AC magnetic field measuring device

Abstract

To enhance detection sensitivity, by suppressing power loss by signal reflection.SOLUTION: A resonance adjusting circuit 50 is configured so that an input impedance being an impedance in which an input side circuit 52 and a reception coil 22 side are viewed from a coaxial cable 51, is equal to an impedance of the coaxial cable 51 in a prescribed frequency by adjusting capacity of a capacitor Cs1, Cp1 of the input side circuit 52, and an output impedance being an impedance in which the output side circuit 55 and an output coil 24 side are viewed from the coaxial cable 51, is equal to an impedance of the coaxial cable 51 in the prescribed frequency by adjusting capacity of a capacitor Cs2, Cp2 of the output side circuit 55.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an AC magnetic field measuring device. [Background technology]

[0002] In brain measurement devices, when detecting MR (Magnetic Resonance) signals with an OPM (optically pumped magnetometer), if the OPM is placed in the static magnetic field of an MRI (Magnetic Resonance Imaging), the MR signals may not be detected with high sensitivity due to differences in the resonance frequency between the electrons of the alkali metal atoms in the OPM and the protons that are the measurement target of the MRI. In response to this, MR signals can be detected with high sensitivity by separating the magnetic field environment of the MR measurement area from the area where the OPM is placed using a circuit called an FT (Flux transformer).

[0003] As a resonant circuit of the FT as described above, a series resonant circuit in which capacitors are inserted in series with the input coil and output coil of the FT is known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 8,305,078 Summary of the Invention [Problem to be solved by the invention]

[0005] Here, in a series resonant circuit such as that described in Patent Document 1, the effect of signal reflection increases as the frequency increases, and if the input coil and output coil of the FT are connected with a twisted pair cable, the amount of signal attenuation also increases, and there is a risk that sufficient detection sensitivity cannot be ensured.

[0006] The present invention has been made in consideration of the above-mentioned situation, and aims to provide an AC magnetic field measuring device that can improve detection sensitivity by suppressing power loss due to signal reflection and signal attenuation in cables. [Means for solving the problem]

[0007] An AC magnetic field measuring device according to one aspect of the present invention includes a receiving coil for detecting a signal and converting it into a current, a resonance adjustment circuit provided corresponding to the receiving coil for outputting a predetermined frequency of the current output from the receiving coil, an output coil for converting the current output from the resonance adjustment circuit into a magnetic signal, and an optically excited magnetic sensor for detecting the magnetic signal output by the output coil, wherein the resonance adjustment circuit has an input side circuit in which a capacitor is provided in series and parallel to the receiving coil, an output side circuit in which a capacitor is provided in series and parallel to the output coil, and a coaxial cable connecting the input side circuit and the output side circuit, and the capacitance of the capacitor of the input side circuit is adjusted so that the input impedance, which is the impedance when looking from the coaxial cable toward the input side circuit and the receiving coil, is the same as the impedance of the coaxial cable at a predetermined frequency, and the capacitance of the capacitor of the output side circuit is adjusted so that the output impedance, which is the impedance when looking from the coaxial cable toward the output side circuit and the output coil, is the same as the impedance of the coaxial cable at a predetermined frequency.

[0008] In an AC magnetic field measuring device according to an aspect of the present invention, the resonance adjustment circuit includes an input circuit having a capacitor connected in series and parallel to the receiving coil, an output circuit having a capacitor connected in series and parallel to the output coil, and a coaxial cable connecting the input circuit and the output circuit. The capacitances of the capacitors in the input circuit and the output circuit are adjusted so that the input impedance seen from the input circuit and the receiving coil side and the output impedance seen from the output circuit and the output coil side are the same as the impedance of the coaxial cable at a predetermined frequency. With this configuration, the input impedance and the output impedance seen from the coaxial cable are the same as the impedance of the coaxial cable at a predetermined frequency, so that the power loss due to signal reflection can be suppressed for the component of the resonance frequency in the signal, and the detection sensitivity of the optically excited magnetic sensor can be improved. As described above, with the AC magnetic field measuring device according to an aspect of the present invention, the detection sensitivity can be improved by suppressing the power loss due to signal reflection.

[0009] The above-mentioned AC magnetic field measuring device further includes a transmitting coil for transmitting a transmission pulse of a predetermined frequency, and a control device for generating an MR image based on a signal detected by the optically excited magnetic sensor, and the receiving coil may detect a nuclear magnetic resonance signal generated by transmitting the transmission pulse and convert it into an electric current. According to this configuration, the brain measuring device can achieve the effect of improving the detection sensitivity described above.

[0010] The frequency band in which the detection sensitivity of the optically excited magnetic sensor is maximized may include the resonance frequency, thereby making it possible to further improve the detection sensitivity of the optically excited magnetic sensor. Effect of the Invention

[0011] According to the present invention, it is possible to provide an AC magnetic field measuring device capable of improving detection sensitivity by suppressing power loss due to signal reflection. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing a configuration of a brain measuring apparatus according to an embodiment. [Diagram 2] 2 is a schematic diagram showing a configuration of an OPM module 23 according to the embodiment. FIG. [Diagram 3] 4 is a flowchart showing the operation of the brain measuring apparatus according to the embodiment. [Figure 4] 4 is a flowchart showing the operation of the brain measuring apparatus according to the embodiment. [Diagram 5] FIG. 13 is a diagram illustrating an example of detection sensitivity according to a detection band. [Figure 6] FIG. 1 is a diagram illustrating a frequency-encoded MR signal. [Figure 7] 1 is a schematic diagram showing a part of a brain measuring device including a resonance adjustment circuit. [Figure 8] FIG. 2 is a diagram illustrating an example of the configuration of a resonance adjustment circuit. [Figure 9] FIG. 2 is a diagram illustrating an example of the configuration of a resonance adjustment circuit. [Figure 10] 10 is a diagram showing a detailed circuit configuration of the resonance adjustment circuit shown in FIG. 9. [Figure 11] 1A to 1C are diagrams illustrating the effects of the brain measuring apparatus according to the embodiment. [Figure 12] FIG. 13 is a diagram showing a detailed circuit configuration of a resonance adjustment circuit according to a modified example. [Figure 13] FIG. 13 is a diagram showing a configuration of a receiving coil according to a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted.

[0014] [Basic configuration of brain measurement device] 1 is a schematic diagram showing the configuration of a brain measuring device M1 according to an embodiment. The brain measuring device M1 is an example of an AC magnetic field measuring device, and is a device that measures brain magnetic fields and MR (Magnetic Resonance) images of a subject. The brain measuring device M1 includes a magnetoencephalograph module having a plurality of OPM (optically pumped magnetometer) modules 1, a plurality of geomagnetic correction magnetic sensors 2, a plurality of active shield magnetic sensors 3, a non-magnetic frame 4, a pair of geomagnetic correction coils 7, a pair of geomagnetic gradient correction coils 8, and a pair of variable magnetic field correction coils 9, and an MRI module having a transmission coil 21, a plurality of reception coils 22, a plurality of OPM modules 23 (detection units), a plurality of output coils 24, and a plurality of resonance adjustment circuits 50. The OPM module 23 as a detection unit may be replaced by an amplifier (not shown). Furthermore, the brain-measuring device M1 includes a control device 5, a coil power supply 6, a pump laser 10, a probe laser 11, amplifiers 12A and 12B, a heater controller 13, an electromagnetic shield 14, and a transmission coil controller 15. In the description with reference to Fig. 1, the description of the resonance adjustment circuit 50 is omitted (the resonance adjustment circuit 50 will be described later).

[0015] In the following description, the direction roughly parallel to the central axis of the subject's head is defined as the Z-axis direction, and the directions perpendicular to the Z-axis and perpendicular to each other are defined as the X-axis direction and the Y-axis direction.

[0016] The OPM module 1 includes an optically excited magnetic sensor 1A, a thermal insulator 1B, and a readout circuit 1C. A plurality of OPM modules 1 are arranged at predetermined intervals along the scalp, for example.

[0017] The optically excited magnetic sensor 1A is a sensor that measures the brain magnetic field by using optical pumping, and has a sensitivity of, for example, about 10 fT to 10 pT. The heat insulating material 1B prevents heat movement and heat transfer of the optically excited magnetic sensor 1A. The readout circuit 1C is a circuit that acquires the detection result of the optically excited magnetic sensor 1A. The optically excited magnetic sensor 1A excites the alkali metal by irradiating a pump light to a cell in which alkali metal vapor is sealed. The excited alkali metal is in a spin polarized state, and when it receives a magnetic field, the tilt of the spin polarization axis of the alkali metal atom changes according to the magnetic field. The tilt of this spin polarization axis is detected by a probe light that is irradiated separately from the pump light. The optically excited magnetic sensor 1A is configured so that a predetermined bias magnetic field is applied in the irradiation direction of the pump light so that the sensor is sensitive to a magnetic field with a frequency in the range of 0 to 200 Hz. The readout circuit 1C receives the probe light that has passed through the alkali metal vapor by a photodiode and acquires the detection result. The readout circuit 1C outputs the detection result to the amplifier 12A.

[0018] The optically excited magnetic sensor 1A may be, for example, an axial gradiometer. The axial gradiometer has a measurement region and a reference region on the same axis and in a direction perpendicular to the subject's scalp (measurement location). The measurement region is, for example, a location closest to the subject's scalp among the locations where the axial gradiometer measures the brain magnetic field. The reference region is, for example, a location at a predetermined distance (for example, 3 cm) from the measurement region in a direction away from the subject's scalp among the locations where the axial gradiometer measures the brain magnetic field. The axial gradiometer outputs the results of measurements in the measurement region and the reference region to the amplifier 12A. Here, if common mode noise is included, its influence is indicated in the output result of the measurement region and the output result of the reference region, respectively. The common mode noise is removed by acquiring the difference between the output result of the measurement region and the output result of the reference region. By removing the common mode noise, for example, when measurement is performed in a magnetic noise environment of 1 pT, the optically excited magnetic sensor 1A can obtain a sensitivity of about 10 fT / √Hz.

[0019] The geomagnetic correction magnetic sensor 2 is a sensor that measures a magnetic field related to geomagnetism at a position corresponding to the optically excited magnetic sensor 1A, and is configured by, for example, a fluxgate sensor having a sensitivity of about 1 nT to 100 μT. The position corresponding to the optically excited magnetic sensor 1A is a position around (near) the area where the optically excited magnetic sensor 1A is arranged. The geomagnetic correction magnetic sensor 2 may be provided in one-to-one correspondence with the optically excited magnetic sensor 1A, or in one-to-multiple correspondence (one geomagnetic correction magnetic sensor 2 for multiple optically excited magnetic sensors 1A). The geomagnetic correction magnetic sensor 2 measures, for example, geomagnetism and a geomagnetic gradient magnetic field (hereinafter simply referred to as a "gradient magnetic field") as a magnetic field related to geomagnetism, and outputs the measurement value to the control device 5. The measurement value of the geomagnetic correction magnetic sensor 2 can be expressed by a vector having a direction and a magnitude. The geomagnetic correction magnetic sensor 2 may continuously perform measurement and output at a predetermined time interval.

[0020] The magnetic sensor 3 for active shield is a sensor that measures a fluctuating magnetic field at a position corresponding to the optically excited magnetic sensor 1A, and is composed of an optically excited magnetic sensor different from the optically excited magnetic sensor 1A, having a sensitivity of, for example, about 100 fT to 10 nT. The position corresponding to the optically excited magnetic sensor 1A is a position around (near) the area where the optically excited magnetic sensor 1A is arranged. The magnetic sensor 3 for active shield may be provided in one-to-one correspondence with the optically excited magnetic sensor 1A, or in one-to-multiple correspondence (one magnetic sensor 3 for active shield for multiple optically excited magnetic sensors 1A). The magnetic sensor 3 for active shield measures a magnetic field of noise (AC) components of, for example, 200 Hz or less as a fluctuating magnetic field, and outputs the measurement value to the control device 5. The measurement value of the magnetic sensor 3 for active shield can be expressed by a vector having a direction and a magnitude. The magnetic sensor 3 for active shield may continuously perform measurement and output at a predetermined time interval.

[0021] The non-magnetic frame 4 is a frame that covers the entire scalp of the subject, whose brain magnetic field is to be measured, and is made of a non-magnetic material such as graphite that has a relative permeability close to 1 and does not disturb the magnetic field distribution. The non-magnetic frame 4 can be, for example, a helmet-type frame that surrounds the entire scalp of the subject and is worn on the subject's head. A plurality of optically excited magnetic sensors 1A are fixed to the non-magnetic frame 4 so as to be close to the subject's scalp. Furthermore, a geomagnetic correction magnetic sensor 2 is fixed to the non-magnetic frame 4 so as to be able to measure the magnetic field related to the geomagnetic field at each position of the plurality of optically excited magnetic sensors 1A, and an active shield magnetic sensor 3 is fixed to be able to measure the fluctuating magnetic field at each position of the plurality of optically excited magnetic sensors 1A. Since the fluctuating magnetic field has less variation in magnetic field strength depending on the position than the static magnetic field, the number of active shield magnetic sensors 3 may be fixed to the non-magnetic frame 4 so that the number of geomagnetic correction magnetic sensors 2 is less. In addition, a receiving coil 22 for detecting nuclear magnetic resonance signals for MR image measurement is fixed to the scalp side of the subject of the multiple optically excited magnetic sensors 1A in the non-magnetic frame 4. This receiving coil 22 detects nuclear magnetic resonance signals of protons, which will be described later, and converts them into electric current. In order to improve the detection sensitivity of the nuclear magnetic resonance signals, it is preferable that the receiving coil 22 is provided on the side of the optically excited magnetic sensor 1A that is closer to the scalp of the subject's head.

[0022] The receiving coil 22 is formed so that the maximum area and the number of turns in a plane perpendicular to the X-axis direction (described in detail later), which is the direction in which the static magnetic field is generated, are constant. The maximum area is determined by, for example, the size, shape (ellipse), position, and axial direction of the receiving coil 22. With this configuration, the sensitivity of the receiving coil 22 becomes uniform.

[0023] The transmission coil 21 is a coil that irradiates an RF pulse (transmission pulse) of a predetermined frequency (for example, about 300 kHz) to the subject's head during MR image measurement. This transmission coil 21 is disposed, for example, outside the non-magnetic frame 4 and above the subject's head.

[0024] The output coil 24 is electrically connected to both ends of the receiving coil 22 via a cable, receives a current flowing between both ends of the receiving coil 22, converts the current back into a magnetic signal, and outputs the magnetic signal. In more detail, the output coil 24 and the receiving coil 22 are connected via a resonance adjustment circuit 50, but a description of the resonance adjustment circuit 50 will be omitted here (the resonance adjustment circuit 50 will be described later).

[0025] Like OPM module 1, OPM module 23 has an optically excited magnetic sensor 23A, a thermal insulator 23B, and a readout circuit 23C. OPM module 23 is arranged, for example, outside non-magnetic frame 4, housed in a magnetic shield 25 that blocks a static magnetic field, which will be described later, together with an output coil 24. Magnetic shield 25 is made of, for example, mu metal or the like, whose relative permeability is greater than 1.

[0026] The optically excited magnetic sensor 23A is a sensor that measures a magnetic signal by utilizing optical pumping. The optically excited magnetic sensor 23A is configured so that a predetermined bias magnetic field is applied in the direction of the pump light irradiation so that the sensor is sensitive to a magnetic field having a frequency in the range of 20 kHz to 500 kHz. For example, a bias magnetic field of about 40 μT is applied so that the sensor is sensitive to a frequency of 300 kHz of the electromagnetic wave emitted by the protons. The readout circuit 23C outputs the detection result by the optically excited magnetic sensor 23A to the amplifier 12B.

[0027] 2 shows a specific example of the configuration of the OPM module 23. The optically excited magnetic sensor 23A includes a longitudinal cell 26 filled with a gas containing an alkali metal whose polarization direction changes depending on the magnetic field to be measured, a heater 27 for heating the entire cell 26 to a predetermined temperature (e.g., 180 degrees), a polarizing beam splitter 28, and a photodetector 29. Pump light L1 is introduced from the outside along the longitudinal direction of the inside of the cell 26, and probe light L2 from the outside is split and irradiated along a direction perpendicular to the longitudinal direction to each of intersection regions 26A divided into a plurality of parts (e.g., four parts) in the longitudinal direction. The magnetic rotation angle of the probe light L2 transmitted through these intersection regions 26A is detected by the polarizing beam splitter 28 and the photodetector 29 provided corresponding to each intersection region 26A. That is, the polarizing beam splitter 28 splits the probe light L2 into two linearly polarized components that are orthogonal to each other, and the photodetector 29 detects the intensities of the two linearly polarized components using two built-in PDs (photodiodes) and detects the magnetic rotation angle of the probe light L2 based on the ratio of the detected intensities. The OPM module 23 is further provided with a circuit board 30, and the magnetic rotation angle of the probe light L2 detected for each intersection region 26A is output via a readout circuit 23C in the circuit board 30.

[0028] The output coil 24 is fixed in the magnetic shield 25 so as to face each intersection region 26A of the cells 26 of the OPM module 23 configured as described above. With this configuration, the magnetic signal BOUT generated by the output coil 24 based on the electromagnetic field BOUT detected by the receiving coil 22 is detected based on the magnetic rotation angle of the probe light L2 that changes according to the tilt of the spin polarization axis of the alkali metal atoms. Here, in the example of FIG. 2, the intersection region 26A is divided into four, but this may be changed to any number. Also, a plurality of cells 26 may be provided in parallel, and the intersection regions 26A may be arranged two-dimensionally (for example, 4×4=16).

[0029] When measuring the brain magnetic field, the control device 5 determines the currents for the various coils based on the measurement values ​​output from the geomagnetic correction magnetic sensor 2 and the active shield magnetic sensor 3, and outputs a control signal for outputting the current to the coil power supply 6. The control device 5 determines the currents for the geomagnetic correction coil 7 and the geomagnetic gradient correction coil 8 based on the measurement values ​​of the multiple geomagnetic correction magnetic sensors 2 so as to generate a magnetic field that cancels out the magnetic field related to the geomagnetism. The control device 5 also determines the current for the variable magnetic field correction coil 9 based on the measurement values ​​of the multiple active shield magnetic sensors 3 so as to generate a magnetic field that cancels out the variable magnetic field. The control device 5 outputs a control signal to the coil power supply 6 according to the determined current.

[0030] Specifically, the control device 5 determines the current for the geomagnetic correction coil 7 so that the average value of the measurement values ​​of the multiple geomagnetic correction magnetic sensors 2 approaches zero (as a result, a magnetic field is generated that is opposite to and has the same magnitude as the geomagnetic field at the position of the optically excited magnetic sensor 1A). The control device 5 outputs a control signal (static magnetic field correction control signal) corresponding to the determined current for the geomagnetic correction coil 7 to the coil power supply 6.

[0031] The control device 5 also determines the current for the geomagnetic gradient correction coil 8 so that the deviation from the average value of the measurement values ​​of the multiple geomagnetic correction magnetic sensors 2 is minimized (as a result, a magnetic field is generated that is opposite in direction to and has the same magnitude as the gradient magnetic field at the position of the optically excited magnetic sensor 1A). The control device 5 outputs a control signal (static magnetic field correction control signal) to the coil power supply 6 according to the determined current for the geomagnetic gradient correction coil 8.

[0032] Furthermore, the control device 5 determines the current for the variable magnetic field correction coil 9 so that the average value of the measurement values ​​of the multiple active shield magnetic sensors 3 approaches 0 (as a result, a magnetic field is generated that is opposite in direction and has the same magnitude as the variable magnetic field at the position of the optically excited magnetic sensor 1A). The control device 5 outputs a control signal (variable magnetic field correction control signal) to the coil power supply 6 according to the determined current for the variable magnetic field correction coil 9.

[0033] Furthermore, the control device 5 obtains information about the magnetism detected by the optically excited magnetic sensor 1A using the signal output from the amplifier 12A. When the optically excited magnetic sensor 1A is an axial gradiometer, the control device 5 may remove common mode noise by obtaining the difference between the output result of the measurement region and the output result of the reference region. The control device 5 may control the operation of the pump laser 10 and the probe laser 11, such as the irradiation timing and irradiation time.

[0034] In addition, when measuring an MR image, the control device 5 determines the current to be supplied to the geomagnetic correction coil 7 and the geomagnetic gradient correction coil 8, which respectively operate as coils for applying a static magnetic field and a gradient magnetic field, and outputs a control signal for outputting the current to the coil power supply 6. That is, the control device 5 determines the current to be passed through the geomagnetic correction coil 7 so as to apply a magnetic field in the X-axis direction of a predetermined strength (for example, 7 mT) to the subject's head as a static magnetic field. In addition, the control device 5 selectively determines the X-axis magnetic field gradient (dBx / dX), the Y-axis magnetic field gradient (dBx / dY), and the Z-axis magnetic field gradient (dBx / dZ) as gradient magnetic fields, and determines the current to be passed through the geomagnetic gradient correction coil 8. This allows the position to be sliced ​​in the MR image to be determined, and the position in the slice plane to be encoded by phase encoding and frequency encoding. In addition, the control device 5 outputs a control signal so as not to supply a current to the variable magnetic field correction coil 9, which removes low-frequency noise, when measuring an MR image.

[0035] Furthermore, when measuring an MR image, the control device 5 controls the transmission coil controller 15 to irradiate the subject's head with a transmission pulse of a predetermined frequency (for example, about 300 kHz when the static magnetic field strength is 7 mT) by outputting a control signal to control the power supplied to the transmission coil 21. As a result, protons on a slice plane (a plane selected by the static magnetic field and the gradient magnetic field) resonate and the spins tilt. Thereafter, the control device 5 controls the power of the transmission coil 21 to be turned off. This makes it possible to obtain an MR image by measuring the return of the spins based on the output of the OPM module 23. More specifically, the control device 5 measures the nuclear magnetic resonance signal from the protons by encoding the position with the frequency and phase using a known spin echo sequence or gradient echo sequence, and converts the measurement result into an MR image using FFT.

[0036] The control device 5 is physically configured to include memories such as RAM and ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and a storage unit such as a hard disk. Examples of such a control device 5 include a personal computer, a cloud server, a smartphone, and a tablet terminal. The control device 5 functions by executing a program stored in the memory with the CPU of a computer system.

[0037] The coil power supply 6 outputs a predetermined current to each of the geomagnetic correction coil 7, the geomagnetic gradient correction coil 8, and the variable magnetic field correction coil 9 in response to a control signal output from the control device 5. Specifically, the coil power supply 6 outputs a current to the geomagnetic correction coil 7 in response to a control signal related to the geomagnetic correction coil 7. The coil power supply 6 outputs a current to the geomagnetic gradient correction coil 8 in response to a control signal related to the geomagnetic gradient correction coil 8. The coil power supply 6 outputs a current to the variable magnetic field correction coil 9 in response to a control signal related to the variable magnetic field correction coil 9.

[0038] The transmitting coil controller 15 is electrically connected to the transmitting coil 21, and supplies power to the transmitting coil 21 in response to a control signal output from the control device 5 so as to irradiate a transmitting pulse of a predetermined frequency.

[0039] The geomagnetic correction coil 7 is a coil for correcting the geomagnetic field among the magnetic fields related to the geomagnetism at the position of the optically excited magnetic sensor 1A. The geomagnetic correction coil 7 generates a magnetic field according to the current supplied from the coil power supply 6 to cancel the geomagnetic field. The geomagnetic correction coil 7 has, for example, a pair of geomagnetic correction coils 7A and 7B. The pair of geomagnetic correction coils 7A and 7B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of geomagnetic correction coils 7A and 7B generate a magnetic field that is opposite to and has the same magnitude as the geomagnetic field at the position of the optically excited magnetic sensor 1A according to the current supplied from the coil power supply 6. The direction of the magnetic field is, for example, the X-axis direction, the Y-axis direction, and the Z-axis direction. The geomagnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude as the geomagnetic field generated by the geomagnetic correction coil 7. In this way, the geomagnetic correction coil 7 corrects the geomagnetic field at the position of the optically excited magnetic sensor 1A.

[0040] The geomagnetic correction coil 7 also serves as a static magnetic field coil for generating a static magnetic field in the X-axis direction during MR image measurement. The geomagnetic correction coil 7 generates a static magnetic field of a predetermined intensity in response to the current supplied from the coil power supply 6.

[0041] The geomagnetic gradient correction coil 8 is a coil for correcting the gradient magnetic field of the magnetic field related to the geomagnetism at the position of the optically excited magnetic sensor 1A. The geomagnetic gradient correction coil 8 generates a magnetic field according to the current supplied from the coil power supply 6, and cancels the gradient magnetic field. The geomagnetic gradient correction coil 8 has, for example, a pair of geomagnetic gradient correction coils 8A and 8B. The pair of geomagnetic gradient correction coils 8A and 8B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of geomagnetic gradient correction coils 8A and 8B generate a magnetic field that is opposite to and has the same magnitude as the gradient magnetic field at the position of the optically excited magnetic sensor 1A according to the current supplied from the coil power supply 6. The directions of the magnetic field are, for example, the X-axis direction, the Y-axis direction, and the Z-axis direction. The gradient magnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude generated by the geomagnetic gradient correction coil 8. In this way, the geomagnetic gradient correction coil 8 corrects the gradient magnetic field at the position of the optically excited magnetic sensor 1A.

[0042] The geomagnetic gradient correction coil 8 also serves as a gradient magnetic field coil for generating a gradient magnetic field during MR image measurement. The geomagnetic gradient correction coil 8 generates a gradient magnetic field having selective gradients in the X-axis direction, the Y-axis direction, and the Z-axis direction according to the current supplied from the coil power supply 6.

[0043] The variable magnetic field correction coil 9 is a coil for correcting the variable magnetic field at the position of the optically excited magnetic sensor 1A. The variable magnetic field correction coil 9 generates a magnetic field in response to the current supplied from the coil power supply 6, and cancels the variable magnetic field. The variable magnetic field correction coil 9 has, for example, a pair of variable magnetic field correction coils 9A and 9B. The pair of variable magnetic field correction coils 9A and 9B are arranged to sandwich the optically excited magnetic sensor 1A (for example, on the left and right of the subject). The pair of variable magnetic field correction coils 9A and 9B generate a magnetic field that is opposite to and has the same magnitude as the variable magnetic field at the position of the optically excited magnetic sensor 1A in response to the current supplied from the coil power supply 6. The direction of the magnetic field is, for example, the X-axis direction, the Y-axis direction, and the Z-axis direction. The variable magnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field that is opposite to and has the same magnitude generated by the variable magnetic field correction coil 9. In this way, the variable magnetic field correction coil 9 corrects the variable magnetic field at the position of the optically excited magnetic sensor 1A.

[0044] The pump laser 10 is a laser device that generates pump light. The pump light emitted from the pump laser 10 is incident on each of the plurality of optically excited magnetic sensors 1A and the optically excited magnetic sensor 23A via fiber branching.

[0045] The probe laser 11 is a laser device that generates probe light. The probe light emitted from the probe laser 11 is incident on each of the multiple optically excited magnetic sensors 1A and the optically excited magnetic sensor 23A via fiber branching.

[0046] The amplifier 12A is a device or circuit that amplifies the signal resulting from the output from the OPM module 1 (specifically, the readout circuit 1C) and outputs the amplified signal to the control device 5.

[0047] The amplifier 12B is a device or circuit that amplifies the signal of the output result from the OPM module 23 (specifically, the readout circuit 23C) and outputs the amplified signal to the control device 5.

[0048] The heater controller 13 is a temperature control device connected to heaters for heating the cells of the photoexcited magnetic sensor 1A and the cells of the photoexcited magnetic sensor 23A, and to thermocouples (not shown) for measuring the temperatures of the respective cells. The heater controller 13 receives cell temperature information from the thermocouples, and adjusts the heating of the heater based on the temperature information to adjust the cell temperature.

[0049] The electromagnetic shield 14 is a shielding member that blocks electromagnetic noise of high frequencies (for example, 10 kHz or more), and is made of, for example, a mesh woven with metal threads, or a non-magnetic metal plate such as aluminum. The electromagnetic shield 14 is disposed so as to surround the OPM modules 1, 23, the transmitting coil 21, the receiving coil 22, the output coil 24, the geomagnetic correction magnetic sensor 2, the active shield magnetic sensor 3, the non-magnetic frame 4, the geomagnetic correction coil 7, the geomagnetic gradient correction coil 8, and the variable magnetic field correction coil 9. This electromagnetic shield 14 can prevent noise in the 300 kHz band, which is the measurement frequency, from entering the receiving coil 22 during MR image measurement, causing an increase in noise. In addition, it can prevent high-frequency noise from entering the optically excited magnetic sensor 1A during brain magnetic field measurement, causing the operation to become unstable.

[0050] [Brain measurement method (brain measurement operation) performed by the brain measurement device] Next, a brain measuring method using the brain measuring apparatus M1 according to the embodiment will be described with reference to Figures 3 and 4. Figures 3 and 4 are flowcharts showing the operation of the brain measuring apparatus M1.

[0051] First, when measurement of the brain magnetic field is started with the non-magnetic frame 4 attached to the subject, the geomagnetic correction magnetic sensor 2 measures the magnetic field related to the geomagnetism, which is a static magnetic field (step S11). The geomagnetic correction magnetic sensor 2 measures the geomagnetism and the gradient magnetic field at each position of the optically excited magnetic sensor 1A, and outputs the measurement values ​​to the control device 5.

[0052] The control device 5 and the coil power supply 6 control the current to the geomagnetic correction coil 7 (step S12). Based on the measurement value of the geomagnetic correction magnetic sensor 2, the control device 5 determines the current to the geomagnetic correction coil 7 so as to generate a magnetic field of the same magnitude and in the opposite direction to the geomagnetic field at the position of the optically excited magnetic sensor 1A. More specifically, the control device 5 determines the current to the geomagnetic correction coil 7 so that, for example, the average value of the measurement values ​​of the multiple geomagnetic correction magnetic sensors 2 approaches zero. The control device 5 outputs a control signal according to the determined current to the coil power supply 6. The coil power supply 6 outputs a predetermined current to the geomagnetic correction coil 7 in response to the control signal output by the control device 5. The geomagnetic correction coil 7 generates a magnetic field in response to the current supplied from the coil power supply 6. The geomagnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field of the same magnitude and in the opposite direction generated by the geomagnetic correction coil 7.

[0053] The control device 5 and the coil power supply 6 control the current to the geomagnetic gradient correction coil 8 (step S13). Based on the measurement value of the geomagnetic correction magnetic sensor 2, the control device 5 determines the current to the geomagnetic gradient correction coil 8 so as to generate a magnetic field having the same magnitude and in the opposite direction to the gradient magnetic field at the position of the optically excited magnetic sensor 1A. More specifically, the control device 5 determines the current to the geomagnetic gradient correction coil 8 so as to minimize the deviation from the average value of the measurement values ​​of the multiple geomagnetic correction magnetic sensors 2. The control device 5 outputs a control signal according to the determined current to the coil power supply 6. The coil power supply 6 outputs a predetermined current to the geomagnetic gradient correction coil 8 in response to the control signal output by the control device 5. The geomagnetic gradient correction coil 8 generates a magnetic field in response to the current supplied from the coil power supply 6. The gradient magnetic field at the position of the optically excited magnetic sensor 1A is canceled by the magnetic field generated by the geomagnetic gradient correction coil 8 having the same magnitude and in the opposite direction.

[0054] The control device 5 judges whether the measured value of the static magnetic field (magnetic field related to geomagnetism) after correction is equal to or less than the reference value (step S14). The measured value of the static magnetic field after correction is the value measured by the geomagnetic correction magnetic sensor 2 after the static magnetic field is corrected by the geomagnetic correction coil 7 and the geomagnetic gradient correction coil 8. The reference value is the magnitude of the magnetic field at which the optically excited magnetic sensor 1A operates normally, and may be, for example, 1 nT. If the measured value of the static magnetic field is not equal to or less than the reference value ("NO" in step S14), the process returns to step S11. If the measured value of the static magnetic field is equal to or less than the reference value ("YES" in step S14), the process proceeds to step S15.

[0055] The active shield magnetic sensor 3 measures the fluctuating magnetic field at each position of the optically excited magnetic sensor 1A, and outputs the measurement value to the control device 5 (step S15).

[0056] The control device 5 and the coil power supply 6 control the current to the variable magnetic field correction coil 9 (step S16). The control device 5 determines the current to the variable magnetic field correction coil 9 based on the measurement value of the active shield magnetic sensor 3 so as to generate a magnetic field having the same magnitude as and in the opposite direction to the variable magnetic field at the position of the optical excitation magnetic sensor 1A. More specifically, the control device 5 determines the current to the variable magnetic field correction coil 9 so that, for example, the average value of the measurement values ​​of the multiple active shield magnetic sensors 3 approaches zero. The control device 5 outputs a control signal according to the determined current to the coil power supply 6. The coil power supply 6 outputs a predetermined current to the variable magnetic field correction coil 9 in response to the control signal output by the control device 5. The variable magnetic field correction coil 9 generates a magnetic field in response to the current supplied from the coil power supply 6. The variable magnetic field at the position of the optical excitation magnetic sensor 1A is canceled by the magnetic field generated by the variable magnetic field correction coil 9 having the same magnitude as and in the opposite direction.

[0057] The control device 5 judges whether the measurement value of the variable magnetic field after correction is equal to or less than the reference value (step S17). The measurement value of the variable magnetic field after correction is the value measured by the active shield magnetic sensor 3 after the variable magnetic field is corrected by the variable magnetic field correction coil 9. The reference value is the noise level at which the brain magnetic field can be measured, and can be, for example, 1 pT. If the measurement value of the variable magnetic field is not equal to or less than the reference value ("NO" in step S17), the process returns to step S15. If the measurement value of the variable magnetic field is equal to or less than the reference value ("YES" in step S17), the process proceeds to step S18.

[0058] The optically excited magnetic sensor 1A measures the brain magnetic field (step S18). The control device 5 outputs the acquired measurement result to a predetermined output destination. The predetermined output destination may be the memory of the control device 5, a storage device such as a hard disk, an output device such as a display, or an external device such as a terminal device connected via a communication interface. Since the static magnetic field (magnetic field related to the geomagnetism) and the variable magnetic field at the position of the optically excited magnetic sensor 1A have been canceled out so as to be equal to or lower than a predetermined reference value, the optically excited magnetic sensor 1A can measure the brain magnetic field while avoiding the influence of the static magnetic field (magnetic field related to the geomagnetism) and the variable magnetic field.

[0059] 4, when the measurement of MR images is started while the non-magnetic frame 4 is still attached to the subject, the control device 5 determines the current to be supplied to the geomagnetic correction coil 7 for applying the static magnetic field, and controls the generation of the static magnetic field in the X-axis direction in the subject's head by outputting a control signal to the coil power supply 6 (step S19). Next, the control device 5 determines the current to be supplied to the geomagnetic gradient correction coil 8 for generating the gradient magnetic field, and controls the generation of the X-axis magnetic field gradient (dBx / dX) by outputting a control signal to the coil power supply 6 (step S20). At the same time, the control device 5 outputs a control signal to the transmission coil controller 15 for controlling the power to be supplied to the transmission coil 21, and controls the transmission pulse to be irradiated to the subject's head (step S21). This excites protons on a predetermined slice plane.

[0060] Furthermore, the control device 5 determines the current to be supplied to the geomagnetic gradient correction coil 8 for generating the gradient magnetic field, and outputs a control signal to the coil power supply 6 to control the generation of the Y-axis magnetic field gradient (dBx / dY) on the slice surface (step S22). This performs phase encoding. Then, the control device 5 determines the current to be supplied to the geomagnetic gradient correction coil 8 for generating the gradient magnetic field, and outputs a control signal to the coil power supply 6 to control the generation of the Z-axis magnetic field gradient (dBx / dZ) on the slice surface (step S23). This performs frequency encoding.

[0061] At the same time, the OPM module 23 outputs a nuclear magnetic resonance signal from the protons via the receiving coil 22 and the output coil 24, and the control device 5 acquires the nuclear magnetic resonance signal data accordingly (step S24). After that, the control device 5 judges whether to acquire nuclear magnetic resonance signal data related to other slice planes (step S25). If the result of the judgment is that nuclear magnetic resonance signal data related to other slice planes is to be acquired ("YES" in step S25), the process returns to step S20. On the other hand, if nuclear magnetic resonance signal data related to other slice planes is not to be acquired ("NO" in step S25), an MR image is acquired by Fourier transforming the nuclear magnetic resonance signal data acquired up to that point (step S26). The control device 5 outputs the acquired MR image to a predetermined output destination. The predetermined output destination may be a memory of the control device 5, a storage device such as a hard disk, an output device such as a display, or an external device such as a terminal device connected via a communication interface.

[0062] [Configuration of resonant adjustment circuit] As described above, the brain-measuring device M1 includes the resonance adjustment circuit 50 (see FIG. 1) as a component included in the MRI module. The following describes the details of the configuration of the resonance adjustment circuit 50. First, the purpose of providing the resonance adjustment circuit 50 will be described with reference to FIGS. 5 and 6.

[0063] FIG. 5 is a diagram showing an example of detection sensitivity according to the detection band. As described above, the brain-measuring device M1 may employ the OPM module 23 as the detection unit, or may employ an amplifier (not shown) as a substitute for the OPM module 23. FIG. 5(a) shows an example of detection sensitivity according to the detection band when an amplifier (not shown) is employed as the detection unit. FIG. 5(b) shows an example of detection sensitivity according to the detection band when the OPM module 23 is employed as the detection unit. In FIGS. 5(a) and (b), the horizontal axis indicates frequency (detection band), and the vertical axis indicates amplitude (detection sensitivity). In FIG. 5(a), the amplitude is shown in absolute value (mV), and in FIG. 5(b), the amplitude is shown in au. The frequency range of the MR signal (nuclear magnetic resonance signal) shown in FIGS. 5(a) and (b) is a range specified by frequency encoding.

[0064] Now, in the example shown in FIG. 5(a) where an amplifier (not shown) is used as the detector, there is a frequency region in the frequency range of the MR signal where the sensitivity (SNR) drops significantly. The FWHM, which is the spectral width at which the sensitivity is 50% of the peak value, is about 4.6 kHz. Also, in the example shown in FIG. 5(b) where an OPM module 23 is used as the detector, there is a frequency region in the frequency range of the MR signal where the sensitivity (SNR) drops significantly, and the FWHM is about 1060 Hz. Thus, in the detector, there is a frequency region in the frequency range of the MR signal where the sensitivity is not sufficient.

[0065] FIG. 6 is a diagram for explaining a frequency-encoded MR signal. By supplying a current to the geomagnetic gradient correction coil 8 for generating a gradient magnetic field, a Z-axis magnetic field gradient (dBx / dZ) is generated as shown in FIG. 6, and frequency encoding is performed. When frequency encoding is performed, the frequency band of the generated MR signal is determined according to the position in the Z-axis direction of the head. That is, as shown in FIG. 6, for example, the lowest side of the head is a range for generating MR signals having a frequency near f1, the upper side of that range is a range for generating MR signals having a frequency near f2 (f2>f1), the upper side of that range is a range for generating MR signals having a frequency near f3 (f3>f2), and the upper side of that range is a range for generating MR signals having a frequency near f4 (f4>f3). In this way, if the generated Z-axis magnetic field gradient and the position of the head are determined, the frequency band of the generated MR signal can be specified, so that by providing a resonance adjustment circuit 50 that extracts a signal of a specific frequency band (resonance frequency) for each position (region), it becomes possible to detect the frequency range of the MR signal without omission and with high sensitivity. Hereinafter, specific configuration examples of the resonance adjustment circuit 50 will be described with reference to FIGS.

[0066] FIG. 7 is a schematic diagram showing a part of the brain measuring device M1 including a plurality of resonance adjustment circuits 50. The plurality of resonance adjustment circuits 50 are provided corresponding to the plurality of receiving coils 22, respectively, and are circuits for extracting and outputting a signal of a predetermined resonance frequency of the current output from the receiving coil 22. In FIG. 7, the resonance adjustment circuits 50a, 50b, 50c, and 50d are illustrated as the plurality of resonance adjustment circuits 50. Note that the resonance adjustment circuits 50 are provided in one-to-one correspondence with, for example, each receiving coil 22, but FIG. 7 shows only a part of the resonance adjustment circuits 50, and only four resonance adjustment circuits 50a, 50b, 50c, and 50d are illustrated.

[0067] The resonance frequency of each of the resonance adjustment circuits 50 is set according to the position of the corresponding receiving coil 22 in the Z-axis direction and the magnetic field gradient in the Z-axis direction generated by controlling the gradient magnetic field. As described above, when frequency encoding is performed by controlling the gradient magnetic field, the frequency band of the MR signal changes along the Z-axis direction (see FIG. 6). Therefore, by setting the resonance frequency of the resonance adjustment circuit 50 in consideration of the position of the receiving coil 22 in the Z-axis direction and the magnetic field gradient, the MR signal output from the receiving coil 22 can be suitably extracted, and the MR signal can be detected with high sensitivity in the downstream OPM module 23. The receiving coil 22 has a diameter of about 30 to 50 mm and is arranged to surround the head. The receiving coils 22 arranged at the same height in the Z-axis direction have the same resonance frequency in the resonance adjustment circuit 50. For example, the resonance frequency of the receiving coil 22 at the center position of the head is set to 300 kHz, and the resonance frequency is shifted by 1.5 kHz for every 30 mm shift in the Z direction. This allows the MR signal emitted from the brain close to each receiving coil 22 to be detected efficiently. The receiver coil 22, which has a diameter of about 30 to 50 mm, has high sensitivity for capturing MR signals generated near the coil, but has low sensitivity to noise generated at positions far from the coil. Therefore, by arranging a large number of such receiver coils 22, the SNR of the MR image can be improved.

[0068] As shown in Fig. 7, OPM modules 23, which are multiple detectors, are provided corresponding to multiple resonance adjustment circuits 50 (resonance adjustment circuits 50a, 50b, 50c, and 50d in Fig. 7), respectively, and detect the resonance frequency signal output from the resonance adjustment circuit 50. The frequency band in which the detection sensitivity of the OPM module 23 is maximum includes the resonance frequency of the corresponding resonance adjustment circuit 50. The output coil 24 is fixed to face the OPM module 23. Each OPM module 23 may be provided in one-to-one correspondence with the resonance adjustment circuit 50, as shown in Fig. 7.

[0069] Alternatively, the multiple OPM modules 23 may include one OPM module 23 that detects all of the signals related to two or more receiving coils 22, each of which has the same resonant frequency related to the resonance adjustment circuit 50, by adjusting the phase. As described above, the resonance frequency of each resonance adjustment circuit 50 is determined taking into consideration the position of the corresponding receiving coil 22 in the Z-axis direction. Therefore, two or more resonance adjustment circuits 50 having the same resonant frequency can be rephrased as two or more resonance adjustment circuits 50 in which the positions of the corresponding receiving coils 22 in the Z-axis direction are the same (or similar). Simply, when signals related to two or more receiving coils 22 are detected by one OPM module 23, there is a risk that the signals will cancel each other out due to the difference in phase between these signals. Therefore, when signals related to two or more receiving coils 22 are detected by one OPM module 23, the phases of these signals are adjusted and detected so that they do not cancel each other out.

[0070] 8 and 9 are diagrams showing an example of the configuration of the resonance adjustment circuit 50. Fig. 8 shows an example of the resonance adjustment circuit 50 in which an amplifier (not shown) is used as the detection unit. Specifically, in and inductor L in In this parallel resonant circuit, a capacitor C is connected in parallel to a receiving coil 22 connected in series with a resistor R in =0.64Ω, inductor L in = 105μH, C = 1 / ω 2 L in From this equation, the value of the capacitor C is set so as to obtain a predetermined resonant frequency (for example, 300 kHz).

[0071] Fig. 9 shows an example of a resonance adjustment circuit 50 in which the OPM module 23 is used as the detection section. The resonance adjustment circuit 50 shown in Fig. 9 has an input side circuit 52 in which capacitors Cs1 and Cp1 are provided in series and parallel to the receiving coil 22, an output side circuit 55 in which capacitors Cs2 and Cp2 are provided in series and parallel to the output coil 24, and a coaxial cable 51 connecting the input side circuit 52 and the output side circuit 55. In the configuration shown in Fig. 9, the receiving coil 22 and the input side circuit 52 are surrounded by the same electromagnetic shield, the output side circuit 55 is surrounded by an aluminum case, and the output coil 24 and the OPM module 23 are surrounded by an OPM magnetic shield.

[0072] Fig. 10 is a diagram showing a detailed circuit configuration of the resonance adjustment circuit shown in Fig. 9. As shown in Fig. 10, the receiving coil 22 has a resistor R in and inductor L in The output coil 24 is connected in series with a resistor R out and inductor L out As described above, the resonance adjustment circuit 50 has an input circuit 52 in which the capacitors Cs1 and Cp1 are provided in series and parallel to the receiving coil 22, an output circuit 55 in which the capacitors Cs2 and Cp2 are provided in series and parallel to the output coil 24, and a coaxial cable 51 connecting the input circuit 52 and the output circuit 55.

[0073] In such a resonance adjustment circuit 50, the capacitances of the capacitors Cs1 and Cp1 of the input side circuit 52 are adjusted so that the input impedance, which is the impedance when the input side circuit 52 and the receiving coil 22 are seen from the coaxial cable 51, becomes the same as the impedance of the coaxial cable 51 at the resonance frequency. Also, in the resonance adjustment circuit 50, the capacitances of the capacitors Cs2 and Cp2 of the output side circuit 55 are adjusted so that the output impedance, which is the impedance when the output side circuit 55 and the output coil 24 are seen from the coaxial cable 51, becomes the same as the impedance of the coaxial cable 51 at the resonance frequency. By adjusting the capacitances of the capacitors Cs1 and Cp1 of the input side circuit 52 and the capacitances of the capacitors Cs2 and Cp2 of the output side circuit 55 in this way, both the input impedance and the output impedance when seen from the coaxial cable 51 become the same as the impedance of the coaxial cable 51 at the resonance frequency, so that the signal reflection of the signal component at the resonance frequency can be suppressed. The impedance of the coaxial cable 51 is, for example, 50Ω.

[0074] 10, specifically, the capacitances of the capacitors Cs1 and Cp1 of the input side circuit 52 are calculated so as to satisfy the following formulas (1) and (2). On the right side of formula (1), 50 indicates the impedance of the coaxial cable 51. ω indicates the angular frequency, which is calculated from the resonance frequency. The capacitances of the capacitors Cs2 and Cp2 of the output side circuit 55 are also calculated based on the same formula. Z+1 / jωCs1=50 (1) Z=(1 / jωCp1) / / (R in +jωL in ) (2)

[0075] Now, for example, L in = 105.7μH, R in =0.77Ω, L OUT = 3.0μH, R OUT = 0.18 Ω, and the resonant frequency = 300 kHz, then from the above equations (1) and (2), it is found that Cp1 = 2.3 nF, Cs1 = 330 pF, Cp2 = 89 nF, and Cs2 = 6.6 nF.

[0076] Next, the effects of the brain-measuring apparatus M1 according to this embodiment will be described.

[0077] In the brain measuring device M1 according to the present embodiment, a resonance adjustment circuit 50 is provided that extracts a signal of a predetermined resonance frequency of the current output from the receiving coil 22. The resonance frequency of each of the multiple resonance adjustment circuits 50 is set according to the position of the corresponding receiving coil 22 in the Z-axis direction and the magnetic field gradient in the Z-axis direction. In this way, the resonance frequency of the resonance adjustment circuit 50 is set in consideration of the position of the corresponding receiving coil 22 and the generated magnetic field gradient, so that the sensitivity of the signal detected in the OPM module 23 provided corresponding to the resonance adjustment circuit 50 can be improved. Here, when the sensitivity is improved by the resonance adjustment circuit 50, the frequency band is limited to about several kHz. In this regard, in the brain measuring device M1 according to the present embodiment, since the multiple receiving coils 22 having different resonance frequencies are arranged according to the magnetic field gradient, it is possible to secure a sufficient frequency band as a whole, that is, FOV (Field of View), while improving the sensitivity. As described above, according to the brain measuring device M1 according to the present embodiment, it is possible to improve the detection sensitivity and secure a sufficient FOV.

[0078] The multiple OPM modules 23 may include one OPM module 23 that detects all of the signals associated with two or more receive coils 22, which have the same resonant frequency associated with the resonance adjustment circuit 50, by adjusting the phase. In this way, signals associated with receive coils 22, which have the same resonant frequency, are detected by the same OPM module 23 while adjusting the phase, thereby reducing the number of OPM modules 23 that are detection units and enabling brain measurement to be performed with a simple configuration.

[0079] The frequency band in which the detection sensitivity of the OPM module 23 is maximized may include the resonance frequency of the corresponding resonance adjustment circuit 50. This can further improve the detection sensitivity of the detection section.

[0080] The receiving coil 22 may be formed so that the maximum area and the number of turns in a plane perpendicular to the X-axis direction in which the static magnetic field is generated are constant. With this configuration, the sensitivity of the receiving coil 22 becomes uniform, and the brain measurement can be performed more appropriately.

[0081] In the resonance adjustment circuit 50 of the brain measuring device M1 according to this embodiment, the capacitances of the capacitors Cs1 and Cp1 of the input circuit 52 are adjusted so that the input impedance, which is the impedance when the coaxial cable 51 is seen from the input circuit 52 and the receiving coil 22 side, is the same as the impedance of the coaxial cable 51 at the resonance frequency, and the capacitances of the capacitors Cs2 and Cp2 of the output circuit 55 may be adjusted so that the output impedance, which is the impedance when the coaxial cable 51 is seen from the output circuit 55 and the output coil 24 side, is the same as the impedance of the coaxial cable 51 at the resonance frequency. In this way, the capacitances of the capacitors Cs1, Cp1, Cs2, and Cp2 of the input circuit 52 and the output circuit 55 are adjusted so that the input impedance when the coaxial cable 51 is seen from the input circuit 52 and the receiving coil 22 side and the output impedance when the coaxial cable 51 is seen from the output circuit 55 and the output coil 24 side are both the same as the impedance of the coaxial cable 51 at the resonance frequency. With this configuration, at the resonant frequency, both the input impedance and the output impedance seen from the coaxial cable 51 are the same as the impedance of the coaxial cable 51, so that power loss due to signal reflection can be suppressed for the resonant frequency component of the signal, and the detection sensitivity of the OPM module 23 can be improved.

[0082] FIG. 11 is a diagram for explaining the action and effect of the brain-measuring device M1 according to the embodiment. FIG. 11(a) shows the noise level in a series resonance configuration (configuration according to a comparative example) in which a capacitor is connected in series between the receiving coil 22 and the output coil 24. FIG. 11(b) shows the noise level in a configuration including the resonance adjustment circuit 50 according to this embodiment (the resonance adjustment circuit 50 shown in FIG. 9 and FIG. 10). In FIG. 11(a) and (b), the horizontal axis shows frequency, and the vertical axis shows magnetic noise density. As shown in FIG. 11(a), in the configuration according to the comparative example, the noise level is 23 fT / Hz when the frequency is 300 kHz. 1 / 2 In contrast, as shown in FIG. 11(b), in the configuration according to this embodiment, the noise level is 2.1 fT / Hz when the frequency is 300 kHz. 1 / 2 As described above, the configuration according to this embodiment can greatly improve the SNR compared to the configuration according to the comparative example. That is, the configuration according to this embodiment can suppress signal reflection between the FT input / output coils (the receiving coil 22 and the output coil 24) and increase the FT gain, thereby increasing the sensitivity of the OPM module 23. Furthermore, signal loss can be reduced even if the coaxial cable 51 between the FT input / output coils is lengthened, so that the degree of freedom in arranging the input / output coils and the OPM module 23 is increased.

[0083] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. For example, the brain measuring device M1 including the resonance adjustment circuit 50 has been described, but the resonance adjustment circuit 50 may be provided in other AC magnetic field measuring devices other than the brain measuring device M1.

[0084] The configuration of the resonance adjustment circuit is not limited to the configuration of FIG. 10. FIG. 12 is a diagram showing a detailed circuit configuration of a resonance adjustment circuit 500 according to a modified example. As shown in FIG. 12, the resonance adjustment circuit 500 provided between the receiving coil 22 and the output coil 240 has an impedance matching circuit 260 connected to the receiving coil 22. The impedance matching circuit 260 makes the rear stage side (the coaxial cable 51 side) as viewed from the receiving coil 22 high impedance. By providing such an impedance matching circuit 260, it is possible to reduce the current flowing through the coil, and inductive coupling between the coils can be reduced when a coil array is used. Even in such a configuration, it is possible to efficiently pass a current through the output coil 240 by matching the impedance between the coaxial cable 51 and the output coil 240.

[0085] The receiving coil may be configured as a phased array coil as shown in Fig. 13. Fig. 13 is a diagram showing the configuration of a receiving coil (phased array coil) according to a modified example. In the phased array coil configuration shown in Fig. 13, the coils 601 are overlapped with each other in consideration of interference between adjacent coils 601, 601. In this way, by arranging the coils 601 with a certain overlap, it is possible to cancel out the induced electromotive forces from the adjacent coils 601 and to minimize the electrical interference. [Explanation of symbols]

[0086] 5...control device, 7...geomagnetic correction coil (static magnetic field coil), 8...geomagnetic gradient correction coil (gradient magnetic field coil), 21...transmitting coil, 22...receiving coil, 23...OPM module (detection unit), 50...resonance adjustment circuit, 51...coaxial cable, 52...input side circuit, 55...output side circuit, Cs1, Cp1, Cs2, Cp2...capacitors, M1...brain measuring device (AC magnetic field measuring device).

Claims

1. a receiving coil for detecting the signal and converting it into a current; a resonance adjustment circuit provided in correspondence with the receiving coil for outputting a predetermined frequency of a current output from the receiving coil; an output coil for converting a current output from the resonance adjustment circuit into a magnetic signal; an optically excited magnetic sensor that detects the magnetic signal output by the output coil; the resonance adjustment circuit includes an input circuit having a capacitor connected in series and parallel to the receiving coil, an output circuit having a capacitor connected in series and parallel to the output coil, and a coaxial cable connecting the input circuit and the output circuit, a capacitance of a capacitor of the input side circuit is adjusted so that an input impedance, which is an impedance when the input side circuit and the receiving coil are viewed from the coaxial cable, becomes equal to an impedance of the coaxial cable at the predetermined frequency; An AC magnetic field measuring device in which the capacitance of the capacitor of the output side circuit is adjusted so that the output impedance, which is the impedance when looking at the output side circuit and the output coil from the coaxial cable, is the same as the impedance of the coaxial cable at the specified frequency.

2. a transmission coil for transmitting a transmission pulse of a predetermined frequency; and a control device for generating an MR image based on a signal detected by the optically excited magnetic sensor.

2. The AC magnetic field measuring device according to claim 1, wherein the receiving coil detects a nuclear magnetic resonance signal generated by transmission of the transmission pulse and converts the signal into a current.

3. 3. The AC magnetic field measuring device according to claim 2, wherein the frequency band in which the detection sensitivity of said optically excited magnetic sensor is maximized includes a resonance frequency.

Images