JP2019010483A - Magnetic field measurement device and measured magnetic field display method - Google Patents

Abstract

To reduce an influence of an environmental magnetic field measured by a magnetic field measurement device using a reference magnetic sensor.SOLUTION: A magnetic field measurement device includes a magnetic sensor arranged inside a magnetic shield room (MSR) 2, a reference magnetic sensor 14 arranged outside the MSR 2, and an arithmetic unit 8 to which magnetic field time series data from the magnetic sensor and environmental magnetic field time series data from the reference magnetic sensor are input. The arithmetic unit acquires the magnitude of an environmental magnetic field included in the magnetic field time series data so as to minimize a predetermined evaluation function of the magnetic field time series data and estimated environmental magnetic field time series data obtained from the environmental magnetic field time series data, and in obtaining the estimated environmental magnetic field time series data, applies a magnetic field reduction effect to the environmental magnetic field time series data for each frequency of the MSR 2 (S103).SELECTED DRAWING: Figure 4

Description

  The present invention relates to a magnetic field measurement apparatus that estimates and removes an approximate value of an environmental magnetic field signal measured by a magnetic field measurement apparatus from an environmental magnetic field signal measured by a reference magnetic sensor.

  2. Description of the Related Art A magnetocardiograph has been developed as a magnetic field measuring apparatus that measures a weak magnetic field from a measurement target of a subject and evaluates the electrophysiological activity of the measurement target non-invasively. A magnetocardiograph is a device that measures the weak magnetic field (magnetomagnetic field) generated by the electromotive force (current dipole) associated with the electrophysiological activity of the heart in a non-contact manner. Evaluation with excellent resolution is possible. An image obtained by imaging the temporal change and spatial distribution of the magnetocardiogram is called a magnetocardiogram. Since the permeability in the living body is almost equal to vacuum, the magnetocardiogram is less susceptible to peripheral organs (bones, lungs, etc.) than the electrocardiogram, and reflects the current associated with the electrophysiological activity of the heart with high sensitivity. To do. Taking advantage of this magnetocardiogram, many clinical effectiveness of the magnetocardiogram has been shown.

  In order to accurately and stably realize the evaluation of the electrophysiological activity of the heart by the magnetocardiogram examination, it is necessary to sufficiently reduce the environmental magnetic field mixed in the magnetocardiogram. The intensity of the magnetocardiogram is very weak, the intensity of adult QRS waves (waves reflecting the electrical activity of the ventricle) is several tens of pT, the intensity of P waves (waves reflecting the electrical activity of the atria) is several pT, The intensity of fetal P wave is about 0.1 pT. On the other hand, the environmental magnetic field is generated by the geomagnetic direct current magnetic field, the magnetic field caused by train sending / returning current, the magnetic field generated by the movement of objects composed of magnetic materials such as automobiles, elevators, and iron doors, and the current of the transmission line. Magnetic field, magnetic field generated by fan / pump rotating body, etc. The environmental magnetic field is much larger than the magnetocardiogram. For example, the geomagnetic DC magnetic field is about 50 μT (more than about 1 million times the magnetocardiogram), and the fluctuation range of the magnetic field due to train sending / returning current is 50 m from the orbit. It is reported as 1.4 μT at the spot. In order to reduce these environmental magnetic fields mixed in the magnetocardiograph, the magnetocardiograph usually has a magnetically shielded room (MSR), a gradiometer of a magnetic sensor unit, an analog filter (high pass). Environmental magnetic field reduction technologies such as filter: HPF, low pass filter (LPF) and digital filter are mounted to reduce the mixed environmental magnetic field.

  The MSR, which is one of the environmental magnetic field reduction techniques, is usually made of a high permeability Fe-Ni alloy permalloy, which has a multilayer structure to achieve high shielding performance (normal biomagnetic measurement). 2 layers). For magnetocardiography near a track or main road, stable measurement is possible by increasing the number of layers of MSR. However, permalloy is an expensive material, and the cost is greatly increased. Moreover, it may be difficult to increase the number of layers of the MSR due to the load resistance at the planned installation position. Therefore, as an environmental magnetic field reduction technique that complements MSR, an environmental magnetic field reduction method (hereinafter referred to as “reduction”) that estimates and removes an approximate value of an environmental magnetic field signal in a biomagnetic measurement device from an environmental magnetic field signal measured by a reference magnetic sensor. Law ") has been developed.

  A system using a superconducting magnetic sensor for measuring the environmental magnetic field installed in the dewar of the biomagnetic measuring device as a reference magnetic sensor for the reduction method (for example, Non-Patent Document 1 and Non-Patent Document 2), installed in or outside the MSR A method using a room temperature magnetic sensor such as a fluxgate magnetometer (for example, Patent Document 1 for the inside of MSR and Non-Patent Document 3 for the outside of MSR) has been proposed.

JP, 2006-217930, A

J. Vrba and S. E. Robinson, “Signal Processing in Magnetoencephalography”, METHODS, vol. 25, pp. 249-271, 2001 Y. Adachi, et al., “Reduction of Non-periodic Environmental Magnetic Noise in MEG Measurement by Continuously Adjusted Least Square Method”, IEEE Transactions on Applied Superconductivity, vol. 11, No.1, pp. 669-672, 2001 Daisuke Oyama et al., Low frequency environmental magnetic noise reduction method using mobile reference sensor, Proc. Of the 30th Annual Meeting of the Biomagnetic Society of Japan, vol. 28, No.1, pp. 170-171, 2015

  In the case of using a superconducting magnetic sensor for measuring the environmental magnetic field as the reference magnetic sensor, the environment is obtained by multiplying the measured magnetic field of the reference magnetic sensor by a linear coupling coefficient and subtracting it from the measured magnetic field of the superconducting magnetic sensor for measuring the biomagnetic field. Realize reduction of magnetic field. Alternatively, the environmental magnetic field can be reduced by multiplying the measurement magnetic field of the reference magnetic sensor and the measurement magnetic field of the superconducting magnetic sensor for biomagnetic field measurement by subtracting them from the measurement values for biomagnetic field measurement. In this case, the weighting coefficient is determined so that the measurement magnetic field of the reference magnetic sensor and the measurement magnetic field of the superconducting magnetic sensor for biomagnetic field measurement most closely match.

  In this method, a reference magnetic sensor is installed near a superconducting magnetic sensor for biomagnetic field measurement, and the reference magnetic sensor and the biomagnetic field measurement magnetic sensor have the same sensor sensitivity. Therefore, the reference magnetic sensor measures an environmental magnetic field similar to the biomagnetic field measurement magnetic sensor, and can reduce the environmental magnetic field with high accuracy. However, since a superconducting magnetic sensor is used as the reference magnetic sensor, the additional cost burden for increasing the number of superconducting magnetic sensors and cooling the reference superconducting magnetic sensor is not small.

  On the other hand, in a method using a room temperature magnetic sensor such as a fluxgate magnetometer arranged in the MSR, a multi-variable polynomial is applied to the measurement magnetic field of the reference magnetic sensor and subtracted from the calculated value for biomagnetism measurement. Realize reduction. In this system, the sensor sensitivity of a room temperature magnetic sensor (such as a sensor using a fluxgate magnetometer and a magnetoresistive effect element) used as a reference magnetic sensor is lower than that of a superconducting magnetic sensor. For this reason, if the environmental magnetic field in the MSR is reduced to a level that cannot be measured by the room temperature magnetic sensor by MSR or other environmental magnetic field reduction techniques, such as active cancellation, the environmental magnetic field cannot be measured, and the reduction effect Cannot be obtained.

  On the other hand, a method of using a room temperature magnetic sensor such as a fluxgate magnetometer as a reference magnetic sensor outside the MSR creates a plurality of measurement values that are time-shifted in the positive and negative directions from the measurement magnetic field of the reference magnetic sensor. The principal component analysis is applied to the measured value, the coefficient after being applied is multiplied by a coefficient, and the environmental magnetic field is reduced by subtracting from the measured value of the superconducting magnetic sensor for biomagnetic field measurement. In this method, an approximate value of the environmental magnetic field measured by the superconducting magnetic sensor for biomagnetic field measurement inside the MSR is estimated from the environmental magnetic field measured outside the MSR.

  However, the environmental magnetic field shielding performance of the MSR has frequency dependence, and the reduction effect varies depending on the frequency of the environmental magnetic field. In this method, the effect of reducing the magnetic field for each frequency of the MSR is not considered, and it is not easy to accurately estimate the approximate value of the environmental magnetic field in the MSR.

  The MSR, the magnetic sensor arranged inside the MSR, the reference magnetic sensor arranged outside the MSR, the magnetic field time series data from the magnetic sensor, and the environmental magnetic field time series data from the reference magnetic sensor are inputted. In the magnetic field measurement device having the arithmetic device, the arithmetic device calculates the magnitude of the environmental magnetic field included in the magnetic field time-series data between the magnetic field time-series data and the estimated environmental magnetic field time-series data obtained from the environmental magnetic field time-series data. In obtaining the predetermined evaluation function to be minimum and obtaining the estimated environmental magnetic field time series data, the magnetic field reduction effect for each frequency of the MSR is applied to the environmental magnetic field time series data.

  Other problems and novel features will become apparent from the description of this specification and the accompanying drawings.

  In a magnetic field measuring apparatus, the influence of an environmental magnetic field is reduced using a reference magnetic sensor.

It is the schematic which shows the whole structure of a magnetocardiograph. It is a figure which shows the example of arrangement | positioning with respect to a test subject's arrangement | sequence of a magnetic sensor. 1 is a schematic diagram showing the overall configuration of a magnetocardiograph according to Example 1. FIG. It is a figure which shows the processing flow which reduces an environmental magnetic field using the magnetic sensor for a reference. It is an environmental magnetic field waveform (channel with the maximum magnetic field strength) from a train measured with a magnetocardiograph. It is the environmental magnetic field waveform from the train reduced by applying the processing flow of a present Example. It is the schematic which shows the whole structure of the magnetocardiograph combining active cancellation. It is an environmental magnetic field waveform (channel with the maximum magnetic field strength) from a train measured with a magnetocardiograph. It is the environmental magnetic field waveform from the train reduced by applying the processing flow of a present Example. It is a figure which shows an example of the display screen which displays the measurement result of a magnetocardiograph. It is a figure which shows an example of the display screen which sets the reduction effect of the environmental magnetic field of a magnetocardiograph.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

  FIG. 1 is a schematic diagram showing an overall configuration of a magnetic field measuring apparatus (a magnetocardiograph). The components of the magnetocardiograph 1 are separately arranged inside and outside the MSR 2. The MSR 2 has a size of about 3 m on each side, for example, and a cryostat in which a plurality of SQUID magnetometers (hereinafter referred to as “magnetic sensors”) are arranged inside the MSR 2 and maintained at a cryogenic temperature. 3, a gantry 4 that holds the cryostat 3, and a bed 5 on which a subject (not shown) lies. The bed 5 moves along the short axis (A direction, y direction) of the bed 5, moves along the long axis (B direction, x direction) of the bed 5, and the vertical direction (C direction, z direction) of the bed 5. The position of the subject and the plurality of magnetic sensors can be easily aligned.

  Outside the MSR 2 are a drive circuit 6 for driving a magnetic sensor disposed in the cryostat 3, an amplifier filter unit 7 for amplifying and filtering the output from the drive circuit 6, and an output signal from the amplifier filter unit 7. Are collected, and the collected data (hereinafter referred to as “magnetic field time-series data”) is analyzed and the arithmetic unit 8 that controls each part of the magnetocardiograph 1 and the analysis result analyzed by the arithmetic unit 8 are analyzed. A display device 9 for displaying is mainly arranged.

  The magnetic sensor of the magnetocardiograph 1 is not limited to the SQUID magnetometer, and a sensor using a magnetoresistive effect element, an optical pumping magnetometer, a fluxgate magnetometer, and a magnetic sensor using a magnetoimpedance element may be applied. it can.

An example of the arrangement of the magnetic sensor array used in the magnetocardiograph 1 and the arrangement with respect to the subject will be described with reference to FIG. The plurality of magnetic sensors constituting the magnetic sensor array are suspended along the z direction on the inner wall at the bottom of the cryostat 3 (see FIG. 1), and a magnetic field component B _z in the z direction perpendicular to the chest wall 10 of the subject is provided. Measure over time. The plurality of magnetic sensors are arranged at equal intervals in the x direction and the y direction so that the amount of change in the distance of the magnetic field can be accurately grasped. It is also possible to apply the magnetic sensor over time measure the magnetic field component B _y of the magnetic field components B _x and y directions parallel x-direction relative to the chest wall 10.

  In the example of FIG. 2, the magnetic sensor array 11 of 64 channels in which the distance between the magnetic sensors is 0.025 m, the measurement surface is 0.175 m × 0.175 m, and the magnetic sensors are arranged in an 8 × 8 array. Show. In the coordinate system of the magnetic sensor array 11, for example, the magnetic sensor array 11 is aligned so that the magnetic sensor located in the seventh row and third column indicated by reference numeral 12 is located directly above the sword-like projection 13 of the chest. Do. The magnetic sensor in the first row and the eighth column is the origin O of the coordinate system.

  As a first embodiment, a magnetic field measurement apparatus that reduces an environmental magnetic field using a reference magnetic sensor installed outside the MSR 2 will be described. Constituent elements common to those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted. FIG. 3 is a schematic diagram showing an overall configuration of a magnetic field measuring apparatus (a magnetocardiograph) in which a reference magnetic sensor is installed outside the MSR.

  Outside the MSR 2, a three-axis fluxgate magnetometer 14 (hereinafter referred to as “reference magnetic sensor”) for measuring an environmental magnetic field, a drive circuit 15 for driving the reference magnetic sensor, and an output from the drive circuit 15 An amplifier filter unit 16 for amplifying and filtering the signal is arranged, and an output signal from the amplifier filter unit 16 is collected by the arithmetic unit 8.

As the reference magnetic sensor 14, a sensor using a magnetoresistive effect element, a sensor using a magnetic impedance element, or a magnetic sensor using an optical pumping magnetometer can be applied. The displacement between the position of the magnetic sensor array 11 of the magnetocardiograph 1 and the position of the reference magnetic sensor 14 is negligible from the position of the source of the environmental magnetic field as the magnetic field strength, but the measurement position is different. Even if only the magnetic field component B _{z in} the z direction of the magnetic sensor of the magnetometer 1 is observed, by correcting with the environmental magnetic components (magnetic field component B _x , magnetic field component B _y , magnetic field component B _z ) in three axes, The influence of the environmental magnetic field on the magnetic sensor of the magnetocardiograph 1 can be determined more accurately. Of course, the present invention can be applied even if the reference magnetic sensor 14 has one or two axes. Further, a plurality of reference magnetic sensors 14 may be used.

  FIG. 4 shows a flowchart of the environmental magnetic field reduction processing procedure using the reference sensor 14 outside the MSR 2. First, processing is started (S101), magnetic field time series data (hereinafter referred to as “cardiac magnetic field time series data”) generated from the subject's heart using a magnetocardiograph (see FIG. 3), and a reference magnetic sensor outside the MSR2. 14 is used to simultaneously measure time series data of environmental magnetic fields (hereinafter referred to as “environmental magnetic field time series data”) (S102).

  The MSR magnetic field reduction effect is applied to the environmental magnetic field time-series data (S103). Since the magnetic field reduction effect of MSR has frequency characteristics, frequency analysis is applied to environmental magnetic field time-series data to convert it to frequency domain data, and the converted frequency domain data is multiplied by the magnetic field reduction rate for each frequency by MSR. Then, frequency analysis is applied to convert the data into time domain data again. Specifically, it is as follows. If the environmental magnetic field time-series data is given by N sample points (digital signal sequence) x (t) (t = 1 to T) sampled at a predetermined time interval, the discrete Fourier spectrum X of x (t) (k) (k = 0, 1,... K−1) can be obtained from (Equation 1).

Here, based on the frequency characteristic D _M (w _k ) of the magnetic field reduction rate of the MSR, K discrete frequency spectra (filter functions) F (k) are set as in (Equation 2).

  By multiplying the filter function F (k) shown in (Expression 2) and the discrete Fourier spectrum X (k) shown in (Expression 1) (Expression 3), the discrete frequency spectrum to which the magnetic field reduction effect of MSR is applied. X ′ (k) can be obtained.

  When the discrete frequency spectrum X ′ (k) is subjected to inverse Fourier transform based on (Equation 4), time series data x ′ (t) of the environmental magnetic field to which the magnetic field reduction effect of MSR is applied can be obtained.

Next, the magnetic field reduction effect of the magnetic sensor (gradiometer structure) of the magnetocardiograph is applied to the environmental magnetic field time-series data to which the magnetic field reduction effect of MSR is applied (S104). The gradiometer structure is a configuration in which a magnetic flux detected by a differential detection coil created with a superconducting wire is transmitted to a SQUID magnetometer. By using the differential detection coil, the spatial gradient of the magnetic field can be detected and the uniform environmental magnetic field can be reduced. Field reduction rate D _g according to the gradiometer, unlike the magnetic field reduction effect of MSR previously described, it can be considered as constant that does not depend on frequency. Therefore, the magnetic field reduction of the gradiometer is obtained by multiplying the time series data x ′ (t) of the environmental magnetic field obtained by applying the MSR magnetic field reduction effect obtained in step S103 by the gradiometer magnetic field reduction rate D _g (Equation 5). The time series data x ″ (t) of the environmental magnetic field to which the effect is applied can be obtained.

Next, the magnetic field reduction effect of the analog filters (HPF and LPF) of the magnetocardiograph is applied to the environmental magnetic field time-series data to which the magnetic field reduction effect of the magnetic sensor is applied (S105). Specifically, it can be obtained by applying a digital filter having a filter type and filter order similar to the analog filter of the magnetocardiograph to the time series data x ″ (t) of the environmental magnetic field obtained in step S104. For example, when an FIR (Finite Impulse Response) filter is used as the digital filter, time series data x ′ ″ (t) of the environmental magnetic field to which the FIR filter is applied can be obtained from (Equation 6). Here, m is a filter order, and a _m is a filter coefficient determined from the filter type and the filter order.

The signals obtained by the processes S103 to S105 so far reflect the reduction effect by the environmental magnetic field reduction technology applied to the magnetocardiograph 1, and the estimated value of the environmental magnetic field measured by the magnetocardiograph (hereinafter referred to as “the magnetocardiograph”). It can be regarded as “estimated environmental magnetic field”. Next, a weight coefficient for each channel that minimizes a difference between the time series data of the magnetocardiograph estimated environmental magnetic field and the cardiac magnetic field time series data actually measured in each channel of the magnetocardiograph is obtained (S106). When a three-axis (x-axis, y-axis, and z-axis) magnetic sensor is used as the reference magnetic sensor 14, a weight coefficient vector W for each axis channel (W = [ _Wx , _Wy , _Wz ]) Can be obtained as a value that minimizes the evaluation function (Equation 7).

In the evaluation function (Equation 7), B ^MCG _{n, t} represents the cardiac magnetic field measured at the t th (t = 1 to T) sampling point in the n th (n = 1 to 64) channel of the magnetocardiograph. Yes. B ^ref _{x, t} , B ^ref _{y, t} and B ^ref _{z, t} are the magnetocardiographic estimation environment at the sampling point t of the magnetocardiograph 1 obtained from the magnetic field signals of the x-axis, y-axis and z-axis reference magnetic sensors. Each represents a magnetic field. W _{x, n} , W _{y, n} and W _{z, n} are relative to the magnetocardiographically estimated environmental magnetic field of the x-axis, y-axis, and z-axis reference magnetic sensors in the n-th (n = 1 to 64) channel. Each represents a weighting factor (hereinafter referred to as “weighting factor for each channel”).

  Finally, the time series data of the magnetocardiographic estimated environmental magnetic field multiplied by the weighting coefficient for each channel is removed from the cardiac magnetic field time series data of each channel measured by the magnetocardiograph (S107), and the removal result is displayed (S108). The process is terminated (S109).

  Note that the frequency analysis method applied to the environmental magnetic field time series data in step S103 includes a well-known fast Fourier transform method, periodogram method, and the like. The Fourier spectrum and power spectral density are calculated, and the frequency of the environmental magnetic field time series data is calculated. Data in the area can be acquired. Further, in order to calculate the magnetic field reduction effect for each frequency of the MSR, for example, a catalog value or actual measurement data of the magnetic shield rate of the MSR can be used. Furthermore, in calculating the magnetic field reduction effect, phase change information for each frequency of the MSR can also be used.

  In order to calculate the magnetic field reduction effect of the magnetic sensor with respect to the environmental magnetic field time-series data in the process S104, for example, a catalog value of magnetic field reduction rate of the magnetic sensor or actually measured data can be used.

  In order to calculate the magnetic field reduction effect of the analog filter with respect to the environmental magnetic field time-series data in step S105, for example, the magnetic field reduction rate obtained from the digital filter of the same type, cutoff frequency, and order as the analog filter of the magnetocardiograph may be used. it can.

  The weighting factor for each channel that minimizes the evaluation function (Equation 7) in step S106 can be obtained by using, for example, the downhill simplex method which is one of linear programming methods.

  The effectiveness of the method for reducing the environmental magnetic field using the reference magnetic sensor arranged outside the MSR described above was confirmed using the measured data of the environmental magnetic field from the train.

  FIG. 5A shows an environmental magnetic field waveform 17 (a channel with the maximum magnetic field strength) measured from a train, measured by a 64-channel magnetocardiograph. The horizontal axis is time (seconds), and the vertical axis is the magnetic field (pT). FIG. 5B shows a magnetic field waveform 18 in which the environmental magnetic field waveform 17 is reduced by applying the processing flow shown in FIG. In the magnetic field waveform 17 of FIG. 5A, a peak of about −150 pT to 100 pT derived from a train was recognized, and the peak to peak value was 219.4 pT. On the other hand, the magnetic field waveform 18 after applying the processing flow of the present embodiment in FIG. 5B exists in the range of −40 pT to 40 pT, and by applying this method, the environmental magnetic field from the train is reduced to the magnetocardiogram level. I understand that The peak-to-peak value of the magnetic field waveform 18 after applying this method was 59.6 pT, and the magnetic field reduction rate by this method was −11.3 dB.

  Based on the actual data analysis result of the environmental magnetic field from the train, the approximate value of the environmental magnetic field measured by the magnetocardiograph in the MSR using the magnetic sensor for reference at the frequency of the MSR using the reference magnetic sensor outside the MSR is obtained. It was shown that the environmental magnetic field measured by the magnetocardiograph can be reduced by estimation.

  Even when the magnetocardiograph 1 employs an environmental magnetic field reduction technique that combines MSR and active cancellation, the method of reducing the environmental magnetic field using the reference magnetic sensor installed outside the MSR of this embodiment is effective. Have

  FIG. 6 is a schematic diagram showing the overall configuration of a magnetic field measuring apparatus (cardiac magnetometer) 1 ′ in which a reference magnetic sensor is installed in addition to the MSR combined with active cancellation. On the outer periphery of the MSR 2 is an active canceling coil 19 that mainly generates a z-component magnetic field around the MSR, and a fluxgate magnetometer 20 (hereinafter referred to as “active canceling” for measuring a z-component environmental magnetic field for active cancellation. And a control circuit 21 that outputs a current necessary to generate a magnetic field for canceling the environmental magnetic field measured by the active canceling magnetic sensor 20 from the active canceling coil 19. Note that a triaxial (x, y and z axis) coil may be applied as the active canceling coil 19. As the active canceling magnetic sensor 20, a sensor using a magnetoresistive effect element may be applied. As the control circuit 21, negative feedback control, PI control, and PID control can be applied.

  The magnetocardiograph 1 'can also reduce the environmental magnetic field according to the flowchart shown in FIG. The effectiveness of the method of the present embodiment was confirmed using measured data of the environmental magnetic field from the train.

  FIG. 7A shows an environmental magnetic field waveform 22 (the channel with the maximum magnetic field strength) from a train measured with a 64-channel magnetocardiograph when active cancellation is applied. The horizontal axis is time (seconds), and the vertical axis is the magnetic field (pT). FIG. 7B shows a magnetic field waveform 23 in which the environmental magnetic field waveform 22 is reduced by applying the processing flow shown in FIG. In the magnetic field waveform 22 of FIG. 7A, a peak of about −20 pT to 20 pT derived from a train was recognized, and the peak to peak value was 31.1 pT. On the other hand, the magnetic field waveform 23 after applying the processing flow of the present embodiment in FIG. 7B exists in the range of −10 pT to 10 pT. By applying this method, the environmental magnetic field from the train is reduced to the magnetocardiogram level. I understand that The peak-to-peak value of the magnetic field waveform 23 after application of this method was 17.3 pT, and the magnetic field reduction rate by this method was −5.1 dB.

  Based on the actual data analysis results of the environmental magnetic field from the train, even in MSR combined with active cancellation, an external reference magnetic sensor is used to measure with a magnetocardiograph in the MSR using the magnetic field reduction effect for each frequency of the MSR. It was shown that the environmental magnetic field measured by the magnetocardiograph can be reduced by estimating the approximate value of the environmental magnetic field.

  In the second embodiment, an interface applied to the magnetic field measurement apparatus according to the first embodiment will be described. First, a method of calculating the degree of coincidence between the time series data of the magnetocardiograph estimated environmental magnetic field multiplied by the weighting coefficient for each channel and the heart magnetic field time series data of each channel measured by the magnetocardiograph will be described.

  When one 3-axis (x, y, and z-axis) reference magnetic sensor is used, the time series data of the magnetocardiograph estimated environmental magnetic field multiplied by the weighting factor for each channel, and each channel measured by the magnetocardiograph The degree of coincidence with the cardiac magnetic field time series data can be obtained from (Equation 8).

In (Equation 8), B ^MCG _{n, t} represents the cardiac magnetic field measured at the t th (t = 1 to T) sampling point in the n th (n = 1 to 64) channel of the magnetocardiograph. B ^ref _{x, t} , B ^ref _{y, t} and B ^ref _{z, t} are the magnetocardiographic estimated environmental magnetic field at the sampling point t of the magnetocardiograph obtained from the magnetic field signals of the x-axis, y-axis and z-axis reference magnetic sensors. Respectively. W _{x, n} , W _{y, n} and W _{z, n} represent weighting factors for each channel with respect to the time series data of the magnetocardiograph estimated environmental magnetic field obtained from (Equation 7).

  This degree of coincidence indicates how much the time series data of the magnetocardiograph estimated environmental magnetic field matches the time series data of the cardiac magnetic field measured by the magnetocardiograph. For example, using this degree of coincidence, it is possible to perform processing such as applying the environmental magnetic field reduction method of the first embodiment to channels having a degree of coincidence of more than%.

  FIG. 8 is an example of a display screen that displays the result of reducing the environmental magnetic field using a reference magnetic sensor outside the MSR. The display screen 24 is displayed on the magnetocardiograph display device 9.

  The display screen 24 of the display device 9 includes an external input data display field 25 for displaying a signal of an external input channel of the arithmetic device 8 to which a reference magnetic sensor is connected, and a magnetocardiograph for displaying a magnetic field signal measured by the magnetocardiograph. It includes a measurement data display field 26 and a magnetocardiogram measurement data (after reduction) display field 27 for displaying magnetocardiographic measurement data in which the environmental magnetic field is reduced using the reference magnetic sensor.

  In the external input data display field 25, the magnetic field signal 28 measured by the reference magnetic sensor is displayed. The channel of the reference magnetic sensor displayed in the external input data display field 25 can be switched using the external input channel selection button 29. In the example shown in the figure, the arithmetic unit 8 can connect 16-channel external inputs. The three-axis (x, y, and z-axis) signals of the reference magnetic sensor are input through the external input channels, respectively, and the reference magnetic sensor corresponding to the “external input channel 65” selected by the external input channel selection button 29 is selected. The signal is displayed as a magnetic field signal 28.

  In the magnetocardiograph measurement data display field 26, the magnetic field signal 30 measured by the magnetocardiograph is displayed. The magnetosensor channel of the magnetocardiograph displayed in the magnetocardiograph measurement data display field 26 can be switched using the magnetocardiograph channel selection button 31. The signal of the magnetic sensor of “cardiograph channel 1” selected by the magnetocardiograph channel selection button 31 is displayed as the magnetic field signal 30.

  In the magnetocardiogram measurement data (after reduction) display field 27, a magnetic field signal 32 obtained by reducing the environmental magnetic field using a reference magnetic sensor is displayed. The magnetosensor channel of the magnetocardiograph displayed in the magnetocardiogram measurement data (after reduction) display field 27 can be switched using the magnetocardiograph channel selection button 33. The magnetocardiogram measurement data (after reduction) of the “magnetometer channel 1” selected by the magnetocardiograph channel selection button 33 is displayed as the magnetic field signal 32.

  The background of each channel display of this magnetocardiograph channel selection button 33 is the degree of coincidence between the time series data of the magnetocardiograph estimated environmental magnetic field estimated using the reference magnetic sensor and the cardiac magnetic field time series data of each channel of the magnetocardiograph. A channel with a high match is displayed in black 34, a channel with a medium match is displayed in gray 35, and a channel with a low match is displayed in white 36. The threshold value of the color matching degree can be arbitrarily set by the system or the operator. Color coding is not limited to three stages. Further, in order to determine whether or not the environmental magnetic field reduction process using the reference magnetic sensor is applicable, a threshold value display field 37 for displaying a threshold value of the matching degree is displayed. The threshold value can be set by the operator. Alternatively, a default value of the apparatus may be determined. When the reduction execution button 38 is pressed, the environmental magnetic field reduction process shown in the first embodiment is performed on the magnetocardiograph channel having a value equal to or higher than the matching degree displayed in the threshold value display field 37. The degree of coincidence indicates the influence of the environmental magnetic field on the magnetocardiographic measurement data. For a channel with a high degree of coincidence (greater than or equal to the threshold value), that is, a channel with a large influence of the environmental magnetic field, the environmental magnetic field is reduced. The measured magnetocardiogram measurement data can be used as it is without performing the environmental magnetic field reduction processing for a channel having a low coincidence (below the threshold value), that is, a channel having a small influence of the environmental magnetic field.

  FIG. 9 is an example of a display screen for setting the environmental magnetic field reduction effect of the magnetocardiograph used for the process of reducing the environmental magnetic field according to the flow of FIG. 4 using the reference magnetic sensor outside the MSR. The display screen 39 is displayed on the display 9 of the magnetocardiograph.

  On the display screen 39 of the display device 9, the frequency characteristic display column 40 of the magnetic shield ratio of the MSR for displaying the frequency characteristic of the shield ratio of the MSR, and the reduction ratio of the magnetic sensor for displaying the magnetic field reduction ratio of the magnetic sensor of the magnetocardiograph Display field 41, Frequency characteristic display field 42 of the high-pass filter reduction rate for displaying the frequency characteristics of the high-pass filter reduction rate of the amplifier filter unit of the magnetocardiograph, Frequency characteristic of the reduction rate of the low-pass filter of the amplifier filter unit of the magnetocardiograph And a frequency characteristic display field 43 of the reduction rate of the low-pass filter to be displayed.

  In the frequency characteristic display column 40 of the magnetic shield rate of the MSR, the magnetic shield rate 44 for each frequency of the MSR is displayed. The magnetic shield rate for each frequency displayed in the frequency characteristic display column 40 of the MSR magnetic shield rate can be set using the magnetic shield rate setting column 45 for each frequency.

  In addition, in the frequency characteristic display column 40 of the magnetic shield ratio of the MSR, the phase change information of the MSR can be displayed when the phase change information is used for calculating the magnetic field reduction effect of the MSR. In this case, a phase change information setting field for each frequency is displayed in the magnetic shield rate setting field 45 for each frequency.

  The high-pass filter reduction rate frequency characteristic display field 42 displays a reduction rate 46 for each frequency of the high-pass filter. The reduction rate for each frequency displayed in the frequency characteristic display column 42 of the reduction rate of the high pass filter includes a filter type setting column 47 for setting the filter type of the high pass filter, an order setting column 48 for setting the order of the high pass filter, and the high pass filter. A value calculated based on the cutoff frequency setting field 49 for setting the cutoff frequency is displayed.

  In the frequency characteristic display column 43 of the reduction rate of the low-pass filter, the reduction rate 50 for each frequency of the low-pass filter is displayed. The reduction rate for each frequency displayed in the frequency characteristic display column 43 of the reduction rate of the low-pass filter is the filter type setting column 51 for setting the filter type of the low-pass filter, the order setting column 52 for setting the order of the low-pass filter, and the low-pass filter. A value calculated based on the cutoff frequency setting field 53 for setting the cutoff frequency is displayed.

1, 1 ′: magnetocardiograph, 2: magnetic shield room (MSR), 3: cryostat, 4: gantry, 5: bed, 6: drive circuit, 7: amplifier filter unit, 8: arithmetic device, 9: display device, 11: Magnetic sensor array, 14: Magnetic sensor for reference, 15: Drive circuit, 16: Amplifier filter unit, 19: Coil for active cancellation, 20: Magnetic sensor for active cancellation, 21: Control circuit

Claims (11)

Magnetic shield room,
A magnetic sensor disposed inside the magnetic shield room;
A magnetic sensor for reference disposed outside the magnetic shield room;
An arithmetic unit for inputting magnetic field time-series data from the magnetic sensor and environmental magnetic field time-series data from the reference magnetic sensor;
The arithmetic device minimizes a predetermined evaluation function between the magnetic field time series data and the estimated environmental magnetic field time series data obtained from the environmental magnetic field time series data with respect to the magnitude of the environmental magnetic field included in the magnetic field time series data. A magnetic field measurement apparatus that applies a magnetic field reduction effect for each frequency of the magnetic shield room to the environmental magnetic field time series data when obtaining the estimated environmental magnetic field time series data. In claim 1,
The magnetic sensor is a superconducting magnetic sensor that detects a uniaxial magnetic field component;
The reference magnetic sensor is a room temperature magnetic sensor that detects magnetic field components of a plurality of axes,
The predetermined evaluation function is a magnetometer measurement device represented as a sum of differences between the magnetic field time-series data and the estimated environmental magnetic field time-series data for each of the plurality of axes. In claim 1,
The arithmetic unit further applies a magnetic field reduction effect based on the structure of the magnetic sensor and a magnetic field reduction effect of an analog filter added to the output of the magnetic sensor when obtaining the estimated environmental magnetic field time-series data. In claim 1,
An active canceling coil that is provided around the magnetic shield room and generates an axial magnetic field detected by the magnetic sensor;
An active canceling magnetic sensor for measuring the environmental magnetic field in the axial direction;
A magnetometer measuring device comprising: a control circuit for generating a magnetic field in the active canceling coil for canceling the environmental magnetism measured by the active canceling magnetic sensor. In claim 1,
A plurality of the magnetic sensors are arranged in an array to form a multi-channel magnetic sensor array,
The arithmetic unit calculates a degree of coincidence between the magnetic field time-series data and the estimated environmental magnetic field time-series data weighted so as to minimize the predetermined evaluation function for each channel of the magnetic sensor array. apparatus. In claim 5,
Having a display device;
The display device displays a measurement magnetic field in which an environmental magnetic field is reduced by the estimated environmental magnetic field time series data weighted so as to minimize the predetermined evaluation function from the magnetic field time series data. In claim 6,
Displaying the degree of coincidence for each of the plurality of channels on the display device;
The said display apparatus is a magnetic field measuring apparatus which displays the magnitude | size of the said matching degree so that identification is possible. In claim 6,
The display device has a setting screen for the arithmetic device to obtain the estimated environmental magnetic field time series data,
A magnetic field measurement apparatus that displays frequency characteristics of a magnetic shield rate of the magnetic shield room on the setting screen. Obtain magnetic field time-series data from each of the multiple-channel magnetic sensor array placed inside the magnetic shield room,
Simultaneously with the acquisition of the magnetic field time series data, the environmental magnetic field time series data is obtained from a reference magnetic sensor arranged outside the magnetic shield room,
Applying the magnetic field reduction effect of the magnetic field measurement device including the magnetic field reduction effect for each frequency of the magnetic shield room to the environmental magnetic field time series data to obtain the estimated environmental magnetic field time series data,
The magnitude of the environmental magnetic field included in the magnetic field time series data is determined so as to minimize a predetermined evaluation function of the magnetic field time series data and the estimated environmental magnetic field time series data,
For each channel of the magnetic sensor array, calculate the degree of coincidence between the magnetic field time-series data and the estimated environmental magnetic field time-series data weighted to minimize the predetermined evaluation function,
Displaying the degree of coincidence for each channel of the magnetic sensor array on a display device;
In response to the setting of a threshold value that determines whether environmental magnetic field reduction processing using the reference magnetic sensor is possible,
For a channel whose coincidence is equal to or greater than the threshold value, a measurement magnetic field in which the environmental magnetic field is reduced by the estimated environmental magnetic field time series data weighted so as to minimize the predetermined evaluation function from the magnetic field time series data. A measured magnetic field display method for displaying the magnetic field time-series data for a channel that is displayed and the degree of coincidence is less than the threshold value. In claim 9,
Each of the magnetic sensor arrays is configured by arranging superconducting magnetic sensors for detecting a uniaxial magnetic field component in an array,
The reference magnetic sensor is a room temperature magnetic sensor that detects magnetic field components of a plurality of axes,
The measured magnetic field display method, wherein the predetermined evaluation function is a sum of differences between the magnetic field time series data and the estimated environmental magnetic field time series data for each of the plurality of weights. In claim 9,
A measurement magnetic field display method including a magnetic field reduction effect based on a structure of a magnetic sensor constituting the magnetic sensor array and a magnetic field reduction effect of an analog filter added to an output of the magnetic sensor as a magnetic field reduction effect of the magnetic field measurement device.

Images