Gradient coil with off center sweet spot for magnetic resonance imaging

• US 5,278,504 A
• Filed: 03/11/1993
• Issued: 01/11/1994
• Est. Priority Date: 06/16/1989
• Status: Expired due to Term

• Current Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Original Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Inventors: Petropoulos, Labros, Hajnal, J. V., Hall, A. S., Morich, Michael A., Patrick, John L.
• Primary Examiner(s): Tokar, Michael J.
• Application Number:
  US08/030,601
• Time in Patent Office: 306 Days
• Field of Search: 324/300, 324/307, 324/309, 324/313, 324/318, 324/319, 324/320, 324/322, 128/653.2, 128/653.5, 335/219
• US Class Current: 324/318
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils

Abstract

An examination region (12) is defined along a z-axis offset from a geometric center (30) of a gradient coil assembly (20). Cylinders of a non-conductive, non-magnetic material support x, y, and z-gradient coils (24, 26, 22) for causing orthogonal magnetic field gradients through the offset examination region. The z-gradient coil (FIG. 2 ) includes a plurality of distributed loop arrays with a winding pattern selected to cause a region of linear magnetic field gradients in the z-direction in a region offset toward a cylinder first end (32). The x and y-gradient coils each include two pairs of oppositely disposed windings (FIG. 3 ) which include a pair of inner spirals (96, 98) offset towards the first end of the cylinder and an outer spiral (90) extending therearound. The outer spiral bows in (92) toward the cylinder first end and fans out (94) toward the second end. In this manner, the x, y, and z-gradient coils create linear magnetic field gradients along x, y, and z-directions which have a region of primary linearity offset from their geometric center.

Claims

1. A magnetic resonance imaging apparatus comprising:

a main magnetic field means for generating a main magnetic field through an examination region;

a magnetic resonance excitation means for selectively exciting magnetic resonance in dipoles disposed in the examination region;

a magnetic field gradient means for generating magnetic field gradients across the examination region, the gradient magnetic field means including;

an x-gradient coil disposed generally along a cylindrical surface extending circumferentially around a central axis for generating linear magnetic field gradients across the examination region in a first direction transverse to the central axis,

a y-gradient coil disposed generally along a cylindrical surface extending circumferentially around the central axis for generating linear magnetic field gradients across the examination region in a second direction transverse to the central axis,

a z-gradient coil disposed generally along a cylindrical surface extending circumferentially around a central axis for generating linear magnetic field gradients across the examination region in a third direction generally parallel to the central axis,

the x, y, and z-gradient coils each having a geometric center midway between end windings thereof along the central axis, the examination region being displaced along the central axis from at least the x- and y-gradient coil geometric centers;

a magnetic resonance signal receiving means for receiving magnetic resonance signals from the resonating dipoles; and

,

a processing means for processing the magnetic resonance signals.

2. The apparatus as set forth in claim 1 wherein the z-gradient coil geometric center is displaced along the central axis from the examination region.

3. The apparatus as set forth in claim 2 wherein the z gradient coil includes:

a first loop array disposed adjacent a first end of the z-gradient coil closest to the examination region;

a second loop array disposed adjacent the first loop array;

a third loop array disposed adjacent the second loop array;

a fourth loop array region disposed adjacent the third loop array;

a fifth loop array disposed adjacent the fourth loop array region; and

,

a coil current supply means for supplying currents through the first, second, third, and fifth loop arrays such that (a) the first loop array has a current density primarily in a first circumferential direction, (b) the second loop array has a net current flux in the first circumferential direction, the second loop array having a smaller net current flux density than the first loop array, (c) the third loop array has current fluxes in the first circumferential direction and in a second circumferential direction opposite to the first,.and (d) the fifth loop array has a net current flux in the second circumferential direction.

4. The apparatus as set forth in claim 3 wherein the z-gradient coil further includes a sixth loop array disposed adjacent the fifth loop array, the coil current supply means supplying a net current flux through the sixth loop array in the second direction.

5. The apparatus as set forth in claim 4 wherein the first, third, and fifth loop arrays have a higher current flux density than the second and sixth current loop arrays.

6. The apparatus as set forth in claim 3 wherein the coil current supply means further supplies current through a fraction of windings of the first loop array in the second circumferential direction.

10. The apparatus as set forth in claim 1 wherein:

the x-gradient coil includes a cylindrical non-magnetic, non-conductive former on which a pair of distributed windings are oppositely disposed; and

the y-gradient coil includes a cylindrical non-magnetic, non-conductive former on which a pair of oppositely disposed distributed windings are mounted, the x and y-gradient coils being telescopically received and oriented substantially 90°

offset from each other.

11. The apparatus as set forth in claim 10 wherein the first and second cylindrical formers are generally elliptical in cross-section.

12. The apparatus as set forth in claim 1 wherein at least one of the x and y-gradient coils include a pair of coils positioned around the imaging region diametrically opposite to one another, each of the coils having its conductors axially spaced at one axial end of the coil.

13. The apparatus as set forth in claim 12 wherein at least one of the x and y-gradient coils includes a second pair of coils disposed diametrically opposite from one another and axially overlapping said first pair of coils.

14. The apparatus as set forth in claim 12 wherein the coils of the first pair of coils have more turns than the coils of the second pair of coils.

15. The apparatus as set forth in claim 13 wherein the coils are saddle coils of generally rectangular shape.

16. The apparatus as set forth in claim 1 wherein the x and y-gradient coils tend to exert a rotational torque relative to the central axis and further including:

a coil means for exerting a rotational torque with is substantially equal and opposite to the x and y-gradient coils rotational torque.

17. The apparatus is set forth in claim 16 wherein the equal and opposite rotational torque coil means includes an array of coils disposed adjacent an end of the x and y-gradient coils most remote from the examination region.

7. The apparatus as set forth in claim wherein at least one of the x and y-gradient coils includes a mirror image pair of distributed windings on opposite sides of the cylindrical surface, each distributed winding including:

a first circumferential loop array extending peripherally therearound;

a second loop array disposed within the peripheral first loop array;

a third loop array disposed within the peripheral first array between the second loop array and a first end.

8. The apparatus as set forth in claim 7 wherein the first loop array bellies-in adjacent the third loop array and fans-out adjacent a second end of the coil.

9. The apparatus as set forth in claim 7 wherein the z magnetic field gradient includes:

a first loop array disposed adjacent a first end of the z-gradient coil closest to the examination region, the first loop array having a net current flow in a first direction;

a second loop array disposed adjacent the first loop array;

a third loop array disposed adjacent the second loop array;

a fourth loop array region disposed adjacent the third loop array;

a fifth loop array disposed adjacent the fourth loop array region; and

a current supply means for supplying currents through the first, second, third, and fifth loop arrays such that the first loop array has a current density primarily in a first circumferential direction, the second loop array has a smaller net current flux than the first loop array in the first circumferential direction, the third loop array has current fluxes in the first circumferential direction and in a second circumferential direction opposite to the first, and the fifth loop array has a net current flux in the second circumferential direction.

18. A gradient coil for a magnetic resonance apparatus, the gradient coil comprising:

a first pair of saddle coils positioned around an imaging volume diametrically opposite one another, the first coil pair having its conductors axially spaced at one axial end of the gradient coil;

a second pair of saddle coils positioned around the imaging volume diametrically opposite one another and overlapping said first pair of saddle coils axially.

19. The apparatus as set forth in claim 18 wherein the saddle coils tend to exert a torque and further including:

a coil means for exerting a torque which is substantially equal and opposite to the saddle coil torque.

20. A gradient coil for a magnetic resonance apparatus, the gradient coil comprising:

a non-magnetic, non-conductive cylindrical former;

a first loop array extending circumferentially around the former adjacent a first end thereof;

a second loop array disposed adjacent the first loop array;

a third loop array disposed adjacent the second loop array;

a fourth loop array region disposed adjacent the third loop array;

a fifth loop array, the first, third, and fifth loop arrays having denser winding pattern than the second loop array and the fourth loop array region.

21. The gradient coil assembly as set forth in claim 20 further including a current supply for supplying current to the loop arrays such that the first and second loop arrays have a primary current flow in a first circumferential direction, said fifth loop array has a primary current flow in a second circumferential direction opposite the first, and the third loop array has current flows in both the first and the second circumferential directions.

22. The apparatus as set forth in claim 20 further including a current supply means for supplying current to the loop arrays, the current supply means supplying current to the first loop array such that a majority of current flows in a first circumferential direction and a minority of current flows closely adjacent the former first end in a second circumferential direction opposite to the first circumferential direction.

23. A gradient coil for a magnetic resonance apparatus, the gradient coil comprising:

a non-magnetic, non-conductive cylindrical former;

a pair of windings disposed opposite each other across the cylindrical former, each of the windings having a first, outer spiral winding which bows in adjacent a first end of the cylindrical former and fans out adjacent a second end of the cylindrical former, a second spiral winding and a third spiral winding, the second and third spiral windings being disposed adjacent to each other and within the first spiral winding, the second and third spiral windings being offset towards the first end of the former.

24. A magnetic resonance method comprising:

passing a main magnetic field through an examination region;

positioning a gradient coil assembly asymmetrically relative to the examination region;

exciting dipoles in the examination region to magnetic resonance;

passing currents along a first pair of double spiral paths defined by the gradient coil assembly with an asymmetric current density which is denser adjacent the examination region to generate gradient magnetic fields along a first direction through the examination region;

passing currents along a second pair of double spiral paths defined by the gradient coil assembly which are offset relative to the first pair with an asymmetric current density which is denser adjacent the examination region to generate gradient magnetic fields along a second direction through the examination region;

passing currents along a multiplicity of circumferential currents paths defined by the gradient coil assembly with an asymmetric current density which is denser adjacent the examination region to generate gradient magnetic fields along a third direction through the examination region.

25. A coil set in a magnetic resonance apparatus for imposing on a magnetic field in a cylindrical volume in which an object to be examined is placed in use of the apparatus a gradient in a direction transverse to the axis of the volume, the coil set effectively comprising a pair of coils correspondingly positioned around said volume diametrically opposite one another, each of said coils having an axial end portion in which the conductors of the coil are spaced apart axially, and which is substantially radially aligned with said volume.

26. The coil set according to claim 25 wherein said conductors in said axial end portion of each coil are positioned in two groups spaced apart axially and include a first pair of identical coils positioned around said volume diametrically opposite one another and a second pair of identical coils positioned around said volume diametrically opposite one another and overlapping said first pair of coils axially.

27. The coil set according to claim 26 wherein each said coil is a saddle coil of rectangular shape.

28. The coil set according to claim 26 in combination with a second coil set for imposing a gradient on said magnetic field in the direction of the axis of said volume comprising a Maxwell coil pair disposed coaxially with said volume in axially spaced relationship so as to lie one on either side of said axial end portions.

29. The coil set according to claim 25 in combination with a bucking coil arrangement which when energized produces forces which counteract forces arising from energization of said coil set.

30. A magnetic resonance apparatus comprising:

a main magnetic field means for generating a static magnetic field through an examination region;

a magnetic resonance excitation means for selectively exciting magnetic resonance in dipoles disposed in the examination region;

a magnetic field gradient means for generating magnetic field gradients across the examination region, the gradient magnetic field means including a z-gradient coil including;

a means for providing a distribution of winding currents flowing generally peripherally around a z-axis, the winding currents being asymmetric with respect to a geometric center of the z-gradient coil, such that the examination region is displaced along a central axis of the z-gradient coil from the geometric center of the z-gradient coil;

a magnetic resonance signal receiving means for receiving magnetic resonance signals from the resonating dipoles; and

,

a processing means for processing the magnetic resonance signals.

31. A z-gradient coil for generating magnetic field gradients along a z-axis through an examination region of a magnetic resonance apparatus, the z-gradient coil comprising:

a non-magnetic, non-conductive cylindrical former mountable parallel to the z-axis;

an electrically conductive winding means extending around the former for carrying a distribution of winding currents which are not mirror image symmetric about a center thereof such that the examination region is displaced along the z-axis from said geometric center.

32. A magnetic resonance apparatus comprising:

a static magnetic field means for generating a static magnetic field through an examination region;

a magnetic resonance excitation means for selectively exciting magnetic resonance in dipoles disposed in the examination region;

x, y, and z-gradient coils which have windings surrounding a central axis through the examination region, the windings extending between opposite end windings and defining a geometric center midway between said end windings, the examination region being displaced along the central axis form the geometric center;

a magnetic resonance signal receiving means for receiving magnetic resonance signals from the resonating dipoles; and

a processing means for processing the magnetic resonance signals.