MAGNETIC STIMULATION (MS) COIL

The present invention relates to a magnetic field shielding arrangement for use with a magnetic stimulation (MS) coil arrangement that allows for modification of the magnetic field profile produced by the windings of the MS coil arrangement.

Magnetic stimulation of tissue is well known and comprises a stimulating coil made up of one or more windings, each winding having a plurality of turns through which a current is passed in order to generate a magnetic pulse which in turn induces electric signals in the tissue. Such a coil arrangement is disclosed in US6179770 where a magnetic stimulator generally comprises a charging circuit, a capacitor, a discharge control and a winding which is of a size and power rating appropriate for the generation of magnetic fields sufficient to cause stimulation of a body portion. The individual winding or plurality of windings may be size adapted to fit partly over the cranium of a human patient in many applications (Transcranial Magnetic Stimulation (TMS)), as well as being used for stimulation of other body parts. The coil arrangement including the one or more windings acts as an inductor and when connected to a stimulator which includes a capacitor provides an input voltage to the inductor which creates a circuit that passes an out-of-phase, sinusoidal voltage and current through the winding(s). An intense sinusoidal magnetic field is formed near the winding and is commonly used to stimulate neurons in patients for medical and research applications. One such magnetic stimulation apparatus is commonly termed a Transcranial Magnetic Stimulation (TMS) apparatus for treatment of the cranium. An example of a coil arrangement is presented in Figure 1 which shows a double winding coil arrangement presented without a covering for clarity purposes that may suitably be positioned on a patient's cranium. The windings 1 are made up of a single wound elongate conductive element 2 formed into the double winding configuration which is connected to a control system 5 including a capacitor. The windings 1 are provided in a housing (not shown) which is moveable to enable appropriate positioning relative to a patient. Magnetic stimulation and particularly TMS magnetic stimulation coil arrangements come in a variety of sizes, shapes and windings having different number of turns for different stimulation applications. It is therefore common to have an array of different coil arrangements that may be utilised by the practitioner dependent on the magnetic pulse strength required for a particular patient or treatment protocol. The provision of a large number of coil arrangements is expensive. Furthermore, smaller coil arrangements for treatment of smaller body areas are expensive and complex to manufacture.

The present invention provides an improved arrangement for enabling use of the same coil arrangement for different patients or treatment protocols.

According to an aspect of the present invention there is a magnetic field shielding arrangement for shielding the magnetic field produced by a magnetic stimulation (MS) coil arrangement designed for providing magnetic stimulation from external of a body, the magnetic field shielding arrangement comprising a magnetic field shield including a shielding portion for partially occluding the magnetic field produced by a MS coil arrangement, the magnetic field shielding arrangement further including a mounting structure for mounting the magnetic field shield relative to an MS coil arrangement.

External of a body means from outside a patent. Such MS coil arrangements are typically hand held or are moved into a position to administer magnetic stimulation to a patient.

Also according to the present invention there is a magnetic stimulation system including a magnetic stimulation (MS) coil arrangement comprising an elongate conductive element formed into a plurality of windings, each winding having a plurality of radially wound turns, and a plurality of magnetic field shielding arrangements according to claim 1, wherein the shielding portion in each of the plurality of magnetic field shields has dimensions to enable transfer therethrough of different magnetic fields and/or different magnetic field profiles, and wherein the mounting structure enables the plurality of magnetic field shielding arrangements to be interchangeably mountable relative to MS coil arrangement dependent on the desired magnetic field and/or magnetic field profile. The present invention enables transformation of the magnetic field profile produced by the windings of a larger coil arrangement to a repeatable magnetic field profile produced by a smaller coil arrangement without the associated manufacture and use challenges associated with producing small magnetic stimulation coils. Further, the present invention enables alteration and manipulation of the magnetic field profile and strength as necessary for a particular treatment.

The magnetic stimulation coil arrangement for which the invention may be utilised is beneficially a Transcranial Magnetic Stimulation (TMS) coil arrangement.

The mounting structure beneficially enables mounting of the magnetic field shield in a repeatable position relative to a MS coil arrangement. The repeatable position is preferably a predetermined distance from a MS coil arrangement. The repeatable position is preferably adjacent a MS coil arrangement.

It will be appreciated that a magnetic field is produced by the windings of a coil arrangement when a current is passed through an elongate conductive element. A winding is a coiled portion of elongate conductive element, and it will be appreciated that at least two, but potentially more windings may be utilised dependent on the application of the invention. Typically for TMS coil arrangements two windings are utilised.

The present invention enables control of the magnetic field strength and/or profile dependent upon the magnetic field shield utilised. Dependent upon the area occluded by the shield the magnetic field strength and/or profile is accordingly controlled. The present invention therefore effectively enables use of the same winding(s) meaning that the practitioner does not have to switch coil arrangements when a different treatment schedule is required. This is particularly beneficial in the event that it is necessary to treat a small area meaning that a small coil arrangement is no longer required as a larger coil arrangement can be utilised by using an appropriate magnetic field shield mounted adjacent to the winding(s) that covers a greater area of the windings. The magnetic field shield preferably includes an insulating covering. The magnetic field shield is beneficially fully covered. The insulating covering provides insulation between the windings of the coil arrangement and the magnetic field shield. The insulating covering also provides electrical insulation between a patient and the magnetic field shield and therefore insulates the patient from current flowing through the magnetic field shield. The insulating covering is preferably contoured to accommodate a patient's head.

The magnetic field shield beneficially comprises a plate. The plate preferably comprises a first side (which may be termed a magnetic field facing face) which the mounting structure positions so that it is adjacent and facing the windings of a magnetic stimulation coil arrangement. An opposing second side (patient facing face) is beneficially utilised for positioning to face a patient.

The magnetic field shielding arrangement (and even more preferably the shielding portion) is preferably configured to at least partially receive the profile of a patient's head. The magnetic field shield beneficially defines a void for receipt of a patient's head.

Accordingly, the magnetic field shield preferably has a profile viewed from a side adapted to receive a patient's head. The magnetic field shield beneficially comprises a longitudinal length and a width, and the magnetic field shield is preferably symmetrical about a transverse axis and preferably a longitudinal axis. A first and second mounting arrangement may be provided spaced apart in the longitudinal axis and preferably may be separated by an aperture in the magnetic field shield.

The magnetic field shielding portion preferably comprises a magnetic field facing face for facing the magnetic field produced by a MS coil arrangement, the magnetic field facing face being non-planar. The magnetic field shield is positioned close to the windings of the magnetic stimulation coil arrangement in use. This has the effect of the current induced in the shield resulting in a significant force attempting to push the magnetic field shield and windings apart. This is due to the opposite direction of the flow of current through the windings compared to the induced current in the magnetic field shield. Accordingly, by making the magnetic field facing face non-planar separation between the magnetic field shield and the windings can be increased at certain points reducing the effect of this force and improving life of the apparatus. This is further beneficial as if treatment is to be carried out of a patient's brain then the shape of a patient's head can be better

accommodated through the provision of a non-planar magnetic field shield.

In use, it will be appreciated due to the non-planar face for facing the magnetic field produced by a MS coil arrangement, the separation between the shielding portion and the MS coil arrangement is preferably variable. The face for facing the magnetic field produced by a MS coil arrangement beneficially comprises a profile defining a bulge therein. The face may be generally convex between a first end and second end. Generally convex should be construed as covering linear portions and also a gap for the aperture for example. The bulge is beneficially shaped to accommodate a patient's head. An opening in the shield is beneficially provided near or at the uppermost tip of the bulge. The face may comprise planar portions and/or curved portions. The benefit is derived through the shielding portion being mounted relative to the MS coil arrangement via the mounting arrangement whereby the distance from the coil arrangement and the windings in particular changes in a longitudinal axis, and in particular wherein this separation reduces to an intermediate point or area in the longitudinal axis.

The magnetic field shield beneficially comprises a longitudinal length and a width, and in a longitudinal axis separation between the face and a MS coil arrangement reduces from a first location to an intermediate location and then increases to a second location. The magnetic field shield beneficially comprises an aperture therein. The aperture enables transfer therethrough of a magnetic field.

There may be provided first and second apertures in the magnetic field shield wherein the first and second apertures are separated by a bridge. Such an arrangement may be useful in the event that it is necessary to mask a part of a treatment area for example. For example, in the event of magnetic stimulation of the brain, an area of the brain can be masked. This is presented with respect to Figures 7 and 8. The magnetic field shield beneficially comprises a first shield portion and a second shield portion. The shield portions are beneficially in a side by side configuration. At least one of the first and second shield portions is beneficially moveable relative to the other of the first and second shield portions in order to change the dimensions of the aperture. It is beneficial that a separation gap is defined between the first and second shield portions. Accordingly, the separation gap is changeable. This provides a significant benefit in that the magnetic field strength and/or profile can be changed through relative movement between the first and second magnetic field shielding portions.

At least one of the first and second shield portions is beneficially arranged to be secured relative to the mounting structure in a plurality of discreet positions. Indicia or other markings may be provided in order that the medical practitioner can accurately position the first and second shield portions relative to each other in order that a known repeatable magnetic field strength and/or profile is achieved.

At least one of the first and second shield portions beneficially comprises a slot therein for enabling adjustment of position relative to the other of the first and second shield portions. This provides a simple mechanism for enabling movement of at least one of the first and second shield portions.

The first and second shield portions are beneficially tilted relative to each other. The effect of this tilting and associated result in a non- planar magnetic field shield means that a patient's head can be better accommodated than a planar shield. It is further beneficial that the magnetic field shield comprises a length having a longitudinal axis and a width having a transverse axis and wherein the first and second shield portions are beneficially tilted about a transverse axis. When secured to a coil arrangement, it is beneficial that the first and second shield portions have a first edge adjacent the aperture which is positioned closer to the windings of the coil arrangement than a second opposing edge which is positioned further away from the windings. The aperture in the magnetic field shield is beneficially defined between the first and second shield portions. It is possible that the aperture is a separation gap between a first shield portion and a second shield portion. In such an embodiment there a physical separation between the first and second shield portions.

The mounting structure beneficially comprises at least one projection extending from the magnetic field shield arranged to project at least partially through and be secured relative to an aperture defined by the radially inner edge of a winding of a magnetic stimulation coil arrangement. It will be appreciated that the radially inner edge of a winding of a magnetic stimulation coil arrangement defines a core through which is provided an opening. It is beneficial that the number of projections matches the number of windings associated with the coil arrangement. Typically, a coil arrangement comprises two windings meaning that the mounting structure comprises two corresponding projections. In one embodiment it will be appreciated that the projection(s) may extend at least partially through the aperture defined by the radially inner edge of a winding of a magnetic stimulation coil arrangement and be secured therein via an interference engagement. In any event, however, it is beneficial that there is a releasable engagement between the mounting structure and the coil arrangement in order that a practitioner can replace the magnetic field shielding arrangement as appropriate and utilise a different magnetic field shield to effect the associated magnetic field produced by a winding. The magnetic field shielding arrangement is beneficially temporarily docked or fixed relative to the windings and is therefore user changeable.

The mounting structure may comprise a first securing portion and a second securing portion for enabling securing of a magnetic stimulation coil arrangement winding therebetween. It is preferable that the first securing arrangement comprises a projection extending from the magnetic field shield arranged to project at least partially through an aperture defined by a radially inner edge of a winding of a magnetic stimulation coil arrangement, and a second securing portion is beneficially configured to releaseably engage with the first securing portion to secure the magnetic field shield therebetween. Complex tools are beneficially not required. The first securing portion beneficially extends from the insulating covering of the magnetic field shield. It will further be appreciated that a magnetic stimulation coil arrangement comprises a winding housing, and the projection extends through the housing and core of the winding.

It is beneficial that the first and second securing portions engage via correspondingly threaded portions.

It will be appreciated that the first securing portion is beneficially integrated with the magnetic field shield. This is beneficial as it means a user of the apparatus does not have the option for incorrectly positioning the magnetic field shield relative to the windings. It is further beneficial that additional first securing portions are provided to match the number of windings. Two or more first securing portions preferably extend through respective cores of respective windings ensuring accurate positioning of the magnetic field shield relative to the windings.

According to a second aspect of the present invention there is a magnetic field shield for partially shielding the magnetic field produced by a magnetic stimulation (MS) coil arrangement, the magnetic field shield including a shielding portion for partially occluding the magnetic field produced by a MS coil arrangement, wherein the shielding portion comprises a magnetic field facing face for facing the magnetic field produced by an MS arrangement, the magnetic field facing face being non-planar.

A significant benefit associated with a non-planar magnetic field facing face is that the magnetic field shield and winding separation can be increased at certain points thus reducing the effect of the force attempting to push the magnetic field shield and windings apart thus improving life of the apparatus. Furthermore, the shape can accommodate a profile of a patient on a patient facing face. The shield is beneficially configured to at least partially receive the profile of a patient's head. The magnetic field shielding portion beneficially defines a void for receipt of a patient's head. The shield beneficially comprises a longitudinal length and a width, and in the longitudinal axis the magnetic field shield rises from a first location to an intermediate location and then falls to a second location.

The magnetic field shield beneficially comprises an aperture therein.

The shielding portion beneficially comprises a first shield portion and second shield portion, and the aperture is a separation gap between the first and second shield portions.

The magnetic field shielding portion beneficially comprises a first shield portion and a second shield portion, and wherein at least one of the first and second shield portions is moveable relative to the other of the first and second shield portions in order to change the dimensions of the aperture. At least one of the first and second shield portions is beneficially arranged to be secured relative to a mounting structure in a plurality of discreet positions, wherein the mounting structure is arranged to mount the magnetic field shield relative to an MS coil arrangement.

At least one of the first and second shield portions beneficially comprises a slot therein for enabling adjustment of position relative to the other of the first and second shield portions.

The present invention also extends to a method of controlling the magnetic field strength and/or magnetic field profile produced by a magnetic stimulation (MS) coil arrangement, the MS coil arrangement comprising an elongate conductive element formed into a plurality of windings, each winding having a plurality of radially wound turns, and mounting a magnetic field shield adjacent the windings for partially occluding the magnetic field produced by the windings.

The method may comprise selecting the magnetic field shield from a plurality of magnetic field shields, each magnetic field shield having a shielding portion for partially occluding the magnetic field produced by the MS coil arrangement, wherein the dimensions of each of the shielding portions are different to enable transfer therethrough of different magnetic fields, and mounting the selected magnetic field shield relative to the MS coil arrangement.

It will be appreciated that there is not direct control of the field produced by the MS coil arrangement meaning the same field is produced by the MS coil arrangement with or without the present invention, however the effect on the patient is changed and thus controlled as the magnetic field produced by the windings is affected by the present invention. The medical practitioner using the apparatus can change the magnetic field shielding arrangement according to the desired treatment protocol for that patient, where the apertures in different shields allow for different magnetic field strengths and/or profiles.

The present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic perspective view of a magnetic stimulation coil arrangement known in the art. Figure 2(a) is a normal double winding and Figures 2(b), (c) and (d) show exemplary embodiments of the present invention showing different magnetic field shielding elements according to the present invention.

Figure 3 is a schematic representation from finite element method modelling of the magnetic field strength corresponding to the peak current in a coil arrangement. Figure 3(a) is representative of a normal coil without the magnetic field shielding element. Figure 3(b) is a schematic representation with the provision of a magnetic field shielding element representative of a plate fully occluding the first side of the double windings, Figures 3(c), (d), (e) and (f) present a magnetic field shielding element having an 8cm aperture, 5cm aperture, 3cm aperture and 1cm aperture in the magnetic field shielding element respectively. Figure 4(a) is an illustration of the current flow in a normal double winding. Figure 4(b) is a presentation of the current flow in a magnetic field shielding element having an aperture therein. Figure 5 is a schematic magnetic field pattern in the (x), (y) and (z) cartesian magnetic field components resembling the magnetic field pattern for a normal double winding as shown in Figure 5(a) and for comparison as presented in Figures 5(b), (c) and (d) the (x), (y) and (z) components of the magnetic field for the structure shown in Figure 2(c) are presented with an 8cm, 5cm and 3 cm aperture respectively.

Figure 6 is a similar representation as presented in Figure 5 however utilising two separate plates with an aperture of 2cm, 4cm and 6cm as presented in Figures 6(a) to (c) respectively. Figure 7 is a schematic plan view of an exemplary embodiment of the present invention illustrating the current flow for this structure where the structure is the same as presented in Figure 2(d).

Figure 8 is a representation of the magnetic field strength produced by the embodiment as presented in Figure 7.

Figure 9(a) and 9(b) are perspective views and side views of a magnetic field shield according to an exemplary embodiment of the present invention adjacent a coil arrangement, and Figure 9(c) is an exemplary presentation of the associated magnetic field pattern.

Figure 10(a), (b) and (c) are perspective illustrations of a magnetic stimulation coil arrangement in exploded view of an exemplary embodiment of the present invention.

Figure 11(a) and (b) are schematic plan views of an exemplary embodiment of the present invention. Figure 12a and 12c and Figures 12b and 12d are schematic exemplary embodiments of a magnetic field shield and arrangement.

Figure 13 is a comparison of a schematic magnetic field pattern in the (x), (y) and (z) Cartesian magnetic field components resembling the magnetic field pattern for a normal double coil winding (Figure 13 a) and compared to that of embodiment of the invention shown in Figure 12d.

Figures 14a and b are a further schematic exploded view and perspective view respectively of a further exemplary embodiment of the present invention.

Referring to Figure 2(a) a typical double winding 1 magnetic stimulation coil arrangement is presented and Figures 2(b), (c) and (d) each show perspective, side and plan views of alternative magnetic field shields according to exemplary embodiments of the present invention. These magnetic field shields are positioned adjacent but not in electrical communication with the windings. As presented in Figure 2(b) first and second windings 1 are shown wherein the magnetic field shield 4 comprises a first magnetic field shield portion 4(a) and second magnetic field shield portion 4(b). The magnetic field shield portions 4(a), 4(b) partially cover a first side of the plurality of radially wound turns of the first and second windings 1. The magnetic field shield portions 4(a), 4(b) are spaced apart from the windings 1. Between the magnetic field shield portions 4(a), 4(b) is provided an aperture in the form of a slit 6 where the magnetic field is unimpeded. The slit 6 in the embodiment presented in figure 2b is provided beneath the central area of the double windings.

Referring to Figure 2(c), a single magnetic field shield 4 is provided having an aperture therein 8. The magnetic field shield 4 is again spaced apart from the windings as shown in the side view and in this embodiment the aperture 8 is aligned beneath the centre of the double winding. The aperture 8 in this embodiment is substantially circular, but it will be appreciated that other shapes may be utilised such as polygon or square. Ref erring to Figure 2(d), a magnetic field shield 4 is presented and comprises two apertures, 8(a), 8(b) again spaced apart from the windings. In this arrangement a bridge 10 separates the first and second apertures 8(a), 8(b). Referring now to Figure 3 presented are representations of the magnetic field strengths corresponding to the peak current provided through the magnetic stimulation coil windings. Figure 3(a) is a presentation of the peak magnetic field for a normal coil arrangement having a double winding presented in side view. It can be seen that both above and below the windings a significant portion of light area is shown identifying the distance outwardly that the magnetic field projects. Referring to Figure 3(b), for making placebo or sham magnetic stimulation coils a typical approach is to place a metal or ferromagnetic sheet under the magnetic stimulation coil windings which shields the magnetic fields from the patient. Such a sheet would be, for example, as presented in Figure 2(c) without the aperture 8. As is clear from Figure 3(b), the magnetic field is shielded below the windings of the magnetic stimulation coil arrangement. It has been found, however, that positioning a hole, aperture, opening or window in the sheet effectively creates a condition whereby a magnetic field pattern of an effectively smaller double winding arrangement is produced. The sheet therefore partially shields the patient from the magnetic field. In the peak magnetic field as presented in Figure 3 the magnetic field in shielding element or sheet 4 is spaced 4mm below the windings of the magnetic stimulation coil arrangement. The magnetic field shielding element 4 itself is nominally provided having a thickness of 1mm. Thickness may be increased having the effect of less heating, however this has to be balanced against the increased volume of material and associated weight. As can be seen in Figures 3(c) to (f) for 8cm, 5cm, 3cm and 1cm diameter circular apertures 8 respectively, the magnetic fields pass through the aperture 8 where the windings meet. Most of the remaining magnetic field is interrupted meaning that it is cancelled and blocked by the magnetic field shield.

Figure 4(a) is a schematic representation of the current flow in a normal double winding magnetic stimulation coil arrangement. This current flow forms a magnetic field around the winding which is sinusoidal. The time varying magnetic field induces a current in the magnetic field shielding elements below it. The induced current (I _p ) flows in the opposite -Indirection to the current in the coil (I _c ). On inspection of the current flowing around the peripheral edge of the aperture and the currents in the double windings (where the windings meet) these resemble the current in the double winding as presented within the dashed box 12 as presented in Figure 4(b).

Referring now to Figure 5, the current distribution as presented in Figure 4 produces a magnetic field pattern in the (x), (y) and (z) cartesian magnetic fields components resembling the magnetic field pattern for a normal double winding magnetic stimulation coil arrangement as presented in Figure 5(a). For comparison, shown is the (x), (y) and (z) components of the magnetic field further shown in Figure 2(c) with an 8cm, 5cm, and 3cm aperture in Figures 5(b), (c) and (d) respectively. As the size of the aperture decreases the distribution of the magnetic fields emerging from the aperture appear as that coming from a double winding of the size of the aperture. This means that utilising the appropriate magnetic field shielding element having the appropriate partial occlusion of the windings means that effectively a smaller double winding magnetic stimulation coil arrangement is achieved without using a different magnetic stimulation coil arrangement. As such, the magnetic field can be controlled through the appropriate magnetic field shield.

As presented in Figure 6, a similar effect may be obtained by using two individual magnetic field shielding portions as depicted in Figure 2(b) to achieve the same goal.

Examples are presented in Figure 6 showing increasing separation (thereby increasing the distance between opposing edges) between 6(a) to 6(c) between the two separate magnetic field shielding portions. Whilst Figure 6 shows that the connection between the magnetic field shielding element is not essential, another embodiment of the invention can be utilised as shown in Figure 2(d). Figure 7 illustrates the current flow for this structure. The time variant current flowing in the windings induces current that flows in the opposite direction through the magnetic field shielding element compared to that in the windings. As presented in Figure 7, a strip of magnetic field shielding element is incorporated between point R and S to form a double aperture in the magnetic field shielding element 8(a), 8(b). The current flowing in the magnetic field shield now meet at point R, then splits to flow around the aperture's 8(a), 8(b) and across the bridge 10 before rejoining at point S. The system works in a similar manner as that as shown in Figure 2(c) however now there must be a zero in the magnetic field beneath the bridge 10 since the current in the windings and the bridge run in opposing directions. A presentation of the magnetic field's strength is shown in Figure 8 again in side view identifying the mid-point in a vertical axis. Such a feature is beneficial in treatments where a part of the body requires treatment and a further part does not require treatment. This may be particularly beneficial in transcranial magnetic stimulation where parts of the brain need to be treated adjacent to parts of the brain that do not require treatment.

It will further be appreciated that the magnetic shield may not be planar, and particularly the face of the shield that opposes the MS coil arrangement is not planar, and such an embodiment is presented in Figures. Referring now to Figures 9(a) to (c) there is a further exemplary embodiment of the present invention. In this embodiment presented is a magnetic field shield 4 comprising first magnetic field shielding portion and second magnetic field shielding portion 4(a), 4(b) respectively. It will be appreciated that in the embodiment presented the aperture 8 is provided by a separation between the magnetic field shielding portions 4(a), 4(b), however it will be appreciated that the magnetic field shield may be a single component having an appearance in plain view as presented, for example, in Figures 2(c), 2(d).

Referring to Figures 9(a) and 9(b) it will be appreciated that in such an embodiment the magnetic field shielding portions 4(a), 4(b) are tilted relative to the windings 1. Tilting of the magnetic field shielding portions 4(a), 4(b) better provides for accommodating the shape of a patient's head. The effect of tilting of the first and second magnetic shield portions 4(a), 4(b) provide for a receiving area for receipt of a patient's body instead of the apparatus sitting on top of a patient's body, particularly the head. As can be seen in Figures 9(a) and 9(b) the portion of the magnetic shield portions 4(a), 4(b) that is adjacent the aperture 8 is provided closest to the windings 1, whereas the opposing peripheral edge spaced in the longitudinal axis is the portion tilted away from the windings the most and as such the separation between the winding 1 and the peripheral edge of the magnetic field shielding portion 4(a), 4(b) is at its most at this point. Referring to Figure 4(c), the magnetic flux density is presented showing the effect of the magnetic shield portions 4(a), 4(b) which have the same effect of prevention of magnetic flux density passing there through, whereas the aperture 8 enables the magnetic field to pass there through. It will further be appreciated that the shielding portion may have a curved profile or other configuration. The profile of the face of the shield opposing the MS coil arrangement may generally be convex, and the opposing side of the shield is generally concave for accommodating the profile of a patient. It will be appreciated that the shield has a longitudinal length, and the separation between the face of the shield opposing the MS coil arrangement reduces from a first location to an intermediate location and then increases to a second location.

A significant benefit associated with this embodiment is that the force that arises due to the flow of current through the magnetic shield opposing the current flow through the windings means the effective force is reduced in terms of pushing apart the windings and the magnetic shield. This is reduced meaning reduced stress upon the equipment and associated longer life for the equipment.

The magnetic field shielding element may further comprise a plurality of apertures in various configurations.

Referring now to Figure 10, presented is a magnetic stimulation coil arrangement 1 wherein a double winding is accommodated within an insulating housing 30. A neck 32 extends from the housing 30 to a magnetic stimulator (as shown in Figure 1). Also presented is a magnetic field shield 4 shown in Figure 10(a) in a position ready for securing relative to the housing 30 and therefore to the relative windings provided in the housing 30. The magnetic field shield is housed in an insulating cover.

Figures 10(b) and 10(c) show the magnetic field shield 4 is secured relative to the housing 30. The mounting arrangement is provided showing first securing portions 34 and second securing portions 36 where the first securing portions 34 are fixedly secured to the face of the magnetic field shielding element that once assembled faces towards the housing 30 and associated windings. The mounting arrangement further comprises a spacing element 38 located intermediate the first and second fixing elements 34, 36 and the magnetic field shielding element 4 in order to space the housing 30 and therefore by default the windings located in the housing 30 apart from the magnetic field shield 4. The first securing portions 34 comprise projections that extend through the centre or core of the windings and through respective openings 40, 42 provided in the housing 30. The first securing portions 34 include a screw thread and preferably engage with a corresponding screw thread of the second securing portions 36. The windings 1 are therefore secured intermediate the first and second securing portions. It will therefore be appreciated that securing of an appropriate magnetic field shield 4 to a housing 30 is simple and fast and the practitioner can therefore readily control the magnetic field profile utilising a magnetic field shield having the appropriate configuration of aperture(s) to control the magnetic field profile. It will be appreciated that there are alternative means of securing the magnetic field shield to the MS coil arrangement. An example would be providing a first securing portion comprising a frame extending from the magnetic field shield and a peripheral edge of the MS coil arrangement that may be elastically deformed for fitting, whereby the frame springs back to secure a part of the peripheral edge of the MS coil arrangement.

Presented in Figures 11(a) and 11(b) is a schematic exemplary embodiment of the present invention. Presented is a magnetic field shield comprising a first shield portion 4a and a second shield portion 4b. An aperture 8 is provided between the first and second shield portions 4a, 4b. Presented in this embodiment both the first and second shield portions 4a, 4b are moveable in the direction indicated by arrow 60 effectively changing the dimensions of the aperture 8 between the first and second shield portions. This has the associated effect of changing the magnetic field strength and/or the magnetic field profile that is enabled to pass to the patient. It will be appreciated that the diagram is schematic in nature and the mounting structure enables securing of the first and preferable second shield portions 4a, 4b in a plurality of discrete positions and markings are beneficially provided for the medical practitioner in order that they may appropriately be sure that the selected aperture dimension will produce the appropriate magnetic field strength/profile. It will further be appreciated that the first and second magnetic field shield portions 4a, 4b may be tilted relative to each other to define a void to receive the body (for example head) of a patient. Referring now to Figure 12, presented in Figure 12(a) and 12(c) are perspective and rear views respectively of an exemplary embodiment of the present invention. In this embodiment the magnetic field shield comprises a longitudinal length wherein the shield is symmetrical about the longitudinal length. In the transverse width, however, the aperture 8 is non-symmetrical and a greater portion of the second winding 2 is presented thereby effecting the magnetic field profile and/or strength. The change of the configuration of the aperture in this regard can be used to manipulate the position on the patient on which treatment occurs. Referring to Figures 12(b) and 12(d), a further exemplary embodiment of the present invention is presented. In this embodiment a portion of the windings 1, 2 are non-shielded and in a particular embodiment the complete winding 2 is exposed meaning that the magnetic field contacting the patient is manipulated as presented in Figure 13(b). In this embodiment therefore there is partial occlusion of the magnetic field produced by the windings 1, 2.

Referring now to Figures 14(a) and 14(b) presented is a further exemplary embodiment of the present invention. Referring initially to Figure 14(a) there is a magnetic stimulation coil arrangement 1 which is provided within a housing 80 having longitudinally extending slots 82 and transversely extending slots 84 in the exemplary embodiment. It will be appreciated that in a simplified embodiment a single transversely extending slot 84 is provided.

Further provided is a magnetic field shield 4 formed of first and second shield portions 4a, 4b. These shield portions 4a, 4b are separated by an aperture 8 in the form of a separation gap. A mounting structure is provided in the form of a first securing portion 34 provided by the first and second shield portions 4a, 4b and a corresponding second securing portion 36. The second securing portion 36 effectively is in the form of first and second covers 86a, 86b positioned on the opposing side of the coil 1 to the first and second shield portions 4a, 4b. The cover portions 86a, 86b are secured by engaging with the first securing portions 34 of the first and second shield portions 4a, 4b. As such, the MS coil provided within the housing 80 is retained therebetween. The cover portions 86a, 86b have retainers 88 in the forms of pins which locate within the housing 80 and project through the longitudinally extending slots 82. This means that the first and second shield portions 4a, 4b are moveable longitudinally relative to the housing 80 and thus the MS coil 1. Each of the respective retainers 88 are connected via an arm 90 to a further retaining element 92 positioned in the transversely extending slots 84. Therefore, the retainers 92 can be independently pushed along the transverse slots 84 to adjust the aperture 8. It will be appreciated that the first and second shield portions 4a, 4b may move

independently of each other as shown in the longitudinal direction by arrow 60 in Figure 14b. It will further be appreciated that it is possible to enable identical movement of the first and second shield portions 41, 4b through movement of a single actuator. Thus, a single retainer 92 maybe provided whereby a first arm 90 projects to a first retainer 88 and a second arm projects from retainer 92 to the second retainer 88. Thus, a "v" is defined between the first and second retainers 88 and the retainer 92. Upon movement transversely through the slot 84 of the retainer 92 the "v" defined by the first and second arms 90 opens and closes thus pushes apart and brings together as appropriate the first and second shield portions 4a, 4b respectively. This means that the first and second shield portions 4a, 4b are moveable concurrently through a single actuator.

The invention may be used for both real and placebo treatment. For example, in the event that it is necessary to give a patient a placebo treatment, a magnetic field shielding element having a very small aperture or opening therein may be provided to achieve stimulation at a very small distance from the windings. As such this gives the effect of scalp stimulation only but provides no actual magnetic stimulation treatment.

It will further be appreciated that the aperture, slit or opening in the magnetic field shielding element may be made to form any shape for tuning the magnetic field.

Furthermore, the aperture, slit or opening may be moved off centre or shifted to tune the shape of the magnetic fields produced. For example, in one embodiment a magnetic field shielding element having plurality of openings therein could have one opening larger than another.

Furthermore, in one embodiment wherein a bridge is provided between first and second openings the magnetic field shielding element may take the form of a resistor whereby the currents are forced to run perpendicular to the current flow in the windings to achieve little cancellation of magnetic fields for tuning purposes.

The present invention has been described by way of example only and it will be appreciated by the skilled addressee that modifications and variations may be made without departing from the scope of protection afforded by the appended claims.