MAGNET ASSEMBLY COMPRISING CLOSED SUPERCONDUCTING HTS SHIMS

Abstract

A magnet assembly in a magnetic resonance apparatus includes a cryostat and a superconducting main field magnet coil system arranged therein for generating a magnetic field in the direction of a z-axis in a working volume. The magnet assembly includes a shim device arranged inside the cryostat for adjusting the spatial variation or homogeneity of the magnetic field generated in the working volume by the magnet coil system. The shim device comprises at least one closed superconducting shim conductor path having an HTS layer. The HTS layer forms a surface that is geometrically developable such that unwrapping onto a plane changes the geodesic distance between any two points on the surface formed by the HTS layer by no more than 10%. The inner and/or outer contour of the geometrical development of the HTS layer describes a non-convex curve.

Claims

1. A magnet assembly in a magnetic resonance apparatus comprising: a cryostat; a superconducting main field magnet coil system positioned in the cryostat for generating a magnetic field along a z-axis in a working volume centered around z=0 on the z-axis; and a shim device positioned inside the cryostat for adjusting a homogeneity of the magnetic field generated in the working volume by the main field magnet coil system, the shim device comprising at least one shim conductor path having a high temperature superconductor (HTS) layer in a closed loop with a winding number of 0 about the z-axis wherein the HTS layer is geometrically developable such that unwrapping the HTS layer onto a plane changes the geodesic distance between any two points on the HTS layer by no more than 10%, and wherein an inner contour or an outer contour of the geometrical development of the HTS layer describes a non-convex curve.

2. The magnet assembly according to claim 1, wherein, the shim conductor path generates a shimming magnetic field with an axial component B.sub.z(r, z, ) with respect to a cylindrical coordinate system about the z-axis, the shimming magnetic field comprising primarily a single field gradient represented in a basis of spherical harmonics about z=0, in addition to a non-negative field gradient of zero degree.

3. The magnet assembly according to claim 1, wherein the single field gradient corresponds to one of the spherical harmonics z.sup.2r.sup.2/2, z.sup.33/2.Math.zr.sup.2, r.Math.cos(), r.Math.sin(), 3rz.Math.cos(), 3rz.Math.sin(), 3r.sup.2.Math.cos(2) or 3r.sup.2.Math.sin(2).

4. The magnet assembly according to claim 1, wherein the shim conductor path passes through two spaces that are separated from one another by a plane perpendicular to the z-axis, and wherein the magnetic fields generated by conductor portions that extend in the two spaces both having a z-component of the same sign at location z=0 on the z-axis.

5. The magnet assembly according to claim 1, wherein the shim conductor path passes through an even number of spaces separated from one another by planes perpendicular to the z-axis, and wherein the magnetic fields generated by conductor portions that extend in each case in two adjacent spaces each having a z-component of opposite sign at location z=0 on the z-axis.

6. The magnet assembly according to claim 1, wherein the shim conductor path passes through four spaces that are separated from one another by a first plane that is perpendicular to the z-axis and by a second plane that contains the z-axis, and wherein the magnetic fields generated by first conductor portions that extend in each case in two spaces adjacent to one another on the first plane each having a z-component of the same sign at location z=0 on the z-axis, and wherein the magnetic fields generated by second conductor portions that extend in each case in two spaces adjacent to one another on the second plane each having a z-component of opposite sign at location z=0 on the z-axis.

7. The magnet assembly according to any claim 1, wherein the shim conductor path passes through four spaces that are separated from one another by a first plane that is perpendicular to the z-axis and by a second plane that contains the z-axis, and wherein the magnetic fields generated by conductor portions that extend in each case in two adjacent spaces each having a z-component of opposite sign at location z=0 on the z-axis.

8. The magnet assembly according to claim 1, wherein the shim conductor path passes through eight spaces that are separated from one another by a first plane that is perpendicular to the z-axis and by a second plane and a third plane that contain the z-axis and are perpendicular to one another, and wherein the magnetic fields generated by first conductor portions that extend in each case in two spaces adjacent to one another on the first plane each having a z-component of the same sign at location z=0 on the z-axis, and wherein the magnetic fields generated by second conductor portions that extend in each case in two spaces adjacent to one another on the second plane or third plane each having a z-component of opposite sign at location z=0 on the z-axis.

9. The magnet assembly according to claim 1, wherein, when projected on a cylinder about the z-axis, the shim conductor path comprises more than two conductor portions that extend in an azimuthal direction and are electrically interconnected by connecting portions extending in other directions.

10. The magnet assembly according to claim 1, wherein the shim conductor path is produced from an HTS strip conductor or from an HTS-coated film, the HTS material comprising a rare earth metal-Barium-Copper oxide (ReBCO) or a Bismuth-Strontium-Calcium-Copper oxide (BSCCO).

11. The magnet assembly of claim 10, wherein the ReBCO comprises Yttrium-Barium-Copper oxide (YBCO) or Gadolinium-Barium-Copper oxide (GdBCO).

12. The magnet assembly according to claim 1, wherein the shim device comprises a plurality of shim conductor paths that are substantially inductively decoupled from one another.

13. The magnet assembly according to claim 12, wherein the mutual inductance L.sub.12 between any two of the shim conductor paths is given by |L.sub.12|/{square root over (L.sub.1L.sub.2)}0.2, L.sub.1 and L.sub.2 being the self-inductance of the two shim conductor paths.

14. The magnet assembly according to claim 1, wherein, when projected on a cylinder about the z-axis, the shim conductor path overlaps or intersects with itself.

15. The magnet assembly of claim 14, wherein the shim conductor path is coiled cylindrically, in more than one layer, around the working volume, or such that the geometrically developable HTS layer exhibits at least one change in direction of revolution about the z-axis.

16. The magnet assembly according to claim 1, wherein the shim device comprises a plurality of shim conductor paths that are arranged so as to be radially above one another relative to the z-axis and extend axially and azimuthally in an identical manner.

17. The magnet assembly according to claim 1, wherein the shim conductor path includes a superconducting switch.

18. The magnet assembly according to claim 1, wherein the superconducting main field magnet coil system comprises coils made of an HTS conductor, and wherein the superconducting main field magnet coil system and the shim device are cooled by a cryocooler to a temperature of between 10 K and 80 K.

19. The magnet assembly according to claim 1, wherein the shim conductor path extends, with respect to the z-axis, radially inside the superconducting main field magnet coil system at least in part.

20. An active shim device comprising: at least one shim conductor path through a high temperature superconductor (HTS) layer in a closed loop that is geometrically developable such that unwrapping the HTS layer onto a plane changes the geodesic distance between any two points on the HTS layer by no more than 10%, wherein an inner contour or an outer contour of the geometrical development of the HTS layer describes a non-convex curve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention is illustrated in the drawings and is explained in greater detail with reference to the drawings.

[0038] FIG. 1 is a schematic view of a magnet assembly according to an example of the invention in a magnetic resonance apparatus.

[0039] FIG. 2A is a schematic view of a shim conductor path for producing an on-axis field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2.

[0040] FIG. 2B is a schematic view of a shim conductor path for producing an on-axis field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2.

[0041] FIG. 3 is a schematic view of a shim conductor path for producing an on-axis field gradient corresponding to the spherical harmonic z.sup.33/2.Math.zr.sup.2.

[0042] FIG. 4A is a schematic view of a shim conductor path for producing an off-axis field gradient corresponding to the spherical harmonic r.Math.sin().

[0043] FIG. 4B is a three-dimensional view of the assembly according to FIG. 4A.

[0044] FIG. 5A is a schematic view of a shim conductor path for producing an off-axis field gradient corresponding to the spherical harmonic 3r.Math.z.Math.sin().

[0045] FIG. 5B is a three-dimensional view of the assembly according to FIG. 5A.

[0046] FIG. 6 is a schematic view of a shim conductor path for producing an off-axis field gradient corresponding to the spherical harmonic 3r.sup.2.Math.cos(2).

[0047] FIG. 7A is a schematic view of a shim conductor path for producing an on-axis field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2.

[0048] FIG. 7B shows the assembly according to FIG. 7A after it has been folded along the middle.

[0049] FIG. 8 is a schematic view of a shim conductor path comprising a superconducting switch and current leads.

DETAILED DESCRIPTION

[0050] FIG. 1 schematically shows an example of the magnet assembly in a magnetic resonance apparatus. A main field magnet coil system 1 is arranged in a cryostat 2 and generates a magnetic field in the direction of a z-axis in a working volume 4 arranged around the point z=0 on the z-axis. A shim device 3 is arranged radially inside the main field magnet coil system 1 for adjusting the spatial variation or homogeneity of the magnetic field generated in the working volume 4 by the main field magnet coil system 1. The shim device 3 comprises at least one closed superconducting shim conductor path which has a winding number of 0 about the z-axis. In other words, the shim conductor path runs as many times clockwise around the z-axis as it does counterclockwise. The HTS layer of the shim conductor path forms a geometrically developable surface, the inner and/or outer contour of which describes a non-convex curve.

[0051] In order to produce a closed superconducting shim conductor path in the magnet assembly according to the invention, a closed conductor path may be blanked out of an HTS-coated film for example.

[0052] In order to produce particular field gradients, a shim conductor path of the magnet assembly may also be wound around the working volume 4 in a plurality of layers. FIG. 2A and FIG. 2B schematically show examples of shim conductor paths 5 and 5, respectively, that produce a field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2 when the shim conductor paths are coiled cylindrically, in two layers, around the working volume 4 and carry an electrical current.

[0053] FIG. 3 shows, as an example, a shim conductor path 5 that has four azimuthally extending portions. The shim conductor path 5 essentially generates, in a working volume 4, a magnetic field having an on-axis field gradient corresponding to the spherical harmonic z.sup.33/2.Math.zr.sup.2 when the shim is coiled cylindrically around the working volume 4 and carries an electrical current. While a single closed superconducting conductor path comprising an HTS film is sufficient here, an analogous shim according to the prior art requires at least two rectangular closed superconducting conductor paths in order to produce a similar field distribution.

[0054] The assembly according to the invention also comprises HTS shims for producing off-axis gradients, as shown, for example, in FIG. 4A, FIG. 4B, FIG. 5A, FIG. 5B, and FIG. 6. Due to the non-convex course of the geometrical development of the HTS layer (e.g., as depicted in FIG. 4A, FIG. 5A, and FIG. 6), it is possible to form shims of this kind each having a single closed superconducting conductor path 5, 5 and 5. FIG. 4A and FIG. 4B each show a conductor path 5 for producing a field gradient corresponding to the spherical harmonic r.Math.sin() when the shim conductor path 5 is coiled cylindrically, in two layers, around the working volume 4 and carries an electrical current. FIG. 5A and FIG. 5B each show a shim conductor path 5 for producing a field gradient corresponding to the spherical harmonic 3r.Math.z.Math.sin() when the shim conductor path 5 is coiled cylindrically around the working volume 4 and carries an electrical current. FIG. 4B and FIG. 5B are three-dimensional views of the shim conductor paths of the corresponding FIG. 4A and FIG. 5A. The shim conductor path 5 in FIG. 6 is arranged cylindrically around the z-axis in two layers and produces a field gradient corresponding to the spherical harmonic 3r.sup.2.Math.cos(2) when the shim conductor path 5 carries an electrical current.

[0055] FIG. 7A and FIG. 7B show a shim conductor path 5 of a magnet assembly, in which the shim conductor path 5 overlaps with itself when projected on a cylinder. FIG. 7A shows the complete geometrical development of the conductor path 5 onto a plane, and FIG. 7B shows the conductor path 5 that has been folded or bent back in the middle, in the azimuthal direction, in the manner in which it is wound around the z-axis on a cylinder. In this way, in this example a shim having a field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2 is achieved using a single symmetrical conductor path.

[0056] The two closed superconducting conductor paths 5 and 5 shown in FIG. 2A and FIG. 2B may also be used together, as a shim for producing a field gradient corresponding to the spherical harmonic z.sup.2r.sup.2/2, for example. For reasons of symmetry, a shim of this kind does not produce field gradients corresponding to spherical harmonics of odd degree, and, simply on account of its shape, each of the two conductor paths 5 and 5 is largely inductively decoupled from shims that produce on-axis field gradients corresponding to spherical harmonics of odd degree.

[0057] In one example, the following geometry for the shim conductor path 5 on a constant radius of 35 mm around the working volume 4 is characterized: The two short azimuthally extending conductor portions of the conductor path 5 each have a z-coordinate of 10 mm, and the long azimuthally extending conductor portion, which is wound twice around the working volume, has a z-coordinate of 36 mm. Furthermore, the width of the conductor path 5 is 4 mm. At a current of 100 A, the on-axis field gradients up to the 8.sup.th degree at location z=0 on the z-axis amount to:

TABLE-US-00001 gradient A.sub.n0 strength (G/cm.sup.n) A.sub.00 19.7 A.sub.10 5.21 A.sub.20 3.38 A.sub.30 0.101 A.sub.40 0.0483 A.sub.50 0.0123 A.sub.60 0.0171 A.sub.70 0.000262 A.sub.80 0.00227

[0058] Apart from the homogenous field contribution A.sub.00, the shim primarily produces a gradient A.sub.10 corresponding to the spherical harmonic z, and a gradient A.sub.20 corresponding to the spherical harmonic z.sup.2r.sup.2/2. Because of the cylindrical symmetry of the shim, off-axis gradients do not occur.

[0059] A shim of this kind can be produced from a 50 mm-wide HTS-coated film. If even wider HTS-coated films are available, the long azimuthally extending conductor portion can be selected so as to be even further from z=0, in order to reduce the gradients of odd degree. Moreover, the inductive coupling to other shims can also be reduced in this way. If the long conductor portion is, for example, at z=42 mm instead of at z=36 mm, the coupling coefficient |L.sub.12|/{square root over (L.sub.1L.sub.2)} to a z-shim reduces from 0.30 to 0.18. In this example, a single-layer conductor path having a rectangular development is assumed as the z-shim, the azimuthal conductor portions of which are located at z=30 mm on a radius of 35 mm.

[0060] Next, a shim consisting of two conductor paths 5 and 5 will be considered. This shim, for reasons of symmetry, does not produce gradients of odd degree. The geometry is as follows: The two short azimuthally extending conductor portions of the shim conductor paths 5 and 5 each have a z-coordinate of 10 mm, and the long azimuthally extending conductor portions have a z-coordinate of 36 mm or +36 mm. The width of the shim conductor paths 5 and 5 is again 4 mm. At a current of 100 A, the shim produces the following field gradients up to the 8.sup.th degree:

TABLE-US-00002 gradient A.sub.n0 strength (G/cm.sup.n) A.sub.00 39.4 A.sub.20 6.76 A.sub.40 0.0968 A.sub.60 0.0342 A.sub.80 0.00453

[0061] FIG. 8 schematically shows, as an example, a shim conductor path 5 of a magnet assembly comprising a superconducting switch 6. In order to charge the shim conductor path 5 with an electrical current, the superconducting switch 6 can be opened, in that it is made to be normally conductive by heating, for example. The current may then be introduced into the conductor path 5 via the current leads 7. After the superconducting switch 6 has been closed, the current flows persistently in the closed superconducting shim conductor path 5, and the power supply via the current leads 7 may be turned off.

LIST OF REFERENCE SIGNS

[0062] 1 main field magnet coil system [0063] 2 cryostat [0064] 3 shim device [0065] 4 working volume [0066] 5, 5, 5, 5, 5, 5, 5 shim conductor path comprising an HTS layer [0067] 6 superconducting switch [0068] 7 current leads