GB2319845A - Opposed pole MRI magnet using HTSC blocks - Google Patents

Abstract

The flux in a C-type MRI magnet is generated by a number of melt-quenched HTSC blocks 31-33 embedded in flux guides 37-39. The blocks may comprise YBCO single crystals and the flux guides may comprise Permite (RTM) material. The blocks may be magnetised by pulses applied to the coils 34-36 either simultaneously or at different times. The blocks may be staggered to prevent interaction of the windings 34-36.

Description

NUCLEAR MAGNETIC RESONANCE IMAGING APPARATUS The present invention relates to Nuclear Magnetic Resonance Imaging Apparatus (known as MRI) apparatus.

In magnetic resonance methods and apparatus a static magnetic field is applied to the body under investigation to define an equilibrium axis of magnetic alignment in the region of the body being examined. A radio frequency field is then applied to the region being examined in a directional orthogonal to the static magnetic field direction in order to excite magnetic resonance in the region, the resulting radio frequency signals being detected and processed. The exciting radio frequency field is applied and resulting signals are detected by radio frequency coils placed adjacent the body.

The present invention is more particularly concerned with the means for generating the main magnetic field. Such means can comprise a permanent or an electro magnet. The present invention is concerned with the construction and composition of a permanent magnet for this purpose.

In order to achieve effective visualisation of the relevant part of the patient it is necessary for a large magnetic field to be generated having good homogeneity in the target region.

To achieve this it has been necessary to employ a massive power supply close to the patient which is potentially undesirable.

The present invention is concerned with reducing the power that is required in order to generate the required magnetic field.

According to the present invention a permanent magnet for use in a nuclear magnetic resonance imaging apparatus has as one of its constituents a melt-quenched high temperature super conducting material.

Preferably the said constituent is melt-quenched yttrium barium copper oxide (YBCO).

According to one aspect ofthe present invention the said magnet comprises a plurality of discrete blocks of material.

According to the second aspect of the present invention the said blocks comprise single crystals.

According to the third aspect of the present invention the said blocks are arranged in the form of an array which is shaped to produce a desired optimum magnetic field pattern.

According to the fourth aspect ofthe present invention the individual blocks of said multiblock core are individually energisable by means of a plurality of electrical coils whereby the magnetic field of the magnet can be shaped.

According to a fifth aspect ofthe present invention there are a second plurality of discrete blocks of magnetic material adapted to assist in shimming the magnet.

The shimming operation is a known technique in MRI whereby the magnet can be finely adjusted to in effect tune it to give optimum performance including to correct for changes in the main magnetic field caused by variations or changes in the relevant characteristics of the patient being examined.

Although the invention is mainly concerned with overcoming the problem referred to above it has possible applications in other fields, for sample dentistry.

How the invention may be carried out will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a side elevational diagrammatic representation of a typical type of MRI magnet to which the present invention may be applied; Figure 2 is a diagrammatic enlarged view of the area circled at B in Figure 1; Figure 3 is a cross sectional view taken at line A-A in Figure 2; Figure 4 is a view similar to Figure 2 of an alternative arrangement according to the present invention; and Figure 5 shows the assembly for one flux drive.

Figure 1 This illustrates in diagrammatic form a typical MRI main magnet 1 having poles 2a, 2b to which the present invention may be applied.

The present invention relates to the construction of the area B of each pole 2a, 2b, this being shown in more detail in Figures 3 and 4.

Figure2 & 3 As indicated earlier the basic concept of the present invention is to have one component ofthe field magnet made of a meltquench superconducting material and more specifically made of yttrium barium copper oxide (YBCO). A characteristic of this material is that it can in effect "trap" flux resulting in it behaving like a permanent magnet. It can therefore be magnetised by the application of a pulse of magnetic field the magnetisation being to the peak value generated by that field, and demagnetised by a reverse pulse or alternatively by raising its temperature so that it is no longer superconducting.

The material YBCO can be magnetised to relatively high fields typically in excess of one Tesla and could be in the region of three or four Tesla's or even higher.

The material YBCO could be incorporated into the magnet in a number of ways and one particular way of doing this will now be described with reference to Figures 2 and 3 which illustrate the area B of Figure 1 in more detail.

In this embodiment ofthe invention the material YBCO is fabricated in the form of small discrete blocks of material which are tile-like in shape, three of these tiles being indicated at 31, 32 and 33 in Figure 2, the tile 32 also being illustrated in the cross sectional view Figure 3. The tiles could be in the form of single crystals of YBCO but this is not essential. Such crystals would typically be in the range of twenty to thirty millimetres in diameter.

Each ofthe tiles 31,32 and 33 would have associated with it an electrical winding 34, 35 and 36 respectively.

It is important that there is no interaction between the windings 34, 35 and 36 and for this reason, in the embodiment of Figure 2, the tiles 31, 32 and 33, and their associated windings, are staggered with respect to one another. Each of the tiles is contained/embedded in a supporting flux guide 37, 38, and 39 respectively which in this embodiment is made of Permite (Registered Trade Mark). The staggered arrangement ofthe tiles shown in Figure 2 is in the x, y plane and such a staggered arrangement would be repeated to cover the desired area/volume.

As shown in Figure 3 the arrangement of flux guides 37, 38 and 39 have neighbouring guides 40, 41, 42 , 43, 44 and 45.

In the arrangement as shown in Figure 2 the tiles 31, 32 and 33 etc. are energised separately by their respective windings. As indicated earlier a disadvantage of exciting the tiles separately is that a current pulse used to excite one tile can generate fields in the region of neighbouring tiles which are in the opposite sense to that in the tile being excited. Therefore the tiles have to be spaced apart with magnetic circuits continuing in front or behind the tile face area by the non-conducting magnetic material of the neighbouring guides 40, 41, 42, 43, 44 and 45 each being made from Permite 75 (Registered Trade Mark), in this embodiment.

The pattern oftiles and the flux guides results in the arrangement shown in Figure 3. This arrangement allows major gaps in the layer of magnetic material so that other layers of tiles are needed to cover the gaps. Typically three, four or more layers may be required.

The tile arrangements described and shown in the drawings are used to modify and shape the field produced at the front poles of the magnet. In other words they act like shims.

One objective of this arrangement is to avoid the flux being shunted round the winding and to prevent magnetic saturation.

As indicated earlier the tiles can be formed into an array shaped to give an optimum field pattern for the purpose required. This array can be energised and de-energised by a single surrounding winding or by multiple windings designed to excite parts of the material section by section. The latter arrangement is the one shown in Figures 2 and 3. With this arrangement there is the advantage that parts of the material can be controllably magnetised to help in shaping the field. As indicated earlier additional tiles beyond those designed to generate the desired field can be incorporated in order to assist with shimming the magnet. In general a magnet will include more than one array of tiles each with at least one excitation drive as previously described with reference to Figures 2 and 3.

However it may be useful in some circumstances to excite all the tiles at the same time, for example, when making a large magnetised plane as a full magnet as might be required in a prepolarising operation where although a large field is required, its quality is not critical.

As it is necessary for the material of the tiles to be superconducting the YBCO has to be cooled to be well below its critical temperature of approximately 90"K. An operating temperature of around 20 to 25 0K is preferable. This temperature can be achieved without the use of cryogens but by using conduction refrigeration employing liquid nitrogen, for example, which is relatively cheap and simple. Incidentally the arrangement envisaged will always have a cryostat but only for insulation purposes, the actual refrigeration being effected by means of gas, as indicated earlier, or possibly by the use of a thermal engine/cycle, e.g. a Joule-Thomson refrigerator.

The conductors used for the purpose of conduction refrigeration are preferably the copper wires used to carry the currents to excite the tiles, although other arrangements could be employed. Relatively high fields of around are Tesla are required so that the conduction wires may have to carry current pulses as large as around 1,000 amps thus generating significant heat which is dissipated in the cables of which the copper wires form part.

As indicated earlier, with the present invention magnetisation can be effected by the use only of a pulse and therefore once this has been done the conductors can be de-energised until next time they are required to change the magnetic field.

In order to distribute the heat load the pulses of current to the tiles 31, 32, 33 etc can be distributed in time. This mode of operation means that the arrangement of the present invention can be used to carry out the so-called shimming operation instead of using the known arrangement where iron flakes (typically 1-2mm thick and 2-3mm diameter) are insertable in recesses or compartments found in a tray to form a shim array tailored to the particular shimming requirement being most relevant in carrying out shimming.

Extra cooling in addition to that provided by the copper conductors, and discussed earlier, can be provided using non-electrical conducting, non-superconducting but thermally conducting ceramic materials. These materials can be used as part of the structure needed to support the tiles mechanically. Mechanical forces in such a structure are significant especially when other tile arrays are used to help produce an increased field. There has already been discussed earlier the problem of the windings 34, 35, 36 etc. generating fields in the region of neighbouring tiles and one solution is shown in Figure 2 of staggering the tiles in the x, y plane. However this itself can give rise to the magnetic field being made non-uniform or dehomogenised.

This effect is mininlised bythe arrangement shown in Figure 3 in which the guides 40, 41, 42 and 43,44 and 45 act as magnetic pole plates the aim of.which is to in effect provide a ground which will simply short the flux round the windings. This is a compromise between not wanting flux around the winding and in the crystal of the tiles and not making guides 40, 41, 42 so shaped that they saturate.

It will be noticed in Figure 3 that the guides 41 and 44 are reduced in area so that they can approach but must stay below saturation so that the tuning effect of the tiles 32 is not masked.

As indicated earlier several tiles can be stacked one on top of the other to give additional magnetisation if needed.

Figure 4 An alternative arrangement for energising the tiles is illustrated in Figure 4 in which adjacent tiles are not staggered, as in Figure 2, but each tile 46, 47 is contained within a narrow neck portion ofthe associated Permite (Registered Trade Mark) flux guide 48 and 49 respectively. Each of the tiles 46, 47, etc. is provided with its associated energisation winding 50, 51, etc. as in the arrangement shown in Figure 3.

The arrangement of Figure 4 is satisfactory if the main core saturation is low as the neck where each of the windings 50, 51 etc. are located ensures that there is no interaction between those coils.

The conductors used to feed the tiles energisation coils may be thicker than the wires used in the windings 34,35,36 etc. which need to be kept small in order to make the windings compact. However the thicker conductors allow better removal of the heat particularly as some conductor paths are relatively long.

Once the arrangement shown is excited the connection from these conductors to the drive electronics can be removed to minimise heat loss.

Figure 5 In the general known type of arrangement already described with reference to Figure 1 a main field magnet arrangement according to the present invention comprises a generally cylindrical magnetic assembly 21 which is located in a cryostat chamber 22.

The tile area 21 is carried at an enlarged end 23a of a flux guide plug 23 which in this embodiment is made of Permite 75 (Registered Trade Mark) manufactured by Hoganas, the other end 23b of which is of smaller cross section to fit within a main electrical energising winding 24. Externally of an enclosing cryostat chamber 22 there is located a Permite (Registered Trade Mark) spreader plate 25 which is at room temperature. The purpose ofthe plate 25 is to act as if like a pole face to shape the flux and in particular as it were "smudge" perturbations in the flux.

The pole and cryostat assemblies shown in Figure 5 are supported from a suitable structure, including enough struts and stiffeners to avoid the units moving from their desired location even under the substantial forces that may be generated during use of the system.

It is necessary to avoid the risk of any currents causing the flux to simply couple around the tiles and thus reduce the external magnitude of the field. This can be achieved by completing the magnetic circuit between individual arrays of tiles except in the region occupied by the patient.

The main drives to the magnet in general are provided by superconducting windings where the currents are transient and non-permanent in nature although they can be operated in persistent mode when the current would be present all the time.

The tile windings could be made from NbTi, NbSn3 or a high temperature superconductor, where "high" means above 20"K. The tile windings are carried on formers which include a Permite (Registered Trade Mark), or equivalent, core. This core is run as near saturation as possible, is then increased in size (so that it is no longer saturated) and split to form the flux guides to each block as shown in Figure 3.

The Permite (Registered Trade Mark) material near the tiles is operated so that the tiles remain below saturation and these tiles are then used to raise the flux in individual parts of the magnetic core as a correction.

The tiles can be used to adjust the local flux positively or negatively.

Figure 5 illustrates an arrangement for one flux drive.

When it is desired to prepolarise the magnet the current leads are kept in position, since fast ramping/collapse is needed, as are the connections to the refrigeration system.

The shim windings contacts can be removed in those situations where they would otherwise only have a marginal effect if it was necessary to have a sudden field removal as a result of an emergency.

The shim module can be shaped at some distance from the main drive coils in order to concentrate and modify the fields locally. In this mode the two can, if necessary, be in separate cryostats, cooled by a common conduction refrigerator, or independently.

Furthermore, the shimming operation could be accomplished by the use of a plurality of small superconducting coils or windings per tile, rather than the single winding illustrated in Figures 24. However in that case the small superconducting windings/coils would need to operate in persistent mode unless continuously supplied and be much more complex in construction than the quasi permanent magnet tiles.

Claims (10)

1. A permanent magnet for use in a nuclear magnetic resonance imaging apparatus has as one of its constituents a melt-quenched high temperature super conducting material.

2. A magnet as claimed in Claim 1 in which the said constituent is yttrium barium copper oxide (YBCO).

3. A magnet as claimed in Claim 1 or 2, which comprises a plurality of discrete blocks of material.

4. A magnet as claimed in Claim 3 in which the said blocks comprise single crystals.

5. A magnet as claimed in Claim 4 in which the said blocks are arranged in the form of an array which is shaped to produce a desired optimum magnet field pattern.

6. A magnet as claimed in Claim 5 in which the individual blocks of said multi-block core are individually energisable by means of a plurality of electrical coils whereby the magnetic field of the magnet can be shaped.

7. A magnet as claimed in any one of Claims 3 to 6 in which there are a second plurality of discrete blocks of magnetic material adapted to assist in shimming the magnet.

8. A nuclear magnetic resonance imaging apparatus including a magnet as claimed in any previous claim.

9. A magnet substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

10. A nuclear magnetic resonance imaging apparatus having the magnet as claimed in Claim 9.