equal to said first magnetization direction of said first permanent magnet, and

at least a fourth generally annular permanent magnet, said at least fourth permanent magnet has an inner diameter larger than said outer diameter of said third permanent magnet, said at least fourth permanent magnet has a magnetization direction equal to said second magnetization direction of said at least second permanent magnet.

80. The magnetic apparatus according to claim 79, wherein said first permanent magnet has a first surface facing said volume, said at least second permanent magnet has a second surface facing said volume, said third permanent magnet has a third surface facing said volume, and said at least fourth permanent magnet has a fourth surface facing said volume, and wherein at least part of said second surface is offset from at least part of said first surface by a first distance along said axis, at least part of said fourth surface is offset from at least part of said third surface by a second distance along said axis, said first distance is substantially equal to said second distance.

81. The magnetic apparatus according to claim 79, wherein said first permanent magnet has a first surface facing said volume, said at least second permanent magnet has a second surface facing said volume, said third permanent magnet has a third surface facing said volume, and said at least fourth permanent magnet has a fourth surface facing said volume, at least part of said second surface is coplanar with at least part of said first surface, and at least part of said third surface is coplanar with at least part of said fourth surface.

82. The magnetic apparatus according to claim 79, wherein said first permanent magnet and said third permanent magnet are selected from a disc-like permanent magnet having a circular cross section in a plane perpendicular to said axis, a regular right polygonal prism-like permanent magnet having a regular polygonal cross-section in a plane perpendicular to said axis and having N sides, a ring-like annular permanent magnet and a annular right regular polygonal permanent magnet having N sides.

83. The magnetic apparatus according to claim 82, wherein N is equal to or larger than eight.

84. The magnetic apparatus according to claim 79, wherein said at least second and said at least fourth permanent magnets are selected from a ring-like annular permanent magnet and an annular right regular polygonal permanent magnet having N sides.

85. The magnetic apparatus according to claim 84, wherein N is equal to or larger than eight.

86. The magnetic apparatus according to claim 79, wherein said first permanent magnet and said third permanent magnet comprise a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.

87. The magnetic apparatus according to claim 86, wherein said segments are equi-angular segments.

88. The magnetic apparatus according to claim 79, wherein said at least second annular permanent magnet and said at least fourth annular permanent magnet comprise a plurality of permanently magnetized segments attached to adjacent segments using a non-conductive adhesive, to reduce eddy currents.

89. The magnetic apparatus according to claim 88, wherein said segments are equi-angular segments.

90. The magnetic apparatus according to claim 79, wherein said first permanent magnet assembly includes a first low magnetic permeability frame for supporting said first permanent magnet and said at least second permanent magnet, and said second permanent magnet assembly includes a second low magnetic permeability frame for supporting said third permanent magnet and said at least fourth permanent magnet.

91. The magnetic apparatus according to claim 90, wherein said first magnetization direction and said second magnetization direction are parallel to said axis and have the same polarity.

92. The magnetic apparatus according to claim 90, wherein said first magnetization direction and said second magnetization direction are parallel to said axis and have opposite polarities.

93. The magnetic apparatus according to claim 54, wherein said at least first electromagnet coil and said at least second electromagnet coil are selected from a circular coil and a regular polygonal shaped coil having N sides.

94. The magnetic apparatus according to claim 93, wherein N is equal to or larger than eight.

95. The magnetic apparatus according to claim 54, wherein said first permanent magnet assembly includes a first low magnetic permeability frame for supporting said at least two coaxial permanent magnets included in said first permanent magnet assembly, and said second permanent magnet assembly includes a second low magnetic permeability frame for supporting said at least two coaxial permanent magnets included in said second permanent magnet assembly.

96. The magnetic apparatus according to claim 54, wherein said first permanent magnet assembly comprises a first adjustment mechanism configured for moving at least one of said at least two coaxial permanent magnets of said first permanent magnet assembly with respect to said volume.

97. The magnetic apparatus according to claim 96, wherein said second permanent magnet assembly comprises a second adjustment mechanism configured for moving at least one of said at least two coaxial permanent magnets of said second permanent magnet assembly with respect to said volume.

98. The magnetic apparatus according to claim 54, further including a first gradient coil assembly disposed between said first permanent magnet assembly and said first electromagnet assembly, and a second gradient coil assembly disposed between said second permanent magnet assembly and said second electromagnet assembly.

99. The magnetic apparatus according to claim 98 wherein each of said first gradient coil assembly and said second gradient coil assembly are non-movably attached within said open magnetic apparatus.

100. The magnetic apparatus according to claim 98, wherein each of said first gradient coil assembly and said second gradient coil assembly includes one or more gradient coils selected from a x-gradient coil, a y-gradient coil, a z-gradient coil, and any combination thereof.

101. The magnetic apparatus according to claim 100, wherein each of said first gradient coil assembly and said second gradient coil assembly is a multi-layer printed circuit assembly, and wherein at least one of said x-gradient coil, y-gradient coil and z-gradient coil of each of said first gradient coil assembly and said second gradient coil assembly is a substantially planar printed circuit coil.

102. The magnetic apparatus according to claim 54, further including an open high magnetic permeability yoke configured for closing the magnetic field lines of said open magnetic apparatus.

103. The magnetic apparatus according to claim 102, wherein said open high magnetic permeability yoke comprises at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.

104. The magnetic apparatus according to claim 102, wherein said open high magnetic permeability yoke is selected from a generally c-shaped yoke, a generally U-shaped yoke, and a generally Y-shaped yoke.

106. The magnetic apparatus according to claim 54, further including one or more support members attached to said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly, for supporting said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly within said open magnetic apparatus.

113. The magnetic apparatus according to claim 98, wherein said first gradient coil assembly is attached to said first electromagnet assembly.

114. The magnetic apparatus according to claim 98, wherein said first gradient coil assembly is attached to said first permanent magnet assembly.

115. The magnetic apparatus according to claim 98, wherein said first gradient coil assembly is attached to a support member, said support member is suitably attached to said open magnetic apparatus.

116. The magnetic apparatus according to claim 98, wherein said second gradient coil assembly is attached to said second electromagnet assembly.

117. The magnetic apparatus according to claim 98, wherein said second gradient coil assembly is attached to said second permanent magnet assembly.

118. The magnetic apparatus according to claim 98, wherein said second gradient coil assembly is attached to a support member, said support member is suitably attached to said open magnetic apparatus.

119. A method for constructing an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of said volume, the method comprising the steps of:

providing a first electromagnet assembly disposed at a first position along said axis, said first electromagnet assembly includes at least a first electromagnet coil, said at least first electromagnet coil is radially symmetric with respect to said axis;

providing a second electromagnet assembly disposed at a second position spaced apart from said first position of said first electromagnet assembly along said axis, said second electromagnet assembly includes at least a second electromagnet coil, said at least second electromagnet coil is radially symmetric with respect to said axis, said at least first electromagnet coil and said at least second electromagnet coil are substantially equidistant from the center of said volume, said first electromagnet assembly and said second electromagnet assembly are configured for generating a first magnetic field within said volume;

providing a first permanent magnet assembly positioned at a third position disposed between said first position and said second position along said axis, said first permanent magnet assembly has an inner surface facing said volume and an outer surface facing said first electromagnet assembly, said first permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets are radially symmetric with respect to said axis;

providing a second permanent magnet assembly opposed to said first permanent magnet assembly, said second permanent magnet assembly is positioned at a fourth position spaced apart from said third position along said axis, said second permanent magnet assembly has an inner surface facing said volume and an outer surface facing said second electromagnet assembly, said second permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets of said second permanent magnet assembly are radially symmetric with respect to said axis; and

positioning said first permanent magnet assembly and said second permanent magnet assembly such that said inner surface of said first permanent magnet assembly and said inner surface of said second permanent magnet assembly define an open region therebetween, said volume is disposed within said open region, said third position of said first permanent magnet assembly and said fourth position of second permanent magnet assembly are substantially equidistant from the center of said volume along said axis, said first permanent magnet assembly and said second permanent magnet assembly generate a second magnetic field superimposed on said first magnetic field to provide said substantially uniform magnetic field within said volume.

120. The method according to claim 119, further including the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of said open magnetic apparatus.

121. The method according to claim 120, wherein said open high magnetic permeability yoke comprises at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.

122. A method for tuning an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of said volume, the method comprising the steps of:

providing a first electromagnet assembly disposed at a first position along said axis, said first electromagnet assembly includes at least a first electromagnet coil, said at least first electromagnet coil is radially symmetric with respect to said axis;

providing a second electromagnet assembly disposed at a second position spaced apart from said first position of said first electromagnet assembly along said axis, said second electromagnet assembly includes at least a second electromagnet coil, said at least second electromagnet coil is radially symmetric with respect to said axis, said at least first electromagnet coil and said at least second electromagnet coil are substantially equidistant from the center of said volume, said first electromagnet assembly and said second electromagnet assembly are configured for generating a first magnetic field within said volume;

providing a first permanent magnet assembly positioned at a third position disposed between said first position and said second position along said axis, said first permanent magnet assembly has an inner surface facing said volume and an outer surface facing said first electromagnet assembly, said first permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets are radially symmetric with respect to said axis;

providing a second permanent magnet assembly opposed to said first permanent magnet assembly, said second permanent magnet assembly is positioned at a fourth position spaced apart from said third position along said axis, said second permanent magnet assembly has an inner surface facing said volume and an outer surface facing said second electromagnet assembly, said second permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets of said second permanent magnet assembly are radially symmetric with respect to said axis;

positioning said first permanent magnet assembly and said second permanent magnet assembly such that said inner surface of said first permanent magnet assembly and said inner surface of said second permanent magnet assembly define an open region therebetween, said volume is disposed within said open region, said third position of said first permanent magnet assembly and said fourth position of second permanent magnet assembly are substantially equidistant from the center of said volume along said axis, said first permanent magnet assembly and said second permanent magnet assembly generate a second magnetic field superimposed on said first magnetic field to provide said substantially uniform magnetic field within said volume; and

controllably moving at least one of said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly relative to said volume for improving the homogeneity of the magnetic field within said volume.

123. The method according to claim 122, further including the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of said open magnetic apparatus, prior to said step of controllably moving.

124. The method according to claim 123, wherein said open high magnetic permeability yoke comprises at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.

125. A method for operating an open magnetic apparatus for producing a predetermined volume of substantially uniform magnetic field directed parallel to an axis of symmetry of said volume, the method comprising the steps of:

providing a first electromagnet assembly disposed at a first position along said axis, said first electromagnet assembly includes at least a first electromagnet coil, said at least first electromagnet coil is radially symmetric with respect to said axis;

providing a second electromagnet assembly disposed at a second position spaced apart from said first position of said first electromagnet assembly along said axis, said second electromagnet assembly includes at least a second electromagnet coil, said at least second electromagnet coil is radially symmetric with respect to said axis, said at least first electromagnet coil and said at least second electromagnet coil are substantially equidistant from the center of said volume, said first electromagnet assembly and said second electromagnet assembly are configured for generating a first magnetic field within said volume;

providing a first permanent magnet assembly positioned at a third position disposed between said first position and said second position along said axis, said first permanent magnet assembly has an inner surface facing said volume and an outer surface facing said first electromagnet assembly, said first permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets are radially symmetric with respect to said axis;

providing a second permanent magnet assembly opposed to said first permanent magnet assembly, said second permanent magnet assembly is positioned at a fourth position spaced apart from said third position along said axis, said second permanent magnet assembly has an inner surface facing said volume and an outer surface facing said second electromagnet assembly, said second permanent magnet assembly includes at least two coaxial permanent magnets, said at least two permanent magnets of said second permanent magnet assembly are radially symmetric with respect to said axis;

positioning said first permanent magnet assembly and said second permanent magnet assembly such that said inner surface of said first permanent magnet assembly and said inner surface of said second permanent magnet assembly define an open region therebetween, said volume is disposed within said open region, said third position of said first permanent magnet assembly and said fourth position of second permanent magnet assembly are substantially equidistant from the center of said volume along said axis, said first permanent magnet assembly and said second permanent magnet assembly generate a second magnetic field superimposed on said first magnetic field to provide said substantially uniform magnetic field within said volume; and

electrically energizing said first electromagnet assembly and said second electromagnet assembly to provide said first magnetic field within said volume, wherein said first magnetic field is superimposed on said second magnetic field to provide said substantially uniform magnetic field within said volume.

126. The method according to claim 125, further including the step of providing an open high magnetic permeability yoke configured for closing the magnetic field lines of said open magnetic apparatus.

127. The method according to claim 126, wherein said open high magnetic permeability yoke comprises at least one material selected from a ferromagnetic material, a high magnetic permeability material, soft iron, a nickel-iron containing alloy, a silicon-iron containing alloy, low carbon iron, and combinations thereof.

128. The method according to claim 126 further including the step of controllably moving at least one of said first electromagnet assembly, said second electromagnet assembly, said first permanent magnet assembly, and said second permanent magnet assembly relative to said high magnetic permeability yoke for improving the homogeneity of the magnetic field within said volume.