Magnetic field measurement device, magnetic field measurement method, and magnetic field measurement program

Abstract

To provide a magnetic field measurement device, a magnetic field measurement method, and a magnetic field measurement program.SOLUTION: The magnetic field measurement device comprises: a magnetic sensor array composed of a plurality of magnetic sensor cells each having a magnetic sensor; a magnetic field acquisition part that acquires measurement data measured by the magnetic sensor array; a signal space separation part that extracts first internal spatial data from a magnetic field spatial distribution indicated by measurement data from some magnetic sensor cells of the magnetic sensor array, and second internal spatial data from a magnetic field spatial distribution indicated by measurement data from a plurality of magnetic sensor cells including some other magnetic sensor cells of the magnetic sensor array; a calculation part calculating projection matrix on the basis of the first internal spatial data; and a noise removal part removing noise of the second internal spatial data with the projection matrix.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a magnetic field measurement device, a magnetic field measurement method, and a magnetic field measurement program. [Background technology]

[0002] Patent Document 1 describes a method for suppressing sensor noise in a spatially oversampling sensor array. [Prior art document] [Patent Documents] Patent Document 1: U.S. Patent Application Publication No. 2019 / 125268 Summary of the Invention

[0003] A first aspect of the present invention provides a magnetic field measurement device. The magnetic field measurement device may include a magnetic sensor array composed of a plurality of magnetic sensor cells, each having a magnetic sensor. The magnetic field measurement device may include a magnetic field acquisition unit that acquires measurement data measured by the magnetic sensor array. The magnetic field measurement device may include a signal space separation unit that extracts first internal space data from the spatial distribution of the magnetic field indicated by measurement data from a portion of the magnetic sensor cells in the magnetic sensor array, and extracts second internal space data from the spatial distribution of the magnetic field indicated by measurement data from a plurality of magnetic sensor cells including another portion of the magnetic sensor cells in the magnetic sensor array. The magnetic field measurement device may include a calculation unit that calculates a projection matrix based on the first internal space data. The magnetic field measurement device may include a noise removal unit that removes noise from the second internal space data using the projection matrix.

[0004] The signal spatial separator may extract second internal spatial data from the spatial distribution of the magnetic field indicated by the measurement data from all the magnetic sensor cells of the magnetic sensor array.

[0005] The signal spatial separation unit may extract third internal spatial data from a spatial distribution of the magnetic field indicated by measurement data from a portion of the magnetic sensor cells different from the portion of the magnetic sensor cells of the magnetic sensor array from which the first internal spatial data is extracted. The calculation unit may calculate one or more projection matrices based on the first internal spatial data and the third internal spatial data.

[0006] The calculation section may calculate a common part between a basis in the time domain of the first internal space data and a basis in the time domain of the third internal space data, and calculate the projection matrix based on the common part.

[0007] The calculation unit may calculate a first projection matrix based on the first internal space data and a second projection matrix based on the third internal space data. The noise removal unit may remove a portion of the noise in the second internal space data using the first projection matrix and remove another portion of the noise in the second internal space data using the second projection matrix.

[0008] The apparatus may include a subtractor that calculates the difference between the first internal space data and the third internal space data, and the calculator may calculate the projection matrix based on the difference.

[0009] The calculation section may calculate a basis of the time domain of the first internal space data by performing singular value decomposition on the first internal space data, and calculate the projection matrix based on the basis.

[0010] The magnetic field measuring device may further include an averaging processor that calculates average values ​​of a plurality of pieces of data for the first internal space data and the second internal space data. The calculator may calculate a projection matrix based on the average value of the first internal space data. The noise remover may remove noise from the average value of the second internal space data using the projection matrix.

[0011] The noise removal unit may remove noise from the second internal space data by projecting the second internal space data onto a signal space using a projection matrix.The magnetic sensor may have a magnetoresistive element.

[0012] A second aspect of the present invention provides a magnetic field measurement method. The magnetic field measurement method may include a magnetic field acquisition step of acquiring measurement data measured by a magnetic sensor array composed of a plurality of magnetic sensor cells, each having a magnetic sensor. The magnetic field measurement method may include a signal space separation step of extracting first internal space data from the spatial distribution of the magnetic field indicated by the measurement data from a portion of the magnetic sensor cells in the magnetic sensor array, and extracting second internal space data from the spatial distribution of the magnetic field indicated by the measurement data from a plurality of magnetic sensor cells including another portion of the magnetic sensor cells in the magnetic sensor array. The magnetic field measurement method may include a calculation step of calculating a projection matrix based on the first internal space data. The magnetic field measurement method may include a noise removal step of removing noise from the second internal space data using the projection matrix.

[0013] A third aspect of the present invention provides a magnetic field measurement program. The magnetic field measurement program causes a computer to function as a magnetic field acquisition unit that acquires measurement data measured by a magnetic sensor array composed of a plurality of magnetic sensor cells, each having a magnetic sensor. The magnetic field measurement program causes the computer to function as a signal space separation unit that extracts first internal space data from the spatial distribution of the magnetic field indicated by measurement data from a portion of the magnetic sensor cells of the magnetic sensor array, and extracts second internal space data from the spatial distribution of the magnetic field indicated by measurement data from a plurality of magnetic sensor cells including another portion of the magnetic sensor cells of the magnetic sensor array. The magnetic field measurement program causes the computer to function as a calculation unit that calculates a projection matrix based on the first internal space data. The magnetic field measurement program also causes the computer to function as a noise reduction unit that uses the projection matrix to reduce noise in the second internal space data.

[0014] The above summary of the invention does not list all of the necessary features of the present invention, and subcombinations of these features may also constitute inventions. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows the configuration of a magnetic field measurement device 10 according to this embodiment. [Figure 2] 1 shows the configuration of a magnetic sensor unit 110 according to this embodiment. [Figure 3] 2 shows the configuration of a magnetic sensor cell 220 in a magnetic sensor array 210 according to this embodiment. [Figure 4] 3 shows an example of input / output characteristics of a magnetic sensor having a magnetoresistive element according to the present embodiment. [Figure 5] 1 shows an example of the configuration of a sensor unit 300 according to this embodiment. [Figure 6] 1 shows an example of input / output characteristics of the sensor unit 300 according to this embodiment. [Figure 7] 1 shows an example of the configuration of a magnetic sensor 520 according to this embodiment. [Figure 8] 10 shows a magnetic flux distribution when a feedback magnetic field is generated in the magnetic sensor 520 according to this embodiment. [Figure 9] 1 shows an example of the arrangement of a plurality of magnetic sensor cells 220 in a magnetic sensor array 210 according to this embodiment. [Figure 10] An example of the definition of space in the magnetic field measuring device 10 according to this embodiment will be shown. [Figure 11] 1 shows the configurations of a magnetic sensor array 210, a sensor data collection unit 230, and a sensor data processing unit 1100 according to this embodiment. [Figure 12] 1 shows a first configuration example of a part of a magnetic field measuring device 10 according to this embodiment. [Figure 13] 10 shows a flow of signal separation of the spatial distribution of a magnetic field by the magnetic field measuring device 10 according to this embodiment. [Figure 14] An example of a flow in which the magnetic field measuring device 10 according to this embodiment removes remaining noise from the internal space data ^Bin that has been subjected to signal space separation will be shown. [Figure 15] 2 shows a second configuration example of a part of the magnetic field measuring device 10 according to the present embodiment. [Figure 16] 10 shows a third example of the configuration of a part of the magnetic field measuring device 10 according to the present embodiment. [Figure 17]10 shows a fourth example of the configuration of a part of the magnetic field measuring device 10 according to the present embodiment. [Figure 18] 10 shows a fifth configuration example of a part of the magnetic field measuring device 10 according to the present embodiment. [Figure 19] 10 shows a sixth configuration example of a part of the magnetic field measuring device 10 according to the present embodiment. [Figure 20] 10 shows the measurement results of the internal space data ^Bin before noise is removed in the magnetic field measuring device 10 of the sixth configuration example according to this embodiment. [Figure 21] 10 shows the measurement results of the internal space data ^Binproj after noise has been removed according to this embodiment. [Figure 22] 99 illustrates an example computer 9900 in which aspects of the present invention may be embodied, in whole or in part. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0017] FIG. 1 shows the configuration of a magnetic field measurement device 10 according to this embodiment. The magnetic field measurement device 10 according to this embodiment extracts multiple different internal space data from the spatial distribution of the magnetic field indicated by magnetic field measurement data measured using a magnetic sensor array. The magnetic field measurement device 10 then removes at least a portion of the noise in the internal space data using a projection matrix calculated from the other internal space data. In this embodiment, the magnetic field measurement device 10 is described as an example of a magnetocardiogram measurement device that measures magnetocardiograms, which are magnetic fields generated by the electrical activity of the human heart. However, this is not limiting. The magnetic field measurement device 10 may be used to measure magnetocardiograms of living organisms other than humans, or to measure biomagnetic fields other than magnetocardiograms, such as brain magnetic fields. The magnetic field measurement device 10 may also be used for magnetic flaw detection to detect surface and subsurface defects in steel materials and welds.

[0018] The magnetic field measuring device 10 includes a main body 100 and an information processing unit 150. The main body 100 is a component for sensing the subject's magnetic field, and includes a magnetic sensor unit 110, a head 120, a driving unit 125, a base 130, and a pole 140.

[0019] The magnetic sensor unit 110 is positioned on the chest of the subject facing the heart during cardiac measurement and senses the subject's cardiac magnetism. The head 120 supports the magnetic sensor unit 110 and positions the magnetic sensor unit 110 facing the subject. The driver 125 is provided between the magnetic sensor unit 110 and the head 120 and changes the orientation of the magnetic sensor unit 110 relative to the head 120 when performing calibration. The driver 125 according to this embodiment includes a first actuator that can rotate the magnetic sensor unit 110 360 degrees around the Z axis in the figure, and a second actuator that rotates the magnetic sensor unit 110 around an axis perpendicular to the Z axis (the X axis in the state shown in the figure). These are used to change the azimuth angle and zenith angle of the magnetic sensor unit 110. As shown as the driver 125 in the figure, the driver 125 has a Y-shape when viewed from the Y axis direction in the figure, and the second actuator can rotate the magnetic sensor unit 110 360 degrees around the X axis in the figure.

[0020] The base 130 is a base that supports other components, and in this embodiment serves as a platform on which the subject stands when measuring the magnetic field of the heart. The pole 140 supports the head 120 at the height of the subject's chest. The pole 140 may be extendable in the vertical direction to adjust the height of the magnetic sensor unit 110 to the height of the subject's chest.

[0021] The information processing unit 150 is a component for processing measurement data obtained by the main body unit 100 and outputting the data by display, printing, or the like. The information processing unit 150 may be a computer such as a PC (personal computer), tablet computer, smartphone, workstation, server computer, or general-purpose computer, or may be a computer system in which multiple computers are connected. Alternatively, the information processing unit 150 may be a dedicated computer designed for processing information from magnetocardiography, or may be dedicated hardware realized by a dedicated circuit.

[0022] 2 shows the configuration of a magnetic sensor unit 110 according to this embodiment. The magnetic sensor unit 110 includes a magnetic sensor array 210 and a sensor data collection unit 230. The magnetic sensor array 210 includes a plurality of magnetic sensor cells 220 and is capable of detecting an input magnetic field in three axial directions. This figure shows a case in which the magnetic sensor array 210 includes a plurality of magnetic sensor cells 220 arranged in each of the X, Y, and Z directions (for example, eight magnetic sensor cells 220 in the X direction, eight magnetic sensor cells 220 in the Y direction, and two magnetic sensor cells 220 in the Z direction, for a total of 128 magnetic sensor cells 220).

[0023] The sensor data collection unit 230 is electrically connected to the multiple magnetic sensor cells 220 included in the magnetic sensor array 210 (not shown), collects sensor data (detection signals) from the multiple magnetic sensor cells 220, and supplies them to the information processing unit 150.

[0024] FIG. 3 shows the configuration of the magnetic sensor cells 220 in the magnetic sensor array 210 according to this embodiment. Each of the magnetic sensor cells 220 includes at least one sensor unit 300 having a magnetoresistive element. This figure illustrates an example in which each of the magnetic sensor cells 220 includes three sensor units 300x-z (collectively referred to as "sensor units 300") and can detect an input magnetic field in three axial directions. However, each of the magnetic sensor cells 220 is not limited to having three sensor units 300x-z. It is sufficient that at least a portion of the magnetic sensor array 210 can detect an input magnetic field in three axial directions. In this case, as will be described later, when spatially sampling each spherical harmonic function using the magnetic sensor array 210, it is necessary to detect the dependency on spatial frequencies related to the angular momentum of the magnetic field. Therefore, it is preferable that the sensor units 300 in the magnetic sensor array 210 are arranged as evenly as possible, at least in the azimuth and zenith directions. For the same reason, it is preferable that the magnetic sensing axes of the sensors in the magnetic sensor array 210 are arranged as evenly as possible, at least in the azimuth and zenith directions. The sensor unit 300x is arranged along the X-axis direction and can detect magnetic fields in the X-axis direction. The sensor unit 300y is arranged along the Y-axis direction and can detect magnetic fields in the Y-axis direction. The sensor unit 300z is arranged along the Z-axis direction and can detect magnetic fields in the Z-axis direction. As shown in the enlarged view indicated by the dashed-dotted lines in this figure, in this embodiment, each sensor unit 300 has magnetic concentrators arranged on both ends of the magnetoresistive element. Therefore, each sensor unit 300 samples the spatial distribution of the magnetic field using a magnetoresistive element arranged in a narrow position between the magnetic concentrators, thereby clearly defining the sampling points in space in each axis direction. The configuration of each sensor unit 300 will be described in detail later.

[0025] The multiple magnetic sensor cells 220 are arranged at equal intervals of Δx along the X-axis direction, Δy along the Y-axis direction, and Δz along the Z-axis direction. The position of each magnetic sensor cell 220 in the magnetic sensor array 210 is represented by a set [i, j, k] of position i in the X-axis direction, position j in the Y-axis direction, and position k in the Z-axis direction. Here, i is an integer satisfying 1≦i≦Nx (Nx indicates the number of arranged magnetic sensor cells 220 in the X-axis direction), j is an integer satisfying 1≦j≦Ny (Ny indicates the number of arranged magnetic sensor cells 220 in the Y-axis direction), and k is an integer satisfying 1≦k≦Nz (Nz indicates the number of arranged magnetic sensor cells 220 in the Z-axis direction). Note that the above description has been given by way of example of a case in which the multiple magnetic sensor cells 220 are arranged at equal intervals along each axial direction. However, this is not limiting. The plurality of magnetic sensor cells 220 may be arranged at different intervals in at least one of the X-axis direction, the Y-axis direction, and the Z-axis direction, for example.

[0026] In this figure, the three-axis directions of the magnetic field detected by the sensor units 300x, 300y, and 300z are the same as the three-dimensional direction in which the magnetic sensor cells 220 are arranged. This makes it easy to understand each component of the distribution of the measured magnetic field. Furthermore, the sensor units 300x, 300y, and 300z are arranged in each magnetic sensor cell 220 so as not to overlap each other when viewed from each of the three-dimensional directions in which the magnetic sensor cells 220 are arranged. Furthermore, in this figure, the sensor units 300x, 300y, and 300z are arranged such that one end is provided on the side of a gap provided between the multiple sensor units 300, and the other end extends in each of the three axial directions so as to be away from the gap. As an example, this figure shows an example in which a gap is provided in the lower left corner of the magnetic sensor cell 220 when viewed from the front, and the sensor units 300x, 300y, and 300z are arranged so that one end is in contact with the gap and the other end is away from the gap, extending in the X-axis, Y-axis, and Z-axis directions. In this figure, the sensor units 300x, 300y, and 300z are arranged along three mutually perpendicular sides from one corner of the cubic magnetic sensor cell 220, with a gap provided at that corner. It is also preferable that the coils or magnetic bodies of the sensor units 300x, 300y, and 300z, described below, are arranged so as not to overlap each other. This clarifies the measurement points and makes it easier to understand each component of the measured magnetic field. Furthermore, the other-axis sensitivities of the sensor units 300x, 300y, and 300z can be considered equivalent to each other. This multi-axis sensitivity is generated by mutual interference between the coils or magnetic materials of the sensor units 300x, 300y, and 300z. However, the three-axis directions of the magnetic field to be detected may differ from the three-dimensional direction in which the magnetic sensor cells 220 are arranged. When these directions are different, there are no restrictions on the arrangement of the sensor units 300 within the magnetic sensor cells 220 or the arrangement direction of the magnetic sensor cells 220, and the degree of freedom in designing the magnetic sensor array 210 can be increased.

[0027] 4 shows an example of the input / output characteristics of a magnetic sensor having a magnetoresistive element according to this embodiment. In this figure, the horizontal axis represents the magnitude B of the input magnetic field input to the magnetic sensor, and the vertical axis represents the magnitude V_xMR0 of the detection signal from the magnetic sensor. The magnetic sensor has, for example, a giant magnetoresistive (GMR) element or a tunnel magnetoresistive (TMR) element, and detects the magnitude of a magnetic field in a predetermined uniaxial direction.

[0028] Such a magnetic sensor has high magnetic sensitivity, which is the slope of the detection signal V_xMR0 relative to the input magnetic field B, and can detect a weak magnetic field of about 10 pT. On the other hand, the detection signal V_xMR0 of the magnetic sensor saturates when the absolute value of the input magnetic field B is about 1 μT, for example, and the range in which the linearity of the input / output characteristics is good is narrow. Therefore, adding a closed loop that generates a feedback magnetic field to such a magnetic sensor can improve the linearity (also called linearity) of the magnetic sensor. Such a magnetic sensor will be described next.

[0029] 5 shows an example of the configuration of the sensor unit 300 according to this embodiment. The sensor unit 300 is provided inside each of the magnetic sensor cells 220, and includes a magnetic sensor 520, a magnetic field generation unit 530, and an output unit 540. Note that part of the sensor unit 300, such as the amplifier circuit 532 and the output unit 540, may be provided on the sensor data collection unit 230 side rather than on the magnetic sensor cell 220 side.

[0030] Like the magnetic sensor described in FIG. 4 , the magnetic sensor 520 includes a magnetoresistive element such as a GMR element or a TMR element. The magnetic sensor 520 also includes magnetic flux concentrators disposed at both ends of the magnetoresistive element. The magnetoresistive element of the magnetic sensor 520 may be configured such that, assuming the positive direction of the magnetic sensing axis is the +X direction, its resistance increases when a magnetic field is input in the +X direction and decreases when a magnetic field is input in the −X direction. That is, by observing the change in the resistance of the magnetoresistive element of the magnetic sensor 520, the magnitude of the magnetic field B input to the magnetic sensor 520 can be detected. For example, if the magnetic sensitivity of the magnetic sensor 520 is S, the detection result of the magnetic sensor 520 for the input magnetic field B can be calculated as S × B. For example, the magnetic sensor 520 is connected to a power source or the like and outputs a voltage drop corresponding to the change in resistance as the detection result of the input magnetic field. The configuration of the magnetic sensor 520 will be described in detail later.

[0031] The magnetic field generation unit 530 applies to the magnetic sensor 520 a feedback magnetic field that reduces the input magnetic field detected by the magnetic sensor 520. The magnetic field generation unit 530 generates, for example, a feedback magnetic field B_FB that is opposite in direction to the magnetic field B input to the magnetic sensor 520 and has approximately the same absolute value as the input magnetic field, thereby operating to cancel out the input magnetic field. The magnetic field generation unit 530 includes an amplifier circuit 532 and a coil 534.

[0032] The amplifier circuit 532 outputs a current corresponding to the detection result of the input magnetic field of the magnetic sensor 520 as a feedback current I_FB. When the magnetoresistive element of the magnetic sensor 520 is configured as a bridge circuit including at least one magnetoresistive element, the outputs of the bridge circuit are connected to the input terminal pair of the amplifier circuit 532. The amplifier circuit 532 outputs a current corresponding to the output of the bridge circuit as a feedback current I_FB. The amplifier circuit 532 includes, for example, a transconductance amplifier, and outputs the feedback current I_FB corresponding to the output voltage of the magnetic sensor 520. For example, if the voltage-current conversion coefficient of the amplifier circuit 532 is G, the feedback current I_FB can be calculated as G × S × B.

[0033] The coil 534 generates a feedback magnetic field B_FB corresponding to the feedback current I_FB. The coil 534 is wound so as to surround the magnetoresistive element of the magnetic sensor 520 and the magnetic concentrator plates disposed on both ends of the magnetoresistive element. It is desirable that the coil 534 generates a uniform feedback magnetic field B_FB throughout the entire magnetic sensor 520. For example, if the coil coefficient of the coil 534 is β, the feedback magnetic field B_FB can be calculated as β × I_FB. Here, the feedback magnetic field B_FB is generated in a direction that cancels out the input magnetic field B, so the magnetic field input to the magnetic sensor 520 is reduced to B - B_FB. Therefore, the feedback current I_FB is expressed by the following equation:

number

[0034] By solving equation (1) for the feedback current I_FB, it is possible to calculate the value of the feedback current I_FB in the steady state of the sensor unit 300. If the magnetic sensitivity S of the magnetic sensor 520 and the voltage-current conversion coefficient G of the amplifier circuit 532 are sufficiently large, the following equation can be calculated from equation (1):

number

[0035] The output unit 540 outputs an output signal V_xMR according to the feedback current I_FB that the magnetic field generation unit 530 flows to generate the feedback magnetic field B_FB. The output unit 540 has, for example, a resistive element with a resistance value R, and outputs a voltage drop caused by the feedback current I_FB flowing through the resistive element as the output signal V_xMR. In this case, the output signal V_xMR is calculated from equation (2) as follows:

number

[0036] As described above, the sensor unit 300 generates a feedback magnetic field that reduces the magnetic field input from outside, thereby substantially reducing the magnetic field input to the magnetic sensor 520. As a result, the sensor unit 300 can prevent the detection signal V_xMR from saturating even if the absolute value of the input magnetic field B exceeds 1 μT, for example, by using a magnetoresistive element having the characteristics shown in FIG. 4 as the magnetic sensor 520. The input / output characteristics of such a sensor unit 300 will now be described.

[0037] FIG. 6 shows an example of the input / output characteristics of the sensor unit 300 according to this embodiment. In this figure, the horizontal axis represents the magnitude B of the input magnetic field input to the sensor unit 300, and the vertical axis represents the magnitude V_xMR of the detection signal from the sensor unit 300. The sensor unit 300 has high magnetic sensitivity and can detect a weak magnetic field of about 10 pT. Furthermore, the sensor unit 300 can maintain good linearity of the detection signal V_xMR even when the absolute value of the input magnetic field B exceeds 100 μT, for example.

[0038] That is, the sensor unit 300 according to this embodiment is configured so that the detection result for the input magnetic field B has linearity within a predetermined range of the input magnetic field B, for example, an absolute value of the input magnetic field B of several hundred μT or less. By using such a sensor unit 300, it is possible to easily detect a weak magnetic signal such as a magnetocardiogram signal.

[0039] FIG. 7 shows an example of the configuration of a magnetic sensor 520 according to this embodiment. As an example, the magnetic sensor 520 according to this embodiment includes a magnetoresistive element 702 and magnetic concentrators 704 and 706 disposed at one end and the other end of the magnetoresistive element 702. The magnetic concentrators 704 and 706 are disposed so as to sandwich the magnetoresistive element 702 therebetween. That is, the magnetic concentrators are disposed at both ends of the magnetoresistive element 702. In FIG. 7 , the magnetic concentrator 704 disposed at the right end of the magnetoresistive element 702 along the magnetic sensing axis in a front view is the magnetic concentrator disposed on the positive side of the magnetic sensing axis, and the magnetic concentrator 706 disposed at the left end of the magnetoresistive element 702 is the magnetic concentrator disposed on the negative side of the magnetic sensing axis. When a magnetic field is input to the magnetic concentrators 704 and 706 from the negative side to the positive side of the magnetic sensing axis, the resistance of the magnetoresistive element 702 may increase or decrease. The magnetic sensing axis may be aligned with the direction of magnetization fixed by the magnetization fixed layer forming the magnetoresistive element 702. The magnetic concentrators 704 and 706 are made of a soft magnetic material such as iron. By disposing the magnetic concentrators 704 and 706 made of a soft magnetic material at one end and the other end of the magnetoresistive element 702, the number of magnetic field lines passing through the magnetoresistive element 702 can be increased, thereby improving the sensitivity of the magnetic sensor 520.

[0040] Although the figure shows an example in which magnetic concentrators are disposed at both one end and the other end of the magnetoresistive element 702, magnetic concentrators may be provided at only one end or the other end of the magnetoresistive element 702. However, to further increase the sensitivity of the magnetic sensor 520, it is preferable to provide magnetic concentrators at both one end and the other end of the magnetoresistive element 702. Furthermore, when magnetic concentrators are provided at both one end and the other end of the magnetoresistive element 702, the position of the magnetoresistive element 702, which is located in a narrow position between the two magnetic concentrators 704 and 706, becomes the magnetic sensing portion, i.e., the spatial sampling point. This makes the magnetic sensing portion clearer, and further improves compatibility with the signal space separation (SSS) technology described later. In this way, by using a magnetic sensor 520 in each sensor unit 300, in which magnetic concentrators 704 and 706 are arranged at both ends of a magnetic resistance element 702, the magnetic field measuring device 10 of this embodiment can sample the spatial distribution of the magnetic field at an extremely narrow position (for example, 100 μm or less) in each axial direction, sandwiched between the magnetic concentrators on both ends, as shown in Figure 3, thereby improving the sampling accuracy (positional accuracy) compared to sampling the spatial distribution of the magnetic field using a SQUID coil (up to 2 cm) that measures biomagnetic fields.

[0041] FIG. 8 shows the magnetic flux distribution when a feedback magnetic field is generated in the magnetic sensor 520 according to this embodiment. In FIG. 8, components having the same functions and configurations as those in FIG. 7 are denoted by the same reference numerals, and descriptions thereof will be omitted except for differences. In the magnetic sensor 520 according to this embodiment, when a feedback current is supplied to the feedback coil 534, the feedback coil 534 generates a feedback magnetic field, generating a magnetic flux distribution as shown by the dashed-dotted line in the figure. The magnetic flux generated by this feedback magnetic field is spatially distributed so as to cancel the spatial distribution of the magnetic field input to the magnetoresistive element 702 and magnetically amplified by the magnetic concentrators 704 and 706. Therefore, when the magnetic concentrators 704 and 706 are disposed at both ends of the magnetoresistive element 702 as shown in the figure, the magnetic field distribution at the position of the magnetoresistive element 702 can be accurately canceled by the feedback magnetic field, thereby achieving a sensor with high linearity between the input magnetic field and the output voltage.

[0042] FIG. 9 shows an example of the arrangement of the plurality of magnetic sensor cells 220 in the magnetic sensor array 210 according to this embodiment. For convenience of explanation, in FIGS. 2 and 3, the magnetic sensor array 210 is shown as being planar. However, in reality, the magnetic sensor array 210 may have a curved surface shape that is curved in at least one direction, as shown in this figure. The plurality of magnetic sensor cells 220 may be configured by being arranged three-dimensionally so as to be located at lattice points included in the curved surface shape. As an example, the magnetic sensor array 210 may be configured by being arranged three-dimensionally so as to be arranged in an arc-shaped cross section.

[0043] That is, the multiple magnetic sensor cells 220 may be arranged in an arc-shaped cross section along the chest of the measured object, with the center of gravity of the measured object at the center. In this case, each magnetic sensor cell 220 is arranged at a lattice point included in a curved surface shape in a three-dimensional lattice space. Note that the lattice points here refer to lattice-like points arranged at equal intervals at predetermined intervals in the X, Y, and Z directions. As an example, each magnetic sensor cell 220 is arranged along a curved surface that has a convex shape in a direction perpendicular to any one of the X, Y, and Z directions when viewed from the X, Y, or Z direction. This figure shows an example in which each magnetic sensor cell 220 is arranged along a curved surface that has a convex shape in the positive direction of the Z axis when viewed from the Y direction. The magnetic sensor array 210 may form a curved surface shape having a convex shape in the positive direction of the Z axis by, for example, arranging each magnetic sensor cell 220 at a lattice point in a three-dimensional lattice space so that each vertex of each magnetic sensor cell 220 is positioned as far in the negative direction of the Z axis as possible, without exceeding a predetermined curved surface having a convex shape in the positive direction of the Z axis.

[0044] More specifically, in the cross-sectional view of this figure, the multiple magnetic sensor cells 220 on the inner side (negative side of the Z axis), i.e., the magnetic sensor cells 220[1,j,1] to 220[8,j,1], are arranged outside the arc indicated by the dashed-dotted line indicated by the reference numeral 915 so as to be arranged outside the inscribed circle of the magnetic sensor array 210 indicated by the reference numeral 910. Also, the multiple magnetic sensor cells 220 on the outer side (positive side of the Z axis), i.e., the magnetic sensor cells 220[1,j,2] to 220[8,j,2], are arranged inside the arc indicated by the dashed-two-dot line indicated by the reference numeral 925 so as to be arranged inside the circumscribed circle of the magnetic sensor array 210 indicated by the reference numeral 920. The centers of these inscribed and circumscribed circles are common and coincide with the coordinate origin in the signal separation calculation described below.

[0045] As described above, the magnetic sensor array 210 may be configured by arranging a plurality of magnetic sensor cells 220, each having a magnetic sensor 520, to form a surface covering at least a portion of the object to be measured. In particular, in the magnetic sensor array 210, the plurality of magnetic sensor cells 220 may be arranged three-dimensionally so as to be located at lattice points between two curved surfaces curved in one direction. Such curved surfaces may be formed in a substantially parabolic shape. This allows the magnetic sensor array 210 to arrange the sensor units 300 not only in one direction facing the heart but also in multiple directions, thereby enabling cardiac magnetism to be sensed from multiple directions. Furthermore, in the magnetic sensor array 210 according to this embodiment, the magnetic sensor cells 220 are formed in a rectangular parallelepiped shape, as an example, and therefore the shape of the magnetic sensor array 210 can be easily changed. That is, the magnetic sensor array 210 according to this embodiment can adopt various shapes that can be configured by arranging the magnetic sensor cells 220 at lattice points, thereby providing a high degree of design freedom. Therefore, as shown in this figure, the magnetic sensor array 210 can easily form a curved surface shape in three-dimensional space by arranging multiple magnetic sensor cells 220 at lattice points included in the curved surface shape in three-dimensional space. The magnetic field measurement device 10 measures the magnetic field by arranging the magnetic sensor array 210 so that the chest of the subject is located toward the center of the curved surface, i.e., so that the heart, which is the source of the magnetic field to be measured, is located toward the center of the curved surface. This allows the magnetic field measurement device 10 to perform signal spatial separation (described below) using measurement data measured at a position close to the heart, which is the source of the magnetic field to be measured, thereby enabling high-accuracy separation of the magnetic field to be measured and the disturbance magnetic field. In this case, it is preferable that the curvature of the curved surface of the magnetic sensor array 210 is approximately equal to the curvature around the chest of the subject, since this allows the magnetic field to be measured at a position closer to the heart, which is the source of the magnetic field to be measured.

[0046] 10 shows an example of the definition of space in the magnetic field measuring device 10 according to this embodiment. The inside of a circle inscribed in the magnetic sensor array 210, indicated by the reference numeral 910, is defined as the internal space, and the outside of the inscribed circle is defined as the external space. When measuring the cardiac magnetism, the magnetic field measuring device 10 positions the magnetic sensor array 210 so that the object to be measured (i.e., the heart of the subject) faces the surface formed by the magnetic sensor array 210 in the internal space. In this case, the cardiac magnetism to be measured is defined as the magnetic field Bin to be measured generated in the internal space.

[0047] Here, when measuring the magnetic field of the heart using the magnetic sensor array 210 and performing signal separation (here, signal separation may also be called signal space separation) on the spatial distribution of the magnetic field indicated by the measurement data, if the measurement target magnetic field Bin can be accurately separated and extracted from other disturbance magnetic fields, the electrical activity of the heart can be more accurately understood. However, when the magnetic field measurement device 10 measures the measurement target magnetic field Bin, various environmental magnetic fields may interfere as disturbance magnetic fields. Such disturbance magnetic fields may include, for example, the external space magnetic field Bout generated in external space. Therefore, in reality, the signal space separated estimate of the measurement target magnetic field Bin, i.e., the internal space data ^Bin (here, "^" means "estimated value"), may contain, in addition to the measurement target magnetic field Bin generated by the electrical activity of the heart, interference components that could not be accurately separated as disturbance magnetic fields and remain. Such interference components may include components caused by the external space magnetic field Bout that should have been separated as the external space data ^Bout.

[0048] The component due to the external space magnetic field Bout increases in proportion to the magnitude of the sensor error. Here, if the magnetic sensor 520 includes a magnetoresistive element 702, errors in the sensor's main axis sensitivity and other axis sensitivity may increase due to manufacturing errors of the magnetoresistive element 702, etc. Furthermore, if the magnetic sensor 520 includes magnetic concentrators 704 and 706 disposed at both ends of the magnetoresistive element 702, errors in the sensor's main axis sensitivity and other axis sensitivity may increase due to misalignment between the magnetic concentrators 704 and 706 and the magnetoresistive element 702. For these reasons, as the sensor error of the magnetic sensor 520 increases, noise due to the external space magnetic field Bout becomes significant. The magnetic field measuring device 10 according to this embodiment removes at least a portion of the remaining noise from the internal space data ^Bin obtained by signal space separation.

[0049] 11 shows the configuration of a magnetic sensor array 210 according to this embodiment, a sensor data collection section 230, and a sensor data processing section 1100. The sensor data processing section 1100 may be included in the information processing section 150 in FIG.

[0050] The magnetic sensor array 210 is configured by arranging a plurality of magnetic sensor cells 220, each having a magnetic sensor 520, to form a surface covering at least a portion of the object to be measured. As an example, each of the plurality of magnetic sensor cells 220 has a plurality of sensor units 300x to 300z as described above. In this figure, of the plurality of magnetic sensor cells 220 that the magnetic sensor array 210 has in each dimensional direction, portions relating to positions [i, j, k], [i+1, j, k], [i, j+1, k], and [i, j, k+1] are shown.

[0051] The sensor data collection unit 230 has a plurality of AD converters 1110 and a clock generator 1112. The plurality of AD converters 1110 are provided corresponding to the plurality of sensor units 300x to 300z of the magnetic sensor cell 220, and convert analog detection signals (sensor output signals V_xMR in FIG. 6) output by the corresponding sensor units 300 into digital measurement data (Vx, Vy, Vz). Here, Vx, Vy, and Vz are measurement values ​​(representing, for example, digital voltage values) obtained by converting the detection signals from the sensor units 300x, 300y, and 300z into digital form.

[0052] The clock generator 1112 generates a sampling clock and supplies a common sampling clock to each of the plurality of AD converters 1110. Each of the plurality of AD converters 1110 performs AD conversion in accordance with the common sampling clock supplied from the clock generator 1112. Therefore, all of the plurality of AD converters 1110 that perform AD conversion on the outputs of the three-axis sensor units 300x to 300z provided at different positions operate in synchronization. This allows the plurality of AD converters 1110 to simultaneously sample the detection results of the three-axis sensor units 300x to 300z provided in different spaces.

[0053] The sensor data processing unit 1100 has a plurality of magnetic field acquisition units 1120 corresponding to each of the plurality of magnetic sensor cells 220, a plurality of calibration calculation units 1130, a plurality of data output units 1140, a basis vector storage unit 1150, a data storage unit 1155, a signal space separation unit 1160, and a calculation processing unit 1170.

[0054] The magnetic field acquiring unit 1120 is connected to three AD converters 1110, each connected to a corresponding magnetic sensor cell 220, and acquires measurement data measured by the sensor units 300x to 300z in the plurality of magnetic sensor cells 220 constituting the magnetic sensor array 210. Specifically, the magnetic field acquiring unit 1120 may be configured using a flip-flop or the like that latches and acquires the digital measurement data (Vx, Vy, Vz) converted by the AD converters 1110 at a predetermined timing T.

[0055] The calibration calculation unit 1130 is connected to the magnetic field acquisition unit 1120 and calibrates the measurement data acquired by the magnetic field acquisition unit 1120 using calibration parameters. An overview of the calibration of measurement data by the calibration calculation unit 1130 is as follows. The magnetic field input to the magnetic sensor cell 220 at position [i, j, k] is denoted by B(Bx, By, Bz), and the detection result of the triaxial magnetic sensor by sensor units 300x, 300y, and 300z is denoted by V(Vx, Vy, Vz). In this case, if the magnetic sensor characteristics of the triaxial magnetic sensor are denoted by a matrix S, the detection result V of the triaxial magnetic sensor can be expressed as follows:

number

[0056] Here, Sxx, Syy, and Szz represent the sensitivities of the sensor units 300x, 300y, and 300z in the principal axis direction (principal axis sensitivity), respectively, and Sxy, Sxz, Syx, Syz, Szx, and Szy represent the sensitivities in the other axis directions (other axis sensitivity). Also, Vos,x, Vos,y, and Vos,z represent the offsets of the sensor units 300x, 300y, and 300z, respectively. Here, the principal axis direction refers to the direction in which the sensor units 300x, 300y, and 300z primarily measure, and the other axis directions refer to directions in which they do not primarily measure. In measuring a magnetic field, the principal axis direction refers to the direction in which the magnetic sensor exhibits maximum sensitivity when a magnetic field is input (input axis direction, sensitivity axis direction). The other axis directions are defined as axes perpendicular to the principal axis direction. For example, when the sensor unit 300x measures in the X-axis direction, the principal axis direction is the X-axis, and the other axes are the Y-axis and Z-axis. Ideally, the magnetic sensor 520 has only the main axis sensitivity, but it may have other axis sensitivity depending on the process, etc. The magnetic sensor 520 also has other axis sensitivity generated by the mutual interference described above. A vertical vector represented by the three components of the main axis sensitivity and other axis sensitivity of the sensor unit 300 is called a sensitivity vector. For example, the sensitivity vector nx of the sensor unit 300x is represented by the three components (Sxx, Sxy, Sxz). In this case, the output of the sensor unit 300x is the dot product of the input magnetic field to the sensor and the sensitivity vector nx. Similarly, the sensitivity vector ny of the sensor unit 300y is represented by the three components (Syx, Syy, Syz), and the sensitivity vector nz of the sensor unit 300z is represented by the three components (Szx, Szy, Szz).

[0057] Each of the sensor units 300 has linearity in the detection result for the input magnetic field within the range of the input magnetic field to be detected, and therefore each element of the matrix S has an approximately constant coefficient that is unrelated to the magnitude of the input magnetic field B. Furthermore, even if the sensor unit 300 has other-axis sensitivity, as long as the detection result of the sensor unit 300 has linearity, each element of the matrix S has an approximately constant coefficient that is unrelated to the magnitude of the input magnetic field B.

[0058] Therefore, the calibration calculation unit 1130 can convert the measurement data V(Vx, Vy, Vz) into magnetic field measurement data B(Bx, By, Bz) indicating the original input magnetic field by using the inverse matrix S-1 of the matrix S and the offset (Vos, x, Vos, y, Vos, z) as shown in the following equation. That is, the calibration calculation unit 1130 calibrates the digital measurement data V from the magnetic field acquisition unit 1120 using the main axis sensitivity, the other axis sensitivity, and the offset. As a result, the calibration calculation unit 1130 corrects the offset, the sensitivity in the main axis direction, and the sensitivity in the other axis direction. Note that this conversion also holds when the sensor units 300x to 300z are equipped with the magnetic flux concentrator plates described above. This is because the magnetic sensor cell 220 is configured as a triaxial magnetic sensor using the sensor units 300x to 300z, which enables conversion using linear algebra. Note that offset calibration may be omitted if a high-pass filter or the like is provided between the output of the sensor unit 300 and the calibration calculation unit 1130 to convert the measurement data V into AC components. That is, the calibration calculation unit 1130 may calibrate the digital measurement data V from the magnetic field acquisition unit 1120 using at least one of the main axis sensitivity, the other axis sensitivity, and the offset. Note that these calibration parameters may be calculated in advance by measuring a known DC or AC magnetic field. Furthermore, the calibration calculation unit 1130 in this embodiment is only required to calibrate the output from each magnetic sensor cell 220 as components expressed in a coordinate system formed by three independent vectors, and does not necessarily need to correct the output to three-axis components expressed in a coordinate system of three orthogonal vectors (so-called Cartesian coordinate system). That is, when all the magnetic sensor cells 220 are measuring the same magnetic field, each calibration calculation unit 1130 corresponding to each magnetic sensor cell 220 may calibrate the digital measurement data V from the corresponding magnetic field acquisition unit 1120 into the same magnetic field measurement data B expressed in three independent axis components.

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[0059] The calibration calculation unit 1130 calculates the inverse matrix S-1 and offset (Vos,x,Vos,y,Vos,z) of the matrix S using the environmental magnetic field measurement data, and converts the magnetic field measurement data acquired by the magnetic field acquisition unit 1120 into magnetic field measurement data B using these calibration parameters and supplies it to the data output unit 1140.

[0060] As described above, since each sensor unit 300 has linearity, the calibration calculation unit 1130 can use approximately constant coefficients to convert the measurement data into magnetic field measurement data B. That is, the approximately constant coefficients used by the calibration calculation unit 1130 can be determined as a set of calibration parameters using environmental magnetic field data.

[0061] The data output unit 1140 supplies the magnetic field measurement data B calibrated by the calibration calculation unit 1130 to the data storage unit 1155 .

[0062] The data storage unit 1155 stores the magnetic field measurement data B supplied from the data output unit 1140 , and supplies the magnetic field measurement data B to the signal space separation unit 1160 .

[0063] The basis vector storage unit 1150 stores in advance basis vectors required for the signal space separation unit 1160 to perform signal separation of the magnetic field measurement data B, and supplies these to the signal space separation unit 1160. Such basis vectors may be calculated from the positions and magnetic sensitivities of the magnetic sensors 520.

[0064] The signal space separation unit 1160 acquires, for each combination, measurement data B from the magnetic sensor cells 220 of different combinations of the magnetic sensor array 210 from the magnetic field measurement data B stored in the data storage unit 1155. The signal space separation unit 1160 extracts first internal space data from the spatial distribution of the magnetic field indicated by the measurement data B from some of the magnetic sensor cells 220 of the magnetic sensor array 210, and extracts second internal space data from the spatial distribution of the magnetic field indicated by the measurement data from the plurality of magnetic sensor cells 220 including some of the magnetic sensor cells 220 of the magnetic sensor array 210. The signal space separation unit 1160 extracts third internal space data from the spatial distribution of the magnetic field indicated by the measurement data B from some of the magnetic sensor cells 220 different from the some of the magnetic sensor cells 220 of the magnetic sensor array 210 from which the first internal space data was extracted. The signal space separating unit 1160 may extract second internal space data from the spatial distribution of the magnetic field indicated by the measurement data B from all the magnetic sensor cells 220 of the magnetic sensor array as a noise removal target. The signal space separating unit 1160 supplies the extracted internal space data to the calculation processing unit 1170.

[0065] The arithmetic processing unit 1170 calculates a projection matrix based on the first internal space data, and removes noise from the second internal space data using the projection matrix. The arithmetic processing unit 1170 outputs the internal space data from which noise has been removed. Details of this arithmetic processing will be described later.

[0066] 12 shows a first example configuration of a portion of the magnetic field measuring device 10. The arithmetic processing unit 1170 has a noise removal unit 1172 and a calculation unit 1174. The signal space separation unit 1160 acquires, from the data storage unit 1155, measurement data B from all the magnetic sensor cells 220 of the magnetic sensor array 210, measurement data B_A from only the magnetic sensor cells 220 of the even channels (even-numbered) of the magnetic sensor array 210, and measurement data B_B from only the magnetic sensor cells 220 of the odd channels (odd-numbered) of the magnetic sensor array 210. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and the magnetic sensor cells 220 of the magnetic sensor array 210 for which corresponding measurement data has been acquired by the signal space separation unit 1160 are indicated by being shaded.

[0067] The signal space separation unit 1160 performs signal separation of the measurement data B. Based on the positions and magnetic sensitivities of the magnetic sensors 520, the signal space separation unit 1160 separates the spatial distribution of the magnetic field indicated by the measurement data, i.e., the spatial distribution of the magnetic field indicated by the magnetic field measurement data B obtained by calibrating the digital measurement data V, into internal space data ^Bin and external space data ^Bout. For example, the signal space separation unit 1160 performs signal separation of the spatial distribution of the magnetic field based on basis vectors calculated from orthonormal functions, the positions of the magnetic sensors 520, and the magnetic sensitivities. In this case, the signal space separation unit 1160 may perform signal separation of the spatial distribution of the magnetic field by expanding the basis vectors into series. More specifically, the signal space separation unit 1160 may calculate the expansion coefficients of the basis vectors using the least squares method. Note that such orthonormal functions may be expressed as spherical harmonic functions. Details of signal separation will be described later. In this way, the signal space separating section 1160 separates the spatial distribution of the magnetic field indicated by the magnetic field measurement data B into internal space data ^Bin and external space data ^Bout.

[0068] Similarly, the signal space separating unit 1160 may perform signal separation on the measurement data B_A and B_B using information from the corresponding magnetic sensor cells 220, and extract internal space data ^Bin_A and ^Bin_B, respectively. The signal space separating unit 1160 supplies the internal space data ^Bin, which is the data after signal separation, to the noise removing unit 1172, and supplies the internal space data Bin_A and ^Bin_B to the calculating unit 1174.

[0069] The calculation unit 1174 calculates a projection matrix based on the internal space data ^Bin_A and the internal space data ^Bin_B using a method such as principal component analysis. For example, the calculation unit 1174 may calculate bases Cin_A and Cin_B from the internal space data ^Bin_A and the internal space data ^Bin_B, respectively, and calculate the projection matrix based on the common part of the bases Cin_A and Cin_B. For example, the calculation unit 1174 performs singular value decomposition on the internal space data Bin_A and ^Bin_B, respectively, to calculate the time domain base Cin_A for the internal space data Bin_A and the time domain base Cin_B for the internal space data Bin_A, respectively. The internal spatial data Bin_A includes the signal components of the object to be measured and the sensor noise of the even channels, and the internal spatial data ^Bin_B includes the signal components of the object to be measured and the sensor noise of the odd channels, so the common part of the basis Cin_A and the basis Cin_B corresponds to the signal components of the object to be measured with the basis corresponding to the sensor noise removed.

[0070] More specifically, the calculation unit 1174 calculates a common fluctuation component as a common part based on the correlation between the bases Cin_A and Cin_B. In this case, the calculation unit 1174 may, for example, perform singular value decomposition on the covariance matrix (or correlation matrix) of the bases Cin_A and Cin_B to determine the correlation coefficient between the bases Cin_A and Cin_B from the magnitude of the singular values, and extract the bases whose correlation coefficient exceeds a predetermined threshold as the common fluctuation component. The calculation unit 1174 then calculates a projection matrix P from the common part of the bases Cin_A and Cin_B, and supplies the projection matrix P to the noise removal unit 1172.

[0071] The noise removal unit 1172 removes noise from the internal space data ^Bin by projecting the internal space data ^Bin onto signal space using the projection matrix P from the calculation unit 1174. The noise removal unit 1172 may output the internal space data ^Binproj from which the noise has been removed. Details of such calculation processing will be described later.

[0072] 13 shows a flow of signal separation of the spatial distribution of the magnetic field by the magnetic field measuring device 10 according to this embodiment. In this figure, the magnetic field measuring device 10 separates the spatial distribution of the magnetic field indicated by the magnetic field measurement data B into internal space data ^Bin and external space data ^Bout.

[0073] In step 1210, the basis vector storage unit 1150 stores basis vectors. As an example, before measuring the magnetic field to be measured, the basis vector storage unit 1150 stores, as basis vectors, signal vectors output by each of the multiple magnetic sensors 520 when the magnetic sensor array 210 detects a magnetic field having a spatial distribution of a spherical harmonic function. That is, the basis vectors may be calculated in advance from the positions and sensitivity vectors of each magnetic sensor 520 in the magnetic sensor array 210 and stored in the basis vector storage unit 1150. That is, the basis vector storage unit 1150 stores, as basis vectors, magnetic field signal vectors obtained by spatially sampling a spherical harmonic function when a predetermined point in space is designated as the coordinate origin. In other words, the basis vector storage unit 1150 calculates in advance, from the positions and sensitivity vectors of each magnetic sensor 520, magnetic field signal vectors that represent the spatial magnetic field in two subspaces (an internal space including the signal source to be measured and an external space) based on the series expansion of a spherical harmonic function, and stores the calculated vectors as basis vectors. Here, a spherical harmonic function is a function obtained by restricting a homogeneous polynomial, which is a solution to an n-dimensional Laplace equation, to a unit sphere, and has orthonormality on the sphere. Note that, as an example, this figure shows a case where step 1210, in which the basis vector storage unit 1150 stores the basis vectors, is the first step in the flow for signal separation of the spatial distribution of the magnetic field by the magnetic field measurement device 10. However, the basis vector storage unit 1150 may store the basis vectors in advance before the flow for signal separation of the spatial distribution of the magnetic field by the magnetic field measurement device 10. The basis vector storage unit 1150 may also store signal vectors that are predetermined based on simulation results or the like as basis vectors.

[0074] Next, in step 1220 , the signal space separator 1160 acquires the magnetic field measurement data B measured by the magnetic sensor array 210 and calibrated by the calibration calculator 1130 from the data storage unit 1155 .

[0075] Furthermore, in step 1230, the signal space separating unit 1160 acquires from the basis vector storage unit 1150 the signal vectors that the basis vector storage unit 1150 stored as basis vectors in step 1210. Note that in this flow, either step 1220 or step 1230 may be performed first.

[0076] In step 1240, the signal space separation unit 1160 performs series expansion on the spatial distribution of the magnetic field indicated by the magnetic field measurement data B acquired in step 1220, using the signal vector acquired in step 1230 as a basis vector. Then, from the vector obtained by series expansion, the signal space separation unit 1160 performs signal separation on the spatial distribution of the magnetic field into internal space data ^Bin (an estimated value of the magnetic field to be measured) and external space data ^Bout (an estimated value of the disturbance magnetic field). Note that the orthonormal function may be a spherical harmonic function. Furthermore, when performing signal separation, the signal space separation unit 1160 calculates the series expansion coefficients of the basis vectors using the least squares method. This will be described in detail below.

[0077] The static magnetic field B(r) can be calculated as the spatial gradient of the potential V(r) using the potential V(r) that satisfies the Laplace equation ΔV(r) = 0, as shown in the following equation: where r is the position vector representing the position from the coordinate origin, Δ is the Laplacian, μ is the magnetic permeability, and ∇ is the operator representing vector differentiation.

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[0078] The solution of the Laplace equation is generally given by the spherical harmonics Yl,m, which are a system of orthonormal functions. Since the solution is in the form of a series expansion using (θ, φ), the potential V(r) can be expressed as follows: where |r| is the absolute value of the position vector r (the distance from the coordinate origin), θ and φ are two argument angles in spherical coordinates, l is the azimuthal quantum number, m is the magnetic quantum number, α and β are multipole moments, and L and L are the series numbers for the space in front of and behind the magnetic sensor array 210, respectively, as seen from the subject. The azimuthal quantum number l is a positive integer, and the magnetic quantum number m is an integer between -l and +l. That is, for example, when l is 1, m is -1, 0, and 1, and when l is 2, m is -2, -1, 0, 1, and 2. Note that because there are no single magnetic poles in a magnetic field, the azimuthal quantum number l in (Equation 7) starts from 1, not 0. The first term in (Equation 7) is a term inversely proportional to the distance from the coordinate origin, and indicates the potential existing in the space in front of the magnetic sensor array 210 as seen from the subject. The second term in (Equation 7) is a term proportional to the distance from the coordinate origin, and indicates the potential existing in the space behind the magnetic sensor array 210 as seen from the measured object.

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[0079] Therefore, according to (Equation 6) and (Equation 7), the static magnetic field B(r) can be expressed by the following equation: Here, the first term in (Equation 8) represents a magnetic field source existing in the space in front of the magnetic sensor array 210 as viewed from the subject, i.e., the cardiac magnetic field (magnetic field to be measured) created by the electrical activity of the heart. Also, the second term in (Equation 8) represents a disturbance magnetic field created by a magnetic field source existing in the space behind the magnetic sensor array 210 as viewed from the subject.

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[0080] When the solution to the Laplace equation is expressed in the form of a series expansion using spherical harmonic functions, the general solution becomes an infinite series. However, as long as a sufficient SNR (signal-to-noise ratio, i.e., the ratio of the magnetic field signal to be measured to the disturbance magnetic field and sensor noise) is obtained to measure the biomagnetic field, it is said that a series of about 10 terms is sufficient in practice. Furthermore, for the signal space separation series in magnetoencephalography, Lin = 8 and Lout = 3 are generally used. Therefore, in this embodiment, Lin and Lout can be values ​​similar to those above. However, the values ​​of Lin and Lout are not limited to these and can be any numerical values ​​that are sufficient to sufficiently suppress the disturbance magnetic field and calculate only the magnetic field to be measured.

[0081] Here, for a total of N sensors used in the magnetic sensor array 210, a1,m and bl,m are defined as follows: where n1, n2, ... nN are sensitivity vectors of each sensor unit. Note that a1,m and bl,m are vectors with dimensions equal to three times the number of magnetic sensor cells 220 (because there are sensor units 300x, y, and z). In other words, they are vectors with dimensions equal to the total number of sensors. As an example, the vectors (a1,m, bl,m) are calculated from the output of each magnetic sensor cell 220 using data corrected by the calibration calculation unit 1130. In this way, the values ​​of a1,m and bl,m calculated including the sensitivity in the main axis directions of the sensor units 300x, y, and z and the sensitivity corrections in the other axis directions (corrected sensitivity vectors) are stored in the basis vector storage unit 1150. In the magnetic field measuring device 10 according to the present embodiment, in which the basis vector storage unit 1150 stores the values ​​of a1,m and b1,m calculated including correction of the magnetic sensitivity (main axis sensitivity, other axis sensitivity), the calibration calculation unit 1130 performs correction on the data acquired by the magnetic field acquisition unit 1120 during operation, thereby making it possible to correct the magnetic sensitivity (main axis sensitivity, other axis sensitivity) of each magnetic sensor cell 220. As another example, when the basis vector storage unit 1150 stores default values ​​of a1,m and b1,m that have not been corrected for the magnetic sensitivity (main axis sensitivity, other axis sensitivity) (using uncorrected sensitivity vectors), the calibration calculation unit 1130 converts the output of each magnetic sensor cell 220 into data in which the magnetic sensitivity has been corrected so as to match the default sensitivity vector of each sensor unit, as shown in (Equation 5), and outputs the data to the data output unit 1140. Thereafter, the signal space separation unit 1160 performs calculations.

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[0082] Since a1,m and bl,m are defined in this way, the sensor output vector Φ output from each magnetic sensor cell 220 at a certain time can be expressed by the following equation.

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[0083] Furthermore, Sin, Sout, Xin, and Xout are defined as follows. That is, Sin is defined as a Lin × (Lin + 2) vector in total, which is a column of vectors a when m = -l to l for each l, from l = 1 to l = Lin. Also, Sout is defined as a Lout × (Lout + 2) vector in total, which is a column of vectors b when m = -l to l for each l, from l = 1 to l = Lout. Also, Xin is defined as a multipole moment α when m = -l to l for each l, from l = 1 to l = Lin. l,m Let Xout be the vector of Lin × (Lin + 2) rows in total, which is the transposed vector of the vectors arranged in columns. Also, let Xout be the multipole moment β when m = -1 to l = Lin, and m = -1 to l for each l. l,m is defined as a vector with a total of Lout × (Lout + 2) rows, which is the transposed vector of the vectors arranged in columns.

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[0084] Then, the sensor output vector Φ can be expressed as the inner product of the matrix S and the column vector X, as shown in the following equation. Here, the matrix S indicates a basis vector, and is obtained, for example, by the signal space separation unit 1160 from the basis vector storage unit 1150 in step 1230. Furthermore, the column vector X indicates a coefficient related to the basis vector.

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[0085] In step 1240, the signal space separation unit 1160 according to this embodiment determines the column vector X that satisfies Φ=SX by least squares approximation using the following equation based on the model equation of the sensor output vector Φ obtained in (Equation 12). This allows the signal space separation unit 1160 to solve the spatial distribution of the magnetic field in step 1240. That is, the signal space separation unit 1160 becomes able to estimate the spatial distribution of the magnetic field. Specifically, the signal space separation unit 1160 can estimate the magnetic field Bin to be measured as internal space data ^Bin=SinXin and the disturbance magnetic field as external space data ^Bout=SoutXout, using the sensor output vector Φ in Equation 12 as magnetic field measurement data B. In this case, the signal space separation unit 1160 may issue a warning that the magnetic field Bin to be measured cannot be estimated with high accuracy if the magnitude of the external space data ^Bout exceeds a predetermined range. This allows the magnetic field measuring device 10 to prevent in advance measurement of the magnetic field Bin to be measured in situations such as when the device is broken or when there is a disturbance magnetic field so large that the magnetic field Bin to be measured cannot be estimated with high accuracy. In this case, the signal space separating unit 1160 may determine that the magnitude of the disturbance magnetic field exceeds a predetermined range when the magnitude of any of the components of SoutXout exceeds a predetermined threshold, or may determine that the magnitude of the disturbance magnetic field exceeds a predetermined range when the sum or average of the magnitudes of the components of SoutXout exceeds a predetermined threshold.

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[0086] The signal space separation unit 1160 may also perform signal separation on the measurement data B_A and B_B in the same manner as in steps 1210 to 1240, extracting internal spatial data ^Bin_A and ^Bin_B, respectively. The signal space separation unit 1160 may perform signal separation on each of the measurement data B, B_A, and B_B in parallel, or may perform signal separation sequentially. Here, ^Bin_A and ^Bin_B may be composed only of data corresponding to the sensors of the even and odd channels, respectively, or may be created by complementing the data of excluded channels. For example, for internal spatial data ^Bin_A using even channels, the coefficients Xin and Xout multiplied by the basis vectors are calculated based on data from only the even channels, but by adding the basis vectors of all channels, including the odd channels, to Sin, it is possible to estimate internal spatial data for all channels.

[0087] Then, the signal space separating unit 1160 supplies the internal space data ^Bin, ^Bin_A, and ^Bin_B, which are data after signal separation in step 1240, to the calculation processing unit 1170.

[0088] As a result, the magnetic field measuring device 10 according to this embodiment can perform signal separation of the spatial distribution of the magnetic field indicated by the magnetic field measurement data measured using the magnetic sensor array 210 configured by arranging a plurality of magnetic sensor cells 220, each having a magnetic sensor 520, so as to form a surface covering at least a part of the object to be measured, into internal space data ^Bin, ^Bin_A, and ^Bin_B and external space data ^Bout. Here, since each of the plurality of sensor units 300 has a magnetic concentrator, the magnetic sensitivity of the sensor unit 300 can be increased and the spatial sampling points can be clarified, thereby further improving compatibility with signal spatial separation technology.

[0089] FIG. 14 shows an example of a flow in which the magnetic field measuring device 10 according to this embodiment removes remaining noise from the internal space data ^Bin that has been subjected to signal space separation.

[0090] In step 1310, the arithmetic processing unit 1170 acquires the internal space data ^Bin, ^Bin_A, and ^Bin_B as time-series data from the signal space separation unit 1160. That is, the arithmetic processing unit 1170 acquires the internal space data ^Bin, ^Bin_A, and ^Bin_B extracted by the signal space separation unit 1160 through signal separation at each time t1, t2, ... tn from the magnetic field measurement data B. Here, the internal space data ^Bin, ^Bin_A, and ^Bin_B are each a matrix whose vertical direction corresponds to each magnetic sensor cell 220 that acquired measurement data and whose horizontal direction corresponds to each time.

[0091] In step 1320, the magnetic field measuring device 10 calculates time-domain bases Cin_A and Cin_B for the internal space data ^Bin_A and ^Bin_B, respectively. For example, the calculation unit 1174 calculates the time-domain bases Cin_A and Cin_B by performing singular value decomposition on the internal space data ^Bin_A and ^Bin_B, respectively. More specifically, the calculation unit 1174 calculates the time-domain bases Cin_A and Cin_B, respectively, by the following equation. That is, the calculation unit 1174 normalizes the transposed matrix of the internal space data ^Bin_A in a certain time domain by dividing it by the Frobenius norm, and applies singular value decomposition. Then, the calculation unit 1174 calculates the left singular vector Uin_A after singular value decomposition as the time-domain basis Cin_A for the internal space data ^Bin_A. Similarly, the calculation unit 1174 divides the transposed matrix of the internal space data ^Bin_B in a certain time domain by the Frobenius norm to normalize it, and then performs singular value decomposition.The calculation unit 1174 then calculates the left singular vector Uin_B after the singular value decomposition as a basis Cin_B in the time domain for the external space data ^Bin_B.

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[0092] In step 1330, the magnetic field measuring device 10 calculates a basis with a strong correlation between the basis Cin_A in the time domain of the internal space data ^Bin_A and the basis Cin_B in the time domain of ^Bin_B, based on a technique such as principal component analysis. For example, the calculation unit 1174 performs singular value decomposition on the covariance matrix (or correlation matrix) of the basis Cin_A and Cin_B, and calculates the coefficient of correlation between the basis Cin_A and Cin_B based on the magnitude of the singular value. Such a correlation coefficient is close to 1 (strong correlation) between the basis corresponding to a time change common to the basis Cin_A and Cin_B of the magnetocardiogram signal or the like, and is close to 0 (uncorrelated) between the basis corresponding to random noise or the like.

[0093] In step 1340, the magnetic field measuring device 10 extracts, as a common part L, a fluctuation component common to the time-domain basis Cin_A in the internal space data ^Bin_A and the time-domain basis Cin_B in the internal space data ^Bin_B. For example, the calculation unit 1174 extracts a part where the correlation coefficient exceeds a predetermined threshold as the common part. In this way, the calculation unit 1174 extracts a basis with a strong correlation as a common fluctuation component based on the strength of correlation between the time-domain basis Cin_A in the internal space data ^Bin_A and the time-domain basis Cin_B in the external space data ^Bin_B. Note that if the predetermined threshold is set too high, the basis of the signal component may be removed, resulting in attenuation of the magnetocardiogram signal, whereas if it is set too low, the basis of noise may also be included. Therefore, it is preferable to set the predetermined threshold to an appropriate value.

number

[0094] In this way, the calculation processing unit 1170 extracts the common part L by performing principal component analysis on the time domain basis Cin_A in the internal space data ^Bin_A and the time domain basis Cin_B in the internal space data ^Bin_B. However, this is not limited to this. The calculation processing unit 1170 may extract a common fluctuation component as the common part L using a different method, such as independent component analysis, instead of principal component analysis.

[0095] In step 1350, the magnetic field measuring apparatus 10 calculates a projection matrix P from the common part L of the bases Cin_A and Cin_B. More specifically, the calculation unit 1174 calculates the projection matrix P for the projection operation to the signal space (SSP: Signal Space Projection) by the following equation. In the following equation, P indicates the projection matrix, and L indicates the extracted common part.

number

[0096] In step 1360, the magnetic field measuring device 10 removes noise from the internal space data ^Bin. For example, the arithmetic processing unit 1170 projects the internal space data ^Bin onto the signal space of the measurement target by multiplying the internal space data ^Bin by a projection matrix P. More specifically, the arithmetic processing unit 1170 removes noise using the following equation. In the following equation, ^Binproj represents the internal space data after noise has been removed by SSP.

number

[0097] According to the magnetic field measuring device 10 of this embodiment, the projection matrix P is calculated from the basis L of the signal components from which noise has been removed, so that only the signal components of the internal space data ^Bin are projected, and the sensor noise components can be efficiently removed.

[0098] FIG. 15 shows a second configuration example of a portion of the magnetic field measurement device 10 according to this embodiment. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and the magnetic sensor cells 220 of the magnetic sensor array 210 whose corresponding measurement data has been acquired by the signal space separation unit 1160 are indicated by hatching. This figure shows the signal space separation unit 1160 and the calculation processing unit 1170 of the magnetic field measurement device 10 of the second configuration example, and other configurations may be similar to those of the magnetic field measurement device 10 of FIGS. 1 to 11. The magnetic field measurement device 10 of the second configuration example does not extract the common part, but calculates two projection matrices and performs two projection calculations. The magnetic field measurement device 10 of the second configuration example may be similar in other configurations and operations to the magnetic field measurement device 10 of the first configuration example. Below, the configurations and operations that differ from those of the magnetic field measurement device 10 of the first configuration example will be mainly described.

[0099] The calculation unit 1174 calculates a projection matrix P1 based on the internal space data ^Bin_A, and calculates a projection matrix P2 based on the internal space data ^Bin_B. For example, similar to step 1320, the calculation unit 1174 calculates a time domain basis Cin_A for the internal space data ^Bin_A by singular value decomposition of the internal space data ^Bin_A. Then, the calculation unit 1174 calculates a projection matrix P1 from the basis Cin_A. In the following equation, P1 represents the projection matrix, and L1 represents the basis Cin_A.

number

[0100] Furthermore, the calculation unit 1174 calculates a time domain basis Cin_B of the internal space data ^Bin_B by singular value decomposition of the internal space data ^Bin_B, as in step 1320. Then, the calculation unit 1174 calculates a projection matrix P2 from the basis Cin_B. In the following equation, P2 represents the projection matrix, and L2 represents the basis Cin_B.

number

[0101] The calculation unit 1174 supplies the calculated projection matrices P1 and P2 to the noise removal unit 1172.

[0102] The noise elimination unit 1172 eliminates a portion of the noise in the internal space data ^Bin using the projection matrix P1 and eliminates another portion of the noise in the internal space data ^Bin using the projection matrix P2. The noise elimination unit 1172 multiplies the internal space data ^Bin by the projection matrix P1 and projects the internal space data ^Bin onto the signal space of the even-numbered channel measurement target, thereby eliminating the odd-numbered channel sensor noise contained in the internal space data ^Bin. Furthermore, the noise elimination unit 1172 multiplies the internal space data ^Bin by the projection matrix P2 and projects the internal space data ^Bin onto the signal space of the odd-numbered channel measurement target, thereby eliminating the even-numbered channel sensor noise contained in the internal space data ^Bin. Note that the noise elimination unit 1172 may first apply either the projection matrix P1 or P2 to the internal space data ^Bin. In this way, the noise elimination unit 1172 can perform noise elimination in multiple steps. The noise removal unit 1172 may output the internal spatial data ^Binproj from which noise has been removed. Furthermore, L1 and L2 may use all of the bases Cin_A and Cin_B, or may be calculated by retaining bases corresponding to signals with large singular values ​​when singular value decomposition is performed on the internal spatial data ^Bin_A and the internal spatial data ^Bin_B, and excluding bases corresponding to noise with small singular values. In this case, too, removing bases with small singular values ​​is advantageous for noise reduction, but removing too many will result in attenuation of signal components, so it is desirable to set an appropriate threshold value for calculation.

[0103] FIG. 16 shows a third configuration example of a portion of the magnetic field measurement device 10 according to this embodiment. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and the magnetic sensor cells 220 of the magnetic sensor array 210 whose corresponding measurement data has been acquired by the signal space separation unit 1160 are indicated by hatching. This figure shows the signal space separation unit 1160 and the calculation processing unit 1170 of the magnetic field measurement device 10 of the third configuration example. The other configurations of the magnetic field measurement device 10 of the third configuration example may be similar to those of the magnetic field measurement device 10 of FIGS. 1 to 11. The magnetic field measurement device 10 of the third configuration example does not extract common parts, but calculates three projection matrices and performs three projection calculations. The magnetic field measurement device 10 of the third configuration example may be similar to the magnetic field measurement device 10 of the first configuration example in other configurations and operations. Below, the configurations and operations that differ from those of the magnetic field measurement device 10 of the first configuration example will be mainly described.

[0104] The signal space separation unit 1160 acquires four different combinations of magnetic field measurement data measured by the magnetic sensor array 210 and calibrated by the calibration calculation unit 1130 from the data storage unit 1155. For example, the signal space separation unit 1160 acquires measurement data B from all the magnetic sensor cells 220 of the magnetic sensor array 210, and measurement data B_A, B_B, and B_C from the magnetic sensor cells 220 of the magnetic sensor array 210, skipping one different column every three columns, from the data storage unit 1155. Specifically, the signal space separation unit 1160 may acquire measurement data B_A from the magnetic sensor cells 220 in the 2nd, 3rd, 5th, 6th, 8th, and so on columns among the magnetic sensor cells 220 in the magnetic sensor array 210, acquire measurement data B_B from the magnetic sensor cells 220 in the 1st, 3rd, 4th, 6th, 7th, and so on columns, and acquire measurement data B_C from the magnetic sensor cells 220 in the 1st, 2nd, 4th, 5th, 7th, 8th, and so on columns. The signal space separation unit 1160 performs signal separation on the measurement data B, B_A, B_B, and B_C in the same manner as in steps 1210 to 1240 of Fig. 13 , and extracts internal space data ^Bin, ^Bin_A, ^Bin_B, and ^Bin_C. The signal space separating unit 1160 supplies the internal spatial data ^Bin to the noise removing unit 1172 , and supplies the internal spatial data ^Bin_A, ^Bin_B, and ^Bin_C to the calculating unit 1174 .

[0105] The calculation unit 1174 calculates a projection matrix P1 based on the internal space data ^Bin_A, and calculates a projection matrix P2 based on the internal space data ^Bin_B, similar to step 1320 and the magnetic field measuring device 10 of the second configuration example. Furthermore, similar to step 1320, the calculation unit 1174 calculates a time-domain basis Cin_C for the internal space data ^Bin_C by singular value decomposition of the internal space data ^Bin_C. Then, the calculation unit 1174 calculates a projection matrix P3 from the basis Cin_C. In the following equation, P3 represents the projection matrix, and L3 represents the basis Cin_C.

number

[0106] The calculation unit 1174 supplies the calculated projection matrices P1, P2, and P3 to the noise removal unit 1172.

[0107] The noise elimination unit 1172 eliminates noise from the internal space data ^Bin by using the projection matrices P1, P2, and P3. For example, the noise elimination unit 1172 eliminates sensor noise from the magnetic sensor cells 220 not used in calculating the projection matrix P1 from the internal space data ^Bin by a projection calculation of the projection matrix P1 (i.e., noise from the magnetic sensor cells 220 in the 1st, 4th, 7th,... columns included in the measurement data ^Bin). The noise elimination unit 1172 eliminates sensor noise from the magnetic sensor cells 220 not used in calculating the projection matrix P2 from the internal space data ^Bin by a projection calculation of the projection matrix P2. The noise elimination unit 1172 removes sensor noise from the magnetic sensor cells 220 not used in calculating the projection matrix P3 from the internal space data ^Bin through a projection operation of the projection matrix P3 (i.e., noise from the magnetic sensor cells 220 in the 3rd, 6th, 8th, and so on columns included in the measurement data ^Bin). The noise elimination unit 1172 can remove noise by multiplying the internal space data ^Bin by the projection matrices P1, P2, and P3, respectively. Note that the order of the projection operation of the projection matrices P1, P2, and P3 on the internal space data ^Bin is not limited. The noise elimination unit 1172 may output the internal space data ^Binproj from which the noise has been removed.

[0108] FIG. 17 shows a fourth configuration example of a portion of the magnetic field measurement device 10 according to this embodiment. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and magnetic sensor cells 220 of the magnetic sensor array 210 for which corresponding measurement data has been acquired by the signal space separation unit 1160 are indicated by hatching. This figure shows the signal space separation unit 1160 and the calculation processing unit 1170 of the magnetic field measurement device 10 of the fourth configuration example. Other configurations of the fourth configuration example may be similar to those of the magnetic field measurement device 10 of FIGS. 1 to 11. The magnetic field measurement device 10 of the fourth configuration example does not extract common parts, but removes noise in the internal space data extracted from the measurement data of some of the magnetic sensor cells 220 of the magnetic sensor array 210 using a projection matrix calculated from the measurement data of other magnetic sensor cells 220 of the magnetic sensor array 210. Other configurations and operations of the magnetic field measurement device 10 of the fourth configuration example may be similar to those of the magnetic field measurement device 10 of the first configuration example. The following mainly describes the configuration and operation that are different from the magnetic field measuring device 10 of the first configuration example.

[0109] The signal space separation unit 1160 acquires, from the data storage unit 1155, measurement data B_A from only the magnetic sensor cells 220 in the even channels (even-numbered) of the magnetic sensor array 210, and measurement data B_B from only the magnetic sensor cells 220 in the odd channels (odd-numbered) of the magnetic sensor array 210. The signal space separation unit 1160 may extract internal space data ^Bin_A and ^Bin_B from the measurement data B_A and B_B, respectively, in the same manner as in steps 1210-1240 of FIG. 13 . The signal space separation unit 1160 supplies the internal space data ^Bin_A to the noise removal unit 1172 and supplies the internal space data ^Bin_B to the calculation unit 1174.

[0110] The calculation unit 1174 calculates the projection matrix P2 based on the internal space data ^Bin_B, in the same way as in step 1320 and the magnetic field measuring apparatus 10 of the second configuration example. The calculation unit 1174 supplies the projection matrix P2 to the noise removal unit 1172.

[0111] The noise elimination unit 1172 eliminates noise from the internal space data ^Bin_A using the projection matrix P2. For example, the noise elimination unit 1172 eliminates sensor noise from the magnetic sensor cells 220 of the even channels that are not used in calculating the projection matrix P2 from the internal space data ^Bin_A by performing a projection operation on the projection matrix P2. The noise elimination unit 1172 can eliminate noise by multiplying the internal space data ^Bin_A by the projection matrix P2. The noise elimination unit 1172 may output the internal space data ^Binproj from which the noise has been eliminated.

[0112] FIG. 18 shows a fifth configuration example of a portion of the magnetic field measurement device 10 according to this embodiment. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and the magnetic sensor cells 220 of the magnetic sensor array 210 for which the signal space separation unit 1160 acquired corresponding measurement data are indicated by hatching. This figure shows the signal space separation unit 1160 and the calculation processing unit 1170 of the magnetic field measurement device 10 of the fifth configuration example. Other configurations of the magnetic field measurement device 10 of the fifth configuration example may be similar to those of the magnetic field measurement device 10 of FIGS. 1 to 11. The magnetic field measurement device 10 of the fifth configuration example includes an averaging processing unit 1165 between the signal space separation unit 1160 and the noise removal unit 1172 and calculation unit 1174, and performs noise removal using the average value of the internal spatial data. Other configurations and operations of the magnetic field measurement device 10 of the fifth configuration example may be similar to those of the magnetic field measurement device 10 of the first configuration example, except for the averaging processing unit 1165. Below, configurations and operations different from those of the magnetic field measurement device 10 of the first configuration example will be mainly described.

[0113] The averaging unit 1165 calculates the average value of multiple pieces of data for the internal spatial data ^Bin, ^Bin_A, and ^Bin_B. The averaging unit 1165 may calculate the average value by taking a moving average of multiple pieces of internal spatial data that are continuous on the time axis and that are supplied from the signal space separating unit 1160, and supply the average value to the noise removing unit 1172. Alternatively, the averaging unit 1165 may divide the internal spatial data ^Bin, ^Bin_A, and ^Bin_B that are continuous on the time axis into specific intervals and perform an arithmetic average of each piece of data. For example, the data may be divided into intervals of 1000 samples each, such as time t1 to t1000, time t1001 to t2000, and the average values ​​may be calculated as the average of the data between times t1 and t1001, the average of the data between t2 and t1002, ..., the average of the data between t1000 and t2000, and supplied to the noise removing unit 1172. The averaging processing unit 1165 calculates the average value of a plurality of pieces of data for each of the internal space data ^Bin, ^Bin_A, and ^Bin_B, thereby making it possible to further suppress noise.

[0114] 14 , the calculation unit 1174 calculates a projection matrix P based on the average values ​​^Bin_A and ^Bin_B of the internal space data, and the noise removal unit 1172 removes noise from the average value ^Bin of the internal space data using the projection matrix P. The noise removal unit 1172 may output the internal space data ^Binproj from which the noise has been removed. In addition, the averaging processing unit 1165 may be provided subsequent to the noise removal unit 1172.

[0115] FIG. 19 shows a sixth configuration example of a portion of the magnetic field measurement device 10 according to this embodiment. This figure shows a portion of the magnetic sensor array 210 on the XY plane, and magnetic sensor cells 220 of the magnetic sensor array 210 for which corresponding measurement data has been acquired by the signal space separation unit 1160 are indicated by hatching. This figure shows the signal space separation unit 1160 and the calculation processing unit 1170 of the magnetic field measurement device 10 of the sixth configuration example. Other configurations of the magnetic field measurement device 10 of the sixth configuration example may be similar to those of the magnetic field measurement device 10 of FIGS. 1 to 11. The magnetic field measurement device 10 of the sixth configuration example includes a subtraction unit 1176 between the signal space separation unit 1160 and the calculation unit 1174, and calculates the projection matrix P using the subtraction result of the data from the signal space separation unit 1160. Other configurations and operations of the magnetic field measurement device 10 of the sixth configuration example may be similar to those of the magnetic field measurement device 10 of the first configuration example. Below, configurations and operations different from those of the magnetic field measurement device 10 of the first configuration example will be mainly described.

[0116] The subtraction unit 1176 calculates the difference ^Bin_A-B between the internal spatial data ^Bin_A and internal spatial data ^Bin_B supplied from the signal space separation unit 1160. The internal spatial data ^Bin_A includes the signal component of the measurement target and sensor noise of the even channels, while the internal spatial data ^Bin_B includes the signal component of the measurement target and sensor noise of the odd channels. Therefore, the difference ^Bin_A-B is data indicating the sensor noise, with the signal component of the measurement target removed. The subtraction unit 1176 supplies the difference ^Bin_A-B to the calculation unit 1174.

[0117] The calculation unit 1174 calculates a basis Cin_A based on the difference ^Bin_A-B from the subtraction unit 1176. As in steps 1310-1320 of Fig. 14, the calculation unit 1174 calculates a basis Cin_A-B in the time domain for the difference ^Bin_A-B by singular value decomposition of the difference ^Bin_A-B. Here, the basis Cin_A-B is a basis vector corresponding to the noise component.

[0118] 14, the calculation unit 1174 calculates the projection matrix P from the basis Cin_A-B. In (Equation 16), the projection matrix P can be calculated as L=Cin_A-B. The calculation unit 1174 supplies the projection matrix P to the noise removal unit 1172.

[0119] The noise removal unit 1172 removes noise from the internal space data ^Bin using the projection matrix P. Here, since the projection matrix P is calculated from noise component data, a projection operation is performed using the projection matrix P as shown in the following equation to project the internal space data ^Bin onto the signal space of the measurement target that is orthogonal to the noise space. In the following equation, I represents a unit matrix. The noise removal unit 1172 may output the internal space data ^Binproj from which noise has been removed.

number

[0120] FIG. 20 shows the measurement results of the internal space data ^Bin before noise removal in the magnetic field measurement device 10 of the sixth configuration example according to this embodiment. The measurement was performed using 300 sensors, sampling 4,000 points at 1-millisecond intervals for a simulated magnetic field signal from a phantom coil, and calculating the internal space data ^Bin through signal spatial separation. In this figure, the horizontal axis represents time, and the vertical axis represents the magnitude of the internal space data ^Bin before noise removal, showing data extracted from some sensors. As shown in this figure, it can be seen that the internal space data ^Bin contains large sensor-derived noise (indicated by arrows in this figure) superimposed on the magnetic field signal (R wave). This large noise is called Barkhausen jump noise, which occurs in magnetoresistive elements such as GMR and TMR when domain walls are trapped by impurities or the like.

[0121] FIG. 21 shows the measurement results of the internal space data ^Binproj after noise removal in the magnetic field measurement device 10 of the sixth configuration example according to this embodiment. The internal space data ^Bin, ^Bin_A, and ^Bin_B were calculated based on the data of the even and odd channels, respectively. Furthermore, the data of the excluded channels of ^Bin_A and ^Bin_B were complemented and output by adding the basis vectors of the excluded channels during calculation, as described above. The time-domain basis Cin_A-B of ^Bin_A-B was obtained by singular value decomposition of ^Bin_A-B, and the top five singular values ​​(300 in total) were extracted, sorted in descending order, to calculate the projection matrix P. In this figure, the horizontal axis represents time, and the vertical axis represents the magnitude of the internal space data ^Binproj after noise removal. As shown in this figure, it can be seen that noise due to Barkhausen jumps was successfully removed by the projection calculation. It should be noted that the same noise removal effect was confirmed for the magnetic field measuring device 10 of the other configuration examples according to this embodiment.

[0122] As described above, when performing signal space separation processing, noise originating from the sensor remains in the internal space data ^Bin extracted by signal space separation. To remove such noise, for example, a method can be considered in which a projection matrix is ​​calculated from the internal space data ^Bin extracted by signal space separation using singular value decomposition, and then the projection matrix is ​​applied to the internal space data ^Bin. However, because the sensor noise remaining in the internal space data ^Bin is in the same signal space as the internal space data ^Bin, the noise component cannot be effectively separated by a projection operation.

[0123] Another possible method is to calculate a projection matrix from measurement data B before signal space separation using singular value decomposition, apply the projection matrix to data B, and then perform signal space separation on measurement data B. However, the data before signal space separation contains a much larger environmental magnetic field (microtesla) while the biomagnetic signal and sensor noise are tiny (picotesla). Therefore, even if you try to separate the bases of the signal and sensor noise using singular value decomposition, it is not possible to accurately calculate anything other than the bases of the environmental magnetic field. Therefore, it is difficult to separate and calculate the bases of the signal and sensor noise and remove the sensor noise using a projection operation.

[0124] In contrast, according to the magnetic field measuring device 10 of this embodiment, after acquiring multiple data sets from different combinations of magnetic sensor cells 220 of the magnetic sensor array 210 and performing signal space separation, a projection matrix is ​​calculated from one data set and noise is removed from the other data sets. This makes it possible to separate noise contained in the signal from noise contained in the projection matrix and perform noise removal, thereby further improving the sensor noise removal performance. The magnetic field measuring device 10 may display various displays related to magnetocardiography on a display or the like using the internal space data ^Binproj from which noise has been removed.

[0125] The magnetic field measurement device 10 of the first to fifth configuration examples, which calculates a projection matrix from signal components, is more suitable for removing noise smaller than the 1 / f noise of the sensor, while the magnetic field measurement device 10 of the sixth configuration example, which calculates a projection matrix from noise components, is more suitable for removing larger noise such as Barkhausen jump. The magnetic field measurement device 10 may include a plurality of the arithmetic processing units 1170 of the first to fifth configuration examples, and may simultaneously perform the calculations of the first to fifth configuration examples, calculate an average value of the obtained multiple types of internal space data ^Binproj, and output the average value as the internal space data ^Binproj. The magnetic sensor cells 220 that acquire the internal space data used by the arithmetic processing unit 1170 may be selected from all the magnetic sensor cells 220 of the magnetic sensor array 210, or may be selected from a portion of the magnetic sensor cells 220 of the magnetic sensor array 210.

[0126] Various embodiments of the present invention may be described with reference to flowcharts and block diagrams, where the blocks may represent (1) stages of a process in which operations are performed or (2) sections of an apparatus responsible for performing the operations. Particular stages and sections may be implemented by dedicated circuitry, programmable circuitry provided with computer-readable instructions stored on a computer-readable medium, and / or a processor provided with computer-readable instructions stored on a computer-readable medium. Dedicated circuitry may include digital and / or analog hardware circuitry, and may include integrated circuits (ICs) and / or discrete circuits. Programmable circuitry may include reconfigurable hardware circuitry, including logical AND, OR, XOR, NAND, NOR, and other logical operations, flip-flops, registers, memory elements such as field programmable gate arrays (FPGAs), programmable logic arrays (PLAs), and the like.

[0127] A computer-readable medium may include any tangible device capable of storing instructions that are executed by an appropriate device, such that the computer-readable medium having instructions stored thereon comprises an article of manufacture containing instructions that can be executed to create means for performing the operations specified in the flowcharts or block diagrams. Examples of computer-readable media may include electronic, magnetic, optical, electromagnetic, and semiconductor storage media. More specific examples of computer-readable media may include floppy disks, diskettes, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), electrically erasable programmable read-only memory (EEPROM), static random access memory (SRAM), compact disc read-only memory (CD-ROM), digital versatile disc (DVD), Blu-ray (RTM) disc, memory stick, integrated circuit card, and the like.

[0128] The computer readable instructions may include either assembler instructions, Instruction Set Architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state-setting data, or source or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk®, JAVA®, C++, etc., and conventional procedural programming languages ​​such as the “C” programming language or similar programming languages.

[0129] The computer-readable instructions may be provided to a processor or programmable circuitry of a general-purpose computer, special-purpose computer, or other programmable data processing apparatus, either locally or over a wide-area network (WAN) such as a local area network (LAN), the Internet, etc., which executes the computer-readable instructions to create means for performing the operations specified in the flowcharts or block diagrams. Examples of processors include computer processors, processing units, microprocessors, digital signal processors, controllers, microcontrollers, etc.

[0130] 22 illustrates an example of a computer 9900 in which aspects of the present invention may be embodied, in whole or in part. Programs installed on the computer 9900 may cause the computer 9900 to function as or perform operations associated with an apparatus or one or more sections of the apparatus according to embodiments of the present invention, and / or to perform a process or steps of a process according to embodiments of the present invention. Such programs may be executed by the CPU 9912 to cause the computer 9900 to perform specific operations associated with some or all of the blocks in the flowcharts and block diagrams described herein.

[0131] The computer 9900 according to this embodiment includes a CPU 9912, a RAM 9914, a graphics controller 9916, and a display device 9918, which are interconnected by a host controller 9910. The computer 9900 also includes input / output units such as a communication interface 9922, a hard disk drive 9924, a DVD-ROM drive 9926, and an IC card drive, which are connected to the host controller 9910 via an input / output controller 9920. The computer also includes legacy input / output units such as a ROM 9930 and a keyboard 9942, which are connected to the input / output controller 9920 via an input / output chip 9940.

[0132] The CPU 9912 operates according to programs stored in the ROM 9930 and RAM 9914, thereby controlling each unit. The graphics controller 9916 retrieves image data generated by the CPU 9912 into a frame buffer or the like provided in the RAM 9914 or into the graphics controller itself, and causes the image data to be displayed on the display device 9918.

[0133] The communication interface 9922 communicates with other electronic devices via a network. The hard disk drive 9924 stores programs and data used by the CPU 9912 in the computer 9900. The DVD-ROM drive 9926 reads programs or data from the DVD-ROM 9901 and provides the programs or data to the hard disk drive 9924 via the RAM 9914. The IC card drive reads programs and data from an IC card and / or writes programs and data to an IC card.

[0134] The ROM 9930 stores therein a boot program or the like that is executed by the computer 9900 upon activation, and / or programs that depend on the hardware of the computer 9900. The input / output chip 9940 may also connect various input / output units to the input / output controller 9920 via a parallel port, a serial port, a keyboard port, a mouse port, etc.

[0135] The programs are provided by a computer-readable medium such as a DVD-ROM 9901 or an IC card. The programs are read from the computer-readable medium, installed in the hard disk drive 9924, RAM 9914, or ROM 9930, which are also examples of computer-readable media, and executed by the CPU 9912. The information processing described in these programs is read by the computer 9900, and causes cooperation between the programs and the various types of hardware resources described above. An apparatus or method may be configured by implementing information manipulation or processing in accordance with the use of the computer 9900.

[0136] For example, when communication is performed between the computer 9900 and an external device, the CPU 9912 may execute a communication program loaded into the RAM 9914 and instruct the communication interface 9922 to perform communication processing based on the processing described in the communication program. Under the control of the CPU 9912, the communication interface 9922 reads transmission data stored in a transmission buffer processing area provided in the RAM 9914, the hard disk drive 9924, the DVD-ROM 9901, or a recording medium such as an IC card, and transmits the read transmission data to the network, or writes received data received from the network to a reception buffer processing area or the like provided on the recording medium.

[0137] The CPU 9912 may also cause all or a necessary portion of a file or database stored on an external recording medium such as a hard disk drive 9924, a DVD-ROM drive 9926 (DVD-ROM 9901), an IC card, etc. to be read into the RAM 9914, and perform various types of processing on the data on the RAM 9914. The CPU 9912 then writes back the processed data to the external recording medium.

[0138] Various types of information, such as various types of programs, data, tables, and databases, may be stored on the recording medium and subjected to information processing. The CPU 9912 may perform various types of processing on data read from the RAM 9914, including various types of operations, information processing, conditional judgment, conditional branching, unconditional branching, information search / replacement, etc., as described throughout this disclosure and specified by the instruction sequences of the programs, and write the results back to the RAM 9914. The CPU 9912 may also search for information in a file, database, etc. on the recording medium. For example, if multiple entries each having an attribute value of a first attribute associated with an attribute value of a second attribute are stored on the recording medium, the CPU 9912 may search for an entry that matches a condition specified by the attribute value of the first attribute from among the multiple entries, read the attribute value of the second attribute stored in the entry, and thereby obtain the attribute value of the second attribute associated with the first attribute that satisfies a predetermined condition.

[0139] The programs or software modules described above may be stored in a computer-readable medium on or near the computer 9900. A recording medium such as a hard disk or RAM provided in a server system connected to a dedicated communication network or the Internet can also be used as a computer-readable medium, thereby providing the programs to the computer 9900 via the network.

[0140] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0141] It should be noted that the execution order of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order. [Explanation of symbols]

[0142] 10 Magnetic field measurement device 100 Main body 110 Magnetic sensor unit 120 head 125 Drive unit 130 Base 140 Pole section 150 Information Processing Department 210 Magnetic Sensor Array 220 Magnetic Sensor Cell 230 Sensor Data Collection Unit 300 Sensor unit 520 Magnetic Sensor 530 Magnetic field generation section 532 Amplifier circuit 534 Coil 540 Output Section 702 Magnetoresistance element 704, 706 Magnetic concentrator 1100 Sensor data processing unit 1110 AD converter 1112 Clock Generator 1120 Magnetic field acquisition section 1130 Calibration calculation unit 1140 Data output unit 1150 Basis Vector Storage Unit 1155 Data storage unit 1160 Signal space separation unit 1165 Average processing unit 1170 Processing Unit 1172 Noise Removal Section 1174 Calculation Unit 1176 Subtraction section 9900 Computer 9901 DVD-ROM 9910 Host Controller 9912 CPU 9914 RAM 9916 Graphics Controller 9918 Display Device 9920 Input / Output Controller 9922 Communication Interface 9924 Hard Disk Drive 9926 DVD-ROM drive 9930 ROM 9940 I / O chip 9942 keyboard

Claims

1. a magnetic sensor array including a plurality of magnetic sensor cells each having a magnetic sensor; a magnetic field acquisition unit that acquires measurement data measured by the magnetic sensor array; a signal space separation unit that extracts first internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a portion of magnetic sensor cells of the magnetic sensor array, and extracts second internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a plurality of magnetic sensor cells including another portion of magnetic sensor cells of the magnetic sensor array; a calculation unit that calculates a projection matrix based on the first internal space data; a noise removal unit that removes noise from the second internal space data using the projection matrix; A magnetic field measuring device comprising:

2. The signal spatial separation unit extracts the second internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from all the magnetic sensor cells of the magnetic sensor array. The magnetic field measuring device according to claim 1 .

3. the signal space separation unit extracts third internal space data from a spatial distribution of a magnetic field indicated by measurement data from a portion of magnetic sensor cells different from the portion of magnetic sensor cells of the magnetic sensor array from which the first internal space data is extracted; The calculation unit calculates one or more of the projection matrices based on the first internal space data and the third internal space data. The magnetic field measuring device according to claim 1 or 2.

4. The calculation unit calculates a common part between a time domain basis of the first internal space data and a time domain basis of the third internal space data, and calculates the projection matrix based on the common part. The magnetic field measuring device according to claim 3 .

5. the calculation unit calculates a first projection matrix based on the first internal space data and calculates a second projection matrix based on the third internal space data; The noise removal unit removes a part of the noise in the second internal space data using the first projection matrix, and removes another part of the noise in the second internal space data using the second projection matrix. The magnetic field measuring device according to claim 3 .

6. a subtraction unit that calculates a difference between the first internal space data and the third internal space data, The calculation unit calculates the projection matrix based on the difference. The magnetic field measuring device according to claim 3 .

7. The calculation unit calculates a basis of the time domain of the first internal space data by performing singular value decomposition on the first internal space data, and calculates a projection matrix based on the basis. The magnetic field measuring device according to claim 1 .

8. an averaging processing unit that calculates an average value of a plurality of data for the first internal space data and the second internal space data, the calculation unit calculates the projection matrix based on an average value of the first internal space data; The noise removal unit removes noise from the average value of the second internal space data using the projection matrix. The magnetic field measuring device according to claim 1 .

9. The noise removal unit removes noise from the second internal space data by projecting the second internal space data into a signal space using the projection matrix. The magnetic field measuring device according to claim 1 .

10. The magnetic sensor has a magnetoresistive element. The magnetic field measuring device according to claim 1 .

11. a magnetic field acquisition stage for acquiring measurement data measured by a magnetic sensor array configured with a plurality of magnetic sensor cells each having a magnetic sensor; a signal spatial separation step of extracting first internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a portion of magnetic sensor cells of the magnetic sensor array, and extracting second internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a plurality of magnetic sensor cells including another portion of magnetic sensor cells of the magnetic sensor array; a calculation step of calculating a projection matrix based on the first internal space data; a denoising step of removing noise from the second internal spatial data using the projection matrix; A magnetic field measurement method comprising:

12. Computer, a magnetic field acquisition unit that acquires measurement data measured by a magnetic sensor array configured with a plurality of magnetic sensor cells each having a magnetic sensor; a signal space separation unit that extracts first internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a portion of magnetic sensor cells of the magnetic sensor array, and extracts second internal spatial data from a spatial distribution of a magnetic field indicated by measurement data from a plurality of magnetic sensor cells including another portion of magnetic sensor cells of the magnetic sensor array; a calculation unit that calculates a projection matrix based on the first internal space data; a noise removal unit that removes noise from the second internal space data using the projection matrix; This magnetic field measurement program functions as a

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