WO2001016616A2

WO2001016616A2 - Active acoustic control for gradient coil structures in mri

Info

Publication number: WO2001016616A2
Application number: PCT/GB2000/003261
Authority: WO
Inventors: Peter Mansfield
Original assignee: Peter Mansfield
Priority date: 1999-08-27
Filing date: 2000-08-25
Publication date: 2001-03-08
Also published as: AU6712200A; GB9920185D0; WO2001016616A3

Abstract

An active acoustic control system for MRI coils includes means for reducing the noise created at the surfaces of support structures within which the coils are mounted.

Description

ACTIVE ACOUSTIC CONTROL FOR GRADIENT COIL STRUCTURES IN MRI

The present invention relates to active acoustic control to reduce emitted noise and more particularly to active acoustic noise control for gradient coils in MRI apparatus.

1. Introduction Active acoustic control is a particular approach proposed by

Mansfield (P. Mansfield, PCT WO 96/31785 Priority 1 Apr 1995) to ameliorate the problem of acoustic noise emission from gradient coil structures used in MRI. Active acoustic control noise levels in typical high speed imaging applications such as echo-planar imaging (EPI) can exceed 130 dB which is dangerous to both patients and to radiographers. In the prior art (P. Mansfield, PCT WO 96/31785 Priority 1 Apr 1995), solutions have been proposed which reduce the typical noise output level by about 20 dB. In this prior art magnetic gradient coils have an additional winding which when activated effectively changes the vibration mode of the coil in such a manner as to eliminate the fundamental vibration mode in the structure. However, higher modes are induced which generate noise output but typically at a lower level.

The prior art describes among other geometries a gradient coil structure comprising flat plates. In our previous attempts at reducing acoustic output only the sound emitted from the flat faces of the plate normal to a high static magnetic field is considered. However, we have now discovered that in development of this type of coil structure a substantial amount of acoustic noise is emitted from the edges of the plate. The present invention is directed to new embodiments which will be introduced and described which reduce the sound output from the plate edge surfaces. Reduction of the residual noise output arising from the higher harmonics of the plate structure itself is also addressed.

The present invention provides a coil structure for active control of acoustic noise reduction in MRI apparatus, the coil structure comprising a first electrical coil for generating a primary gradient magnetic field, said electrical coil comprising a first conductor mounted within a support structure, said support structure having at least one surface and including a further surface arrangement mounted adjacent to said at least one surface of said structure, said further arrangement being attached to said surface by a suitable first soft material of low elastic constant, said further surface arrangement being arranged to maintain the further surface arrangement substantially static in space, including an at least second electrical coil for controlling the acoustic output said at least second electrical coil being driven by a second electrical current to provide additional acoustic noise reduction, but with no substantial effect on the magnitude and character of said primary gradient magnetic field and in which the support structure comprises at least two weakly coupled mechanical structures separated by a gap which is filled with a suitable second soft anisotropic orthotropic material or an air gap, the second of said weakly coupled mechanical structures providing support means for the electrical return path of said first and second electrical conductors.

In a specific embodiment the further surface arrangement comprises a surface conductor arrangement being electrically driven to produce a suitable surface current distribution to maintain the further surface conductor arrangement substantially static in space.

In a further embodiment the further surface arrangement comprises a rigid air tight encasing static cylinder which completely surrounds the weakly coupled mechanical structures when said structures are coupled by a suitable second soft anisotropic or orthotropic material.

The present invention also provides a method of active control of acoustic noise reduction in MRI, said method comprising the steps of adding an outer structure to a noise generating surface of a magnetic coil structure, said structure being attached to a soft material arranged in a position intermediate between the magnetic coil structure and said outer structure, the arrangement being such as to maintain the outer structure substantially static in space to thereby reduce the noise emitted from said noise generating surface.

The method may also comprise the step of sandwiching between the noise generating surface and the outer structure a soft material having anisotropic or orthotropic properties and in a preferred embodiment may comprise the step of mounting conductors within said magnetic coil structure in anti- vibration mounts.

The present invention also provides apparatus for active control of acoustic noise reduction in MRI apparatus for use with the above coil structure comprising means for supplying a plurality of currents of suitable amplitude and phase to said electrical coil and to said further surface conductor arrangement, said apparatus preferably providing means for detecting the spatial distribution of the acoustic noise amplitude and phase output of the coil structure and means for displaying said acoustic amplitude and phase noise output.

The coil structure may comprise a magnetically screened fingerprint coil structure wherein the primary and screening current conductor tracks are split into sectors around a polar axis and independent sectors along the z-axis, each sector being mechanically supported on its own mechanically independent support segment.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which :-

Fig. 1 is (a) a sketch of prior art arrangement showing an outer loop of conductor carrying current I_t and an inner re-entrant loop of conductor carrying current ι₂e^ιφ ' . Both conductors are firmly embedded in a rigid plastic support material, (b) Sketch of plate pair 1(a) looking along the y-axis. The Lorentz forces, F,F', on the wires carrying currents Ij and I₂ deform the plates as indicated by the dotted lines. Deformations of the plate faces produce sound S which is emitted along the z-axis. Elongations of the plates along the x-axis produce an approximately flat piston-like displacement of the edges resulting in sound S' emitted from the plate edges;

Fig. 2 shows a flat plate assembly comprising firmly embedded conductors carrying currents I₂ and I₃ but with the addition of current carrying surfaces applied to the edges of the flat plate structure and carrying currents I_{l 5} I₄. The additional conductors are weakly coupled to the plate edges using a suitable anti-vibration mounting as indicated;

Fig. 3 shows a new embodiment of the flat plate structure applicable to rectangular or arcuate sections in which the primary coil windings carrying currents I₂, I₂' with phases <p₂ and φ₂ ' are lightly supported within the plates with a soft material of low elastic constant, k₂, k₂' , producing a weak coupling to the main support structure. Note that the edge compensation plates can have rim guards to prevent sound emissions along the z-axis. The mode control windings carry currents I₃, I₃' with phases <p₃, φ₃ ' and are lightly supported by anti-vibration mounts with couplings k₃, k₃';

Fig. 4 shows a model of a wire carrying current I, the coupling material (cross-hatched) and the supporting plate structure (hatched). F is the stress per unit length applied to the conductor of mass m per unit length and k is the coupling constant. F' is the reduced stress per unit length experienced by the supporting plate. The magnetic field is B;

Fig. 5 is a sketch of a particular embodiment of the general concept outlined in Fig. 3. In this arrangement the supporting plate structure comprises two sheets of material in which the conductors carrying currents I₂ and I₃ of Fig. 3 are sandwiched together between anti-vibration mounts as indicated. In addition the plates are prevented from moving in a so-called breathing mode along the z-axis by use of fixed spacers. The edge compensation plates carry additional conductor surfaces with currents Ij and I₄ flowing and the surfaces are lightly mounted using an anti- vibration material. Rim guards may also be fitted. This arrangement reduces the acoustic radiation arising from the piston effect on the plate edges;

Fig. 6 shows sketches of mechanisms for immobilising the lightly mounted conductors within a solid support structure, (a) Removable pegs or bolts. These couple the conductor to the plate support structure as indicated, (b) Lateral motion is restricted by a cam pressing on one edge of the conductor. The other edge is forced into a docking recess as shown by an activating lever, (c) Sketch showing an arrangement using an electro-viscous compound forming a brake to immobilise the conductor relative to the plate support structure;

Fig. 7 (a) shows a cut-away sketch illustrating the nested coaxial layers of material and conductor surfaces comprising a cylindrical distributed gradient coil. The outer surface of the coil is marked A. An air gap separating the outer coil from an inner coil is marked O and the inner surface of the inner coil is marked A' . As indicated such a coil system would include full acoustic compensation, (b) Shows a detailed annotated section showing the various surfaces and materials comprising the section A, O, A' as in (a) above. In all cases the hatched and cross- hatched regions consist of soft mounting material, the clear regions consist of a solid polymer support matrix and the dotted region is the air gap.

Also indicated are the conductors carrying currents I_x I₄ and I₄' I ;

Fig. 8 (a) shows typical conductor tracks for a distributed fingerprint primary coil and (b) associated magnetic screen. All tracks are shown in unrolled form, that is to say the axes are z,a# andz,b# where a and b are the radii of the primary and screen coils respectively. Quadrants Q_a and Q_b show the conductor tracks wired in series as an example, (c) Is a sketch showing how the conductor tracks form into segments and is illustrated for the two lower quadrants of 8(a) above; for example in 8(c) they are mechanically split along the z-axis at points A, B, C into four sections so as to be mechamcally independent of each other but coupled electrically through flexible couplings as indicated. In addition the coil is split around the a<9 - axis into quadrants. Each coil is divided into two sub-loops along the a<9 - axis as indicated. This means that fewer flexible connections are required in a practical embodiment, (d) Shows a view of the nested coil arrangement looking along the z-axis and indicates the structure split into four mechanically isolated gradient plate quadrants. In this arrangement it is emphasised that the quadrants have their own flexible electrical connections, including the z-gradient coil, the tracks of which are shown as dotted vertical lines in Fig. 8(c) above; Fig. 9 (a) is a sketch showing part of a simplified system compared with Fig. 7. In this sketch the x,y and z gradient plate quadrants are fixed to each other but lightly supported on solid rings through a soft coupling material. The magnetic screening plates are supported on one ring structure, H, and the primary gradient plates are supported on a second inner ring structure, H' . The plates for both the primary and the screen coils have additional conductor tracks made from a thinner material mounted directly onto the gradient plates via soft mounting in order to compensate for the piston effect. Air gaps, O, between the magnetic screen support structure and the primary gradient support structure are filled with absorbent material, (b) Shows a detailed annotation of the various layers through a reduced or simplified distributed gradient coil system. The annotation letters are the same as used in Fig. 7(b) but with missing layers corresponding to strand (i). We are thus left with layers A, B, F, G, H, O, H', G', F', B' and A' . Also indicated are the currents I_ι(xyz), I₂(xyz), I₂'(xyz) and ^'(xyz) corresponding in the case of I₂' to the three primary gradient plates, and in the case of Ij to the three piston effect compensation plates for the magnetic screen gradient cluster, (c) Detail of the ring support structure for both the magnetic screening plates and the primary gradient coil plates. The piston effect compensation conductors are indicated by A. B shows the soft coupling to the magnetic screening plates F. The supporting spur, G, is rigidly attached to F and weakly coupled via a soft anti-vibration material to the pair of ring supports, H,H, as shown. A similar arrangement in reverse is shown for the primary gradient plate set F' which is coupled to the piston effect compensation plates A' through a soft coupling material B';

Fig. 10 (a) is a block schematic of gradient driver circuitry for an acoustically controlled gradient coil. Also shown is a microphone array for signal reception and measurement. Further details are described in the text, (b) Visual display arrangement for microphone array using bar or analogue indicators to display the acoustic amplitudes Y - V_n. Also shown are indicators for signal phase, ψ^ - ψ_^ . (c) An arrangement of delays Dl - D4 and pulse generators PG1 - PG4 suitable for driving the gradient drivers of (a) above. The circuit is inserted between P_x - P₄ and P - P₄' . Switches $>_x - S₄ isolate the circuit from the amplitude and phase control unit of Fig. 10(a). The input trigger T is derived from the coherence trigger and pulse envelope generator output T of (a) above;

Fig. 11 shows an equivalent diagram for the arrangement shown in

Fig. 3 in which we have specifically fixed the outer edges of the plate in a manner equivalent to Fig. 3 when driven correctly with l_!,I₄,I₄' and I,' . The equivalent arrangement for the inner edges is also shown in which the mid-plane dividing the plate structure passes through the plane x = 0 and is static; and

Fig. 12 is a sketch of part of a cylindrical magnetically screened gradient coil arrangement in which the conductor tracks are mounted on flat segments in the form of a regular octagon as shown. Panel P represents one segment of the primary coil arrangement together with its piston effect compensation. Panel P' represents a corresponding panel of the magnetically screened coil together with its piston effect compensation plate. For both the primary and magnetically screened coils orthotropic materials are used to mount the piston effect compensation plate to the primary or the screened coil plate. General support of the plate structure is not indicated but would follow arrangements similar to those described in Fig. 9.

2. Active control of sound output from edges

In the prior art of Fig. 1(a) wires are embedded in a plastic material and driven with currents as indicated. The plane of the flat plate 10 is normal to the magnetic field B which points along the z-axis. In this configuration Lorentz forces, F,F' , on the wires produce transverse stresses 10 in the plate which in turn cause strains in the structure resulting in emission of sound along the z-axis. Elongations of the plates along the x-axis, Fig. 1(b), and along the y-axis (not shown) produce sound SN resulting from the edge displacements. In the structure shown in Fig. 1(b) the plate 10 comprises two sections 12,14. Current I_x flows in an outer loop, and current ι₂e^ιφ flows in the inner loop. When φ is close to zero each half of the plate structure in Fig. 1(b) undergoes translational motion in opposite directions along the x-axis and the y-axis so that the plate edges behave like flat pistons. The piston effect is responsible for a surprisingly substantial amount of sound emission. In our first embodiment, therefore, we propose to add third and fourth conductor surfaces 20,22 to the edges of each plate as shown in Fig. 2, the third and fourth conductors being mounted on soft material 24 so that they may move independently of the main plate assembly. If the main plate assembly 12,14 has a mass M and the third or fourth conductor strip has a mass m, then currents Ij or I₄ equal in magnitude to (m/M)I₂ or (m/M)I₃ respectively are typically required in order to oscillate the third or fourth conductors in anti-phase to the moving edge of the plate. The gradient strength efficacy of such an arrangement is only marginally affected if m/M « 1.

In order to minimise the piston effect it is necessary to adjust the currents Ij and I₄ so that the surfaces on which they flow remain effectively static in space and, therefore, do not emit sound. However, it may be desirable to have some residual motion in order to cancel any wave amplitude which may pass through the edge compensation plates. The sound output from a flat plate structure which uses first and fourth currents Ij, I₄ as described above is significantly altered with respect to the overall sound reduction when I₂ and I₃ above are switched in phase or anti-phase. The overall noise reduction is typically 20 dB. This is a worthwhile reduction in its own right but one would like to see greater reductions. The difficulty with the arrangement shown in Fig. 2 is that higher modes of vibration exist naturally in the plate and, therefore, the plate will respond emitting residual sound. Without changing the characteristics of the plate itself it is hard to visualise how we can improve beyond a = 20 dB reduction. In the next section we discuss changes to the plate structure in order to make further improvements.

3. Modifications to plate and wire support

In a new embodiment we consider the modified plate structure as shown in Fig. 3 in which all conductors are driven with currents, I - I₄, IC- I₄' with phases φ - φ₄ and φ_λ ' - φ_Λ \ We now make further modifications in which the conductors carrying currents I₂,I₂' and I₃,I₃' are no longer rigidly embedded in the plate, but are supported on a light material 30 with an elastic constant k_n. For rectangular plates, k_n = k_n' = k, I₂' = I₂exp(i^₂ + ^π) ^and ⁼ I₃exp(i^₃ + π). The wires when activated are, therefore, allowed to move relative to the rigid plate structure and this effectively reduces the stress or force coupling to the plate from an original value when the wires are rigidly fixed, F, to a modified stress or force of F' . We note that in addition to edge compensation plates, we also include rim guards 32 to reduce residual sound emission along the z-axis. The problem of reduced coupling can be analysed in the simple case of a mass 40 coupled by a springy material 42 to the plate assembly. This is sketched in Fig. 4. In this figure F is the stress or force per unit length due to Lorentz forces, m is the mass per unit length of the conductor and k is the coupling constant. We thus have a driven oscillator system and we may as an approximation ignore damping so that the equation of motion for this simplified system is mD²x + kx = Fsinω t, [1]

where D = d/dt is the differential operator. The steady state solution of this forced vibrational equation gives a displacement x given by

where ω₀ ² =k m . The transmitted force or reduced force F' applied to the structure is given by

F' = kx = RFsinfi> t [3]

where the stress reduction factor R is given by

2 2 τ _ ^ωo _ ^fo m

^R'1XX-7X ^[4]

If f₀ = 100 Hz and f = 3 kHz, the reduction factor R = 60 dB. Therefore, sizeable additional reductions can be obtained in acoustic output by allowmg the wires to move within the plate structure. By evaluating Eq. [2] we can show that for a magnetic field strength B = 3.0 T and a current of 20 A the typical wire displacement is less than 1 μm. For increased current it is possible in the above arrangement to keep the displacement to around 1 μm by increasing the mass of the conductor. This can be done by increasing the cross-sectional area of the conductor wire but additional heavy materials such as lead could also be attached to the conductor wire in order to increase its effective mass.

We note that if damping is included in Eq.[l], Eq.[2] becomes

where the phase angle ε is given by ηω tan £ = - ,.2 [6] ω₀ - ω

and where η is the viscous damping coefficient 4. Modifications to Fig. 4

The particular modifications described in Fig. 3 can be enacted but in Fig. 5 we show a further modification which incorporates the basic ideas discussed but which is easier to manufacture. It comprises two separate plates 50,52 which sandwich the conductors carrying currents I₂ and I₃, together with acoustic isolators or anti-vibration mounts consisting of thin strips of a suitable soft material plus a sound absorber. In order to stop the plates from moving in a breathing mode along the z-axis, fixed spacers 56 are placed between the plates and attached. As in Fig. 3(a) the piston effect at the plate edges is compensated by first and fourth conductor plates 58 carrying currents I_x and I₄ together with rim guards as described previously.

5. Other geometries In our prior art we have described a number of geometries in which active acoustic control can be beneficial. The types of structure described there include magnetically screened gradient coils comprising arcuate segments forming a cylindrical array as well as true distributed or fingerprint magnetically screened coils comprising a primary coil and a magnetic screening coil. In this section we extend the latest approaches in active acoustic control to include the other geometries as described above.

6. Arcuate sections

Compact transverse gradient coil systems may be made by deforming the straight plate sections as described in Fig. 1 into arcuate sections with cross-sections similar to Figs. 2 and 3. The distinguishing features of this arrangement over our prior art are that the wires are not firmly embedded in the supporting plate structure but softly mounted within the supporting structure and that additional current surfaces are applied at the inner, outer and end edges in order to ameliorate the acoustic piston effect. One practical embodiment of the arcuate plate arrangement is a sandwiched plate structure as described in Figs. 3 and 5 for the rectangular plate sections. Thus, apart from deforming the straight wires of the rectangular plate structure into arcuate sections, all features as described in Sections 2-4 are applicable.

7. Matching

Since the gradient coil conductors are weakly coupled to the supporting structure the problem of acoustic matching in arcuate structures within the solid supporting structure is easier, especially when the anti- vibrational coupling material is different for the inner, middle and outer conductors. In general for an arcuate cross-section sketched in Fig. 3 we have eight currents, 1^ I₄, I₄' and 1/ being the edge compensation currents, I₂, I₂' being the gradient producing currents and I₃ and I₃' being the acoustic mode control currents.

In the most general arrangement, all currents, I_x - I₄ and their relative phases, φ_x - φ ', are different. However, if I₃ and I₃' are physically close they can have equal amplitude and opposite phase. Because of the arcuate curvature I_{2 ≠} -I₂' in general. However, the reduced Lorentz force transmitted to the support structure can be varied for I₂, I₃ and I₃' and I₂' by adjusting the coupling constant k_n for the particular conductor. Furthermore, variation of k_n means that apart from phase shift all currents can have the same amplitude, I. The coupling k_n can be varied by changing the material, but it may be more practical to vary the thickness and width of the anti-vibration pads.

8. Extension of acoustic control concept

In the original definition of acoustic control described in our prior art an additional conductor is included in the gradient coil design in order to allow the generation of additional acoustic waves which in effect change the vibrational mode of the plate and thereby reduce the acoustic output.

In the present work we have introduced additional conductors carrying currents I₃ and I₄ applied to the edges of the plates to reduce the acoustic output by the piston effect. These currents can, of course, be switched off remotely. We now extend the concept of acoustic control to effect the coupling of the main gradient conductors to the supporting frame. That is to say we introduce the concept of effectively switching the coupling constant from its normally very low value to a much higher value, thereby coupling the vibrations produced by the Lorentz forces of currents I_j and I₂ more strongly to the supporting structure. The ability to switch the coupling constant will be a valuable attribute both for the demonstration of efficacy of the coil system, and also in the setting up and alignment procedures necessary for optimum behaviour of the coil assembly.

The simplest mechanism for changing the coupling constant is simply to have pegs 60 which are placed in holes passing through the support structure and the conductor wires sandwiched between. The pegs when in position will strongly couple the vibrational movements of the sandwiched conductor to the main plate support structure 50. This is illustrated in Fig. 6(a). In Fig. 6(b) an alternative method of immobilising the movement of the sandwiched conductor is shown. Here lateral motion is restricted by cam 62 action on one edge of the conductor, the other edge being forced into a docking recess 64. Rotation of the cam 62 will lock the conductor and this may be performed remotely and electrically by utilising a dc current carrying conductor 66 as illustrated. This exploits the fact that this wire is residing in a strong magnetic field which will produce a sizeable Lorentz force depending on the dc current strength and direction. Reversal of the current will, of course, have the opposite effect and will unlock or release the clamped wire. It is, of course, assumed that the lever activation current is switched off between the clamping and unclamping actions so that the desired magnetic field gradient is not compromised. A third method of restricting the conductor motion thereby effectively increasing the coupling constant is to use an electro-viscous compound 68. Such compounds exist and operate by a variety of mechanisms in such a way as to effectively lock solid under the effect of a locally applied electrical field produced by a voltage +V. This is sketched in Fig. 6(c).

In everything described so far we have not included magnetic screening of the gradient coil structure whether it be rectangular or arcuate. For small gradient coils such as purpose-built head coils for brain scanning it may not be so important to have magnetic screening in addition to active acoustic control. But in many applications especially in whole-body imaging where much more of the magnetic volume is used, and as a consequence the gradient coil has a larger diameter, the requirement for active magnetic screening increases.

9. Acoustic control in magnetically screened coils Our strategy for reducing acoustic noise output in gradient coil systems comprises four main strands: (i) Our prior art method in which acoustic vibrational mode changes are introduced within the solid structure; (ii) Compensation of the edge or piston effect which is not addressed in (i) above, (iii) Allowing some controlled motion of the primary gradient wires within the solid support structure by light mounting through anti-vibration mounts and (iv) A fourth strand in our strategy is to introduce strategically placed absorbent materials in and around the coil structure itself so that residual noise levels are not reflected off hard surfaces. All four strands form part of our general strategy in ameliorating the acoustic noise problem. All four strands may be applied to magnetically screened gradient coils as a general concept but it is emphasised that all four strands of our general approach may not, in general, be required. In other words if two or three of these strands produce sufficient acoustic attenuation then there may not be a necessity to invoke the fourth strand.

In magnetically screened coils of the distributed kind, so-called fingerprint coils, there will in general be three nested primary coils producing gradients G_x, G_y and G₂, and three nested magnetic screening coils producing screening of G_x, G_y and G_z. The G_z gradients are usually circular coaxial coils of the Maxwell type but with many distributed turns for each half of the coil. The transverse gradients G_x and G_y are similar in design but with a 90° axis rotation. Therefore, in addressing the general problem of acoustic control in such coils it will suffice to consider just one gradient, for example the G_x gradient coil and its magnetic screen.

Strand (i)

In our prior art the primary gradient and its magnetic screen are rigidly mounted on or cast within two coaxial cylinders each of which can oscillate radially and independently of each other. This is ensured by having a small air gap between the inner cylinder which carries the primary gradient coil and the outer cylinder which carries on its outer surface the magnetic screen. Two additional coils are rigidly attached to the inner and outer cylinders on the outer and inner surfaces of the inner and outer cylinder respectively. This pair of coils being in close proximity will produce no net effective magnetic field at the centre of the coil assembly but will effectively quench the fundamental mode of radial oscillation of the pair of cylinders, thereby reducing acoustic output from the end faces of these nested cylinders.

Strand (ii) The above prior art arrangement will in general reduce the acoustic noise output but does not address the radiation from the outer cylindrical surface or indeed the inner cylindrical surface which is in closest proximity to the patient. Noise from these surfaces arise from the pison effect described previously and to ameliorate this noise source we add additional coils immediately adjacent to the main gradient producing coils, but of a lighter structure and weakly coupled to the gradient coils using anti-vibration mounts. Just as in the case described previously in connection with rectangular plates these additional compensating coils will vibrate independently of the main cylindrical vibrations referred to above and if driven by suitable currents could be made to effectively remain stationary in space, thereby reducing any radial components of acoustic noise. If these additonal coils are of low mass m compared with the mass M of the mount cylinders for the primary gradient and its magnetic screen then the compensating currents will in general be much lower and typically in the ratio m/M of the primary current or of the screened current. Furthermore, if the additional coils have the same current track patterns as the primary and magnetic screen coils the effect on both the quality of the primary gradient and the efficacy of magnetic screening will be zero. Only the magnitude will be changed since it is assumed that the weak coupling of these additional coils is undamped and, therefore, no phase effects or negligible phase effects will be introduced between the compensating currents and the primary and screen currents.

Strand (iii) In strict analogy with the rectangular plate structures described previously we envisage as one embodiment that the primary and magnetic screening gradient coils would be mounted within the inner and outer cylinders in suitably machined compartments. In such an arrangement only the outer surfaces of the cylinders would move and under strand (ii) above the piston effect compensation coils would be applied directly to the cylinders themselves. In this arrangement as for the rectangular plates the primary and magnetic screening coils, together with the mode quenching coils, would all be lightly mounted to the cylinder and effectively sealed in their own enclosure allowing them to move independently of the cylinder itself. The mass of these coils can be increased by adding a layer of lead to each of the coil plates. This would form an exact cylindrical analogy to the situation described above for rectangular plates with edge compensation. However, technically such an arrangement may be difficult to realise and excessively expensive because of additional machining costs. We, therefore, propose a modification to this scheme in which the primary and magnetic screening coils are lightly supported on a frame structure rather than on solid cylinders, and the piston effect compensation coils are applied directly to the primary and magnetic screening gradient coils.

Strand (iv)

The fact that there is now no solid cylinder between the primary coil and the magnetic screening coil means that there will be surface or piston radiation into the cavity between the two conductor surfaces and this may be effectively absorbed by using a suitable absorbent material. In this arrangement, therefore, we propose dispensing with the strand (i) and the fundamental mode quenching coils on the assumption that acoustic radiation in the annular space between the two coils can be completely absorbed and that the supporting frames which hold the plates in position are sufficiently weakly coupled to the plates that they do not emit substantial sound along the z-axis.

In the arrangement described so far we consider the whole of the primary coil and the whole of the screen coil as being integral components. However, because of the spiral nature of the current tracks on our conductor surfaces and because of the presence of a large coaxial magnetic field B parts of the primary and screen coils will have opposite radial forces, that is to say parts of the same coil will experience a couple which will tend to deform or buckle the surface. In order to overcome this problem we section the conductor surfaces in such a way as to make sure that each section has either a radially outward or radially inward force. This means cutting through certain conductor tracks and reconnecting using flexible copper contacts in the form of braided or other suitable conductor material which allow each segment of the conductor surface to move independently of the rest of the surface.

Magnetically Screened Acoustically Controlled Transverse Gradient Coil

In Fig. 7 we show a sketch of part of a magnetically screened fully acoustically controlled transverse gradient coil of the fingerprint design comprising a series of nested conductors and cylinders as illustrated in Fig. 7(a). The sketch is cut away to illustrate the detail of the structure. The outer cylindrical surface A and the inner surface A' define the build of the gradient coil. In fact the assembly comprises essentially two cylinders and a gap O between the two. Figure 7(b) is a detailed sketch of the various layers which we now describe in detail.

The various layers are annotated A,B,C,D to N, the gap is designated by O and the inner cylinder is annotated N' ,M',L'.... to A' . Starting with the outer cylinder A denotes an outer conductor track carrying current I,; B is a soft supporting material which weakly couples A to C; C is the outer cylinder surface, this cylinder D is made of a rigid supporting plastic material; E and G are soft supporting materials; F is a conductor surface carrying current I₂ sandwiched between E and G; H is the rigid cylindrical supporting plastic; I and K are soft supporting materials; J is a conductor surface carrying a current I₃; L is the inner surface of the outer cylinder of rigid plastic; M is a soft supporting material which carries a conductor N with a current I₄; O is the gap between the inner and outer cylinders, and then in reverse order we have a similar arrangement but on smaller diameters for the entities N' to A' . The primary gradient current is provided by I₂' and the magnetic screen is provided by I₂. The currents I₃ and I₃' provide active acoustic control under strand (i). The currents I_l5 I₄, I₄' and 1 provide active acoustic control to compensate the cylindrical surface piston effect under strand (ii).

In the arrangement envisaged we consider distributed coils of the fingerprint type so that the primary coil F' carrying current I₂' is similar to that represented in Fig. 8(a). Here the conductor tracks for all four segments of the primary coil are rolled out flat as indicated in Fig. 8(a). The conductor F carrying magnetic screening current I₂ is also of the distributed fingerprint design and an actual conductor track when rolled out flat for all four segments is sketched in Fig. 8(b).

It will be noticed that for both 8(a) and 8(b) when the magnetic field is applied along the z-axis the forces on the wires in these distributed coils can be directed into or out of the conductor surface, even for the same segment. This means that to avoid couples acting on each segment they must be split at planes A,B and C as indicated in Fig. 8(c). That is to say the cylindrical structure must be cut into at least four segments along the z-axis as shown. A further complication arises when we have x and y gradient coils in which the y coil is rotated through 90° with respect to the x coil. If all parts of the coil structure are to be free to vibrate independently it means that in addition to splitting the coil former along the z-axis it is also necessary to split it into four segments around the polar axis. This is sketched in Figs. 8(c) and 8(d).

While we have concentrated on the primary gradient conductor and its associated magnetic screen we now emphasise that the additional piston effect compensating conducting surfaces and the associated currents should have tracks similar to Figs. 8(a) and 8(b) or 8(c). However, since the current required for compensation of the piston effect is very low, we now anticipate that these additional current conductor surfaces will be of much thinner gauge metal sheet, whereas the major current carrying conductors will be of substantial thickness to mitigate any heating effects. If the outer and inner conductors A and A' are close enough to F and F' then the conductor tracks will be identical to Figs. 8(a) and (b) and in this case none of the advantageous effects of magnetic screening are vitiated since the effect of Ij and IC will be to marginally reduce the magnetic field produced by I₂ and I₂\ thus rescaling of I₂ and I₂' will completely restore the primary field and also the external magnetic field cancellation.

Reduced Transverse Gradient System

We now describe a reduced version of the gradient system described in Fig. 7. Certain supporting structures have been removed, in particular the active acoustic screening under strand (i) has been taken out and the rigid supporting cylinders removed. A sketch of part of the reduced system is shown in Fig. 9(a), and this comprises suitably split gradient conductor plates now for all three axes, x,y,z, and the associated magnetic screens. The conductor tracks on these plates are punched from or etched from copper sheet, the sheet being supported on a thin rigid plastic sheet of glass-reinforced epoxy typically 1 mm thick. The whole assembly is then rolled to the required diameter and the x,y and z gradient plates are nested and stuck together to form a single unit. The gradient plates in this arrangement are supported on a ring structure, the details of which are shown in Fig. 9(c). The reduced structure is annotated in Fig.

9(b) and includes only elements A, B, F, G, H, O, H', G\ F', B' and A'.

In this arrangement F and F' comprise the nested conductor assemblies for the magnetic screen and primary gradient coil respectively.

Conductors A and A' comprise the nested conductors for piston effect compensation of the conductors F and F' .

It is noted that conductors A and A' are mounted directly on the primary and screened gradient plates F,F' via a soft, lightly coupled mounting B and B' . F, F' are made of thicker copper or other metal sheet. The mass of the nested x,y and z conductor segments may be increased by adding a layer of lead or lead alloy sheet. The gap between F and F' comprises the support rings and absorbent acoustic foam. Details of this arrangement and the ring support mechanism are shown in Fig. 9(c). In the arrangement shown pairs of rings sandwich spurs, G' ,G, which are rigidly fixed to the primary coil plates F' and the magnetic screening plates F. The spurs are weakly coupled to the ring supports via soft anti- vibration material. Also weakly coupled to the conductor clusters F and F' are the conductor clusters A and A' via a soft support anti- vibration material indicated as B and B' in Fig. 9(c).

Gradient Coil Driver and Signal Reception

Figure 10(a) shows circuitry 100 for driving gradient coils in the active acoustic control mode. The arrangement, in this case driven by a network analyser 102 as an example, shows the audio frequency output into a coherence trigger 104 and pulse envelope generator and timer, the output of which is then fed to an amplitude and phase control box 106.

This circuit provides four control voltages Vj - V₄ with signal phases φ -φ₄ . The voltage output and phases are all variable. The signals are sent through gradient driver amplifiers 108 to four power amplifiers, Aj - A₄, the outputs of which drive the various coil inputs with currents

I_l ,φ₁ -I₄,φ₄ . Noise from the gradient coil is picked up by a microphone array m₁ m_n. Each microphone has a preamplifier, PA_X PA_n, with outputs Q_x Q_n, and phases ψ_v...ψ_n . These outputs may be switched sequentially on to the network analyser input Q for sequential signal monitoring or alternatively they may be connected to corresponding inputs Q₁' ...Q_π' of a visual display unit, Fig. 10(b), giving an instant parallel output visual indication 150 of the simultaneously received signals. Additional visual output display units 152 may be incorporated to measure simultaneous phases ψ_λ ...ψ_n of the acoustic signals from m_x m_n. The array may be one or two-dimensional. The AF input signal to the amplitude and phase control unit may be pulsed on and off for short periods and the pulse waveform may be triggered from a zero crossing of the AF input signal to avoid large rapid discontinuities in the power amplifiers. The pulses so generated would be sinusoidal or cosinusoidal signals contained within an envelope, the amplitude and leading controlling edges of which may be shaped to include a rectangular trapezoidal or other waveform.

In some applications of active acoustic control it is desirable to have a pulse modulated gradient rather than a sinusoidally modulated gradient. In this case the actual pulse gradient waveform delays are generated from a series of pulse generators, PG1 PG4, as indicated in

Fig.10(c). In order to compensate for propagation delays either through the amplifier system or especially through the acoustic material comprising the gradient system, it is in general desirable to have the pulse gradient waveforms delayed by delays Dl - D4. The pulse circuitry in

Fig.10(c) would be placed in the circuit of Fig. 10(a) between the points

Pi P₄ and P,' P₄' . When inserted in this way switches Si S₄ may be switched to isolate the AF signals from the amplitude and phase control unit and couple the gradient drivers directly to the outputs of the pulse generators PG1....PG4. The pulse generators are triggered through their respective delays from the trigger input T which may be derived from the coherence trigger and pulse envelope generator of Fig.10(a).

Further Embodiments

In our earlier discussions on soft mounting and edge compensation effects in Figs. 3, 4 and 5, the soft mounting material has elastic constants, kj k* and k 1^' , in for example Fig. 3. These are effectively the spring constants of the material but we stress that in such soft mounting materials there will, in general, be a change in the thickness or width of the material due to Poisson's ratio σ . If Poisson's ratio is isotropic then changes in the width of soft mounting along the x-axis, in for example Fig. 4, will give rise to a thickening of the material through the effect of the component, ^ , which will launch an acoustic wave at right-angles to the applied force F, in this case along the z-axis. This means that although general radiation from the edge face is reduced, isotropic Poisson effects will to some extent vitiate this compensation. We, therefore, propose that the soft mounting material is chosen so as to have an anisotropic or strictly speaking an orthotropic Poisson's ratio σ and in particular that in the principal coordinate axes system where there are six components of σ there are at least two components of σ , for example σ_^, σ_yz which are zero. In certain circumstances it is possible to choose materials or fabricate materials in which a further one or two components of σ are zero, namely σ_xy and/or σ_yx. In cellular materials forming tubules, the remaining two components of σ , namely σ_zx and σ ^ are expected to have values of 1 if the tube axis is z. If zero stress is applied along the z-axis there should be substantially no acoustic radiation coming from the soft mounting material and no acoustic radiation coming from the end edges of the plate or the rim guards. Use of materials with anisotropic Poisson's ratios could also have benefits when choosing the main conductor mounting as in Fig. 5 and Fig. 3 where we refer specifically to the soft mounting k₂,k₃ and k₂' and k₃' . In the specific arrangement of conductor supports between plate pairs, as shown in Fig. 5, acoustic radiation from the anti-vibration mounts could be substantially reduced by choosing an anisotropic soft mounting for these conductors as well.

Of course, in addition to the use of anisotropic celluar materials for soft mounts, the acoustic output from the main coil plates may be greatly reduced by use of anisotropic cellular materials as described in our prior art (P. Mansfield, PCT WO 99/00692 Priority 7 March 1998).

In order to clarify our invention with regard to Figs. 2, 3 and 5, reference is now made to Fig. 11 which is equivalent to Fig. 3 but does not include the edge compensation conductor plates and rim guards since the latter effectively maintain the edges of the plates static and fixed in space. In Fig. 11 , therefore, we simply note that the end faces 110,112 are fixed and furthermore that the inner plates carrying currents I₄ and I₄' are removed altogether since the effect of these inner currents is to maintain the inner edges static through the mid-plane. The equivalent mechanical arrangement is as shown in Fig. 11 where the inner gap has a width 2g and the outer compensation surfaces have a thickness g. In addition Fig. 11 includes the anisotropic components of the Poisson's ratio for the coupling material and edge compensation, σ _lxz, and also for the plate material itself, σ_2xz. It will be readily appreciated that if the end faces of the plate are fixed and if σ _lx2 = 0, no acoustic radiation will be emitted along the z-axis or indeed along the y-axis provided the edges remain flat. The only radiation that could occur from this plate configuration is that which is generated due to strains in the supporting plate material itself. The amplitude of these strains is controlled by σ_M. It is apparent, therefore, that if the plate material itself is made of an anisotropic material in which cr_2xz = 0, there will be no strain in the plate along the z-axis and, therefore, no acoustic emission along the z-axis. Making the supporting plate material from suitable anistropic material may be expensive and it may be preferable to use ordinary plastic materials or laminate materials in which σ_2xz ≠ 0.

Even in this case provided σ _lxz = 0 and provided that the plate is correctly driven with currents l₂ , ₂ and I₃,^₃ etc there will still be substantial reduction of acoustic noise emitted along the z-axis. This acoustic noise will be further reduced by using soft mounting of the conductors as indicated in Fig. 11 in which the spring constant or stiffness of the coupling material 14 is k₂,k₃,k₃' and k₂' . It is noted that if σ_2xz is not equal to zero there will, in general, be a change in the length of the plate (not shown), and this change in length along the y-axis will also give rise to acoustic emission from the plate ends due to a piston effect which may also be compensated for in a manner similar to that described for movement of the edges along the x-axis.

A potential problem in the arrangement shown in Fig. 9 is that the curved conductor layer and its support will tend to change the radius of curvature on application of current in the conductors. This could have a deleterious effect when considering the piston effect compensation conductor which would be supplied with current in the opposite direction. This has, therefore, the opposite effect in terms of radius of curvature to that produced by the primary current and magnetic screen. In order to function correctly it may be required that the supporting plastic layer be made of extremely stiff material minimising any geometric distortion.

This effect may be ameliorated to some extent in the following embodiment shown in Fig. 12.

Here instead of having a cylindrical surface we propose that the surface is made up of flat segments in the form of a regular polygon in the form of an octagon, or even a regular hexadecagon. The advantage of this arrangement is that the primary current plates and the piston effect compensation plates are all flat. Provided that the forces on these plates can be reasonably uniformly distributed, the efficacy of piston effect compensation should be much better. In this arrangement the separate segments should be free to move independently which means that the conductor tracks on these flat segments must be coupled together using a flexible braided copper conductor or equivalent. In order to make the plates less flexible it may be necessary to make the plate thicknesses of both the primary gradient coils and the piston effect compensation plates of thick and strong material.

Extending the concepts embodied in Fig. 11 to cylindrical structures, it may be possible to replace the piston effect compensation conductor surfaces in Figs. 7, 8d and 9 by rigid static encasing cylinders on the outer, A, and inner, A', surfaces. These rigid cylinders would be capped each end by rigid static annular spacers so that the entire gradient assembly is encased by the rigid structure. Contact of the gradient coil assembly on all surfaces to the rigid enclosure would be via the soft supporting material, for example B, M, in fig. 7b. Provided the static enclosure is airtight little to no noise should be emitted from the gradient coil assembly, either radially or axially. This assumes that anisotropic materials are used as in Fig. 11.

The same encasement procedure can be used to enclose the octagon arrangement of Fig. 12, thereby allowing removal of the piston effect compensation conductor surfaces.

Claims

1. A coil structure for active control of acoustic noise reduction in MRI apparatus, the coil structure comprising a first electrical coil for generating a primary gradient magnetic field, said electrical coil comprising a first conductor mounted within a support structure, said support structure having at least one surface and including a further surface arrangement mounted adjacent to said at least one surface of said structure, said further arrangement being attached to said surface by a suitable first soft material of low elastic constant, said further surface arrangement being arranged to maintain the further surface arrangement substantially static in space, including an at least second electrical coil for controlling the acoustic output said at least second electrical coil being driven by a second electrical current to provide additional acoustic noise reduction, but with no substantial effect on the magnitude and character of said primary gradient magnetic field and in which the support structure comprises at least two weakly coupled mechanical structures separated by a gap which is filled with a suitable second soft anisotropic orthotropic material or an air gap, the second of said weakly coupled mechanical structures providing support means for the electrical return path of said first and second electrical conductors.

2. A coil structure as claimed in claim 1 in which the further surface arrangement comprises a surface conductor arrangement being electrically driven to produce a suitable surface current distribution to maintain the further surface conductor arrangement substantially static in space.

3. A coil structure as claimed in claim 2 in which when said gap is an air gap, a surface conductor arrangement is provided on each side of said air gap.

4. A coil structure as claimed in claim 1 in which the further surface arrangement comprises a rigid air tight encasing static cylinder which completely surrounds the weakly coupled mechanical structures when said structures are coupled by a suitable second soft anisotropic or orthotropic material.

5. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in claim 1 to claim 3 in which said first soft material is anisotropic or orthotropic.

6. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 1 to 5 in which the coil structure is in the form of a rectangular coil structure comprising a series of thin flat rectangular plates, the plate surfaces of which are normal to the magnetic field direction.

7. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 1 to 5 in which the coil structure comprises a series of thin flat arcuate plates, the plate surfaces of which are normal to the magnetic field direction.

8. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in claim 1 to 7 in which said electrical return conductor path of said first electrical coil forms a primary gradient magnetic screening coil.

9. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 1 to 5 in which the coil structure is in the form of a cylindrical regular polygon supporting a plurality of flat plate segments the planes which are parallel to the magnetic field.

10. A coil structure as claimed in any one of claims 1 to 5 in which the coil structure is in the form of concentric cylinders supporting the conductors for producing the main magnetic field gradient, the magnetic gradient screen and the surface current distribution to reduce acoustic noise output.

11. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 1 to 10 in which said conductors are supported within said support structure in anti-vibration mounts.

12. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in claim 11 in which said support structure comprises first and second substantially parallel plates, said first and second plates being supported a defined space apart by spacers and by material comprising sound absorbent material.

13. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in claim 12 in which motion of said conductor is restrained within said first and second plates by peg, bolt, cam lever or electro- viscous brake means.

14. A method of active control of acoustic noise reduction in MRI, said method comprising the steps of: adding an outer structure to a noise generating surface of a magnetic coil structure, said structure being attached to a soft material arranged in a position intermediate between the magnetic coil structure and said outer structure, the arrangement being such as to maintain the outer structure substantially static in space to thereby reduce the noise emitted from said noise generating surface.

15. A method of active control of acoustic noise reduction in MRI as claimed in claim 14 in which said outer structure comprises a surface conductor arrangement which is electrically driven to produce a suitable surface current distribution to maintain the surface conductor arrangement substantially static in space.

16. A method of active control of acoustic noise reduction in MRI as claimed in claim 14 in which the outer structure comprises a rigid air tight encasing static cylinder.

17. A method of active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 14 to 16 comprising the step of sandwiching between the noise generating surface and the outer structure a soft material having anisotropic or orthotropic properties.

18. A method of active control of acoustic noise reduction in MRI apparatus as claimed in any one of claims 14 to 17 comprising the step of mounting conductors within said magnetic coil structure in anti-vibration mounts.

19. Apparatus for active control of acoustic noise reduction in MRI apparatus for use with the coil structure of claim 2 comprising means for supplying a plurality of currents of suitable amplitude and phase to said electrical coils and to said further surface conductor arrangement.

20. Apparatus for active control of acoustic noise reduction in MRI apparatus as claimed in claim 19 including means for detecting the spatial distribution of the acoustic noise amplitude and phase output of the coil structure and means for displaying said acoustic noise amplitude and phase output.

21. A coil structure for active control of acoustic noise reduction in MRI apparatus as claimed in claim 1 in which said coil structure comprises a magnetically screened fingerprint coil structure wherein the primary and screening current conductor tracks are split into sectors around a polar axis and independent sectors along the z-axis, each sector being mechanically supported on its own mechanically independent support segment.