Magnetic shielding device and magnetic shielding method

A magnetic shielding device includes: a passive shield having an inner space; a first coil that cancels a magnetic field entering in the inner space; a first magnetic sensor that measures the magnetic field entering in the inner space; a second magnetic sensor located in a position farther from the first coil than the first magnetic sensor; and a controller that controls the first coil so that a gradient between a first magnetic field measured by the first magnetic sensor and a second magnetic field measured by the second magnetic sensor be less than a predetermined threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

The entire disclosures of Japanese Patent Applications No. 2013-188097 filed on Sep. 11, 2013, and No. 2012-204462 filed on Sep. 18, 2012 are expressly incorporated by reference herein.

BACKGROUND

The invention relates to magnetic shielding device and magnetic shielding method.

2. Related Art

Diagnostic imaging is widely used in medical procedures. Since the diagnostic imaging is a non-invasive procedure, the diagnostic imaging is preferably used for a sensitive organ such as a heart or a brain. Magnetic field source imaging is one example of the diagnostic imaging. These organs generate current caused by activities of neuron. Measurement of the magnetic field caused by the current reflects status of these organs. For example, Magnetocardiogram (MCG), measurement of the magnetic filed generated by a heart, and Magnetoencephalogram (MEG), measurement of the magnetic field generated by a brain, are known.

There are two major problems for measuring these magnetic fields. The first problem relates to a sensitivity of a measuring device. To measure these magnetic fields, a high sensitivity is required. The second problem relates to an external magnetic field. Since an external magnetic field caused by the earth magnetism, for example, is hundred thousands times greater than a magnetic field generated by a living body, a magnetic shielding device to shield the external magnetic field is required.

SUMMARY

The invention provides a magnetic shielding device and a magnetic shielding method for reducing a gradient of a magnetic field of an inner space of a magnetic shield.

According to one aspect of the invention, there is provided a magnetic shielding device, including: a passive shield having an inner space; a first coil that cancels a magnetic field entering in the inner space; a first magnetic sensor that measures the magnetic field entering in the inner space; a second magnetic sensor located in a position farther from the first coil than the first magnetic sensor; and a controller that controls the first coil so that a gradient between a first magnetic field measured by the first magnetic sensor and a second magnetic field measured by the second magnetic sensor be less than a predetermined threshold.

The passive shield may have at least one opening.

The second magnetic shield may be located nearer a target space compared with the first magnetic sensor in a measurement direction of a magnetic field measuring device installed in the target space, the target space being a part of the inner space.

The threshold may be determined based on a range of a magnetic field used in a device installed in a target space, the target space being a part of the inner space.

The threshold may cause difference in a magnetic field between edges of a target space to be less than 10 nT, the target space being a part of the inner space.

The magnetic shielding device may further include a second coil that cancels a magnetic field in a target space, the target space being a part of the inner space.

The magnetic shielding device may further include a first driver circuit that drives the first coil; and a second driver circuit that drives the second coil.

The passive shield may have at least one opening, and the first driver circuit may provide current causing the first coil to cancel a magnetic field at an edge face of the opening.

The first magnetic sensor may be located at the edge face, and the first driver circuit may provide current that cancels a magnetic field measured by the first magnetic sensor.

The first magnetic sensor may be located at the center of the edge face.

The second driver circuit may provide current causing the second coil to cancel a magnetic field in the inner space.

The second magnetic sensor may be located in the inner space, and the second driver circuit may provide current causing the second coil to cancel a magnetic field measured by the second magnetic sensor.

A dynamic range of a magnetic field generated by the first coil may be greater than a dynamic range of a magnetic field generated by the second coil.

A resolution in a magnetic field generated by the second coil may be higher than a resolution of a magnetic field generated by the first coil.

The passive shield may includes two openings, the magnetic shielding device may include two first coils, and each of the two first coils may cancel a magnetic field near one of the two openings, respectively.

The first driver circuit may provide current having the same magnitude to the two first coils.

According to another aspect of the invention, there is provided a method in magnetic shielding device including a passive shield having an inner space, a first coil that cancels a magnetic field entering in the inner space, a first magnetic sensor that measures the magnetic field entering in the inner space, a second magnetic sensor located in a position farther from the first coil than the first magnetic sensor, the method including: controlling the first coil so that a gradient between a first magnetic field measured by the first magnetic sensor and a second magnetic field measured by the second magnetic sensor be less than a predetermined threshold.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

First Exemplary Embodiment

Configuration

FIG. 1shows an example of an external view of magnetic shielding device1according a first exemplary embodiment. Magnetic shielding device1is used to shield a device such as a magnetic measuring device from an external magnetic field, which is a magnetic field caused by an object other than an object to be measured. Magnetic shielding device1includes passive shield11, external coil12, internal coil13, and magnetic sensor14.

Passive shield11is made of conductive material having a high conductivity such as Aluminum. Such material shields a magnetic field by eddy current.

Passive shield11may be made of ferromagnetic substance, such as permalloy, ferrite, or amorphous of iron, chromium, or cobalt. Passive shield11has tubular shape whose cross section has square shape. A term “inner space” refers to a space surrounded by passive shield11and a term “outer space” refers to a space other than the inner space. The inner space is so large that the inner space can accommodate a magnetic field measuring device (for example, a magnetocardiographic device) and a tested body (for example, a person). Among the inner space, a space where magnetic field is a target to be cancelled, is referred to as a “target space” (not shown inFIG. 1). In magnetic shielding device1, a magnetic field measuring device is installed and used at the target space.

For the description below, a three dimensional orthogonal coordinate system is defined. In the coordinate system, x-axis, y-axis, and z-axis are defined as a direction along the width direction, the depth direction, and the height direction of passive shield11, respectively. In this example, passive shield11has two openings, opening111A and opening111B.

External coil12A and external coil12B are coils to cancel a magnetic field entering the inner space of passive shield11. External coil12A and external coil12B are located near opening111A and opening111B in the outer space, respectively.

Magnetic sensor14A is a sensor to measure a magnetic field entering the inner space. In this example, magnetic sensor14is located at the center of opening111A. Further, magnetic sensor14A is used to measure a gradient of the magnetic field as described above.

FIG. 2shows an example of a configuration of magnetic shielding device1. In the figure, driver circuit15and control unit16are shown in addition to a schematic of a cross section along y-axis. Internal coil13A and internal coil13B are coils to cancel a magnetic field in the target space. In the example, external coil12and internal coil13are Helmholtz coils. In other words, internal coil13provides spatially homogeneous magnetic field so as to control an offset of the magnetic field in the target space.

Magnetic sensor14B is a sensor to measure a magnetic field in the inner space. Magnetic sensor14B is located at a position nearer the target space (more specifically, the center of the target space), compared with magnetic sensor14A. In this example, magnetic sensor14B is located outside the target space. More specifically, magnetic sensor14B is located at a position nearer the target space in a measurement direction (for example, x-axis) of the magnetic field measuring device installed in the target space, compared with magnetic sensor14A. It is to be noted that magnetic sensor14A is located at a position nearer external coil12, compared with magnetic sensor14B.

Driver circuit15A is a circuit to drive external coil12. Driver circuit15B is a circuit to drive internal coil13. Control unit16controls driver circuits15A and15B, in response to a magnetic field measured by magnetic sensor14. More specifically, control unit16controls driver circuit15A (in other words, external coil12) so that a gradient (or difference) between a magnetic field measured by magnetic sensor14A and a magnetic field measured by magnetic sensor14B is less than a predetermined threshold. The threshold may be stored in a memory of magnetic shielding device1in advance, or may be input by a user.

FIG. 3shows an example of a flowchart illustrating an operation of magnetic shielding device1. The operation shown inFIG. 3is triggered by activating magnetic shielding device, for example.

In step S101, magnetic shielding device1measures a magnetic field near the opening of passive shield11. More specifically, control unit16obtains a signal showing results of measurement of the magnetic field, from magnetic sensor14A.

In step S102, control unit16controls the coils so that the magnetic field near the opening is less than a predetermined threshold. More specifically, control unit16outputs to driver circuit15A a signal to provide external coil12current for canceling the magnetic field measured by magnetic sensor14A. Driver circuit15A provides current to external coil12in accordance with the signal output from control unit16. The current provided to external coil12generates a magnetic field having a direction opposite to a direction of the magnetic field measured by magnetic sensor14A. The current provided to external coil12has a magnitude to generate a magnetic field having the same magnitude as the magnetic field measured by magnetic sensor14A. The relationship between the magnitude of current and the magnitude of a magnetic field is stores in a ROM (Read Only Memory) of control unit16.

In this example, control unit16also controls internal coil13. For example, control unit16outputs a signal causing driver circuit15B to provide current so as to cancel the magnetic field measured by magnetic sensor14B.

In step S103, control unit determines whether the magnitude of a magnetic field near the opening is less than or equal to a predetermined threshold (for example, 10 nT (nano tesla)). More specifically, control unit16obtains a signal showing the measured magnetic field from magnetic sensor14A. Control unit16determines whether the magnitude of a magnetic field shown by the signal is less than or equal to the threshold, If it is determined that the magnitude of the magnetic field is greater than the threshold (step S103: YES), control unit16transfers the operation to step S102, If it is determined that the magnitude of the magnetic field is less than or equal to the threshold (step S103: NO), control unit16transfers the operation to step S104.

In step S104, control unit16measures a gradient of a magnetic filed in the inner space. More specifically, control unit16obtains a signal showing the measured magnetic field of the inner space from magnetic sensor14B. Control unit16calculates the gradient of a magnetic filed between magnetic sensors14A and14B, using the magnetic filed measured by magnetic sensor14A, the magnetic field measured by magnetic sensor14B, and a distance between magnetic sensors14A and14B. The distance between magnetic sensors14A and14B is stored in the ROM of control unit16. Alternatively, the distance between magnetic sensors14A and14B may be measured and input by a user.

In step S105, control unit16determines the gradient in the inner space is less than or equal to a predetermined threshold. If it is determined that the gradient in the inner space is less than or equal to the threshold (step S105: YES), control unit16transfers the operation to step S106. If it is determined that the gradient in the inner space is greater than the threshold (step S105: NO), control unit16transfers the operation to step S107.

It is to be noted that the threshold of the gradient used in step S105is determined, for example, according to a use range (for example, a dynamic range of a measurement in a magnetic field measuring device) of a device (for example, a magnetic field measuring device). More specifically, the threshold is determined so that the difference in the magnetic field between edges of the target space is less than a predetermined value (for example, 10 nT) corresponding to a dynamic range of a magnetic field measuring device. For example, in a case that the dynamic range of the magnetic field measuring device is 10 nT, the threshold is determined so that the difference, which is equal to a product of the gradient and the length in a direction of x-axis, in the magnetic field between edges of the target space in x-axis is less than the dynamic range. For more detailed example, if the target space has 1 m length in x-axis direction, the threshold is determined as 10 nT/m. It is to be noted that the threshold of the magnetic field may be less than 10 nT, for example, 3 nT.

In step S106, control unit16maintain current provided to external coil12and internal coil13. Control unit16transfers the operation to step S103.

In step S107, control unit16controls at least one of external coil12and internal coil13so that the magnitude of the gradient of the magnetic field decrease. In this process, control unit16controls a gradient of a magnetic field in the target space, with the magnitude of the magnetic field in the target space being less than or equal to a predetermined threshold (for example, 10 nT. Note that the threshold may be different from the threshold used in step S103). In this example, since magnetic sensor14B is outside the target space, magnetic sensor14B cannot directly measures the magnetic field in the target space. Therefore, control unit16estimates a magnetic field in the inner space. Control unit16controls at least one of external coil12and internal coil13so that the estimated magnitude of the magnetic field in the inner space is less than or equal to a threshold, and the gradient of the magnetic field in the inner space is less than or equal to a threshold.

Any algorithm can be used for estimating a magnetic field in the inner space. For example, if the magnetic field measured by magnetic sensor14B is greater than the magnetic field measured by magnetic sensor14A, control unit16controls current provided to external coil12so that the magnetic field generated by external coil12increases. If the magnetic field measured by magnetic sensor14B is less than the magnetic field measured by magnetic sensor14A, control unit16controls current provided to external coil12so that the magnetic field generated by external coil12decreases. After controlling the current, control unit16estimates the magnetic field in the target space. The magnetic field in the target space is estimated using results of measurements by magnetic sensors14A and14B, and the distance between magnetic sensors14A and14B. If the estimated magnitude of the magnetic field is greater than the threshold, control unit16controls current provided to internal coil13so that the magnetic field in the inner space is reduced (in other words, offset of the magnetic field in controlled).

After the control, control unit16transfers the operation to step S102. It is to be noted that the current provided to internal coil13may be controlled in addition to or instead of the control of external coil12in step S107.

FIG. 4shows an example of a decreased gradient of the magnetic field. The vertical axis and the horizontal axis show the magnetic field and spatial position, respectively. The dashed line shows a profile without decreasing the gradient, and the solid line shows a profile with decreasing the gradient. According to the operation shown inFIG. 3, the magnetic field entering the inner space is reduced less than a threshold, and the gradient of the magnetic field of the inner space is reduced less than a threshold. In other words, magnet shielding device1can provide an inner space where the gradient of the magnetic field is reduced.

3. Examples of Configuration of Magnetic Sensors

The spatial configuration (or layout) of the magnetic sensors are not restricted to an example shown inFIG. 2. Other examples will be described in the following. InFIGS. 5 to 9, target space112is shown in the figures. Internal coil13and driver circuit15B is not shown in the following figures.

FIG. 5shows an example of a configuration of magnetic sensors. In this example, magnetic sensor14A is located not in the inner space but out of the passive shield (in the outer space). It is to be noted that magnetic sensor14B may be located outside target space112.

FIG. 6shows another example of a configuration of magnetic sensors. In this example, a single external coil (external coil12B) is driven. External coil12A is not driven.

FIG. 7shows yet another example of a configuration of magnetic sensors. In this example, two sets of sensors are used. The first set of sensors measure the magnetic field entering the inner space. The second set of sensors measure the magnetic field of the inner space. Magnetic sensors14A and14C measure the magnetic field entering the inner space. In this example, passive shield11has two openings. Magnetic sensor14A measures a magnetic field near the first opening and magnetic sensor14B measures a magnetic field near the second opening. Magnetic sensors14B and14D measure the magnetic field of the inner space. Magnetic sensor14B is located nearer magnetic sensor14A, compared with magnetic sensor14D. Magnetic sensor14D is located nearer magnetic sensor14B, compared with magnetic sensor14A. In this example, the gradient of the magnetic field between magnetic sensors14A and14B and the gradient of the magnetic field between magnetic sensors14C and14D, are controlled so as to be less than a predetermined threshold, respectively.

FIG. 8shows yet another example of a configuration of magnetic sensors. In this example, three magnetic sensors are used to measure the gradient of the magnetic field. Magnetic field14A measures the magnetic field entering the inner space. Magnetic sensors14B and14E measure the magnetic field of the inner space. Magnetic sensors14B and14E are located at different position each other. In this example, the gradient of the magnetic field is determined using three items of data measured by three sensors and a predetermined algorithm (for example, least square algorithm). It is to be noted that four or more magnetic sensors may be used to determine the gradient of the magnetic field.

In another example ofFIG. 8, magnetic sensor14A may not be used to determine the gradient of the magnetic field. In other words, magnetic sensor14A may be used only for controlling external coil12A (or measuring the magnetic field entering the inner space). In such a case, the gradient of the magnetic field is determined by magnetic sensors14B and14C.

FIG. 9shows yet another example of a configuration of magnetic sensors. In this example, magnetic sensor14A is located at a position outside passive shield11. Since magnetic sensor14A is used to estimate the magnetic field entering the inner space, magnetic sensor14A may be located far from the opening, not close to the opening.

In the above examples, magnetic sensor14B is located outside the target space. However, magnetic sensor14B may be located in the target space. Further, magnetic sensor14B may not be located near the target space. In a case that a magnetic sensor is located in the target space, the magnetic sensor should have higher sensitivity than the second magnetic sensor measuring the magnetic field entering the inner space, so that the magnetic field measured by the magnetic sensor is less than a threshold. However, in the present embodiment, two magnetic sensors may be located near the opening and the sensitivities of these two sensors may be the same level. Further, in a case that a magnetic sensor is located outside the target space, the target space can be used effectively.

The invention is not restricted to the embodiment described above. Various modifications may be applied to the embodiment. Some modifications will be described in the following. Two or more modification may be combined.

The shape of passive shield11is not restricted to the example described in the embodiment above. For example, the number of openings and the shape of the openings are not restricted to the example described above. Passive shield11may not have an opening, or may have s single opening. Alternatively, an opening may be covered by a cover.

The cross section perpendicular to x-axis is not restricted to a square. For example, the cross section perpendicular to x-axis may be a polygon other than a square, a circle, or an ellipse. Further, passive shield11may be made of multi-layered material.

Internal coil13may be omitted. In a case that magnetic shielding device1does not have internal coil13, the gradient of the magnetic shield is controlled by current provided to external coil12. Further, the shape of external coil12and internal coil13are not restricted to the example described above. The shape of these coils may be a polygon other than a square, a circle, or an ellipse.

In the above embodiment, only an x-axis component of the magnetic field is cancelled. However, two or more axes components may be cancelled or controlled. In such a case, magnetic shielding device1has two or more sets of internal coils13. For example, magnetic shielding device1may have three sets of internal coils13, for x-, y-, and z-axes. Each component of the magnetic field may be cancelled or controlled.

A method for controlling a coil is not restricted to the example described above. In the above embodiment, each coil is feedback controlled. A coil may be controlled by a method other than the feedback control.

B. Second Exemplary Embodiment

FIG. 10shows an example of an external view of magnetic shielding device1according a second exemplary embodiment. Magnetic shielding device1includes passive shield11, external coil12, internal coil13, magnetic sensor14, driver circuit15, and control unit16. Magnetic shielding device1includes both the passive shield (passive shield11) and the active shield (external coil12and internal coil13). In magnetic shielding device1, a magnetic field cancelled by external coil12is reduced by passive shield11and is further reduced by internal coil13.

Passive shield11is made of conductive material having a high conductivity such as Aluminum. Passive shield11may be made of ferromagnetic substance, such as permalloy, ferrite, or amorphous of iron, chromium, or cobalt. Passive shield11has inner space110and at least one opening111. Passive shield11has tubular shape whose cross section has square shape, with two openings111. Inner space110is so large that the inner space can accommodate a magnetic field measuring device (for example, a magnetocardiographic device) and a tested body (for example, a person). External coil12is a coil to cancel a magnetic field near the opening111. Internal coil13is a coil to cancel a magnetic field in inner space110. Helmholtz coils (a set of two coils) are used as external coil12and internal coil13, for example. Magnetic sensor14measures a magnetic field. A unique magnetic sensor14is used for each of external coil12and internal coil13. In this example, magnetic sensor14A corresponds to external coil12and magnetic sensor14B corresponds to internal coil13.

FIG. 11shows an example of a configuration of magnetic shielding device1. For the description below, a three dimensional orthogonal coordinate system is defined. In the coordinate system, x-axis, y-axis, and z-axis are defined as a direction along the width direction, the depth direction, and the height direction of passive shield11, respectively. Passive shield11has two opening s111A and111B, at the end in a direction of x-axis. External coil12A is installed near opening111A, and external coil12B is installed near opening111B. These coils are wound around passive shield11. In this example, the edge face of external coil12A is identical with the edge face111S of opening111A. Further, the edge face of external coil12B is identical with the edge face111S of opening111B. Internal coils13A and13B are located in inner space110so as to sandwich a target space (in this example, the target space includes the center of inner space110) whose magnetic field it to be cancelled,

FIGS. 12A and 12Bshow an example of a spatial configuration (layout) of magnetic sensor14.FIG. 12Ashows a view from a viewpoint on a positive x-axis.FIG. 12Bshows a schematic cross section along x-y plane. Magnetic sensor14A is located at the center (the center of gravity) of opening111, for example. Magnetic sensor14A measures x-component of a magnetic field near opening111. Magnetic sensor14B is located in the target space. Magnetic sensor14B measures x-component of a magnetic field in the target space. It is to be noted that, in this example, resolution (in magnetic field) of magnetic sensor14B is higher than that of magnetic sensor14A.

Referring toFIG. 10again. Driver circuit15A is a circuit to drive external coil12. Driver circuit15B is a circuit to drive internal coil13. Control unit16controls driver circuits15A and15B, in response to a magnetic field measured by magnetic sensor14. Control unit16is a computer device including a CPU, a ROM, and a RAM.

Control unit16receives a signal showing results of the measurement (more specifically, showing a magnitude and direction of the magnetic field) of the magnetic field, from magnetic sensors14A and14B. Control unit16controls driver circuit15A so that the magnitude of a magnetic field at edge face111S is less than a predetermined threshold. Further, control unit16controls driver circuit15B so that the magnitude of a magnetic field in inner space110is less than a predetermined threshold.

Driver circuit15A provides current to drive external coil12so that a magnetic field at edge face111S is cancelled. Here, to “cancel” a magnetic field means to control a magnetic field so that the magnetic field is within a predetermined range including zero. The predetermined range is determined based on the resolution of magnetic sensor14A in a magnetic field. The predetermined range is, for example, less than or equal to 1 pT (pico tesla). Driver circuit15A provides to coils12A and12B, current having the same direction and the same magnitude. By providing current, the magnitude of the x-components of the magnetic field in inner space110is reduced compared with a case where no external coil is used, another case where the magnetic field at the center of the inner space is cancelled, or still another case where external coil12generates a magnetic field having the same magnitude and the opposite direction with the external magnetic field.

Driver circuit15B provides current to drive internal coil13so that a magnetic field in the inner space110is cancelled. As described above, to “cancel” a magnetic field means to control a magnetic field so that the magnetic field is within a predetermined range including zero. Here, the predetermined range is determined based on the resolution of magnetic sensor14B in a magnetic field. The predetermined range is, for example, less than or equal to 1 pT (pico tesla). Driver circuit15B provides to coils13A and13B, current having the same direction and the same magnitude. By providing current, the magnitude of the x-components of the magnetic field in inner space110is reduced compared with a case where no internal coil is used.

It is to be noted that, in this case, the dynamic range of a magnetic field generated by external coil12is greater than the dynamic range of a magnetic field generated by internal coil13. Further, the resolution (in a generated magnetic field) of internal coil is greater than that of external coil12.

FIG. 13shows an example of a flowchart illustrating an operation of magnetic shielding device1. The following operation is triggered by activating magnetic shielding device1, for example.

In step S1, control unit measures a magnetic field at edge face111S. More specifically, control unit16obtains a signal showing a measured magnetic field (a magnitude and a direction of the magnetic field).

In step S2, control unit16controls driver circuit15A to provide current to external coil12. More specifically, control unit16outputs a signal causing driver circuit15A to provide to external coil12current to generate a magnetic field canceling a magnetic field measured by magnetic sensor14A. Driver circuit15A provides current to external coil12according to the signal output from control unit16. The current has a direction by which a magnetic field opposite to a magnetic field measured by magnetic sensor14A is generated. The current has a magnitude by which a magnetic field having the same magnitude as a magnetic field measured by magnetic sensor14A is generated. The relationship between the magnitude of current provided to external coil12and a magnetic field generated by external coil12is stored in the ROM, for example.

In step S3, control unit16measures a magnetic field at edge face111S.

In step S4, control unit determines whether the magnitude of the measured magnetic field at edge face111S is cancelled. If it is determined that the magnitude of the measured magnetic field is cancelled (step S4: YES), control unit16transfers the operation to step S5. If it is not determined that the magnitude of the measured magnetic field is cancelled (step S4: NO), control unit16transfers the operation to step S2again.

In step S5, control unit16measures a magnetic field in inner space110. More specifically, control unit16receives a signal from magnetic sensor14B showing the measured magnetic field.

In step S6, control unit16controls driver circuit15B to provide current to internal coil13. More specifically, control unit16outputs a signal causing driver circuit15B to provide to internal coil13current to generate a magnetic field canceling a magnetic field measured by magnetic sensor14B. Driver circuit15B provides current to internal coil13according to the signal output from control unit16. The current has a direction by which a magnetic field opposite to a magnetic field measured by magnetic sensor14B is generated. The current has a magnitude by which a magnetic field having the same magnitude as a magnetic field measured by magnetic sensor14B is generated. The relationship between the magnitude of current provided to internal coil13and a magnetic field generated by internal coil13is stored in the ROM, for example.

In step S7, control unit16measures the magnetic field in inner space110again.

In step S8, control unit determines whether the magnitude of the measured magnetic field in inner space110is cancelled. If it is determined that the magnitude of the measured magnetic field is cancelled (step S8: YES), control unit16terminates the operation shown inFIG. 13. If it is not determined that the magnitude of the measured magnetic field is cancelled (step S8: NO), control unit16transfers the operation to step S6again.

According to processes in steps S1to S4, the magnetic field in inner space110decreased near zero, with an order of the resolution of external coil12, According to processes in steps S5to S8, the magnetic field in inner space110decreased near zero, with an order of the resolution of internal coil12. According to the present exemplary embodiment, the external magnetic field is more effectively cancelled compared with a case with external coil12alone or a case with internal coil13alone.

The invention is not restricted to the embodiment described above. Various modifications may be applied to the embodiment. Some modifications will be described in the following. Two or more modification may be combined.

The coils of the shielding device are not restricted to the above described example. The shielding device may have a coil to control a gradient of a magnetic field in the target space, in addition to or instead of the above described coils.

FIG. 14shows an example of an external view of magnetic shielding device2according a modification of the second exemplary embodiment. Magnetic shielding device2includes gradient coil17, magnetic sensor14C, and a circuit15C. Description for elements common with magnetic field shield device1is omitted. Gradient coil17is a coil to control a gradient of magnetic field in inner space110. For example, two coils17A and17B are used. Magnetic sensor14C is a sensor corresponding to gradient coil17. Magnetic sensor14C is located at a face where gradient coil17is located, for example. Magnetic sensor14C measures x-component of the magnetic field at the face where gradient coil17is located. Driver circuit15C is a circuit to drive gradient coil17.

In this modification, control unit16receives a signal showing the measured magnetic field, from magnetic sensor14C. Control unit16calculates a gradient of the magnetic field based on the magnetic field measured by magnetic sensor14C. Control unit16controls driver circuit15C so that the gradient of the magnetic field is cancelled. Driver circuit15C provides current to drive gradient coil17according to a signal output from control unit16. Here, to “cancel” a gradient of a magnetic field means controlling the gradient to be within a predetermined range including zero. The predetermined range relates to a range to be considered as that the magnetic field is constant (or uniform). Driver circuit15C provides current having opposite directions, to coils17A and17B. By controlling current provided to coils17A and17B, the gradient of the magnetic field in the inner space between coils17A and17B is reduced, compared with a case where gradient coil17is not used.

FIG. 15shows an example of a flowchart illustrating an operation of magnetic shielding device2. In magnetic shielding device2, steps S9to S12are further executed in addition to steps S1to S8shown inFIG. 13.

In step S9, control unit16calculates a gradient of the magnetic field in inner space110. More specifically, control unit16receives a signal showing the magnetic field measured by magnetic sensor C, and calculates the gradient A according to the following equation (3).
A=Hmax−Hmin(3)

Here, Hmax and Hmin denote the maximum and the minimum magnetic field measured by magnetic sensor14C.

In step S10, control unit16controls driver circuit15C to provide current to gradient coil18, as well as measuring magnetic fields H1and H2generated by gradient coils17A and17B, respectively. Control unit16controls driver circuit15C so that the measured magnetic fields satisfy the following equation (4).
α=|H1−H2|−(Hmax−Hmin)=0  (4)

More specifically, control unit16outputs a signal causing driver circuit15C to provide current so that the gradient of the magnetic field is cancelled. Driver circuit15C provides current to a gradient coil17according to the signal output from control unit16.

In step S11, control unit16calculates the gradient of the magnetic field in inner space110again.

In step S12, control unit16determines whether the gradient of the magnetic field in inner space is cancelled. If it is determined that the gradient is cancelled (step S12: YES), control unit16terminates the operation. If it is not determined that the gradient is not cancelled (step S12: NO), control unit16transfers the operation to step S10again. According to processes in steps S9to S12, the gradient of the magnetic field in inner space110become near zero.

It is to be noted that magnetic sensor14C may be omitted in a case that gradient coil17is used. In such a case, control unit16may control gradient coil18based on a signal from magnetic sensor14B.

The cross section perpendicular to x-axis is not restricted to a square. For example, the cross section perpendicular to x-axis may be a polygon other than a square, a circle, or an ellipse. Passive shield may not have two openings111at the edges. At least one opening may be covered by a cover. Further, passive shield11may be made of multi-layered material.

The shape of external coil12, internal coil13, and gradient coil17is not restricted to the example shown in the above exemplary embodiments. The shape may be a polygon other than a square, a circle, or an ellipse.

Each of external coil12, internal coil13, and gradient coil17is not restricted to a Helmholtz coil. A single coil may be used external coil12and internal coil13.

In the above embodiment, only an x-axis component of the magnetic field is cancelled. However, two or more axes components may be cancelled or controlled. In such a case, magnetic shielding device1has two or more sets of internal coils13. For example, magnetic shielding device1may have three sets of internal coils13, for x-, y-, and z-axes. Each component of the magnetic field may be cancelled or controlled.

The number of magnetic sensor14and the layout thereof are not restricted to examples shown in the above exemplary embodiments. For example, magnetic sensor14A may be located at the center of gravity of one of opening111A and111B.

A method for controlling a coil is not restricted to the example described above. In the above embodiment, each coil is feedback controlled. A coil may be controlled by a method other than the feedback control.

C. Third Exemplary Embodiment

The first exemplary embodiment and the second exemplary embodiment may be combined. For example, gradient coil17may be used in addition to or instead of internal coil13in the first exemplary embodiment.