GB2540386A

GB2540386A - Method and apparatus for adjusting magnetic field homogeneity of an actively shielded cylindrical superconducting magnet

Info

Publication number: GB2540386A
Application number: GB1512363.1A
Authority: GB
Inventors: Noys Jonathan; Robson Edward
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2015-07-15
Filing date: 2015-07-15
Publication date: 2017-01-18
Anticipated expiration: 2035-07-15
Also published as: GB201512363D0; GB2540386B

Abstract

A method for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet, comprising an inner magnet assembly (21) itself comprising end coils (10) and inner coils (14) aligned on a common axis (A-A) between the end coils, said inner coils and said end coils defining a bore (18); and one or more shield coils (12) of greater diameter than the end coils (10) and inner coils (14). The method comprises the steps of: applying electric current to the shield coils, the inner coils and the end coils to generate a magnetic field within an imaging region (20) defined within the bore (18); measuring a homogeneity of the magnetic field within the imaging region (20) and adjusting a position of at least one of the shield coils (12) with respect to the inner magnet assembly (21). Alternatively the shield coils can be removed or replaced in order to change constructional details such as the number of coil turns or diameter, or in the case of coil defects.

Description

METHOD AND APPARATUS FOR ADJUSTING MAGNETIC FIELD HOMOGENEITY OF AN ACTIVELY SHIELDED CYLINDRICAL SUPERCONDUCTING MAGNET

The present invention relates to methods and apparatus for adjusting the homogeneity of an electromagnet. The present invention particularly relates to actively-shielded superconducting magnets for MRI (Magnetic Resonance Imaging) systems .

Fig. 1 schematically illustrates an axial cross-section through a coil structure of an actively-shielded superconducting magnet for MRI systems. The coil structure is essentially rotationally symmetrical about common axis A-A. In this description, the term "axial" and the like will be used to denote a direction parallel to the common axis A-A, and the term "radial" and the like will be used to denote a direction perpendicular to the common axis A-A, in a plane which passes through the axis A-A.

The relative sizes, dimensions and positions of the coils are indicated for explanation only.

End coils 10 carry current in a "forward" direction, and generate the majority of the resultant magnetic field within the bore 18 of the magnet. Shield coils 12 carry current in a "reverse" direction, opposite to the "forward" direction, and limit the strength of a stray field emanating from the magnet. Inner coils 14 serve to add to the strength of the magnetic field in the bore 18 of the magnet, and to provide a homogeneous magnetic field within an imaging volume 20. Typically, all inner coils 14 carry current in the "forward" direction, although some conventional magnets include some inner coils 14 which carry current in the "reverse" direction.

In the context of an MRI imaging magnet, it is very important that the magnetic field within the imaging volume 20 should be as homogenous as possible. For this reason, it is conventional to carry out a shimming process before the magnet is encased in a cryostat. Typically, this shimming is carried out by relative movement of the end coils 10. A relatively low current, such as 1 ampere, is passed through the coil structure comprising inner coils 14, end coils 10 and shield coils 12, and the homogeneity of the resultant magnetic field in the imaging volume 20 is measured. Known algorithms are applied to the result, and a movement is calculated for one or both of the end coils 10, relative to the inner coils 14 and the shield coils 12, to improve the homogeneity of the magnetic field in the imaging volume 20. The calculated relative movement is applied to the appropriate end coil(s). Preferably, the process is repeated iteratively until a satisfactory homogeneity is achieved, or until no further significant improvement can be obtained by relative movement of the end coils. Conventionally, although not illustrated in Fig. 1, inner coils 14 are mounted on a former, for example wound into cavities defined in the material of the former. End coils 10 are conventionally either included in the same former, or are mounted in separate journals which are attached to the former by mechanical means such as bolting. Relative movement of the end coils may be achieved by placing or removing non-magnetic shims between an axial end-surface of the end coil and the adjacent surface of the journal or former. Alternatively, pieces of magnetic material, for example of iron, may be placed inside the cryostat with the magnet, at positions calculated from the homogeneity measurement by known algorithms (e.g. US patent application 2012/0098538).

Recent superconducting magnet designs have included end coils 10 bonded to the inner coils 14, either directly or through the intermediary of a supporting structure such as spacers or a tube. In such superconducting magnets, it is not possible to adjust the relative position of the end coils 10 with respect to the inner coils 14. It has therefore not been possible to adjust the homogeneity of the magnet structure in the manner described above.

The present invention therefore provides methods and apparatus for adjusting the homogeneity of a superconducting magnet structure which does not require relative movement of the end coils 10 with respect to inner coils 14.

The present invention accordingly provides methods and apparatus as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, given by way of examples only, wherein:

Fig. 1 represents a conventional arrangement of coils in a cylindrical superconducting magnet;

Fig. 2 represents an arrangement of coils according to an embodiment of the present invention;

Fig. 3 represents an arrangement of coils and an adjustable mounting for the shield coils, according to an embodiment of the present invention;

Fig. 4 represents an arrangement of coils and an adjustable mounting for the shield coils, according to a second embodiment of the present invention;

Fig. 5 represents an arrangement of coils and an adjustable mounting for the shield coils, according to a third embodiment of the present invention;

Fig. 6 represents an arrangement of coils and an adjustable mounting for the shield coils, according to a fourth embodiment of the present invention; and

Fig. 7 represents an arrangement of coils and an adjustable mounting for the shield coils, according to a further embodiment of the present invention.

Fig. 2 schematically illustrates a coil arrangement which may be improved according to an embodiment of the present invention. Similarly to the arrangement of Fig. 1, the coil arrangement of Fig. 2 is symmetrical about axis A-A. In this coil arrangement, an inner coil assembly 21 comprises end coils 10 bonded to inner coils 14 by a structure 22 which may include spacers, rings or other pieces which are bonded to the coils and retain the coils in fixed relative positions. Although, as with the conventional method described above, the magnetic field homogeneity in the imaging volume 20 may be measured, it is not possible to correct for inhomogeneity in the magnetic field by adjusting the position of the end coils 10 with respect to the inner coils 14.

However, in accordance with the present invention, shield coils 12 may be adjusted in position, or replaced, according to the measured magnetic field homogeneity in the imaging volume 20, to improve that homogeneity. The illustrated embodiment has two shield coils 12, each axially positioned towards a respective axial end of the inner coil assembly 21 which comprises end coils 10 and inner coils 14. In other embodiments, more or fewer than two shield coils may be provided and adjusted in position, or replaced, according to the measured magnetic field homogeneity in the imaging volume 20, to improve that homogeneity.

In certain embodiments, the shield coils 12 may be displaced axially with respect to the inner coils 14 and end coils 10. In certain embodiments, the shield coils may be displaced radially with respect to the inner coils 14 and end coils 10. In certain embodiments, respective shield coils may each be displaced by a differing amount. In certain embodiments, respective shield coils may each be displaced by a same amount.

As illustrated, a centreline 31 between shield coils 12 may be misaligned from centreline 30 of the inner coil assembly 21. In this way, the homogeneity of the resultant magnetic field in the imaging volume 20 will vary. The misalignment of centreline 31 of shield coils 12 with respect to the centreline 30 of the inner coil assembly 21 may be obtained by displacement of one or other, or both, of the shield coils 12.

Alternatively, or in addition, the shield coils 12 may be replaced by other shield coils of more or fewer turns, or smaller or larger diameter, or different cross-sectional aspect ratio, in order to improve the homogeneity of the field in the imaging volume 20.

Mechanical retaining structures will be provided with the coils of Fig. 2, to hold them in their required relative positions, as will be understood by those skilled in the art, yet are not illustrated in the drawing for clarity of illustration of the present invention. The mechanical retaining structures may comprise structural elements mounting the shield coils to the inner coil assembly 21. Alternatively, the mechanical retaining structures may comprise structural elements mounting the shield coils to a cryostat enclosing the coil structure.

Fig. 3 shows a more detailed illustration of an embodiment of the present invention. Features common with Fig. 1 or Fig. 2 carry corresponding reference numerals. In this embodiment, shield coils 12 are located within annular journals 24. The annular journals may be mounted with fixed separation in a fixed position relative to the inner coil assembly 21 of inner coils 14 and end coils 10. The coils 12 themselves may be a relatively loose axial fit inside the journals. This may be achieved by winding and resin-impregnating the coils in a demountable mould, removing the coils from the respective mould and building the journals 24 around them. Alternatively, the coils may be wound and impregnated in situ within the journals 24, with a removable axial spacer which is removed after impregnation. Care must be taken to ensure that the coil does not become bonded to the material of the journal 24.

In this embodiment, in use, the shield coils 12 are subjected to an axially outward force, driving them into contact with axially outer thrust faces 26 of the journals. According to an aspect of the invention, non-magnetic shim pieces 28 may be placed between a shield coil 12 and its respective thrust face such that, in use, the axial position of the shield coil is displaced with respect to the assembly 22 of inner coils 14 and end coil 10, inwards towards the magnet centre line 30 by the thickness of the shims 28 used.

In other embodiments, depending on coil design and layout, the shield coils may be subjected to an axially inward force, driving them into contact with opposite, axially inner, thrust faces of the journals. Shim pieces 28 may, in such embodiments, be placed between a shield coil 12 and its respective axially-inner thrust face such that, in use, the axial position of the shield coil is displaced with respect to the assembly 22 of inner coils 14 and end coil 10, outwards away from the magnet centre line 30 by the thickness of the shims 28 used.

Alternatively, or in addition, the journals 24 may be removable, such that a replacement journal containing a different shield coil 12 may be substituted in order to improve homogeneity of the magnetic field in the imaging volume 20.

Alternatively, or in addition, axial end walls 32 of the journals may be removable, so that one or more of the shield coils 12 may be removed from the journals and replaced with different shield coil(s) in order to improve homogeneity of the magnetic field in the imaging volume 20.

In further variants, the separation between the journals 24 may be adjustable. The axial positioning of the journals may be adjustable.

Fig. 4 schematically illustrates another embodiment of the present invention. Features corresponding to features of Figs. 1-3 carry corresponding reference labels. In this embodiment, the position of each shield coil 12 is not adjustable within its journal 24, but the position of the journal is adjustable upon its mechanical retaining structure. In such embodiments, the shield coils 12 may each be wound and impregnated within their journal 24. Part of the mechanical retaining structure is illustrated, being the part which defines the axial position of each journal, and so defines the axial position of each shield coil. The illustrated part 34 of the mechanical retaining structure may be a tie rod, and may be duplicated at intervals around the circumference of the shield coils 12. The illustrated part of the mechanical retaining structure provides a mechanical support structure of adjustable separation, as will now be described.

Tie rods 34, or similar, are bolted to the respective journals. Typically, the tie rods have threaded ends 36 which mate with threaded holes in the journals. Flanges 38 define a contact location with journals 24. The shim coils are thereby fixed in place by fasteners retaining the journals 24 in position in relation to flanges 38 on the tie rods 34. Shims 36 of non-magnetic material may be placed between the flanges 38 of the tie bars 34 and the journals 24. The shims 3 6 may be U-shaped to allow them to be inserted and removed without detaching the tie rods 34. Shims 36 may be added and removed by loosening the fasteners, adding or removing an appropriate thickness of shims 36 between the flanges 38 and the journals 24 and re-tightening the fasteners onto the shims. In this way, the axial position of each journal 24, and the associated shield coil 12, may be adjusted. The use of shims provides a predictable and repeatable arrangement for moving the shim coils by a certain distance.

Fig. 5 schematically illustrates another embodiment of the present invention. Features corresponding to features of

Figs. 1-4 carry corresponding reference labels. In this embodiment, the position of each shield coil 12 is not adjustable within its journal 24, but the position of the journal is adjustable upon its mechanical retaining structure. In such embodiments, each shield coil 12 may be wound and impregnated within the journal 24. Part of the mechanical retaining structure is illustrated, being the part which defines the axial position of each journal 24, and so defines the axial position of each shield coil 12. The illustrated part of the mechanical retaining structure may be a tie rod 34, and this may be duplicated at intervals around the circumference of the shield coils 12. The tie rod(s) 34 of the mechanical retaining structure provide(s) a mechanical support structure of adjustable separation, as will now be described.

Journals 24 are provided with axial through-holes or slots 40. Tie rods 34, or similar, pass through the through-holes or slots 40 and are retained by a respective mechanical stop 39 at respective extremities. Mechanical stop 39 may be a nut or other fastener or similar. Shims 36 of non-magnetic material may be placed between the stop 39 and the journals 24. The shims 36 may be U-shaped to allow them to be inserted and removed without detaching the tie rods 34. Shims 3 6 may be added and removed by loosening stop 39, adding or removing an appropriate thickness of shims and retightening the stop 39 onto the shims. In this way, the axial position of each journal 24, and the associated shield coil 12, may be adjusted.

In the illustrated embodiment, in use, the shield coils 12 are subjected to an axially-outward force which will tend to move the journals axially away from the centre-line 30, to compress the shims 36.

The use of shims provides a predictable and repeatable arrangement for moving the shield coils 12 by a certain distance, but in a variant of this embodiment, shims are not used and the variation in the axial position of the shield coils 12 may be provided by adjusting the position of the stops 39.

Fig. 6 shows another embodiment of an arrangement of coils in a cylindrical superconducting magnet according to the present invention. Features common with features of Figs. 1-5 carry common reference numerals. In this embodiment, shield coils 12 are each bonded to an adjacent crust 42 of a filler material such as resin-impregnated glass cloth or resin-impregnated glass beads. The crust 42 may be provided on an axially-adjacent surface as shown, or may be provided on a radially-adjacent surface (not shown). A tension rod 34, similar to those described in relation to Figs. 2-5 is shown. The tension rod 34 may be provided with flanges 39 at its extremities. The crust 42 may include complementary threaded holes to mate with the tension rod 34, or the crust may include inserts to receive fasteners to attach to the ends of the tension rod 34. Shims 36 of nonmagnetic material may be introduced between the flanged tie bars 34 and the crust 42 to define the axial position of the shim coils 12 relative to the inner coil assembly 21 comprising end coils 10 and inner coils 14. The shims 36 may be U-shaped to allow them to be inserted and removed without detaching the tie rods 34. Shims 36 may be added and removed by loosening fasteners or unscrewing the tie-rod 34 as appropriate, adding or removing an appropriate thickness of shims and re-fastening the tie-rod 34 onto the shield coils 12. In this way, the axial position of each shield coil 12, may be adjusted.

Fig. 7 shows another embodiment of an arrangement of coils in a cylindrical superconducting magnet according to the present invention. Features common with features of Figs. 1-6 carry common reference numerals. In this embodiment, shield coils 12 are each bonded to an adjacent crust 42 of a filler material such as resin-impregnated glass cloth or resin-impregnated glass beads. The crust 42 may be provided on an axially-adjacent surface as shown, or may be provided on a radially-adjacent surface (not shown).

Partial tension rods 46 are each respectively attached to the crust 42 of the associated shield coil 12. The partial tension rods 4 6 may be threaded at their axially inner extremities, in opposite directions so that they may be threaded into turnbuckle 48. The partial tension rods 46 are attached at their axially outer ends to the crust 42 of a respective shield coil 12. This may be by providing a threaded end to the partial tension rods and complementary threaded holes in the crust 42 to receive the threaded ends of the partial tension rod 46, or the crust 42 may include threaded inserts 48 to receive the threaded ends of the partial tension rod 46. Alternatively, the axially outer ends of the partial tension rods 4 6 may be bonded into holes formed in the crust 42. By rotating the turnbuckle 48, shield coils 12 may be brought closer together or driven further apart as required to improve the homogeneity of the magnetic field in the image region 20. The turnbuckle and opposite threads on the axially inner ends of the partial tension rods 46 means that the shield coils can be adjusted towards one another, or away from one another, but cannot be adjusted individually.

The present invention accordingly provides methods and apparatus for adjusting the axial position of shield coils in order to improve homogeneity of a magnetic field inside an imaging region 20. The present invention also provides methods and apparatus for removing and replacing shield coils in order to improve homogeneity of a magnetic field inside an imaging region 20. It may also be found beneficial to enable adjustment of the radial positions of the shield coils, for example to correct for error in axial alignment of shield coils 12 with inner assembly 22 comprising end coils 10 and inner coils 14 or to intentionally introduce an axial misalignment to compensate for some other defect causing inhomogeneity in the magnetic field in the imaging region 20. The mechanical arrangements for enabling radial movement of the shield coils may resemble to the described arrangements for axial movement of shield coils, for example comprising tension rods arranged radially, or at least partially radially, at 90° intervals around the circumference of the shield coil. Furthermore, in certain embodiments of the present invention, arrangements may be made for moving the shield coils 12 by tilting at least one of them with respect to the common axis (A-A).

While the present invention has been described with reference to an inner magnet assembly comprising end coils 10 and inner coils 14 which cannot be moved with respect to one another, the present invention may also be applied to magnets which do not have this characteristic.

Claims

1. A method for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet, said actively-shielded cylindrical superconducting magnet comprising : - an inner magnet assembly (21) itself comprising end coils (10) and inner coils (14) aligned on a common axis (A-A) between the end coils, said inner coils and said end coils defining a bore (18); and - one or more shield coils (12) of greater diameter than the end coils (10) and inner coils (14), the method comprising the steps of: - applying electric current to the shield coils, the inner coils and the end coils to generate a magnetic field within an imaging region (20) defined within the bore (18); - measuring a homogeneity of the magnetic field within the imaging region (20) and - adjusting a position of at least one of the shield coils (12) with respect to the inner magnet assembly (21), thereby to improve the homogeneity of the magnetic field within the imaging region (20) .

2. A method according to claim 1 wherein the step of adjusting a position of at least one of the shield coils comprises adjusting a position of the least one of the shield coils in a direction parallel to the common axis (A-A).

3. A method according to claim 1 wherein the step of adjusting a position of at least one of the shield coils comprises tilting at least one of the shield coils with respect to the common axis (A-A).

4. A method according to claim 1 wherein the step of adjusting a position of at least one of the shield coils comprises adjusting a position of the least one of the shield coils in a direction perpendicular to the common axis (A-A).

5. A method according to claim 2 wherein the least one of the shield coils (12) is provided within a journal (24), and the adjustment of the position of the shield coil is performed by addition or removal of shims (28) between the shield coil (12) and a thrust face (26) of the journal.

6. A method according to claim 2 wherein the least one of the shield coils is provided within a journal (24), and the adjustment of the position of the shield coil is performed by adjustment of the position of the journal with respect to the inner magnet assembly (22).

7. A method according to claim 6 wherein the step of adjustment of the position of the journal is performed by addition or removal of shims (36) from a part of a mechanical retaining structure (34) holding the journal in place.

8. A method according to claim 2 wherein at least two shield coils (12) are provided, each provided with a crust (42), a number of tension bars (34) being attached to each crust around the perimeter of the shield coils, wherein the step of adjustment of the position of the shield coil is performed by addition or removal of shims (36) from a mechanical retaining structure comprising the tension bars (34) joined to the crust.

9. A method according to claim 2 wherein at least two shield coils (12) are provided, each provided with a crust (42), a number of tension bars (34) being attached to each crust around the perimeter of the shield coils, wherein the step of adjustment of the position of the shield coil is performed by adjustment of the length of the tension bars (34) joined to the crust.

10. A method for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet, said actively-shielded cylindrical superconducting magnet comprising: - an inner magnet assembly (22) itself comprising axially-aligned end coils (10) and inner coils (14) between the end coils, said inner coils and said end coils defining a bore; and - one or more shield coils (12) of greater diameter than the end coils (10) and inner coils (14), the method comprising the steps of: - applying electric current to the shield coils, the inner coils and the end coils to generate a magnetic field within an imaging region (20) defined within the bore; - measuring a homogeneity of the magnetic field within the imaging region (20) and - replacing at least one of the shield coils with respect to the inner magnet assembly, thereby to improve the homogeneity of the magnetic field within the imaging region (20).

11. A method according to claim 10, wherein the least one of the shield coils is mounted within a journal (24), and the step of replacing at least one of the shield coils comprises removing the journal and coil assembly and replacing it with a different journal and coil assembly.

12. A method according to claim 11, wherein the least one of the shield coils is mounted within a journal (24), and the step of replacing at least one of the shield coils comprises removing the coil from the journal and replacing it with a different coil within the journal.

13. An arrangement for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet, said actively-shielded cylindrical superconducting magnet comprising: - an inner magnet assembly (22) itself comprising end coils (10) and inner coils (14) aligned on a common axis (A-A) between the end coils, said inner coils and said end coils defining a bore; and - one or more shield coils (12) of greater diameter than the end coils (10) and inner coils (14), - a mechanical retaining structure holding end coils, inner coils and shield coils in relative positions, - characterised in that the mechanical retaining structure is adjustable such that the relative position of at least one of the shield coils may be adjusted with respect to the inner magnet assembly (22).

14. An arrangement for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet according to claim 13 wherein the mechanical retaining structure provides adjustment of a position of at least one of the shield coils in a direction parallel to the common axis (A-A).

15. An arrangement for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet according to claim 13 wherein the mechanical retaining structure provides tilting at least one of the shield coils with respect to the common axis (A-A) .

16. An arrangement for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet according to claim 13 wherein the mechanical retaining structure provides adjusting a position of the least one of the shield coils in a direction perpendicular to the common axis (A-A) .

17. An arrangement according to claim 14 wherein at least one of the shield coils is provided within a journal (24), and shims (28) are placed between the shield coil (12) and a thrust face (26) of the journal.

18. An arrangement according to claim 14 wherein at least one of the shield coils is provided within a journal (24), and the mechanical retaining structure provides adjustment of the position of the shield coil by adjustment of the position of the journal with respect to the inner magnet assembly (22) .

19. An arrangement according to claim 18 wherein shims (36) are provided in a part of the mechanical retaining structure (34) holding the journal in place, to provide adjustment of the position of the journal.

20. An arrangement according to claim 14 wherein at least two shield coils (12) are provided, each provided with a crust (42), a number of tension bars (34) being attached to each crust around the perimeter of the shield coils, and shims (36) are provided in a mechanical retaining structure comprising the tension bars (34) joined to the crust, thereby enabling adjustment of the position of the shield coils.

21. An arrangement according to claim 14 wherein at least two shield coils (12) are provided, each provided with a crust (42), a number of tension bars (34) of adjustable length being attached to each crust around the perimeter of the shield coils.

22. An arrangement for adjusting the magnetic field homogeneity of an actively-shielded cylindrical superconducting magnet, said actively-shielded cylindrical superconducting magnet comprising: - an inner magnet assembly (22) itself comprising end coils (10) and inner coils (14) aligned on a common axis (A-A) between the end coils, said inner coils and said end coils defining a bore; and - one or more shield coils (12) of greater diameter than the end coils (10) and inner coils (14), - a mechanical retaining structure holding end coils, inner coils and shield coils in relative positions, - characterised in that at least one of the shield coils is removable form the mechanical retaining structure and is replaceable with a different shield coil.

23. An arrangement according to claim 22, wherein the least one of the shield coils is mounted within a journal (24), the journal and shield coil being removable from the mechanical retaining structure and replaceable with a different journal and shield coil assembly.

24. An arrangement according to claim 22, wherein the least one of the shield coils is mounted within a journal (24), the shield coil being removable from the journal and replaceable with a different shield coil. Amendment to the claims have been filed as follows: CLAIMS