The present invention relates to superconducting magnets for generating homogeneous magnetic fields, as typically used in MRI imaging systems. In particular, it relates to the mechanical means used to support magnet coils within a cryostat, and the effect of such mechanical means upon the homogeneity of the resultant magnetic field. Fig. 1 shows a conventional arrangement of a superconducting magnet within a cryostat without a cryogen vessel. A cooled superconducting magnet coil structure 110, comprising magnet coils 10 is provided within outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the magnet coils 10 and the OVC 14. A cooling arrangement is provided to cool the magnet coil structure 110. In some known arrangements, refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in Fig. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen to a much lower temperature, typically in the region of 4-10K.

A negative electrical connection 21a is usually provided to the magnet coils 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20. Mechanical support for magnet coil structure 110 and thermal radiation shields 16 is provided by tensile rods or bands 30 connected between the OVC 14 and the coil structure 110, passing through respective holes in the thermal radiation shield(s) 16. Tension of these tensile rods and bands is adjusted to ensure that the magnet coils 10 and thermal radiation shields 16 are firmly held in place. A similar structure may be provided to retain the thermal radiation shields. Due to thermal contraction, the dimensions of various components change when the superconducting magnet is cooled. Components of differing materials will contract at different rates. As the tension of tensile rods and bands 30 is typically adjusted at room temperature, this tension will vary as the magnet cools. Care must be taken to ensure that contraction of the various components on cooling is taken in to account when tensioning the tensile rods and bands . The tension of the tensile rods and bands 30 is typically set prior to shipping of the magnet to its intended site of installation. The tensions are usually high to reduce the risk of damage to any part of the magnet or cryostat structure caused by movement, particularly of the heavy coil structure, relative to the OVC due to shock loads encountered during shipping. The tensions of the tensile rods and bands 30 cannot be adjusted later without breaking the vacuum within the OVC.

A gradient coil assembly 24 is provided within the bore of the OVC. Typically, the gradient coil assembly comprises a number of coils of resistive wire, such as copper, arranged to generate oscillating magnetic gradient fields in three orthogonal dimensions. These coils are typically embedded in a thermosetting resin. Slots 26 are provided within the material of the gradient coil assembly to hold shim trays 28. Pieces of ferromagnetic material known as shims are placed in shim trays 28 at selected locations, as is well known to those skilled in the art. These shims influence the magnetic field generated by the magnet, and computer simulation is usually employed in an iterative process known as shimming, involving energizing the magnet, measuring the homogeneity of the resultant field, de-energizing the magnet and adjusting the distribution of shims within the shim trays. Other arrangements and methods are known for carrying out shimming, such as by using resistive or superconductive shim coils to influence the total magnetic field generated by the superconducting magnet.

It has been found, however, that distortion to the magnet coil assembly 110 and the magnet coils 10 themselves is caused by differential thermal contraction of magnet coils, OVC and tensile rods and bands. This distortion results in a degradation of expected magnetic field homogeneity. Such degradation in homogeneity may adversely affect the images produced by the MRI system, or may make MRI imaging impossible . A homogeneous imaging volume is an essential requirement of a high performing MRI system. The actual shape and position of superconducting coils is affected by many factors, such as build tolerances, distortion due to Lorentz forces when in operation, thermal contraction, loads applied by tensile support rods and bands 30. The effects of all these inputs must be corrected for.

The homogeneity of the magnetic field in an imaging volume is conventionally improved by passive shimming using ferro- magnetic material, and/or active shimming which involves changing current in shim coils whether resistive or superconducting. These methods have disadvantages, however, including technical challenges, cost implications, time consuming iterations of energising and de-energising, field drift due to temperature change, and reduced patient comfort due to restricted available bore diameter d due to the radial thickness of the shimming arrangements.

The present invention accordingly seeks to reduce the adverse effects of distortion of superconducting magnet coils on homogeneity of a magnetic field generated by a superconducting magnet, and 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, given by way of non-limiting example only, in conjunction with the appended drawings, wherein:

Fig. 1 shows a radial cross-section of a conventional superconducting MRI magnet in a cryostat;

Fig. 2 represents a radial cross-section of magnet coils as designed;

Fig. 3 represents a first type of distortion of the magnet coils;

Fig. 4 represents a second type of distortion of magnet coils 10;

Figs. 5A-5B represent cross-sections through tensile rod or band tension adjusting arrangements;

Fig. 6 shows a cross-section through a compressive support element as may be employed in the present invention;

Figs. 7, 8 represent cross-sections of certain embodiments of support adjustment mechanisms;

Fig. 9 represents an arrangement of tensile support elements; and

Fig. 10 shows an axial half-cross-section of a coil assembly 110 which may be used in the present invention.

As discussed above, it is conventional for significant tension to be applied to tensile rods and bands 30 which restrain magnet coils 10 and thermal radiation shields 16 within the OVC, to prevent damage to the magnet and cryostat structure during shipping due to shock loads . Such tension may cause distortion of the magnet coils 10, which in turn causes distortion of the generated magnetic field. This causes a reduction in homogeneity of the produced magnetic field, which reduces the ability of the system to produce effective MRI images. Conventionally, adjustment of the tension in the tensile bands and rods is performed prior to shipping. According to an aspect of the present invention, tension in tensile support rods and bands is adjusted once the magnet has arrived at its installation site, as part of the shimming procedure. Such adjustment allows for adjustment of tension in the tensile support rods and bands to compensate for expected distortion of the coils due to differential thermal contraction of the coils, the tensile support rods and bands and the OVC, during cooling of the magnet, and other influences such as manufacturing tolerances and the local magnetic environment.

According to an aspect of the present invention, arrangements are made for adjustment of tension of tensile rods and bands 30 without the need to access the interior of the OVC and with the superconducting magnet cooled to operating temperature. Some example arrangements will be described in more detail below .

Fig. 2 schematically represents a radial cross-section of magnet coils 10 as designed. The coils have a circular cross- section, and tensile bands or rods 30, 32 retain the magnet coils in position. Notably, tensile bands or rods 30, 32 are only schematically illustrated. In practice, there may be more than four of them, they will not all lie within a radial plane and they will not all extend radially to an axis A-A of the magnet coils. Fig. 3 represents a first type of distortion of the magnet coils 10. Here, the coils 10 are deformed radially outward in the direction of tensile support rods or bands 30, while being deformed radially inward in the direction of tensile support rods or bands 32. Such deformation will affect the homogeneity of the magnetic field produced by the magnet. The illustrated deformation may represent an unwanted distortion of the magnet due to differential thermal contraction of coils, OVC and suspension elements. Alternatively, such distortion may represent distortion intentionally applied to the magnet coils 10 in performance of a method according to the present invention in order to improve the homogeneity of the magnetic field of the magnet. Fig. 4 represents a second type of distortion of the magnet coils 10. Here, the coils 10 are deformed radially outward in the direction of tensile support rods or bands 32, while being deformed radially inward in the direction of tensile support rods or bands 30. Such deformation will affect the homogeneity of the magnetic field produced by the magnet. The illustrated deformation may represent an unwanted distortion of the magnet due to differential thermal contraction of coils, OVC and suspension elements. Alternatively, such distortion may represent distortion intentionally applied to the magnet coils 10 in performance of a method according to the present invention in order to improve the homogeneity of the magnetic field of the magnet.

Fig. 5A represents a cross-section through a tensile rod or band tension adjusting arrangement 50 according to an embodiment of the present invention. A hole 52 is provided through the wall of OVC 14. A radially outer end 54 of tensile support rod, or an arrangement for adjusting the tension in a tensile support band, 30, passes through the hole 52. A retaining structure 56 retains the tensile support rod or tensile support band adjuster in position relative to a retainer platform 58. A bellows structure 60 encloses a vacuum region 62 around the tensile support rod or tensile support band adjuster to prevent leakage of air into the OVC 14. The structure may be sealed against air ingress by an o-ring (not illustrated) or by the welding of bellows 60 to the tensile support rod, or arrangement for adjusting the tension in a tensile support band.

As mentioned above, such arrangements allow adjustment of the tension of retaining rods or bands without the need to access the interior of the OVC. Indeed, adjustment of the tension of tensile rods or bands may be carried out with the magnet at field, that is, with current flowing in cooled superconducting coils 10 to generate the required magnetic field. Adjusting bolts 63 are threaded through retainer platform 58 and bear against a support surface 64 on the wall of the OVC 14. Support surface 64 may be specially strengthened, or may have a low-resistance coating applied, such as a layer or disc of polytetrafluoroethylene PTFE.

To increase tension on the tensile support rod or tensile support band adjuster, 54, adjusting bolts 63 may be driven so as to pull retainer platform 58 radially outward away from the OVC. Within a given range of motion, bellows 60 can expand and contract to maintain the vacuum region 62 sealed against air influx. The tension on tensile support rod or tensile support band adjuster, 54, and air pressure will serve to retain adjusting bolts 63 against the support surface 64, although a retaining structure may be provided if desired. A retaining structure such as shown at 66 may be provided to prevent movement of the tension adjusting arrangement 50 across the surface of the OVC 14.

Similarly, to reduce tension on the tensile support rod or tensile support band adjuster, 54, adjusting bolts 63 may be driven so as to allow retainer platform 58 to move radially inward towards the OVC .

Locking nuts 68 are preferably provided on the adjusting bolts 62 to prevent unintentional movement of the bolts and adjustment of tension on tensile support rod or tensile support band adjuster, 30.

Nut 70 is threaded onto a threaded end of the tensile support rod or tensile support band adjuster, 54, and bears onto a support surface 72. Support surface 72 preferably comprises a part-spherical underside 74 bearing against a complementary bearing surface 76 within retaining structure 56. This allows a certain degree of angular displacement of the tensile support rod or tensile support band adjuster, 54, relative to the OVC 14.

An additional or alternative method for adjusting tension on tensile support rod or tensile support band adjuster, 54, may be achieved by tightening or loosening nut 70. A locking nut 78 may be provided to prevent unintentional adjustment of the nut 70.

Fig. 5B represents an alternative arrangement, similar to the arrangement of Fig. 5A, but having a pivot point 79 about which the retainer platform 58 may rotate, as determined by adjustment of one or more bolts 63. In such embodiments, the angular displacement permitted by part-spherical surfaces 74, 76 is necessary to avoid flexure of tensile support rod or tensile support band adjuster, 54.

Fig. 6 shows a cross-section through a compressive support element as may be employed in the present invention. Instead of, or in addition to, tensile support rods or tensile support bands 30, 32, compressive supports such as shown at 80 may be used. Rather than exerting a force on the magnet coils 10 which would tend to pull the coils towards the OVC, compressive supports 80 exert a force on the magnet coils 10 which tend to push the coils away from the OVC. Such supports will have a similar effect to the tensile supports discussed earlier, although operating in a respective opposite direction .

In the example schematically illustrated in Fig. 6, compressive support 80 includes an enclosure 82 carrying a threaded hole 84 into which a threaded compressive support member 86 is threaded. Compressive support member 86 passes through a hole 88 in the wall of the OVC 14 to bear upon magnet coil structure 10 at a bearing surface 92. Preferably, bearing surface 92 is protected by reinforcement such as a patch of polytetrafluoroethylene PTFE. The bearing surface 92 should not be radially adjacent a coil 10, as any movement or distortion of the coil may result in a quench. The bearing surface 92 should be located on part of a support structure of magnet coil assembly 110, conventional in itself, axially located between magnet coils 10. A bellows arrangement 90 is provided to seal the hole 88 in the OVC against influx of air. The compressive support member 86 passes through a hole 94 in thermal radiation shield (s) 16. By rotating threaded compressive support member 86 in threaded hole 84, the support member 86 may be driven toward or away from the magnet assembly, increasing or reducing compressive force bearing upon the magnet assembly. Figs. 7, 8 represent cross-sections of certain embodiments of support adjustment mechanisms.

In the arrangement of Fig. 7, compressive support member 86 itself rotates with respect to bellows 90. Care must be taken to ensure that a vacuum-tight seal is maintained between compressive support member 86 and bellows 90. In the illustrated embodiment, enclosure 82 is retained against the OVC by one or more retainer 96.

In the arrangements discussed with reference to Figs. 5-7, the 5 bellows arrangement 90 may be replaced by an o-ring, suitably retained in contact with a suitably smooth and round surface of tensile support rod or tensile support band adjuster, 30, or compressive support member 86. In arrangements using bellows, an o-ring may be used to provide vacuum sealing while 10 allowing relative rotation between the bellows 90 and the tensile support rod or tensile support band adjuster, 30, or compressive support member 102.

In the arrangement of Fig. 8, enclosure 82 retains a threaded 15 drive bolt 98 in position, partly threaded into a cavity 100 in an end of a compressive support member 102. Adjustment of the compressive force applied by compressive support member 102 is achieved by rotating threaded drive bolt 98 within cavity 100. In this arrangement, bellows 90 can be sealed to 20 the compressive support member 102 by welding, to provide a reliable vacuum-tight seal, as there is no need for the compressive support member 102 to rotate against the bellows 90. Bellows 90 maintains a vacuum-tight seal, preventing influx of air despite movement of compressive support member 25 102.

While described with particular reference to adjustment of compressive support members 86, 102, the support adjustment mechanisms of Figs. 7-8 may be employed to adjust tension on 30 tensile support rods or tensile support band adjusters, 54.

Clearly, the tension in the tensile support rod or tensile support band adjuster, 54, or the compression in compressive support members 86, 102 can be adjusted from outside of the OVC.

35 While Figs. 3-4 schematically represent the concept of coil assembly distortion due to tension and compression in support elements, Fig. 9 represents a more realistic arrangement of tensile support elements 30, 130. The illustrated arrangement provides restraint in all axes, preventing radial or axial motion of the coil assembly 110 with respect to the OVC, and rotation of the coil assembly about its axis A-A. The methods of the present invention may of course be applied to such an arrangement, with suitable calculation of the effects and interactions of tensions or compressive forces within respective support elements.

Fig. 10 shows an axial half-cross-section of a coil assembly 110 which may be used in the present invention. As shown, coils 10 are separated and mechanically joined by mechanical joining sections 120, for example of resin-impregnated glassfibre cloth. Such an arrangement may be referred to as "serially bonded". Tensile support elements 30 are preferably joined to the mechanical joining sections 120, rather than to the coils themselves. A simple, non-magnetic nut-and-bolt arrangement 112 may be used with non-magnetic washers 114 and brackets 116.

Fig. 10 also illustrates an example interface between coil assembly 110 and compressive support element 86. Bearing surface 92 may comprise a PTFE disc or similar bonded to a mechanical support section 120.

A method of transporting and shimming a magnet according to an embodiment of the present invention may proceed as follows.

Once the magnet has been assembled in its OVC, the support structure elements, comprising a combination of some or all of: tensile rods, tensile bands and compressive elements, is tensioned to a tension sufficient to protect the coil assembly 110 and the cryostat from damage in transit. Such a step is conventional in itself.

The magnet is then transported to its intended installation site. Once installed, the tension on the support structure may be reduced, for example to a pre-determined value. Distortion of the coils assembly 110 due to tension of the support structure would then reduce. The magnet is then energised, by ramping-up operating current into the coils 10. As is conventional, magnetic field strength measurements are performed to measure the homogeneity of the generated magnetic field in an imaging region. A computer implemented simulation may then be performed to calculate how homogeneity of the magnetic field within the imaging region may be improved by adjustment of tension in the support structure elements. Adjustments are made to the tension in certain support structure elements, according to the calculation of the simulation. These adjustments of tension may be made with the magnet energised.

Another set of measurements is then performed, and another simulation performed using the results of such measurement. Further adjustments may be made to the tension in the support structure elements if the measured homogeneity is not sufficient .

It may be found, after some experience, that a technician becomes capable of recognising which of the available adjustments will improve the homogeneity of a particular measured magnetic field. The technician may then perform such adjustments, and perform a further measurement of magnetic field homogeneity without necessarily performing magnetic field simulations. Further iterations of measuring field homogeneity, simulation of possible adjustments to support structure elements, and adjusting the tension in support structure elements may be performed as required. By performing this tension adjustment on the magnet once it has been sited, any variation in the magnetic field due to the local magnetic environment can be corrected at the same time. For example, steel girders used in the construction of a building in which the MRI system is to be installed may cause deformation of the magnetic field within the imaging region. Any such deformation is corrected for during the above-described iterative tension adjustment method .

While this is an iterative process and may require several stages of adjustment of tension in support structure elements, it may be unnecessary to de-energise the magnet between iterations, and adjustment of tension within the support elements may be carried out with the magnet at field. Of course, care must be taken to ensure that only non-magnetic tools are used in the vicinity of the magnet when at field, to avoid any danger to technicians performing the adjustments.

After one or more iterations of adjustment of tension in the support elements, it may be determined, or preferred, that any further improvement in homogeneity of the magnetic field in the imaging region should be achieved by conventional shimming techniques, such as passive shimming by placing pieces of ferromagnetic material within shim trays, or by adjusting current flowing in superconductive or resistive shim coils.

As homogeneity of the magnetic field in the imaging region may be improved by adjustment of the tension in support elements, according to the present invention, there is a reduced need for the ability of any other shimming arrangement. Accordingly, radially thinner shim trays may be used, which in turn may enable a larger available bore diameter d to be provided, for increased patient comfort. It may be found that the provision of further active or passive shim arrangements is unnecessary, and that the radial space formerly employed to accommodate these shim arrangements may become available for increasing the available bore diameter d to be provided, for increased patient comfort. The elimination of other shim arrangements may also reduce the system cost by adding shimming functionality to the suspension system which is already provided, and reduce the complexity of installation and shimming.

The adjustment of tension in support elements as provided by the present invention may serve to deform the coils structure 110 and the magnet coils themselves, as discussed with reference to Figs. 3-4. Multiple sets of support elements may be provided along the axial length of the coil structure 110 to allow different deformations to be applied at different axial locations. In addition, the adjustment of tension in support elements as provided by the present invention may serve to move the coil assembly 110 with respect to the OVC . This movement will change the coil assembly's position with respect to the structure of the cryostat and any further shimming arrangements, and so will affect the homogeneity of the magnetic field in the imaging region.

In certain arrangements, some support elements may be arranged essentially axially. By varying the tension on such axially- directed support elements, a slight adjustment may be made to the axial length of the coils assembly, and to the axial spacing between magnet coils.

In some arrangements, two sets of support elements may be provided : one to provide support of the coils assembly against gravity, and another set to provide deformation of the coil assembly. All of such support elements may be tensioned for shipping, with the second set of support elements being loosened on installation, so as to provide the required controlled deformation of the coils structure. The support elements provided for homogeneity adjustment may be arranged essentially radially, to facilitate deformation, while the support elements provided for support against gravity may be arranged essentially tangentially, for stability. In other arrangements, the coils assembly 110 may be split into a plurality of pieces, for example separate end coils and a central assembly including several coils, similar to the arrangement of Fig. 10. Each of these pieces may be separately suspended within the OVC, and adjustment of tension on support elements for each piece will enable different deformation to be applied to each piece, and a certain extent of relative motion may be achieved.

As well as adjusting the homogeneity of the magnetic field in the imaging region, motion and deformation of coils according to the present invention may serve to move the homogeneous region relative to the coil assembly by a certain extent.

Stiffeners may conventionally be added in to a serially-bonded coil assembly to reduce distortion of the coil assembly. According to some embodiments of this invention, those stiffeners could be removed, and a more flexible coil assembly may be provided, which may allow more significant deformation and a wider range of adjustment for improving homogeneity and other purposes according to the present invention.

Although the preceding description has made particular reference to coil structures 110 in which coils are bonded together by joining structures 120 in a self-supporting "serially bonded" arrangement, the present invention may also be applied to coil structures which are assembled onto a former, provided that the former is flexible to a certain extent. Such formers include turned aluminium formers, and formers constructed of folded sheet material such as aluminium.

Although the examples described above relate to adjustment of homogeneity in an imaging region of a solenoidal magnet, the present invention may be applied to non-solenoidal magnets, such as the well-known open C-shaped magnets. The invention provides adjustment of tension in support elements to cause deformation of superconducting magnet coils, thereby controlling the homogeneity of a generated magnetic field within an imaging region. The present invention accordingly provides methods and apparatus for improving the homogeneity of a magnetic field of a superconducting magnet within an imaging region by adjusting the tension in support elements, in turn causing controlled deformation of magnet coils of the superconducting magnet. Distortion of magnet coils caused by tension in suspension elements is modified by adjusting tension in the suspension tension elements, to optimise the magnetic field homogeneity in the imaging region. While described in terms of superconducting magnets, the methods and structures of the present invention may also be applied to resistive electromagnets used for imaging.