Magnetoencephalograph and method for measuring brain magnetic field

Abstract

To provide a magnetoencephalograph system and a magnetoencephalography, capable of making high-accuracy measurement without using a magnetically shielded room.SOLUTION: A magnetoencephalograph system M1 includes a plurality of optical excitation magnetic sensors 1A that measures a brain magnetic field, a plurality of correcting magnetic sensors 2 that measures geomagnetism and a variable magnetic field at each position of the plurality of optical excitation magnetic sensors 1A, a correcting coil for correcting the geomagnetism and the variable magnetic field, a control device 4 that determines current for the correcting coil so as to generate a magnetic field that cancels the geomagnetism and the variable magnetic field on the basis of a measured value of the geomagnetism and a measured value of the variable magnetic field by the plurality of correcting magnetic sensors 2 to output a control signal according to the determined current, and a coil power supply 5 that outputs current to the correcting coil according to the control signal output by the control device 4.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a magnetoencephalograph and a method for measuring a magnetic field of the brain. [Background technology]

[0002] Conventionally, a superconducting quantum interference device (SQUID) has been used as a magnetoencephalograph to measure a weak brain magnetic field. In recent years, magnetoencephalographs using optically excited magnetic sensors instead of SQUIDs have been researched. Optically excited magnetic sensors measure a weak magnetic field by using the spin polarization of alkali metal atoms excited by optical pumping. For example, Patent Document 1 discloses a magnetoencephalograph that uses an optically pumped magnetometer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5823195 Summary of the Invention [Problem to be solved by the invention]

[0004] Measurements using magnetoencephalography are performed in a magnetically shielded room to prevent the effects of magnetic noise, which is stronger than the brain's magnetic field. However, the installation of a magnetically shielded room is limited due to its weight, cost, and other factors.

[0005] The present disclosure has been made in consideration of the above-mentioned circumstances, and has an object to provide a magnetoencephalograph and a method for measuring a brain magnetic field that are capable of performing measurements with high accuracy without using a magnetically shielded room. [Means for solving the problem]

[0006] A magnetoencephalometer according to one embodiment of the present disclosure includes a plurality of optically excited magnetic sensors that measure a brain magnetic field, a plurality of correction magnetic sensors that measure the geomagnetic field and a variable magnetic field at each position of the plurality of optically excited magnetic sensors, a correction coil for correcting the geomagnetic field and the variable magnetic field, a control device that determines a current to be applied to the correction coil based on the measurement values ​​of the geomagnetic field and the variable magnetic field measured by the plurality of correction magnetic sensors so as to generate a magnetic field that cancels out the geomagnetic field and the variable magnetic field and outputs a control signal according to the determined current, and a coil power supply that outputs a current to the correction coil according to the control signal output by the control device.

[0007] In the magnetoencephalograph according to one aspect of the present disclosure, the geomagnetic field and the variable magnetic field are measured at the respective positions of the multiple optically excited magnetic sensors that measure the brain magnetic field. Then, a current to a correction coil is determined so as to generate a magnetic field that cancels the geomagnetic field and the variable magnetic field based on the measured values ​​of the geomagnetic field and the variable magnetic field, and a control signal corresponding to the determined current is output. When a current corresponding to the control signal is output to the correction coil, a magnetic field is generated in the correction coil. As a result, the geomagnetic field and the variable magnetic field are canceled by the magnetic field generated in the correction coil at the positions of the multiple optically excited magnetic sensors. In this way, the geomagnetic field and the variable magnetic field are canceled at the positions of the multiple optically excited magnetic sensors, and the multiple optically excited magnetic sensors can measure the brain magnetic field in a state where the influence of the geomagnetic field and the variable magnetic field are avoided. With such a magnetoencephalograph, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room.

[0008] The correction coil may have a geomagnetic correction coil for correcting the geomagnetism, and a variable magnetic field correction coil for correcting the variable magnetic field. The control device may determine a current for the geomagnetic correction coil so as to generate a magnetic field that cancels the geomagnetic field based on the measurement value of the geomagnetic field, and may determine a current for the variable magnetic field correction coil so as to generate a magnetic field that cancels the variable magnetic field based on the measurement value of the variable magnetic field. Even in this case, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room. In addition, the number of turns of the geomagnetic correction coil for correcting the geomagnetic field having a strength on the order of 10 μT and the number of turns of the variable magnetic field correction coil for correcting the variable magnetic field having a strength on the order of 10 nT can be optimized, respectively, so that the geomagnetic field and the variable magnetic field can be corrected with high accuracy.

[0009] The correction coil may have a geomagnetic gradient correction coil for correcting the geomagnetic gradient. The control device may determine a current for the geomagnetic gradient correction coil based on the measured value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic gradient. In this case, uniform geomagnetic correction (zeroth-order correction) is performed by controlling the current for the geomagnetic correction coil, and further, correction of the geomagnetic gradient taking into account the difference in the position of each optically excited magnetic sensor (first-order correction) is performed by controlling the current for the geomagnetic gradient correction coil. In this way, the geomagnetic field and the geomagnetic gradient are canceled out step by step, so that the geomagnetic field can be corrected with high accuracy.

[0010] The correction coil may have a variable magnetic field gradient correction coil for correcting the gradient of the variable magnetic field. The control device may determine a current for the variable magnetic field gradient correction coil so as to generate a magnetic field that cancels the gradient of the variable magnetic field based on the measurement value of the variable magnetic field. In this case, a uniform variable magnetic field correction (zeroth-order correction) is performed by controlling the current for the variable magnetic field correction coil, and further, a correction of the gradient of the variable magnetic field (first-order correction) is performed taking into account the difference in the position of each optically excited magnetic sensor by controlling the current for the variable magnetic field gradient correction coil. In this way, the variable magnetic field and the gradient of the variable magnetic field are canceled out step by step, so that the variable magnetic field can be corrected with high accuracy.

[0011] The correction coil may be configured by a pair of coils arranged to sandwich the multiple optically excited magnetic sensors. In this case, the geomagnetism and the variable magnetic field at the positions of the multiple optically excited magnetic sensors sandwiched between the pair of correction coils are effectively corrected. This makes it possible to appropriately correct the geomagnetism and the variable magnetic field with a simple configuration.

[0012] The correction coil may be a coil system arranged around each of the multiple optically excited magnetic sensors in three orthogonal directions. The control device may determine the current for the coil system so as to generate a magnetic field that cancels the geomagnetism and the gradient of the geomagnetism based on the measurement value of the geomagnetism. In this case, a coil system is arranged for each optically excited magnetic sensor in correspondence with each of the three components of the geomagnetism (x-axis, y-axis, and z-axis). Then, by controlling the current for each coil system, a magnetic field that cancels the x-axis component, y-axis component, and z-axis component of the geomagnetism is generated for each optically excited magnetic sensor, and the geomagnetism and the gradient of the geomagnetism are corrected from three directions. This allows the current to be finely controlled for each optically excited magnetic sensor, improving the correction accuracy of the geomagnetism. In addition, since only the geomagnetism in the area related to the operation of the multiple optically excited magnetic sensors is corrected, the increase in power consumption related to unnecessary correction can be suppressed.

[0013] The corrective magnetic sensor may be a fluxgate sensor that outputs a measurement value of the geomagnetic field as a DC component and a measurement value of the variable magnetic field as an AC component. The dynamic range of the fluxgate sensor may include a geomagnetic field having a strength on the order of 10 μT and a variable magnetic field of a specific frequency (e.g., a commercial frequency) having a strength on the order of 10 nT. The variable magnetic field of the specific frequency is a particularly large magnetic field among variable magnetic fields. Such a fluxgate sensor can suitably measure the geomagnetic field and the variable magnetic field of the specific frequency.

[0014] The optically excited magnetic sensors may be axial gradiometers having a measurement region and a reference region on the same axis in a direction perpendicular to the object to be measured. In this case, the effect of common mode noise is indicated in the output result of the measurement region and the output result of the reference region, respectively, so that the common mode noise can be removed by obtaining the difference between the two output results. This improves the measurement accuracy of the brain magnetic field.

[0015] The multiple optically excited magnetic sensors and the multiple corrective magnetic sensors may be fixed to a helmet-type non-magnetic frame that is worn on the subject's head and has a relative permeability close to 1 and does not disturb the magnetic field distribution. In this case, the non-magnetic frame worn on the head and the sensors fixed to the non-magnetic frame move in response to the movement of the subject's head, so that even if the subject's head moves, it is possible to appropriately correct the geomagnetic field and the variable magnetic field at the positions of the multiple optically excited magnetic sensors and measure the brain magnetic field.

[0016] The magnetoencephalograph may further include an electromagnetic shield for blocking high-frequency electromagnetic noise. In this case, high-frequency electromagnetic noise that is not the subject of measurement by the magnetoencephalograph can be prevented from entering the multiple optically excited magnetic sensors. This allows the multiple optically excited magnetic sensors to operate stably.

[0017] A brain magnetic field measuring method according to one aspect of the present disclosure includes steps of measuring the geomagnetic field and variable magnetic field at each position of a plurality of optically excited magnetic sensors, determining a current to a correction coil based on the measured geomagnetic field and the measured variable magnetic field so as to generate a magnetic field that cancels out the geomagnetic field and the variable magnetic field, and outputting a control signal corresponding to the determined current, outputting a current to the correction coil in response to the control signal, and measuring the brain magnetic field using the plurality of optically excited magnetic sensors.

[0018] In a brain magnetic field measuring method according to an aspect of the present disclosure, the geomagnetic field and the variable magnetic field are measured at each position of a plurality of optically excited magnetic sensors that measure the brain magnetic field. Then, a current to a correction coil is determined so as to generate a magnetic field that cancels the geomagnetic field and the variable magnetic field based on the measured values ​​of the geomagnetic field and the variable magnetic field, and a control signal corresponding to the determined current is output. Then, when a current corresponding to the control signal is output to the correction coil, a magnetic field is generated in the correction coil. As a result, the geomagnetic field and the variable magnetic field are canceled by the magnetic field generated in the correction coil at the positions of the plurality of optically excited magnetic sensors. In this way, the geomagnetic field and the variable magnetic field are canceled at the positions of the plurality of optically excited magnetic sensors, so that the plurality of optically excited magnetic sensors can measure the brain magnetic field in a state where the influence of the geomagnetic field and the variable magnetic field are avoided. According to such a brain magnetic field measuring method, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room.

[0019] In the brain magnetic field measuring method, outputting a control signal may include determining a current for a geomagnetic correction coil based on a measurement value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic field, determining a current for a variable magnetic field correction coil based on a measurement value of the variable magnetic field so as to generate a magnetic field that cancels the variable magnetic field, and outputting a control signal for geomagnetic correction and a control signal for variable magnetic field correction according to the determined current. Even in this case, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room. In addition, the number of turns of the geomagnetic correction coil for correcting the geomagnetic field having a strength on the order of 10 μT and the number of turns of the variable magnetic field correction coil for correcting the variable magnetic field having a strength on the order of 10 nT can be optimized, respectively, so that the geomagnetic field and the variable magnetic field can be corrected with high accuracy.

[0020] The brain magnetic field measuring method may further include a step of determining a current for a geomagnetic gradient correction coil based on a measured value of the geomagnetic field so as to generate a magnetic field that cancels the gradient of the geomagnetic field, and outputting a control signal for geomagnetic gradient correction according to the determined current. In this case, uniform geomagnetic correction (zeroth-order correction) is performed by controlling the current for the geomagnetic gradient correction coil, and further, correction of the geomagnetic gradient taking into account the difference in the positions of each optically excited magnetic sensor (first-order correction) is performed by controlling the current for the geomagnetic gradient correction coil. In this way, the geomagnetic field and the gradient of the geomagnetic field are canceled out step by step, so that the geomagnetic field can be corrected with high accuracy.

[0021] The brain magnetic field measuring method may further include a step of determining a current for a variable magnetic field gradient correction coil so as to generate a magnetic field that cancels the gradient of the variable magnetic field based on the measured value of the variable magnetic field, and outputting a control signal for the variable magnetic field gradient correction according to the determined current. In this case, a uniform variable magnetic field correction (zeroth-order correction) is performed by controlling the current for the variable magnetic field gradient correction coil, and further, a correction of the gradient of the variable magnetic field (first-order correction) is performed by controlling the current for the variable magnetic field gradient correction coil, taking into account the difference in the positions of the optically excited magnetic sensors. In this way, the variable magnetic field and the gradient of the variable magnetic field are canceled out step by step, so that the variable magnetic field can be corrected with high accuracy. Effect of the Invention

[0022] According to the present invention, it is possible to provide a magnetoencephalograph and a brain magnetic field measuring method capable of performing measurements with high accuracy without using a magnetically shielded room. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram showing a configuration of a magnetoencephalograph according to an embodiment. [Diagram 2] FIG. 1 is a schematic diagram showing an arrangement for connection between a fluxgate sensor and a controller. [Diagram 3] 4 is a flowchart showing the operation of the magnetoencephalograph according to the embodiment. [Figure 4]FIG. 13 is a schematic diagram showing the configuration of a magnetoencephalograph according to another embodiment. [Diagram 5] FIG. 2 is a diagram showing the arrangement of a coil system. [Figure 6] 13 is a flowchart showing the operation of a magnetoencephalograph according to another embodiment. [Figure 7] FIG. 13 is a schematic diagram showing the configuration of a magnetoencephalograph according to a modified example. [Figure 8] FIG. 13 is a schematic diagram showing the configuration of a magnetoencephalograph according to a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted.

[0025] 1 is a schematic diagram showing the configuration of a magnetoencephalometer M1 according to an embodiment. The magnetoencephalometer M1 is a device that measures brain magnetic fields by using optical pumping while generating a magnetic field that cancels magnetic noise. The magnetoencephalometer M1 includes a plurality of OPM (optically pumped atomic magnetometer) modules 1, a plurality of corrective magnetic sensors 2, a non-magnetic frame 3, a control device 4, a coil power supply 5, a pair of geomagnetic correction coils 6, a pair of geomagnetic gradient correction coils 7, a pair of variable magnetic field correction coils 8, a pair of variable magnetic field gradient correction coils 9, a pump laser 10, a probe laser 11, an amplifier 12, a heater controller 13, and an electromagnetic shield 14.

[0026] The OPM module 1 includes an optically excited magnetic sensor 1A, a thermal insulator 1B, and a readout circuit 1C. A plurality of OPM modules 1 are arranged at predetermined intervals along a measurement target (for example, the scalp).

[0027] The optically excited magnetic sensor 1A is a sensor that measures the brain magnetic field by using optical pumping, and has a sensitivity of, for example, about 10 fT to 10 pT. The heat insulating material 1B prevents heat transfer and heat transfer of the optically excited magnetic sensor 1A heated to 180 degrees by a heater (not shown). The readout circuit 1C is a circuit that acquires the detection result of the optically excited magnetic sensor 1A. The optically excited magnetic sensor 1A excites the alkali metal by irradiating a pump light to a cell in which an alkali metal vapor is sealed. The excited alkali metal is in a spin polarized state, and when it receives a magnetic field, the tilt of the spin polarization axis of the alkali metal atom changes according to the magnetic field. The tilt of the spin polarization axis is detected by a probe light that is irradiated separately from the pump light. The readout circuit 1C receives the probe light that has passed through the alkali metal vapor by a photodiode and acquires the detection result. The readout circuit 1C outputs the detection result to the amplifier 12.

[0028] The optically excited magnetic sensor 1A may be, for example, an axial gradiometer. The axial gradiometer has a measurement region and a reference region on the same axis and in a direction perpendicular to the measurement target. The measurement region is, for example, a portion of the axial gradiometer that measures the brain magnetic field, which is closest to the subject's scalp. The reference region is, for example, a portion of the axial gradiometer that measures the brain magnetic field, which is a predetermined distance (for example, 3 cm) from the measurement region in a direction away from the subject's scalp. The axial gradiometer outputs the results of measurements in the measurement region and the reference region to the amplifier 12. Here, if common mode noise is included, its influence is indicated in the output result of the measurement region and the output result of the reference region. The common mode noise is removed by acquiring the difference between the output result of the measurement region and the output result of the reference region. By removing the common mode noise, for example, when measurement is performed in a magnetic noise environment of 1 pT, the optically excited magnetic sensor 1A can obtain a sensitivity of about 10 fT / √Hz.

[0029] The correction magnetic sensor 2 measures the geomagnetism and the variable magnetic field at a position corresponding to the optically excited magnetic sensor 1A. The correction magnetic sensor 2 is a fluxgate sensor having a sensitivity of, for example, several pT to 100 μT. The dynamic range of the fluxgate sensor may include the geomagnetism having an intensity on the order of 10 μT and the variable magnetic field of a specific frequency (for example, a commercial frequency of 50 Hz or 60 Hz) having an intensity on the order of 10 nT. The variable magnetic field of the specific frequency is a particularly large magnetic field among the variable magnetic fields. The position corresponding to the optically excited magnetic sensor 1A is a position around (near) the area where the optically excited magnetic sensor 1A is arranged. The correction magnetic sensor 2 may be provided in one-to-one correspondence with the optically excited magnetic sensor 1A, or in one-to-multiple correspondence (one correction magnetic sensor 2 for multiple optically excited magnetic sensors 1A).

[0030] As shown in FIG. 2, the correction magnetic sensor 2 branches the measurement values ​​of the geomagnetism and the variable magnetic field and outputs them to the control device 4. The correction magnetic sensor 2 outputs the measurement value of the geomagnetism as a DC (direct current) component and outputs the measurement value of the variable magnetic field as an AC (alternating current) component. The wiring between the correction magnetic sensor 2 and the control device 4 is branched to form a line L1 and a line L2. A capacitor C and an amplifier A are arranged on the line L2 in order from the side closer to the correction magnetic sensor 2. The capacitor C cuts (blocks) the DC component. The amplifier A amplifies the AC component from which the DC component has been cut from the output of the correction magnetic sensor 2. The DC component is output to the control device 4 via the line L1. The AC component is amplified on the line L2 and then output to the control device 4. The AC component remains on the line L1 as well, but is weaker than the DC component, so its effect on the DC component can be ignored. Alternatively, a low-pass filter that blocks the AC component and passes only the DC component may be provided on the line L1. Each measurement value of the correction magnetic sensor 2 can be represented by a vector having a direction and a magnitude in the control device 4. The control device 4 calculates the gradient of the geomagnetic field (hereinafter referred to as the "geomagnetic gradient") based on the measurement values ​​of the geomagnetic field by the multiple correction magnetic sensors 2. The control device 4 also calculates the gradient of the variable magnetic field (hereinafter referred to as the "variable magnetic field gradient") based on the measurement values ​​of the variable magnetic field by the multiple correction magnetic sensors 2. The correction magnetic sensor 2 may continuously perform measurement and output at a predetermined time interval.

[0031] Returning to FIG. 1, the non-magnetic frame 3 is a frame that covers the entire scalp of the subject, whose brain magnetic field is to be measured, and is made of a non-magnetic material such as graphite, whose relative permeability is close to 1 and does not disturb the magnetic field distribution. The non-magnetic frame 3 can be, for example, a helmet-type frame that surrounds the entire scalp of the subject and is worn on the subject's head. A plurality of optically excited magnetic sensors 1A are fixed to the non-magnetic frame 3 so as to be close to the subject's scalp. Furthermore, a correction magnetic sensor 2 is fixed to the non-magnetic frame 3 so as to be able to measure the geomagnetism and fluctuating magnetic field at each position of the plurality of optically excited magnetic sensors 1A.

[0032] The control device 4 controls the current supplied to the correction coils based on the geomagnetic field measurement values ​​and the variable magnetic field measurement values ​​obtained by the multiple correction magnetic sensors 2. That is, the control device 4 determines the current to be supplied to the correction coils so as to generate a magnetic field that cancels the geomagnetic field and the variable magnetic field based on the geomagnetic field measurement values ​​and the variable magnetic field measurement values, and outputs a control signal according to the determined current to the coil power supply 5. The correction coils are coils for correcting the geomagnetic field and the variable magnetic field. The correction coils include a geomagnetic correction coil 6, a geomagnetic gradient correction coil 7, a variable magnetic field correction coil 8, and a variable magnetic field gradient correction coil 9.

[0033] Specifically, the control device 4 determines the current for the geomagnetic correction coil 6 so that the average value of the geomagnetic measurement values ​​obtained by the multiple correction magnetic sensors 2 approaches zero (so that, as a result, a magnetic field is generated that is opposite in direction to and approximately equal in magnitude to the geomagnetic field at the position of the optically excited magnetic sensor 1A). The control device 4 outputs a control signal (a control signal for geomagnetic correction) to the coil power supply 5 according to the determined current for the geomagnetic correction coil 6.

[0034] The control device 4 also determines the current for the geomagnetic gradient correction coil 7 so that the deviation from the average value of the geomagnetic measurement value obtained by the correction magnetic sensor 2 is minimized (as a result, a magnetic field is generated that is opposite to and has the same magnitude as the geomagnetic gradient at the position of the optically excited magnetic sensor 1A). The control device 4 outputs a control signal (a control signal for geomagnetic gradient correction) to the coil power supply 5 according to the determined current for the geomagnetic gradient correction coil 7.

[0035] Furthermore, the control device 4 determines the current for the variable magnetic field correction coil 8 so that the average value of the measurement values ​​of the variable magnetic field by the multiple corrective magnetic sensors 2 approaches zero (as a result, a magnetic field is generated that is opposite in direction and has the same magnitude as the variable magnetic field at the position of the optically excited magnetic sensor 1A). The control device 4 outputs a control signal (a control signal for variable magnetic field correction) corresponding to the determined current for the variable magnetic field correction coil 8 to the coil power supply 5.

[0036] Furthermore, the control device 4 determines the current for the variable magnetic field gradient correction coil 9 so that the deviation from the average value of the measurement value of the variable magnetic field by the corrective magnetic sensor 2 is minimized (as a result, a magnetic field is generated that is opposite to and has the same magnitude as the variable magnetic field gradient at the position of the optically excited magnetic sensor 1A). The control device 4 outputs a control signal (a control signal for correcting the variable magnetic field gradient) corresponding to the determined current for the variable magnetic field gradient correction coil 9 to the coil power supply 5.

[0037] Furthermore, the control device 4 obtains information about the magnetism detected by the optically excited magnetic sensor 1A using the signal output from the amplifier 12. When the optically excited magnetic sensor 1A is an axial gradiometer, the control device 4 may remove common mode noise by obtaining the difference between the output result of the measurement region and the output result of the reference region. The control device 4 may control the operation of the pump laser 10 and the probe laser 11, such as the irradiation timing and irradiation time.

[0038] The control device 4 is physically configured to include memories such as RAM and ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and a storage unit such as a hard disk. Examples of such a control device 4 include a personal computer, a cloud server, a smartphone, and a tablet terminal. The control device 4 functions by executing a program stored in the memory with the CPU of a computer system.

[0039] The coil power supply 5 outputs a predetermined current to each of the correction coils in response to a control signal output from the control device 4. Specifically, the coil power supply 5 outputs a current to the geomagnetic correction coil 6 in response to a control signal related to the geomagnetic correction coil 6. The coil power supply 5 outputs a current to the geomagnetic gradient correction coil 7 in response to a control signal related to the geomagnetic gradient correction coil 7. The coil power supply 5 outputs a current to the variable magnetic field correction coil 8 in response to a control signal related to the variable magnetic field gradient correction coil 8. The coil power supply 5 outputs a current to the variable magnetic field gradient correction coil 9 in response to a control signal related to the variable magnetic field gradient correction coil 9.

[0040] The geomagnetic correction coil 6 is a coil for correcting the geomagnetic field at the position of the optically excited magnetic sensor 1A. The geomagnetic correction coil 6 generates a magnetic field in response to the current supplied from the coil power supply 5 to cancel the geomagnetic field. The geomagnetic correction coil 6 has, for example, a pair of geomagnetic correction coils 6A and 6B. The pair of geomagnetic correction coils 6A and 6B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of geomagnetic correction coils 6A and 6B generate a magnetic field in the opposite direction and of the same magnitude as the geomagnetic field at the position of the optically excited magnetic sensor 1A in response to the current supplied from the coil power supply 5. The direction of the magnetic field is, for example, from one geomagnetic correction coil 6A to the other geomagnetic correction coil 6B. The geomagnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field generated by the geomagnetic correction coil 6 in the opposite direction and of the same magnitude. In this way, the geomagnetic correction coil 6 corrects the geomagnetic field at the position of the optically excited magnetic sensor 1A.

[0041] The geomagnetic gradient correction coil 7 is a coil for correcting the geomagnetic gradient at the position of the optically excited magnetic sensor 1A. The geomagnetic gradient correction coil 7 generates a magnetic field in response to a current supplied from the coil power supply 5 to cancel the gradient. The geomagnetic gradient correction coil 7 has, for example, a pair of geomagnetic gradient correction coils 7A and 7B. The pair of geomagnetic gradient correction coils 7A and 7B are arranged on either side of the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of geomagnetic gradient correction coils 7A and 7B generate a magnetic field that is opposite to and has the same magnitude as the geomagnetic gradient at the position of the optically excited magnetic sensor 1A in response to a current supplied from the coil power supply 5. The direction of the magnetic field is, for example, from one geomagnetic gradient correction coil 7A to the other geomagnetic gradient correction coil 7B. The geomagnetic gradient at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude as the geomagnetic gradient generated by the geomagnetic gradient correction coil 7. In this way, the geomagnetic gradient correction coil 7 corrects the geomagnetic gradient at the position of the optically excited magnetic sensor 1A.

[0042] The variable magnetic field correction coil 8 is a coil for correcting the variable magnetic field at the position of the optically excited magnetic sensor 1A. The variable magnetic field correction coil 8 generates a magnetic field in response to the current supplied from the coil power supply 5, and cancels the variable magnetic field. The variable magnetic field correction coil 8 has, for example, a pair of variable magnetic field correction coils 8A and 8B. The pair of variable magnetic field correction coils 8A and 8B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of variable magnetic field correction coils 8A and 8B generate a magnetic field that is opposite to and has the same magnitude as the variable magnetic field at the position of the optically excited magnetic sensor 1A in response to the current supplied from the coil power supply 5. The direction of the magnetic field is, for example, from one variable magnetic field correction coil 8A to the other variable magnetic field correction coil 8B. The variable magnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude generated by the variable magnetic field correction coil 8. In this way, the variable magnetic field correction coil 8 corrects the variable magnetic field at the position of the optically excited magnetic sensor 1A.

[0043] The variable magnetic field gradient correction coil 9 is a coil for correcting the variable magnetic field gradient at the position of the optically excited magnetic sensor 1A. The variable magnetic field gradient correction coil 9 generates a magnetic field in response to a current supplied from the coil power supply 5, and cancels the variable magnetic field gradient. The variable magnetic field gradient correction coil 9 has, for example, a pair of variable magnetic field gradient correction coils 9A and 9B. The pair of variable magnetic field gradient correction coils 9A and 9B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of variable magnetic field gradient correction coils 9A and 9B generate a magnetic field that is opposite to and has the same magnitude as the variable magnetic field gradient at the position of the optically excited magnetic sensor 1A in response to a current supplied from the coil power supply 5. The direction of the magnetic field is, for example, from one variable magnetic field gradient correction coil 9A to the other variable magnetic field gradient correction coil 9B. The variable magnetic field gradient at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude as the variable magnetic field gradient generated by the variable magnetic field gradient correction coil 9. In this manner, the variable magnetic field gradient correction coil 9 corrects the variable magnetic field gradient at the position of the optically excited magnetic sensor 1A.

[0044] The pump laser 10 is a laser device that generates pump light. The pump light emitted from the pump laser 10 is incident on each of the multiple optically excited magnetic sensors 1A via fiber branching.

[0045] The probe laser 11 is a laser device that generates probe light. The probe light emitted from the probe laser 11 is incident on each of the multiple optically excited magnetic sensors 1A via fiber branching.

[0046] The amplifier 12 is a device or circuit that amplifies the signal resulting from the output from the OPM module 1 (specifically, the readout circuit 1C) and outputs the amplified signal to the control device 4.

[0047] The heater controller 13 is a temperature control device connected to a heater (not shown) for heating the cell of the optically excited magnetic sensor 1A and a thermocouple (not shown) for measuring the temperature of the cell. The heater controller 13 receives cell temperature information from the thermocouple and adjusts the heating of the heater based on the temperature information to adjust the cell temperature.

[0048] The electromagnetic shield 14 is a shielding member that blocks high-frequency (e.g., 10 kHz or higher) electromagnetic noise and is made of, for example, a mesh of woven metal threads or a nonmagnetic metal plate such as aluminum. The electromagnetic shield 14 is disposed so as to surround the optically excited magnetic sensor 1A, the corrective magnetic sensor 2, the nonmagnetic frame 3, the geomagnetic correction coil 6, the geomagnetic gradient correction coil 7, the variable magnetic field correction coil 8, and the variable magnetic field gradient correction coil 9.

[0049] Next, a method for measuring a brain magnetic field using the magnetoencephalometer M1 according to the embodiment will be described with reference to Fig. 3. Fig. 3 is a flowchart showing the operation of the magnetoencephalometer M1.

[0050] The correction magnetic sensor 2 measures the geomagnetism (step S11). The correction magnetic sensor 2 measures the geomagnetism at each position of the optically excited magnetic sensor 1A, and outputs the measurement value of the geomagnetism to the control device 4. The control device 4 calculates the geomagnetic gradient based on the measurement values ​​of the geomagnetism by the multiple correction magnetic sensors 2.

[0051] The control device 4 and the coil power supply 5 control the current to the geomagnetic correction coil 6 (step S12). Based on the geomagnetic measurement value by the correction magnetic sensor 2, the control device 4 determines the current to the geomagnetic correction coil 6 so as to generate a magnetic field of the same magnitude and in the opposite direction to the geomagnetic field at the position of the optically excited magnetic sensor 1A. More specifically, the control device 4 determines the current to the geomagnetic correction coil 6 so that, for example, the average value of the geomagnetic measurement values ​​by the multiple correction magnetic sensors 2 approaches zero. The control device 4 outputs a control signal according to the determined current to the coil power supply 5. The coil power supply 5 outputs a predetermined current to the geomagnetic correction coil 6 in response to the control signal output by the control device 4. The geomagnetic correction coil 6 generates a magnetic field in response to the current supplied from the coil power supply 5. The geomagnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field of the same magnitude and in the opposite direction generated by the geomagnetic correction coil 6.

[0052] The control device 4 and the coil power supply 5 control the current to the geomagnetic gradient correction coil 7 (step S13). Based on the geomagnetic measurement value by the correction magnetic sensor 2, the control device 4 determines the current to the geomagnetic gradient correction coil 7 so as to generate a magnetic field having the same magnitude and in the opposite direction to the geomagnetic gradient at the position of the optically excited magnetic sensor 1A. More specifically, the control device 4 determines the current to the geomagnetic gradient correction coil 7 so as to minimize the deviation from the average value of the geomagnetic measurement value by the correction magnetic sensor 2, for example. The control device 4 outputs a control signal according to the determined current to the coil power supply 5. The coil power supply 5 outputs a predetermined current to the geomagnetic gradient correction coil 7 in response to the control signal output by the control device 4. The geomagnetic gradient correction coil 7 generates a magnetic field in response to the current supplied from the coil power supply 5. The geomagnetic gradient at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field generated by the geomagnetic gradient correction coil 7 having the same magnitude and in the opposite direction.

[0053] The control device 4 determines whether the geomagnetic measurement value after correction is equal to or less than the reference value (step S14). The geomagnetic measurement value after correction is the geomagnetic measurement value by the correction magnetic sensor 2 after the geomagnetic correction coil 6 and the geomagnetic gradient correction coil 7 have corrected the geomagnetic field. The reference value is the magnitude of the magnetic field at which the optically excited magnetic sensor 1A operates normally, and may be, for example, 1 nT. If the geomagnetic measurement value is not equal to or less than the reference value ("NO" in step S14), the process returns to step S11. If the geomagnetic measurement value is equal to or less than the reference value ("YES" in step S14), the process proceeds to step S15.

[0054] The corrective magnetic sensor 2 measures the variable magnetic field (step S15). The corrective magnetic sensor 2 measures the variable magnetic field at each position of the optically excited magnetic sensor 1A, and outputs the measurement values ​​of the variable magnetic field to the control device 4. The control device 4 calculates the variable magnetic field gradient based on the measurement values ​​of the variable magnetic field by the multiple corrective magnetic sensors 2.

[0055] The control device 4 and the coil power supply 5 control the current to the variable magnetic field correction coil 8 (step S16). Based on the measurement value of the variable magnetic field by the corrective magnetic sensor 2, the control device 4 determines the current to the variable magnetic field correction coil 8 so as to generate a magnetic field having the same magnitude and the opposite direction to the variable magnetic field at the position of the optically excited magnetic sensor 1A. More specifically, the control device 4 determines the current to the variable magnetic field correction coil 8 so that, for example, the average value of the measurement values ​​of the variable magnetic field by the multiple corrective magnetic sensors 2 approaches zero. The control device 4 outputs a control signal according to the determined current to the coil power supply 5. The coil power supply 5 outputs a predetermined current to the variable magnetic field correction coil 8 in response to the control signal output by the control device 4. The variable magnetic field correction coil 8 generates a magnetic field in response to the current supplied from the coil power supply 5. The variable magnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field generated by the variable magnetic field correction coil 8 having the same magnitude and the opposite direction.

[0056] The control device 4 and the coil power supply 5 control the current to the variable magnetic field gradient correction coil 9 (step S17). Based on the measurement value of the variable magnetic field by the corrective magnetic sensor 2, the control device 4 determines the current to the variable magnetic field gradient correction coil 9 so as to generate a magnetic field having the same magnitude as and in the opposite direction to the variable magnetic field gradient at the position of the optically excited magnetic sensor 1A. More specifically, the control device 4 determines the current to the variable magnetic field gradient correction coil 9 so as to minimize the deviation from the average value of the measurement value of the variable magnetic field by the corrective magnetic sensor 2, for example. The control device 4 outputs a control signal according to the determined current to the coil power supply 5. The coil power supply 5 outputs a predetermined current to the variable magnetic field gradient correction coil 9 in response to the control signal output by the control device 4. The variable magnetic field gradient correction coil 9 generates a magnetic field in response to the current supplied from the coil power supply 5. The variable magnetic field gradient at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field having the same magnitude as and in the opposite direction generated by the variable magnetic field gradient correction coil 9.

[0057] The control device 4 judges whether the measurement value of the variable magnetic field after correction is equal to or less than the reference value (step S18). The measurement value of the variable magnetic field after correction is the measurement value of the variable magnetic field by the corrective magnetic sensor 2 after the variable magnetic field has been corrected by the variable magnetic field correction coil 8. The reference value is a noise level at which the brain magnetic field can be measured, and may be, for example, less than 1 nT, more specifically, 10 pT. If the measurement value of the variable magnetic field is not equal to or less than the reference value ("NO" in step S18), the process returns to step S15. If the measurement value of the variable magnetic field is equal to or less than the reference value ("YES" in step S18), the process proceeds to step S19.

[0058] The optically excited magnetic sensor 1A measures the brain magnetic field (step S19). Since the geomagnetism and the variable magnetic field at the position of the optically excited magnetic sensor 1A have been cancelled out so as to be equal to or less than a predetermined reference value, the optically excited magnetic sensor 1A can measure the brain magnetic field while avoiding the influence of the geomagnetism and the variable magnetic field.

[0059] FIG. 4 is a schematic diagram showing the configuration of a magnetoencephalograph M2 according to another embodiment. The magnetoencephalograph M2 is a device that measures a brain magnetic field by using optical pumping while generating a magnetic field that cancels magnetic noise, similar to the magnetoencephalograph M1. The magnetoencephalograph M2 includes an OPM module 1, a corrective magnetic sensor 2, a non-magnetic frame 3, a control device 4, a coil power supply 5, a variable magnetic field correction coil 8, a variable magnetic field gradient correction coil 9, a pump laser 10, a probe laser 11, an amplifier 12, a heater controller 13, an electromagnetic shield 14, and a coil system 15 (correction coil). In the magnetoencephalograph M2, instead of the geomagnetic correction coil 6 and the geomagnetic gradient correction coil 7 of the magnetoencephalograph M1, a coil system 15 is arranged for each OPM module 1 (optically excited magnetic sensor 1A). Here, the arrangement of the coil system 15 will be described with reference to FIG. 5.

[0060] FIG. 5 is a diagram showing the arrangement of the coil system 15 associated with the magnetoencephalograph M2. The coil system 15 is a coil system (for example, a three-axis Helmholtz coil or a planar coil system) capable of applying magnetic fields in three orthogonal directions that are arranged orthogonal to each other and in a circular arrangement. The coil system 15 specifically includes coil systems 15X, 15Y, and 15Z. In FIG. 5, the coil systems 15X, 15Y, and 15Z are arranged with respect to the OPM module 1 as shown by dotted lines. In this manner, the coil systems 15X, 15Y, and 15Z are arranged for each OPM module 1 (optically excited magnetic sensor 1A) so as to be orthogonal to each other and in a circular arrangement. The coil system 15X is a coil for correcting the x-axis component of the geomagnetism and the geomagnetic gradient shown in FIG. 5. Similarly, the coil systems 15Y and 15Z are coils for correcting the y-axis and z-axis components of the geomagnetism and the geomagnetic gradient, respectively.

[0061] Returning to FIG. 4, only the points of the magnetoencephalograph M2 that are different from the magnetoencephalograph M1 will be described. The control device 4 determines the current for the coil system 15X, 15Y, 15Z for each of the optically excited magnetic sensors 1A so that the average value of the geomagnetic measurement values ​​by the multiple correction magnetic sensors 2 approaches zero. The control device 4 also determines the current for the coil system 15X, 15Y, 15Z for each of the optically excited magnetic sensors 1A so that the deviation from the average value of the geomagnetic measurement values ​​by the correction magnetic sensors 2 is minimized. The control device 4 determines the current for the coil system 15X based on the geomagnetic measurement values ​​by the correction magnetic sensors 2 so that a magnetic field is generated that is opposite to and has the same magnitude as the x-axis component of the geomagnetic field and the geomagnetic gradient at the position of the optically excited magnetic sensor 1A. The control device 4 outputs a control signal (control signal for geomagnetic field correction) according to the determined current to the coil power supply 5. The control device 4 also determines a current for the coil system 15Y based on the geomagnetism measured by the correction magnetic sensor 2 so as to generate a magnetic field having the same magnitude and in the opposite direction to the geomagnetism and the y-axis component of the geomagnetism gradient at the position of the optically excited magnetic sensor 1A. The control device 4 outputs a control signal (a control signal for geomagnetism correction) according to the determined current to the coil power supply 5. Furthermore, the control device 4 determines a current for the coil system 15Z based on the geomagnetism measured by the correction magnetic sensor 2 so as to generate a magnetic field having the same magnitude and in the opposite direction to the z-axis component of the geomagnetism and the geomagnetic gradient at the position of the optically excited magnetic sensor 1A. The control device 4 outputs a control signal (a control signal for geomagnetism correction) according to the determined current to the coil power supply 5.

[0062] The coil power supply 5 outputs a predetermined current to each of the coil systems 15X, 15Y, and 15Z in response to the control signal output by the control device 4. Specifically, the coil power supply 5 outputs a current to the coil system 15X in response to the control signal related to the coil system 15X. The coil power supply 5 outputs a current to the coil system 15Y in response to the control signal related to the coil system 15Y. The coil power supply 5 outputs a current to the coil system 15Z in response to the control signal related to the coil system 15Z.

[0063] The coil system 15 generates a magnetic field in response to the current supplied from the coil power supply 5, and cancels the geomagnetism and geomagnetic gradient. Specifically, the coil system 15X generates a magnetic field in the opposite direction and of the same magnitude as the x-axis components of the geomagnetism and geomagnetic gradient at the position of the optically excited magnetic sensor 1A in response to the current supplied from the coil power supply 5. The x-axis components of the geomagnetism and geomagnetic gradient at the position of the optically excited magnetic sensor 1A are canceled by the magnetic field generated by the coil system 15X in the opposite direction and of the same magnitude. Similarly, the coil systems 15Y and 15Z generate magnetic fields in the opposite direction and of the same magnitude as the y-axis and z-axis components of the geomagnetism and geomagnetic gradient at the position of the optically excited magnetic sensor 1A, respectively, and cancel the geomagnetism and geomagnetic gradient. In this way, the coil system 15 corrects the geomagnetism and geomagnetic gradient at the position of the optically excited magnetic sensor 1A. Note that the magnetic information obtained by the control device 4 does not include the magnetic field generated by the coil system 15.

[0064] The electromagnetic shield 14 is disposed so as to surround the optically excited magnetic sensor 1A, the corrective magnetic sensor 2, the non-magnetic frame 3, the variable magnetic field correction coils 8, the variable magnetic field gradient correction coils 9, and the coil system 15.

[0065] Next, a method for measuring a brain magnetic field using the magnetoencephalometer M2 according to the embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart showing the operation of the magnetoencephalometer M2.

[0066] The correction magnetic sensor 2 measures the geomagnetism (step S21). The correction magnetic sensor 2 measures the geomagnetism at each position of the optically excited magnetic sensor 1A, and outputs the measurement value of the geomagnetism to the control device 4. The control device 4 calculates the geomagnetic gradient based on the measurement values ​​of the geomagnetism by the multiple correction magnetic sensors 2.

[0067] The control device 4 and the coil power supply 5 control the current to the coil system 15 for each optically excited magnetic sensor 1A (step S22). The control device 4 determines the current to the coil system 15 based on the measurement value of the geomagnetism by the correction magnetic sensor 2 so as to generate magnetic fields of the same magnitude and in the opposite directions for the components of the geomagnetism and the geomagnetic gradient in the three directions (x-axis, y-axis, and z-axis) at the position of the optically excited magnetic sensor 1A. More specifically, the control device 4 determines the current to the coil systems 15X, 15Y, and 15Z for each optically excited magnetic sensor 1A so that, for example, the average value of the measurement values ​​of the geomagnetism by the multiple correction magnetic sensors 2 approaches zero. In addition, the control device 4 determines the current to the coil systems 15X, 15Y, and 15Z for each of the multiple optically excited magnetic sensors 1A so that the deviation from the average value of the measurement values ​​of the geomagnetism by the correction magnetic sensor 2 is minimized. The control device 4 outputs a control signal corresponding to the determined current for each of the coil systems 15X, 15Y, and 15Z to the coil power supply 5. The coil power supply 5 outputs a predetermined current to each of the coil systems 15X, 15Y, and 15Z in response to a control signal output by the control device 4. The coil systems 15X, 15Y, and 15Z each generate a magnetic field in response to the current supplied from the coil power supply 5. The three-directional components of the geomagnetic field and the geomagnetic gradient at the position of the optically excited magnetic sensor 1A are cancelled out by magnetic fields of the same magnitude and in opposite directions generated by each of the coil systems 15X, 15Y, and 15Z.

[0068] A test operation of the optically excited magnetic sensor 1A is performed (step S23). The optically excited magnetic sensor 1A acquires a measurement value of the residual magnetic field through the test operation and outputs the measurement value to the control device 4. The measurement value of the magnetic field is a value measured by the optically excited magnetic sensor 1A after the geomagnetism and the geomagnetic gradient are corrected by the coil system 15.

[0069] The control device 4 determines whether the measured value of the magnetic field is equal to or less than the reference value (step S24). The reference value is a level at which the optically excited magnetic sensor 1A operates normally, and may be, for example, 0.3 nT. If the measured value of the magnetic field is not equal to or less than the reference value ("NO" in step S24), the process returns to step S21. If the measured value of the magnetic field is equal to or less than the reference value ("YES" in step S24), the process proceeds to step S25.

[0070] The following steps S25 to S29 are the same as steps S15 to S19, and therefore the explanation is simplified. The corrective magnetic sensor 2 measures the variable magnetic field (step S25). The control device 4 controls the current to the variable magnetic field correction coil 8 (step S26). The control device 4 controls the current to the variable magnetic field gradient correction coil 9 (step S27). The control device 4 determines whether the measurement value of the variable magnetic field after correction is equal to or less than the reference value (step S28). If the measurement value of the variable magnetic field is not equal to or less than the reference value ("NO" in step S28), the process returns to step S25. If the measurement value of the variable magnetic field is equal to or less than the reference value ("YES" in step S28), the process proceeds to step S29. The optically excited magnetic sensor 1A measures the brain magnetic field (step S29).

[0071] [Effects] Next, the effects of the magnetoencephalograph according to the above-described embodiment will be described.

[0072] The magnetoencephalometers M1 and M2 of this embodiment include a plurality of optically excited magnetic sensors 1A that measure the brain magnetic field, a plurality of correction magnetic sensors 2 that measure the geomagnetic field and variable magnetic field at each position of the plurality of optically excited magnetic sensors 1A, a correction coil for correcting the geomagnetic field and the variable magnetic field, a control device 4 that determines a current to be applied to the correction coil based on the measurement values ​​of the geomagnetic field and the variable magnetic field measured by the plurality of correction magnetic sensors 2 so as to generate a magnetic field that cancels out the geomagnetic field and the variable magnetic field, and outputs a control signal according to the determined current, and a coil power supply 5 that outputs a current to the correction coil according to the control signal output by the control device 4.

[0073] In the magnetoencephalometers M1 and M2, the geomagnetic field and the variable magnetic field are measured at the respective positions of the optically excited magnetic sensors 1A that measure the brain magnetic field. Then, a current to the correction coil is determined so as to generate a magnetic field that cancels the geomagnetic field and the variable magnetic field based on the measured values ​​of the geomagnetic field and the variable magnetic field, and a control signal according to the determined current is output. When a current according to the control signal is output to the correction coil, a magnetic field is generated in the correction coil. As a result, the geomagnetic field and the variable magnetic field are canceled by the magnetic field generated in the correction coil at the positions of the optically excited magnetic sensors 1A. In this way, the geomagnetic field and the variable magnetic field at the positions of the optically excited magnetic sensors 1A are canceled, so that the optically excited magnetic sensors 1A can measure the brain magnetic field in a state where the influence of the geomagnetic field and the variable magnetic field are avoided. With such magnetoencephalometers M1 and M2, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room. In addition, the correction magnetic sensor 2 allows the sensor for measuring the geomagnetic field and the variable magnetic field to be shared, so that an increase in the number of sensors can be suppressed.

[0074] The correction coil includes a geomagnetic correction coil 6 for correcting the geomagnetic field and a variable magnetic field correction coil 8 for correcting the variable magnetic field. The control device 4 determines the current for the geomagnetic correction coil 6 so as to generate a magnetic field that cancels the geomagnetic field based on the measured value of the geomagnetic field, and determines the current for the variable magnetic field correction coil 8 so as to generate a magnetic field that cancels the variable magnetic field based on the measured value of the variable magnetic field. Even in this case, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room. In addition, the number of turns of the geomagnetic correction coil 6 for correcting the geomagnetic field having a strength on the order of 10 μT and the number of turns of the variable magnetic field correction coil 8 for correcting the variable magnetic field having a strength on the order of 10 nT can be optimized, respectively, so that the geomagnetic field and the variable magnetic field can be corrected with high accuracy.

[0075] The correction coil has a geomagnetic gradient correction coil 7 for correcting the geomagnetic gradient. Based on the measured value of the geomagnetic field, the control device 4 determines the current to the geomagnetic gradient correction coil 7 so as to generate a magnetic field that cancels the geomagnetic gradient. In this case, uniform geomagnetic correction (zeroth-order correction) is performed by controlling the current to the geomagnetic correction coil 6, and further, correction of the geomagnetic gradient (first-order correction) is performed taking into account the difference in the position of each optically excited magnetic sensor 1A by controlling the current to the geomagnetic gradient correction coil 7. In this way, the geomagnetic field and the geomagnetic gradient are canceled out step by step, so that the geomagnetic field can be corrected with high accuracy.

[0076] The correction coil has a variable magnetic field gradient correction coil 9 for correcting the gradient of the variable magnetic field. The control device 4 determines the current for the variable magnetic field gradient correction coil 9 based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the gradient of the variable magnetic field. In this case, uniform variable magnetic field correction (zeroth-order correction) is performed by controlling the current for the variable magnetic field correction coil 8, and further, correction of the gradient of the variable magnetic field (first-order correction) is performed taking into account the difference in the position of each optically excited magnetic sensor 1A by controlling the current for the variable magnetic field gradient correction coil 9. In this way, the variable magnetic field and the gradient of the variable magnetic field are cancelled out step by step, so that the variable magnetic field can be corrected with high accuracy.

[0077] The correction coil is composed of a pair of coils arranged to sandwich the multiple optically excited magnetic sensors 1A. In this case, the geomagnetism and the variable magnetic field at the positions of the multiple optically excited magnetic sensors 1A sandwiched between the pair of correction coils are effectively corrected. This makes it possible to appropriately correct the geomagnetism and the variable magnetic field with a simple configuration.

[0078] The correction coil is a coil system 15 arranged around each of the multiple optically excited magnetic sensors 1A in three orthogonal directions. The control device 4 determines the current for the coil system 15 so as to generate a magnetic field that cancels the geomagnetism and the gradient of the geomagnetism based on the measurement value of the geomagnetism. In this case, the coil system 15 is arranged for each optically excited magnetic sensor 1A in correspondence with each of the three components of the geomagnetism (x-axis, y-axis, and z-axis). Then, by controlling the current for each coil system 15, a magnetic field that cancels the x-axis component, y-axis component, and z-axis component of the geomagnetism is generated for each optically excited magnetic sensor 1A, and the geomagnetism and the gradient of the geomagnetism are corrected from three directions. This allows the current to be finely controlled for each optically excited magnetic sensor 1A, improving the correction accuracy of the geomagnetism. In addition, since only the geomagnetism in the area related to the operation of the multiple optically excited magnetic sensors 1A is corrected, the increase in power consumption related to unnecessary correction can be suppressed.

[0079] The corrective magnetic sensor 2 is a fluxgate sensor that outputs a measurement value of the geomagnetic field as a DC component and a measurement value of a fluctuating magnetic field as an AC component. The dynamic range of the fluxgate sensor can include a geomagnetic field having a strength on the order of 10 μT and a fluctuating magnetic field of a specific frequency (e.g., a commercial frequency) having a strength on the order of 10 nT. The fluctuating magnetic field of the specific frequency is a particularly large magnetic field among fluctuating magnetic fields. Such a fluxgate sensor can suitably measure the geomagnetic field and the fluctuating magnetic field of the specific frequency.

[0080] The optically excited magnetic sensors 1A are axial gradiometers having a measurement area and a reference area on the same axis in a direction perpendicular to the object to be measured. In this case, the effect of common mode noise is shown in the output result of the measurement area and the output result of the reference area, respectively, so that the common mode noise can be removed by obtaining the difference between the two output results. This improves the measurement accuracy of the brain magnetic field.

[0081] The multiple optically excited magnetic sensors 1A and the multiple corrective magnetic sensors 2 are fixed to a helmet-type non-magnetic frame 3 that is worn on the subject's head and has a relative permeability close to 1 and does not disturb the magnetic field distribution. In this case, the non-magnetic frame 3 worn on the head and the sensors fixed to the non-magnetic frame 3 move in response to the movement of the subject's head, so that even if the subject's head moves, it is possible to appropriately correct the geomagnetic field and variable magnetic field at the positions of the multiple optically excited magnetic sensors 1A and measure the brain magnetic field.

[0082] The magnetoencephalometers M1 and M2 may further include an electromagnetic shield 14 for blocking high-frequency electromagnetic noise. In this case, high-frequency electromagnetic noise that is not the subject of measurement by the magnetoencephalometers M1 and M2 can be prevented from invading the multiple optically excited magnetic sensors 1A. This allows the multiple optically excited magnetic sensors 1A to operate stably.

[0083] A brain magnetic field measuring method according to one embodiment of the present disclosure includes the steps of measuring the geomagnetic field and variable magnetic field at each position of a plurality of optically excited magnetic sensors 1A, determining a current to a correction coil based on the measured values ​​of the geomagnetic field and the variable magnetic field so as to generate a magnetic field that cancels out the geomagnetic field and the variable magnetic field, and outputting a control signal corresponding to the determined current, outputting a current to the correction coil in response to the control signal, and measuring the brain magnetic field using the plurality of optically excited magnetic sensors 1A.

[0084] In the brain magnetic field measuring method according to one aspect of the present disclosure, the geomagnetic field and the variable magnetic field are measured at each position of the multiple optically excited magnetic sensors 1A that measure the brain magnetic field. Then, a current to the correction coil is determined so as to generate a magnetic field that cancels the geomagnetic field and the variable magnetic field based on the measured values ​​of the geomagnetic field and the variable magnetic field, and a control signal corresponding to the determined current is output. Then, when the current corresponding to the control signal is output to the correction coil, a magnetic field is generated in the correction coil. As a result, the geomagnetic field and the variable magnetic field are canceled by the magnetic field generated in the correction coil at the positions of the multiple optically excited magnetic sensors 1A. In this way, the geomagnetic field and the variable magnetic field are canceled at the positions of the multiple optically excited magnetic sensors 1A, so that the multiple optically excited magnetic sensors 1A can measure the brain magnetic field in a state where the influence of the geomagnetic field and the variable magnetic field are avoided. According to such a brain magnetic field measuring method, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room.

[0085] In the brain magnetic field measuring method, outputting a control signal includes determining a current for the geomagnetic correction coil 6 so as to generate a magnetic field that cancels the geomagnetic field based on the measured value of the geomagnetic field, determining a current for the variable magnetic field correction coil 8 so as to generate a magnetic field that cancels the variable magnetic field based on the measured value of the variable magnetic field, and outputting a control signal for geomagnetic correction and a control signal for variable magnetic field correction according to the determined current. Even in this case, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room. In addition, the number of turns of the geomagnetic correction coil 6 for correcting the geomagnetic field having a strength on the order of 10 μT and the number of turns of the variable magnetic field correction coil 8 for correcting the variable magnetic field having a strength on the order of 10 nT can be optimized, respectively, so that the geomagnetic field and the variable magnetic field can be corrected with high accuracy.

[0086] The brain magnetic field measuring method further includes a step of determining a current for the geomagnetic gradient correction coil 7 based on the measured value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic gradient, and outputting a control signal for geomagnetic gradient correction according to the determined current. In this case, uniform geomagnetic correction (zeroth-order correction) is performed by controlling the current for the geomagnetic correction coil 6, and further, correction of the geomagnetic gradient (first-order correction) is performed taking into account the difference in the positions of the optically excited magnetic sensors 1A by controlling the current for the geomagnetic gradient correction coil 7. In this way, the geomagnetic field and the geomagnetic gradient are cancelled out step by step, so that the geomagnetic field can be corrected with high accuracy.

[0087] The brain magnetic field measuring method further includes a step of determining a current for the variable magnetic field gradient correction coil 9 so as to generate a magnetic field that cancels the gradient of the variable magnetic field based on the measured value of the variable magnetic field, and outputting a control signal for the variable magnetic field gradient correction according to the determined current. In this case, uniform variable magnetic field correction (zeroth-order correction) is performed by controlling the current for the variable magnetic field gradient correction coil 8, and further, correction of the gradient of the variable magnetic field (first-order correction) is performed taking into account the difference in the positions of the optically excited magnetic sensors 1A by controlling the current for the variable magnetic field gradient correction coil 9. In this way, the variable magnetic field and the gradient of the variable magnetic field are cancelled out step by step, so that the variable magnetic field can be corrected with high accuracy.

[0088] [Variations] The present disclosure has been described in detail above based on the embodiments. However, the present disclosure is not limited to the above embodiments. Various modifications of the present disclosure are possible without departing from the spirit and scope of the present disclosure.

[0089] In the embodiment, the correction magnetic sensor 2 has a configuration in which the measurement value is branched and output to the control device 4, but the correction magnetic sensor 2 may be configured to output the measurement value to the control device 4 without branching. In such a configuration, the control device 4 may extract a DC component and an AC component based on the measurement value output from the correction magnetic sensor 2, and obtain each component as a measurement value of the geomagnetism and a measurement value of the variable magnetic field. In this case, the wiring for branching can be omitted.

[0090] The correction magnetic sensor 2 may be a fluxgate sensor that measures the geomagnetism and an optically excited magnetic sensor that measures the fluctuating magnetic field. The optically excited magnetic sensor is a different sensor from the optically excited magnetic sensor 1A that measures the brain magnetic field. The optically excited magnetic sensor has higher sensitivity than a fluxgate sensor or the like, and can measure the fluctuating magnetic field with high accuracy. As a result, the fluctuating magnetic field and the fluctuating magnetic field gradient can be corrected more precisely.

[0091] Although the variable magnetic field correction coil 8 has been described as having a pair of variable magnetic field correction coils 8A and 8B, it may be arranged as a coil system for each OPM module 1 (optically excited magnetic sensor 1A) like coil system 15. In this case, the control device 4 determines a current for the variable magnetic field correction coil 8 so as to generate a magnetic field that is opposite to and has the same magnitude as the three directional (x-axis, y-axis, and z-axis) components of the variable magnetic field at the position of the optically excited magnetic sensor 1A. The control device 4 outputs a control signal corresponding to the determined current for each of the variable magnetic field correction coils 8 arranged as a coil system to the coil power supply 5.

[0092] The optically excited magnetic sensor 1A may have a plurality of measurement points within one housing. In this case, it is possible to narrow the distance between the measurement points, and it is possible to measure the brain magnetic field with high spatial resolution.

[0093] Although the coil system 15 associated with the magnetoencephalograph M2 has been described as correcting the geomagnetic field and the geomagnetic gradient, it may also correct only the geomagnetic field. The geomagnetic gradient may also be corrected by further providing a geomagnetic gradient correction coil 7. Even in this case, the brain magnetic field can be measured with high accuracy without using a magnetically shielded room.

[0094] FIG. 7 is a diagram showing the configuration of the magnetoencephalograph M3 according to the modified example. The magnetoencephalograph M3 is different from the magnetoencephalograph M1 in that the variable magnetic field correction coil 8 and the variable magnetic field gradient correction coil 9 are omitted. The geomagnetic correction coil 6 may correct the geomagnetic field by DC current and superimpose an AC current component to correct the variable magnetic field. Similarly, the geomagnetic gradient correction coil 7 may correct the geomagnetic gradient by DC current and superimpose an AC current component to correct the variable magnetic field gradient. According to such magnetoencephalograph M3, for the same reason as the magnetoencephalographs M1 and M2, it is possible to measure the brain magnetic field with high accuracy without using a magnetically shielded room. In addition, since the increase in the number of correction coils can be suppressed, it is possible to appropriately correct the geomagnetic field and the variable magnetic field with a simple configuration.

[0095] FIG. 8 is a diagram showing the configuration of the magnetoencephalograph M4 according to the modified example. The magnetoencephalograph M4 is different from the magnetoencephalograph M2 in that the variable magnetic field correction coil 8 and the variable magnetic field gradient correction coil 9 are omitted. The correction coil may be only the coil system 15. The control device 4 may determine the current for the coil system 15 so as to generate a magnetic field that cancels the geomagnetic field and the gradient of the geomagnetic field, as well as the gradient of the variable magnetic field and the gradient of the variable magnetic field, based on the measurement value of the geomagnetic field and the measurement value of the variable magnetic field. In this case, the coil system 15 is disposed for each optically excited magnetic sensor 1A in correspondence with each of the components of the geomagnetic field and the variable magnetic field in three directions (x-axis, y-axis, and z-axis). Then, by controlling the current for each coil system 15, a magnetic field that cancels each of the components of the geomagnetic field and the variable magnetic field in the x-axis direction, the y-axis direction, and the z-axis direction is generated for each optically excited magnetic sensor 1A, and the geomagnetic field and the gradient of the geomagnetic field, as well as the gradient of the variable magnetic field and the gradient of the variable magnetic field are corrected from the three directions. This allows for more precise control of the current for each optically excited magnetic sensor 1A, improving the accuracy of correcting the geomagnetism and fluctuating magnetic field. In addition, because only the geomagnetism and fluctuating magnetic field in the area related to the operation of the multiple optically excited magnetic sensors 1A are corrected, it is possible to suppress an increase in power consumption associated with unnecessary correction. [Explanation of symbols]

[0096] 1A...optically excited magnetic sensor, 2...correction magnetic sensor, 3...non-magnetic frame, 4...control device, 5...coil power supply, 6...geomagnetic correction coil, 7...geomagnetic gradient correction coil, 8...variable magnetic field correction coil, 9...variable magnetic field gradient correction coil, 14...electromagnetic shield, 15...coil system.

Claims

1. A plurality of optically excited magnetic sensors for measuring brain magnetic fields; a plurality of correcting magnetic sensors for measuring the geomagnetism and the fluctuating magnetic field at the respective positions of the plurality of optically excited magnetic sensors; A correction coil for correcting the geomagnetic field and the fluctuating magnetic field; a control device that determines a current for the correction coil so as to generate a magnetic field that cancels out the geomagnetism and the variable magnetic field based on the measurement values ​​of the geomagnetism and the variable magnetic field obtained by the plurality of correction magnetic sensors, and outputs a control signal according to the determined current; a coil power supply that outputs a current to the correction coil in response to the control signal output by the control device; Equipped with the correction coils include a geomagnetic correction coil for correcting the geomagnetic field, a geomagnetic gradient correction coil for correcting the gradient of the geomagnetic field, and a variable magnetic field correction coil for correcting the variable magnetic field, The control device determines a current for the geomagnetic correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic field, determines a current for the geomagnetic gradient correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the gradient of the geomagnetic field, and determines a current for the variable magnetic field correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the variable magnetic field. Magnetoencephalography.

2. A plurality of optically excited magnetic sensors for measuring brain magnetic fields; a plurality of correcting magnetic sensors for measuring the geomagnetism and the fluctuating magnetic field at the respective positions of the plurality of optically excited magnetic sensors; A correction coil for correcting the geomagnetic field and the fluctuating magnetic field; a control device that determines a current for the correction coil so as to generate a magnetic field that cancels out the geomagnetism and the variable magnetic field based on the measurement values ​​of the geomagnetism and the variable magnetic field obtained by the plurality of correction magnetic sensors, and outputs a control signal according to the determined current; a coil power supply that outputs a current to the correction coil in response to the control signal output by the control device; Equipped with the correction coils include a geomagnetic correction coil for correcting the geomagnetic field, a variable magnetic field correction coil for correcting the variable magnetic field, and a variable magnetic field gradient correction coil for correcting a gradient of the variable magnetic field, The control device determines a current for the geomagnetic correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic field, determines a current for the variable magnetic field correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the variable magnetic field, and determines a current for the variable magnetic field gradient correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the gradient of the variable magnetic field. Magnetoencephalography.

3. 3. The magnetoencephalograph according to claim 1, wherein the correction coil is constituted by a pair of coils arranged to sandwich the optically excited magnetic sensors.

4. The correction coil is a coil system arranged around each of the optically excited magnetic sensors in three orthogonal directions, the control device determines, based on the measurement of the geomagnetic field, a current for the coil system so as to generate a magnetic field that cancels the geomagnetic field and a gradient of the geomagnetic field. The magnetoencephalograph according to claim 1 .

5. 5. The magnetoencephalograph according to claim 1, wherein the corrective magnetic sensor is a fluxgate sensor that outputs a measurement value of the geomagnetism as a DC component and outputs a measurement value of the fluctuating magnetic field as an AC component.

6. 6. The magnetoencephalograph according to claim 1, wherein the plurality of optically excited magnetic sensors are axial gradiometers having a measurement region and a reference region on the same axis in a direction perpendicular to the object to be measured.

7. 7. The magnetoencephalometer according to claim 1, wherein the plurality of optically excited magnetic sensors and the plurality of corrective magnetic sensors are fixed to a helmet-type non-magnetic frame that is worn on the head of the subject and has a relative permeability close to 1 and does not disturb the magnetic field distribution.

8. 8. The magnetoencephalograph according to claim 1, further comprising an electromagnetic shield for blocking high-frequency electromagnetic noise.

9. Measuring the geomagnetic field and the varying magnetic field at each position of a plurality of optically excited magnetic sensors; determining a current for a correction coil based on the measured value of the geomagnetism and the measured value of the variable magnetic field so as to generate a magnetic field that cancels the geomagnetism and the variable magnetic field, and outputting a control signal according to the determined current; outputting a current to the correction coil in response to the control signal; measuring a brain magnetic field by the plurality of optically excited magnetic sensors; Including, outputting the control signal includes determining a current for a geomagnetic correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic field, determining a current for a geomagnetic gradient correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the gradient of the geomagnetic field, determining a current for a variable magnetic field correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the variable magnetic field, and outputting a control signal for geomagnetic correction, a control signal for geomagnetic gradient correction, and a control signal for variable magnetic field correction according to the determined currents. Methods for measuring brain magnetic fields.

10. A step of measuring the geomagnetic field and a varying magnetic field at each position of a plurality of optically excited magnetic sensors; determining a current for a correction coil based on the measured value of the geomagnetism and the measured value of the variable magnetic field so as to generate a magnetic field that cancels the geomagnetism and the variable magnetic field, and outputting a control signal according to the determined current; outputting a current to the correction coil in response to the control signal; measuring a brain magnetic field by the plurality of optically excited magnetic sensors; Including, outputting the control signal includes determining a current for a geomagnetic correction coil based on the measurement value of the geomagnetic field so as to generate a magnetic field that cancels the geomagnetic field, determining a current for a variable magnetic field correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the variable magnetic field, determining a current for a variable magnetic field gradient correction coil based on the measurement value of the variable magnetic field so as to generate a magnetic field that cancels the gradient of the variable magnetic field, and outputting a control signal for geomagnetic correction, a control signal for variable magnetic field correction, and a control signal for variable magnetic field gradient correction according to the determined currents. Methods for measuring brain magnetic fields.

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