GB2483854A

GB2483854A - Shimming a Magnetic Field

Info

Publication number: GB2483854A
Application number: GB201015752A
Authority: GB
Inventors: Robert Andrew Slade
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2010-09-21
Filing date: 2010-09-21
Publication date: 2012-03-28
Anticipated expiration: 2030-09-21
Also published as: GB2483854B; GB201015752D0

Abstract

A cylindrical superconducting magnet arrangement uses a shim arrangement (6, 12, 14, 16) for modifying the homogeneity of a magnetic field within a homogeneous field region (50). The shim arrangement uses shim channels (6, 10) extending axially within the bore in a volume between the magnetic field generator and the homogeneous field region. Repositionable shim pieces (12) comprising magnetic material and shim trays (40) containing shim plates (38) are located in single or separate channels. The repositionable shim pieces have means to retain each shim piece in an axial position. There is also an arrangement to move each shim piece axially while at field.

Description

I

ARRANGEMENTS AND METHOD FOR SHIMMING A MAGNETIC FIELD

The present invenfion relates to apparatus and methods for shimming a magnetic field in a cylindrical superconducting magnet. In applications such as magnetic resonance imaging (MRI), it is necessary to provide a very homogeneous background magnetic field. For example, a magnetic field of flux density 0.1T or more must have an inhomogeneity of about 40 parts per million or less peak to peak over an imaging volume of, for example, a 50cm diameter sphere.

Conventionally, small pieces of ferromagnetic material, such as sheet mild steel, are strategically arranged in calculated positions around the imaging volume to compensate for inhomogeneity in the magnetic field produced by the magnet, in a process known to those skilled in the art as "passive shimming". For example, a typical MR1 magnet may be cylindrical in shape, formed of coils of superconducting wire and housed within a cylindrical cryogen vessel, itself housed within a hollow cylindrical outer vacuum chamber (OVC) which thermally isolates it from ambient temperature. Within the bore of the outer vacuum chamber is positioned a cylindrical gradient coil assembly. This is typically a moulded artefact containing resistive coils within a potting material such as an epoxy resin, and is used to produce orthogonal magnetic field gradients. The resistive coils include gradient coils themselves, and radially outside the gradient coils, gradient shield coils may optionally be provided to reduce the magnitude of magnetic field from the gradient coils reaching the outer vacuum container (OVC). Within the moulded artefact are provided shim channels. These are holes, typically of rectangular cross-section, and typically provided between the gradient coils and the gradient shield coils. Shim trays, of similar rectangular cross-section, are located within the shim slots. Each shim tray contains a number of shim pockets along its length. Pieces of sheet ferromagnetic material, called shim plates, typically mild steel with reproducible magnetic properties, such as that used in transformer laminations, are placed within the shim pockets of the shim trays, and the shim trays loaded into the gradient coil assembly. The pieces of ferromagnetic material affect the magnetic field produced by the magnet, and may be used to improve the homogeneity of the resultant magnetic field. A shim algorithm is used to calculate the number and distribution of the shim plates required to reduce the inhomogeneity of the magnetic field within the imaging volume to the desired level. The shim trays may also or alternatively be placed between the radially outer surface of the gradient coil assembly and the bore of the OVC, or between the radially inner surface of the gradient coil assembly and a body (RF) coil within the bore of the gradient coil assembly.

Fig. 1 schematically represents an end view of a conventional superconducting magnet arrangement. A hollow cylindrical superconducting magnetic field generator comprises an outer vacuum container 1 which contains a superconducting magnet, and cooling means, for example a refrigerator 2, to keep the superconducting magnet sufficiently cold that superconductivity is possible. An access turret 3 is schematically represented, and enables access into the outer vacuum container 1, to reach the magnet, to add or remove cryogen, to pass cables or pipes as necessary.

Hollow cylindrical gradient coil assembly 4 is located within bore 5 of the cylindrical outer vacuum container 1, and provides a bore within which an object to be imaged may be placed, such as a patient. Gradient coil assemblies are commonly provided in the bore of cylindrical outer vacuum containers, for example as used in cylindrical MR1 (magnetic resonance imaging) magnets. A number of shim channels 10 are provided within the material of the gradient coil assembly. These channels extend through the gradient coil assembly, in axial directions-that is to say, directions parallel to the axis A-A (Fig. 3) of the cylindrical outer vacuum container 1. ln the present description and the appended claims, the term "radial' will be used to indicate directions perpendicular to the axis A-A (Fig. 3) of the cylindrical outer vacuum container 1.

The shim channels may also or alternatively be placed between the radially outer surface of the gradient coil assembly and the bore surface of the OVC, or between the radially inner surface of the gradient coil assembly and the homogeneous region. A body (RF) coil may be positioned within the bore of the gradient coil assembly, and the shim channels may be provided in positions radially located between the gradient coil assembly and the body (RF) coil.

Shimming conventionally proceeds as follows. A magnet is initially brought to field, and the resulting magnetic field variation is measured over the imaging volume, typically using an array of nuclear magnetic resonance (NMR) probes. Bringing the magnet to field involves gradually increasing electric current flowing through the superconducting coils, a process known as ramping-up. The ramping-up process takes time, and consumes cryogen coolant, as heating occurs within the cryogen vessel. In addition to the time spent ramping, which is typically at least half an hour, potentially several hours, the magnet must be allowed to reach equilibrium, which takes a further oneto two hours.

Once the magnetic field variation has been measured, which may be performed using an NMR field camera to map the flux density on the surface of a sphere and decompose this into a sum of spherical harmonics to describe the inhomogeneity, known algorithms may be used to calculate a suitable distribution of shim plates to improve the homogeneity of the magnetic field within the imaging volume. The current in the superconducting magnet is then removed. This "ramping-down" procedure consumes time, and cryogen, similarly to the ramping-up procedure described above. When the magnet has been ramped down, the shim trays are removed from the gradient coil assembly; shim plates are placed in calculated positions in the shim pockets in the shim trays. The shim trays are then replaced in the gradient coil assembly.

The shim plates cannot be loaded or removed at field for safety reasons: significant forces, of hundreds of newtons, are experienced as the shim plates move through a steep magnetic field gradient at the open ends of the bore of a cylindrical magnet. Some experiments have been done on removing and replacing the shim trays with the magnet "at field", but these have proved less than satisfactory.

The process of ramping up and measuring the magnetic field homogeneity is then repeated. It is unlikely that such shimming will achieve an adequately homogeneous magnetic field in a single iteration due to small errors in the accuracy of positioning of individual shim plates. Typically, two or three shim iterations are required, needing three or four ramping up and two or three ramping down procedures. This is time consuming and wasteful of cryogen coolant.

lt is desired to reduce repeated ramping cycles, to save time on installing or re-commissioning a superconducting magnet, and reduce consumption of cryogen. Quenches, when a superconducting magnet reverts to a resistive state and loses its stored energy as heat into the cryogen, typicafly occur only during ramping. By reducing the need for ramp cycles, the likelihood of a quench is also reduced by the present invention. The method and apparatus of the present invention is applicable to cylindrical superconducting magnets.

Fig. 2 illustrates an alternative arrangement for shimming of a cylindrical superconducting magnet as described in co-pending UK patent application GBI 004361.0. According to that patent application, there is provided a cylindrical superconducting magnet arrangement comprising: a hollow cylindrical superconducting magnetic field generator 1 having a bore 5; further comprising a shim arrangement for modifying the homogeneity of a magnetic field within a homogeneous field region 10, the shim arrangement itself comprising: shim channels 6 extending axially within the bore, in a volume between the magnetic field generator and the homogeneous field region, at least one shim piece comprising magnetic material located within each shim channel; retaining means for retaining each shim piece in an axial position; an arrangement for moving each shim piece axially within the corresponding shim channel while the magnetic field generator is generating a magnetic field, thereby to relocate the shim pieces within the corresponding shim channels, whereby to modify the homogeneity of the magnetic field over the homogeneous field region; wherein the retaining means serve to retain each shim piece in its adjusted axial position. This arrangement allows the pieces of shim material to be moved while the magnet is at field, and significantly shortens the time required for shimming a cylindrical magnet. As each element remains within its shim channel, only small forces are required and there is no safety issue.

However, this arrangement suffers from certain disadvantages of its own. Firstly, the range of inhomogeneity which may be shimmed is rather less than when using conventional shim trays. . Secondly, no solution is provided in the case that the rearrangement of shim pieces within shim channels is insufficient to adequately shim the magnet. Finally, the arrangement is incompatible with existing gradient coil assemblies, which generally have a small number of shim channels, each of rectangular cross-section.

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

ln particular, the present invention provides a shimming arrangement compri&ng both shim trays similar to those conventionally used, and shim pieces repositionable within shim channels, similar to those described in co-pending UK patent application GBI 004361.0. Among other benefits, this enables shim trays to be used to shim the magnet in the factory, and allows the repositionable shim pieces to compensate for inhomogeneity introduced by coil movement during transport or caused by the magnetic environment of the site.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Fig. I shows a first conventional shim arrangement for a cylindrical superconducting magnet; Fig. 2 shows a second conventional shim arrangement for a cylindrical superconducting magnet; Fig. 3 shows a shim arrangement for a cyhndrical superconducflng magnet according to an embodiment of the present invention; Fig. 4 shows a more detailed view of part of the shimming arrangement of Fig. 3; Fig. 5 shows a partial axial cross-section through a superconducting magnet and gradient coil assembly comprising a shimming arrangement, according to an embodiment of the invention; Figs. 6-7 show examples of retention or clamping devices useful in some embodiments of the present invention; Fig. 8 shows a more detailed partial perspective view of part of the shimming arrangement of an embodiment of the present invention; Figs. 9-14 schematically illustrate shim pieces useful in embodiments of the present invention; Figs. ISA-I SB show an example of a shim piece and a complementary shim channel profile, according to an embodiment of the present invention; Figs. 16A-16B show another example of a shim piece and a complementary shim channel profile, according to an embodiment of the present invention; and Fig. 17 shows an example of a shim piece having through-holes allowing circulation of a fluid for temperature regulation.

Fig. 18 shows an axial component of flux density as a function of the position of a shim plate; Fig. 19 shows the force on a shim piece at various axial positions; and Fig. 20 shows a partial axial cross-section through a superconducting magnet and gradient coil assembly comprising a shimming arrangement, according to another embodiment of the invention.

According to the present invention, both shim trays carrying arrangements of pieces of ferromagnetic material, typicafly conventional shim plates, and repositionable shim pieces, are provided within a sing'e magnet.

In preferred embodiments, both a shim tray and a repositionable shim element are provided within a single shim channel.

This aflows a wider range of inhomogeneities to be corrected, yet allows on-site shimming with a single ramping step.

Fig. 3 shows a schematic view of an example embodiment of the present invention. ln this example. a conventional gradient coil assemby 4 is used, having a Umited number, for example sixteen, of shim chann&s 10, each having a rectangular cross-section. In each shim channel 10, two channels 6 are provided for repositionable shim pieces, along with a shim tray 14 of essentially conventional structure. Assuming that the shim channels 10 are of conventional size, the width of the shim trays 14 must of course be reduced to accommodate the channels 6 for repositionable shim pieces. For example, a reduction in width of 20-40% may be found suitable.

Fig. 4 shows a cross-section of a shim channel according to the embodiment of Fig. 3. Here, a conventional shim channel 10 of rectangular cross-section contains a narrow shim tray 14, with channels 6 for repositionable shim elements positioned on either side of it. Preferably, non-magnetic tubes 7, such as plastic tubes, are provided, on either side of the shim tray 14, to define the channels 6 for the repositionable shim elements.

The non-magnetic tubes 7 have the added benefit of keeping the narrow shim tray 14 in position within the shim channel 10. Alternatively, or in addition, the shim tray 14 may be provided with protrusions, for example at its ends, which project from its edges, so as to retain the shim tray 14 in position within the shim channel 10, and to guide, yet not significantly restrict, the range of movement of the repositionable shim pieces.

In other arrangements, a channel 6 for repositionable shim elements may be provided on one side only of the shim tray, with a wider shim tray.

This, and alternative, embodiments will now be described in more detail.

The present invention relates to a particular type of shim arrangement for a superconducting magnet, and a method for shimming the field of the magnet using such shim arrangement. The present invention applies to shim devices located within shim channels, and is not limited to shim channels formed within gradient coil assemblies. lndeed, shim channels of the present invention may be provided within any convenient body, or in the spaces between bodies, e.g. between OVC bore surface and gradient coil assembly outer diameter, and extend within a volume between a magnetic field generator 1 and a homogeneous field region 10, for example, a 50 cm diameter sphere. Shim channels may be placed between the radially outer surface of the gradient coil assembly and the bore surface of the OVC, or between the radially inner surface of the gradient coil assembly and the homogeneous region. A body (RF) coil may be positioned within the bore of the gradient coil assembly, and the shim channels may be provided radially between the gradient coil assembly and the body (RF) coil.

Fig. 5 shows an axial cross-section through a superconducting magnet arrangement similar to that shown in Fig. 3, according to an embodiment of the present invention. Axis A-A is shown, and the whole assembly is substantially rotationally symmetrical about this axis. Centrally within the magnet assembly, an imaging region 10 is shown. The superconducting magnet I is required to produce a very homogeneous magnetic field within the imaging region, which may also be referred to as the homogeneous region. The shimming devices of the present invention are provided to compensate for inhomogeneities in the magnetic field produced by the magnet in the imaging region.

According to certain features of embodiments of the present invention, shown in Fig. 5, repositionable shim pieces 12 are provided, two in each shim channel 6. As illustrated for example in Fig. 3, shim channels 6 may be defined within shim channels 10, adjacent to shim trays 14. Each shim piece 12 is connected to a rod 14 which is long enough to allow an operator to move the shim piece 12 along the shim channel for at least half its length, preferably its fufi length. C'amps 16 are provided near the ends of each shim channel, for examp'e on the ends of the gradient coi' assembly 4. These clamps are accessible to the user, enabling the rods 14 to be clamped at any desired position, thereby retaining the corresponding shim piece 12 in a selected position within the shim channel 6. Clamps 16 may be replaced by any suitable retaining means. The shim pieces are positioned so as to affect the magnetic field produced by the superconducting magnet in the imaging region so as to modify its homogeneity. According to a feature of the invention, the position of the shim pieces 12 within the shim channels 6 may be modified with the magnet at field. While the shim pieces may be moved axially within the shim channels, no shim pieces need be removed from the shim channels, and no shim pieces need be introduced into the shim channels during shimming and while the magnet is at field. The present invention provides shimming by rearranging the repositionable shim pieces which are already present, in addition to initial shimming by conventional arrangement of shim plates within shim trays.

The present invention employs the fact that forces acting on the shim pieces 12 are relatively small when they are inside the shim channel 6 with the magnet at field, those forces being of the order of a few newtons. On the other hand, it has been observed that much greater forces (hundreds of newtons) act on the shim pieces as they are removed from the shim channel, or inserted into the shim channel. This is a result of a large magnetic field strength gradient present at the ends of the shim channels, due to their proximity to the open ends of the cylindrical magnetic field generator, compared with a much smaUer magnetic field strength gradient along the length of the shim channels. As a result, it has been found to be easy and safe to move the shim pieces 12 along the shim channels 6, provided that no attempt is made to remove the shim pieces from the shim channels, and no attempt is made to introduce further shim pieces into the shim channels.

According to the present invention, therefore, an array of shim pieces 12, for example of iron or steel, are introduced into the shim channels 6 before ramping up the magnetS The position of the shim pieces may be adjusted with the magnet at field, using a suitable arrangement for moving each shim piece along the corresponding shim channel. For example, this may be achieved by driving rods 14 attached to each shim piece 12 and accessible to an operator, in to or out of the associated shim channel as schematically indicated by arrows 17. The rods 14 may be retained, for example by a clamp arrangement 16, in a desired position, to fix the position of the corresponding shim piece.

ln the embodiment illustrated in Fig. 5, two rods 14 are inserted in each shim channel 6, one from each end and each joined at its axially inner end to a respective shim piece 12. In an example embodiment, the rods 14 are flexible fibreglass rods, such as used in drain cleaning equipment, cut to a suitable length, at least half the length of the shim channel 6, but preferably the full length. The shim pieces 12 may each be attached to a corresponding rod 14 by clamping, adhesive bonding, or the use of sprung barbs. Any retaining means may be used, provided that it allows the shim piece to be driven to-and-fro within the shim channel 6 against a force of up to a maximum of about lOON. The shim pieces may be of iron, or suitable steel or any other magnetic material as appropriate. Each shim piece may be shaped as deemed appropriate, for example as a disc, plate, ball, cylinder, cuboid and so on.

The shim pieces 12 and the attached rods 14 are installed within the shim channels 6 before the magnet is ramped up. This installation may be performed at the factory, before the magnet assembly is transported to the customer site.

In operaUon, the shim pieces 12 can be moved to-and-fro within the shim channel by puffing or pushing on the attached rod 14. The rod can be clamped in any required position with a suitable simple clamp 16 at the opening of the shim channel.

When the magnet is commissioned, or re-commissioned, the magnet may preferably be ramped to field with the rods 14 pulled out to their fullest extent, bringing the shim pieces 12 as far away from the imaging region 10 as possible, to minimise their effect on the magnetic field in the imaging region 10. The shim pieces 12 are secured in these positions by clamping 16 the attached rods 14 near the open end of each shim channel. The magnet is then ramped to field, and its magnetic field plotted over the surface of the imaging region 10. The force acting on each shim piece 12 in these positions can be shown to be small and safe. Bore magnet homogeneity may be improved by shimming using shim pieces placed within shim trays, as is conventional.

Further shimming may be performed as required by moving the positions of selected shim pieces 12 by calculated or estimated distances, whereby the homogeneity of the magnetic field within the imaging region 10 may be modified. Calculation or estimation of the optimum positions for the various shim pieces may be done based on the experience of a skilled user, or may be based on mathematical algorithms. A suitable shim algorithm can be used to find the required axial position of each shim piece 12. The objective is to make the resultant influence of the shim pieces on the magnetic field in the homogeneous region equal and opposite to the inhomogeneity of the field from the superconducting magnet within that region, as with conventional passive shimming.

It is possible to adjust the axial position of each shim piece with millimetre resolution, unlike conventional shim trays with fixed pockets which have a resolution of several centimetres.

The shim pieces 12 may be moved to their calculated required positions simply by using a ruler to measure the remaining amount of rod 14 protruding from the end of the shim channel 6. Alternatively, the rods themselves may be marked with a scale indicating the position of the attached shim piece 12 with respect to the open end of the shim channel 6, or with respect to the axial centre of the imaging region, for example. According to the present invention, this repositioning of the shim pieces 12 may be carried out with the magnet at field, and movement of each shim piece may require a force of typically no more than about lOON, afthough this maximum force wiU increase with increasing magnetic field gradients and increasing size of the shim pieces. None of the shim pieces are removed from the shim channels during shimming, and no further shim pieces are added to the shim channels during shimming.

Once the shim pieces 12 have been relocated into their calculated or estimated required positions, the rods 14 are clamped in place and the magnetic field of the imaging region 10 may be re-plotted. lf the magnetic field in the imaging region is still not sufficiently homogeneous, the process of calculating or estimating required positions for the shim pieces, moving the shim pieces to their required positions and re-plotting the magnetic field of the imaging region may be repeated. This process may be carried out with the magnet at field. No ramping of the magnet is required, so this is a very fast and efficient method of shimming.

When the modified homogeneity of the magnetic field in the imaging region is satisfactory, the protruding ends of the rods 14 may be simply cut off.

Depending on the flexibility of the material of the rods, they may aRernatively be bent through 90 degrees and tied out of the way with tie-wraps. Whatever method is used, the ends of the gradient coil assembly, the shim channels 6, rods 14 and clamps 16 are typically hidden by bore end "looks" covers, conventionally provided to conceal the working parts of the magnet assembly from operators and patients.

The use of flexible fibreglass rods 14 clamped in simple V-shaped slots 18, for example using an elastomer retainer 20 under tension is envisaged, as shown in Fig. 6. Any other suitable arrangements may be used, such as a screw clamp 22 shown in Fig. 7, which is preferably of a non-magnetic material such as brass or nylon.

Air, or other fluid, of a controlled temperature may be circulated through the shim channels 6 to keep the shim pieces 12 at a stable temperature, thus avoiding changes in their magnetization. The shim pieces should either be made a relatively loose fit in the shim channel, or through-holes should be provided through the shim pieces, as described below with relation to Fig. 17.

Repositionable shim pieces which are not required for modifying the homogeneity of the magnetic field within the imaging volume can be left at an end of the shim channel, to minimise their effect on the magnetic field in the imaging region 10.

Fig. 8 shows a more detailed, perspective, view of an embodiment of the present invention, similar to that illustrated in Fig. 4. The surface of the shim channel 10 is shown in phantom. In this example, the shim channel is not of conventional rectangular cross-section, but is of oval cross-section, having two parallel sides 16, 18 and two semi-circular ends 20.

Correspondingly, the shim tray 22 is not of conventional rectangular cross-section. Although the cross-section has two parallel sides 24, the ends 26 of the cross-section have arcuate, for example semicircular, recesses, defining corresponding channels 28 along edges of the shim tray. These channels 28 co-operate with the semi-circular ends 20 of the cross-section of the shim channel to define a channel 6 to accommodate a corresponding one or more repositionable shim pieces 12. Preferably, two repositionable shim pieces 12 are provided in each channel; a respective one being repositionable from each end of the channel by movement of the corresponding rod 14% In use, bare magnet homogeneity is corrected in the factory by ramping the magnet to field, plotting over the imaging volume, calculating an appropriate positioning of shim plates 38 within the shim trays 40. The magnet would then be ramped down and the shim plates loaded into the shim trays, which would then be loaded into the channels 10 in the gradient coil assembly. As it is known that the magnet's homogeneity will in any case be different at the customer site, due to a number of factors, there is no requirement to ramp up and check the effect of the shim plates on homogeneity in the factory. This "pre-shimming" hence requires only one cycle of ramping up and ramping down of the current in the magnet during factory test, with attendant savings in cryogen consumption and time. Once on the customer site, the repositionable shim elements 12 may be re-arranged to provide the required level of homogeneity. This approach enables satisfactory pre-shimming of the bare magnet homogeneity in the factory with a reduced number of ramping cycles, saving time and cryogen consumption.

Once installed on site, any further shimming which may have become necessary due to shipping or the local ferromagnetic environment on site may be achieved by repositioning the repositionable shim elements within their channels, according to the methods previously described. No changes would be made to the position of the shim plates in the shim trays, which would remain loaded during installation and shimming on site.

lf it is found that the repositioning of the repositionable shim elements 12 within their channels 6 is not sufficient to correct the homogeneity of the magnetic field to a desired degree, then a conventional cycle of ramping down the magnet, re-positioning shim plates 38 within the shim tray 40 and ramping up the magnet may be performed, as in the conventional method. However, in the majority of cases, it will not be necessary to adjust the shim plates in the shim tray.

Should any further shimming be found necessary, for example after service operations on the magnet, then the required re-shimming may typically be performed simply by re-positioning the repositionable shim pieces 12 within their channels 6.

ln further embodiments, such as shown in Fig. 9, both conventional shim trays in rectangular channels 10, and repositionable shim elements in separate channels 6 may be provided. The shim channels 10 for trays and shim channels 6 for repositionable shim pieces may alternate, as illustrated, or two shim channels for repositionable elements may be provided for each shim channel for a shim tray; or whatever ratio provides the desired combination of shimming capability.

ln an example manufacturing process of the gradient coil assembly 4, the wire and supporting structures are assembled into a "skeleton" which also includes mould parts required for forming shim channels 10, 6. The skeleton is placed in a large cylindrical mould which is then and flooding with epoxy resin or other suitable potting compound. The conventional rectangular shim channels 10 used for shim trays may be formed by metal mould pieces which are removed after the potting compound has set using a large hydraulic press.

The shim channels 6 used for repositionable shim elements according to the present invention may be provided by round 8mm diameter inside diameter non-magnetic tubes, such as plastic tubes, which would be left in place. The plastic tubes would be effectively glued in place by the potting compound.

Many other arrangements are possible, having shim channels 10 for shim trays and shim channels 6 for repositionable shim pieces, within the scope of the present invention.

Although the repositionable shim elements have been depicted as simple cylinders, which is one viable embodiment, numerous variations may be employed. Some such variations are described in co-pending UK patent application GBI 004361.0 as follows: Fig. 10 shows an example of a simple shim piece 612 as may be employed in the present invention. The shim piece is composed of a cylinder 22 of magnetic material, such as iron or steel, attached to a suitable rod 14.

The radial cross-sectional diameter of the cylinder enables the shim piece 612 to fit snugly within the shim channel 6, but yet sufficiently loosely that it can be moved axially within the shim channel with little applied force. As illustrated in Fig. 5, each shim channel 6 may house two of these shim pieces, each introduced from a respective end of the shim channel. By providing rods 14 which are longer than the half-length of the shim channel, two shim pieces may be located in the same axial half of a shim channel, providing increased range of shimming, if required.

Further refinements are possible if it is found that additional degrees of freedom are needed to find shimming solutions which provide an acceptable magnetic field homogeneity in the imaging region 10.

Fig. 11 shows an example of a shim piece 712 having eccentrically placed shim material. The cylindrical shape of the shim piece is similar to that of the shim piece 612 of Fig. 10. However, the cylinder is divided axially into two halves. One half 24 of the cylinder is composed of a magnetic material, while the other half 26 is composed of a non-magnetic material. The rod 14 is joined at the centre of the radial cross-section of the cylinder. The shimming effect of such a shim piece is equivalent to a half-cylinder of magnetic material, and the non-magnetic half-cylinder is provided only to ensure that the magnetic half-cylinder remains in position within the shim channel. By rotating the rod 14, the radial position of the centroid of the magnetic material 26 can be adjusted by a few millimetres. ln a similar embodiment a flat is put on one side of an otherwise cylindrical shim piece, effectively resulting in an off-centre magnetic effect. The effect of shim pieces on the shimmed magnetic field in the homogenous region is very sensitive to the radial position of each shim piece, so this feature of shim pieces whose effect is off-centre within the radial cross-section of the shim channels can be used to fine-tune the shimming. A particular advantage of this arrangement as compared to moving the axial location of the shim material is that the harmonic corrections contributed by a shim piece will vary in strength as it is rotated, but will remain in the same relative proportions.

ln another optional feature, the volume of the shim piece(s) in adjacent shim channels could be varied, for example by varying the radial cross-section or axial length of the shim pieces. One particularly advantageous arrangement provides consecutive shim channels with shim pieces in a repeating pattern of shim effect, for example (1, 3, 5, 3, 1)(3, 1, 5, 1, 3)(5, 3, 1, 3, 5) ... that is, the volume of the shims may vary in the indicated proportion and/or differing materials may be used to obtain the indicated ratio of shim effects. Placing small shim pieces near to large shim pieces, in terms of their angular position around the bore 5 of the magnet, gives addition& freedom to the optimization algorithm. For example, if the optimization algorithm needs to concentrate a large quantity of shim material in one spot to compensate a local region of inhomogeneity in the magnetic field, then all the shim pieces in a (1,3,5,3,1) group can be moved axially into positions corresponding to the local inhomogeneity, giving a total of 2x1 ÷2x3÷513 units. lf only a small quantity of iron is needed, then only a single I unit shim piece would be moved. Other combinations of elements from the local group can be used to achieve a quasi-continuous variation between these two extremes, covering all steps in the range I to 13 units. For example: 4=3÷1 or 9=5+3÷1. Furthermore the shim algorithm can choose either one of the shim elements of a certain value from a given group, as best suited to suppressing the local inhomogeneity.

Even more freedom is provided by alternating between (1,3,5,3,1) and (3.1,5,1,3) and (5,3,1,3,5)groups. it is desirable to use a repeating pattern, not random, for simplicity of the shim algorithm. This technique allows a discrete array of shim pieces to provide a quasi-continuous variation.

As iHustrated in Figs. 12A-13, more than one shim element can be placed in each axial half of a shim channel 6. In the arrangement illustrated in Fig. I 2A, the shim-piece of Fig. 10 has been divided diametrically in half. Each half 28, 30 is attached to its own respective rod 14. The two shim pieces 28, 30 may be placed together at a same axial position, giving a shim effect similar to the effect obtained from a shim piece as shown in Fig. 10. Alternatively, the two shim-pieces 28, 30 may be positioned at different axial positions, being slid apart within the shfrn channel. As best seen in Fig. 1 2B, edges 32 of the shim pieces 28, 30 may be chamfered to assist the shim pieces moving past one another. lf desired, the two parts 28, 30, may be made of unequal size, allowing further refinement of the shimming ability of the shim arrangement.

Fig. 13 illustrates a different arrangement with a similar effect. In Fig. 13, shim piece 912 has a radial cross section divided into two concentric parts: an outer, hollow cylinder 34 and an inner cylinder 36, each having its own rod 14.

The two parts 34, 36 may be of equal volumes, so having substantially equal shimming effects, or may be of differing volumes, allowing further refinement of the shimming ability of the shim arrangement. One or both of the parts 34, 36 may have chamfered edges 38 to assist the shim pieces to move past one another.

Fig. 14 shows an alternative arrangement which allows two shim pieces to be positioned independently within a single shim channel from one end of the shim channel. Two cylindrical shim pieces 40, 42 are provided, each with its own rod 14. The shim pieces are inserted into a shim channel 6 so that shim piece 42 is axially nearer the centre of the shim channel 6 than shim piece 40 is. Shim piece 40 has a through-hole 44, through which the rod 14 of shim piece 42 may pass, enabling separate control of the position of shim piece 42.

Shim piece 42 may be provided with a similar though-hole to equalise the shim effect of the two pieces, or to simplify manufacture.

While the various optional features of the shim pieces described with reference to Figs. 10-14 have been described with particular reference to cylindrical shim pieces, the shim pieces may alternatively be shaped as a disc, plate, ball, cuboid and so on.

Figs. 15A-153 and 16A-163 illustrate alternative embodiments in which an alternative to the rods 14 is provided for moving the shim pieces within the shim channel. Rather than having a rod 14 rigidly attached to the shim pieces, the shim pieces of Figs. 15A-153 and 16A-16B have a threaded rod 46 passing axially through the respective shim piece 1112, 1212. The shim piece 1112, 1212 has a co-operating threaded through-hole through which the threaded rod passes. Rather than moving the shim piece within a shim channel by pushing or pulling on a fixed rod, the shim pieces of Figs 15A, 16A are moved by rotating the threaded rod 46. The shim-pieces are unable to rotate within the shim channels, and are driven axially along the shim channel by operation of the co-operating threads on the threaded rod 46 and the threaded through-hole. A threaded rod of length equal to half the axial length of the shim channel may be introduced from each end of the shim channel, along with a shim piece 1112, 1212. The threaded rod need not protrude from the end of the shim channel, but must be provided with a means for causing it to rotate, for example, a handle or wheel. An electric motor, for example a stepper motor, may be provided to rotate the threaded rod 46, adding a degree of automation to the process of shimming. In these embodiments, it is important to ensure that the shim piece 1112, 1212 itself cannot rotate within the shim channel. This may be simply achieved by using shim pieces and shim channels which do not have a circular radial cross-section. In the embodiment of Figs. 1SA-15B, the shim piece 1112 and the shim channel both have a square radial cross-section. ln the embodiment of Figs. 16A-16B, the shim piece and the shim channel have radial cross-sections provided with keys 48 which allow the shim piece to move axially within the shim channel, but prevent it from rotating. in embodiments employing a threaded rod 46, it may be found unnecessary to provide separate retaining means. Interaction between each threaded rod and the threaded through-hole within the respective shim piece may be found sufficient to retain the shim piece in position. The threaded rod will need to be axially retained in position within the shim channel, for example by a suitable mounting at the opening of the shim channel.

Fig. 17 shows an example of a shim piece 1312 having through-holes 50, permitting a flow of air or other fluid through shim pieces, allowing their temperature to be controlled. The magnetic properties of materials vary with their temperature, and the gradient coils heat due to ohmic dissipation during operation, so it may become important to stabiUse the magnetic effect of the shim pieces by stabilising their temperature.

Although the present invention has been described with reference to the movement and retention in position of the shim pieces, either by simple rods 14 and damps 16, or threaded rods 46 and threaded shim pieces 1112, 1212, any suitable arrangement may be provided for moving each shim piece axially within its shim channel, and retaining it in position! As discussed above, the shim pieces may be of differing radial cross-sectional sizes, and the shim channels may be of correspondingly different radial cross-sectional areas.

The shim channels 10, 6 themselves may be positioned in the radial interval of the gradient coil assembly between gradient coils and gradient shield coils.

However, the shim arrangement of the present invention may alternatively be positioned radially within the gradient coil assembly, inside the gradient coils between the gradient coils and the bore of the gradient coil assembly, or they may be placed within the gradient coil assembly, between the gradient shield coils and the bore surface of the outer vacuum chamber. ln other embodiments, the shim arrangement of the present invention may be positioned not within the gradient coil assembly, for example radially inside the gradient coil assembly, or they may be placed radially outside the gradient coil assembly, between the gradient coil assembly 4 and the bore surface of the outer vacuum chamber (OVC). This latter position may be found to provide better stability because the material of the shim pieces is not directly heated by the gradient coil, and can be more easily cooled. A greater mass of shim material would be needed for the same shimming effect at such an increased radial separation from the imaging region 10.

The present invention is also compatible with so-called "active" shimming: the technique of adjusting currents in additional coils suitably located within the system, either resistive or superconducting, to improve the homogeneity, as will be known to those familiar with the art. Indeed, it is envisaged that the present invention will be used in conjunction with conventional first order active shimming, for example by reserving a fraction of the available gradient coil current for shimming purposes rather than imaging use, and applying a DC offset to reduce first order errors. The shim algorithm can calculate the gradient coil currents required to provide the optimum shim distribution solution.

The present invention accordingly provides a shim arrangement, and a corresponding shimming method, in which shim pieces are present within shim channels, and shimming progresses by relocating shim pieces within the shim channels. No shim pieces are added or removed during the shimming process, which preferably progresses with the magnet at field. Where a shim piece is not required for shimming, the shim piece is simply moved to its furthest extent away from the imaging region 10, so as to minimise its effect on the magnetic field within the imaging region. According to the present invention, repositionable shim pieces are placed within the shim channels 6 before the magnet is ramped up. Shimming is initially performed by adding shim plates into shim trays as is conventional, but further shimming may be performed by repositioning those repositionable shim pieces with the magnet at field and without removing the repositionable shim pieces. This saves much time and money, by avoiding the need to ramp the magnet down and back up again between shimming iterations.

While the present invention has been particularly described with reference to the shimming of magnetic fields within an imaging region of a magnet used for MRI imaging, it may be applied to the improvement of the homogeneity of a magnetic field within any homogeneous field region, regardless of the purpose

to which the resultant homogeneous field is put.

It is a known problem that the homogeneity of passively shimmed magnets is subject to temperature instability caused by heating, or heat-induced motion, of the shim pieces after initial shimming has been carried out as described above. The shim arrangement of the present invention is suitable to correct for variations in the homogeneity of the field in the imaging region after initial shimming is complete. More particularly, automated drift compensation may be provided to periodically measure the homogeneity of the magnetic field in the homogeneous field region, typically using MRI methods, to calculate a movement of the repositionable shim pieces which would compensate for any degradation in the measured homogeneity and to move the repositionable shim pieces in accordance with the calculated movement.

The following documents have been found to offer rather different solutions to similar problems: US6313634 US6617853 US7224167 and US20070216413A1.

To correct for degradation in homogeneity of the magnetic field in the homogenous field region due to temperature-induced change in magnetization or position of the passie shim pieces, additional features may be provided, according to certain embodiments of the present invention, to actively move the shim elements axially by small amounts as part of a feedback loop.

A sensor may be provided to detect and measure change in the magnetic field homogeneity. The sensor may be an MRI phantom of conventional structure in the patient table, or any other conventional alternative. A calculation device is also required, to calculate movements of shim pieces which would correct the detected inhomogeneities. Such calculation devices are known and available to those skilled in the art. An actuator is also required, to move shim elements by the calculated movements. Such actuators may be pneumatic, electrical, or indeed any suitable devices. Ideally, the actuators would be remote from the homogeneous field region, to avoid interference with the field in that region. Conveniently, rods 14 or threaded rods 46 as described above may be used, in conjunction with appropriate shim devices, and electric motors, such as stepper motors, may be arranged to drive the rods 14 or threaded rods 46 as appropriate to cause the required movement of shim pieces.

One significant shortcoming of conventional passive shimming arrangements is that the shims are difficult to access, and the shims may only be altered by ramping down the magnet, modifying the distribution of shim material and ramping the magnet up again. This is such an onerous task that passive shimming is rarely adjusted to correct for changes in the homogeneous field regions after initial shimming, caused for example by changes in the quality or position of iron in the magnet's environment, for example, the re-positioning of iron girders during building work.

According to the present invention, repositionable shim pieces are provided, along with conventional shim plates in shim trays. This allows a wide range of inhomogeneities to be initially shimmed yet allows the positions of the repositionable shim pieces to be adjusted with the magnet at field. The measurement of field homogeneity, calculation of corrective movements of the shim pieces, and corresponding movement of the shim pieces may be carried out automatically according to certain embodiments of the present invention.

The present invention opens the attractive possibility of actively controlling the positions of the repositionable shim pieces during imaging to compensate for temperature drift. ldeally this would be done with cable drives attached to flexible rods 14 or stepper motors arranged to rotate threaded rods 42. The associated motors could be mounted on the OVC 1, magnetically shielded from the homogeneous field region. Alternatively pneumatic or hydraulic positioning and movement may be used.

When applied to an MRI or NMR imaging system, an active feedback loop can be envisaged in which an image signal from one or more samples in the imaging region, conveniently located in a patient table for example, is monitored, harmonics calculated and shim position corrections determined and applied to compensate drift in real time. It would not be necessary to adjust the positions of all of the repositionable shim pieces 12 to control many harmonics. It is believed that movement of as few as 8 or 16 repositionable shim pieces would provide substantial drift compensation.

These embodiments of the invention accordingly provide a method and apparatus for adjusting the shimming of a magnet at field using an array of repositionable shim pieces attached to moveable rods in an array of shim channels, which may be located within a gradient coil assembly, for example between gradient coils and gradient shield coils while stiH providing a significant range of shimming ability due to the concurrent use of conventional shim plates in shim trays. The axial positions of some or all repositionable shim elements can be controlled by actuators in a feedback loop to maintain homogeneity of the magnetic field in the homogeneous field region. This may usefully compensate for changes due to temperature, for example caused by heating of the gradient coil assembly in use, or any other source of change in homogeneity of the magnetic field in the imaging region.

As an aid to understanding the interaction of the magnetic field with repositionable shim pieces, Fig. 18 shows the axial component of flux density (in micro Tesla) as a function of the z position of a saturated cuboid shim plate made of silicon steel with dimensions 65x80x0.28mm, the shim being displaced 0.16m from the measurement point in the radial direction.

Fig. 19 shows the force on a repositionable shim piece at various axial positions, and indicates that the force acting on the shim piece is tolerably low until an attempt is made to remove the repositionable shim piece from the magnet. The force acting on a shim piece is given by: = MV{B9 where M is the magnetization, V is the volume and dB/dz is the flux density gradient. In particular, Fig.19 shows how the force on a cylindrical shim of volume 1.4cm3 varies along the length of a shim slot at radius 0.4m in a typical I.5T solenoid superconducting magnet.

Fig. 20 illustrates a further embodiment of the present invention. It has been found desirable, in certain circumstances, to provide differently-sized repositionable shim pieces. For example, if all repositionable shim pieces are of a same size, it may be found impossible to shim out certain inhomogeneities. As illustrated in Fig. 20, guide tubes 200 may be temporarily attached over one or more of the shim channels 6 for repositionable shims 12. Repositionable shims 12 of differing sizes may be introduced into, or removed from, the shim channels 6 by applying appropriate forces to the rods 14. As discussed above, only a relatively small force is required to adjust the position of a repositionable shim within the shim channel, but a much larger force is required to introduce shims into, or remove shims from, the shim channels. Due to the larger forces involves, guide tubes should be used for safety and to assist with the process of inserting repositionable shim pieces into the corresponding shim channels 6. The guide tubes 200 should be firmly, but removably attached to the shim channels 6, for example by mechanical clamping or by providing complementary threads at respective ends of the guide tubes 200 and shim channels 6.

An example shimming process using the embodiment of Fig. 20 may proceed as follows: Bare magnet homogeneity is shimmed in the factory using conventional shim plates in conventional shim trays, and a conventional method, for example ramping up the magnet, plotting the resultant magnetic field over the imaging region, ramping the magnet down, calculating appropriate masses and positions of shim pieces to improve homogeneity of the magnetic field in the imaging region, loading shim plates into corresponding positions in the shim trays, loading the shim trays into the shim channels. The process may be repeated if necessary to achieve acceptable magnetic field homogeneity in the imaging region. The magnet is then shipped to its installation site.

The magnet is ramped up on site, without any repositionable shim pieces 12 in the corresponding shim channels 6. The magnetic field homogeneity over the imaging region is then plotted.

A range of repositionable shim pieces is made available. These may be of a same composition, with differing lengths adfor of differing compositions, but in any case the range of shim pieces has a range of magnetic effects. For example, a range of between three and six different types of repositionable shim element may be found sufficient. The required type and axial position of repositionable shim element is calculated for each shim channel 6.

Corresponding repositionable shim pieces are loaded into the shim channels with the magnet still at field. Preferably, a long guide tube 200 is attached to each shim channel 6 in turn to assist with insertion of the corresponding repositionable shim element. Each repositionable shim element would then be loaded into the guide tube in a region of low magnetic field strength, e.g. 2-3 metres from the magnet. The repositionable shim piece is and then pushed down the guide tube 200 into the shim channel 6 within the magnet. Once the repositionable shim piece is close to the magnet, it would be attracted into the shim channel 6 due to magnetic forces. Once the repositionable shim piece 12 is inside the shim channel 6, the guide tube 200 is removed and the position of the repositionable shim element may be adjusted as described above.

A similar process may be employed even with repositionable shim pieces of uniform size, in circumstances where it is found desirable to remove certain repositionable shim pieces in order to provide improved homogeneity.

Claims

CLAIMS1. A cyhndrical superconducting magnet arrangement comprising: -a hoow cylindrical superconducting magnetic field generator (1) having a bore (5); further comprising -a shim arrangement (6, 12, 14, 16) for modifying the homogeneity of a magnetic field within a homogeneous field region (50), the shim arrangement itseff comprising: -shim channels (6, 10) extending axially within the bore, in a volume between the magnetic field generator and the homogeneous field region, the shim channels having located therein: -repositionable shim pieces (12) comprising magnetic material; and -shim trays (40) themselves containing shim plates (38), wherein the repositionable shim pieces are themselves provided with: -retaining means (16; 46) for retaining each shim piece in an axial position; and -an arrangement (14; 46) for moving each shim piece axially within the corresponding shim channel while the magnetic field generator is generating amagnetic field,thereby to relocate the shim pieces within the corresponding shim channels, whereby to modify the homogeneity of the magnetic field over thehomogeneous field region (10); wherein-the retaining means (16: 46) serve to retain each shim piece in its adjusted axial position.
2. A cylindrical superconducting magnet arrangement according to claim I wherein at least one shim channel (10) contains both a shim tray and at least one repositionable shim piece.
3. A cylindrical superconducting magnet arrangement according to claim 2 wherein a shim channel (10) contains a shim tray (40), with channels (6) for repositionable shim elements (12) positioned on either side of the shim tray.
4. A cylindrical superconducting magnet arrangement according to claim 3, wherein non-magnetic tubes (7) are provided, on either side of the shim tray (40), to define the channels (6) for the repositionable shim elements.
5. A cylindrical superconducting magnet arrangement according to claim 3 or claim 4 wherein the shim tray (40) is provided with protrusions which project from its edges, so as to retain the shim tray in position within the shim channel (10).
6. A cylindrical superconducting magnet arrangement according to claim 2 wherein at least one of the shim channels is of oval cross-section, having two parallel sides (16, 18) and two semi-circular ends (20); the shim tray 22 has a cross-section having two parallel sides (24) and ends (26) of the cross-section have arcuate recesses, defining corresponding channels (28) along edges of the shim tray, these channels (28) co-operating with the semi-circular ends (20) of the cross-section of the shim channel to define a channel (6) to accommodate a corresponding one or more repositionable shim pieces (12).
7. A cylindrical superconducting magnet arrangement according to claim 1 wherein shim trays (40) are provided in channels (10) for shim trays, and repositionable shim elements (12) are provided in separate channels (6).
8. A cylindrical superconducting magnet arrangement according to claim 7 wherein the channels (10) for shim trays and the separate channels (6) alternate circumferentially around the bore.
9. A cylindrical superconducting magnet arrangement according to claim 7 wherein two separate channels (6) are provided for each channel (10) for shim trays.
10. A cylindrical superconducting magnet arrangement according to any of claims 7-9 wherein the channels (10) for shim trays are of rectangular cross-section.
11. A cylindrical superconducting magnet arrangement according to any preceding claim wherein the arrangement (14; 46) for moving each repositionable shim piece axially within the corresponding shim channel is operable to adjust the axial position of each shim piece within the corresponding shim channel without adding or removing any shim pieces to or from the shim channels.
12. A cylindrical superconducting magnet arrangement according to claim 1, further comprising a hollow cylindrical gradient coil assembly (4) located within the bore and comprising coils for generating magnetic field gradients in the homogeneous field region (50) located within the bore.
13. A cylindrical superconducting magnet arrangement according to claim 12, wherein the shim channels (6) are positioned between a radially outer surface of the gradient coil assembly and the bore wall of the hollow cylindrical superconducting magnetic field generator.
14. A cylindrical superconducting magnet arrangement according to claim 12, wherein the shim channels (6, 10) are positioned between a radially inner surface of the gradient coil assembly and the homogeneous region.
15. A cylindrical superconducting magnet arrangement according to claim 12, wherein the shim channels (6, 10) are positioned between a radially inner surface of the gradient coil assembly and body (RF) coil within the bore of the gradient coil assembly.
16. A cylindrical superconducting magnet arrangement according to claim 12, wherein the shim channels (6, 10) are formed within the gradient coil assembly (4) and extend axially therethrough.
17. A cylindrical superconducting magnet arrangement according to claim 16, wherein the gradient coil assembly comprises gradient coils and gradient shield coils, positioned radially outside the gradient coils; the gradient coils and gradient shield coils are embedded within a potting material (8) and the shim channels (6, 10) are positioned radially between the gradient coils and the gradient shield coils.
18. A cylindrical superconducting magnet arrangement according to claim 16, wherein the gradient coil assembly comprises gradient coils and gradient shield coils, positioned radially outside the gradient coils; the gradient coils and gradient shield coils are embedded within a potting material (8) and the shim chann&s are formed within the potting material, radially outside the gradient coils and the gradient shield coils.
19. A cylindrical superconducting magnet arrangement according to claim 6, wherein the gradient coil assembly comprises gradient coils and gradient shield coils, posoned radially outside the gradient coils, the gradient coils and gradient shield coils are embedded within a potting material (8) and the shim channels are positioned radially within the gradient coils and the gradient shield coils.
20. A cylindrical superconducting magnet arrangement according to claim 12 wherein the shim channels are formed in a structure separate from the gradient coil assembly.
21. A cylindrical superconducting magnet arrangement according to any preceding claim wherein the arrangement for moving each repositionable shim piece along the corresponding shim channel (6) comprises a number of rods (14) attached to respective repositionable shim pieces such that the position of a repositionable shim piece within the shim channel may be adjusted by driving a corresponding rod in to or out of the shim channel.
22. A cylindrical superconducting magnet arrangement according to claim 21 wherein the retaining means (16; 46) for retaining each repositionable shim piece in position comprises a clamp arrangement (16) acting to hold each rod near an end of the shim channel.
23. A cylindrical superconducting magnet arrangement according to any of claims I to 20 wherein the arrangement (46) for moving each repositionable shim piece along the corresponding shim channel (6) comprises a number of threaded rods (46), each co-operating with a threaded through-hole within a respective repositionable shim piece, such that the position of a repositionable shim piece within the shim channel (6) may be adjusted by rotating the threaded rod with respect to the respective repositionable shim piece.
24. A cylindrical superconducting magnet arrangement according to claim 13 wherein the retaining means (16; 46) for retaining each repositionable shim piece in position comprises an interaction between each threaded rod (46) and the threaded through-hole within the respective repositionable shim piece.
25. A cylindrical superconducting magnet arrangement according to any preceding claim, further comprising an arrangement for directing a flow of fluid at a controlled temperature through at least one of the shim channels.
26. A cylindrical superconducting magnet arrangement according to claim 23 or any claim dependent on claim 23, wherein the repositionable shim pieces (1112; 1212) and the corresponding shim channels (116, 126) have corresponding non-circular radial cross-sections.
27. A cylindrical superconducting magnet arrangement according to any preceding claim, wherein at least one of the shim channels (6) contains a plurality of repositionable shim pieces (28, 30; 34, 36), each of said repositionable shim pieces being provided with a respective arrangement for moving the reposftionable shim piece and retaining means for retaining the repositionable shim piece in position, said repositionable shim pieces having complementary radial cross-sectional areas, such that the plurality of shim pieces may overlap within the shim channel.
28. A cylindrical superconducting magnet arrangement according to claim 27 wherein the plurality of repositionable shim pieces, taken together when overlapping, have a combined radial cross-section substantially the same as the radial cross-section of the shim channel.
29. A cylindrical superconducting magnet arrangement according to claim 27 or claim 28 wherein said plurality of repositionable shim pieces (28, 30) have radial cross-secfional areas corresponding to sectors of the radial cross-sectional area of the shim channel.
30. A cylindrical superconducting magnet arrangement according to claim 27 or claim 28 wherein said plurality of repositionable shim pieces include first shim piece (34) having an axial through-hole, and a complementary second shim piece (36) radially dimensioned to pass through the through-hole in the first repositionable shim piece.
31. A cylindrical superconducting magnet arrangement according to any of claims 1-26 wherein at least one of the shim channels contains a plurality of repositionable shim pieces (40, 42), each of said repositionable shim pieces being provided with a respective arrangement for moving the repositionable shim piece and retaining means for retaining the repositionable shim piece in position, said repositionable shim pieces being arranged at different positions within the shim channel, one or more through holes (44) being provided to allow an arrangement (14) for moving one repositionable shim piece (42) to pass through another repositionable shim piece (40).
32. A cylindrical superconducting magnet arrangement according to claim 25 or any claim dependent on claim 25 wherein one or more of the repositionable shim pieces (1312) is provided with through-holes (50) allowing the fluid to pass through the repositionable shim piece.
33. A cylindrical superconducting magnet arrangement according to any preceding claim, wherein the volume and/or type of magnetic material in the repositionable shim pieces of consecutive shim channels (6) varies, such that the relative shim effect is provided substantially in the ratio 1, 3, 5, 3, 1, 3, 1,5,1,3,5,3,1,3,5... and so on.
34. A cylindrical superconducting magnet arrangement according to any preceding claim, wherein the magnetic field generator generates a magnetic field within the homogeneous field region with a magnetic flux density of 0.11 or more.
35. A cylindrical superconducting magnet arrangement according to any preceding claim, further comprising an arrangement for correcting degradation in homogeneity of the magnetic field in the homogenous field region, comprising: one or more sensors arranged to detect and measure change in themagnetic field homogeneity;a calculation device arranged to calculate desired movements of certain repositionable shim pieces suitable to correct the detected change inthe magnetic field homogeneity; andan actuator arranged to move the certain repositionable shim pieces by the calculated desired movements.
36. A cylindrical superconducting magnet arrangement according to any preceding claim, wherein the repositionable shim pieces comprise repositionable shim pieces of different magnetic properties.
37. A cylindrical superconducting magnet arrangement according to any preceding claim, provided with at least one guide tube (200) for temporarily attachment over the shim channels (6) for repositionable shims (12), such that repositionable shims (12) may be introduced into, or removed from, the shim channels (6) by applying appropriate forces to the rods (14)while the magnet is at field.
38. A cylindrical superconducting magnet arrangement according to claim 37 wherein the guide tube(s) (200) is removably attached to the shim channel(s) (6), by mechanical clamping or by complementary threads at respective ends of the guide tube(s) (200) and shim channel(s) (6).
39. An MRI system comprising a cylindrical superconducting magnet arrangement according to any preceding claim.
40. A method for modifying the homogeneity of a magnetic field within a homogeneous field region (10) within a cylindrical superconducting magnet arrangement comprising a hollow cylindrical superconducting magnetic field generator (1) having a bore: the method comprising: -providing shim channels (6, 10) extending axially within the bore, in a volume between the magnetic field generator and the homogeneous field region, -providing at least one repositionable piece of shim material (12) located within each shim channel (6): -activating the magnetic field generator to generate a magnetic field within thehomogeneous field region:-measuring the homogeneity of the magnetic field over the homogeneousfield region:-deactivating the magnetic field generator:-placing shim plates (38) within shim trays (40) and positioning the shim trays within at least some of the shim channels (10); -reactivating the magnetic field generator to generate a magnetic field withinthe homogeneous field region;-with the magnetic field generator still active, moving at least some of the repositionable shim pieces along the corresponding shim channels (6) thereby modifying the homogeneity of the magnetic field over the homogeneous field region, such that each repositionable shim piece remains within the corresponding shim channel throughout this step, and no further shim pieces are introduced into the shim channels during this step; and -retaining the shim pieces in their position by use of retaining means.
41. A method according to claim 40 further comprising the step of: -measuring the modified homogeneity of the magnetic field over thehomogeneous field region.
42. A method according to claim 40 or claim 41 wherein the magnetic field generator generates a magnetic field within the homogeneous field region with a magnetic flux density of 0.1 T or more, and the modified homogeneity of the magnetic field over the homogeneous field region has an inhomogeneity of 40 parts per million or less, peak to peak.
43. A method for modifying the homogeneity of a magnetic field within a homogeneous field region (10) within a cylindrical superconducting magnet arrangement comprising a hollow cylindrical superconducting magnetic field generator (1) having a bore: the method comprising: -providing shim channels (6, 10) extending axially within the bore, in a volume between the magnetic field generator and the homogeneous field region, -activating the magnetic field generator to generate a magnetic field within thehomogeneous field region:-measuring the homogeneity of the magnetic field over the homogeneousfield region:-deactivating the magnetic field generator:-placing shim plates (38) within shim trays (40) and positioning the shim trays within at least some of the shim channels (10); -reactivating the magnetic field generator to generate a magnetic field withinthe homogeneous field region;-with the magnetic field generator still active, inserting at least one repositionable piece of shim material (12) into a shim chann& (6); -with the magnetic field generator still active, moving at least some of the repositionable shim pieces along the corresponding shim channels (6) thereby modifying the homogeneity of the magnetic field over the homogeneous field region, such that each repositionable shim piece remains within the corresponding shim channel throughout this step; and -retaining the shim pieces in their position by use of retaining means.
44. A method according to claim 43 wherein the repositionable shim pieces (12) are selected, according to a desired shimming effect, from a range of different types of repositionable shim pieces, each having a different magnetic characteristic.
45. A method according to claim 44 wherein the repositionable shim pieces (12) are of a same composition, having a range of differing lengths.
46. A method according to any of claims 43-45 in which a long guide tube (200) is removably attached to a shim channel (6) to assist with insertion of the corresponding repositionable shim element, and is removed once the repositionable shim piece (12) is inside the shim channel (6).
47. A method according to claim 43 wherein the homogeneous field region comprises a 50cm diameter sphere.