GB2545436A

GB2545436A - A Cylindrical superconducting magnet

Info

Publication number: GB2545436A
Application number: GB1522122.9A
Authority: GB
Inventors: Jan Marie Kruip Marcel
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2015-12-15
Filing date: 2015-12-15
Publication date: 2017-06-21
Anticipated expiration: 2035-12-15
Also published as: GB201522122D0; GB2545436B

Abstract

A cylindrical superconducting magnet comprising a plurality of axially-aligned superconducting coils 12a-f and a ferromagnetic (e.g. iron) shield 11. The ferromagnetic shield comprises a cylindrical outer surface (5b, figure 2) concentric with the coils and two end surfaces (5a and 5c, figure 2), 10 each end surface including a hole, concentric with the coils. A cryogenic refrigerator 8 may be mounted on the surface of the ferromagnetic shield 11. The system may include a bore tube (7, figure 2) sealed to the ferromagnetic shield 11 and enclosing an evacuated volume (a vacuum). Radiation therapy equipment, such as a linear accelerator (LINAC) 13, a deflection unit 14, a multi-leaf 15, a beam focussing arrangement 16 or a collimator 17, may be disposed on the outer surface of the shield 11. The purpose of the invention is to restrain stray fields generated by superconducting magnets 12a-f.

Description

A CYLINDRICAL SUPERCONDUCTING MAGNET

The present invention relates to superconducting magnets, and particularly relates to shielding arrangements for restraining stray fields generated by superconducting magnets .

Fig. 1 schematically represents a conventional actively-shielded superconducting magnet arrangement. Annular inner coils la, lb, lc, Id, le are arranged about a common axis. Annular shield coils 2a, 2b are also arranged about the same axis, but have a larger radius. The inner coils generate the main magnetic field, while the shield coils generate an opposing magnetic field which restrains the stray magnetic field emanating from the superconducting magnet structure.

The coils are accommodated within an enclosure 3, which may be a cryogen vessel or a thermal shield, depending on the type of cooling provided for the superconducting magnet. A cryogen vessel is required for bath-cooled magnets where the coils are partially immersed in liquid cryogen, but is not provided where conduction cooling is provided, for example, in which case the enclosure 3 may be a thermal radiation shield.

An outer vacuum container (OVC) is provided, enclosing a vacuum region to thermally insulate the superconducting magnet arrangement from ambient temperature.

This arrangement of central coils la - le and shield coils 2a - 2b results in a magnet construction which requires about double the amount of superconducting wire than would be the case to produce inner coils la-le to generate a similar magnetic field but without shielding. This adds significant cost to the manufacture of the structure.

As is conventional, the superconducting coils la-le may be assembled from several lengths of superconducting wire, electrically joined together. These electrical joints typically require the use of a solder, which is diamagnetic. The diamagnetism of the solder causes distortion of the magnetic field. This may be compensated for by shimming, but the screening currents generated in the solder decrease with time, changing the field disturbance and so reducing the effectiveness of the shimming applied to compensate.

Superconducting magnet coils must be cooled to a cryogenic temperature to become superconducting. This may be achieved by partial immersion in boiling cryogen, but a cryogenic refrigerator is needed to re-cool and re-condense the cryogen. In case of conduction cooled magnets, the refrigerator is clearly needed for the conduction cooling.

Cryogenic refrigerators used to cool superconducting magnets typically include a mechanical displacer which contains a paramagnetic rare earth metal. An example of such a refrigerator is the SHI RDK 408 . The mechanical displacer moves in an oscillatory fashion. Because the displacer is magnetised by the presence of the magnetic field of the superconducting magnet structure, the oscillatory movement will result in a cyclic variation of the magnetic field in an imaging region of a MRI system employing such a magnet and refrigerator. A conventional approach to mitigating this effect is to mount the refrigerator further from the inner coils and further from the imaging region. This may be attempted by mounting the refrigerator in a side-sock: an additional volume attached to the OVC. This complicates the manufacture of the OVC.

Typically, the refrigerator is driven by a synchronous motor, which can only operate in magnetic fields of lOOmT or less. This restricts the possible positioning of the refrigerator to regions well away from the imaging region.

The present invention provides an alternative shielding arrangement which produces regions of low magnetic field close to the inner coils and does not require the provision of superconducting shield coils. US5309106 and W02012055890 describe conventional combined MRI/radiation therapy equipment.

The present invention also provides equipment for MRI systems and for combined MRI/radiation therapy equipment.

Accordingly, the present invention provides equipment as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of non-limiting examples only, with reference to the accompanying drawing, wherein:

Fig. 1 schematically shows a conventional actively-shielded superconducting magnet;

Fig. 2 schematically shows principal components of a superconducting magnet according to an embodiment of the present invention;

Fig. 3 illustrates a magnetic field contour of an embodiment of the present invention as shown in Fig. 2;

Fig. 4 illustrates a magnetic field contour of a conventional superconducting magnet structure as shown in Fig. 1;

Fig. 5 schematically illustrates principal components of a combined MRI and radiation therapy equipment according to an embodiment of the present invention; and

Fig. 6 illustrates a magnetic field strength contour in an embodiment of the invention as shown in Fig. 5.

With reference to Fig. 2, an embodiment of the present invention comprises axially-aligned superconducting coils 6a, 6b, 6c, 6d, 6e, 6f. No shield coils are provided. A feature of the invention is a ferromagnetic shield 5a, 5b, 5c, of a ferromagnetic material such as iron. In this embodiment, the ferromagnetic shield comprises an outer cylindrical wall 5b and two annular end-plates 5a, 5c. The annular end plates have openings which approximately correspond in size to the inner diameter of the superconducting coils 6a-6f. The shield is fitted with a bore tube 7, sealed in a vacuum-tight manner to the annular end-plates 5a, 5c. An evacuated volume is thereby defined by the ferromagnetic shield with the bore tube 7. A vacuum is created within the shield as thermal insulation for the superconducting magnet. Thus, the ferromagnetic shield 5a, 5b, 5c, and its bore tube 7 may perform the function of the OVC, and it is no longer necessary to provide a separate OVC.

Also shown in Fig. 2 is a cylinder 9 on which the coils are wound. It may act both as a mechanical support and as a thermal bus. Radiation shield 10 is typically kept at 35 K. The cold stage 11 is the coldest part of the coldhead, and may be thermally linked to cylinder 9 to cool the coils. Such a link is not shown in the drawing, though.

The outer radius of the outer cylindrical wall 5b of the ferromagnetic shield exceeds the outer radius of the superconducting coils 6a-6f by a distance of typically 400mm 1000mm. In the illustrated embodiment, a cryogenic refrigerator 8 is mounted on the outer cylindrical wall 5b of the ferromagnetic shield, and is largely accommodated between the shield outer cylindrical wall 5b and the coils 6a-6f.

Fig. 3 illustrates a lOmT magnetic field strength contour in an embodiment such as shown in Fig. 2. For comparison, Fig. 4 illustrates a lOOmT magnetic field strength contour in a conventional superconducting magnet arrangement as shown in Fig. 1. As can be seen from a comparison of these two contours, the ferromagnetic shield of the present invention encloses a very low magnetic field strength volume just outside the superconducting coils, while the conventional actively-shielded arrangement provides a much stronger magnetic field throughout the corresponding volume.

By comparing Figs. 2 and 3, the cryogenic refrigerator 8 is seen to reside in a volume which has a magnetic field strength of less than lOmT, which is a significant improvement even over positioning in a side sock on an OVC such as illustrated in Fig. 4. As the magnetic field strength is low over the length of the refrigerator, there are no issues due to magnetization of the displacer in the refrigerator .

Other field-sensitive components may also be placed within the low-field region within the shield 5a, 5b, 5c. For example, the solder joints mentioned above may be placed in the low-field region, and would not cause such field disturbance as conventional, and described above. Other field sensitive components such as superconducting switches may also be placed in the low-field region within the ferromagnetic shield.

Fig. 5 schematically illustrates a further embodiment of the present invention. In this embodiment, inner coils 12a - 12f are provided, aligned along an axis. Cryogenic refrigerator 8 is again mounted on the outer cylinder of magnetic shield 11. A bore tube, similar to the bore tube 7 in Fig. 2 will be provided, but is not shown in Fig. 5. Radiation therapy equipment 13, 14, 15, 16, 17 is also mounted on the outer cylindrical surface of the ferromagnetic shield 11, in this case radially opposite the refrigerator 8, but this need not be the case.

The radiation therapy equipment may comprise one or more of the following components: a linear accelerator (LINAC) 13, a deflection unit 14, a multi leaf 15, a beam focussing arrangement 16 and collimator 17. Collimator 17 is sensitive to background magnetic fields, and according to this embodiment of the invention, is placed in a region of low background field.

Fig. 6 shows a lOmT magnetic field contour in an embodiment of the invention as shown in Fig. 5. As can be appreciated from a comparison of Figs. 5 and 6, the radiation therapy equipment 13, 14, 15, 16, 17 is located in a region of low magnetic field strength: less than lOmT in this example. The cryogenic refrigerator 8 is also located in the region of low magnetic field strength.

The present invention accordingly provides a cylindrical superconducting magnet. The present invention may be applied to other types of magnet comprising a shield of ferromagnetic material. In the case of a cylindrical magnet, the ferromagnetic shield comprises a cylindrical outer surface concentric with the coils and two end surfaces, each end surface including a hole, concentric with the coils.

Preferably, the shield should be dimensioned such that the ferromagnetic material is not saturated. The associated limits of saturation depend on the quantity and quality of ferromagnetic material used, and the strength of the magnetic field generated, as will be apparent to those skilled in the art.

The outside radius of the ferromagnetic shield is preferably between 400mm and lOOOmmm larger than the outer radius of the coils. That will allow space for location of the cryogenic refrigerator 8, or any other equipment which may be desired to be placed in the low-field region generated according to the present invention. The inner radius of the shield allows space for mounting the required equipment - for example, for radially mounting a cryogenic refrigerator as shown in Fig. 2 .

The minimum outer radius of the ferromagnetic shield is therefore constrained by the needs to enclose a required space for mounting equipment and to have a sufficient thickness of shield material that the shield material is not saturated. Preferably, the outer diameter of the ferromagnetic shield is at least 400mm but no more than 1000mm more than the outer diameter of the coils.

The present invention provides an effectively shielded superconducting magnet without the need for active shield coils, reducing the consumption of superconducting wire. The positioning of the refrigerator is simplified, and may lead to an improved symmetry in temperature across the coils. The superconducting magnet structure of the present invention allows improved freedom in placement of joints and switches, as a volume is available within the ferromagnetic shield with a low magnetic field strength. The volume between the coils and the outer cylindrical surface of the ferromagnetic shield allows for the location of many components which are sensitive to magnetic fields.

In some embodiments, the shield is constructed to perform the function of the OVC: to be vacuum-tight and to contain an insulating vacuum. This may result in an overall cost saving, as the cost of producing the ferromagnetic shield may be outweighed by the saving in not providing an OVC or shield coils .

Claims

1. A cylindrical superconducting magnet comprising a plurality of axially-aligned superconducting coils (6a-6f; 12a-12f) and a shield (5a, 5b, 5c; 11) of ferromagnetic material, the ferromagnetic shield comprising a cylindrical outer surface (5b) concentric with the coils and two end surfaces (5a, 5c), each end surface including a hole, concentric with the coils.

2. A cylindrical superconducting magnet according to claim 1 wherein an outside radius of the cylindrical outer surface of the ferromagnetic shield is between 400mm and lOOOmmm larger than the outer radius of the superconducting coils.

3. A cylindrical superconducting magnet according to claim 1 or claim 2, further comprising a cryogenic refrigerator (8) mounted on the cylindrical outer surface of the ferromagnetic shield.

4. A cylindrical superconducting magnet according to any preceding claim, further comprising a bore tube (7) sealed to the ferromagnetic shield and enclosing an evacuated volume.

5. A cylindrical superconducting magnet according to any preceding claim, further comprising radiation therapy equipment (13, 14, 15, 16, 17) mounted on the outer cylindrical surface of the ferromagnetic shield (11).

6. A cylindrical superconducting magnet according to any claim 5, wherein the radiation therapy equipment comprises one or more of the following components: a linear accelerator (LINAC) (13); a deflection unit (14); a multi-leaf (15)/ beam focussing arrangement (16); and collimator (17).