Improvements in and regarding field nulling

Abstract

In an apparatus for zeroing a magnetic field in a zeroing region in an external surrounding magnetic field, a plurality of separate magnetic field generating elements 102 are arranged at respective separate locations surrounding the zeroing region to generate respective zeroing magnetic fields extending into the zeroing region. A plurality of magnetic field sensing elements 103 are arranged at respective separate locations within the zeroing region to sense respective values ​​of the magnetic field in the zeroing region. A feedback control unit 150 controls the value of the respective zeroing magnetic field generated by each of the plurality of magnetic field generating elements by driving the magnetic field generating elements with respective currents that reduce the value of the magnetic field detected by each magnetic field sensing element to a value not exceeding a preset threshold corresponding to a preset zeroing of the magnetic field in the zeroing region in response to the value of the magnetic field sensed by the plurality of magnetic field sensing elements.

Description

[Technical field]

[0001] This application claims priority to International Application No. PCT / EP2021 / 080826, filed November 5, 2021, the contents and elements of which are incorporated herein by reference for all purposes. The present invention relates to a method and apparatus for magnetic field nulling, and more particularly, but not exclusively, to a method and apparatus for magnetic field nulling in Magnetoencephalography (MEG) applications. [Background technology]

[0002] Magnetoencephalography (MEG) is a well-established medical technique for mapping the electrical activity of the brain by recording the magnetic fields that this activity produces. The magnetic fields produced by the brain are very weak and require a substantially absent (or negligible) ambient magnetic environment to be detectable. Magnetic field detection sensors used in MEG are often based on superconducting quantum interference devices (SQUIDs). These devices require cooling with liquid helium, which is very costly. An alternative sensor technology is the optically pumped magnetometer (OPM), which does not require cooling and therefore reduces the cost of MEG equipment.

[0003] Typically, an environment with substantially no (or negligible) ambient magnetic fields (e.g., for use with MEG) is created using a large and very expensive magnetically shielded room (MSR), which typically has walls, floors, and ceilings made of magnetic shielding materials. Furthermore, MSRs may be equipped with additional systems to counteract residual magnetic fields present within the MSR that may result from the limitations of the MSR or from equipment within the room. The cost of MSRs is so high that few facilities can afford MEG equipment.

[0004] During MEG measurements, the patient must come to the MSR, as the MEG device cannot be moved to the patient together with the MSR. In the MSR, preferably only non-magnetic objects should be used to reduce the extra ambient magnetic field in the room. Moreover, the MSR has a very limited space and no windows. This limits the possibilities to bring in tools that can be used for patient interaction, which is important to confirm the brain response. Due to the fixed nature of the MSR, it is not possible to study the patient in a different environment, e.g. outside the building or in an environment that provides psychological comfort. This is especially critical if the MSR environment is not comfortable for claustrophobic patients or frightening for children.

[0005] "Long-term patient monitoring, for example during sleep, is very difficult in an MSR. Moreover, MSR does not allow combining MEG measurements with other simultaneous measurements that require large equipment for other techniques. For example, MEG and low-field MRI can be measured simultaneously, eliminating the need to use an MSR."

[0006] Additionally, within the MSR, some time-varying magnetic field sources are unavoidable, such as the patient's beating heart, the power lines supplying power to the MEG equipment (e.g., computers) within the MSR, etc. Where the MSR itself requires additional compensation of these residual internal magnetic fields, this can be achieved using Helmholtz coils or biplanar coils placed within the MSR.

[0007] The present invention has been made in view of the above circumstances. Summary of the Invention [Means for solving the problem]

[0008] The inventor has realised concepts known from the field of superdiamagnetism. Superdiamagnetism occurs in certain superconductors, resulting in the exclusion of external magnetic fields from the interior region, i.e. the internal magnetic field. It is known that when a superconductor is placed in an external magnetic field, it behaves as a nearly perfect diamagnet. It excludes the external magnetic field so that the magnetic flux lines of the external field avoid the spatial region occupied by the superconductor. This happens because the external magnetic field generates currents on the surface of the superconductor, which in turn generate an opposing magnetic field. The external magnetic field and the surface-generated magnetic field cancel each other out.

[0009] The inventors have realised that knowledge of the external magnetic field of a surface is sufficient to make it possible to cancel or reduce the magnetic field in the volume enclosed by the surface to very small or negligible values. This is supported by Stokes' theorem. With reference to FIG. 1, an external magnetic field 1a skirts around a diamagnetic material 1b, but does not penetrate it. The reason for this effect lies in Stokes' theorem, which relates the integral of the curl of a vector field on a surface to the line integral of the vector field around the boundary of the surface. In other words, the line integral of a vector field on a loop is equal to the flux of curls of the vector field through the surface enclosed by the loop. FIG. 1 shows diagrammatically a loop C encircling a perfectly conductive two-dimensional surface 1c. The surface is made up of continuous surface elements or facets (ΔS i ) of the surface 1c. Suppose this surface 1c is stretched along a spherical surface (e.g., as in making a chewing gum balloon) to create a "bubble" with an opening defined by a boundary loop C, and then the size of the opening C is reduced to zero diameter. The result, according to Stokes' theorem, is a sphere with no magnetic field inside it. The inventors have now decided to apply this idea to a conceptual reference surface having an array of adjacent, discrete magnetic field generating elements (e.g., conductive loops), each of which represents a surface element or facet (ΔS) of the conceptual reference surface (e.g., the aforementioned spherical bubble) that surrounds or encases a region of space in which the ambient magnetic field is to be nulled. i) corresponding to each of the surfaces 1c of the reference plane 100. This conceptual reference plane may completely encompass the spatial region and may be conceptually divided into a number of successive surface facets covering all or at least most of the conceptual surface, and the separate magnetic field generating elements may be arranged in or on each of the successive surface facets, respectively. This results in a structure that approximates part or all of the "bubble" structure consisting of surface 1c of FIG. 1.

[0010] In the broadest sense, the present invention is based on the idea of ​​achieving the cancellation (or nulling) of magnetic fields in a volume in a similar manner to the principles underlying super-diamagnetism. By providing an array of magnetic field generating elements surrounding a volume and applying feedback control between these sensors and an array of magnetic field sensing elements in the volume, the cancellation (or "nulling") of the magnetic field in the volume can be substantially achieved by controlling the current supplied to the magnetic field generating elements as required. In other words, the inventors have found that by achieving the cancellation (or "nulling") of the magnetic field in a volume in which an array of magnetic field generating elements is disposed over the entire surface, it is possible to effectively obtain indirect information of the magnetic field at the surface of the volume. Once nulling is achieved, it means that the magnetic field at the surface of the volume is countered by the array of magnetic field generating elements on the surface in an approximately equal amount across the surface.

[0011] The present invention can, for example, eliminate the need for MSR in MEG systems, providing a cheaper and more compact solution. The present invention can be applied to nulling / cancelling magnetic fields with time-varying magnetic field gradients. The present invention can simultaneously null / cancel both the geomagnetic field and gradient fields arising from ambient magnetic sources as well as time-varying and / or spatially moving magnetic sources.

[0012] In a first aspect, the present invention provides an apparatus for nulling a magnetic field in a nulling region within an ambient (e.g. external) magnetic field, comprising: a plurality of separate magnetic field generating elements disposed at respective separate locations surrounding the nulling region for generating respective nulling magnetic fields extending into the nulling region; a plurality of magnetic field sensing elements disposed at a plurality of respective distinct locations within the nulling region for sensing respective values ​​of the magnetic field within the nulling region; a feedback controller for controlling, in response to values ​​of the magnetic field sensed by the plurality of magnetic field sensing elements, a value of the respective nulling magnetic field generated by each of the plurality of magnetic field generating elements by driving the magnetic field generating element with a respective current that reduces the value of the magnetic field detected by each magnetic field sensing element to a value not exceeding a preset threshold corresponding to a preset nulling of the magnetic field in the nulling region; It is possible to provide an apparatus comprising:

[0013] The feedback controller can be designed to determine a set of optimal respective currents to be used to drive the corresponding magnetic field generating elements by applying an orthogonal projection algorithm. The feedback controller can be configured to use the optimal respective currents to generate a set of basis vectors to be used in controlling the value of each of the respective zeroing magnetic fields generated by each of the respective magnetic field generating elements. The feedback controller can be configured to use the set of basis vectors in calculating each of the optimal currents to be used to drive the magnetic field generating elements of the device. The set of basis vectors can be based on a plurality of measurements of the magnetic field generated in the zeroing region by each of the magnetic field generating elements individually (e.g. measured separately by the magnetic field sensing elements) when driven by a preset calibration current. The orthogonal projection algorithm can be designed to generate an orthogonal basis set based on the plurality of measurements of the magnetic field generated in the zeroing region by each of the magnetic field generating elements individually when driven by a preset calibration current.

[0014] The inventors have found that the orthogonal projection method can be very efficient and accurate in calculating the appropriate drive current to generate the magnetic field to effectively null / cancel the magnetic field. Surprisingly, this technique requires only one calculation of the drive current value, which may take, for example, less than 250 μs. This is in contrast to other techniques that require several (sometimes many) iterative calculations of the drive current value over and over again in an iterative optimization process (i.e., iterative refinement of the nulling) to find a better value of the drive current. These iterative processes have been found to be much slower than the orthogonal projection method.

[0015] Thus, external magnetic fields (e.g., the Earth's magnetic field) can be nulled or cancelled. External magnetic gradient fields can be nulled or cancelled, or external time-varying magnetic fields can be nulled or cancelled. The invention provides a system capable of creating a substantially magnetic field-free condition (or a condition of appropriately negligible magnetic field) in a large volume relative to the system size. A feedback sensor can control the magnetic field generating elements (e.g., by controlling the current in the coils) such that the magnetic field in the nulling space is reduced by orders of magnitude. The invention can be applied to nulling regions with complex magnetic field gradients. Thus, the invention can be applied to avoid the need to use magnetically shielded rooms (MSRs) during use of magnetoencephalography (MEG) and / or to provide additional active nulling. The relative volume of the nulling space to the volumetric space enclosed by the array of magnetic field generating elements can be increased compared to other methods that utilize MSRs or Helmholtz coils. The magnetic field in the nulling region can be reduced to very low magnetic field levels with high magnetic field homogeneity. It has been found that for very low magnetic field levels with high magnetic field homogeneity, further improvements can be obtained simply by increasing the number of magnetic field generating elements in the array.

[0016] Preferably, the feedback control unit comprises an input terminal (e.g. for receiving a feedback signal) connected to each output terminal (e.g. for outputting a magnetic field measurement signal) of the magnetic field sensing element, and one or more signal output terminals (e.g. for outputting a control signal or a drive current) connected to an input terminal of the associated magnetic field generating element. For example, the one or more output terminals of the feedback control unit can be directly connected to each input terminal of the associated magnetic field generating element. For example, each input terminal of one associated magnetic field generating element can be directly connected to one associated (e.g. dedicated) output terminal of the feedback control unit, for example such that the magnetic field generating element is the only magnetic field generating element connected to that particular output terminal of the feedback control unit. Through this connection between a given output terminal of the feedback control unit and an input terminal of one associated magnetic field generating element, a respective current can be transmitted that is directly used by the associated magnetic field generating element in generating a magnetic field required to reduce the value of the magnetic field detected by each magnetic field sensing element. Alternatively, the one or more output terminals of the feedback control unit may be indirectly connected to the input terminals of each of the associated magnetic field generating elements via (one or more) intermediate current supply units configured to receive a current supply control signal from the feedback control unit and to supply a specified current to the (or each) specified magnetic field generating element in response to receiving the current control signal, the specified current and the specified magnetic field generating element being specified in the current supply control signal.

[0017] Preferably, the feedback control unit is configured to analyze each signal from each magnetic field sensing element to determine how to adjust or regulate the supply current to the associated magnetic field generating element in a direction that reduces the sensed magnetic field reading from the magnetic field sensing element.

[0018] The feedback control is preferably configured to continuously monitor (e.g. continuously for a given duration or intermittently) the magnetic field values ​​provided by the magnetic field sensing elements in the nulling volume and to continuously control the supply currents to the respective corrective magnetic field generating elements, thereby providing an active magnetic field zeroing.

[0019] Preferably, the plurality of magnetic field generating elements comprises at least 10 distinct magnetic field generating elements, more preferably at least 50 distinct magnetic field generating elements, even more preferably at least 200 distinct magnetic field generating elements. Preferably, the plurality of magnetic field generating elements comprises at least about 100 elements, but not more than about 4000 elements, or not more than about 3000 elements, or not more than about 2000 elements.

[0020] Preferably, the magnetic field generating element comprises a conductive coil adapted to transmit the electric current. Each magnetic field generating element may comprise a conductive coil. A coil of a magnetic field generating element may comprise a circular coil or a coil of another shape, for example a polygonal coil (such as a hexagon). A coil of a magnetic field generating element may be substantially flat or planar, with all the turns of the coil lying substantially in the same plane. The diameter of each coil of the array of magnetic field generating elements (e.g. the diameter of the coil loop or turn) may be substantially the same as the diameter of the other coils of the array of magnetic field generating elements, or may be the same as the diameter of every other coil of the array of magnetic field generating elements. In the case of a Dual Geodesic Icosahedron lattice, the coils located at the hexagonal parts / facets of the lattice may be larger than the coils located at the pentagonal parts / facets of the lattice. Preferably, the array of magnetic field generating elements may include a mix of coils with different diameters. This is especially true when using a geodesic grid, which helps to achieve better grid coverage. If the grid that defines the arrangement of the magnetic field generating coils is based on Platonic solids, good grid coverage can be achieved by using coils with the same diameter.

[0021] Alternatively, a coil of a magnetic field generating element may be configured with its turns arranged in a stack (e.g., helical spiral) such that all turns of the coil are substantially planar and plane-parallel, but successive turns of the coil are vertically dispersed in a direction perpendicular to the plane of the turns. The degree of vertical dispersion is preferably small compared to the extent of the lateral dimension of each coil. In other words, each coil is preferably "thin" to save space. The lateral dimension (e.g., diameter) of the coil turns is at least 4 times (e.g., 4 times width compared to thickness) larger than the longitudinal dimension of the coil perpendicular to the lateral dimension, more preferably at least 5 times, even more preferably at least 7 times, and even more preferably at least 10 times.

[0022] A coil may comprise a single winding or loop of a conductive wire, track or strip, or multiple windings or loops of a conductive wire, track or strip. A coil may comprise a current input terminal, a current output terminal, and one or more windings or loops electrically connecting the two terminals, such that a current input via the input terminal flows through the windings or loops to the current output terminal. The multiple windings or loops of a coil may share substantially the same shape, size and orientation with each other by repeating a common loop or winding shape. Alternatively, the multiple windings or loops of a coil may define a spiral shape with a gradually increasing or decreasing spiral radius (e.g. a flat spiral).

[0023] Preferably, each coil is configured to point towards the centre or centre of gravity of the array of magnetic field generating elements. The direction in which a coil "points" can be considered as the direction perpendicular to a plane containing the diameter of the coil. The direction in which a coil "points" can be considered as the direction parallel to the winding axis of the coil (i.e. the central axis around which the windings of the coil are wound, e.g. the axis of symmetry of the coil). Thus, the direction in which a coil "points" can be defined as the direction parallel to the magnetic field that is generated through the centre of the coil when driven with a current. The direction of that magnetic field can be controlled by controlling the flow direction of a given current through the coil. The direction of the current through a coil can be defined, for example, by whether the current flows clockwise or counterclockwise when the coil is viewed in a direction towards the centre of the array of magnetic field generating elements. For example, a given current in a given coil can be defined as "positive" when viewed from one side of the coil and "negative" when viewed from the other side (i.e., reversing the viewing direction) or when the coil is rotated 180 degrees to present the opposite side to a stationary viewer (i.e., flipping the "face" of the coil).

[0024] Most preferably, the orientation of each coil in the array of separate magnetic field generating elements is different from the other coils in the array depending on the position of that coil in the array. In other words, the position of a coil in the array also determines the orientation of that coil (i.e. the direction in which it "points") relative to the center of gravity of the array. Most preferably, the direction of the current used to drive an individual coil is different from the other coils in the array depending on the position of that coil in the array. In other words, the position of a coil in the array also determines the direction of the current passed through that coil. The direction of the current in a coil can be defined relative to the local coordinate system of that coil. An individual coil local coordinate system can be arranged to be in a rotated orientation or position relative to the coil local coordinate system of the coils adjacent to it (or all coils) in the array of coils.

[0025] Preferably, each coil of the array of magnetic field generating elements is electrically isolated (e.g., insulated) from every other coil of the array of magnetic field generating elements. This allows each coil to be driven independently of every other coil, allowing great flexibility in controlling the array of magnetic field generating elements to achieve nulling. Adjacent coils in the array of magnetic field generating elements may at least partially overlap, or may be arranged such that no coil in the array overlaps with any other adjacent coil in the array.

[0026] Optionally, the coil group of the array of magnetic field generating elements may comprise a number of subgroups each comprising three coils pointing in three mutually perpendicular directions. For example, the directions in which each coil of a subgroup points may correspond to three local orthogonal axes (i.e. x, y and z coordinate directions centered in the middle of the subgroup). Preferably, the direction in which any coil of the subgroup points is substantially perpendicular to the direction in which each of the other coils of the subgroup points, such that no two coils of the subgroup point in the same direction. This arrangement of the subgroup coils allows additional control over the vector direction of a portion of the local magnetic field in and around the area occupied by each subgroup coil group. One coil of each subgroup coil group may be configured to point towards the center or centroid of the array of magnetic field generating elements.

[0027] Preferably, the plurality of magnetic field generating elements are arranged at each of the distinct locations in a first array formed along a three-dimensional reference plane that surrounds the zeroed region. Preferably, the plurality of magnetic field sensing elements are arranged at each of the distinct locations that define a second array formed along a three-dimensional reference plane. It should be understood that the reference plane may or may not be a physical surface. If the reference plane is a physical surface (i.e. it coincides with a physical surface), the surface may provide the function of a support surface on which the magnetic field generating elements are arranged (e.g. attached) to constrain and hold the positions of the magnetic field generating elements to follow a desired array pattern. The support surface may be a continuous surface or a framework, in which case the magnetic field generating elements are attached at locations on the framework that coincide with the locations of the conceptual reference plane. For example, the support surface may be a spherical shell surface that coincides with a conceptual spherical reference plane. Alternatively, the support surface may comprise a three-dimensional polyhedral framework with vertices that coincide with points on the surface of a sphere. Magnetic field generating elements may be attached to each vertex or each group of vertices of this support surface, or to edges of the framework extending between the vertices (eg edges of polygons that form the conceptual facets of a polyhedron).

[0028] The support surface (e.g., a continuous surface or framework) may be mounted on a rotating support assembly that allows rotation of the support surface about at least one spatial axis, or at least two orthogonal spatial axes, or all three orthogonal spatial axes. The rotating support assembly may include an Alt-Az mount that allows independent adjustment of the azimuth and altitude pointing positions of the support surface. This allows the support surface (e.g., framework) (along with any field generating coils and array of field sensing elements thereon) to be oriented against the direction of the Earth's magnetic field vector, which may assist in achieving better nulling.

[0029] Preferably, the second array (i.e. the array defined by the plurality of magnetic field sensing elements) is arranged to be substantially concentric with the first array (i.e. the array of magnetic field generating elements). The inventors have found that this arrangement allows for particularly accurate and effective zeroing within the nulling region. The distinct locations in the first array may be defined according to a regular grid. The locations may coincide with (or be defined by) notional vertices or facets of a polyhedron. The distinct locations in the second array may be defined according to a regular grid. Preferably, the plurality of magnetic field generating elements are arranged at the distinct locations substantially equidistant from the center of the nulling region, thereby defining a substantially spherical array surrounding the nulling region.

[0030] Preferably, the plurality of magnetic field sensing elements are positioned at each of the distinct locations substantially equidistant from a centre of the nulling region, thereby defining a substantially spherical array within the nulling region.

[0031] Preferably, the diametric dimension of the array of magnetic field sensing elements is at least about 40% of the diametric dimension of the array of magnetic field generating elements. The inventors have found that when this condition is applied, particularly effective magnetic field nulling can be achieved. Preferably, the diametric dimension of the array of magnetic field sensing elements is between about 40% and about 90% of the diametric dimension of the array of magnetic field generating elements. More preferably, the diametric dimension of the array of magnetic field sensing elements is between about 40% and about 80% of the diametric dimension of the array of magnetic field generating elements. Preferably, the diametric dimension of the array of magnetic field sensing elements is between about 40% and about 90% of the diametric dimension of the array of magnetic field generating elements, and the array of magnetic field generating elements comprises at least about 200 elements and not more than about 2000 elements. The inventors have found that when one or more of these conditions are applied, particularly effective magnetic field nulling can be achieved.

[0032] Preferably, the threshold is 5×10 -9 Less than Tesla, or more preferably 5×10 -10 Less than Tesla.

[0033] The magnetic field sensing element may comprise one or more of a Hall effect sensor (e.g. for μT magnetic fields), a magneto-impedance sensor (e.g. to cover the range μT to nT), a fluxgate sensor (e.g. to cover the range μT to nT), an Optically Pumped Magnetometer (OPM) sensor (e.g. to cover the range nT to fT). Three such sensors can be used as a sensor feedback group with a wide sensing range.

[0034] The number of magnetic field sensing elements may be less than the number of magnetic field generating elements. The feedback controller may be configured to generate magnetic field values ​​associated with one or more locations within the nulling region by interpolation between magnetic field values ​​sensed by magnetic field sensing elements of the array of magnetic field sensing elements. The controller may be configured to define a notional interpolation sphere determined by a radius from a target point (coordinate) within the nulling region between which magnetic field values ​​are to be interpolated. The controller may be configured to determine which of the magnetic field sensing elements are located within the notional sphere (and increase the radius of the sphere until at least two sensors are within the sphere). The controller may be configured to calculate an interpolated value of the magnetic field at the target location by three-dimensional interpolation of magnetic field values ​​received from the sensors within the interpolation sphere.

[0035] The feedback control may be configured to generate a set of basis vectors used in controlling the value of each nulling magnetic field generated by each of the plurality of magnetic field generating elements. The feedback control may be configured to use the set of basis vectors in calculating each current used to drive the magnetic field generating elements of the device. The feedback control may be configured to supply current to each magnetic field generating element in turn, while all other magnetic field generating elements do not receive current from the control. The feedback control may be configured to obtain measurements of the magnetic field in the device sensed by each of the magnetic field sensing elements at each fixed location in the nulling region from the magnetic field sensing elements. The feedback control may be configured to use these measured magnetic field values ​​as calibration values ​​to determine basis vectors corresponding to the magnetic field generating elements supplied with current. The control may be configured to repeat this process separately for each magnetic field generating element of the array of coils, so that each magnetic field generating element is separately responsible for being a single magnetic field generating element supplied with current. For each element of the array of m magnetic field generating elements, the controller may be configured to generate a respective basis vector based on the calibration magnetic field values ​​received from all n sensors. For example, the basis vector corresponding to the first coil may be:

number

[0036] The m coil array, the feedback control unit, and a basis vector matrix constructed using each basis vector corresponding to all of the m magnetic field generating elements

number

[0037] The feedback controller can be designed to perform active cancellation of the ambient magnetic field by calculating the value and direction of the drive current to be supplied to each magnetic field sensing element based on these unit vectors. The feedback controller calculates each measurement of the magnetic field from one magnetic field sensing element with all sensing elements then operating in the active cancellation function.

number

number

number

number

[0038] The feedback controller may adjust α in the above equation to reduce the values ​​of the magnetic field measured by the magnetic field sensing elements collectively (e.g., as an average therebetween) or individually so that they do not exceed a desired pre-set threshold corresponding to an appropriate level of magnetic field cancellation / nulling. j It is possible to change the value of the current applied to each magnetic field generating element by changing the value of

[0039] The feedback control unit adjusts the following optimization equation with a parameter α j , which determines the current α that must be used to drive each magnetic field generating element. j It can be designed to calculate the value of

number

[0040] The orthogonal projection algorithm finds a suitable "optimum" drive current for the field nulling / cancellation current by applying the above equation with the parameter α j Note that the optimization may be performed directly as a result of solving (e.g., inverting) j It should be understood that this refers to calculating the value of the parameter α. This is not the same as the prior art "optimization" techniques involved in the much less efficient iterative field minimization techniques, which often require multiple iterations. In other words, the orthogonal projection technique discussed in this application does not require a large number of iterations to be performed. j can be considered to immediately obtain what is needed, without any need for subsequent "iterative" calculations to find "better" values ​​for these parameters.

[0041] The feedback control section detects the reading V from the magnetic sensing element. k These readings can be placed in a reading matrix by the controller (e.g., [V1, V2, V3, ..., V N ]). The magnetic field measurements may include measurements of three orthogonal field components (Sx, Sy, Sz) by each of the magnetic sensing elements. The feedback controller may be designed to calculate a dot product of the reading matrix and the orthogonal basis vectors of the magnetic field generating elements as follows:

number

number

[0042] In this way, multiple (e.g., three) sensor readings can be generated for each sensor location, corresponding to multiple orthogonal magnetic field directions (e.g., x, y, z). The elements of the orthogonal basis vectors are related to the magnetic field components generated by the magnetic field generating elements (e.g., in the three orthogonal directions x, y, z) as follows:

number

[0043] The feedback controller can be designed to invert the optimization equation using the resulting dot products, and the controller can calculate individual values ​​of α corresponding to the currents used to individually drive the magnetic field generating elements. j This has proven to be a particularly time-efficient method of processing.

[0044] Usually, only a few iterations (typically only one) are required to obtain the best current solution for the nulling / cancellation of the external magnetic field. The orthogonal basis for the orthogonal projection method is preferably generated in the calibration process. The orthogonal basis only needs to be generated once and the controller can then use it for all active nulling / cancellation using sensor data received from the magnetic field sensor array during the active nulling / cancellation operation. The calibration process (i.e., determining the calibration magnetic field, generating the orthogonal basis) can be performed (e.g., once) in an environment that is substantially free of environmental or ambient magnetic fields (i.e., in a magnetically shielded room (MSR)). This can be performed (i.e., recalibration) when the device is manufactured or periodically throughout the life of the device as needed and appropriate. Once calibrated, the device can then be used outside the magnetically shielded room (MSR).

[0045] Alternatively, the calibration can be performed in a standard environment (i.e., not within the MSR) that includes magnetic field gradients from the Earth's magnetism and the local environment (e.g., electrical components, electronics, etc.) To perform the calibration in such an environment, the device can be designed as follows:

[0046] The feedback control unit is (a) when no drive current is supplied to any of the magnetic field generating elements (i.e., the measured value corresponds to the value of the environmental / geomagnetic field), or (b) when a preset drive current is supplied to each of the magnetic field generating elements; obtaining a correction value corresponding to a measurement of a magnetic field in the device sensed by each of the magnetic field sensing elements at each fixed location in the nulling region; and subtracting said correction value from the calibration value generated by supplying current to each magnetic field generating element in turn alone (as described above) while all other magnetic field generating elements receive no current from said control unit; The set of basis vectors can be generated by:

[0047] This results in corrected calibration values ​​that are used to define the basis vectors. In this way, the readings of the magnetic field sensing elements (feedback sensors) can be effectively set to zero during the calibration process (i.e., the readings are zeroed with respect to the internal magnetic field).

[0048] The feedback control unit can be configured to stop subtracting the correction value from the measurements generated by each magnetic field generating element after (i.e. immediately after) the calibration process is completed. This means that the current readings of the magnetic field generating elements are returned to their true readings (i.e. the "zeroing" is not performed), after which the orthogonal projection process may be performed. As a result, the basis vectors of the orthogonal projection are inherently subtracted from the internal magnetic field present under the conditions (a) or (b). The experimental results shown in Figures 27(a), (b) and (c) were generated as a result of the calibration performed as described above without the MSR.

[0049] The predetermined drive current (mentioned in point (b) above) may be the same for each of the magnetic field generating elements or may be different between each different magnetic field generating element. The advantage of a predetermined drive current is that it generates a magnetic field around the magnetic field sensing element that can partially null the environmental / earth magnetic field to a degree sufficient to reduce the strength of the magnetic field to a value that is included in the dynamic range of the magnetic field sensing element. It should be noted that for some magnetic field sensing elements, the typical strength of the environmental / earth magnetic field far exceeds the dynamic range of the element and the element would be saturated without the use of a predetermined drive current. This allows the generation (calibration) of basis vectors and the subsequent orthogonal projection to proceed using magnetic field generating elements with any desired dynamic range.

[0050] Thus, although it may be advantageous to calibrate the device in an MSR if one is available and convenient, the present invention can provide a means of performing calibration in the absence of an MRS. In fact, this also means that calibration can be performed every time the device is used. The advantage of this is that mechanical changes in the positioning of the magnetic field generating elements and / or magnetic field sensing elements are eliminated. Also, temperature changes in the device may affect the performance of the magnetic field sensing elements, but these changes will also be taken into account by the calibration process.

[0051] In a second aspect, the present invention provides a method for zeroing a magnetic field in a nulling region in an ambient (e.g. external) magnetic field, comprising: providing a plurality of separate magnetic field generating elements disposed at respective separate locations surrounding the nulling region for generating respective nulling magnetic fields extending into the nulling region; providing a plurality of magnetic field sensing elements disposed at a plurality of respective distinct locations within said nulling region for sensing respective values ​​of the magnetic field within said nulling region; controlling, in response to values ​​of the magnetic field sensed by the plurality of magnetic field sensing elements, a value of each of the nulling magnetic fields generated by each of the plurality of magnetic field generating elements by driving the magnetic field generating elements with a respective current that reduces the value of the magnetic field detected by each of the magnetic field sensing elements to a value not exceeding a preset threshold corresponding to a preset nulling of the magnetic field within the nulling region. A method comprising:

[0052] Preferably, in accordance with the present invention, the magnetic field generating elements comprise conductive coils and the method comprises transmitting the electrical current through each of the coils.

[0053] The method may include providing the plurality of magnetic field generating elements as arranged at each of the distinct locations in a first array formed along a three-dimensional reference plane that circumscribes the nulling region.

[0054] The invention can include providing the plurality of magnetic field sensing elements as arranged at each of the distinct locations defining a second array formed along a three-dimensional reference plane.

[0055] The method can include providing the second array substantially concentric with the first array.

[0056] Preferably, in the method, each of said distinct locations in said first array is defined according to a regular grid. Preferably, in the method, each of said distinct locations in said second array is defined according to a regular grid.

[0057] The method may include providing the plurality of magnetic field generating elements arranged at each of the distinct locations substantially equidistant from a center of the nulling region, thereby defining a substantially spherical array surrounding the nulling region.

[0058] The method may include providing the plurality of magnetic field sensing elements disposed at each of the distinct locations approximately equidistant from a center of the nulling region, thereby defining a generally spherical array within the nulling region.

[0059] Preferably, in this method, the diametric dimension of the array of magnetic field sensing elements is at least 40% of the diametric dimension of the array of magnetic field generating elements.

[0060] The method may include providing the plurality of magnetic field generating elements to comprise at least 10 distinct magnetic field generating elements, more preferably at least 50 distinct magnetic field generating elements, and even more preferably at least 200 distinct magnetic field generating elements.

[0061] One method may include receiving an input in a feedback control unit corresponding to an output from the magnetic field sensing element, and providing an output from the feedback control unit including a control signal that is input to an associated magnetic field generating element.

[0062] Preferably, the threshold is 5×10 -9 Less than Tesla, or more preferably 5×10 -10 Less than Tesla.

[0063] The method may comprise configuring to generate a magnetic field value associated with one or more locations within the nulling region by interpolation between magnetic field values ​​sensed by magnetic field sensing elements of the array of magnetic field sensing elements. The method may include defining a notional interpolation sphere defined by a radius from a target point (coordinate) within the nulling region between which magnetic field values ​​are to be interpolated. The method may include determining which of the magnetic field sensing elements are located within the notional sphere (and increasing the radius of the sphere until at least two sensors are within the sphere). The method may include calculating an interpolated value of the magnetic field at the target location by three-dimensional interpolation of magnetic field values ​​sensed by the sensors within the interpolation sphere.

[0064] In another aspect, the present invention may provide a magnetoencephalography (MEG) device comprising one or more magnetoencephalography (MEG) sensors configured to be placed around a patient's head and a device according to the first aspect of the present invention for nulling a magnetic field within a nulling area dimensioned to accommodate the patient's head. The device according to the first aspect of the present invention may be configured to cancel ambient magnetic fields (e.g. magnetic fields arising from the earth's magnetic field and surrounding objects) within the nulling area including the magnetic field sensing elements of the device and the patient's head. In a further aspect, the present invention may provide a medical imaging device comprising the above-mentioned MEG device, and / or a brain activity mapping device comprising the above-mentioned MEG device, and / or a biomagnetic sensing device comprising the above-mentioned MEG device, and / or a neurofeedback device comprising the above-mentioned MEG device, and / or a brain-computer interface device comprising the above-mentioned MEG device.

[0065] For example, when used in MEG applications, magnetic field nulling using the proposed invention can be performed within an MSR. This method can be used to cancel residual magnetic fields (e.g., emanating from the device) within the MSR. When used in MEG applications or other medical applications, the array of magnetic field generating elements can be sized and configured to enclose a volume of space (e.g., a volumetric volume) sufficient to accommodate the entire patient or a body part of the patient other than the head. The array of magnetic field sensing elements can be similarly sized and configured.

[0066] The present invention, in any of its aspects, can provide shielding for magnetocardiography (MCG), magnetomyography (MMG), or magnetoneurography (MNG). The present invention, in any of its aspects, can provide shielding for nerve signal propagation in the lumbar spine for magnetospinography (MSG). The present invention, in any of its aspects, can provide shielding for cervical spinal cord evoked field (SCEF) measurements. The present invention, in any of its aspects, can provide shielding for magnetoencephalography-based brain-computer interfaces (BCI).

[0067] In some of these applications, shielding using the proposed invention can be produced covering the entire human or animal body or different body parts.

[0068] In yet another aspect, the present invention may provide a method of magnetoencephalography (MEG) comprising positioning one or more magnetoencephalography (MEG) sensors around a patient's head and zeroing a magnetic field within a nulling region dimensioned to accommodate the patient's head in accordance with the second aspect of the invention.

[0069] Combinations of the described aspects and preferred features are encompassed by the invention unless such combinations are expressly impermissible or explicitly avoided. [Brief description of the drawings]

[0070] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments and experiments illustrating the principles of the present invention are discussed below with reference to the accompanying drawings.

[0071] [Figure 1] Diagram showing a schematic representation of the diamagnetic effect and the principle of Stokes' theorem in two dimensions. [Diagram 2] (a) and (b) Schematic showing an element of a magnetic field nulling device and a cross-section of the device, respectively. [Diagram 3] (a) and (b) Schematic showing an element of a magnetic field nulling device and a cross-section of the device, respectively. [Figure 4] FIG. 2 shows elements of a magnetic field nulling device in a cross-sectional view of the device. [Diagram 5] (a) and (b) Schematic showing an element of a magnetic field nulling device and a cross-section of the device, respectively. [Figure 6] 1A and 1B show, respectively, magnetic field lines representing the ambient external magnetic field and the nulling region produced by a magnetic field nulling device, and a schematic representation of the diamagnetic effect. [Figure 7] FIG. 2 shows a magnetic field strength space plot illustrating the ambient external magnetic field and the nulling region produced by a magnetic field nulling device. [Figure 8] Illustrates (a), (b), (c) and (d) the magnetic field strength within the nulling region produced by the magnetic field nulling device. [Figure 9] A diagram showing an array of magnetic field generating elements of a magnetic field zeroing device and zeroed regions within the array, along with a plot of the current values ​​applied to the magnetic field generating elements of the magnetic field zeroing device to achieve the zeroed regions. [Figure 10] A diagram showing an array of magnetic field generating elements of a magnetic field zeroing device and zeroed regions within the array, along with a plot of the current values ​​applied to the magnetic field generating elements of the magnetic field zeroing device to achieve the zeroed regions. [Figure 11] 4(a)-(d) are plots of current values ​​applied to a magnetic field generating element of a magnetic field zeroing device to achieve a zeroed region. [Figure 12] FIG. 2 shows a plot of the magnetic field strength and magnetic field gradient produced by a magnetic field zeroing device within the zeroing region of the device as a function of the number of magnetic field generating elements provided within the magnetic field zeroing device. [Figure 13] (a)-(d) Plots of the average and maximum magnetic field strengths and the average and maximum magnetic field gradients generated by a magnetic field zeroing device within the zeroing region of the device as a function of the number of magnetic field generating elements provided in the magnetic field zeroing device for different lattice types. [Figure 14](a), (b) Cross-sectional views of an array of magnetic field generating elements of a magnetic field zeroing device, each comprising one of two different arrays of magnetic field sensing elements of the magnetic field zeroing device. [Figure 15] (a) A plot of the magnetic field gradient achieved within the zeroing region of a spherical array of magnetic field generating elements of a magnetic field zeroing device as a function of the diameter of the concentric spherical array of magnetic field sensing elements within the array of magnetic field generating elements, and (b) a plot of the optimal diameter of the concentric spherical array of magnetic field sensing elements as a function of the number of magnetic field generating elements within the spherical array of magnetic field generating elements to which Figure 16(a) pertains. [Figure 16] (a)-(d) Plots of the average and maximum magnetic field strengths and the average and maximum magnetic field gradients generated by a magnetic field zeroing device within the zeroing region of the device as a function of the diameter of a concentric spherical array of magnetic field sensing elements within the array of magnetic field generating elements of the magnetic field zeroing device. [Figure 17] FIG. 16(b) shows a plot of the optimum diameter of the concentric spherical array of magnetic field sensing elements as a function of the number of magnetic field generating elements in the spherical array of magnetic field generating elements to which FIGS. 16(a)-(d) relate. [Figure 18] FIG. 13 illustrates a schematic process for calculating currents to be applied to an array of magnetic field generating elements using measurements from an array of magnetic field sensing elements. [Figure 19] FIG. 2 shows a schematic diagram of components of a field nulling device. [Figure 20] FIG. 2 is a schematic diagram illustrating a process for controlling components of a field zeroing device. [Figure 21] 19(a)-(c) show field nulling plots produced by the process of FIG. 18. [Figure 22] (a), (b) Schematic images of a virtual magnetic field sensing element array generated using a sub-array of real magnetic field sensing elements of a magnetic field zeroing device. [Figure 23] 1(a), (b) and (c) show magnetic field zeroing devices with different numbers of magnetic field generating elements, along with cross-sectional views of the magnetic field within the nulling region of the device. [Figure 24] FIG. 1 shows a magnetic field zeroing device. [Diagram 25] FIG. 1 shows a magnetic field zeroing device. [Figure 26] 25 is a close-up view of a portion of the field nulling apparatus of FIG. 24. [Figure 27] 27(a), (b) and (c) are graphical illustrations of some examples of sensor signals generated by the sensors of the magnetic field zeroing device of FIG. 26. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] BRIEF DESCRIPTION OF THE DRAWINGS Aspects and embodiments of the present invention are discussed below with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this document are incorporated herein by reference.

[0073] FIG. 2(a) shows an apparatus for nulling a magnetic field in a nulling region in an external ambient magnetic field. FIG. 2(b) shows the apparatus in cross section. The apparatus comprises a plurality of separate magnetic field generating coil elements 8, each comprising a circular loop of common diameter and arranged at a respective separate location on a first notional spherical shell reference plane surrounding the nulling region to generate a respective nulling magnetic field extending into the nulling region. Each magnetic field generating coil element 8 comprises a current input terminal (not shown) and a current output terminal (not shown) for inputting and outputting a driving current to the coil.

[0074] A plurality of magnetic field sensing elements 6, e.g., OPM sensors, are disposed at a plurality of respective distinct locations within the nulling region for sensing respective values ​​of the magnetic field within the nulling region. The magnetic field sensing elements are also disposed at respective distinct locations on a second notional spherical shell reference surface surrounding the nulling region. The first and second notional spherical shell reference surfaces are concentric, and the diameter of the first reference spherical shell surface is approximately 2.5 times the diameter of the second reference spherical shell surface.

[0075] The patient's head 2 is positioned within the nulling region in the spherical shell array of magnetic field sensing elements 6, coinciding with the centres of the first and second reference spherical shells.

[0076] A feedback control unit (not shown, see reference numeral 150 in FIG. 19) is designed to control the value of each zeroing magnetic field generated by each of the plurality of magnetic field generating elements 8, depending on the value of the magnetic field sensed by the plurality of magnetic field sensing elements 6, by driving the magnetic field generating elements with respective currents that reduce the value of the magnetic field detected by each magnetic field sensing element to a value not exceeding a preset threshold corresponding to a preset zeroing of the magnetic field in the zeroing region.

[0077] Each magnetic field generating coil 8 is configured to point towards the centre or centre of gravity of the array of magnetic field generating elements. In other words, each coil "points" in a direction perpendicular to a plane containing the diameter of the coil. The direction in which a coil "points" is parallel to the winding axis of the coil (i.e. the central axis around which the coil's windings are wound, e.g. the coil's axis of symmetry). Thus, the direction in which a coil "points" defines a parallel orientation to the magnetic field that is generated through the centre of the coil when driven with a current. The direction of that magnetic field is controlled by controlling the flow direction of a given current through the coil.

[0078] Each coil 8 of the array of magnetic field generating elements is electrically isolated and insulated from every other coil in the array of magnetic field generating elements, and each coil is driven independently of every other coil. Adjacent coils in the array of magnetic field generating elements are arranged such that no coil in the array overlaps with any other adjacent coil in the array. In other examples, adjacent coils may at least partially overlap.

[0079] In the figure, the patient 2 is wearing an MEG sensor cap 4, in this case an OPM sensor covering the patient's head. The magnetic field sensors 6 are arranged around the MEG sensor cap 4 at a safe distance. The patient's brain magnetic field is not visible to the magnetic field sensors 6. The magnetic field sensors 6 and the coils 8 each form a spherical grid with an opening at the bottom to accommodate the patient's head 2. It should be noted that the invention is flexible in the sense that the extent of the grid arrangement of the magnetic field generating elements does not have to cover the entire spherical shell reference surface shape, and various conceptual reference surface shapes can be used. For example, the conceptual reference surface may be spheroidal or cylindrical. Since the cancellation / nulling is achieved by electronic control of the current in the magnetic field generating elements 8, various arrangements of the magnetic field generating elements can be used. This allows for a flexible shaping of the arrangement of the magnetic field generating elements.

[0080] 3(a) and (b) show diagrams of a coil array with an opening 11 for receiving a patient's head 2 (e.g., with a magnetoencephalography cap 4 with an OPM sensor). The opening 11 is defined by a large auxiliary coil opening that helps achieve better cancellation by weakening the magnetic flux around the opening. The coil array can perform without such an auxiliary coil. Note that the field generating coil array includes coil elements with different coil diameters, from a maximum coil diameter 18a to a minimum coil diameter 18b. Using coils of various diameters allows the coils to be packed more tightly over the notional reference spherical shell surface.

[0081] Figure 4 is a cross-sectional view of the example of Figure 3(b), showing a surface 22 (generated by calculation / simulation) that bounds a volumetric volume within which all points have magnetic field values ​​less than 5 nT. Thus, a magnetic field strength of 5 nT exists around the OPM sensor 6 and is within the dynamic range of the sensor.

[0082] A more complex version of the invention is presented in Fig. 5(a) and (b). Fig. 5(b) is a cross-sectional view of Fig. 5(a), where the patient 2 is wearing the MEG sensor cap. The magnetic field sensing elements 212 (e.g. OPM sensors) are placed at a safe distance so as not to detect the patient's brain magnetic fields. The number of these sensors is reduced in this example because they are associated with the current control of a larger group of magnetic field generating elements 216 and 214. In particular, the array of magnetic field generating elements (coils) comprises two sets of concentric subarrays of magnetic field generating elements, of which the inner subarray of magnetic field generating elements 214 is concentric with and surrounded by the outer subarray of magnetic field generating elements 216. Both the inner and outer subarrays are mounted on the surface of a physical spherical shell support surface 218a, 218b, respectively. Both the inner and outer subarrays comprise magnetic field generating elements regularly arranged on their respective notional spherical shell reference surfaces. Here, the coils of each subarray of magnetic field generating elements 214, 216 comprise a number of subgroups 210. Each subgroup within each subarray comprises three coils pointing in three mutually perpendicular directions. The directions of each coil of a subgroup correspond to three local orthogonal axes (i.e., x, y and z coordinate directions centered at the middle of the subgroup). The direction of any coil in a subgroup is generally perpendicular to the direction of each of the other coils in the subgroup, so that no two coils in the subgroup point in the same direction. This arrangement of the coils in each subgroup can be used to control the vector direction of a portion of the local magnetic field in and around the area occupied by the coils of a given subgroup. One coil in each subgroup is configured to point toward the center or center of gravity of the subarray of magnetic field generating elements of which it is a part.

[0083] FIG. 6(b) shows the external magnetic field 1a bypassing but not penetrating the diamagnetic material 1b, discussed above with reference to FIG. 1. For comparison, FIG. 6(a) shows a cross-sectional view of an external magnetic field 40 (generated by calculation / simulation) bypassing an array of magnetic field generating coils according to an embodiment of the invention. An exploded view of the magnetic field generated by two magnetic field generating coils of a coil array is shown, in which the two coils are diametrically opposed across the center of gravity of the coil array. The upper of the two selected coils is at positive y coordinates and has a local coordinate system with unit vector (a1, b1), while the lower coil is at positive y coordinates and has a local coordinate system with unit vector (a2, b2). It can be seen that the local coordinate system of the lower coil corresponds to the local coordinate system of the upper coil rotated 180 degrees around the z axis, with a2=-a1 and b2=-b1. The direction of the current is indicated for each coil by a symbol (X) (41, 43) indicating a current in the negative z direction (i.e., into the plane of the page) and a symbol (+) (42, 44) indicating a current in the positive z direction (i.e., out of the plane of the page). It can be seen that the current directions closest to the coordinate origin in each of the two coil local coordinate systems are opposite, indicating that each current has an opposite sign.

[0084] A three-dimensional visualization (generated by calculation / simulation) of a similar coil array zeroing operation according to an embodiment of the present invention is shown in FIG. 7. In FIG. 7, the surface height represents the magnetic flux density. A coil array with 252 coils and a diameter of 1 m was used in FIG. 7. A uniform magnetic field of 50 μT was cancelled in the coils to a negligible value. The magnetic field vector was parallel to the Y-axis. In both FIG. 6(a) and FIG. 7, a cross section was taken through the center of the coil array and includes the xy plane at z=0. The zeroed region 53 is clearly visible. One of several depressions along the periphery of the zeroed region, "depression" 51, corresponds to the location on the surface where the magnetic field generating coils are located. This shows how large the zeroed region is relative to the diameter of the coil array. The ambient external magnetic field 50 is shown at the surface with an undisturbed magnetic field strength of 50 μT. The magnetic field dips 52 aligned along the y-axis on each side of the coil array are a response to canceling the effect of a uniform magnetic field also oriented in the positive y-axis direction.

[0085] 8(a), (b), (c) and (d) show examples (generated by calculation / simulation) of the gradient cancellation performance of an embodiment of the present invention. The greyscales of FIG. 8(a) and (c) are the greyscales of FIG. 8(b) and (d) multiplied by 10,000, respectively. The result of FIG. 8(a) corresponds to a peripheral external magnetic field gradient source with the magnetic field pointing in the positive direction of the x-axis of FIG. 8(a). FIG. 8(b) shows the result of nulling / cancelling the magnetic field of FIG. 8(a). The absence of white indicates that the magnetic field from the source has been reduced by a factor of less than 10,000. The result of FIG. 8(c) corresponds to a peripheral external magnetic field gradient source with the magnetic field pointing in the positive direction of the y-axis of FIG. 8(c). FIG. 8(d) shows the result of nulling / cancelling the magnetic field of FIG. 8(c). The absence of white indicates that the magnetic field from the source has been reduced by a factor of less than 10,000.

[0086] FIG. 9 shows a magnetic field generating coil array 60 in which the locations of the coils form a Fibonacci lattice. The array has 252 coils in an array with a diameter of 1 m. The three-dimensional geometry 61 (generated by calculation / simulation) within the coil array shows regions of magnetic field less than 5 nT, providing nulling / cancellation against a uniform external ambient magnetic field (e.g. the Earth's magnetic field) of 50 μT oriented along the y-axis running from left to right across the figure. The magnitude and relative direction (positive / negative) of the drive current 62 applied to each coil in the coil array is shown versus the y-coordinate value of each coil, where the y-axis runs from left to right across the figure, with the coordinate origin corresponding to the center of the coil array.

[0087] FIG. 10 shows a magnetic field generating coil array 70 in which the locations of the coils form a double geodesic icosahedral lattice. The array has 252 coils in an array with a diameter of 1 m. The three-dimensional geometry 71 (generated by calculation / simulation) within the coil array shows regions of magnetic field less than 5 nT, which nulls / cancels a uniform external ambient magnetic field (e.g., the Earth's magnetic field) of 50 μT oriented along the y-axis running from left to right in the figure. The magnitude and relative direction (positive / negative) of the drive current 72 applied to each coil in the coil array is shown versus the y-coordinate value of each coil, where the y-axis runs from left to right in the figure, with the origin of the coordinates corresponding to the center of the coil array. Note that the double geodesic icosahedral lattice requires approximately 50% less current than the Fibonacci lattice. Table 1 shows a comparison of the performance parameters for the two coil arrays. [Table 1]

[0088] Figures 11(a), (b), (c) and (d) show other examples (generated by calculation / simulation) of the drive current applied to a spherical array of magnetic field generating coils as a function of the y-axis coordinate of a given coil. In each case the coil array grid (81, 83, 85, 88) is a double geodesic icosahedral array with 212 coils. The arrows 82, 84, 86 and 87 represent different magnetic field distributions / sources. The magnetic field vectors to be cancelled are parallel to the y-axis. Figure 11(a) corresponds to a uniform magnetic field (three arrows 82). The drive currents supplied to each coil of the array to achieve nulling are almost perfectly linearly proportional to the y-axis position coordinate of a given coil. Figure 11(b) corresponds to a point magnetic field (one arrow 84) ​​lying on the negative y-axis. The linearity between the drive current and the y-coordinate of the coils in Figure 11(a) is now replaced by a curved one. Figure 11(c) corresponds to a point magnetic field 86 located on the positive x-axis but oriented parallel to the positive y-axis, and Figure 11(d) corresponds to a point magnetic field 87 located at positive x and negative y coordinates but oriented parallel to the positive y-axis.

[0089] Figure 12 shows typical values ​​of magnetic field and magnetic field gradient (generated by calculation / simulation) achieved within the nulling region of each example of the invention as a function of the number of magnetic field generating coils of the Fibonacci lattice arrangement. Figures 13(a), (b), (c) and (d) show further examples of this for a Fibonacci lattice (Fig. 13(a) magnetic field value, Fig. 13(c) magnetic field gradient) and a double geodesic icosahedral lattice (Fig. 13(b) magnetic field value, Fig. 13(d) magnetic field gradient). In each case, both maximum and average magnetic field values ​​or magnetic field gradients are shown. These figures show how increasing the number of coils improves the performance of the coil array, and also show the large influence of the lattice type. The coil array had a diameter of 1 m and was acting against a uniform magnetic field of 50 μT. The area evaluated was a sphere of diameter 0.35.

[0090] It can be seen that using the double geodesic icosahedral lattice performs better than using the Fibonacci lattice.

[0091] 14(a) and (b) show cutaway views of a spherical shell array of magnetic field generating coils (90, 92) surrounding a spherical shell array of magnetic field sensing elements (91, 94), the former arranged to be substantially concentric with the surrounding spherical shell array of magnetic field generating coils. The patient's head 2 is located at the center of the array of magnetic field sensing elements. The inventors have found that this concentric arrangement allows for particularly accurate and effective zeroing in the nulling region. Each distinct location of the coils (91, 92) of the coil array and each distinct location of the sensors in the sensor array (91, 94) are each defined by a regular grid, which locations coincide with (or are defined by) the conceptual vertices or facets of a polyhedron or points on a sphere. Two physical polyhedral or spherical support structures (not shown) support the elements of each array at said locations, respectively. The plurality of magnetic field generating coils (90, 92) are disposed at respective discrete locations that are approximately equidistant from the center of the nulling region, thereby defining a generally spherical array surrounding the nulling region, and similarly, the plurality of magnetic field sensing elements (91, 94) are disposed at respective discrete locations that are approximately equidistant from the center of the nulling region, thereby defining a generally spherical array within the nulling region.

[0092] FIG. 14(a) shows a cross-sectional view of a device in which the diameter of the array of magnetic field sensing elements 91 (0.35 m diameter of the spherical array of sensors) relative to the diameter of the array of magnetic field generating coils 90 (1.0 m diameter of the spherical array of coils) is smaller than the diameter of the array of magnetic field sensing elements 94 (0.61 m diameter of the spherical array of sensors) relative to the diameter of the array of magnetic field generating coils 92 (1.0 m diameter of the spherical array of coils) of the device shown in cross-section in FIG. 14(b). The inventors have found that there is an optimum location (e.g., diameter of the array) for sensors located within the surrounding coil array such that performance is dramatically improved. It has been found that the optimum location depends on the diameter of the coil array, the number of coils in the coil array, the type of lattice that defines the coil array, and the shape of the coils. For example, for a Fibonacci lattice with 1200 coils as shown in Figures 14(a) and (b), it was found that the optimal position of the sensors in the sensor array corresponds to an array with a concentric diameter of 0.61 times the diameter of the surrounding coil array, as shown in Figure 14(b), when the sensor array is a spherical shell array.

[0093] In general, the optimal sensor location corresponds to a concentric diameter of the sensor array that has a value in the range of at least about 30% to about 90% of the diameter of the surrounding magnetic field generating coil array. Figures 15(a) and (b) show how the value of the magnetic field gradient in the nulled region of the external ambient magnetic field (Figure 15(a)) varies as a function of the diameter of the magnetic field sensor array (with the coil array diameter fixed) and the variation of the optimal sensor array diameter (Figure 15(b)) as a function of the number of coils in the array (with the coil array diameter varied), respectively. Here, the coil array is a Fibonacci lattice. For example, with 2000 coils, if the sensor array diameter is about 68% of the coil array diameter, the nulled magnetic field is less than 1.6 pT in the nulled volume, and the value of this nulled magnetic field only increases to 0.95 nT by increasing the diameter of the sensor sphere to about 0.81% of the coil array diameter. This example means that if a coil array sphere with a diameter of 1 m is used, the diameter of the sensor array sphere is 0.81 m. Figure 15(b) shows how the optimum diameter of the sensor array sphere changes as a function of the number of magnetic field generating coils used in the spherical Fibonacci array of coils. The relationship is well approximated by a quadratic function, where the diameter of the sensor array changes as a quadratic function of the number of coils in the coil array.

[0094] 16(a) and (c) show another example of how the magnetic field value (FIG. 16(a)) and magnetic field gradient (FIG. 16(c)) in the nulled area of ​​an array with 200 coils in the external ambient magnetic field change as a function of the diameter of the magnetic field sensor array when the coil array is a fixed 1.0 m diameter Fibonacci lattice and the nulled ambient magnetic field is a uniform 50 μT magnetic field. Both the maximum magnetic field value of 95 and the average magnetic field value of 96, as well as the maximum magnetic field gradient value of 95 and the average magnetic field gradient value of 100 are shown. FIG. 16(b) and (d) show another example of how the magnetic field value (FIG. 16(b)) and magnetic field gradient (FIG. 16(d)) in the nulled area of ​​an array with 600 coils in the external ambient magnetic field change as a function of the diameter of the magnetic field sensor array when the coil array is a fixed 1.0 m diameter double geodesic icosahedral lattice and the nulled ambient magnetic field is a uniform 50 μT magnetic field. Both maximum 98 and average 97 magnetic field values ​​are shown, as well as maximum 102 and average 101 magnetic field gradient values. It can be clearly seen that in each case there exists an optimal sensor array diameter that corresponds to the smallest magnetic field and magnetic field gradient values. Figure 17 shows how the optimal sensor array sphere diameter values ​​identified from Figures 16(a)-(d) vary as a function of the number of magnetic field generating coils used in the spherical Fibonacci array of coils. The relationship is well approximated by a quadratic order, with the sensor array diameter varying as a quadratic function of the number of coils in the coil array.

[0095] As a result, the inventors have found a strong synergy between the diameter of the array of magnetic field generating elements and the diameter of the array of magnetic field sensing elements required to achieve optimal nulling of the magnetic field in the nulling region. In other words, an optimal placement of the sensors of the feedback system of the present invention provides an optimal feedback value of the magnetic field to the control system, and optimally controls the current used to drive the magnetic field generating elements in reducing the magnetic field in the nulling region. The diameter of the array of magnetic field sensing elements is preferably between about 40% and about 80% of the diameter of the array of magnetic field generating elements. For example, the diameter of the array of magnetic field sensing elements may be between about 40% and about 90% of the diameter of the array of magnetic field generating elements, and the array of magnetic field generating elements may include at least about 200 elements and not more than about 2000 elements. The inventors have found that particularly effective magnetic field nulling can be achieved when one or more of these conditions are applied.

[0096] Fig. 18 shows a process for determining the value of the current to be supplied to the magnetic field generating coils of the coil array. Fig. 19 shows an apparatus for zeroing a magnetic field in a nulling area in an external surrounding magnetic field, comprising a plurality of separate magnetic field generating elements 102 arranged at respective separate locations surrounding the nulling area to generate respective nulling magnetic fields extending into the nulling area. A plurality of magnetic field sensing elements 103 are arranged at a plurality of respective separate locations in the nulling area to sense respective values ​​of the magnetic field in the nulling area. A feedback control 150 is configured to control the value of each of the zeroing magnetic fields generated by each of the plurality of magnetic field generating elements depending on the value of the magnetic field sensed by the plurality of magnetic field sensing elements. The feedback control is designed to drive the magnetic field generating elements 102 with respective currents that reduce the value of the magnetic field detected by each of the magnetic field sensing elements 103 to a value not exceeding a preset threshold corresponding to a preset zeroing of the magnetic field in the nulling area.

[0097] The feedback controller is designed to calculate the appropriate drive current as follows: Consider the following items: The minimization of the magnetic field readings of each sensor 103 is sufficient to cancel the magnetic field inside the nulling volume of interest. The magnetic field reading of one sensor 103 is the sum of the total magnetic field from all magnetic field generating elements 102 plus the external ambient magnetic field. · Each sensor in the coil position is fixed so that this spatial component of the magnetic field equation remains constant. The only variable that is changing is the drive current, which depends proportionally on the magnetic field.

[0098] The contribution of each magnetic field generating element 102 (e.g. coil) to the total sensor 103 can be calibrated. If the magnetic field is cancelled, the feedback value of the magnetic field sensed by the sensors for the volume enclosed by the sensors will also be zero. There is a proportional dependency between the current and the magnetic field. The following equation describes the value of the magnetic field at a given location, which is a fixed position for each sensor expressed in cylindrical coordinates:

number

[0099] In other words, each coil in turn is driven independently by a current, while all other magnetic field generating elements receive no current from the control. The magnetic field generated by the single coil is sensed by each of the magnetic field sensing elements 103 at their respective fixed locations within the nulling region. These measured electric field values ​​are calibration values ​​and define a basis vector for the single coil under consideration. These values ​​are input from each magnetic field sensing element 103 to the control unit 150, which stores them. The control unit is configured to repeat this process separately for each magnetic field generating element 102 of the array of coils, so that each magnetic field generating element 102 is responsible separately for becoming a single magnetic field generating element supplied with current. For each coil 102 the control unit 150 generates a basis vector containing the calibration magnetic field values ​​for all n sensors 103, which for the first coil are:

number

[0100] An array of m coils results in an array of these basis vectors constructed with each basis vector corresponding to all m coils.

number

number

number

number

number

[0101] Usually, only a few iterations (typically only one) are needed to obtain the best current solution for the zeroing / cancellation of the external magnetic field. The orthogonal basis for the orthogonal projection method is generated in a calibration process. The orthogonal basis only needs to be generated once, and the controller can then use it for all active cancellation / zeroing using the sensor data received from the magnetic field sensor array during the active cancellation / zeroing operation. This is a very fast and simple process. Therefore, any magnetic field distortions due to differences and inaccuracies between them are eliminated. Figure 20 summarizes this process, which includes the following steps: Step 160: The controller 150 outputs individual drive currents to the magnetic field generating coils 102 in sequence, while the other coils receive no current. Step 161: The magnetic field in the nulling region is measured by the magnetic field sensitive sensor array 103. Step 162: The control unit 150 calculates the individual drive currents to be applied to each coil 102 by solving the optimization equation, and drives each coil with the drive currents thus calculated.

[0102] Figures 19(a), (b) and (c) show examples of two very different magnetic sources used as magnetic field generating elements in a regular spherical array 102 of 0.5 m diameter with 32 magnetic field generating elements arranged in a double geodesic icosahedron surrounding an array of magnetic field feedback sensors 103. The magnetic field in the nulling region of the array shown in Figure 19(b) corresponds to the case where each magnetic field generating element is configured to generate its local magnetic field with a circular current (e.g. a circular coil of 14 cm diameter). In contrast, the magnetic field in the nulling region of the array shown in Figure 19(c) corresponds to the case where each magnetic field generating element is configured to generate its local magnetic field as a magnetic dipole field. In both cases nulling was obtained over an 8 cm diameter nulling region, reducing the external ambient magnetic field of over 50 nT to an average magnetic field of about 0.5 nT and an average magnetic field gradient of 0.027 nT / cm (FIG. 19(b)), or to an average magnetic field of about 1.0 nT and an average magnetic field gradient of 0.044 nT / cm (FIG. 19(c)). This reinforces the point that the magnetic field generating element does not have to be a coil, but could instead be any suitable magnetic field generating source configured to generate, for example, a dipole magnetic field.

[0103] FIG. 22 shows an example of how the orthogonal projection method described above can use a smaller number of magnetic field sensing elements (feedback sensors) 103 than the number of magnetic field generating coils 102. Specifically, if the number of sensors 103 is reduced to about 40% of the number of magnetic field generating coils 102, the performance of the method may begin to degrade. To overcome this problem, the control unit 150 can be configured to generate magnetic field calibration values ​​associated with a conceptual "virtual" magnetic field sensor 105 located at any point within the zeroed region by interpolating the calibration magnetic field values ​​actually received by the physical sensors 103 of the sensor array. The control unit 150 can be configured to define a conceptual interpolation sphere 106 (see FIG. 22(b)) defined by a radius from a point of interest (coordinates: x, y, z) within the zeroed region into which the virtual sensor calibration magnetic field values ​​should be interpolated. The control unit 150 can determine which real magnetic field sensors 103 are located within the conceptual sphere and increase the radius of the sphere until at least two real sensors are within the sphere. Once the controller has identified the number of real sensors in the interpolation sphere, it calculates a virtual value of the calibration field at the target location (x,y,z) by 3D interpolation of the real calibration field values ​​received from the real sensors in the interpolation sphere. This is then used in the process described above with reference to FIG. 20. Any number of separate target locations (coordinates: x,y,z) can be selected, such as locations corresponding to any arrangement on the notional sphere 108 (see FIG. 22(a)), with any number of virtual (i.e., interpolated) calibration field values ​​generated. The interpolation can be as simple as summing the calibration field values ​​provided by the real sensors in the interpolation sphere (i.e., the sum of the appropriate x,y,z components of each field) and dividing the result (for each field component) by the number of real sensors in the interpolation sphere. In cases where the difference in numbers between coils 102 and feedback sensors 103 is large, the orthogonal projection method can be improved by introducing a virtual feedback sensor that is thus created using readings from the real feedback sensors. A heat map 108 covering the conceptual sphere on which the feedback sensors (virtual and real) are placed shows a greyscale map representing the magnitude of the magnetic field flux density.

[0104] 23(a), (b) and (c) show examples of implementation of the method, where FIG. 23(a) shows a device 30 with a coil array with 32 coils and 16 real sensors. Field zeroing results 31 are shown for a spherical nulling area of ​​4 cm. An average magnetic field of 1.35 nT is obtained with an average gradient of 0.0043 nT / cm. FIG. 23(b) shows a device 32 with a coil array with 92 coils, 16 real sensors and 76 virtual sensors. Field zeroing results 33 are shown for a spherical nulling area of ​​4 cm. An average magnetic field of 0.04 nT is obtained with an average gradient of 0.001 nT / cm. FIG. 23(c) shows a device 34 with a coil array with 212 coils, 16 real sensors and 196 virtual sensors. Field zeroing results 35 are shown for a spherical nulling area of ​​4 cm. An average magnetic field of 0.02 nT is obtained with an average gradient of 0.001 nT / cm.

[0105] FIG. 24 shows a support framework 110 for supporting an array of magnetic field generating coils 102 and an array of magnetic field sensing elements 103. The sensor mountings can be adjusted in distance to the center of the array by pressing in with bolts. The support framework comprises a non-magnetic polyhedral frame. This is just one example of how the coils and feedback sensors can be mounted. The type of grid is a 32-sided double geodesic icosahedron. Each feedback sensor 103 is mounted on one of a number of adjustable mounting rods 111 with a fixed end at the center of the local facet of the grid on which the feedback sensor is mounted and a free end within the volume enclosed by the grid. Each mounting rod points radially towards the interior of the volume enclosed by the array. To adjust for optimal performance, the proximity of a given sensor to the coil grid center can be adjusted by adjusting the length of the mounting rod. For example, in this example, each mounting rod comprises a cylindrical tube containing a cylindrical sensor that can be moved back and forth along the axis of the mounting rod to telescope the length of the mounting rod. The lattice support framework thus holds the coils and sensors in place. Each coil can be secured in place in a variety of ways, including with adhesives, locking systems, grippers, etc.

[0106] Figure 25 shows another example of the use of the device according to the invention in an OPM-based cap-based magnetoencephalography shielding device. The patient's head 2 is within a hemispherical array of flat coils 114 mounted on a transparent hemispherical support surface 115 made of non-magnetic material. For example, the coil wires can be formed by evaporation, printing, foil etching, etc. A passive magnetic shield 113, e.g. a layer of mu metal and aluminum, covers the magnetic field generating coil array. Support arms and housings 116 for electrical cables are provided.

[0107] The present invention allows reducing the cost of sensors to cancel / null weak magnetic fields to achieve nulled regions in the μT~nT range or nT~fT range.

[0108] FIG. 26 shows an example of a magnetic field zeroing device constructed using a support framework according to the design described above with reference to FIG. 24. The device supported an array of magnetic field generating coils 102 and an array of magnetic field sensing elements 103. Thirty-two magnetic field generating coils 102 and sixteen magnetic field sensing elements 103 were used in this device. The magnetic field generating coils were arranged to cover a geodesic icosahedron with a diameter of 0.5 m. The magnetic field sensing elements were placed in appropriately adjusted positions to obtain a spherical nulling volume with a diameter of 8 cm, and the magnetic field zeroing device was designed to obtain a zero magnetic field throughout the nulling volume. The magnetic field zeroing device was installed in a typical building environment exposed to the Earth's magnetic field of about 50 μT. In addition, the magnetic field zeroing device was surrounded by noisy magnetic objects from electrical mains sockets and cables located nearby. These magnetic objects combined with the Earth's magnetic field to generate a magnetic field within the spherical nulling volume, which was substantially nulled by the action of the magnetic field zeroing device.

[0109] FIG. 26 shows a close-up view of the magnetic field zeroing device, showing the array of magnetic field sensing elements 103 in the zeroing device. Each magnetic sensing element 103 has three magneto-impedance sensors, each arranged to measure one of the magnetic field components along three orthogonal axes, Sx, Sy and Sz, along the x, y and z axes, respectively. An example of a suitable magnetic impedance sensor is the Nanotesla sensor manufactured by Aichi Steel Corporation, available under model number MI-CB-1DH. These sensors were modified to operate in direct current mode (rather than alternating current mode) according to instructions provided by Aichi Steel Corporation, which are readily available to those skilled in the art. The magnetic impedance sensor was operated in DC operating mode and calibrated in a low magnetic field environment. A fluxgate magnetic sensing element 123 (manufactured by Bartington Instruments, USA, product name "MAG-13") was placed at the center of the array of magnetic field sensing elements as an additional reference sensor.

[0110] Because the magnetic impedance sensor used in this example has a narrow dynamic range, the earth's magnetic field was first cancelled by appropriately orienting the array of magnetic field generating coils 102 and the array of magnetic field sensing elements 103 of the zeroing device as a whole. This was done by orienting the support framework (and the array of magnetic field generating coils 102 and the array of magnetic field sensing elements 103 mounted thereon) in the opposite direction to the earth's magnetic field vector and applying current to the coils of the array of magnetic field generating coils 102 as described above with reference to the configuration shown in Figure 11(a). This brought the initial magnetic field in the spherical nulling space within the dynamic range of the magnetic impedance sensor.

[0111] The field zeroing device was then subjected to the calibration process described above with reference to Figures 18 and 19 to obtain the orthogonal basis matrices as described above. Scalar products were then calculated and the inverse matrix was calculated from the products and stored for use in the active cancellation process.

[0112] The control unit (reference numeral 150 in FIG. 19) determines the current α j We apply the "negative" values ​​of

number

[0113] To calculate the “negative” value of the coil current in each active cancellation loop, the readings from the magnetic sensing elements are divided into [V1, V2, V3, …, V N The magnetic field measurements include measurements of three orthogonal field components (Sx, Sy, Sz) by each of the sensors, which are represented within the elements of the measurement matrix as follows:

number

[0114] The dot product of the matrix with an orthogonal basis was calculated as follows:

number

number

[0115] Using the resulting dot products, the controller is configured to invert the "orthogonal projection" optimization equation above. The result is individual values ​​of α j It consists of a matrix containing

[0116] In this example, a 2.8GHz quad-core controller with product code "PXIe-8861" from National Instruments was used. To read out the magnetic sensing elements, this was equipped with two PXI-6349 simultaneous analog-to-digital converters (ADCs), also from National Instruments with product code "PXI-6349". To provide a current source to provide the drive current for the magnetic field generating coils, a digital-to-analog converter (DAC), also from National Instruments with product code "PXIe-6739", was placed. The magnetic field (Sx, Sy, Sz) etc. were measured, and a "negative" current α j The process of applying these currents to the coils took less than 250 μs, and the process can easily be sped up using a field programmable gate array (FPGA).

[0117] Figure 27(a) shows the readings (Sx, Sy, Sz) obtained from 16 magnetic sensing elements 103, which corresponds to 48 (i.e. 16 x 3 = 48) single magneto-impedance magnetic sensors, perturbed by a non-uniform magnetic field.

[0118] Figure 27(b) shows the same readings after one calculation of the orthogonal projection (zeroing) algorithm. The reference magnetic sensor readings (Sx, Sy, Sz) from fluxgate magnetic sensing element 123 ("MAG-13") are shown in Figure 27(c). The vertical scale in these figures is measured in volts (V), with a reading of 2.5V corresponding to a measured value of 5μT.

[0119] It has been found that when active cancellation is performed within the magnetic field nulling device, the support framework can be rotated to any position relative to the Earth's magnetic field and still approach zeroing of the magnetic environment within the nulling region without degradation in quality, even in the presence of magnetic objects or AC magnetic sources. Additionally, it has been found that errors due to changing characteristics or tolerances in physical dimensions of the electronic components of the device have an equivalent effect as external magnetic field variations, but the device responds to counteract the effects those errors would otherwise have on the zeroing of the magnetic environment.

[0120] Each feature disclosed in the foregoing description, the following claims, or the accompanying drawings may, where appropriate, be expressed in its specific form or in terms of a means for performing a disclosed function or a method or process for obtaining a disclosed result, but the features may be utilized individually or in any combination of several features to realize the invention in various of its forms.

[0121] Although the present invention has been described in connection with exemplary embodiments, many equivalent modifications and variations will be apparent to those skilled in the art in view of the present disclosure. Accordingly, the exemplary embodiments of the present invention described above should be considered as illustrative and not restrictive. Various changes can be made to the embodiments without departing from the spirit and scope of the present invention.

[0122] For the avoidance of doubt, any theoretical explanations provided herein are intended to enhance the understanding of the reader, and the inventors do not wish to be bound by any of these theoretical explanations.

[0123] The headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described.

[0124] Throughout this specification, including the claims which follow, the words "comprise" and "include", and variations thereof (such as "comprises", "comprising", "including", etc.), unless the context otherwise requires, shall be interpreted as meaning the inclusion of stated integers or steps or groups of integers or steps, but not the exclusion of other integers or steps or groups of integers or steps.

[0125] As used herein and in the appended claims, the singular forms "a," "an," and "the" include the plural referents unless the context clearly dictates otherwise. When expressing a range in this specification, a particular value as the beginning and / or another particular value as the end may be used as the beginning and / or the other particular value as the end, respectively. When such a range is expressed, the range in which the particular value is the very beginning and / or the other particular value is the very end is another embodiment. Similarly, when values ​​are expressed as approximations by use of the antecedent "about," it should be understood that the particular values ​​form another embodiment. The relationship between the term "about" and the numerical values ​​is arbitrary, and may mean, for example, ±10%.

[0126] As used herein, "diameter" refers to a straight line passing through the center of a structure, arrangement, body or figure, particularly, but not limited to, a circle or a sphere.

Claims

1. 1. An apparatus for nulling a magnetic field in a nulling region within an ambient (e.g., external) magnetic field, comprising: a plurality of separate magnetic field generating elements disposed at respective separate locations surrounding the nulling region for generating respective nulling magnetic fields extending into the nulling region; a plurality of magnetic field sensing elements disposed at a plurality of respective distinct locations within the nulling region for sensing respective values ​​of the magnetic field within the nulling region; a feedback controller for controlling, in response to values ​​of the magnetic field sensed by the plurality of magnetic field sensing elements, a value of the respective nulling magnetic field generated by each of the plurality of magnetic field generating elements by driving the magnetic field generating element with a respective current that reduces the value of the magnetic field detected by each magnetic field sensing element to a value not exceeding a preset threshold corresponding to a preset nulling of the magnetic field in the nulling region; An apparatus comprising:

2. The apparatus of claim 1 , wherein the magnetic field generating element comprises a conductive coil adapted to carry the electrical current.

3. 3. The apparatus of claim 1, wherein the plurality of magnetic field generating elements are arranged at each of the distinct locations in a first array formed along a three-dimensional reference plane that surrounds the zeroing region.

4. 4. The apparatus of claim 1, wherein the plurality of magnetic field sensing elements are disposed at each of the distinct locations defining a second array formed along a three-dimensional reference plane.

5. 5. The apparatus of claims 3 and 4, wherein the second array is configured to be substantially concentric with the first array.

6. An apparatus according to any one of claims 3 to 5, wherein each of the distinct locations in the first array is determined according to a regular grid.

7. An apparatus according to any preceding claim, wherein each of the distinct locations in the second array is determined according to a regular grid.

8. 8. The apparatus of claim 1, wherein the plurality of magnetic field generating elements are disposed at each of the distinct locations approximately equidistant from a center of the nulling region, thereby defining a substantially spherical array surrounding the nulling region.

9. 9. The apparatus of claim 1, wherein the plurality of magnetic field sensing elements are disposed at each of the distinct locations approximately equidistant from a center of the nulling region, thereby defining a substantially spherical array within the nulling region.

10. An apparatus according to any preceding claim, wherein the diameter of the array of magnetic field sensing elements is at least 30% of the diameter of the array of magnetic field generating elements.

11. An apparatus according to any preceding claim, wherein the plurality of magnetic field generating elements comprises at least 10 distinct magnetic field generating elements.

12. Apparatus according to any preceding claim, wherein the plurality of magnetic field generating elements comprises at least 50 separate magnetic field generating elements.

13. Apparatus according to any preceding claim, wherein the plurality of magnetic field generating elements comprises at least 200 individual magnetic field generating elements.

14. An apparatus as described in any one of claims 1 to 13, wherein the feedback control section has an input terminal connected to the output of the magnetic field sensing element and an output terminal connected to the input terminal of an associated magnetic field generating element.

15. The threshold is 5×10 －9 15. The apparatus of any one of claims 1 to 14, which is sub-Tesla.

16. The threshold is 5×10 －10 16. The apparatus of any one of claims 1 to 15, which is sub-Tesla.

17. 1. A method for nulling a magnetic field in a nulling region within an ambient (e.g., external) magnetic field, comprising: providing a plurality of separate magnetic field generating elements disposed at respective separate locations surrounding the nulling region for generating respective nulling magnetic fields extending into the nulling region; providing a plurality of magnetic field sensing elements disposed at a plurality of respective distinct locations within said nulling region for sensing respective values ​​of the magnetic field within said nulling region; controlling, in response to values ​​of the magnetic field sensed by the plurality of magnetic field sensing elements, a value of each of the nulling magnetic fields generated by each of the plurality of magnetic field generating elements by driving the magnetic field generating elements with a respective current that reduces the value of the magnetic field detected by each of the magnetic field sensing elements to a value not exceeding a preset threshold corresponding to a preset nulling of the magnetic field within the nulling region. A method for providing the above.

18. The method of claim 17 , wherein the magnetic field generating elements comprise conductive coils, the method including transmitting the current through each of the coils.

19. 19. The method of claim 17 or 18, comprising providing the plurality of magnetic field generating elements as arranged at each of the distinct locations in a first array formed along a three-dimensional reference plane that surrounds the zeroing region.

20. A method according to any of claims 17 to 19, comprising providing the plurality of magnetic field sensing elements arranged at each of the distinct locations defining a second array formed along a three-dimensional reference plane.

21. 21. The method of claims 19 and 20, comprising providing the second array substantially concentrically with the first array.

22. A method according to any of claims 19 to 21, wherein each of the distinct locations in the first array is arranged according to a regular lattice.

23. A method according to any of claims 19 to 22, wherein each of the distinct locations in the second array is arranged according to a regular lattice.

24. 24. A method according to any of claims 17 to 23, comprising providing the plurality of magnetic field generating elements arranged at each of the distinct locations substantially equidistant from a centre of the nulling region, thereby defining a substantially spherical array surrounding the nulling region.

25. 25. A method according to any of claims 17 to 24, comprising providing the plurality of magnetic field sensing elements arranged at each of the distinct locations substantially equidistant from a centre of the nulling region, thereby defining a substantially spherical array within the nulling region.

26. A method according to any one of claims 17 to 25, wherein the diametric dimension of the array of magnetic field sensing elements is at least 30% of the diametric dimension of the array of magnetic field generating elements.

27. A method according to any of claims 17 to 26, comprising providing the plurality of magnetic field generating elements to comprise at least 10 separate magnetic field generating elements.

28. A method according to any of claims 17 to 27, comprising providing the plurality of magnetic field generating elements to comprise at least 50 separate magnetic field generating elements.

29. A method according to any of claims 17 to 28, comprising providing the plurality of magnetic field generating elements to comprise at least 200 distinct magnetic field generating elements.

30. A method according to any of claims 17 to 29, comprising receiving an input in a feedback control corresponding to an output from the magnetic field sensing element, and providing an output from the feedback control including a control signal that is input to an associated magnetic field generating element.

31. The threshold is 5×10 －9 The method of any one of claims 17 to 30, wherein the temperature is less than Tesla.

32. The threshold is 5×10 －10 The method of any one of claims 17 to 31, wherein the temperature is less than Tesla.

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