JP2016006817A - Magnetic shield device and magnetic shield method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic shield device having an excellent magnetic field measurement space by controlling a gradient of inflow magnetic field.SOLUTION: The magnetic shielding device includes a passive shield 11, a correction target space 150 designated inside the passive shield, external coils 12A and 12B as a first coil for correcting the magnetic field inside the passive shield 11, a first magnetic sensor 14A, a second magnetic sensor 14B arranged at a position more inside than the magnetic sensor 14 inside the passive shield 11, and a control unit 20. The first magnetic sensor 14A and the second magnetic sensor 14B measure the gradient of the magnetic field inside the passive shield 11. The control unit 20 controls the external coils 12A and 12B on the basis of the measurement result of the first magnetic sensor 14A and the second magnetic sensor 14B.

Description

  The present invention relates to a magnetic shield device and a magnetic shield method.

Image diagnosis is widely used in the medical field. Since image diagnosis is non-invasive, there is little burden on the human body, and it is particularly preferred when examining organs important for life activity such as the heart and brain.
A living organ always generates a weak current due to the activity of neurons, and the state of the organ can be known by measuring the magnetic field generated by this current.
Specifically, the magnetic field caused by a living body is extremely weak, about 0.1 pico tesla (1 × 10 ⁻¹³ T) to about 10 pico tesla (1 × 10 ⁻¹¹ T).
One technique for detecting and imaging such a magnetic field is magnetic field source drawing. For example, a method for recording a magnetic field formed by the movement of the heart is known as magnetocardiography (MCG). A method for measuring and recording a magnetic field formed by brain activity is known as magnetoencephalography (MEG).

On the other hand, the external magnetic field such as geomagnetism is, for example, about 10 micro Tesla (1 × 10 ⁻⁵ T) to about 100 micro Tesla (1 × 10 ⁻⁴ T), compared with the magnetic field generated from the brain and heart. It is 100,000 times larger. Therefore, such an external magnetic field becomes noise in highly sensitive magnetic field measurement.
That is, in order to detect a weak magnetic field from a living body, it is necessary to cut off an external magnetic field that becomes noise after using a highly sensitive magnetic field measuring device, and to shield an external magnetic field, a magnetic shield device Must be used. As a magnetic shield device that shields an external magnetic field, for example, techniques described in Patent Documents 1 to 8 are known.

  According to the techniques described in Patent Documents 1 to 8, a cancellation magnetic field is generated so that the magnetic field in the space to be controlled (the space in which the magnetic field measurement device is arranged) inside the magnetic shield device is minimized, or The canceling magnetic field is generated so that the magnetic field flowing into the opening of the magnetic shield device is minimized.

JP-A-6-167583 JP 2002-94280 A JP 2002-257914 A JP 2005-294537 A JP 2007-129049 A JP 2008-282893 A JP 2009-175067 A Japanese Patent No. 4377666

  However, the techniques described in Patent Documents 1 to 8 have a problem that it is difficult to create an environment having a very weak magnetic field gradient inside the magnetic shield. Specifically, in the prior art, the magnetic field from the outside can be reduced. However, since the magnetization component of the magnetic shield is not considered, there is a problem that a gradient occurs in the magnetic field state inside the magnetic shield.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  Application Example 1 A magnetic shield device according to this application example includes a passive shield, a correction target space defined in the passive shield, a first coil for correcting a magnetic field in the passive shield, and a first magnetic sensor. And a second magnetic sensor disposed inside the passive shield with respect to the first magnetic sensor, and a control unit, wherein the first magnetic sensor and the second magnetic sensor are in the passive shield. The magnetic field gradient is measured, and the control unit controls the first coil based on measurement results of the first magnetic sensor and the second magnetic sensor.

  According to this configuration, each magnetic field strength is measured by the first magnetic sensor and the second magnetic sensor, and the magnetic field gradient inside the passive shield is measured. Based on the measured value of the magnetic field gradient, the control unit estimates the maximum magnetic field gradient in the correction target space. Furthermore, the control unit adjusts the current amount of the first coil so that the maximum magnetic field gradient in the correction target space is equal to or less than a predetermined threshold value. By setting the magnetic field gradient in the correction target space to be equal to or less than a predetermined threshold value, it is possible to provide a space in the correction target space in which magnetic field measurement is performed with a very weak magnetic field gradient.

  Application Example 2 In the magnetic shield device according to this application example, it is preferable that the control unit controls the first coil so that the magnetic field gradient is equal to or less than a threshold value.

According to this configuration, by setting the magnetic field gradient to a predetermined threshold value or less, the magnetic field gradient can be set to a predetermined threshold value or less in the correction target space inside the passive shield as compared with the first magnetic sensor.
That is, it is possible to provide a space for performing magnetic field measurement in a state where the magnetic field gradient is extremely weak in the correction target space.

  Application Example 3 According to the magnetic shield device according to this application example, the first magnetic sensor and the second magnetic sensor measure the magnetic field gradient along the first direction, and the axis of the first coil is It is preferable that it is along the first direction.

  According to this configuration, the direction of the magnetic field gradient measured by the first magnetic sensor and the second magnetic sensor is the same as the axial direction of the first coil. Therefore, the magnetic field gradient can be easily adjusted by controlling the current amount of the first coil.

  Application Example 4 According to the magnetic shield device according to this application example, the passive shield has an opening, the coordinates of the opening in the first direction are set as the origin, and the opening along the first direction. When the direction from the portion toward the inside of the passive shield is a positive direction of the first direction, the first magnetic sensor has a projection cross section orthogonal to the first direction of the opening with respect to the first direction. It is preferable that it is disposed within the range of -0.5 to 1.0 times the square root of the area.

  According to this configuration, it is possible to increase the degree of freedom of arrangement of the first magnetic sensor, and to make the magnetic field gradient equal to or less than a predetermined threshold even in the correction target space of the passive shield having an opening.

  Application Example 5 According to the magnetic shield device of this application example, the second magnetic sensor is 0 times the square root of the area of the projection cross section perpendicular to the first direction of the opening with respect to the first direction. It is preferable that the coordinates of the second magnetic sensor with respect to the first direction be more positive than the coordinates of the first magnetic sensor with respect to the first direction.

  According to this configuration, the degree of freedom of arrangement of the second magnetic sensor can be increased, and a magnetic field gradient equal to or less than a predetermined threshold can be obtained even in the correction target space of the passive shield having an opening.

  Application Example 6 According to the magnetic shield device according to this application example, the correction target space is more positive with respect to the coordinates relating to the first direction than the coordinates relating to the first direction of the first magnetic sensor. preferable.

  According to this configuration, even in the correction target space of the passive shield, the magnetic field gradient can be set to a predetermined threshold value or less.

  Application Example 7 In the magnetic shield device according to this application example, it is preferable that a second coil for adjusting the magnetic field strength of the correction target space is further provided inside the passive shield.

  According to this configuration, it is possible to adjust the magnetic field strength of the correction target space to a predetermined value by adjusting the current amount of the second coil. That is, it is possible to provide a magnetic field measurement space for performing magnetic field measurement in a state where the magnetic field strength is weaker in the correction target space.

  Application Example 8 A magnetic shielding method according to this application example includes a first step of measuring a magnetic field gradient in a passive shield, and a second step of setting the magnetic field gradient to a predetermined threshold or less based on the first step. And a third step of measuring the magnetic field strength in the passive shield, and a fourth step of setting the magnetic field strength to a predetermined threshold value or less.

  According to this method, the magnetic field gradient below a predetermined threshold can be obtained in the correction target space of the passive shield. It is also possible to adjust the magnetic field strength of the correction target space to a predetermined value. That is, the correction target space can be a magnetic field measurement space in which magnetic field measurement is performed in a state where the magnetic field gradient is extremely weak.

The perspective view of an outline of a magnetic shield device. The general-view figure including the control part of a magnetic shielding apparatus. The figure which shows an example of a structure of a control circuit. The operation | movement flowchart of a magnetic shielding apparatus. The distribution map of the magnetic field intensity of a magnetic shielding apparatus. The general-view figure of the magnetic shielding apparatus which showed the 2nd example of arrangement | positioning. The general-view figure of the magnetic shielding apparatus which showed the 3rd example of arrangement | positioning. The general-view figure of the magnetic shielding apparatus which showed the 4th example of arrangement | positioning. The general-view figure of the magnetic shielding apparatus which showed the 5th example of arrangement | positioning.

  DESCRIPTION OF EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings. Note that the drawings to be used are appropriately enlarged or reduced so as to clearly recognize the portion to be described and promote understanding of the reader.

<Embodiment>
FIG. 1 is a schematic perspective view of a magnetic shield device 100 according to this embodiment. The magnetic shield device 100 is used, for example, to shield a magnetic measurement device from an external magnetic field such as geomagnetism when measuring a weak current emitted from a living body as magnetism.
The magnetic shield device 100 includes a passive shield 11, external coils 12 </ b> A and 12 </ b> B that correct an internal magnetic field of the passive shield 11, a first magnetic sensor 14 </ b> A and a second magnetic sensor 14 </ b> B, and an internal portion disposed inside the passive shield 11. The coils 13A and 13B and the control unit 20 (see FIG. 2) are included. Here, the external coils 12A and 12B are examples of the “first coil” according to the present invention, and the internal coils 13A and 13B are examples of the “second coil” according to the present invention.

  The passive shield 11 has a cylindrical shape with a hollow inside, and is disposed on a pedestal (not shown), for example, with its axial direction substantially horizontal. The cross-sectional shape of the passive shield 11 in the axial direction is generally a quadrangle. The passive shield 11 is open at both ends in the axial direction, and has one end as an opening 11A and the other end as an opening 11B. As for the size of the passive shield 11, the length in the axial direction is about 200 cm, and one side of the openings 11A and 11B is about 90 cm. The passive shield 11 having such a size allows a “human body” as an object to be measured to enter and exit from either of the openings 11A and 11B and lie inside thereof.

  The passive shield 11 is made of a ferromagnetic material (permalloy, ferrite, or iron, chromium, or cobalt-based amorphous material) having a relative permeability of, for example, several thousand or more, or a high-conductivity conductor (for example, aluminum) and an eddy current. It is desirable to have a magnetic field reducing effect depending on the effect. Note that the above-described ferromagnetic material and high conductivity conductor can be alternately laminated. In this embodiment, aluminum and permalloy are alternately formed in two layers, and the total thickness is set to about 20 to 30 mm.

  A region surrounded by the passive shield 11 is defined as an internal space 110, and a region excluding the internal space 110 (outside the passive shield 11) is defined as an external space. The internal space 110 has a space that can accommodate an object to be measured (for example, a human body) and a high-sensitivity magnetic sensor (not illustrated) (for example, an optically pumped magnetic sensor) that measures a magnetic field emitted from the object to be measured. Of the internal space 110, a region where a high-sensitivity magnetic sensor is arranged and a weak magnetic signal from, for example, the heart or brain is measured is particularly referred to as a correction target space 150. More details will be described later.

  In the following description, a three-dimensional orthogonal coordinate system is used to facilitate understanding of the present embodiment. The axial direction of the passive shield 11 is defined as the X direction, the depth direction is defined as the Y direction, and the so-called height direction orthogonal to the X direction and the Y direction is defined as the Z direction. A line connecting the centers of the vertical sections (YZ planes) at each position in the axial direction of the passive shield 11 and a line obtained by extending the lines are hereinafter defined as a central axis 110A. In the case of a substantially cylindrical shape like the passive shield 11 of the present embodiment, a line connecting the center point of the opening 11A and the center point of the opening 11B becomes the center axis 110A.

  Further, the position of the opening 11A is defined as the origin in the X direction, and the direction from the opening 11A to the opening 11B is defined as the positive X direction. Here, the X direction is an example of the “first direction” according to the present invention. For example, when the opening 11A is not perpendicular to the central axis 110A, that is, when the opening 11A has an inclination with respect to the central axis 110A, the ridge line surrounding the opening 11A is in the most positive X direction. A certain position is the origin in the X direction. That is, the position in the X direction in which the same shape as the projection plan view of the passive shield 11 on the YZ plane appears as a cross-sectional shape is called an origin.

The external coils 12A and 12B are coils for correcting the magnetic field flowing into the internal space 110. The inflow magnetic field is a magnetic field in which an external magnetic field enters the internal space 110 from the openings 11A and 11B. The inflow magnetic field is in a direction perpendicular to the vertical section of the openings 11A and 11B (that is, a direction along the first direction). To become the strongest. The external coils 12A and 12B are Helmholtz coils, for example, and generate a magnetic field by a current supplied from a power source (not shown).
The external coil 12A is disposed on the opening 11A side, and the external coil 12B is disposed on the opening 11B side, which is the remaining end, so as to surround the passive shield 11. In the present embodiment, the diameters of the external coils 12A and 12B are about 120 cm. It is desirable that the external coil 12A is disposed at substantially the same position as the position of the opening 11A in the X direction, that is, at the origin position in the X direction. It is desirable that the external coil 12B is disposed at substantially the same position as the opening 11B in the X direction.

  The external coils 12 </ b> A and 12 </ b> B are arranged in a positional relationship in which the axes passing through the centers of the external coils 12 </ b> A and 12 </ b> B are substantially the same direction as the direction of the central axis 110 </ b> A of the passive shield 11. As a result, the direction in which the inflow magnetic field becomes stronger and the direction of the magnetic field corrected by the external coils 12A and 12B substantially coincide with each other, so that the magnetic field gradient can be corrected efficiently. Note that the external coils 12A and 12B may be configured by surrounding the passive shield 11 with a non-magnetizable frame or the like (not shown) and winding the coil around the frame portion. Further, the shape of the external coils 12A and 12B is not limited to the shape similar to the cross-sectional shape in the direction of the central axis 110A of the passive shield 11, and may be, for example, a circular shape.

  The first magnetic sensor 14A is, for example, a semiconductor sensor that measures an inflow magnetic field from the external space to the internal space 110. The first magnetic sensor 14A is provided on the axis of the central axis 110A, and is disposed at substantially the same position as the opening 11A in the X direction.

  The second magnetic sensor 14B is a semiconductor sensor, for example, for measuring the magnetic field strength of a certain region of the internal space 110. The second magnetic sensor 14B is provided on the axis of the central axis 110A, like the first magnetic sensor 14A. Both the first magnetic sensor 14A and the second magnetic sensor 14B have a size of about 2 cm square, and are respectively arranged inside the passive shield 11 by a non-magnetizable jig (not shown).

  The second magnetic sensor 14B is located inside the passive shield 11 with respect to the first magnetic sensor 14A. In other words, it is located in the positive X direction with respect to the first magnetic sensor 14A. By measuring the magnetic field strength with the first magnetic sensor 14A and the second magnetic sensor 14B, the magnetic field gradient along the axial direction of the central axis 110A can be measured. Hereinafter, the magnetic field gradient obtained by the first magnetic sensor 14A and the second magnetic sensor 14B will be described as an inflow magnetic field gradient.

  In summary, the first magnetic sensor 14A and the second magnetic sensor 14B are provided on the central axis 110A. In addition, since the axes passing through the centers of the external coils 12A and 12B are also arranged in the same direction as the central axis 110A, the direction of the inflow magnetic field gradient and the direction of the magnetic field that can be corrected by the external coils 12A and 12B. Is the same.

  The internal coils 13A and 13B are coils used to adjust the magnetic field strength of the correction target space 150, and are arranged in the internal space 110 so as to sandwich both ends of the correction target space 150. The internal coils 13 </ b> A and 13 </ b> B are preferably arranged so that the axis passing through the center of each coil overlaps the central axis 110 </ b> A of the passive shield 11.

  The internal coils 13A and 13B are Helmholtz coils, for example, and generate a uniform magnetic field by flowing a current supplied from a power source (not shown) to the coils. Since it is a Helmholtz coil, a uniform magnetic field strength can be formed on an axis connecting at least the centers of the internal coils 13A and 13B of the correction target space 150. That is, by adjusting the amount of current flowing through the internal coil 13A and the internal coil 13B, the magnetic field strength of the correction target space 150 can be adjusted to an arbitrary value.

  Since the correction target space 150 is not of a property that can clearly define the position, in the passive shield 11 having the internal coils 13A and 13B, the space sandwiched between the internal coil 13A and the internal coil 13B is the correction target space. It is defined as 150 (area where dot hatching is applied in FIGS. 1 and 2). The correction target space 150 is located in the positive X direction with respect to the first magnetic sensor 14A.

  As described above, in the present embodiment, the external coils 12A and 12B, the internal coils 13A and 13B, the first magnetic sensor 14A, and the second magnetic sensor 14B are arranged along the central axis 110A of the passive shield 11. Has been.

  Once the description of the magnetic shield device 100 has been completed, the purpose of this embodiment will be described more clearly. As a result of intensive research by the present inventor, for example, in order to measure a weak magnetic field that is less than 1 / 100,000 of an external magnetic field with high sensitivity, such as a magnetic field from a living body, It has been found that it is important that the magnetic field gradient in the correction target space 150 is not more than a certain threshold value.

  That is, in the conventional techniques as described in Patent Documents 1 to 8, a canceling magnetic field is generated so that the magnetic field of the correction target space 150 is minimized, or from the opening 11A (11B) of the magnetic shield device 100. The canceling magnetic field was generated so that the inflow magnetic field of the current was minimized. Then, a difference between, for example, the end of the correction target space 150 and the opening 11 </ b> A appeared as a magnetic field gradient in the correction target space 150. The inventor has found that the magnetic field gradient in the correction target space 150 hinders highly sensitive magnetic field measurement. Therefore, in order to measure the weak magnetic field as described above with high sensitivity, it is first important to control the magnetic field gradient in the correction target space 150 to a certain threshold value or less. It is important to minimize the magnetic field strength in the correction target space 150 according to the object to be measured after being set to a certain threshold value or less. The threshold will be described later.

  It has also been confirmed by the inventors' experiments that the magnetic field gradient in the correction target space 150 can be estimated from the inflow magnetic field gradient. In other words, it was confirmed that the magnetic field gradient in the correction target space 150 can be made equal to or less than the predetermined threshold value by making the inflow magnetic field gradient equal to or less than a certain threshold value.

  In order to avoid misunderstanding, the inflow magnetic field gradient and the magnetic field gradient in the correction target space 150 are numerically different. This is because the inflow magnetic field gradient is a magnetic field gradient in the vicinity of the opening 11 </ b> A, and is usually larger than the magnetic field gradient in the correction target space 150 due to the influence of a leakage magnetic field from the passive shield 11.

  The relative positional relationship in the X direction between the first magnetic sensor 14A and the second magnetic sensor 14B, which is important in estimating the magnetic field gradient in the correction target space 150, will be described in more detail. According to the experiment by the present inventor, it is possible to suitably measure or estimate the inflow magnetic field gradient by arranging the first magnetic sensor 14A and the second magnetic sensor 14B at positions having the following relationships. I was able to confirm. The preferable measurement or estimation here makes it possible to estimate the magnetic field gradient in the correction target space 150 (specifically, the magnetic field gradient between the internal coil 13A and the internal coil 13B) based on the inflow magnetic field gradient. This refers to measurement.

In showing the area | region which can arrange | position the 1st magnetic sensor 14A and the 2nd magnetic sensor 14B, let S be the cross-sectional area of 11 A of opening parts. In this case, the sectional area S of the opening 11A is defined as the area of the projected section on the YZ plane with respect to the X direction. As described above, when the center position of the opening 11A is the origin in the X direction and the direction from the opening 11A to the opening 11B is the positive X direction (+ X direction), the position of the first magnetic sensor 14A is from the origin. With respect to the + X direction, it is less than 1.0 times (+ √S) the square root of the area (S) of the projection cross section of the opening 11A onto the YZ plane, and is absolute with respect to the −X direction from the origin. It is preferable to be located in a range of 0.5 times (0.5 × √S) or less of the square root of the area (S) of the projected cross section on the YZ plane. That is, if the range (X coordinate) in which the first magnetic sensor 14A can be arranged is L1, L1 corresponds to the range of the expression (1). In the present embodiment, the position of L1 is 0 cm.
−0.5 × √S ≦ L1 <+ √S (1)

Further, the position of the second magnetic sensor 14B is located in a range of 1.0 times (+ √S) or less of the square root of the area (S) of the projection cross section on the YZ plane with respect to the + X direction from the position of the origin. It is suitable. That is, if the range in which the second magnetic sensor 14B can be arranged is L2, L2 corresponds to the range of Expression (2). However, the position of the second magnetic sensor 14B in the + X direction must always be more positive than the first magnetic sensor 14A. In this embodiment, the position of L2 is about 60 cm.
0 <L2 ≦ + √S (2)

FIG. 2 shows an overview including the control unit of the magnetic shield device 100 according to the present embodiment. FIG. 2 shows a control unit 20 in addition to the schematic cross-sectional view of the magnetic shield device 100 in FIG. 1 viewed from the Y-axis direction. In the following description, the description of the same part as in FIG. 1 is omitted.
In FIG. 2, the control unit 20 includes the measurement circuit 15, the drive circuit 16, and the control circuit 17, but is not limited to this configuration. That is, any one or more of these may be included, and other circuits may be included. In addition, although the X, Y, and Z directions are illustrated in FIG. 2, these are not taken into account when representing the positional relationship of the measurement circuit 15 and the like constituting the control unit 20.

The measurement circuit 15 is connected to the first magnetic sensor 14A, the second magnetic sensor 14B, and a high-sensitivity magnetic sensor (not shown) located in the correction target space 150, respectively. Here, the high-sensitivity magnetic sensor means a magnetic sensor having a measurement accuracy equal to or higher than that of the first magnetic sensor 14A and the second magnetic sensor 14B. In FIG. 2, a plurality of sensors such as the first magnetic sensor 14 </ b> A and the second magnetic sensor 14 </ b> B are connected to one measurement circuit 15. However, each magnetic sensor has an independent measurement circuit 15. You may do it.
The measurement circuit 15 can measure the magnetic field strength in the first magnetic sensor 14A and the second magnetic sensor 14B, for example, the distance from the position of the origin.

  The drive circuit 16 is connected to the external coils 12A and 12B and the internal coils 13A and 13B, respectively. In FIG. 2, four coils of the external coils 12A and 12B and the internal coils 13A and 13B are connected to one drive circuit 16, but each coil has an independent drive circuit 16. May be. The drive circuit 16 supplies an alternating current and a direct current to the coil from a power source (not shown) to generate a magnetic field.

  The control circuit 17 is connected to the measurement circuit 15 and the drive circuit 16. The control circuit 17 is an apparatus for measuring the inflow magnetic field gradient according to the magnetic field intensity measured by the measurement circuit 15 and controlling the drive circuit 16 so that the inflow magnetic field gradient is equal to or less than a predetermined threshold value.

  FIG. 3 is a diagram illustrating an example of the configuration of the control circuit 17. The control circuit 17 includes a CPU (Central Processing Unit) 21, a long-term storage circuit (for example, a nonvolatile memory such as a ROM (Read Only Memory) or a flash memory) 22, and a temporary storage circuit (for example, a RAM (RAM such as DRAM or SRAM). Random Access Memory)) 23, an input unit 24, and an output unit 25. Each component is electrically connected to each other via a bus 26.

  The CPU 21 reads and executes the control program stored in the long-term storage circuit 22 based on the magnetic field intensity acquired from the measurement circuit 15 and controls the drive circuit 16. As an example, the long-term memory circuit 22 holds a threshold value of the inflow magnetic field gradient. The temporary storage circuit 23 provides a work area when the CPU 21 executes the control program. The input unit 24 receives the magnetic field strength and the like from the measurement circuit 15 via an operation unit (not shown). The output unit 25 outputs a drive signal (for example, the magnitude of an alternating current) to the drive circuit 16. That is, the CPU 21 is optimal for the drive circuit 16 so that the inflow magnetic field gradient calculated based on the magnetic field strength measured by the first magnetic sensor 14A, the second magnetic sensor 14B, and the measurement circuit 15 is less than the threshold value. Command can be sent.

<Operation Method of Magnetic Shield Device 100>
Next, the operation of the magnetic shield device 100 will be described with reference to FIG. FIG. 4 is an operation flowchart of the magnetic shield device 100. The process of FIG. 4 is started when the magnetic shield device 100 is driven, for example. That is, it means a state in which the external coils 12A and 12B and the internal coils 13A and 13B are operated by the drive circuit 16 based on the performance of the passive shield 11 (for example, indicated by “high magnetic permeability × thickness”).

<Inflow magnetic field gradient measurement (S10)>
A method for measuring the inflow magnetic field gradient will be described. As described above, the inflow magnetic field gradient in step S10 refers to the difference between the magnetic field intensities measured by the first magnetic sensor 14A and the second magnetic sensor 14B, between the first magnetic sensor 14A and the second magnetic sensor 14B. Divided by the distance.

  First, the first magnetic sensor 14A and the measurement circuit 15 measure the magnetic field strength (magnetic field flowing from the external space to the internal space 110) at the position where the first magnetic sensor 14A is installed, and the distance from the origin. (In the present embodiment, since the first magnetic sensor 14A is disposed at the origin, the distance from the origin is zero). Subsequently, the magnetic field intensity at the position where the second magnetic sensor 14B is installed (the magnetic field intensity of the internal space 110) and the distance from the origin are measured by the second magnetic sensor 14B and the measurement circuit 15.

The control circuit 17 acquires the magnetic field strength and distance between the first magnetic sensor 14A and the second magnetic sensor 14B measured by the measurement circuit 15, and measures the inflow magnetic field gradient. The distance between the first magnetic sensor 14A and the second magnetic sensor 14B is not limited to the case where the distance is measured by the measurement circuit 15. For example, it can be measured in advance by the user and input to the long-term storage circuit 22 via the input unit 24 of the control circuit 17.
Incidentally, the inflow magnetic field gradient here is not necessarily limited to a linear linear gradient. In other words, the present invention can be applied even in a state having substantially a first-order or higher-order gradient component, for example, a second-order gradient.
The process proceeds to step S20 “judgment magnetic field gradient threshold determination”.

<Threshold determination of inflow magnetic field gradient (S20)>
The control circuit 17 determines whether or not the inflow magnetic field gradient measured in step S10 is equal to or less than a predetermined threshold value. The predetermined threshold here is obtained from the relationship between the inflow magnetic field gradient obtained in step S10 and the magnetic field gradient allowed in the correction target space 150 (that is, the region where the brain magnetic field or the like is measured).
Usually, in the passive shield 11, the magnetic field distribution of the ferromagnetic material constituting the passive shield 11 is different between the vicinity of the opening 11 </ b> A (11 </ b> B) and the correction target space 150 due to the structure of the passive shield 11. . Therefore, the inflow magnetic field gradient obtained in step S10 does not exist uniformly up to the correction target space 150, but the magnetic field gradient becomes more gradual as it proceeds from the opening 11A (11B) to the correction target space 150. It is also known that
As described above, it has been confirmed by experiments of the present inventors that the magnetic field gradient in the correction target space 150 can be estimated from the inflow magnetic field gradient. In other words, it has been confirmed that the magnetic field gradient in the correction target space 150 can be made equal to or less than a predetermined threshold value by making the inflow magnetic field gradient equal to or less than a certain threshold value.

Specifically, the magnetic field gradient in the correction target space 150 is estimated using the magnetic field strengths of the first magnetic sensor 14A and the second magnetic sensor 14B and the mutual distance as parameters, that is, the inflow magnetic field gradient measured in step S10. It has been experimentally confirmed that this is possible. That is, as an example, when the projection cross section of the opening 11A has an area S, and the inflow magnetic field gradient is about 50 nT / cm, the range from 1 to 1.6 times the square root of the area of the projection cross section from the origin (this In the case of the embodiment, since S = 8100 cm ² , it has been confirmed that the magnetic field gradient of the correction target space 150 in the range of approximately 90 cm to 140 cm from the origin is ² nT / cm or less.

  In other words, a correlation between the inflow magnetic field gradient and the magnetic field gradient in the correction target space 150 is obtained by previously obtaining an inflow magnetic field gradient and the magnetic field gradient in the correction target space 150 in the inflow magnetic field gradient. be able to. That is, by storing the correlated data and the relational expression derived based on the data in the long-term storage circuit 22 of the control circuit 17, for example, the determination in step S20 can be easily performed.

Here, the threshold value of the magnetic field gradient allowed in the correction target space 150 is determined according to the measurement range of a magnetic measurement device (high-sensitivity magnetic sensor) such as a magnetocardiograph used in the correction target space 150.
More specifically, the difference in magnetic field strength at both ends in the X direction of the correction target space 150 (both ends can be defined by the internal coil 13A and the internal coil 13B) is a predetermined value corresponding to the measurement range of the magnetometer. It is determined to be below the value. For example, assuming that the measurement range in the magnetic measurement apparatus is 10 nT, the difference between the magnetic field strengths at both ends in the X direction of the correction target space 150 (the magnetic field gradient of the correction target space 150 and the length of the correction target space 150 in the X direction). The threshold value of the magnetic field gradient allowed in the correction target space 150 is determined so that the product) is equal to or less than the measurement range.

That is, when the measurement range in the magnetic measurement apparatus is 10 nT and the length of both ends in the X direction of the correction target space 150 is 1 m, the threshold value of the magnetic field gradient in the correction target space 150 is 10 nT / m. The correction target space 150 having a magnetic field gradient that is equal to or lower than the magnetic field gradient allowed in the correction target space 150 is hereinafter referred to as a correction target space 150 in which the magnetic field gradient is canceled.
That is, making the inflow magnetic field gradient equal to or less than the threshold value has the same meaning as making the correction target space 150 a space in which the magnetic field gradient is canceled.

  When the control circuit 17 determines that the inflow magnetic field gradient is equal to or less than the threshold value obtained by the above method, the control circuit 17 advances the process to step S30 “external coil maintenance state”. If it is determined that the value is larger than the threshold value, the process proceeds to step S40 “external coil control state”.

<External coil maintenance state (S30)>
If the control circuit 17 determines that the inflow magnetic field gradient is equal to or less than the threshold value, the control circuit 17 causes the current flowing in the external coils 12A and 12B to the drive circuit 16 to maintain the current inflow magnetic field gradient. Let the amount maintain. Subsequently, the process proceeds to step S50 “Measurement of magnetic field in correction target space”.

<External coil control state (S40)>
When the control circuit 17 determines that the inflow magnetic field gradient is larger than the threshold value, the control circuit 17 adjusts the amount of current flowing through the external coils 12A and 12B so that the magnetic field gradient is equal to or less than the threshold value. Specifically, a signal for causing the external coils 12 </ b> A and 12 </ b> B to supply a current that causes a predetermined inflow magnetic field gradient to the drive circuit 16. The drive circuit 16 supplies current to the external coils 12A and 12B based on a signal from the control circuit 17. The relationship between the magnitude of the current supplied to the external coils 12A and 12B and the magnetic field generated in the external coils 12A and 12B is stored in advance in the long-term memory circuit 22 of the control circuit 17, for example.

Note that any method may be used for making the inflow magnetic field gradient used for this control equal to or less than the threshold value. For example, when the magnetic field strength measured by the second magnetic sensor 14B is larger than the magnetic field strength measured by the first magnetic sensor 14A, or conversely, the magnetic field strength measured by the second magnetic sensor 14B is The control circuit 17 can control the amount of current supplied to the external coils 12A and 12B in any magnetic field gradient direction, such as when the magnetic field intensity is smaller than that measured by the magnetic sensor 14A.
And it returns to step S10 "inflow magnetic field gradient measurement" again. Steps S10, S20, and S30 are looped until the control circuit 17 determines that the magnetic field gradient is equal to or less than the threshold value.
If it is determined that the threshold value is not more than the threshold value, the process proceeds to step S50 “magnetic field measurement of the correction target space”.

<Magnetic field measurement in correction target space (S50)>
Subsequently, the measurement circuit 15 performs magnetic field measurement of the correction target space 150 with a high-sensitivity sensor (not shown) (for example, an optical pumping magnetic sensor) disposed in the correction target space 150.
By the steps up to step S30, the correction target space 150 is already a space where the magnetic field gradient has been canceled. However, since the residual magnetic field is generated by the passive shield 11 in the correction target space 150, the magnetic field strength of the correction target space 150 may be increased by at least the residual magnetic field strength. Note that the cancellation of the magnetic field gradient does not mean that the magnetic field strength is substantially zero. The cancellation of the magnetic field gradient means that the magnetic field strength is uniform.
Proceed to step S60.

<Threshold determination of magnetic field strength (S60)>
The control circuit 17 acquires the magnetic field strength of the correction target space 150 from the measurement circuit 15 and determines whether or not the magnetic field strength of the correction target space 150 is equal to or less than a threshold value. As described above, the correction target space 150 is not in a state where the magnetic field intensity is substantially zero. In other words, due to the presence of the residual magnetic field, an offset is applied by the magnetic field intensity of the residual magnetic field, and this offset state becomes the background when performing magnetic field measurement, and a suitable magnetic field measurement environment corresponding to the object to be measured is established. It may not be provided.

The threshold value used as a criterion for determining whether or not the threshold value is equal to or smaller than the threshold value varies depending on the sensitivity of the magnetic field measuring device selected according to the object to be measured, but at least cancels the residual magnetic field remaining in the correction target space 150. That is, a value that can make the magnetic field strength substantially zero. By setting the magnetic field intensity to substantially zero, a suitable magnetic field measurement environment can be provided.
Note that the value of approximately zero here cannot be said to be absolutely zero because it includes a measurement error and the absolute sensitivity of the magnetic field measurement device required differs depending on the object to be measured. Because.

  When the control circuit 17 determines that the magnetic field strength of the correction target space 150 is equal to or less than the threshold value, the control circuit 17 proceeds to step S <b> 70 “internal coil maintenance state”. If it is determined that the value is larger than the threshold value, the process proceeds to step S80 “internal coil control state”.

<Internal coil maintenance state (S70)>
When the control circuit 17 determines that the magnetic field strength of the correction target space 150 is equal to or less than the threshold value, the control circuit 17 flows to the internal coils 13A and 13B with respect to the drive circuit 16 in order to maintain the current magnetic field strength. Maintain the amount of current. Then, the operation of the magnetic shield device 100 ends. Since the inflow magnetic field gradient and the magnetic field strength in the correction target space 150 may vary depending on the state of the external space, the process proceeds from step S70 to step S10 “inflow magnetic field gradient measurement” again, and proceeds to steps S10 to S80. It is also possible to loop the process flow to reach.

<Internal coil control state (S80)>
When the control circuit 17 determines that the magnetic field strength of the correction target space 150 is greater than the predetermined threshold value, the control circuit 17 determines the amount of current flowing through the internal coils 13A and 13B so that the magnetic field strength is equal to or less than the threshold value. Adjust. Specifically, the drive circuit 16 outputs a signal for causing the internal coils 13A and 13B to supply a current that provides a predetermined magnetic field strength.
The drive circuit 16 supplies current to the internal coils 13A and 13B based on the signal from the control circuit 17. The relationship between the magnitude of the current supplied to the internal coils 13A and 13B and the magnetic field generated inside the internal coils 13A and 13B is stored in advance in the long-term storage circuit 22 of the control circuit 17, for example.
Then, the process proceeds to step S50 “magnetic field measurement of the correction target space”, and steps S50, S60, and S80 are looped until the control circuit 17 determines that the magnetic field strength is equal to or less than the threshold value.
When it is determined that the threshold value is not more than the threshold value, the process proceeds to step S70 “internal coil maintaining state”, and the operation of the magnetic shield device 100 is terminated.

  The relationship between the magnetic field gradient and the magnetic field intensity controlled by the processing flow of FIG. 4 will be further described with reference to FIG. FIG. 5 shows an example of the distribution of the magnetic field strength of the magnetic shield device 100 controlled by the processing flow of FIG. The description overlapping with the preceding figure is omitted.

  FIG. 5A is a schematic view of a magnetic shield device 100 similar to FIG. 2, and FIG. 5B is a magnetic field intensity distribution diagram corresponding to the X direction of FIG. 5A. In FIG. 5B, the vertical axis represents the magnetic field strength, and the horizontal axis represents the opening 11A as the origin in the X direction, and the direction from the first magnetic sensor 14A toward the second magnetic sensor 14B as the positive X direction. It represents the relative positional relationship of time. For convenience, the position of the first magnetic sensor 14A in the X direction is L1 (actually, the origin is X = 0), and the position of the second magnetic sensor 14B is L2. It should be noted that L1 and L2 may be in the range of the above formulas (1) and (2). The intersection of the broken line descending in the direction perpendicular to FIG. 5B from each member in FIG. 5A and each graph in FIG. 5B represents the approximate magnetic field strength in the member. Yes.

  A graph G10 indicated by a broken line in FIG. 5B shows an example of an approximate magnetic field strength distribution in the initial state, that is, before the driving of the magnetic shield device 100 shown in FIG. The magnetic field gradient (so-called inflow magnetic field gradient) between L1 and L2 is larger than the threshold value determined in step S20 in FIG. 4, and the magnetic field gradient in the correction target space 150 is not canceled out. . Further, the magnetic field strength of the correction target space 150 is also larger than the threshold value determined in step S60 of FIG.

  A graph G20 indicated by a one-dot chain line in FIG. 5B shows an approximate magnetic field strength distribution in step S30 “external coil maintaining state” in FIG. That is, it shows a state where the processes of Step S10 and Step S20 (including Step S40) have been completed. The magnetic field gradient between L1 and L2 is equal to or less than the threshold value determined in step S20 in FIG. 4, and the correction target space 150 is in a state where the so-called magnetic field gradient is canceled. However, the magnetic field intensity of the correction target space 150 is larger than the threshold value determined in step S60.

A graph G30 indicated by a solid line in FIG. 5B shows an approximate magnetic field strength distribution in step S70 “internal coil maintaining state” in FIG. That is, the graph G20 further shows a state where the processes of step S50 and step S60 (including step S80) have been completed. The magnetic field intensity in the correction target space 150 is equal to or less than the threshold value determined in step S60 of FIG. 4, and shows a state where a suitable magnetic field measurement environment is formed.
In the graph G30, the displacement of the magnetic field strength of the convex portion is seen at both ends of the correction target space 150, that is, around the internal coils 13A and 13B. This is a generated magnetic field generated when the internal coils 13A and 13B using Helmholtz coils are driven, but does not affect the distribution of the magnetic field strength in the correction target space 150.

According to the magnetic shield device 100 according to the embodiment described above, the following effects can be obtained. (1) By controlling the inflow magnetic field gradient, a so-called magnetic field gradient-cancelled space can be formed in the correction target space 150. Therefore, the correction target space 150 can be set as a suitable magnetic field measurement environment.
(2) In order to make the magnetic field intensity of the correction target space 150 equal to or less than the threshold value, control can be performed using known Helmholtz coils as the internal coils 13A and 13B. That is, the easily obtained internal coils 13A and 13B can make the correction target space 150 substantially zero magnetic field, and for example, a minute magnetic signal from the heart or brain can be measured with a high S / N ratio.
(3) The first magnetic sensor 14A and the second magnetic sensor 14B only need to have the same measurement sensitivity. For example, a high-sensitivity sensor (not shown) disposed in the correction target space 150 (for example, an optical pump) Type magnetic sensor). Therefore, it is possible to provide the magnetic shield device 100 with reduced manufacturing costs.

  The present invention is not limited to the above-described embodiment, and various embodiments are possible. Hereinafter, some modified examples will be described. In addition, description of the part which overlaps with the part demonstrated by the said embodiment is abbreviate | omitted.

(Modification 1)
<Arrangement example of magnetic sensor>
The arrangement of the first magnetic sensor 14A and the like for measuring the magnetic field strength is not limited to that shown in FIG. Several arrangements of the first magnetic sensor 14A and the like will be described below.

  FIG. 6 is a schematic view of a magnetic shield device 200 showing a second arrangement example. In this example, only one external coil 12A is driven and the other external coil 12B is not driven. Therefore, the control of the inflow magnetic field gradient by the control circuit 17 can be simplified.

  FIG. 7 is a schematic view of a magnetic shield device 300 showing a third arrangement example. In this example, two sets of a magnetic sensor for measuring the magnetic field flowing into the passive shield 11 and a magnetic sensor for measuring the magnetic field in the internal space 110 are provided. The first magnetic sensors 14A and 14C are magnetic sensors for measuring the strength of the magnetic field flowing from the external space into the internal space 110, and the second magnetic sensors 14B and 14D are magnetic for measuring the magnetic field strength of the internal space 110. It is a sensor. In this example, the first magnetic sensors 14A and 14C and the second magnetic sensors 14B and 14D are disposed on a center axis 110A (not shown).

The first magnetic sensor 14A and the second magnetic sensor 14B measure the inflow magnetic field gradient on the side of the opening 11A, which is one end of the passive shield 11, and the magnetic sensor 14C and the magnetic sensor 14D are on the side of the opening 11B on the other end. Measure the inflow magnetic field gradient. And it controls by the control circuit 17 so that each inflow magnetic field gradient may become below a threshold value.
According to the arrangement example of the third magnetic sensor, the inflow magnetic field gradient can be controlled to be equal to or less than the threshold value with respect to the inflow magnetic field from the openings 11A and 11B at both ends of the passive shield 11. Even in the passive shield 11 having no symmetry, the inflow magnetic field gradient can be easily controlled.

  FIG. 8 is a schematic view of a magnetic shield device 400 showing a fourth arrangement example. Three magnetic sensors for measuring the magnetic field strength are arranged along a central axis 110A (not shown). The first magnetic sensor 14A, the magnetic sensor 14E, and the second magnetic sensor 14B are arranged in this order from the opening 11A of the passive shield 11 toward the inside. In this example, the inflow magnetic field gradient can be measured by the second magnetic sensor 14B and the magnetic sensor 14E disposed in the internal space 110 without using the first magnetic sensor 14A closest to the opening 11A. On the other hand, the magnetic field strength can be measured by each of the three first magnetic sensors 14A and the second magnetic sensors 14B and 14E, and the inflow magnetic field gradient can be measured by a predetermined calculation (for example, the least square approximation method).

  In measuring the inflow magnetic field gradient, more detailed magnetic field gradients may be measured using four or more magnetic sensors. The arrangement direction of the second magnetic sensor 14B and the magnetic sensor 14E arranged in the internal space 110 may not be along the direction of the central axis 110A (not shown). For example, depending on the structure of the passive shield 11, the measurement sensitivity of the magnetic field gradient by the second magnetic sensors 14B and 14E may change depending on the direction of the arrangement.

  FIG. 9 is a schematic view of a magnetic shield device 500 showing a fifth arrangement example. The magnetic sensor for measuring the strength of the inflow magnetic field only needs to exist within the range (formula (1)) in which the first magnetic sensor 14A described above can be arranged. The direction of arrangement with the magnetic sensor 14B does not have to be along the center axis 110A (not shown). This is because the first magnetic sensor 14A only needs to be able to measure the inflow magnetic field from the external space to the internal space 110.

(Modification 2)
The cross-sectional shape of the passive shield 11 in the X direction is not limited to that described in the above embodiment. For example, it may be a polygonal shape or a circular shape, or a shape obtained by combining the outer periphery with a straight line and a curved line. Specifically, the cross-sectional shape of the passive shield 11 may be a square, a pentagon, a hexagon, a heptagon, an octagon, or the like as long as it is a polygon, in addition to a circle or an ellipse.
Further, the passive shield 11 may not have the openings 11A and 11B, or only one of the openings 11A and 11B. Alternatively, the openings 11A and 11B may be covered with a lid or the like. By providing the shape of the passive shield 11 with a degree of freedom, the magnetic shield device 100 can be applied to various magnetic measurement devices.

(Modification 3)
The internal coils 13A and 13B may be omitted. Even when the magnetic shield device 100 does not have the internal coils 13A and 13B, the inflow magnetic field gradient can be controlled using the external coils 12A and 12B, and the correction target space 150 in which the magnetic field gradient is canceled can be formed. is there. In the case where the internal coils 13A and 13B are not provided, for example, a region where the high-sensitivity magnetic sensor is disposed may be set as the correction target space 150.

(Modification 4)
In the above-described embodiment, the example in which the magnetic field in the X direction (that is, one axis) is corrected has been described. On the other hand, in the internal space 110, the inflow magnetic field gradient may be corrected by measuring the magnetic field strength of two or more axes. In this case, the magnetic shield device 100 has two or more sets of external coils. Specifically, for example, in the case of three axes, the magnetic shield device 100 uses two external coils for each of the three axes of the X axis, the Y axis, and the Z axis (a total of six), and the three axis directions You may measure the magnetic field of these components. Compared with the case of measuring the inflow magnetic field gradient of only one axis, the inflow magnetic field gradient can be measured with higher accuracy.

(Modification 5)
The passive shield 11 is not limited to the case where the central axis 110A is installed so as to be substantially horizontal. For example, it may be arranged like a telephone box so that the central axis 110A is substantially vertical.

(Modification 6)
The external coils 12A and 12B are Helmholtz coils as an example, but may have other configurations. For example, a plurality of external coils may be arranged at regular intervals between the external coil 12A and the external coil 12B.

  DESCRIPTION OF SYMBOLS 11 ... Passive shield, 11A, 11B ... Opening part, 12A, 12B ... External coil, 13A, 13B ... Internal coil, 20 ... Control part, 14A, 14C ... 1st magnetic sensor, 14B, 14D ... 2nd magnetic sensor, 15 ... Measurement circuit, 16 ... Drive circuit, 17 ... Control circuit, 20 ... Control unit, 21 ... CPU, 22 ... Long term memory circuit, 23 ... Temporary memory circuit, 24 ... Input unit, 25 ... Output unit, 26 ... Bus, 100 , 200, 300, 400, 500 ... magnetic shield device, 110 ... internal space, 110A ... central axis, 150 ... space to be corrected.

Claims (8)

A passive shield,
A correction target space defined inside the passive shield;
A first coil for correcting the magnetic field in the passive shield;
A first magnetic sensor;
A second magnetic sensor disposed inside the passive shield with respect to the first magnetic sensor;
A control unit,
The first magnetic sensor and the second magnetic sensor measure a magnetic field gradient in the passive shield;
The said control part controls the said 1st coil based on the measurement result of a said 1st magnetic sensor and a said 2nd magnetic sensor, The magnetic shield apparatus characterized by the above-mentioned.   The magnetic shield device according to claim 1, wherein the control unit controls the first coil such that the magnetic field gradient is equal to or less than a threshold value. The first magnetic sensor and the second magnetic sensor measure a magnetic field gradient along a first direction,
3. The magnetic shield device according to claim 1, wherein an axis of the first coil is along the first direction. 4. The passive shield has an opening;
When the coordinate in the first direction of the opening is the origin, and the direction from the opening to the inside of the passive shield along the first direction is the positive direction of the first direction,
The first magnetic sensor is arranged within a range of -0.5 to 1.0 times the square root of the area of the projection cross section perpendicular to the first direction of the opening with respect to the first direction. The magnetic shield device according to claim 1, wherein the magnetic shield device is a magnetic shield device. The second magnetic sensor is disposed within a range of 0 to 1.0 times the square root of the area of the projection cross section perpendicular to the first direction of the opening with respect to the first direction,
5. The magnetism according to claim 1, wherein a coordinate of the second magnetic sensor related to the first direction is more positive than a coordinate of the first magnetic sensor related to the first direction. Shield device.   6. The magnetism according to claim 1, wherein the correction target space is more positive with respect to the coordinates relating to the first direction than the coordinates relating to the first direction of the first magnetic sensor. Shield device.   The magnetic shield device according to claim 1, further comprising a second coil that adjusts the magnetic field strength of the correction target space inside the passive shield. A first step of measuring a magnetic field gradient in the passive shield;
A second step of setting the magnetic field gradient to a predetermined threshold or less based on the first step;
A third step of measuring the magnetic field strength in the passive shield;
A fourth step of setting the magnetic field intensity to a predetermined threshold value or less;
A magnetic shielding method comprising:

Images