GB2284058A - Curved yoke MRI magnet - Google Patents

Abstract

An MRI magnet 2 has a yoke 8 which is smoothly curved to prevent flux loss, as well as to reduce the size of the magnet whilst retaining the useful volume. Imaging apparatus including the magnet 2 may be enclosed in a mobile RF screened enclosure 30 (Fig. 4) having an RF sealing flange which mates with screened patient vehicles 50, 70 (Fig. 5). <IMAGE>

Description

APPARATUS FOR MAGNETIC RESONANCE MEASUREMENT The present invention is concerned with the field of magnetic resonance measurement, and in particular magnetic resonance imaging (MRI) systems for providing images of sections of a subject, either animate or inanimate, under examination. In one aspect it relates to magnets for use in such MRI systems and in another to the improved shielding of MRI systems.

Most magnets used in MRI systems are superconducting solenoidal magnets. However, some, as in the present invention, employ permanent magnets or electromagnets incorporating an iron or steel yoke. The latter type when used in MRI devices typically consist of two slabs of magnet material spaced apart with a gap between them and a yoke which extends around them to support them both.

Generally the structure is large enough to enable a person to be positioned longitudinally between the slabs and within the gap. Magnets used in nuclear magnetic resonance require very high flux densities over the fairly large gaps (large enough in medical applications to accommodate a limb or body) and a highly homogenous magnetic field. Thus the magnets must be produced to great accuracy.

Various types of magnets have been proposed. Patent documents EP-A-0,161,782, US-A-4,870,380 and US-A4,829,252, show examples of magnets typically used. In the first two of these the magnets are shown with thetyoke generally made up of three or four flat pieces forming either three or four sides of a rectangle. The two slabs of magnet material are then attached on opposing plates of the yoke to face each other. In the third document the slabs of magnet material are mounted on opposing rectangular plates which are spaced apart by pillars positioned at the corners of the plates. Both types of yoke suffer from the disadvantage that, in order to accommodate the greatest length required by the device, for instance the shoulders of a person, the overall size of the magnet can be rendered quite large. A possible further disadvantage in both types of yoke is that the right angles in the flux return path in the yoke allow flux to escape.

This results in a lower central field and a higher external (leakage) field; both of which are undesirable. Thus the magnet becomes less efficient and no unshielded electronic equipment sensitive to magnetic fields, eg cathode ray tubes and relays, can be used close to it.

One of the disadvantages of using MRI is that radio frequency shielding is required to ensure that no extraneous radio waves are present. These tend to generate noise and faults or interference in the image which could lead to incorrect deductions. Usually the shielding, a Faraday cage, is incorporated around the entire room in which the imaging takes place, and this can be very expensive.

Thus current MRI systems are generally large and are installed in specially adapted rooms which must be set aside for the purpose. They usually consist of several units in individual cabinets which are connected together but which are fixed in location. For instance, the controls might be provided in one cabinet, the magnet necessary for imaging in a second cabinet, and the circuitry for using the magnet and processing the data received for imaging in a third cabinet. However, because it is necessary to shield the subject of the imaging from extraneous radio waves the apparatus still has to be used in a specially shielded room.

Accordingly, it is an aim of the present invention to overcome some of the disadvantages found in the prior art.

According to a first aspect of the present invention there is provided a magnetic resonance measurement magnet, comprising two opposed and spaced apart flux generating magnetic poles and a flux return yoke making a magnetic circuit with the poles, wherein the yoke comprises curved portions forming part of the magnetic circuit.

According to another aspect of the invention there is also provided an MRI device incorporating a magnet as defined above.

According to a further aspect there is provided a non-magnetic shielded vehicle for use with a scanning or measuring device, comprising: a vehicle base; a shielding mesh mated with said vehicle base to prevent the transmission of radiowaves to within the vehicle; an aperture through which a subject for examination may extend from out of said vehicle to said scanning or measuring device; and sealing means around said aperture for co-operation with means provided on said device for providing a radio frequency seal between them.

The vehicle could be a wheelchair or trolley.

Further there is provided an MRI device used in combination with such a vehicle.

The present invention will be further described, by way of non-limitative example, with reference to the accompanying drawings, in which: Figure 1 shows a front elevation of a magnet according to one aspect of the present invention; Figure 2 shows a cross-section of the magnet of Figure 1 in the plane ZZ'; Figures 3a and 3b show magnets of alternative shapes from that of Figure 1; Figure 4 shows a system enclosure according to a second aspect of the present invention; and Figure 5a and 5b show a shielded wheelchair and a shielded trolley respectively according to embodiments of a third aspect of the present invention.

Figures 1 and 2 show a magnet according to a first aspect of the present invention. Figure 1 is an end elevation looking at the magnet in the same direction from which a subject to be imaged will be placed in the gap between pole pieces 12 of the two magnets. Figure 2 is a cross-section taken through the magnet looking in a plane perpendicular to that of Figure 1.

The magnet is intended for the imaging of extremities of the adult human body such as legs, arms and heads or for the imaging of whole babies. It may also be used for imaging extremities of large animals or whole small animals or subjects of biological or non-biological natures in physical, chemical, or industrial laboratories.

The magnet 2, as shown in Figure 1, is symmetrical about the planes XX', YY', and ZZ'. These planes intersect in the centre of the magnet and the line parallel to ZZ' through the centre is denoted as the axis of the magnet.

The main body of the magnet consists of a flux return yoke 8 and two opposing magnet poles 4 and pole pieces 12 spaced apart and with a gap 18 between them.

The poles each comprise a slab 4 of permanent magnet material, which is magnetised parallel to the axis. The material for the permanent magnets is typically Ferrite, Neodymium Boron Iron, Samarium Cobalt or Alnico. The polarity of the magnetisation is such that opposite poles face one another North to South. By this means magnetic flux flows from one magnet slab towards the other. The slabs are mounted on base plates 6 made of a steel. The term steel is intended to include any ferromagnetic material which has a high magnetic permeability. Each slab 4 may be a single magnetised piece or alternatively could be made of a set of magnetised bricks which are butted together to form the composite slab. The slabs have circular outer boundaries or approximately circular boundaries, for example, for a composite slab composed of bricks, the outer boundary could be a polygon which approximates a circle reasonably closely.

Alternatively the permanent magnet slabs may be replaced with electromagnets consisting of annular coil structures carrying electrical current surrounding a cylindrical steel core in contact with the base plates 6 and pole pieces 12. The coil may be made from a normal conductor - typically copper or aluminium, or a cooled superconducting winding.

The pole pieces comprise steel front plates 12 attached to the magnetic slabs or poles 4 on the surfaces of the poles facing the gap 18. The pole pieces 12 are cylindrically symmetric about the axis of the magnet. The sides of the pole pieces facing into the gap 18 are profiled and have annular features which determine the distribution of the magnetic flux within the air gap.

Typically the outsides of the pole faces have a substantially prominent lip feature 16 known as the Rose Shim, which is the largest annular feature. This helps in providing a uniform field in the centre of the gap.

Recesses within the lips of the pole faces optimise field homogeneity within the magnet imaging volume. The pole faces are additionally shaped and include magnetic materials to compensate for the magnetic effects of the system enclosure and the external environment.

The central region of the air gap 18 is known as the imaging volume, where the variations in the uniformity of the magnetic field (the inhomogeneity) must typically be fewer than 50 parts per million in order to achieve satisfactory imaging performance.

The flux return yoke 8 is steel and its presence, as part of the magnetic circuit, greatly enhances the flow of flux across the gap separating the two pole pieces. Thus the strength of the magnetic field in the gap is increased.

The yoke consists of the two steel base plates 6 joined at both ends by two curved portions. At both ends of the yoke, on either side of the poles, there are large extended volumes 23. In this embodiment the curved portions are semi-circular but other smoothly curving shapes are possible. The yoke may be designed to fulfil the dual roles of providing both a magnetic flux return and a rigid structure which is mechanically self-supporting.

The arced return path geometry of the magnet minimises overall magnet depth whilst providing effective stress distribution within the return yoke so that the magnet may fit in a supporting enclosure. The magnet return yoke is designed so that the magnetic flux does not saturate the material.

Variations of the shape of yoke are illustrated in Figures 3a and 3b. In these designs the yoke has a continuous curved cylindrical form, in one case circular and in the other elliptical. The base plate 6 is then shaped in such a way to be in contact with the rest of the yoke 8 over an area at least as large as the area of the magnetic slabs 4. Alternatively the yoke 8 is cast or machined such that the section joining the curved outside of the yoke to the flat of the baseplate is incorporated into the yoke construction.

One advantage of using a yoke of any of these shapes is that there is significantly better access to the magnet in the XX' direction and the access dimension in this direction can significantly exceed that of an MRI system based on a typical solenoidal superconducting magnet. Thus the magnet can accommodate objects which need to be imaged and which are extended in this direction. However because the yoke is curved away from the top and base the magnet does not suffer from the size problem found in the prior art. The large extended volumes 23, as well as allowing improved manoeuvrability, can also be used to accommodate any ancillary equipment which may be needed or useful to accompany the subject while scanning is in progress. They also afford excellent visual and physical access to the subject such that, for example, a doctor could carry out procedures on a subject whilst the scan is in progress (Interventional MRI).

Furthermore the use of curved yoke pieces in the yokes and the lack of right angles in the magnetic circuit reduces flux leakage.

The whole structure is arranged mechanically such that the two pole pieces are concentric and parallel with one another. The preferred embodiment is a rigid mechanical structure for the magnet with sufficiently stringent tolerances that this alignment may be achieved without adjustment of the poles. This has the advantage of stability and less potential for flux leakage due to small gaps between neighbouring elements of the magnetic circuit.

Alternatively the poles could be made adjustable by means of jacking screws or other means such that relative lateral displacement of the poles is possible in the X and Y directions, and the relative tilt of the planes of the poles could also be adjusted.

The adjustment mechanism would allow relative translation and rotation of the two pole faces in three dimensions so that field homogeneity may be optimised and any effects of the external environment on field homogeneity minimised. Adjustments may be carried out with the magnet assembly in situ within an enclosure.

If the permanent magnet embodiment is used thermal insulation material may be used on the magnets with the aim of keeping the temperature of the permanent magnet material as stable as possible because the magnetisation of a permanent magnetic material depends on temperature, and any temperature instability is reflected in an undesirable drift in the magnetic field in the imaging volume. In addition, heaters and/or coolers may be installed inside the magnet structure which are designed to activate automatically as part of a servo mechanism when sensors indicate that the temperature is departing from a stable target value.

An MRI system requires a set of three independent coils, called gradient coils, which can modulate the strength of the main magnetic field with superimposed linear gradient fields. An MRI system also needs a transmit coil which can generate radiofrequency (RF) magnetic fields in the subject being imaged, and a receive coil which can pick up the RF nuclear magnetic resonance signals which are emitted from the subject.

In an MRI system employing a magnet according to the present invention, the gradient coils 20 are most conveniently designed such that each of the coils lie on a plane which can be stacked together and mounted near or within the profiled faces of the pole pieces. This removes the coils from the main imaging volume and so improves access. Similarly the radio frequency (RF) transmitting and receiving coils 22 may be designed to lie on a plane and be mounted parallel to the faces of the pole pieces in the same fashion as the gradient coils 20.

The overall magnet structure may be mounted with its axis (and hence the main magnetic field direction vertical as shown in Figure 1, or horizontal, or at some intermediate oblique angle. The direction may be selected as being most convenient for any given application - for example imaging of the knee or head.

Figure 4 shows a single system enclosure which includes the magnet of the first aspect of the present invention. In this, because the flux leakage is reduced in the magnet and the magnet itself is of a reduced size, the whole MRI imaging system, save for the control panel and monitor, is provided in the single movable cabinet which is shown in Figure 4.

The cabinet 30 has at its front an imaging aperture 32 through which the subject to be imaged is placed. This aperture may or may not be in the same shape as the magnet yoke. The useful space in the described magnet is a dumbbell shape with the extended volumes in the flared ends allowing better access, such that a person's head might be able to lean in to examine the subject closely. The magnet is situated behind this aperture and any subject entering the aperture will then be within the magnetic gap and thus the imaging volume. The cabinet 30 may be a standard 19" rack enclosure 34 or any other industry standard and is radio frequency sealed to ensure correct results. Around the front of the aperture 32 there is a radio frequency sealing flange 36 which, as discussed later, is intended to mate with flanges on shielded trolleys or wheelchairs.

Another advantage of the elongated access to the magnet, as shown in Figure 4, is that a person having his leg scanned can stand into the magnet, putting weight onto that leg. There are many instances when imaging under such loadbearing conditions are useful or necessary for investigating the function of, for example, the knee joint.

This loadbearing mode is not at all easily attained with other magnet geometries.

The self contained magnetic resonance imaging system and enclosure, being both compact and mobile, could readily be used in industrial, veterinarial and medical fields. In one embodiment it takes its power solely from storage batteries and this allows complete portability and the ability to operate almost anywhere. Alternatively it could take electrical power from a conventional single-phase electrical supply.

Within the magnet there are radio frequency gradient and shim coil assemblies. Internal electronics are situated in enclosures above the magnet and include a computer system which computes a sequence of analogue control voltages to provide the radio frequency coils and gradient coils with a timed sequence of waveforms which allow encoding of spatial information within the generated magnetic resonance signal. Other systems allow for the reception of echo signals from a set of protons within the magnetic gap and process these to provide the magnetic resonance image. By placing the electronics here a standard layout may be provided and the lengths of connecting cables may be minimised.

The magnet is surrounded by system insulation 38 and a thermal control system as mentioned earlier to ensure that a correct temperature is maintained. Associated with this are cooling fans 40 at the top of the cabinet which also ensure that the rest of the internal electronics do not get too warm. Where the ducts to the fans encounter the outside of the cabinet there is a radio frequency wave guide seal and possibly further temperature monitoring.

Similar seals are located at air inlets in the base of the cabinet. The imaging system is controlled by a console (not shown) which includes a monitor for displaying the image, via leads connected to the port 42 on the base of the cabinet. The whole device is mobile, mounted on castors 44.

The system housing is based on industry standard RF shielded rack technology with additional mechanical load distribution and strengthening structures. The enclosure provides radio frequency and magnetic shielding of the magnetic resonance system from the external environment.

The internal temperature, humidity and gases are accurately monitored and controlled to the user's specifications by appropriate sensors. Wave guide cut-off seals and filters are provided at all inlet and outlets to prevent ingress and egress of radiofrequency signals. If desired the enclosure can be locked.

Because the device is not intended to be able to image an entire body the magnet can be of a limited size and further because of the construction of the magnet described earlier the housing may contain the required circuitry for MRI and thus the entire MRI system may be contained within a single readily movable cabinet. This is an improvement over the large immovable or unwieldy systems of the past.

Figures 5a and 5b show two vehicles for use with the system enclosure described above. However, the vehicles may be used with any MRI system which is adapted correctly.

Indeed its use is not necessarily limited to MRI but may be used with any system where the shielding of a patient or subject is necessary.

Figure 5a shows a shielded wheelchair 50 in which a patient 52 is sitting in position for imaging of the leg 54 in an MRI system 56. The shielded wheelchair comprises a non-magnetic wheelchair 58 on which is mounted conductive radio frequency shielding mesh 60, such that together they form a continuous Faraday cage around the patient. The mesh may be conveniently sandwiched in a clear thermoplastic material providing good visual transparency while maintaining RF integrity. Ventilation slots 62 are provided to allow a patient to breathe but these must be designed not to allow the intrusion of radio waves. This may be achieved using mesh without its plastic covering, or by the use of extended metallic tubes acting as waveguide cut-off filters. The largest aperture to the cage is provided at the front (or wherever else it may be required) and is surrounded by a mating flange 64. This mating flange is intended to mate with a similar flange 36 which surrounds the imaging aperture 32 which leads to the magnet of an MRI device 56. The patient's leg or other limb or part to be imaged protrudes through the aperture in the cage and is manoeuvred into the imaging aperture 32 such that the two flanges 64, 36 mate and provide a radio frequency seal. As the imaging magnet will also be frequency shielded the whole "system11 will be a radio frequency shielded environment.

Figure Sb shows a shielded trolley 70 for similar use to the wheelchair of Figure 5a. In this the patient 52 lies down on a standard non-magnetic trolley 72 on which is mounted a thermoplastic sandwiched radio frequency shielding mesh 74 to form a Faraday cage around the patient. Ventilation slots 62, as before, are provided in the thermoplastic and a radio frequency seal and mating flange 76 is again provided around an aperture which allows a particular part of a body to protrude beyond the cage for imaging. The flange 76 mates with a flange 36 in the MRI device 56, around the imaging aperture 32 to provide a radio frequency seal. In Figure 5b a head is shown protruding through the aperture on the trolley 70, although a trolley could be used for imaging arms or legs or any other part requiring imaging.

In the case of the wheelchair the imaging aperture is shown having its widest dimension vertical and in-the case of the trolley it is more likely to be horizontal. The orientation will depend on the shielded vehicle used and what is to be imaged. Devices using magnets produced according to the first aspect of the invention are ideally suited for use with such vehicles since they can be large enough to allow the entrance of a limb even at an angle.

In the case of either type or other types of RF screened or shielded vehicle or when using any type of RF screened cage, it may be necessary to provide extra space within the screened area to allow another screened person access to the subject being scanned.

The advantage of using such shielded vehicles is that they are totally mobile and allow the MRI device to be used anywhere rather than just in a specifically prepared room.

Thus they save a great deal on the cost of providing such a device.

Claims (32)

1. A magnetic resonance measurement magnet, comprising two opposed and spaced apart flux generating magnetic poles and a flux return yoke making a magnetic circuit with the poles, wherein the yoke comprises curved portions forming part of the magnetic circuit.

2. A magnet according to claim 1 wherein said yoke is continuously curved along the path of the magnetic circuit.

3. A magnet according to claim 2 wherein said yoke is elliptical.

4. A magnet according to claim 3 wherein said yoke is circular.

5. A magnet according to claim 1 wherein said yoke comprises two opposed flat plates joined together by curved plates at both ends, and wherein said magnet slabs are mounted on said flat plates.

6. A magnet according to claim 5, wherein said curved plates are semi-circular.

7. A magnet according to any one of the preceding claims wherein the yoke is of constant cross section.

8. A magnet according to any one of the preceding claims wherein said poles are permanent magnets.

9. A magnet according to any one of claims 1 to 7 wherein said poles are electromagnets.

10. A magnet according to any one of the preceding claims, wherein said poles are fitted with opposing pole pieces.

11. A magnet according to claim 10, wherein said pole pieces are identical and aligned.

12. A magnet according to claim 10 or 11, wherein said pole pieces are substantially cylindrical.

13. A magnet according to any one of claims 10 to 12 further comprising radio frequency and gradient coils mounted parallel to and on or near said pole pieces.

14. A magnet according to any one of claims 10 to 13, further comprising manoeuvring means for manoeuvring said pole pieces with respect to each other in respect of lateral shift and tilt of planes.

15. A magnet according to any one of the preceding claims further comprising thermal insulation to reduce temperature changes and magnetic field drift caused thereby.

16. A magnet according to any one of the preceding claims further comprising an active servo control mechanism to stabilise the temperature of the magnet.

17. A magnet according to any one of the preceding claims further comprising means for monitoring and/or controlling the temperature, humidity, and/or composition of gases of the environment of the magnet.

18. A magnet substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.

19. A magnetic resonance imaging device comprising a magnet according to any one of the preceding claims.

20. A device according to claim 19, wherein the device is mobile.

21. A device according to claim 19 or 20, wherein said magnet is mounted such that its axis may be vertical, horizontal or at an intermediate angle to suit varying imaging applications.

22. A device according to claim 19, 20 or 21 adapted to receive a storage battery power supply.

23. A magnetic resonance imaging device substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.

24. A non-magnetic shielded vehicle for use with a scanning or measuring device, comprising: a vehicle base; an RF shield mounted on said vehicle base to prevent the transmission of radiowaves to within the vehicle; an aperture through which a subject for examination may extend from out of said vehicle to said scanning or measuring device; and sealing means around said aperture for co-operation with means provided on said device for providing a radio frequency seal between them.

25. A shielded vehicle according to claim 24, wherein the vehicle base is a wheelchair.

26. A shielded vehicle according to claim24, wherein the vehicle base is a trolley.

27. A shielded vehicle according to any one of claims 24 to 26 for use with a magnetic resonance imaging device.

28. A shielded vehicle according to any one of claims 24 to 27 further comprising ventilation holes within said RF shield.

29. A shielded vehicle substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.

30. A magnetic resonance imaging device used in combination with a shielded vehicle according to any one of claims 24 to 29.

31. A device according to claim 30 further comprising a magnet according to any one of claims 1 to 18.

32. A magnetic resonance imaging device according to any one of claims 19 to 23 used in combination with a shielded vehicle according to any one of claims 24 to 29.