JP4676809B2 - Magnetometer for biomagnetic measuring device - Google Patents

Description

  The present invention relates to a magnetometer used in a biomagnetic measurement device such as a cardiac magnetic measurement system that measures a magnetic field generated from each part of a living body such as a human body.

  Using a superconducting quantum interference device (SQUID = Superconducting Quantum Interference Device) as a magnetic sensor, measure the distribution of the weak magnetic field generated from the living body, and estimate the position of the active current inside the living body from the measurement results. A multi-channel biomagnetism measuring apparatus that images the distribution (a matrix arrangement of a large number of magnetic sensors) is known.

  A biomagnetic measuring device using a SQUID magnetic sensor is used as, for example, a cardiac magnetic measuring device (cardiograph) for measuring the magnetic field distribution of the heart, and above the subject's chest is filled with liquid He and liquid nitrogen as refrigerants. And a plurality of magnetic sensors including a SQUID magnetic sensor and a detection coil connected to the SQUID magnetic sensor. The liquid He is continuously supplied from an automatic supply device outside the magnetic shield room.

The output of the SQUID magnetic sensor outputs a voltage that has a specific relationship with the strength of the biomagnetic field (which can be thought of as the magnetic flux density) generated from the subject and detected by the detection coil, and the output is FLL (Flux Locked loop ) Is input to the circuit. This FLL circuit cancels the change of the biomagnetic field (biomagnetism) input to the SQUID via the feedback coil so as to keep the output of the SQUID constant. Then, by converting the current passed through the feedback coil into a voltage, a voltage output having a specific relationship with the change in the biomagnetic field signal can be obtained. Thus, since the system which detects via a feedback coil is taken, the weak magnetic field can be detected with high sensitivity (patent document 1).
Japanese Patent Laid-Open No. 2004-209303

  In the conventional biomagnetic measuring device, a SQUID magnetic sensor is used to measure the biomagnetic field, but in order to use the SQUID magnetic sensor, a large cooling device must be prepared, This is one of the factors that make a magnetic measuring device expensive.

  In addition, the SQUID magnetic sensor is exposed to a cryogenic temperature and is capable of detecting a weak magnetic field with high sensitivity, so it is inevitably contained in a container filled with liquid nitrogen or liquid helium. Since it is the outer surface of the container filled with cryogenic liquid nitrogen or liquid helium that contacts or is close to the subject's chest, it is never pleasant to the subject.

  Furthermore, in order to eliminate the influence of external magnetism, the subject enters the magnetic shield room. However, since the magnetic shield room is expensive, a reduction in size is desired. However, since the container filled with liquid nitrogen or liquid helium has a certain size, it has hindered further downsizing of the magnetic shield room.

  The present invention has been made in view of such a point of view. A magnetometer for a biomagnetic measurement device capable of measuring a weak biomagnetic field at room temperature with high sensitivity by eliminating the influence of noise as much as possible without using a cooling device. It is something to be offered.

  The present invention relates to a magnetometer for a biomagnetism measuring apparatus, which outputs a magnetic field measurement signal by applying an excitation signal to the magnetic sensor and receiving an output signal from a detection coil of the magnetic sensor. A flux gate type magnetic sensor is used as a magnetic sensor.

  A fluxgate type magnetic sensor has a configuration in which an excitation coil and a detection coil are wound around a core made of a magnetic material having a high magnetic permeability, for example, a ring shape, and is used as a magnetic sensor such as a Hall element or a magnetoresistive element. It is known that it has features such as higher resolution and superior temperature stability.

  In addition, the fluxgate type magnetic sensor has been conventionally used for measurement of a DC magnetic field and thus has no idea of being used for biomagnetic measurement. However, focusing on the fact that the noise characteristic is attenuated by 1 / f, particularly the heart Biomagnetism measurement is enabled by quantitatively evaluating the noise level in the frequency band of the magnetic field.

Furthermore, the sensitivity of the conventional fluxgate magnetic sensor is at most 10 ⁻¹⁰ Tesla (T), and the noise level is on the order of 100 pTrms / rtHz, and the sensitivity of the SQUID magnetic sensor is even smaller than 10 ⁻¹⁰ T of the cardiac magnetic field. It is known that measurement at 10 ⁻¹³ (T) is possible.

  Therefore, the conventional fluxgate type magnetic sensor is too noisy to detect a magnetic field from a living body, for example, a magnetic field from the heart.

  The present invention aims to reduce the noise of a fluxgate type magnetic sensor and, as a result, obtain a sensitivity equivalent to that of a SQUID magnetic sensor, so that a weak biomagnetic field can be measured at room temperature.

  Specifically, when an excitation voltage is applied to, for example, an excitation coil wound around a ring core constituting the flux gate type magnetic sensor, a hysteresis curve is obtained as a BH characteristic as shown in FIG.

  The flux gate type magnetic sensor measures the magnetic field by applying an alternating magnetic field to the core, which is a magnetic detection member, and detecting a component whose change in magnetic flux density is asymmetric by the measurement magnetic field.

  The size of the magnetic domain, which is a magnet unit in the magnetic detection member, is not uniform and the reversal of the magnetic axis is non-uniform, so that non-uniform saturation occurs.

  In the saturation region, the saturation of each magnetic domain varies and unevenness occurs in the magnetic permeability change of the magnetic detection member (core). However, since the magnetic flux density is proportional to the product of the current and the magnetic permeability, the uneven magnetic permeability change is It is considered that the fluctuation of the magnetic field causes fluctuations and becomes a factor of noise generation of the magnetometer output.

  On the other hand, the excitation current continues to rise until the magnetic detection member (core) is saturated, and at the time of saturation, the magnetic permeability of the core decreases, the inductance decreases, and the impedance decreases, so the current further increases. .

  By the way, in the conventional excitation circuit, since this current increase is abruptly performed in the saturation region, it is estimated that the fluctuation of the magnetic flux density is made redundant and the output noise is deteriorated.

  Therefore, in the present invention, the fluctuation of the magnetic flux density is suppressed by slowing the increase of the excitation current in the saturation region and delaying the increase of the current until each magnetic domain is saturated, and at the conventional frequency of 1 Hz, about 100 pTrms / rtHz. The output noise level of a fluxgate magnetic sensor could be reduced to 2-3 pTrms / rtHz.

  On the other hand, the cardiac magnetic field is composed of several waveforms, and their peaks are about 10 pT to 100 pT. The noise characteristics of the fluxgate type magnetic sensor are dominated by 1 / f noise and decrease in proportion to the square root of the frequency. Since the frequency range of the cardiac magnetic field is 10 Hz or more, it becomes 1 pTrms / rtHz or less. By filtering, each waveform component can be identified and the cardiac magnetic field can be measured.

A first configuration of a magnetometer for a biomagnetic field measurement apparatus that realizes the object of the present invention is used in a biomagnetic field measurement apparatus that measures a magnetic field emitted from a subject that is a living body. A flux gate type magnetic sensor located in the vicinity of the measurement site of the subject, in which an excitation coil and a detection coil are wound around a core made of a magnetic material, and an excitation signal is applied to the magnetic sensor and the magnetic sensor A magnetometer circuit that receives a signal output from the detection coil and outputs a magnetic field measurement signal. The magnetometer circuit has a sine wave as an excitation voltage waveform applied to the excitation coil, and a peak of the voltage. By matching the region where the derivative of the voltage change is zero with the timing at which the core starts to be saturated in the saturated region where the core is saturated as the exciting current passing through the exciting coil increases. And having a suppressing current variation suppressor variation of the excitation current.

A second configuration is characterized in that, in the first configuration described above, the measurement frequency region of the fluxgate type magnetic sensor is set to the magnetic field frequency region of the measurement site of the subject as described in claim 2. To do.

The third configuration, as described in claim 3, in the construction described above, the fluxgate magnetic sensor is characterized in that the plurality is arranged in a matrix.

  According to the magnetometer for the biomagnetism measuring device of the present invention, cardiac magnetic field measurement by a fluxgate type magnetic sensor operating at room temperature is possible, and a cooling device and a coolant which are necessary for cooling to a cryogenic temperature are unnecessary, As a result, not only can the biomagnetic measuring device be reduced in size, but also the cost can be reduced.

  FIG. 1 is a block diagram showing a schematic configuration of a magnetometer for a biomagnetic measuring device according to the present invention, and FIG. 2 is a block diagram showing the details thereof.

  In FIG. 1, the fluxgate type magnetic sensor 6 is arranged in a matrix (8 × 8 in the present embodiment) as shown in FIG. 3, for example, and is attached to the sensor mounting base plate 17, for example, lying on its back on the bed. It is located directly on the chest of the subject B in a different posture or with a slight gap.

  The fluxgate type magnetic sensor (hereinafter abbreviated as FG sensor) 6 receives an excitation signal from a fluxgate type magnetometer circuit (hereinafter abbreviated as magnetometer circuit) 7 and outputs a detection signal to the magnetometer circuit 7. .

  As shown in FIG. 3, an active current (cardiac current) flows through each part of the heart of the subject B, which is a human body, and a cardiac magnetic field is formed. The FG sensor 6 is located in this cardiac magnetic field.

  As shown in FIG. 2, the FG sensor 6 is configured by winding an excitation coil (not shown) and a detection coil 9 around a core 8 formed as a magnetic detection member, for example, in a ring shape. The detection coil 9 detects an asymmetric component of the magnetic flux generated in the magnetic detection material as an electric signal, reflecting the measurement magnetic field.

  In FIG. 2, the excitation frequency reference signal is output from the excitation / phase signal generation circuit 15 to the excitation circuit 13, and the phase frequency reference signal is output from the excitation / phase signal generation circuit 15 to the phase detector 11. An excitation signal is applied to the excitation coil (not shown) of the core 8 by the excitation circuit 13, and an alternating magnetic field is generated in the core 8.

  Since the FG sensor 6 is positioned in the cardiac magnetic field, which is a measurement magnetic field, an electrical signal of an asymmetrical component of the magnetic flux reflecting the cardiac magnetic field is output from the detection coil 9 to the magnetometer circuit 7. And is output to the second harmonic amplifying circuit 10.

  The secondary harmonic output circuit 10 amplifies the secondary harmonic induced by the asymmetric component of the magnetic flux detected by the detection coil 9 and extracts the measurement magnetic field as an alternating electrical signal. The electric signal of the second harmonic extracted by the second harmonic output circuit 10 is detected by the phase detector 11 and converted into a DC voltage signal.

  The DC voltage signal output from the phase detector 11 is converted into a current signal by the feedback circuit 12 and supplied to the detection coil 9. The detection coil 9 gives the core 8 a feedback magnetic field in a direction to cancel the measurement magnetic field by the fed back current signal. As a result, the measurement magnetic field is converted into an electric signal, and a feedback loop is formed that is fed back.

  This feedback loop amplifies the difference between the measurement magnetic field and the feedback magnetic field and cancels it, so that the feedback magnetic field is equal to the measurement magnetic field, and the current that creates it is proportional to the measurement magnetic field. This current is applied to the output side of the phase detector 11, and the DC voltage output from the phase detector 11 corresponds to the measurement magnetic field. By taking this as the output of the magnetometer, the magnetic field is measured. The fluxgate magnetometer measures the magnetic field based on the above operation principle. The DC voltage corresponding to the measurement magnetic field from the phase detector 11 is output as a magnetic field measurement signal to a processing circuit (not shown) of the biomagnetic field measurement device through a bandpass filter (BPF) 14 that filters the magnetocardiogram frequency band. The magnetocardiographic waveform data is displayed by a display device (not shown), a recording device or the like.

  The excitation circuit 13 shown in FIG. 2 includes the circuit block shown in FIG.

  In FIG. 4, the excitation circuit 13 amplifies the excitation reference frequency signal to the excitation power by the power amplifier 1 and removes the DC component included in the output of the power amplifier 1 by the DC component removal capacitor 2. The exciting current from which the DC component has been removed by the DC component removing capacitor 2 is maintained in the output of the power amplifier 1 by the current limiting resistor 3.

  In the conventional excitation circuit, the excitation current from the current limiting resistor 3 is directly supplied to the excitation coil of the core 8. In this embodiment, the excitation coil of the core 8 is supplied through the current fluctuation suppression circuit 5.

  As shown in FIG. 5 and FIG. 6A, the current fluctuation suppressing circuit 5 moderates the increase of the excitation current in the saturation region and delays the increase of the current until each magnetic domain has been saturated. It has the effect of suppressing fluctuations. The current fluctuation suppressing circuit 5 is configured by, for example, an LC resonance circuit that generates a sine wave.

  As described above, the FG sensor 6 measures the magnetic field by applying an alternating magnetic field to the core, which is a magnetic detection member, and detecting a component in which the change in magnetic flux density is asymmetric due to the measurement magnetic field. And the size of the magnetic domain which is a magnet unit in a magnetic detection member is not uniform, and since the inversion of the magnetic axis is non-uniform | heterogenous, nonuniformity arises also in the saturation method.

  In the saturation region, the saturation of each magnetic domain varies and unevenness occurs in the magnetic permeability change of the magnetic detection member (core). However, since the magnetic flux density is proportional to the product of the current and the magnetic permeability, the uneven magnetic permeability change is It is considered that the fluctuation of the magnetic field causes fluctuations and becomes a factor of noise generation of the magnetometer output.

  On the other hand, the excitation current continues to rise until the magnetic detection member (core) is saturated, and at the point of saturation, the magnetic permeability of the core decreases, the inductance decreases, and the impedance decreases, so the current further increases. .

  By the way, in the conventional excitation circuit, since this current increase is abruptly performed in the saturation region, it is estimated that the fluctuation of the magnetic flux density is made redundant and the output noise is deteriorated.

  On the other hand, in the present embodiment, the current fluctuation suppression circuit 5 is provided in the excitation circuit 13 to moderate the increase in the excitation current in the saturation region and to delay the increase in the current until each magnetic domain has been saturated. The fluctuation of density change is suppressed.

That is, the magnetometer for biomagnetism measurement according to the present invention is a method for suppressing the sudden increase of the saturation current in the saturation region for the excitation to the FG sensor 6 operating at room temperature, and the excitation voltage waveform for driving the core is a sine wave. In addition, it can also be expressed as a method of reducing the current increase rate in the saturation region by matching the region that is the peak and the derivative of the voltage change is zero with the saturation region. Current fluctuation suppression circuit 5, in turn, generates a sine wave, yet Ri circuit der to adjust the peak position, specifically, the excitation voltage waveform applied to the exciting coil a sine wave, and its voltage Fluctuation of the excitation current by matching the region where the derivative of the voltage change that is the peak of zero is zero with the timing at which the core starts saturation in the saturation region where the core saturates as the excitation current flowing through the excitation coil increases It is a circuit that suppresses

  FIG. 6 is a waveform diagram of the excitation current, FIG. 6A is a waveform diagram of the excitation circuit including the current fluctuation suppression circuit 5 of the present embodiment shown in FIG. 4, and FIG. 6B is a current fluctuation suppression circuit 5. It is a wave form diagram by the conventional excitation circuit which is not. In FIG. 6, the horizontal axis represents time, and the vertical axis represents excitation current. In FIG. 6, the area indicated by symbol A corresponds to the area A shown in FIG.

As shown in FIG. 6 (a), the excitation voltage waveform to be applied to the excitation coil of the core is a sine wave by the excitation circuit 13 provided with the current fluctuation suppressing circuit 5 of the present embodiment, and the peak thereof, By matching the region where the voltage change derivative is zero to the saturation region , that is, by matching the timing at which the core begins to saturate in the saturation region where the core saturates as the exciting current passing through the exciting coil increases, The current increase rate of the excitation current at 30 mA was 30 mA / μsec. In contrast, the current increase rate of the excitation current in the saturation region by the conventional excitation circuit shown in FIG. 6B was 60 mA / μsec. That is, the current increase rate in the saturation region could be reduced by half.

  FIG. 7 shows waveforms of noise level characteristics measured by setting the excitation circuit 13 including the current fluctuation suppressing circuit 5 in the noise level measurement system.

  In FIG. 7, the horizontal axis represents frequency (Hz), the vertical axis represents sensitivity (dB), and the magnetometer sensitivity is 60 pT / mV. In the waveform diagram of FIG. 7, it was 39.1 μVrms / rtHz at 1 Hz. This value was 39.1 (μVrms / rtHz) × 60 (pT / mV) = 2.3 pTrms / rtHz, and the output noise level of the fluxgate magnetic sensor 6 could be reduced to 2 to 3 pTrms / rtHz.

  On the other hand, the cardiac magnetic field is composed of several waveforms, and their peaks are about 10 pT to 100 pT. The noise characteristics of the fluxgate type magnetic sensor are dominated by 1 / f noise, and decrease in proportion to the square root of the frequency. By filtering, each waveform component can be identified and the cardiac magnetic field can be measured. FIG. 8 shows a magnetocardiographic waveform measured using the magnetometer of the present embodiment. In FIG. 8, Q, R, and S waves occur when the heart's ventricle is electrically excited, and T waves occur when the heart's ventricle is awakened from electrical excitation.

The block diagram which shows schematic structure of the magnetometer in embodiment of this invention. The block diagram which shows the detailed structure of the magnetometer of FIG. The figure explaining the measurement of the cardiac magnetic field of a human body using the magnetometer of FIG. The block diagram which shows the detail of the excitation circuit of FIG. The BH characteristic diagram of a fluxgate type magnetic sensor. FIG. 5A is a waveform diagram of an excitation current, FIG. 5A is a waveform diagram of the excitation circuit of FIG. 4, and FIG. 5B is a waveform diagram of a conventional excitation circuit. The characteristic diagram of the output noise level of the magnetometer of FIG. FIG. 2 is a waveform diagram of a magnetocardiogram measured by a biomagnetometer (magnetometer) using the magnetometer of FIG. 1.

Explanation of symbols

B Human body 1 Power amplifier 2 DC component removal capacitor 3 Current limiting resistor 4 Core 5 Current fluctuation suppression circuit 6 Flux gate type magnetic sensor 7 Flux gate type magnetometer circuit 8 Core 9 Detection coil 10 Second harmonic amplifying circuit 11 Phase detector 12 Feedback circuit 13 Excitation circuit 14 Band pass filter (BPF)
15 Excitation / phase signal generation circuit 16 Amplifier

Claims (3)

A fluxgate that is used in a biomagnetic field measurement device that measures a magnetic field emitted from a subject, which is a living body, and is positioned in the vicinity of the measurement site of the subject and in which an excitation coil and a detection coil are wound around a core made of a magnetic material And a magnetometer circuit for supplying an excitation signal to the magnetic sensor and outputting a magnetic field measurement signal when an output signal from a detection coil of the magnetic sensor is input. The magnetometer circuit includes: The excitation voltage waveform applied to the excitation coil is a sine wave, and the region where the differential of the voltage change that is the peak of the voltage is zero is a saturation region where the core is saturated with an increase in the excitation current applied to the excitation coil by the core be adjusted to the timing for starting the saturation of, for biomagnetic field measuring apparatus characterized by having a suppressing current variation suppressor variation of the exciting current Magnetometer. The magnetometer for a biomagnetic field measurement apparatus according to claim 1, wherein a measurement frequency region of the fluxgate type magnetic sensor is set to a magnetic field frequency region of a measurement site of a subject. The magnetometer for a biomagnetic field measurement apparatus according to claim 1 or 2, wherein a plurality of the fluxgate magnetic sensors are arranged in a matrix.

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