LOW-STRAY-FIELD PERMANENT MAGNET ARRANGEMENT FOR MR APPARATUSES

Abstract

A magnetic resonance arrangement with a permanent magnet system and having magnet elements, pole piece elements and yoke elements of magnetic material arranged cylinder-symmetrically with respect to the z axis. The yoke elements have a first lid (11′) and a second lid (11″) and also a hollow cylindrical drum (12) arranged axially between the lids. The yoke elements enclose the measuring volume in the axial and radial direction. The magnet elements each include a pair of cylinder-symmetrical axial magnets (13′, 13″) and also radial magnet rings (14′, 14″). The axial magnets are each arranged axially adjoining the lids and are arranged radially within the radial magnet rings and respectively axially further away from the measuring volume than the radial magnet rings. The outer diameter of the axial magnets is less than or equal to the inner diameter of the radial magnet rings.

Claims

1. Magnet arrangement in a magnetic resonance apparatus with a permanent magnet system configured to generate a homogeneous magnetic field in a z-axis direction in a measuring volume, the permanent magnet system comprising: magnet elements, pole piece elements and yoke elements of magnetic material arranged cylinder-symmetrically with respect to the z axis, wherein the yoke elements comprise a first lid, a second lid and at least one hollow cylindrical drum arranged axially between the lids with respect to the z axis, wherein the yoke elements enclose the measuring volume both axially and radially, wherein the magnet elements each comprise at least one pair of cylinder-symmetrical axial magnets and one pair of cylinder-symmetrical radial magnet rings, wherein the axial magnets are each arranged axially adjoining a side of the first lid facing towards the measuring volume and/or adjoining a side of the second lid facing towards the measuring volume, wherein the axial magnets are arranged radially within the radial magnet rings and respectively axially further away from the measuring volume than the radial magnet rings, and wherein: external diameter of the axial magnets≦internal diameter of the radial magnet rings.

2. Magnet arrangement according to claim 1, wherein the yoke elements, the magnet elements and the pole piece elements each have circular cylinder symmetry or cylinder symmetry with polygonal cross-sectional surfaces perpendicular to the z axis.

3. Magnet arrangement according to claim 1, wherein the yoke elements comprise a plurality of hollow cylindrical drums arranged axially one alongside another.

4. Magnet arrangement according to claim 1, wherein the radial magnet rings and/or the axial magnets form segments or wedges that extend radially and/or parallel to a circumference of the drum in an azimuthal direction.

5. Magnet arrangement according to claim 1, wherein the at least one hollow cylindrical drum of the yoke elements has on an outer jacket surface thereof at least one circumferential groove and/or at least one circumferential step, and/or has a stepped profile on an inner surface thereof.

6. Magnet arrangement according to claim 1, wherein the at least one hollow cylindrical drum of the yoke elements has at least one aperture and/or at least two access openings arranged radially opposite one another with respect to the z axis.

7. Magnet arrangement according to claim 1, wherein the first lid and/or the second lid of the yoke elements is discus-shaped or tapers conically in an axial direction away from the measuring volume.

8. Magnet arrangement according to claim 1, wherein the first lid and/or the second lid of the yoke elements has, along the z axis, a centring portion protruding in an axial direction towards the measuring volume.

9. Magnet arrangement according to claim 1, wherein the first lid and/or the second lid of the yoke elements has a side protruding towards the measuring volume, on which side is formed an annular groove, a step and/or a shoulder as a limit stop or for securing the axial magnets.

10. Magnet arrangement according to claim 1, wherein the pole piece elements are configured axially in two parts with a pole plate and a pole cap, or in three parts with a pole plate, a pole cap and a pole ring.

11. Magnet arrangement according to claim 1, wherein the pole piece elements are fixed respectively adjacently on the first lid or on the second lid of the yoke elements by at least one non-magnetic fastening element.

12. Magnet arrangement according to claim 11, wherein the at least one non-magnetic fastening element comprises a pin, a bolt or a screw.

13. Magnet arrangement according to claim 1, wherein at least one adjusting element is provided on an abutment face between at least one of the lids and the drum, respectively, with which an abutment angle and/or a distance between the lids and the drum, is adjustable, to provide a homogenized magnetic field.

14. Magnet arrangement according to claim 13, wherein the at least one adjusting element comprises a plurality of adjusting screws, and wherein an abutment angle and/or a distance between the lids and the drum is adjustable in a range≦5 mm.

15. Magnet arrangement according to claim 1, wherein the radial magnet rings are arranged on an inner surface of the hollow cylindrical drum that faces towards the measuring volume.

16. Method for producing a magnet arrangement according to claim 1, comprising: making available the lids and the at least one hollow cylindrical drum of the yoke elements, arranging the radial magnet rings on an inner surface of the at least one drum, fixing the axial magnets and the pole pieces on the first lid and on the second lid of the yoke elements, moving the axial magnets, fixed on the lids, together with the pole pieces into the respective radial magnet rings, which are located radially within the at least one drum, such that the lids bear on the at least one drum of the yoke elements, and the axial magnets are arranged at a defined position at least partially radially within the radial magnet rings.

17. Method according to claim 16, further comprising homogenizing the magnetic field by tilting the first lid and/or the second lid relative to the z axis and/or rotating the first lid and/or the second lid about the z axis.

18. Method according to claim 16, further comprising mounting the radial magnet rings on an inner face of the at least one hollow cylindrical drum at positions predefined by projections or steps.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The invention is shown in the drawing and is explained in more detail on the basis of illustrative embodiments. In the drawing:

[0047] FIG. 1A shows a schematic perspective half-section view of a magnet arrangement according to the invention;

[0048] FIG. 1B shows the same as FIG. 1A, but without pole pieces;

[0049] FIG. 1C shows the same as FIG. 1B, but without lid assemblies;

[0050] FIG. 2 shows lid assembles as in FIG. 1A;

[0051] FIG. 3A shows yoke assemblies with drum and lids, but without magnets;

[0052] FIG. 3B shows the same as FIG. 3A, but with a plurality of drum assemblies;

[0053] FIG. 4A shows a half-sectioned drum assembly in a cylindrical configuration;

[0054] FIG. 4B shows the same as FIG. 4A, but with a conical edge shape;

[0055] FIG. 4C shows the same as FIG. 4A, but with two circumferential grooves;

[0056] FIG. 4D shows the same as FIG. 4A, but with two circumferential steps;

[0057] FIG. 5A shows a complete drum assembly of cylindrical configuration with diametrically arranged rectangular apertures;

[0058] FIG. 5B shows the same as FIG. 5A, but with apertures having slit-shaped cutouts;

[0059] FIG. 5C shows the same as FIG. 5A, but with H-shaped apertures;

[0060] FIG. 5D shows the same as FIG. 5A, but with a wider configuration of the apertures;

[0061] FIG. 6A shows a segmented magnet ring assembly with 8× division along the circumference;

[0062] FIG. 6B shows the same as FIG. 6A, but with 16× division along the circumference and 4× axial division;

[0063] FIG. 7A shows a non-segmented magnet ring assembly with axial magnetization indicated by arrows;

[0064] FIG. 7B shows the same as FIG. 7A, but with 16× segmentation along the circumference;

[0065] FIG. 7C shows the same as FIG. 7B, but with radial magnetization indicated by arrows;

[0066] FIG. 8A shows a half-sectioned drum assembly with radial magnet segments;

[0067] FIG. 8B shows the same as FIG. 1A, but with magnetizations of the magnet segments indicated by arrows;

[0068] FIG. 9 shows the same as FIG. 8B, but in a sectional front view, and with a central fastening element and indicated measuring volume;

[0069] FIG. 10A shows as half-sectioned pole piece element in a one-part design;

[0070] FIG. 10B shows the same as FIG. 10A, but in a two-part design with separate pole ring;

[0071] FIG. 10C shows the same as FIG. 10B, but in a three-part design with an additional separate pole plate; and

[0072] FIG. 11 shows the same as FIGS. 1A-1C, but with fastening elements and adjusting elements for adjusting the lid elements.

DETAILED DESCRIPTION

[0073] The magnet arrangement according to the invention, as shown schematically in a perspective half section in FIGS. 1A and 11, has its main use as a constituent part of a magnetic resonance apparatus with a permanent magnet system for generating a homogeneous magnetic field in the direction of a z axis in a (shown explicitly in FIG. 9) measuring volume MV. The permanent magnet system has magnet elements, pole piece elements and yoke elements of magnetic material that are arranged with cylinder symmetry with respect to the z axis. According to the invention, the magnet arrangement is characterized in that the yoke elements comprise a first lid 11′ and a second lid 11″ and also at least one hollow cylindrical drum 12; 12′,12″,12″; 22a; 22b; 22c; 22d arranged axially between the lids 11′, 11″ with respect to the z axis, for which different advantageous embodiments are also shown in FIGS. 3A, 3B, 4A-4D, 5A, and 5D.

[0074] The yoke elements enclose the measuring volume MV both in the axial direction and in the radial direction. The magnet elements comprise at least one pair of cylinder-symmetrical axial magnets 13′, 13″ and a pair of cylinder-symmetrical radial magnet rings 14′, 14″, wherein the axial magnets 13′, 13″ are arranged axially adjoining the side of the first lid 11′ and adjoining the side of the second lid 11″ facing towards the measuring volume MV. The axial magnets 13′, 13″ are arranged radially within the radial magnet rings 14′, 14″ and are each axially further away from the measuring volume MV than are the radial magnet rings 14′, 14″. The external diameter of the axial magnets 13′, 13″ is smaller than or equal to the internal diameter of the radial magnet rings 14′, 14″.

[0075] The yoke elements, the magnet elements and the pole piece elements can each have circular cylinder symmetry or cylinder symmetry with polygonal cross-sectional surfaces perpendicular to the z axis.

[0076] The radial magnet rings 14′, 14″ and/or the axial magnets 13′, 13″ can be divided segment-like or wedge-like in the radial direction and/or parallel to the drum circumference in the azimuthal direction, as can be seen clearly in FIGS. 1A to 1C, 6A to 8B and 11.

[0077] It will also be seen from FIGS. 1A to 1C, 3A, 3B, 4C and 4D that the hollow cylindrical drums 12; 12′,12″,12″′; 22c; 22d of the yoke elements can have, on their outer jacket surfaces, at least one circumferential groove 12a and/or at least one circumferential step 12b and/or can have a step profile 12c on their inner surfaces.

[0078] It will likewise be clearly seen from FIGS. 1A to 2, 4C and 4D that the first lid 11′ and/or the second lid 11″ of the yoke elements can have, on its side protruding towards the measuring volume MV, an annular groove, a step 11a′; 11a″ or a shoulder 11b′; 11b″ as a limit stop or for holding the axial segments 13′, 13″.

[0079] As is shown in FIG. 2 for example, the first lid 11′ and/or the second lid 11″ of the yoke elements can be discus-shaped or can taper conically in the axial direction away from the measuring volume MV.

[0080] In embodiments of the invention that are not specially depicted in the drawing, the first lid 11′ and/or the second lid 11″ of the yoke elements can have, along the z axis, a centring portion that protrudes in the axial direction towards the measuring volume MV.

[0081] As is shown in FIG. 3A, the yoke elements can have a single hollow cylindrical drum 12. However, as is shown in FIG. 3B, they can also have a plurality of hollow cylindrical drums 12′, 12″, 12″′ arranged one after another in the axial direction.

[0082] FIGS. 3A, 3B and 5A to 5D show embodiments of the invention in which the hollow cylindrical drums 12; 12′,12″,12′″ of the yoke elements each have apertures 121; 12ii; 12i′; 12ii′; 12i″; 12ii″; 12i″′; 12ii′ or access openings lying diametrically opposite each other in pairs with respect to the z axis, which apertures or access openings can have different geometrical configurations.

[0083] FIGS. 6A and 6B illustrate the many possibilities of a segmented configuration of the magnet elements that can be used in a magnet arrangement according to the invention. Thus, FIG. 6A shows a segmented magnet ring assembly with 8× division along the circumference, while FIG. 6B shows an assembly with 16× division along the circumference and 4× axial division.

[0084] FIGS. 7A to 8B each show a magnet ring assembly with magnetization indicated by arrows, specifically: [0085] FIG. 7A shows a non-segmented magnet ring assembly with axial magnetization, [0086] FIG. 7B shows an assembly with 16× segmentation along the circumference and axial magnetization, [0087] FIG. 7C shows an assembly with 16× segmentation along the circumference and radial magnetization, [0088] FIG. 8A shows a drum assembly with radial magnet segments; and [0089] FIG. 8B shows magnet segments both with axial and also with radial magnetization.

[0090] FIG. 9 moreover shows a sectional front view of an arrangement according to the invention with an indicated measuring volume and with magnetization arrows that are continued schematically to magnetic flux lines. It can also be seen that the pole piece elements are fixed on the respectively adjacent first lid 11′ or second lid 11″ of the yoke elements by a central fastening element 16′, 16″ of non-magnetic material, in particular by a pin, a bolt or a screw.

[0091] FIG. 10A shows a half-sectioned pole piece element in a one-part design.

[0092] FIG. 10B shows a (likewise half-sectioned) pole piece element in a two-part design with a separate pole ring 15c.

[0093] FIG. 10C shows a three-part design of a pole piece element with a pole plate 15a, a pole cap 15b and an additional pole rings 15c.

[0094] FIG. 11, finally, shows a magnet arrangement according to the invention as has also already been shown in FIG. 1. Moreover, however, the magnet arrangement also contains fastening elements and adjusting elements for adjusting the lid elements. In particular, on the abutment surfaces between the lids 11′, 11″ and the drum 12, 12″, respectively, a plurality of adjusting elements 17′, 17″ in the form of adjusting screws are provided, with which the abutment angle and/or the distance between the lids 11′, 11″ and the drum 12, 12″, respectively, preferably in the range of ≦1 mm, can be adjusted and, consequently, the magnetic field can be homogenized. In this case, it is possible to use adjusting screws 17′ which can be equipped for example with a hexagonal head and with which the lids 11′, 11″ can be raised by screwing these adjusting screws 17′ into a corresponding thread in the respective lid 11′, 11″. By using further screws 17″, which can be equipped for example with an Allen socket, the lids 11′, 11″ are braced against the respective drum 12, 12″.

[0095] To produce a magnet arrangement according to the invention, the following method can be used: [0096] making available the lids 11′, 11″ and hollow cylindrical drum(s) 12; 12′,12″,12′″; 22a; 22b; 22c; 22d of the yoke elements [0097] arranging the radial magnet rings 14′, 14″ on the inner surface of the drum(s) 12; 12′,12″,12″′; 22a; 22b; 22c; 22d [0098] fixing the axial magnets 13′, 13″ and the pole pieces on the first lid 11′ and on the second lid 11″ of the yoke elements [0099] moving the axial magnets 13′, 13″, fixed on the lids 11′, 11″, together with the pole pieces into the respective radial magnet rings 14′, 14″, which are located radially within the drum(s) 12; 12′,12″,12″′; 22a; 22b; 22c; 22d, such that the lids 11′, 11″ bear on the hollow cylindrical drum(s) 12; 12′,12″,12″′; 22a; 22b; 22c; 22d of the yoke elements, and the axial magnets 13′, 13″ are arranged at a defined position at least partially radially within the radial magnet rings 14′, 14″.

[0100] In order to homogenize the magnetic field, the first lid 11′ and/or the second lid 11″ is tilted relative to the z axis and/or rotated in the radial direction.

[0101] The radial magnet rings 14′, 14″ are mounted on the inner face of the hollow cylindrical drum(s) 12; 12′,12″,12′; 22a; 22b; 22c; 22d at positions predefined by projections, steps 12b or grooves 12a.

LIST OF REFERENCE SIGNS

[0102] 11′ first lid [0103] 11″ second lid [0104] 11a′; 11a″ annular groove [0105] 11b′; 11b″ shoulder [0106] 12, 12′, 12″ hollow cylindrical drum(s) [0107] 12a circumferential groove [0108] 12b circumferential step [0109] 12c step profile [0110] 12i; 12ii aperture [0111] 13′ first cylinder-symmetrical axial magnet [0112] 13″ second cylinder-symmetrical axial magnet [0113] 14′, 14″ cylinder-symmetrical radial magnet rings [0114] 15a pole cap [0115] 15b pole plate [0116] 15c pole ring [0117] 16′, 16″ fastening elements [0118] 17′, 17″ adjusting elements [0119] MV measuring volume [0120] z z axis