Patent Description:
In particular the disclosure relates to an electromagnet assembly for a Magnetic Resonance Imaging apparatus.

In Magnetic Resonance Imaging a plurality of electromagnets comprising superconducting coils is energised to generate a magnetic field which is both strong and spatially confined. This causes interaction between the electromagnets, thus subjecting them to electromagnetic loads. Moreover, transportation and installation may also subject the electromagnets to loads. In order to maintain the desired performance of the electromagnets, such loads need to be managed by the means used for supporting the coil.

A known means for supporting electromagnets involves a mechanically-supported journal into which the coil is wound. This arrangement, however, may not sufficiently restrain relative movement of the journal and the electromagnets.

Another known means for supporting electromagnets involves a tensile support wrapped around the electromagnets. This arrangement may not provide sufficient rotational restraint to effectively manage transportation loads. Additionally such an arrangement may be difficult to configure to adequately achieve small tolerances required for spatially locating the electromagnets. Any deviation from an optimal relative position of the electromagnets may result in local stress concentration detrimental to performance.

Hence an arrangement which allows for accurate and secure mounting of an electromagnet of a MRI device is highly desirable.

<CIT>, <CIT> and <CIT> describe arrangements for supporting superconducting coils.

According to the present disclosure there is provided an apparatus as set forth in the appended claims.

Accordingly there is provided an electromagnet assembly <NUM> for a Magnetic Resonance Imaging MRI apparatus as defined in claim <NUM>. The electromagnet assembly <NUM> comprises a pair of electromagnets <NUM>, each comprising a coil <NUM> configured to generate a magnetic field, the coil <NUM> having a first axial end face <NUM>. Each electromagnet further comprises a support element <NUM> configured for mounting the coil <NUM> in the MRI apparatus, wherein the support element <NUM> is bonded to the first axial end face <NUM> of the coil <NUM>.

Thus the bonded support element provides a means for securing the coil to a support structure and, thereby, inhibit axial and rotational movement of the electromagnet. The support element <NUM> may comprise a bonding face <NUM> by which the support element <NUM> is bonded to the first axial end face <NUM>. The bonding face <NUM> may be configured to have a reduced stiffness towards its periphery and a higher stiffness towards a central area of the bonding face.

Thus the support element is configured to reduce a stress concentration which may arise at the periphery of the bonding face in response to a force acting on the electromagnet.

A plurality of support elements <NUM> may be bonded to the first axial end face.

The plurality of support elements <NUM> may be equally spaced around the first axial end face <NUM>.

According to other examples, the plurality of support elements may be irregularly spaced. That is to say, the plurality of support elements <NUM> may be unequally distributed around the first axial end face <NUM>.

The coil <NUM> may comprise a second axial end face <NUM> opposite to the first axial end face <NUM>. A further support element <NUM>' may be bonded to the second axial end face <NUM>.

The coil <NUM> and the support element <NUM> may be bonded using an adhesive suitable for cryogenic applications.

The coil <NUM> and the support element <NUM> may be resin-impregnated to form a monolithic structure.

According to the invention, the electromagnet <NUM> further comprises a support structure <NUM> comprising a first support plate <NUM>, <NUM>, wherein the support element <NUM> is mounted to the first support plate <NUM>, <NUM>.

According to the invention, the support structure <NUM> comprises a second support plate, the first and second support plates being arranged to form a pair of spaced-apart support plates <NUM>, <NUM> forming a cavity <NUM> therebetween. The support element <NUM> is received into the cavity <NUM> and mechanically fastened to the support plates <NUM>, <NUM>.

An extension tube <NUM> may be fitted around a pin <NUM> for making a pinned connection between the support plates <NUM>, <NUM> and the support element <NUM>. The extension tube <NUM> may be configured to abut against the support plates <NUM>, <NUM> and the pin <NUM> to maintain compression on the support element <NUM> when the coil <NUM> is in its superconducting state.

Thus the extension tube is configured to maintain compression in a case where the support element has a greater coefficient of thermal contraction than the pin.

The support plates <NUM>, <NUM> may be configured to press the support element <NUM> and the further support element <NUM>' against the coil <NUM> when the coil is in its superconducting state.

Thus the support element <NUM> and the further support element <NUM>' are configured to secure the coil. In use, the support element and the further support element may cooperate with the support plates to press against the first axially outer surface of the coil and the second axially outer surface of the coil.

The support element <NUM> and the support plates <NUM>, <NUM> may be configured to make a pinned connection for securing the electromagnet <NUM> to the support structure <NUM>.

According to another example, there may be provided an MRI apparatus comprising the electromagnet assembly <NUM> as described above.

According to another example which is outside the scope of the invention as defined by the claims, there may be provided a method of manufacturing an electromagnet for an MRI apparatus. The method may comprise winding a superconductor wire into a mould to form a coil <NUM>, arranging a support element <NUM> against a first external surface <NUM> of the coil <NUM>, impregnating the coil <NUM> and the support element <NUM> with a thermosetting resin, and removing the coil <NUM> and the support element <NUM> from the mould.

The method of manufacturing may comprise placing a further support element <NUM>' against a second external surface <NUM> of the coil <NUM>, the second external surface <NUM> being opposite to the first external surface <NUM> of the coil <NUM>.

The present application is concerned with an electromagnet suitable for being mounted in an electromagnet assembly which restrains the electromagnet axially and circumferentially, thereby preventing distortion of the electromagnet arrangement along or around its assembly axis A-A (described below).

<FIG> shows a schematic illustration of a perspective view of an electromagnet assembly <NUM> according to the present disclosure.

In use, the electromagnet assembly <NUM> forms part of a Magnetic Resonance Imaging (MRI) apparatus (or 'scanner'). For such a purpose the electromagnet assembly may be contained within a housing which, in operation, contains an inert gas as a coolant, for example helium. Hence the housing forms a cryogen vessel, which enables the electromagnet assembly to be cooled to sufficiently low temperatures to optimise performance.

The electromagnet assembly <NUM> is generally rotationally/circumferentially symmetrical, defining an assembly axis A-A, a radial direction and a circumferential direction. Accordingly, "axial" refers to a direction parallel to the assembly axis, while "radial" refers to a direction perpendicular to the assembly axis, and "circumferential" refers to a direction perpendicular to both the assembly axis and the radial direction around the assembly axis A-A. The electromagnet assembly extends axially (or "lengthwise") along the assembly axis.

The electromagnet assembly <NUM> comprises a pair of electromagnets <NUM> and a support structure <NUM> configured to carry the electromagnets.

The support structure <NUM> is arranged to retain the electromagnets <NUM> in a particular spatial (i.e. relatively spaced) arrangement, preventing each electromagnet from moving along the assembly axis A-A or rotating about the assembly axis. For this purpose, the electromagnets are mechanically fastened to the support structure.

According to the present example, the electromagnets <NUM> are mounted to two brackets <NUM>, <NUM> of the support structure. In alternative examples a different number of brackets greater than two may be provided. For example, greater than three brackets may be provided. The number of brackets used may be (at least in part) dependent on the expected loads and the allowable deflection of the electromagnets.

<FIG> shows a cross-sectional view of the electromagnet assembly <NUM> wherein the electromagnets <NUM> are mounted to the support structure <NUM>.

According to the present example, the electromagnets <NUM> are configured to generate a magnetic field which, in use, actively shields the magnetic field which is generated by a main magnet of the MRI apparatus. Each electromagnet <NUM> comprises a coil <NUM> (or 'shield coil') configured to generate the magnet field.

Further, each electromagnet comprises a support element <NUM>, which is mountable in the MRI apparatus. The support element is configured to restrain the coil against forces caused, for example, in response to electromagnetic interaction between coils. For the present example it is assumed that electromagnetic interaction causes a force on each coil which is directed outwardly with respect to the electromagnet assembly <NUM>. That is, each coil experiences a force pressing the coil against its respective support element. Further, the support element also restrains the coil against rotational forces as may be caused, for example, during transportation.

The coil <NUM> is formed of a superconductor wire wound into an annular structure. The coil is therefore essentially rotationally symmetrical. The coil may be described in terms of an axial direction, a radial direction and a circumferential direction. When considered in the context of the MRI apparatus, these directions correspond to those described above in relation to the MRI apparatus as a whole. In particular, the coil possesses rotational symmetry about the assembly axis A-A.

<FIG> show the coil <NUM> and the support element <NUM>.

The coil <NUM> has a first axial end face <NUM> and, opposite thereto, a second axial end face <NUM>. The pair of axial end faces delimits an extent of the coil along the axial direction. Similarly, the coil has a first radial surface <NUM> (or 'inner radial surface') and, opposite thereto, a second radial surface <NUM> (or 'outer radial surface'). The pair of radial surfaces delimits an extent of the coil along the radial direction.

The support element <NUM> is fixed to the coil <NUM> and provides a means for mounting the coil in the MRI apparatus and, more specifically, to the support structure <NUM>. The support element has a bonding face <NUM> which is bonded to the coil. The support element, or at least the bonding face, is configured to be bonded using an adhesive suitable for cryogenic applications. According to the present invention, the support element is bonded to the first axial end face <NUM> of the coil.

The support element <NUM> possesses a pair of mating surfaces <NUM>, <NUM> configured for being brought into abutment/contact with the support structure <NUM>. According to the present example, the mating surfaces <NUM>, <NUM> are substantially flat so as to sit flat against a corresponding portion of the support structure <NUM>.

According to the present invention, in which the support element <NUM> is bonded to the first axial end face <NUM>, the mating surfaces are separated by a distance which is greater than the radial extent (or 'radial thickness') of the coil <NUM>. Thereby the curvature of the coil may be accommodated between the mating surfaces. Radial expansion of the coil, in response to energising, is also accommodated by suitably spacing the mating surfaces.

The support element <NUM> is configured for mounting the coil to the support structure <NUM>. Accordingly, the support element <NUM> comprises means for mechanically securing to the support structure <NUM>. Any suitable means may be used such as, for examples, bolts, pins or other suitable mechanical elements. In the present example the support element comprises an aperture <NUM> for receiving a pin and thus make a pinned connection with the support structure. By way of example, a pin <NUM> is shown in <FIG>. However, for clarity, the pins (or other mechanical elements) are not shown in <FIG>.

<FIG> show the electromagnet <NUM> mounted in the support structure <NUM>. More particularly, <FIG> shows a radial cross-section of the support structure, the coil <NUM> and the support element <NUM>, while <FIG> shows a corresponding tangential cross-section.

The support structure <NUM> comprises a pair of support plates <NUM>, <NUM> configured to receive the electromagnet <NUM>. More particularly, an upper support plate <NUM> and a lower support plate <NUM> are arranged to define a passage (or 'cavity') <NUM> into which the support element <NUM> of the electromagnet is received. According to the present example, the support plates are arranged generally parallel but may alternatively be arranged non-parallel to provide for a different stiffness of the support structure. The passage so formed is suitably shaped to receive the support element <NUM> and, according to the present example, defined by substantially flat sections <NUM>, <NUM> of the support plates <NUM>, <NUM> which match the substantially flat mating surfaces <NUM>, <NUM> of the support element. As explained with reference to <FIG>, the example electromagnet assembly <NUM> comprises two brackets <NUM>, <NUM>. Each bracket comprises a pair of support plates <NUM>, <NUM>.

According to the present example, where a pair of electromagnets <NUM> is carried by the support plates <NUM>, each support plate extends from a first electromagnet <NUM> to a second electromagnet <NUM>'.

When received between the support plates <NUM>, <NUM>, the support element <NUM> is anchored to both support plates. Suitably an aperture <NUM>, <NUM> is provided in each in each support plate <NUM>, <NUM>. For securing the support element, the aperture <NUM> of the support element <NUM> is aligned with the apertures <NUM>, <NUM> of the support structure. A pin is then fitted through the aligned apertures <NUM>, <NUM>, <NUM>, thus bringing the electromagnet assembly <NUM> from an unlocked configuration into a locked/fixed configuration. In the locked configuration, relative movement between the support element and the support structure are restrained (or 'inhibited').

A bracing element <NUM> may be provided which extends between the support plates <NUM>, <NUM>. The size and/or location of the bracing element is chosen to modify the stiffness of the support elements, where necessary, in order to accommodate for expected loads acting on the electromagnet assembly <NUM>. Such loads may be caused by ferrous building material present where the electromagnet assembly will be operated (for example metal reinforcement rods or girders in floors and walls). Other loads may be caused internally, particularly in response to interaction between the magnets. More particularly, an inwardly-acting coaxial force may be caused on each electromagnet <NUM>. That is, the electromagnet assembly <NUM> may also be configured to operate in compression if the electromagnetic load direction is reversed. Sufficient bracing between the plates will be required to prevent buckling, and the bracing element may be sized and/or located suitably.

For applications relating to Magnetic Resonance Imaging, it is considered convenient to assemble an MRI apparatus completely, or as much as feasible, before transportation to its destination. This may save days or weeks of assembly time. Also, without requiring initial assembly and final assembly, this may reduce the amount of coolant required, thus saving precious resources resulting in a 'greener' final product. During transportation of a fully or partially assembled apparatus, however, rotational loads may act on the coils <NUM>. If unchecked, the forces can misalign or deform the coils and, consequently, adversely affect performance of the assembly.

However the provision of the bonded support elements <NUM> provides axial restraint and rotational (or 'circumferential') restraint of the electromagnet <NUM> relative to the support structure <NUM>.

Additionally, during operation the electromagnetic interaction between the coils <NUM> is restrained by the interaction of the support elements <NUM> and support structure <NUM>, thus preventing relative axial and radial movement of the coils, thereby optimising performance of the device.

Hence the bonded support elements <NUM> of the present disclosure provide support for the coils <NUM> against rotational loads to inhibit distortion of the assembly during transit and in operation, thus increasing the chance of optimised performance of the final product.

The exemplary electromagnet <NUM> according to the present disclosure may be manufactured using any suitable process of manufacturing.

An example process by which the electromagnet <NUM> may be manufactured relates to the known process of resin-impregnation. As part of this process, superconductor wire is wound into a mould. The support element is arranged with the coil so formed, locating the support element against an external surface of the coil. Where a plurality of support elements is located on the external surface, they may be arranged with equal spacing.

The resulting structure is then impregnated with a thermosetting resin and the resin allowed or caused to set. Subsequently the electromagnet is removed from the mould as a monolithic structure.

According to the example process described, the step of forming the coil by winding superconductor wire is essentially unaffected by the later addition of the support element. That is, the coil is formed into a shape optimised for performance, essentially without consideration for bonding to or being carried by the support element.

The bonding between the coil <NUM> and the support element <NUM> may also be achieved using resin-impregnation, either at the same time as forming the coil, or once the coil is formed. Resin-impregnation is known to provide great structural strength to a coil, but according to the present disclosure said structural strength is used also for bonding the coil and the support element. Notably, the bond may be reinforced, for example through the addition glass fibre to the resin, to provide a bond sufficiently strong for the intended application. In particular, the bond may be made strong enough to withstand a tensile force which may be caused in response to a reversal of the electromagnetic interaction between the electromagnets <NUM>.

Alternatively the support element <NUM> is secured to a resin-impregnated coil <NUM> in a separate manufacturing step. This may be achieved using an adhesive suitable for, where desired, cryogenic applications.

The prepared electromagnet <NUM> is mounted to the support structure <NUM> to form the electromagnet assembly <NUM>. This process includes locating the support element <NUM> of the electromagnet <NUM> in the passage <NUM> formed between the support plates <NUM>, <NUM>, aligning the aperture <NUM> of the support element with the apertures <NUM>, <NUM> of the support plates, and fitting a pin through the aligned apertures. It is noted that shape-matching of pins, particularly round pins, in corresponding apertures is efficiently achievable to high accuracy. This provides for a predictable load distribution to ensure optimal performance of the electromagnets.

Particularly for applications relating to cryogenic applications, which includes Magnetic Resonance Imaging, the electromagnets <NUM> are cooled below a critical temperature at which the coils <NUM> enter a superconducting state. For efficient cooling it is beneficial to expose a large surface area of the coils to the coolant. Conveniently, the electromagnet <NUM> according to the present disclosure allows for relatively small support elements so that a relatively large surface area remains exposed.

<FIG> show an alternative arrangement of the electromagnet assembly <NUM> which is configured to support particularly against compressive loads.

<FIG> is a radial cross-sectional view of the electromagnet assembly <NUM>, while <FIG> is a plan view of the electromagnet assembly, and <FIG> shows a tangential cross-section of the electromagnet <NUM>. Similar to <FIG>, <FIG> are partial views in the sense that only a right-hand side of the electromagnet assembly <NUM> is depicted. That is to say, the essentially identical lefthand side is omitted from the figure.

According to the example shown in <FIG>, the support plates <NUM>, <NUM> extend partway between the pair of electromagnets <NUM>. This is in contrast to the example of <FIG> in which the support plates extend all of the way from the first electromagnet to the second electromagnet. Put another way, in the example of <FIG> each electromagnet <NUM> is provided with its own pair of support plates <NUM>, <NUM>. A bracing element <NUM> is provided between each pair of support plates <NUM>, <NUM>. A rod <NUM>, which may also be provided in the above example, extends between the bracing elements <NUM>.

The example of <FIG> also illustrates an alternative shape of the support element <NUM>. According to this example, the bonding face <NUM> possesses reduced stiffness towards the periphery delimiting the bonding face. That is to say, the support element <NUM> is formed to be more flexible on its bonding face <NUM> towards a periphery of the bonding face than towards a central area of the bonding face, the central area being delimited by the periphery. This arrangement is suitable for minimising "edge effects". "Edge effects" may arise in response to stresses exerted on the coil <NUM>, irrespective of whether compressive or tensile loads are concerned. As regards a compressive load where the coil <NUM> is pressed against the bonding face <NUM>, the edge effects may result in a stress concentration at the periphery (or 'edge') of the bonding face which may cause damage to the coil or the support element. As regards a tensile load where the coil is pulled along an axial direction away from the bonding face, a stress concentration at the periphery of the bonding face may cause damage to the bond between the coil and the bonding face. Reducing edge effects will thus also reduce the likelihood of failure of the bond between the support element <NUM> and coil <NUM>.

<FIG> shows a radial cross-sectional view of a part of an alternative example of an electromagnet assembly <NUM>' according to an embodiment of the present invention. According to the example depicted in <FIG>, the electromagnet assembly is configured to operate in compression. That is, in use the electromagnetic load direction acts to bring the coils <NUM> together along the assembly axis A-A. To improve resistance to buckling, the support plates <NUM>, <NUM> and the bracing element <NUM> are formed integrally. The support plates <NUM>, <NUM> and the bracing element <NUM> may comprise solid or hollow sections. For example the support plates <NUM>, <NUM> and the bracing element <NUM> may be solid or hollow section extrusions. Additionally, the passage/cavity <NUM> defined between the support plates <NUM>, <NUM> correspond to a local cut-out sized to accommodate the support element <NUM> only. This arrangement therefore provides for a relatively large bracing element and a relatively small passage <NUM>, providing a particularly high resistance to buckling under compression. With suitable configuration and/or choice of materials, for example those of the coil surface <NUM> and the bonding face <NUM>, this example electromagnet assembly may also be operable in tension, where the integral bracing element <NUM> supports the induced loads.

<FIG> illustrates an alternative shape for the support element <NUM>. According to this example, the support element has a generally trapezoid cross-sectional shape. The legs (or sides) of the trapezoid shape are convex to further reduce stiffness towards the periphery.

<FIG> shows another alternative shape for the support element <NUM> configured to reduce edge effects. According to this example, the support element comprises cutaway sections towards the periphery of the bonding face <NUM>, which provides the periphery of the bonding face with greater flexibility.

<FIG> shows another alternative example of an electromagnet assembly <NUM>' according to the present disclosure. According to this example, the electromagnet <NUM> is mounted in the support structure <NUM> such that a compressive load exerted by the pinned connection is maintained after cooling the system below the critical temperature of the superconductor coil <NUM>. Where the support element <NUM> has a higher coefficient of thermal contraction than the pin <NUM> or pins used to make the pinned connection, as the system is cooled compression cannot be maintained so that the electromagnet is not fully secured. According to the present example, an extension tube <NUM> is fitted around the pin, wherein the extension tube (or 'sleeve') has a lower coefficient of thermal contraction than the pin. The extension tube is configured to abut against the support plates <NUM>, <NUM> and the pin <NUM> to compress the support plates against the support element even when cooled, and thus maintain compression.

<FIG> shows a perspective view of a variant of the electromagnet <NUM>. In the examples described above, at least one support element <NUM> is bonded to the first axial end face <NUM>. In other examples, at least one support element <NUM> may be bonded to each axial end face <NUM>, <NUM> or, more generally, to each pair of opposing external surfaces of the coil. That is, a first support element is bonded to a first outer surface of the coil, a second support element is bonded to a corresponding location of a second outer surface which is opposite to the first outer surface. According to <FIG>, the first support element <NUM> is bonded to the first axial end face <NUM>, while the second support element <NUM>' is bonded to the second axial end face <NUM>.

It is considered convenient to provide a support structure configured to compress the coil along the assembly axis A-A when the support structure and the coil are cooled and/or the coil is energised. For this purpose, the support structure may be configured to possess greater thermal contraction than the coil along the axial direction. Additionally or alternatively, the support structure and the coil are configured so that an expansion of the coil on being energised results in the support structure pressing the support elements against the coil. Conveniently, the described arrangement may reduce misalignment of the coil on being energised and may improve durability of the bonding.

In relation to the examples where the support elements <NUM> are bonded to the axial end faces, the first support element is located at a particular circumferential location on the first axial end face, and a second support element is located in on essentially the same circumferential location on the second axial end face.

Where a support element <NUM> is bonded to a radial surface, the bonding face <NUM> of said support element is suitably curved, convex or concave, dependent on the particular radial surface to which it is mounted. Further, the support structure is suitably adapted for mounting an electromagnet <NUM> having a support element or elements <NUM> on a radial surface or surfaces <NUM>, <NUM>.

Claim 1:
An electromagnet assembly (<NUM>) for an MRI apparatus, the electromagnet assembly (<NUM>) being generally rotationally symmetrical, thereby defining an assembly axis, a radial direction and a circumferential direction, the electromagnet assembly (<NUM>) comprising a pair of electromagnets (<NUM>) and a support structure (<NUM>) configured to carry the electromagnets (<NUM>), each electromagnet (<NUM>) comprising:
- a coil (<NUM>) configured to generate a magnetic field, the coil (<NUM>) having a first axial end face (<NUM>; <NUM>), and
- a support element (<NUM>; <NUM>') configured for mounting the coil (<NUM>) in the MRI apparatus;
wherein the support element (<NUM>; <NUM>') is bonded to the first axial end face (<NUM>; <NUM>) of the coil (<NUM>);
characterised in that
the support structure (<NUM>) comprises at least two brackets (<NUM>, <NUM>) to which the electromagnets (<NUM>) are mounted, each bracket comprising a pair of spaced-apart support plates (<NUM>, <NUM>);
wherein each support plate extends from a first electromagnet to a second electromagnet of said pair of electromagnets or wherein each electromagnet (<NUM>) is provided with its own pair of support plates which extend partway between the pair of electromagnets (<NUM>), a bracing element (<NUM>) being provided between each pair of support plates and a rod (<NUM>) extending between the bracing elements;
wherein each pair of spaced-apart support plates is spaced apart from one another in the radial direction by a distance which is greater than a radial thickness of each coil and forming a cavity (<NUM>) therebetween; and
wherein the support element (<NUM>) is received into the cavity (<NUM>) and mechanically fastened to the support plates (<NUM>, <NUM>).