GB2257521A

GB2257521A - Electromagnets

Info

Publication number: GB2257521A
Application number: GB9114465A
Authority: GB
Inventors: Peter John Feenan; Rory John Warner
Original assignee: Magnex Scientific Ltd
Current assignee: Magnex Scientific Ltd
Priority date: 1991-07-04
Filing date: 1991-07-04
Publication date: 1993-01-13
Anticipated expiration: 2011-07-04
Also published as: GB2257521B; GB9114465D0

Abstract

Eddy currents in surrounding conductors C (eg housings) are avoided or reduced by the use of an arrangement of four magnets 1, 2, 3, 4. The fields produced by the magnets interact so as to leave surrounding areas at least partly free of magnetic fields that would otherwise induce eddy currents in conductors. <IMAGE>

Description

ELECTROMAGNETS There is continuing need for electromagnets for various purposes, e.g. electromagnets containing sampling volumes(s) for receiving portions of patients to be subjected to nuclear magnetic resonance (NMR). Various apparatus are known for enabling NMR signals for applications. Some applications provide imaging of sources of NMR signals (e.g. of protons or phosphorus in medical imaging of a patient's body), and/or determining qualities of said sources (e.g. relaxation times or frequency spectra).

A first aspect of the present invention provides an electromagnet (hereinafter called a gradient electromagnet), optionally suitable for NMR apparatus, comprising: (a) at least one first coil means, for at least partly surrounding a volume of space (e.g. a sampling volume of NMR apparatus), so as to give a first magnetic field component in said volume; (b) at least one second coil means, spaced apart (e.g.

axially spaced apart) from said first coil means by a first reference separation, said second coil means being for at least partly surrounding said volume, so as to give a second magnetic field component in said volume; (c) at least one third coil means, for at least partly surrounding said first coil means and being spaced apart from said first coil means by a second reference separation, this separation being transverse to said first reference separation; (d) at least one fourth coil means, for at least partly surrounding said second coil means and being spaced apart from said second coil means by a third reference separation, this separation being transverse to said first reference separation; wherein: magnetic field components produced by time varying currents in said third and fourth coil means will interact with magnetic field components produced by time varying currents in said first and second coil means, external of said first, second, third, and fourth coil means, optionally so as to reduce magnetic field at conductor means external of said first, second, third and fourth coil means, such that there will be substantially zero or zero eddy currents induced in said conductor means by time varying currents in said first, second, third, and fourth coil means, said interaction enabling said volume to be at least partly free from magnetic fields that are time varying in an unwanted manner because of said eddy currents, the resultant spatial variation of magnetic field in said volume being optionally at least substantially linear in said first reference direction by means of said first reference separation.

A second aspect of the invention provides electromagnet apparatus, comprising at least one said gradient electromagnet of the first aspect of the invention.

A third aspect of the invention provides NMR apparatus, comprising at least one gradient electromagnet of the first aspect of the invention, or at least one electromagnet apparatus of the second aspect of the invention, said conductor means optionally being comprised by said NMR apparatus. The NMR apparatus may include reference electromagnet means for giving a suitably constant and uniform background magnetic field in said sampling volume.

A fourth aspect of the invention provides a method of enabling a predetermined resultant magnetic field in said volume, comprising utilising at least one gradient electromagnet of the first aspect of the invention, or at least one electromagnet apparatus of the second aspect of the invention, or an NMR apparatus of the third aspect of the invention.

In general, the present invention may be embodied in any suitable manner. Time varying currents supplied to the various coil means may be provided in any suitable manners, e.g. in parallel or in series, or via a single power supply or a plurality of power supplies. Said conductor means may be embodied in any suitable manner for any suitable application, e.g. be a metal annular or cylindrical wall in an annular or cylindrical container for any suitable cryogenic material that is liquid below substantially 100 K, e.g. liquefied Helium or liquefied Nitrogen, with a cryostat comprised by NMR apparatus.

Such a wall may be an aluminium radiation shield.

The present invention may be utilised in any suitable manner, e.g. for providing a compact NMR apparatus, which may be suitable for one or more stationary locations (e.g.

a static location in a building) or a mobile location comprised by transport means (e.g. on a lorry).

Any said coil means may be embodied in any suitable manner, e.g. as a printing or winding. Any said coil means may comprise at least one turn constituted in any suitable manner(s). At least one turn of any said coil means may comprise the shape of any suitable 2-dimensional figure, e.g. a substantially circular shape, a substantially elliptical shape, or a substantially quadrilateral shape (for instance substantially oblong or substantially square). Such a coil means may be annular coil means comprising at least one turn of e.g.

substantially oblong shape. At least one turn of any said coil means may comprise the shape of any suitable 3 dimensional figure, e.g. when a said 2-dimensional figure is conformed with (e.g. folded over) at least a portion of the exterior of a former, for instance by suitably bending or otherwise conforming the 2-dimensional figure over at least a portion of the exterior of the former, e.g. a cylindrical former. A said conformed coil means supported relative to a portion of the former's exterior (e.g. seated on the exterior) is hereinafter called a saddle coil means. A said cylindrical former may support at least one said annular coil means and at least one said saddle coil means.

Said at least one first coil means and said at least one third coil means may be at least substantially concentric. Said at least one second coil means and said at least one fourth coil means may be at least substantially concentric. All said first, second, third and fourth coil means may be at least substantially coaxial.

In some embodiments, said first reference separation may be a Z axial distance in the range substantially 15 to substantially 100 cm, e.g. substantially 60 cm. Said second reference separation may be a transverse distance in the range substantially 1 to substantially 10 cm, e.g substantially 1.5 cm. Said third reference separation may te a transverse distance in the range substantially 1 to substantially 10 cm, e.g. substantially 1.5 cm.

Preferably, when said second reference separation is defined by said first coil means having a radius R1 and said third coil means having a radius R3, the ratio R3/R1 is in the range substantially 1.03 to substantially 1.3, e.g. substantially 1.06. Preferably, when said third reference separation is defined by said second coil means having a radius R2 and said fourth coil means having a radius R4, the ratio R4/R2 is in the range substantially 1.03 to substantially 1.3, e.g. substantially 1.06. R1, R2 may each be in the range substantially 5 to substantiallly 60 cm, e.g. substantially 25 cm.

Some electromagnet apparatus of the second aspect of the present invention comprise: (a) at least one first said gradient electromagnet, for providing at least one first magnetic field gradient in a first predetermined direction; (b) at least one second said gradient electromagnet, for providing at least one second magnetic field gradient in a second predetermined direction; and (c) at least one third said gradient electromagnet, for providing at least one third magnetic field gradient in a third predetermined direction, said first, second, third gradients being at least substantially where the direction of the total magnetic field is along the Z axial direction.

At least one said first gradient electromagnet may comprise at least one first said saddle coil means supported by a said former. At least one said second gradient electromagnet may comprise at least one second said saddle coil means supported by the same or another said former. At least one said third gradient electromagnet may comprise at least one said annular coil means supported by the same or another said former.

Some examples of electromagnet apparatus of the second aspect of the present invention may comprise: at least two said formers, one at least partly within the other, each said former having at least one respective said saddle coil means and optionally at least one respective said annular coil means.

Some examples of electromagnetic apparatus of the second aspect of the invention may comprise: at least two said first saddle coil means, respectively on opposite sides of said cylindrical former; on least two said second saddle coil means, respectively at opposite sides of said cylindrical former, and arcuately disposed (e.g. substantially 90 or substantially 180 ) relative to the at least two first saddle coil means; and at least one said annular coil means, disposed relative to said first and second coil means.

When two inner and outer said cylindrical formers are present, the ratio of the radius of the outer former to the radius of the inner former may be in the range substantially 1.03 to substantially 1.3, e.g substantially 1.06. Preferably, the two formers are at least substantially coaxial. Preferably the two formers are at least substantially concentric.

At least one single annular coil means may be disposed at any suitable side location of a centre line of said volume, this center line being e.g. in the X,Y or Z axial direction. Said first and second coil means may be disposed at suitable opposite side locations of said centre line.

Some examples of said electromagnet or said electromagnet apparatus of the present invention may be embodied as substantially rigid assembles. For example, after all said coil means are positioned on two respective said cylindrical formers, the two formers may be assembled in their correct inner and outer relationships, and the resultant assembly treated with fixative means, e.g.

comprising vaccum impregnation with epoxy resin, etc. A resultant "fixed" assembly gives rigidity and weight, so as to reduce vibration of the assembly that would otherwise occur when said time varying currents are passed in the presence of background magnetic field of said NMR apparatus. An assembly may include at least one shim winding, e.g. on one or more said formers, thereby giving one example of utilising space between formers. Coolant ducts (e.g. water pipes) may be provided in e.g. such space so as to remove heat generated in various coil means. The NMR apparatus may comprise superconductive shim windings, etc. to 'fine tune' the background magnetic field homogeneity.

The boundary surface of said sampling volume of an NMR apparatus may be of any suitable shape, e.g. be substantially cylindrical, substantially ellipsoidal, or substantially spheroidal. For an ellipsoidal shape, the ratio of major axis to minor axis may be up to substantially 4/1, etc. At least one optional ferromagnetic means and/or at least one said coil means may constitute a limiting dimension for access to the sampling volume. Said limiting dimension may be in a direction along said X,Y, or Z axis. A dimension (e.g. a diameter or a major axis) of the sampling volume may be substantially 0.2 to substantially 0.75 of the distance of said limiting dimension from a said axis (e.g. the X axis).

The value of homogeneity of magnetic field within said volume may be defined as the maximum difference in magnetic field intensity anywhere in said volume. The difference may be parts per million.

Said volume may have any suitable shape, e.g. may be uniform or vary along an axial direction.

In the accompany drawings, which are by way of example of the present invention: Fig. 1 shows one example of a gradient electromagnet.

Fig. 2 shows one example of a substantially oblong coil before forming into a saddle coil.

Fig. 3 shows one example of an electromagnet apparatus.

Fig. 4 shows one example of an NMR apparatus for receiving an electromagnet apparatus of Fig. 3.

In Fig. 1, a first annular coil 1 of windings substantially radius R1 surrounds a sampling volume V. A second annular coil 2 of windings substantially radius R2 is axially separated from coil 1 by an axial separation (e.g. a Z axial direction as shown) which may be any suitable distance e.g. substantially 25 cm. Coil 2 surrounds sampling volume V. A third annular coil 3 of windings substantially radius R3 surrounds coil 1. Coil 3 and coil 1 are at least substantially concentric. The ratio R3/R1 may be any suitable value, e.g. substantially 1.06. A fourth annular coil of windings substantially radius R4 surrounds coil 2. Coil 4 and coil 2 are at least substantially concentric. The ratio R4/R2 may be any suitable value, e.g. substantially 1.06. All coils 1,2,3,4 are substantially coaxial.

Magnetic field components produced by time varying currents in coils 3 and 4 will interact with magnetic field components produced by time varying currents in coils 1 and 2, external of coils 1,2,3,4, so as to reduce magnetic field at conductor means C external of coils 1,2,3,4, such that there will be substantially zero or zero eddy currents induced in conductor means C by time varying currents in coils 1,2,3,4. This interaction enables volume V to be at least partly free from magnetic fields that are time varying in an unwanted manner because of said eddy currents. The resultant magnetic field in volume V is at least substantially linear in the axial distance of separation between coils 1 and 2. The gradient electromagnet of Fig. 1 may be used as a Z-axis gradient coil, etc.

In Fig. 2, a flat coil 21 is substantially oblong.

This coil has the shape of a 2-dimensional figure, which may be conformed with at least a portion of the exterior of a cylindrical former so as to provide a saddle coil (see Fig. 3). The conforming may be done in any suitable manner, e.g. by folding the coil 21 suitably.

In Fig. 3, a cylindrical former 31 of substantially radius R31 is for insertion into a cylindrical former 41 of substantially radius R41. The ratio R41/R31 may be any suitable value, e.g. substantially 1.06. Formers 31 and 41 will be at least substantially coaxial and at least substantially concentric. On former 31 are two X coils 0 Xl,XlA substantially 180 o arcuately separated relative to each other. There are also corresponding X coils X2,X2A on former 31. Coils X1,XlA,X2,X2A are saddle coils obtained from flat coils 21 of Fig. 2. On former 31 are two Y coils V1,Y1A substantially 180 0 arcuately separated relative to each other. There are also corresponding Y coils Y2,Y2A on former 31. Coils Y1,Y1A,Y2,Y2A are saddle coils obtained from flat coils 21 of Fig. 2.On former 31 are two Z annular coils Z1, Z2. On former 41 are two coils X3,X3A substantially 180 arcuately separated relative to each other. There are also corresponding X coils X4, X4A on former 41. Coils X3,X4A,X4,X4A are saddle coils obtained from flat coils 21 of Fig. 2. On former 41 are two Y coils y3,Y3A substantially 180 arcuately separated relative to each other. There are also corresponding Y coils Y4,Y4A on former 41. Coils Y3,Y3A,Y4,Y4A are saddle coils obtained from flat coils of Fig. 2. On former 41 are two Z annular coils Z3, Z4.

Insertion of former 31 and its mounted coils into former 41 (and hence into the coils mounted onto former 41) gives an electromagnet apparatus that is an assembly comprising Z,Y,Z gradients coils. The assembly may be embodied as a substantially rigid assembly, e.g. as described earlier above utilising fixative means, etc. e.g. by vaccum impregnation using epoxy resin. Water pipes (not shown) may be provided to remove heat generatable in the windings during use. Shim windings (not shown) may be incorporated into the assembly, e.g. on one or both of formers 31, 41.

An electromagnet assembly of Fig. 3 may be utilised for insertion into NMR apparatus of Fig. 4. In Fig. 4, a cryostat outer cylindrical case or casing 51 has a port 52 for passage of liquefied Helium or liquefied Nitrogen to and from an internal annular container 53 within casing 51. Between casing 51 and container 53 are two circumjacent cylindrical aluminium radiation shields 54, 55. Container 53 surrounds and is radially separated from sampling volume V. Flanges 58, 59, 60, 61 are for securing two bore tubes 62, 63 to respective opposite ends of casing 51, for supporting the shields 54, 55. An electromagnet assembly 71 (Fig. 3) is shown for insertion into left end plate 64. There is a corresponding right end plate 65. The NMR apparatus may include reference electromagnet means (not shown) for giving a suitably constant and uniform background magnetic field in sampling volume V.

The present invention as exemplified in the drawings may be embodied in any suitable manner.

The present invention includes equivalents and modifications within the disclosures of the present application. For example, at least one gradient electromagnet of the present invention may be present in a cryostat comprised by NMR apparatus.

In general, the first, second, third, and fourth aspects of the present invention may be embodied in any suitable manners. Some preferred embodiments of the first, second, and third aspects of the invention may be compact in size compared with conventional NMR apparatus and conventional parts and fittings for such apparatus.

NMR apparatus of the present invention may be especially suitable when a patient's feet are first inserted into sampling volume V so that the knees would be in sampling volume V while the patients's head would be outside sampling volume V, for instance during diagnosis and/or monitoring of arthritis in elderly patients.

Another application would be when a patient's head would be first inserted into sampling volume V. In general, any such NMR apparatus may be termed "Patients's appendage(s) NMR apparatus" for any suitable appendages, e.g. arms, elbows, feet, fingers, hands, heads, knees, legs. The resultant apparatus may be compact compared with conventional NMR apparatus.

Claims

1. An electromagnet (optionally suitable for NMR apparatus), comprising: (a) at least one first coil means, for at least partly surrounding a volume of space (optionally a sampling volume of NMR apparatus), so as to give a first magnetic field component in said volume; (b) at least one second coil means, spaced apart (optionally axially spaced apart) from said first coil means by a first reference separation, said second coil means being for at least partly surrounding said volume, so as to give a second magnetic field component in said volume; (c) at least one third coil means, for a least partly surrounding said first coil means and being spaced apart from said first coil means by a second reference separation, this separation being transverse to said first reference separation;; (d) at least one fourth coil means, for at least partly surrounding said second coil means and being spaced apart from said second coil means by a third reference separation, this separation being transverse to said first reference separation; wherein: magnetic field components produced by time varying currents in said third and fourth coil means will interact with magnetic field components produced by time varying currents in said first and second coil means, external of said first, second, third, and fourth coil means, optionally so as to reduce magnetic field at conductor means external of said first, second, third, and fourth coil means, such that there will be substantially zero or zero eddy currents induced in said conductor means by time varying currents in said first, second, third, and fourth coil means, said interaction enabling said volume to be at least partly free from magnetic fields that are time varying in an unwanted manner because of said eddy currents, the resultant spatial variation of magnetic field in said volume being optionally at least substantially linear in said first reference direction by means of said first reference separation.

2. An electromagnet as claimed in claim 1, adapted such that time varying currents supplied to the various said coil means will be provided in manner(s) chosen from parallel supply, series supply, a single power supply, and a plurality of power supplies.

3. An electromagnet as claimed in claim 1 or 2, comprising said conductor means.

4. An electromagnet as claimed in claim 3, wherein said conductor means is a metal annular or cylindrical wall in an annular or cylindrical container for cryogenic material C liquid below substantially 100 K, optionally with a cryostat comprised by NMR apparatus.

5. An electromagnet as claimed in claim 3 or 4, wherein said wall is an aluminium radiation shield.

6. An electromagnet as claimed in any one of claims 1 to 5, wherein any said coil means is chosen from printing(s) and winding(s).

7. An electromagnet as claimed in any one of claims 1 to 6, wherein at least one turn of any said coil means comprises a shape chosen from 2-dimensional figures.

8. An electromagnet as claimed in claim 7, wherein at least one said 2-dimensional figure is chosen from substantially circular shapes, substantially elliptical shapes, and substantially quadrilateral shapes.

9. An electromagnet as claimed in claim 8, wherein at least one said quadrilateral shape is chosen from substantially oblong shapes and substantially square shapes.

10. An electromagnet as claimed in any one of claims 1 to 9, wherein at least one said coil means is an annular coil means.

11. An electromagnet as claimed in claim 10, wherein said annular coil means comprises at least one turn of substantially oblong shape.

12. An electromagnet as claimed in claim 10 or 11, wherein at least one said annular coil means is disposed at any suitable side location of a centre line of said volume.

13. An electromagnet as claimed in any one of claims 1 to 12, wherein at least one turn of any said coil means comprises a shape chosen from 3-dimensional figures.

14. An electromagnet as claimed in claim 13, wherein at least one said 3-dimensional figure is a said 2dimensional figure conformed with at least a portion of the exterior of a former.

15. An electromagnet as claimed in claim 14, wherein said conformation has been provided by bending a said 2dimensional figure over at least a portion of the exterior of a said former.

16. An electromagnet as claimed in claim 14 or 15, wherein a said former is a cylindrical former.

17. An electromagnet as claimed in any one of claims 14 to 16, wherein a said conformed coil means (termed a saddle coil means) is supported relative to a portion of a said former's exterior.

18. An electromagnet as claimed in claim 17, wherein a said conformed coil means is seated on the exterior of a said former.

19. An electromagnet as claimed in any one of claims 16 to 18, wherein a said cylindrical former supports at least one said annular coil means and at least one said conformed coil means.

20. An electromagnet as claimed in any one of claims 1 to 19, wherein at least one said first coil means and at least one said second coil means are disposed at suitable opposite side locations of a centre line of said volume.

21. An electromagnet as claimed in any one of claims 1 to 20, wherein at least one said first coil means and at least one said third coil means are substantially concentric.

22. An electromagnet as claimed in any one of claims 1 to 21, wherein at least one said second coil means and at least one said fourth coil means are substantially concentric.

23. An electromagnet as claimed in any one of claims 1 to 22, wherein all said first, second, third, and fourth coil means are at least substantially coaxial.

24. An electromagnet as claimed in any one of claims 1 to 23, wherein said first reference separation is Z axial distance.

25. An electromagnet as claimed in claim 24, wherein said first reference separation is in the range substantially 15 to substantially 100 cm.

26. An electromagnet as claimed in claim 25, wherein said first reference separation is substantially 60 cm.

27. An electromagnet as claimed in any one of claims 1 to 26, wherein said second reference separation is in the range substantially 1 to substantially 10 cm.

28. An electromagnet as claimed in claim 27, wherein said second reference separation is substantially 1.5 cm.

29. An electromagnet as claimed in any one of claims 1 to 28, wherein said third reference separation is in the range substantially 1 to substantially 10 cm.

30. An electromagnet as claimed in claim 29, wherein said third reference separation is substantially 1.5 cm.

31. An electromagnet as claimed in any one of claims 1 to 30, wherein said second reference separation is defined by said first coil means having a radius R1 and said third coil means having a radius R3, the ratio R3/R1 being in the range substantially 1.03 to substantially 1.3.

32. An electromagnet as claimed in claim 31, wherein said ratio R3/R1 is substantially 1.06.

33. An electromagnet as claimed in any one of claims 1 to 32, wherein said third reference separation is defined by said second coil means having a radius R2 and said fourth coil means having a radius R4, the ratio R4/R2 being in the range substantially 1.03 to substantially 1.3.

34. An electromagnet as claimed in claim 33, wherein said ratio R4/R2 is substantially 1.06.

35. An electromagnet as claimed in any one of claims 31 to 34, wherein R1, R2 are each in the range substantially 5 to substantially 60 cm.

36. An electromagnet as claimed in claim 35, wherein R1, R2 are each substantially 25 cm.

37. An electromagnet, substantially as hereinbefore described with reference to and as shown in Fig. 1 and/or Fig. 2 of the accompanying drawings.

38. Electromagnet apparatus, comprising at least one electromagnet as claimed in any one of claims 1 to 37.

39. Electromagnet apparatus as claimed in claim 38, comprising: (a) at least one first electromagnet as claimed in any one of claims 1 to 37, for providing at least one first magnetic field gradient in a first predetermined direction; (b) at least one second electromagnet as claimed in any one of claims 1 to 37, for providing at least one second magnetic field gradient in a second predetermined direction; and (c) at least one third electromagnet as claimed in any one of claims 1 to 37, for providing at least one third magnetic field gradient in a third predetermined direction; wherein said first, second, third gradients are at least substantially where the direction of the total magnetic field will be along a Z axial direction.

40. Electromagnet apparatus as claimed in claim 39, wherein at least one first said electromagnet comprises at least one first said saddle coil means supported by a said former.

41. Electromagnet apparatus as claimed in claim 39 or 40, wherein at least one said second electromagnet comprises at least one second said saddle coil means supported by a former, which may be the same former according to claim 40 or another former.

42. Electromagnet apparatus as claimed in any one of claims 39 to 41, whcrein at least one said third electromagnet comprises at least one said annular coil means supported by the same former according to claim 40 or 41 or another former.

43. Electromagnet apparatus as claimed in any one of claims 40 to 42, wherein there are at least two said formers, one at least partly within the other, each said former at least one respective said saddle coil means and optionally at least one respective said annular coil means.

44. Electromagnet apparatus as claimed in any one of 39 to 43, wherein there are: at least two said first saddle coil means, respectively on opposite sides of a said cylindrical former; at least two said second saddle coil means, respectively at opposite sides of said cylindrical former, and arcuately disposed relative to said at least two first saddle coil means; and at least one said annular coil means, disposed relative to said first and second coil means.

45. Electromagnet apparatus as claimed in claim 44, wherein said arcuate disposition is in the range 0 0 substantially 90 to substantially 180

46. Electromagnet apparatus, substantially as hereinbefore described wth reference to and as shown in Fig. 3 of the accompanying drawings.

47. NMR apparatus, comprising at least one electromagnet as claimed in any one of claims 1 to 37, or at least one electromagnet apparatus as claimed in any one of claims 38 to 46, said volume optionally being a sampling volume of said NMR apparatus.

48. NMR apparatus as claimed in claim 47, wherein said NMR apparatus comprises said conductor.

49. NMR apparatus as claimed in claim 47 to 48, comprising reference electromagnet means for giving a suitably constant and uniform background field in said sampling volume.

50. NMR apparatus as claimed in any one claims 47 to 49, wherein said NMR apparatus is adapted for use at one or more stationary locations, optionally a static location in a building.

51. NMR apparatus as claimed in any one of claims 47 to 50, wherein said NMR apparatus is adapted for use at the mobile location comprised by transport means, optionally a lorry.

52. NMR apparatus, substantially as hereinbefore described with reference to and as shown in Fig. 4 of the accompanying drawings.

53. NMR apparatus as claimed in any one of claims 47 to 52, adapted for sampling patients appendages.

54. A method of enabling a predetermined resultant magnetic field in a volume of space, comprising utilising at least one electromagnet as claimed in any one of claims 1 to 37, or at least one electromagnet apparatus as claimed in any one of claims 38 to 46, or an NMR apparatus as claimed in any one of claims 47 to 53.

55. A method of enabling a predetermined resultant magnetic field in a volume of space, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.