GB2586489A

GB2586489A - Method and structures for coil adjustment

Info

Publication number: GB2586489A
Application number: GB1912003.9A
Authority: GB
Inventors: John Disney Mallet Michael
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2019-08-21
Filing date: 2019-08-21
Publication date: 2021-02-24
Anticipated expiration: 2039-08-21
Also published as: GB201912003D0; GB2586489B

Abstract

A superconducting magnet assembly 10, comprises: axially aligned coils 12, 14, 16, 18 separated from one another by spacer blocks 20 bonded to respective axial faces of the coils; wherein each spacer block 20 is divided into two parts along an interface plane which corresponds to a plane which is tilted at an angle to provide respective thrust faces. The magnet may have at least three coils 12, 16, 18 with spacer blocks 20 configured such that the rotation of the middle coil 16 causes an axial movement of coil 16 relative to coils 12 and 18 whilst keeping the same distance between coils 12 and 18. Clamp means, of a fixed or removable nature, may be provided which can prevent further movement of the coils. The spacer blocks 20 may support assembly structures other than just the coils. Rollers may be used to allow the rotation of one or more coils to obtain desired coil axial position adjustments. Also disclosed is a method, suitable for adjusting the magnetic field homogeneity of a superconducting magnet assembly, comprises: measuring the magnetic field homogeneity produced by a superconducting magnet assembly; calculating an axial adjustment of at least one of the coils which would improve the magnetic homogeneity; and rotating at least one coil to achieve the calculated coil axial adjustment.

Description

METHOD AND STRUCTURES FOR COIL ADJUSTMENT

The present invention relates to superconducting magnets comprising several axially aligned coils. In particular, it relates to cylindrical magnets for imaging systems such as M RI or NMR imaging systems.

As is well known, a superconducting magnet for an imaging system must provide a high-strength magnetic field of high homogeneity. However, tolerances in the manufacture of a cylindrical superconducting magnet comprising several axially aligned coils mean that a produced magnet may have a field homogeneity insufficient to produce the required imaging field homogeneity in the finished system.

The present invention provides methods and apparatus for adjusting the field homogeneity of cylindrical superconducting magnets comprising axially-aligned coils.

The introduction of manufacturing tolerances during the construction of a superconducting MRI magnet mean that the target field homogeneity is extremely unlikely to be achieved without some form of corrective adjustment to remove inhomogeneities, which may be represented as unwanted contaminant harmonics. Corrective adjustment may comprise balancing the magnet by changing the relative positions of the coils, which can dramatically reduce the unwanted contaminant harmonics and improve the likelihood that the magnet can be successfully brought to an acceptable field homogeneity by the end of the manufacturing process by a conventional shimming technique.

Active and passive shimming techniques are well-known for adjusting the homogeneity of magnetic fields produced by superconducting magnets. Active shimming comprises adding electrical current to small correcting coils provided for the purpose, while passive shimming involves adding pieces of ferromagnetic material to adjust the magnetic field shape. However, active or passive shimming of a superconducting magnet can have a limited beneficial effect if the field inhomogeneity, which can be expressed as "unwanted" or "contaminant" harmonics, is too large in magnitude.

"Balancing" of the magnet by changing the relative positions of the coils can dramatically reduce the unwanted contaminant harmonics and improve the likelihood that the magnet can be successfully shimmed after manufacture, by a conventional active or passive shimming method.

Typically, during the manufacture of cylindrical superconducting magnets comprising axially-aligned coils, a room-temperature plot of the generated magnetic field is performed. Once the magnet coils have been constructed and located in their intended positions, a relatively small DC electrical current is passed through the magnet coils. As the coils are at room temperature for this stage, they are not superconducting. The electrical current passes through the sheathing, typically copper or aluminium, in which superconducting threads are embedded. The magnetic field strength is plotted over a closed surface, typically a spherical surface representing the imaging region in the completed imaging system.

The homogeneity of the magnetic field over that closed surface is then calculated, and a computer simulation, well known to those of skill in the art, is performed to calculate a suitable correction for inhomogeneities in the magnetic field strength which may be thus found.

The calculated correction may then be applied to the magnet structure. This step may be known as "balancing" the magnet. It typically comprises a mechanical adjustment to one or more of the coils, such that the position of, and / or number of turns in, a particular coil is adjusted.

The required balancing may be performed in one or more of the following manners: Turns balancing: in which the number of turns on the coils is adjusted, to effectively add or remove ampere-turns at specific locations in the magnet assembly.

"Prep" balancing: in which physical shims such as metal or composite material gaskets are introduced into, or removed from, spaces between coils to move the relative axial location of the magnet coils. In practice, this technique is usually only applied to end coils due to difficulty in accessing other coils. Former balancing: in which the physical size of former components or components of a supporting structure is changed, so as to move the axial location of the magnet coils. Such method is typically applied only to shield coils, due to the relative ease of access for shield coils.

These approaches generally operate stepwise in nature: for turns balancing, typically a full turn is either added or removed from a coil; in "prep" balancing, a gasket or mechanical shim is either added or removed; in former balancing, an adjustment mechanism is provided and is typically adjusted by one full turn of the adjuster at a time, so the available variation is stepwise in nature.

The turns balance approach is may be applied to magnet coils that are still on a winding machine, so a magnetic field measurement cannot usually be made to verify the effect of removing/adding a turn. Turns balancing in this manner may be applied by physically measuring coil geometries and relative positions, to give an indication of what the magnet harmonics would be with those particular coil dimensions. A computer simulation would typically be employed to calculate the effects of any particular coil geometry. In some instances, turns may be removed from an impregnated coil. The prep balancing or former balancing approaches usually require partial disassembly of the magnet assembly to change components, so can be time consuming.

The present invention accordingly aims to alleviate at least one of the disadvantages related to conventional magnet balancing methods, and provides apparatus and/or methods for magnet balancing as set out in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments of the present invention, given by way of non-limiting examples only, wherein: Fig. 1 schematically illustrates a magnet assembly according to an embodiment of the present invention with all coils in respective "neutral" positions; Fig. 2 schematically illustrates a spacer block according to a feature of the present invention, in a "neutral" position; Fig. 3 schematically illustrates a spacer block according to a feature of the present invention, in a first displaced position, having a greater axial dimension than in the "neutral" position; Fig. 4 schematically illustrates a spacer block according to a feature of the present invention, in a second displaced position, having a lesser axial dimension than in the "neutral" position; Fig. 5 schematically illustrates a magnet assembly according to an embodiment of the present invention with one of the coils rotated to provide axial displacement with respect to its original axial position; Fig. 6 schematically illustrates another embodiment of the present invention; Fig. 7 schematically illustrates an arrangement of rollers which may be used to effect rotation of the coils; and Fig. 8 schematically illustrates an embodiment of the present invention in which arrangements are provided for adjusting the axial location of shield coils.

The present invention provides methods and apparatus for magnet balancing by coil rotation. Preferably, the coils may be rotated in a linear fashion, so there is no stepwise effect. The apparatus of the present invention allows a single coil within a coil set to be rotated, with the effect of axially adjusting the position of the moved coil without affecting the positions of the remaining coils.

Fig. 1 schematically represents a magnet assembly 10 comprising a coil set, which is provided with arrangements for magnet balancing by axial displacement by coil rotation according to an embodiment of the present invention. In a finished imaging system, magnet assembly 10 will be enclosed within a cryostat and cooled to the superconducting temperature of the superconducting magnet coils. Magnet assembly 10 is essentially rotationally symmetric about axis A-A. Magnet assembly 10 comprises a number of discrete coils or coil assemblies 12, 14, 16, 18. Coils 12, 14 are end coils and coils 16, 18 are inner coils. All coils are shown to have the same dimensions, which is unlikely to be the case in a real superconducting magnet design, but magnet assembly 10 is shown in this manner here for ease of illustration.

The respective coils 12, 14, 16, 18 are axially spaced apart to their required positions by spacer blocks 20 which are bonded to respective axial surfaces of the adjacent coils. Spacer blocks 20 form the load transfer and physical constraint for the coil assembly.

In a completed imaging system including such a superconducting magnet assembly, the magnet assembly 10 is located inside a cryostat and cooled to operating temperature. The various coils have to be terminated -that is, electrically joined to one another and to further electrical equipment, not represented in the drawings.

Figs. 2 and 3 show more detailed representations of an example spacer block 20. In Figs. 2 and 3, the spacer block 20 is represented as having an overall cuboid form, but may in practice be arcuate to match the curvature of the circumference of the attached coils. According to a feature of the present invention, the spacer block 20 is divided into two parts 20a, 20b along an interface plane 22 which corresponds to a radial XY plane (see representation of XYZ directions in Fig. 1), tilted in the Z direction by a tilt angle a to form respective thrust faces of the two parts of the spacer block 20. In the illustrated embodiment, the two parts 20a and 20b are essentially identical, though this need not be the case. In the illustrated "neutral" position, the two parts 20a and 20b are aligned on top of one another in the axial direction Z, and the spacer block 20 has a neutral axial dimension d.

As illustrated in Fig. 3, a first part 20a of the spacer block may be rotated in the circumferential direction c with respect to the second part 20b. In the example of Fig. 3, this rotation causes the axial dimension of the block 20 to take an increased value d+, due to the angle ce of the interface plane 22 with respect to the radial plane represented by axially outer surfaces 24 of the spacer block 20. In a preferred embodiment, a is selected to produce approximately 1mm axial movement for approximately 1 degree rotation in the circumferential direction c, e.g. arctan (1/(1000*pi /360)) "-6.5 degrees for the inner magnet, where 1 degree approximately corresponds to lcm of motion in the circumferential direction c, or approximately arctan (1/10) such that displacement of one part 20a, 20b of the spacer block with respect to the other part by a distance of lOmm will result in a change of axial dimension d of lmm.

On the other hand, as illustrated in Fig. 4, the first part 20a of the spacer block may be rotated in the opposite circumferential direction c with respect to the second part 20b. In the example of Fig. 4, this rotation causes the axial dimension of the spacer block 20 to take a decreased value d-, due to the angle a of the interface plane 22 with respect to the radial plane represented by axially outer surfaces 24 of the spacer block 20. In a preferred embodiment, a is approximately tan-1(1/10), such that displacement of one part 20a, 20b of the spacer block with respect to the other part by a distance of

S

10mm in the circumferential direction c will result in a change of axial dimension d of approximately 1mm.

Fig. 5 schematically illustrates the arrangement of Fig. 1 wherein coil 16 has been rotated in the direction of arrows 26, resulting in the axial displacement of coil 16 in the direction of arrow 28. As a result of that displacement, spacer blocks 20 between coils 12, 16 have a reduced axial dimension d-and spacer blocks 20 between coils 16, 18 have an increased axial dimension d+. As all spacer blocks 20 have the same angle a of the interface plane 22, the amount by which reduced axial dimension d-is reduced as compared to neutral axial dimension d equals the amount by which increased axial dimension d+ is increased as compared to neutral axial dimension d, that is to say, d+ + d-= 2d. The effect of this is of course that rotation of coil 16 as illustrated results in the axial displacement of coil 16, but does not change the axial positions of the remaining coils 12, 14, 18.

In the case of end coils 12, 14, if one is rotated to change the axial dimension d of the associated spacer blocks, then the magnet assembly 10 as a whole will become longer or shorter in the axial direction Z. Similar, but independent, rotation of coil 18 will result in the axial displacement of coil 18, without changing the axial positions of the other coils. A limit will of course be reached, when the axial displacement between first 20a and second 20b parts of a spacer block reaches a maximum extent before the overlap between first 20a and second 20b parts of the spacer block becomes too small to support an axial mechanical load which will be placed on the spacer blocks when the magnet is in use. It may be preferred to provide spacer blocks 20 with a larger circumferential extent, with a reduced tilt angle a.

As illustrated in Fig. 6, in an embodiment of the invention, the spacer blocks 20 effectively join together to form a single, annular spacer block in which each part 20a, 20b has a sawtooth surface. In such an embodiment, the "neutral" position shown Figs. 1 and 2 would correspond to a position midway in the range of motion, as illustrated.

Such axial movement of individual coils, as provided by the present invention, allows unwanted contaminant harmonics to be eliminated, thereby improving the homogeneity of the magnetic field produced by the magnet assembly 10. The required axial movement, and the corresponding degree of rotation may be calculated by a computer simulation as well known to those skilled in the art.

In a preferred embodiment, the coefficient of friction between first and second parts 20a, 20b of the spacer block is sufficient that it is unnecessary to bond the first and second parts 20a, 20b of the spacer block together once their relative position is finalised. Preferably, the coefficient of friction is low enough to allow rotation of each coil, but high enough such that an expected axial force experienced by the coils in use is too low to overcome the rotational friction and cause rotational displacement of any of the coils. In such an embodiment, it would be possible to remove and replace a single coil from the assembly if necessary. A clamping arrangement may be applied to clamp all of the coils axially to prevent further relative motion of the coils. If such clamping arrangement is removable, then it would still be possible to remove and replace a single coil from the assembly if necessary. If friction between first and second parts 20a, 20b of the spacer block is deemed insufficient alone, removable fastenings may be used to fix the coils in their amended positions, such as an arrangement of bolts or screws. Whichever method is chosen, once the coil positions are finalised, it should be impossible for further or unintentional movement to occur between the coils. This may be achieved by an adhesive bond between the first and second parts 20a, 20b of respective spacer blocks.

The present invention provides a simple manner of providing an appreciable range of axial adjustment of each coil individually, without the need to disassemble the magnet assembly. Use of the methods and apparatus of the invention is believed to reduce the maximum required shim mass for a magnet and reduce the likelihood of a magnet being found unshimmable after assembly.

As illustrated in Fig. 7, rotation of the coils may be enabled for example by resting each coil on a respective pair of rollers 30a 30b which support the weight of the respective coil, or a set of three or more rollers 30a, 30b, 30c pressing radially onto a respective coil at certain locations around the inner or outer circumference of a coil.

In an embodiment, such rollers 30a, 30b, 30c are controlled by a computer, such that axial adjustment of coils according to the present invention may be performed automatically. A device performing a room temperature plot of the magnetic field homogeneity over an imaging region of the magnet assembly 10 may be linked to control of rollers 30a, 30b, 30c effecting rotation of the coils 12, 14, 16, 18. Axial movement of the coils by rotation, as provided by the present invention, may thereby be effected automatically and in real time, such that measurement of the field homogeneity over the imaging volume gives rise to calculation of coil rotation for improving the field homogeneity; the calculated rotation is automatically applied to the coils, and the field homogeneity over the imaging volume can be measured again, and another step of applying rotation to magnet coils can be performed. This may be repeated as necessary until an optimal combination of coil positions is reached.

Multiple degrees of freedom are provided, to enable effective balancing of the magnet prior to termination and cryostat assembly. No disassembly of the coil assembly 10 would be required to achieve an appropriate balance solution. The range of axial movement required to achieve a suitable balance solution may be of order of ten millimetres. The tilt angle a of the spacer block 20 should be chosen such that adjusted coil axial positions can be achieved whilst not requiring excessive coil rotation. Excessive coil rotation might lead to difficulty in later termination stages of the magnet assembly.

In particular example embodiments of the present invention, spacer blocks 20 may be of a resin impregnated glass fibre material such as that known as G10, or TUFNOL (RTM) phenolic cotton laminated plastic.

The circumferential length of spacer blocks 20, and their number, would be chosen dependent upon the axial mechanical load the blocks are required to support. A minimum number of two spacer blocks 20 is required, a maximum number can be any number greater than two, but in preferred embodiments the number of spacer blocks 20 is between three and twenty.

The invention can be applied to every coil in a magnet coil assembly or only to a subset of coils e.g. the end coils or the shield coils.

While coils such as shown at 16, 18 may be rotated to adjust their axial position without changing the overall axial length of the magnet assembly 10, any adjustment of end coils 12, 14 will result in a change to the axial length of the magnet assembly 10. This may be achieved, according to the present invention, as angled spacer blocks 20 allow rotary movement for axial displacement of a coil.

Similarly, as illustrated in Fig. 8, the axial position of shield coils 32,34 may be adjusted by rotation in conjunction with angled spacer blocks 20 as discussed above. Since shield coils 32,34 are provided at or near axial extremities of the magnet assembly, a cylindrical spacer 36 or other similar mechanical mounting arrangement may be provided. Angled spacer blocks 20 are placed between the mechanical mounting arrangement 36 and each shield coil 32, 34. By rotating each shield coil 32, 34, in the manner described above for main magnet coils, the shield coil 32, 34 may be moved axially towards or away from the mechanical mounting arrangement 36. This allows scope for balancing of the shield coils.

The spacer blocks 20 may also be used to provide a suitable mounting location from which other parts of the magnet assembly can be fixed. For example, a supporting structure for shield coils or electrical termination components, or indeed magnet suspension arrangements.

The invention accordingly provides methods and apparatus for coil adjustment in a cylindrical superconducting magnet by rotation of respective magnet coils.

Claims

CLAIMS: 1. A superconducting magnet assembly (10) comprising a number of axially aligned coils (12, 14, 16, 18) separated from one another by spacer blocks (20) bonded to respective axial faces of respective coils, characterised in that each spacer block (20) is divided into two parts (20a, 20b) along an interface plane (22) which corresponds to a radial XY plane, tilted in the Z direction by a tilt angle (a) to form respective thrust faces of the two parts of the spacer block (20).
2. A superconducting magnet assembly according to claim 1, wherein the superconducting magnet assembly comprises at least three coils, such that a first coil (16) is located between a second coil (12) and a third coil (18), and in that spacer blocks (20) are provided between the first coil and the second coil, and between the first coil and the third coil, and the spacer blocks have corresponding tilt angle (a), whereby rotation of the first coil (16) with respect to second coil (12) and third coil (18) results in axial displacement of the first coil with respect to the second coil and the third coil, but wherein the relative positions of the second coil and the third coil to one another remain unchanged.
3. A superconducting assembly according to any preceding claim wherein a clamping arrangement is provided to clamp the coils axially to prevent further relative motion of the coils.
4. A superconducting assembly according to claim 1 or claim 2 wherein removable fastenings are provided to fix the coils in position.
5. A superconducting assembly according to claim 1 or claim 2 wherein an adhesive bond is provided between the first and second parts (20a, 20b) of respective spacer blocks.
6. A superconducting assembly according to any preceding claim wherein at least some of the spacer blocks (20) are used to provide a mounting location from which other parts of the magnet assembly are fixed.
7. An arrangement for adjusting the magnetic field homogeneity of a superconducting magnet assembly according to any preceding claim, comprising a pair of rollers for each of the axially aligned coils, such that the respective coil may be rested upon the pair of rollers, and that by rotation of the respective pair of rollers, rotation of the respective coil may be achieved.
8. An arrangement for adjusting the magnetic field homogeneity of a superconducting magnet assembly according to any preceding claim, comprising a set of three or more rollers pressing radially onto a respective coil at certain locations around the circumference of that coil, such that the respective coil may be retained upon the set of rollers, and that by rotation of the respective set of rollers, rotation of the respective coil may be achieved.
9. A method for adjusting the magnetic field homogeneity of a superconducting magnet assembly (10) according to any of claims 1-6, comprising the steps of: measuring the magnetic field homogeneity of a magnetic field produced by the superconducting magnet assembly over a closed volume; - calculating an axial adjustment of at least one of the coils (16) which would improve the magnetic field homogeneity of the magnetic field produced by the superconducting magnet assembly over the closed volume; and - rotating the at least one coil to achieve the calculated axial adjustment.
10. A method for adjusting the magnetic field homogeneity of a superconducting magnet assembly (10) according to claim 9 wherein the steps of measuring, calculating and rotating are repeated to achieve further improvement of the magnetic field homogeneity of the magnetic field produced by the superconducting magnet assembly over the closed volume.
11. A method for adjusting the magnetic field homogeneity of a superconducting magnet assembly (10) according to claim 9 or claim 10, wherein axial adjustment of coils is performed automatically, in that a device measuring the magnetic field homogeneity of the magnetic field produced by the superconducting magnet assembly over the closed volume is linked to control of rollers effecting rotation of the coil (16), wherein measurement of the field homogeneity over the closed volume gives rise to calculation of coil rotation for improving the field homogeneity; and the calculated rotation is automatically applied to the coils.