GB2465991A

GB2465991A - Gradient Coil Suspension for MRI Magnet

Info

Publication number: GB2465991A
Application number: GB0822152A
Authority: GB
Inventors: Mark James Le Feuvre
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-12-04
Filing date: 2008-12-04
Publication date: 2010-06-09
Also published as: GB0822152D0

Abstract

An MRI cooled superconducting magnet (10) housed within an outer vacuum chamber (OVC) (14). The OVC has a bore tube (22), an outer cylindrical surface and annular end surfaces (30) joining the bore tube to the outer cylindrical surface to form a substantially closed container. The magnet arrangement further comprises a cylindrical gradient coil unit (20) retained within the bore tube (22) of the OVC. The cylindrical gradient coil unit has a bore for accommodating a target for imaging. The gradient coil unit (22) is suspended within the bore tube (22) by attachment to the end surfaces (30) of the OVC. This arrangement reduces eddy currents, acoustic noise amplification and stray fields within the OVC.

Description

GRADIENT COIL SUSPENSION IN MAGNET ARRANGEMENT

The present invention relates to an arrangement for supporting a gradient coil within the bore of a cylindrical magnet, such as those used for nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI).

As is well known to those skilled in the art, an MRI imaging system must generate a number of superimposed magnetic fields. Firstly, a main magnet must generate a background field which is strong and very homogeneous. This field does not vary during the imaging process.

Gradient magnetic fields are provided, which are time-varying magnetic fields, and may vary in all three dimensions x, y, z. Conventionally, the z direction is the axis of symmetry of a cylindrical magnet, corresponding to the head-to-toe direction of a human patient placed within the bore of the magnet. These gradient magnetic fields are usually generated by a number of electromagnet coils, cylindrical and saddle-shaped, to generate the required field variations in three dimensions. All of the gradient electromagnet coils are conventionally formed together, embedded within a resin body or similar to form a single gradient coil unit. This resin body, or similar, including several electromagnet coils, is referred to below as a "gradient coil". To perform imaging, resonance must be induced in the atoms of the target. This is achieved by a body coil, also known as an RF coil, which generates a magnetic field which oscillates with very high frequency (in the radio-frequency range). Typically, the body coil generates the radio-frequency magnetic field to create resonance in the target. The body coil is then turned off, and used to detect resonating magnetic fields in the target. From the timing of the detected resonance and the three-dimensional gradient fields, an image is built up.

The methods used for creating images are well known to those skilled in the art, but form no part of the present invention and so will not be discussed further.

Fig. 1 shows an axial cross-section through a conventional arrangement of a cylindrical superconducting magnet cooled within a cryostat. The structure is essentially rotationally symmetrical about axis AA. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. The cryogen vessel 12 is typically partially filled with a liquid cryogen 15.

Gradient coil 20 is provided within a bore tube 22 of the OVC 14. It has a central bore for accommodating a patient for imaging. It must be securely retained in position.

A conventional arrangement for retaining the gradient coil uses injection moulded wedges 26, typically twelve at each end, equally spaced around the circumference of the gradient coil. These wedges 26 are pushed into the circumferential gap around the outside of the gradient coil 20, between the gradient coil and the bore tube 22 of the OVC 14, to rigidly fix the gradient coil in position. This rigid fixation provides a path for the mechanical vibration to pass into the surrounding OVC structure through wedges 26. Such transmitted vibration increasing the noise of the system and also creating secondary induced eddy currents within the material due to its movement within the strong magnetic environment.

During an imaging cycle within an MRI imaging system, the gradient coil is pulsed with electrical current at frequencies of up to 2 kHz.

This of course generates a magnetic field at a corresponding frequency, which interacts with the background magnetic field provided by superconducting magnet 10. This operation of the gradient coil creates large non symmetrical internal forces within the gradient coil 20 as well as an alternating stray magnetic field that penetrates out into the OVC structure 14 and beyond. These phenomena result in firstly, direct mechanical vibration of the gradient coil due to the interaction of the oscillating gradient field and the stable background field. This mechanical vibration transfers into the OVC bore tube 22, causing alarming noise to a patient, discomfort to an operator, and possibly interfering with the imaging process. The mechanical movement of conductive materials such as the bore tube 22 of the OVC 14 within the background magnetic field will cause eddy currents to be induced within them, in turn causing heating, and magnetic fields which may interfere with imaging. Secondly, relative motion between the gradient coil and surrounding conductive material will cause induced eddy currents, again causing unwanted heating, magnetic field interferences and electromagnetically induced vibrations, by interaction of the background field with magnetic fields caused by the eddy currents within these conductive structures.

Induced eddy currents flowing in the material of the OVC or thermal radiation shield disturb the magnetic field used for imaging, causing ghosting of the image.

Stray gradient magnetic field oscillations may penetrate the OVC and induce eddy currents and vibrations within the thermal shield 16, which results in heating of the thermal radiation shield, some of which heat may reach the cryogen vessel 12 resulting in increased boil-off of the liquid cryogen 15.

The present invention accordingly aims to address some of the drawbacks of the known arrangements by providing an arrangement for retaining a gradient coil 20 within the bore tube 22 of a cylindrical magnet, which reduces the amount of mechanical vibration transmitted to the OVC and other parts of the cryo stat.

In some embodiments of the invention, arrangements are provided to reduce the stray gradient magnetic field reaching components of the cryo stat.

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

The objects, characteristics and advantages of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Fig. 1 shows an axial cross-section through a conventional arrangement of a cylindrical superconducting magnet cooled within a cryo stat; Fig. 2 shows an example of a gradient coil suspension arrangement according to the present invention, schematically in axial cross-section; Fig. 2A shows a partial axial cross section of an assembly of an OVC bore tube, end spinning and a backing piece; Fig. 3 shows an example of a cradle piece according to an embodiment of the invention; Figs. 3A and 3B show example mounting arrangements for a body coil within the gradient coil; Fig. 4 shows two cradle pieces in place on opposite ends of a gradient coil, in the positions schematically illustrated in Fig. 2; and Figs. 5 and 6 each show an enlargement of a part of Fig. 2, modified to illustrate further embodiments of the invention.

One of the aims of the present invention is to reduce the mechanical vibration path between the OVC 14 and the gradient coil 20. A second aim of the present invention is to reduce the stray field produced at the ends of the gradient coil, and to reduce interaction of this stray field with the material of the OVC 14.

The Lorentz force is the force acting on an electric charge moving in a magnetic field. During an imaging procedure, very high Lorentz forces are experienced within the gradient coil 20 as pulsed electric current flows.

These Lorentz forces cause the whole gradient coil 20 to distort into a number of different eigenmodes depending upon the frequency of the current pulses applied to the gradient coil 20.

Eigenmodes are the free resonant modes of vibration, which are essentially shape configurations that the gradient coil will form at certain resonant frequencies. These are characterised with larger amplitudes and therefore cause larger vibration and noise effects within the system dynamics than vibrations associated with other frequencies.

In conventional arrangements such as shown in Fig. 1, these mechanical vibrations are transferred directly into the OVC bore tube 22 through the wedges 26. As a result, the OVC bore tube vibrates thus producing eddy currents. These eddy currents in turn interact with the static magnetic field and cause vibration throughout the OVC. Further to this, the stray field from the gradient coil 20 can penetrate the OVC at certain frequencies, whereupon the stray field causes heating of the thermal shield 16, leading to heating of the cryogen vessel 12 and increased boil-off of liquid cryogen 15.

In recent years, there has been a tendency for magnets of MRI systems to become shorter, with the effect that current systems have gradient coils 20 of much the same length as main magnet 10.

Accordingly, the stray magnetic field which spills from the ends of the gradient coil 20 may interact with the largely planar, essentially circular, end surfaces 30, 32 of the OVC 14 and shield 16. These end surfaces may be known as end spinnings, as they are typically of metal and are formed by a spinning process. Such interaction of the stray gradient fields with the end spinnings adds to the acoustic noise generated during imaging and induces further boil-off of liquid cryogen 15 due to heating of the thermal shield 16. These effects are known to vary in severity with the frequency of application of current to the gradient coil 20.

According to the present invention, in order to reduce the transmission of mechanical vibrations from the gradient coil to the bore 22 of the OVC 14, the gradient coil is suspended within the bore tube by a cradle attached to the end spinnings 30 of the OVC. An example of a magnet arrangement according to an embodiment of the prior art is shown in Fig. 2. Features corresponding to features of Fig. 1 carry corresponding reference numerals.

As shown in Fig. 2, an annular cradle piece 36 is attached between each end of the gradient coil 20 and the corresponding end spinning of the OVC 14. A radially inner flange 38 of each cradle piece 36 is attached to an end surface of the gradient coil 22, while a radially outer flange 40 is attached to the end spinning of the OVC 14. The radially inner and outer flanges are connected by an axially extending piece 42. Preferably, damping fixtures 44 are provided between the cradle piece and the gradient coil, and between the gradient coil and the end spinning 30. The damping fixtures 44 may, for example, be bolts passing through a pair of rubber washers, and through a hole in the cradle piece, the cradle piece being sandwiched between the rubber washers. The damping fixtures serve to isolate the gradient coil vibrations from the OVC spinnings.

The cradle piece 36 is preferably attached to the end spinning 30, for example by bolting, from a known quiet point', that is, a position on the end spinnings that is structurally stiff and sufficiently strong to resist the structural forces exerted during gradient coil current pulsing. An example quiet point, as illustrated in Fig. 2, is believed to be close to the joint between the bore tube and the end spinning. Typically, at that joint, a backing piece 35 is provided, joining the end spinning 30 and the bore tube 22. All three pieces are typically welded together. An example arrangement is shown in part-axial cross section in Fig. 2A. The extra material provided by the backing piece and the welding, and the geometry of the structure at that point, mean that the OVC is particularly stiff at that point.

Fig. 3 shows a perspective view of an example cradle piece 36 according to an embodiment of the invention. Typically, the cradle piece is metallic, but non-magnetic, for example being of aluminium. The axially extending piece 42 may be provided with a stiffened section, for example formed by winding a composite material such as resin-wetted glass fibre filament around the cradle piece itself. This would be a relatively inexpensive process, as the cradle piece fulfils the function of a winding mandrel. The thickness and extent of the stiffened section may be designed, or customised of necessary, to resist the various possible eigenmodes of vibration which may be induced by current switching within the gradient coil at various frequencies. Damping fixtures 44 for inclusion between the cradle piece and the end spinning are clearly visible.

Similar damping fixtures would be provided between the inner flange 38 and the gradient coil 20.

Just visible in Fig. 3 are mounting holes 50 for mounting a body coil, or RF coil as follows. In order to isolate the body coil from the gradient coil, the body coil may be mounted to the OVC by mounting fixtures passing through the cradle via holes 50. Alternatively, the body coil may be retained by extended fixation arms reaching from the body coil, out through the cradle piece to attach to the OVC end spinnings radially outside of the cradle piece. Example arrangements are shown in Figs. 3A and 3B, showing possible mounting arrangements for body coil 80 on mounting fixtures 82 or fixation arms 84.

Fig. 4 shows two cradle pieces 36, attached to respective ends of a gradient coil 20. In a real installation, at least one of the cradle pieces 36 would be attached to the gradient coil 20 only after the gradient coil is positioned within the bore 22 of the OVC.

Fig. 5 illustrates an improved embodiment of the present invention.

Fig. 5 represents an enlarged portion of the arrangement of Fig. 2, wherein vacuum seals 46 are provided, both between the cradle piece 36 and the gradient coil 20, and between the cradle piece 36 and the end spinning 30.

One such vacuum seal 46 is visible in Fig. 4. In this embodiment, a vacuum may be created in the cylindrical space between the gradient coil and the bore tube 22. Provision of such a vacuum further reduces noise and mechanical vibration transferred from the gradient coil 20 to the OVC 14 as the former air gap no longer acts as a vibration medium.

In a particularly advantageous embodiment, the functions of the vacuum seals 46 and the damping fixtures 44 may be combined. For example, a resilient ring, for example of rubber, may be provided on the faces of the inner and outer flanges 38, 40 of each cradle piece 36.

Mechanical fasteners such as bolts may pass through holes in the resilient ring to screw into complementary threaded holes in the gradient coil 20.

When the fasteners are tightened, the resilient rings are compressed to form gas-tight seals, and fulfil the functions of both the vacuum seals 46 and the damping fixtures 44.

In a further development of this concept, the mechanical fixings may be omitted. Once a vacuum has been created in the space between the gradient coil 20 and the bore tube 22, atmospheric pressure may be sufficient to maintain the vacuum seal and retain the gradient coil in position.

In Fig. 6, dotted line 52 schematically illustrates the provision of services to the gradient coil 20. Such services may include electrical -10-current supply and return for each coil and cooling water supply and return paths.

In certain embodiments of the present invention, and as illustrated in Fig. 6, an isolated electrically conductive annular plate 54 is mounted onto each cradle piece 36, offset away from the OVC end spinnings. These annular plates act as a magnetic damper to restrain mechanical vibrations within the OVC end spinnings 30.

A "magnetic damper" means an electrically conductive plate which resists movement. The very movement of a conductive surface in a magnetic field induces eddy currents which produce their own magnetic field, which resists the movement of the conductive surface. This damping is occurs best when the conductive surface cuts magnetic flux lines perpendicularly. Although the flux lines are quite complex at the end of the magnet, the conductive annular plate 54 parallel to the OVC end spinning will cause a significant degree of damping.

In certain embodiments, the dimensions and material of the plates 54 may be selected to provide shimming of the background magnetic field in the z-direction.

The present invention, in each of its various embodiments, provides at least some of the following advantages.

Mechanical vibration of the OVC is reduced. Particularly, vibration within the bore tube 22 is reduced. This is achieved by the gradient coil 20 being mechanically isolated from the bore tube 22, and mounted onto the OVC 14 at quiet points outside of the bore tube. This reduced mechanical -11 -vibration and in turn reduces eddy currents within the material of the bore tube, reduces heating of the thermal radiation shield 16, and reduces boil-off of the liquid cryogen 15. The mechanical vibration of the OVC is likely to interfere with the imaging procedure, so any reduction in the mechanical vibrations is likely to improve the imaging procedure.

Acoustic noise levels due to vibration of the OVC bore tube 22 and end spinnings 30 are reduced, due to the gradient coil 20 being mechanically isolated from the bore tube, and mounted onto the OVC at quiet points outside of the bore tube, and due to reduced eddy currents in the material of the OVC. The reduced noise levels should reduce anxiety in patients, reduce discomfort for the operator and make the imaging procedure more productive as the patient is likely to be more co-operative.

The reduced mechanical vibration of the OVC bore tube 22 leads to reduced induced eddy currents within the OVC and thermal radiation shield bore tubes. This in turn leads to reduced boil-off of liquid cryogen.

The positioning of the metallic cradle pieces intercepts some of the stray gradient field, which reduces the stray gradient field interaction with the end spinnings 30, 32 of the OVC 14 and the thermal radiation shield 16. This reduction in stray field interaction reduces eddy currents and heating in the shield, thus reducing boil-off of the liquid cryogen.

As the weight of the gradient coil is not borne by the bore tube 22 of the OVC 14, but by the end spinnings 30, and gradient stray field is reduced by the presence of the electrically conductive cradle pieces, thinner metallic OVC end spinnings, or non-conductive OVC end spinnings, may be used without greatly increasing boil-off.

Stray field from the gradient coil is conventionally somewhat absorbed by the OVC ends, thus protecting the highly conductive thermal radiation shield ends from the stray field, which if exposed would result in increased heating of the cryogen vessel, cryogen boil-off and potential image ghosting interference. By partially shielding the gradient coil's stray field from the OVC end spinnings, due to the cradle 36 of the present invention, some of the conductive material of the OVC end spinnings may be removed without affecting the stray field getting to the thermal radiation shield.

The mounting of cradle pieces 36 on the OVC end spinnings 30 effectively increases the stiffness of the OVC end spinnings, further enabling them to be made of thinner material. The use of thinner materials reduces weight, material cost and transport cost.

Similarly, the reduced mechanical loading on the bore tube 22 means that thinner material may be used for the bore tube.

While the present invention has been described with reference to a limited number of particular embodiments, numerous modifications and variations will be apparent to those skilled in the art.

For example, the present invention may be applied to cooled magnets which are not partially immersed in liquid cryogen 15 in a cryogen vessel. Some known superconducting magnets are cooled by direct mechanical cooling, or by a closed cooling loop, also known as a thermo-siphon. Such magnets are usually still housed within an OVC, and a thermal radiation shield is typically provided between the OVC and the -13 -magnet. References to boiled off cryogen in the description may be read as excess thermal influx when applied to such embodiments.

The described embodiments all relate to cylindrical magnets which have a horizontal axis. The present invention may also be applied to magnets arranged with a vertical axis, although the cradle pieces and end spinnings would then need to be designed to bear the weight of the gradient coil in the axial direction.

The described embodiments all have a single gradient coil unit, in which x, y and z gradient coils are all embedded in a single body. The present invention may, however, be applied to arrangements in which multiple separate electromagnet coil units are provided. For example, each of the x, y, z gradient coils may be physically separate, or the gradient coil may be divided in any other appropriate way.

The present invention has been particularly discussed with reference to magnetic resonance imaging of human patients. However, the present invention may be applied to imaging systems for imaging animals, or inanimate objects, for example in nuclear magnetic resonance spectroscopy.

While the present invention has been described with particular reference to the use of annular cradle pieces, the present invention may be embodied with any suitable means of mechanically connecting the gradient coil unit to the end spinnings of the OVC. For example, a number of brackets of strip metal may be attached and radially distributed around the end of the gradient coil unit, attached to suitable points on the OVC end spinnings. Alternatively, sector-shaped brackets may be provided.

-14 -These may be assembled into adjoining relationship, such that the finished arrangement resembles a single cradle piece, or the sector-shaped brackets may be arranged at intervals around the periphery of the gradient coil unit. Different arrangements may be provided at the ends of the gradient coil unit. For example, a cradle piece may be provided at one end of the gradient coil, while strip metal mounting brackets may be used at the other end.

Claims

CLAIMS1. A cylindrical magnet arrangement comprising a cooled superconducting magnet (10) housed within an outer vacuum chamber (OVC) (14) having a bore tube (22), an outer cylindrical surface and annular end surfaces (30) joining the bore tube to the outer cylindrical surface to form a substantially closed container, the magnet arrangement further comprising a cylindrical gradient coil unit (20) retained within the bore tube (22) of the OVC, the cylindrical gradient coil unit having a bore for accommodating a target for imaging, characterised in that the gradient coil unit (22) is suspended within the bore tube (22) by attachment to the end surfaces (30) of the OVC.
2. A magnet arrangement according to claim 1 which is essentially symmetrical about an axis (AA) which passes through the centre of the bore of the gradient coil and the bore tube (22) of the OVC.
3. A magnet arrangement according to any of claims 1-2, wherein the gradient coil unit (20) comprises multiple separate electromagnet coils, arranged to provide varying magnetic field gradients in orthogonal planes.
4. A magnet arrangement according to claim 3, wherein the electromagnet coils are embedded within a single body.
5. A magnet arrangement according to any preceding claim, wherein the gradient coil unit (22) is suspended by an annular cradle piece (36) attached between an end of the gradient coil 20 and the corresponding end spinning (30) of the OVC (14). -16-
6. A magnet arrangement according to claim 5, further comprising damping fixtures (44) between the cradle piece (36) and the gradient coil unit (20), and between the gradient coil unit (20) and the end spinning (30).
7. A magnet arrangement according to claim 5 or claim 6 wherein the cradle piece (36) comprises: -a radially inner flange (38) attached to an end surface of the gradient coil unit (20); -a radially outer flange (40) attached to the end spinning (30) of the OVC 14; and -an axially extending piece (42) joining the radially inner and outer flanges.
8. A magnet arrangement according to claim 7 wherein the cradle piece is formed of a metal and the axially extending piece (42) is 9. A magnet arrangement according to any preceding claim further comprising a body coil mounted within the bore of the gradient coil unit (20), wherein the body coil is suspended within the bore of the gradient coil unit (20) by attachment to the end surfaces (30) of the OVC.10. A magnet arrangement according to claim 9 when dependent on any of claims 5-8, wherein the cradle piece is provided with holes (50) and mounting fixtures pass through the holes (50) to join the body coil to the end surfaces of the OVC.11. A magnet arrangement according to claim 9 when dependent on any of claims 5-8, wherein the body coil is retained by extended fixation arms reaching from the body coil, out through the cradle piece to attach to the OVC end surfaces radially outside of the cradle piece.12. A magnet arrangement according to claim 5 or any claim dependent on claim 5, wherein vacuum seals (46) are provided, both between the cradle piece (36) and the gradient coil unit (20); and between the cradle piece (36) and the end surface (30).13. A magnet arrangement according to claim 12 wherein a resilient ring is provided on the face of each of the inner and outer flanges (38, 40) of the cradle piece (36); and wherein mechanical fasteners pass through holes in the resilient ring such that the resilient rings are compressed to form gas-tight seals, and fulfil the functions of both the vacuum seals (46) and damping fixtures (44).14. A magnet arrangement according to claim 12 wherein a vacuum exists in the cylindrical space between the gradient coil unit (20) and the OVC bore tube (22).15. A magnet arrangement according to claim 14 wherein a resilient ring is provided on the face of each of the inner and outer flanges (38, 40) of the cradle piece (36); such that the resilient rings are compressed to form gas-tight seals, and fulfil the functions of both the vacuum seals (46) and damping fixtures (44).16. A magnet arrangement according to claim 5 or any claim dependent on claim 5, wherein an isolated electrically conductive annular -18-plate (54) is mounted onto each cradle piece (36), offset away from the OVC end surfaces (30).17. A magnet arrangement according to any preceding claim wherein the cooled superconducting magnet (10) is partially immersed in liquid cryogen (15) in acryogen vessel (12) within the OVC (14).18. A magnet arrangement according to any of claims 1-16 wherein the cooled superconducting magnet (10) is cooled by direct mechanical cooling.19. A magnet arrangement according to any of claims 1-16 wherein the cooled superconducting magnet (10) is cooled by a closed cooling loop.20. A magnet arrangement according to any preceding claim, provided with a thermal radiation shield (16) within the OVC (14), surrounding the cooled superconducting magnet (10).21. A magnet arrangement substantially as described and/or as illustrated in Figs 2-6 of the accompanying drawing.Amendment to the claims have been filed as follows 1. A cylindrical magnet arrangement comprising a cooled superconducting magnet (10) housed within an outer vacuum chamber (OVC) (14) having a bore tube (22), an outer cylindrical surface and annular end surfaces (30) joining the bore tube to the outer cylindrical surface to form a substantially closed container, the magnet arrangement further comprising a cylindrical gradient coil unit (20) retained within the bore tube (22) of the OVC, the cylindrical gradient coil unit having a bore for accommodating a target for imaging, characterised in that the gradient coil unit (22) is suspended within the bore tube (22) by an annular cradle piece (36) attached between an end of the gradient coil (20) and the corresponding end surface (30) of the OVC (14). a)2. A magnet arrangement according to claim 1 which is essentially symmetrical about an axis (AA) which passes through the centre of the bore of the gradient coil and the bore tube (22) of the OVC. (03. A magnet arrangement according to any of claims 1-2, wherein the gradient coil unit (20) comprises multiple separate electromagnet coils, arranged to provide varying magnetic field gradients in orthogonal planes.4. A magnet arrangement according to claim 3, wherein the electromagnet coils are embedded within a single body.5. A magnet arrangement according to any preceding claim, further comprising damping fixtures (44) between the cradle piece (36) and the gradient coil unit (20), and between the gradient coil unit (20) and the end spinning (30).6. A magnet arrangement according to any preceding claim wherein the cradle piece (36) comprises: -a radially inner flange (38) attached to an end surface of the gradient coil unit (20); -a radially outer flange (40) attached to the end spinning (30) of the OVC 14; and -an axially extending piece (42) joining the radially inner and outer flanges.7. A magnet arrangement according to claim 6 wherein the 0) cradle piece is formed of a metal and the axially extending piece (42) is wound with a composite material. L()8. A magnet arrangement according to any preceding claim further comprising a body coil mounted within the bore of the gradient coil unit (20), wherein the body coil is suspended within the bore of the gradient coil unit (20) by attachment to the end surfaces (30) of the OVC.
9. A magnet arrangement according to claim 8, wherein the cradle piece is provided with holes (50) and mounting fixtures pass through the holes (50) to join the body coil to the end surfaces of the OVC.
10. A magnet arrangement according to claim 8, wherein the body coil is retained by extended fixation arms reaching from the body coil, out through the cradle piece to attach to the OVC end surfaces radially outside of the cradle piece.
11. A magnet arrangement according to any preceding claim, wherein vacuum seals (46) are provided, both between the cradle piece (36) and the gradient coil unit (20); and between the cradle piece (36) and the end surface (30).
12. A magnet arrangement according to claim 11 wherein a resilient ring is provided on the face of each of the inner and outer flanges (38, 40) of the cradle piece (36); and wherein mechanical fasteners pass through holes in the resilient ring such that the resilient rings are compressed to form gas-tight seals, and fulfil the functions of both the vacuum seals (46) and damping fixtures (44).0)
13. A magnet arrangement according to claim 11 wherein a vacuum exists in the cylindrical space between the gradient coil unit (20) 14) and the OVC bore tube (22). (014. A magnet arrangement according to claim 13 wherein a resilient ring is provided on the face of each of the inner and outer flanges (38, 40) of the cradle piece (36); such that the resilient rings are compressed to form gas-tight seals, and fulfil the functions of both the vacuum seals (46) and damping fixtures (44).15. A magnet arrangement according to any preceding claim, wherein an isolated electrically conductive annular plate (54) is mounted onto each cradle piece (36), offset away from the OVC end surfaces (30).16. A magnet arrangement according to any preceding claim wherein the cooled superconducting magnet (10) is partially immersed in liquid cryogen (15) in acryogen vessel (12) within the OVC (14).17. A magnet arrangement according to any of claims 1-15 wherein the cooled superconducting magnet (10) is cooled by direct mechanical cooling.18. A magnet arrangement according to any of claims 1-15 wherein the cooled superconducting magnet (10) is cooled by a closed cooling loop.19. A magnet arrangement according to any preceding claim, 0) provided with a thermal radiation shield (16) within the OVC (14), surrounding the cooled superconducting magnet (10).LU20. A magnet arrangement substantially as described and/or as (0 o illustrated in Figs 2-6 of the accompanying drawing.