Magnetic measurement device

Abstract

To reduce a current value and a frequency of current flowing to an excitation coil in a magnetic measuring device of a type for detecting a change in magnetization generated by exciting a measurement object.SOLUTION: A magnetic measuring device 1 includes an excitation coil C1 capable of linearly responding to a change in magnetization of a magnetic body P by applying an AC excitation magnetic field to a measurement object including the magnetic body P, a detection coil C0 for generating a primary detection signal S1 by detecting a primary AC detection magnetic field generated by a change in magnetization of the magnetic body P, a magnetic field generation coil C2 for generating a secondary AC detection magnetic field on the basis of the primary detection signal S1, and a magnetic sensor 16 for generating a secondary detection signal S2 including a non-sinusoidal wave component by detecting a secondary AC detection magnetic field. Accordingly, without a nonlinear response to a change in magnetization of the magnetic body P, the secondary detection signal S2 including a non-sinusoidal wave component can be obtained, thus reducing a current value and a frequency of current flowing to an excitation coil.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a magnetic measurement device, and more particularly to a magnetic measurement device that can be used for magnetic particle imaging. [Background technology]

[0002] A magnetic particle imaging device is known as a type of magnetic measurement device that detects magnetization changes caused by exciting a measurement object (see Non-Patent Document 1). A magnetic particle imaging device is equipped with an excitation coil that applies an AC excitation magnetic field to a measurement object containing magnetic particles, and a magnetic sensor that detects an AC detection magnetic field caused by magnetization changes of the excited magnetic particles. The magnetic sensor is applied with not only an AC detection magnetic field but also an AC excitation magnetic field, but by making the magnetization changes of the magnetic particles respond nonlinearly, it is possible to separate the detection signal component and excitation component (noise component) contained in the output signal of the magnetic sensor by signal processing. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] B. Gleich and J. Weizenecker, Nature, 435, 1214 (2005) Summary of the Invention [Problem to be solved by the invention]

[0004] However, to make the magnetization change of a magnetic body such as a magnetic particle respond nonlinearly, it is necessary to apply a very strong AC excitation magnetic field to the measurement object. Moreover, to detect the magnetization change caused by the nonlinear response with high sensitivity, the frequency of the AC excitation magnetic field must be set to a high frequency, for example, 20 kHz or higher. Therefore, when the measurement object is relatively large, such as the human body, an extremely large high-frequency current must be passed through the excitation coil, which is not practical.

[0005] Therefore, an object of the present invention is to reduce the current value and frequency of the current flowing through the excitation coil in a magnetic measurement device of the type that detects a change in magnetization caused by exciting an object to be measured. [Means for solving the problem]

[0006] The magnetic measuring device according to the present invention is characterized by comprising a first coil that applies an AC excitation magnetic field to a measurement object including a magnetic body to cause a linear response of the magnetization change of the magnetic body; a first magnetic sensor that generates a primary detection signal by detecting a primary AC detection magnetic field generated by the magnetization change of the magnetic body; a second coil that generates a secondary AC detection magnetic field based on the primary detection signal; and a second magnetic sensor that generates a secondary detection signal including a non-sinusoidal component by detecting the secondary AC detection magnetic field.

[0007] According to the present invention, the primary detection signal generated by the first magnetic sensor is converted back into a magnetic field, and this magnetic field is detected by the second magnetic sensor, so that the secondary detection signal containing a non-sinusoidal component can be obtained without causing the magnetization change of the magnetic body to respond nonlinearly. This makes it possible to detect the magnetization change of the magnetic body with high sensitivity even if the current value and frequency of the current flowing through the first coil are small.

[0008] The magnetic measurement device according to the present invention may further include a third coil for canceling the AC excitation magnetic field applied to the first magnetic sensor, thereby reducing noise components contained in the primary detection signal.

[0009] The magnetic measurement device according to the present invention may further include a signal processing circuit that detects harmonic components of the secondary detection signal, thereby making it possible to remove noise components contained in the secondary detection signal. [Effects of the Invention]

[0010] Thus, according to the present invention, in a magnetic measurement device of the type that detects magnetization changes caused by exciting an object to be measured, it is possible to reduce the current value and frequency of the current flowing through the excitation coil. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram for explaining the configuration of a magnetic measurement device 1 according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram for explaining the change in magnetization of the magnetic body P. As shown in FIG. [Figure 3] FIG. 3 is a circuit diagram of the magnetic sensor 16. [Figure 4] FIG. 4 is a schematic graph for explaining the change in magnetization of the magnetic body P, in which the vertical axis represents the magnetization M and the horizontal axis represents the magnetic field H. [Figure 5] FIG. 5 is a schematic graph for explaining the change in the secondary detection signal S2, in which the vertical axis represents voltage V and the horizontal axis represents magnetic field H. [Figure 6] FIG. 6 is a graph showing the waveforms of each signal, where (a) shows the waveform of the AC excitation current i1, (b) shows the waveform of the primary detection signal S1, (c) shows the waveform of the secondary detection signal S2, and (d) to (h) show the waveforms of the third, fifth, seventh, ninth, and eleventh harmonics contained in the secondary detection signal S2. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0013] FIG. 1 is a schematic diagram for explaining the configuration of a magnetic measurement device 1 according to an embodiment of the present invention.

[0014] The magnetic measurement device 1 according to this embodiment is a device for detecting a magnetic body P within a measurement object located in a measurement region A, and as shown in FIG. 1, includes magnets 11 and 12 that generate a gradient DC magnetic field φ, an excitation circuit 13 connected to an excitation coil C1, and a detection coil C0 that detects a primary AC detection magnetic field generated by a change in magnetization of the magnetic body P. The magnetic body P may be a nano-sized magnetic nanoparticle. If magnetic nanoparticles are used as the magnetic body P, the measurement object can be the human body.

[0015] The magnets 11 and 12 are arranged with their south and north poles facing each other so that the intensity of the gradient DC magnetic field φ in the measurement area A is nearly zero. Coils may be used instead of the magnets 11 and 12. A mechanism for spatially moving the measurement area A may also be provided. The excitation circuit 13 is a circuit that passes an AC excitation current i1 through the excitation coil C1, thereby applying an AC excitation magnetic field to the measurement area A. The waveform of the AC excitation current i1 is sinusoidal. As will be described later, the intensity of the AC excitation magnetic field is set to an intensity at which the magnetization change of the magnetic body P located in the measurement area A responds linearly. A linear response means that the magnetization change of the magnetic body P occurs in a non-saturated region. Therefore, the magnetization change is not necessarily completely linear, and as long as the magnetization change occurs in a non-saturated region, some non-linear components may be included.

[0016] FIG. 2 is a schematic diagram for explaining the change in magnetization of the magnetic body P. As shown in FIG.

[0017] In the example shown in FIG. 2(a), when an excitation magnetic field is applied to the magnetic body P, the magnetic body P, which has magnetization M in a predetermined direction, rotates, resulting in a change in the direction of the magnetization M. In addition, in the example shown in FIG. 2(b), when an excitation magnetic field is applied to the magnetic body P, the magnetization M inside the magnetic body P rotates. In either of these cases, applying an excitation magnetic field to the magnetic body P changes the direction of the magnetization M of the magnetic body P. Furthermore, application of an excitation magnetic field may cause both the rotation of the magnetic body P itself shown in FIG. 2(a) and the rotation of the magnetization M inside the magnetic body P shown in FIG. 2(b). In this embodiment, the change in the direction of the magnetization M caused by applying an excitation magnetic field to the magnetic body P is defined as a "magnetization change."

[0018] The magnetization change of the magnetic body P generates a primary AC detection magnetic field. The primary AC detection magnetic field is detected by a detection coil C0, which is a first magnetic sensor, and a primary detection signal S1 is generated. In this embodiment, the detection coil C0 is used as the magnetic sensor that detects the primary AC detection magnetic field, but the magnetic sensor that detects the primary AC detection magnetic field is not limited to this and may be a magnetic sensor using a magneto-sensitive element. In addition, the AC excitation magnetic field is also applied to the magnetic body P located outside the measurement area A, but since the direction of the magnetization M in the area outside the measurement area A is fixed by the inclined DC magnetic field φ having a predetermined strength, substantially no magnetization change occurs. Therefore, the detection coil C0 can selectively detect the magnetization change of the magnetic body P located in the measurement area A.

[0019] Furthermore, the AC excitation magnetic field applied to the detection coil C0 is canceled by the cancel coil C3. A cancel current i3 flows through the cancel coil C3 by the compensation circuit 14, thereby canceling out the AC excitation magnetic field applied to the detection coil C0. However, it is difficult to completely cancel out the AC excitation magnetic field applied to the detection coil C0, and some noise components resulting from the AC excitation magnetic field are superimposed on the primary detection signal S1.

[0020] The primary detection signal S1 is input to the amplifier circuit 15. The amplifier circuit 15 is an analog circuit including a differential amplifier, a filter circuit, etc., and supplies an AC detection current i2 to the magnetic field generating coil C2 based on the primary detection signal S1. This causes a secondary AC detection magnetic field to be generated from the magnetic field generating coil C2. Here, since the amplifier circuit 15 is an analog circuit, there is almost no delay, and the secondary AC detection magnetic field is generated in almost real time in response to the primary AC detection magnetic field. The secondary AC detection magnetic field is detected by the second magnetic sensor 16, and a secondary detection signal S2 is generated.

[0021] FIG. 3 is a circuit diagram of the magnetic sensor 16.

[0022] As shown in FIG. 3, the magnetic sensor 16 is composed of magnetically sensitive elements 21-24 connected in a full bridge configuration. The magnetically sensitive elements 21-24 may be magnetoresistive elements such as TMR (tunnel magnetoresistance) elements, GMR (giant magnetoresistance) elements, or AMR (anisotropic magnetoresistance) elements, as well as Hall elements and MI (magneto-impedance) elements, which are highly sensitive even at low frequencies and undergo magnetic saturation. The magnetic sensor 16 is configured so that the secondary AC detection magnetic field generated by the magnetic field generating coil C2 is applied to the magnetically sensitive elements 21 and 22 and the magnetically sensitive elements 23 and 24 in opposite directions. This causes the magnetic sensor 16 to output a secondary detection signal S2 corresponding to the secondary AC detection magnetic field. The magnetic sensor 16 is not limited to a full-bridge configuration of four magnetically sensitive elements, but may also be a half-bridge configuration of two magnetically sensitive elements, or a single magnetically sensitive element.

[0023] The secondary detection signal S2 is supplied to a signal processing circuit 18 via an amplifier 17. The signal processing circuit 18 generates a tertiary detection signal S3 by extracting harmonic components contained in the secondary detection signal S2. The tertiary detection signal S3 is the final output signal of the magnetic measurement device 1 according to this embodiment, and indicates a change in magnetization of the magnetic body P located in the measurement region A. Then, by using an imaging device that images the tertiary detection signal S3, it is possible to configure a magnetic particle imaging device.

[0024] The above is the configuration of the magnetic measurement device 1 according to this embodiment. Next, the operation of the magnetic measurement device 1 according to this embodiment will be described.

[0025] First, the excitation circuit 13 applies an AC excitation current i1 to the excitation coil C1 so that the magnetization change of the magnetic body P located in the measurement area A responds linearly. In other words, the AC excitation current i1 supplied to the excitation coil C1 is set to a current amount that is sufficiently smaller than the current amount required to cause the magnetization change of the magnetic body P to respond nonlinearly.

[0026] FIG. 4 is a schematic graph for explaining the change in magnetization of the magnetic body P, in which the vertical axis represents the magnetization M and the horizontal axis represents the magnetic field H.

[0027] As shown in Figure 4, when the amplitude of the magnetic field H is set to H1, the magnetization M of the magnetic body P is saturated, and the magnetization M of the magnetic body P responds nonlinearly between magnetization m1 and magnetization m2. In this case, the detection signal component contained in the primary detection signal S1 becomes a non-sinusoidal wave. Here, if the measurement object is the size of a human body, a strong magnetic field of approximately 6 mT is required to cause the magnetization M of the magnetic body P made of magnetic nanoparticles to respond nonlinearly. On the other hand, when the amplitude of the magnetic field H is set to H2(

[0028] In this way, the amount of AC excitation current i1 is kept to a current amount at which the magnetization change of the magnetic body P responds linearly, and therefore the amount of AC excitation current i1 is significantly reduced compared to when the magnetization change of the magnetic body P responds nonlinearly. Here, if the object to be measured is the size of a human body, a magnetic field of, for example, 0.1 mT is sufficient to cause the magnetization M of the magnetic body P made of magnetic nanoparticles to respond linearly. In other words, the amount of current is less than 1 / 10 of when the magnetization change of the magnetic body P responds nonlinearly.

[0029] As described above, in this embodiment, the excitation coil C1 linearly responds to the magnetization change of the magnetic body P, and therefore the detection signal component of the primary detection signal S1 generated by the detection coil C0, which is caused by the primary AC detection magnetic field, is a sine wave. The primary detection signal S1 also contains noise components caused by the AC excitation magnetic field that have not been completely canceled. However, because the noise components are sufficiently suppressed by the cancellation coil C3, their level is sufficiently small, and the detection signal component is dominant. The primary detection signal S1 is converted into an AC detection current i2 by the amplifier circuit 15, which generates a secondary AC detection magnetic field from the magnetic field generating coil C2. The secondary AC detection magnetic field is detected by the magnetic sensor 16, and a secondary detection signal S2 is generated.

[0030] ​FIG. 5 is a schematic graph for explaining the change in the secondary detection signal S2, where the vertical axis represents voltage V and the horizontal axis represents magnetic field H.

[0031] As shown in FIG. 5, the amplitude of the detection signal component included in the secondary AC detection magnetic field is H3. For the component with an amplitude of H3 in the secondary AC detection magnetic field, the magnetoresistance effect of the magnetosensing elements 21 to 24 saturates, and the voltage V of the secondary detection signal S2 responds non-linearly between voltage v1 and voltage v2. In this case, the detection signal component included in the secondary detection signal S2 becomes a non-sine wave. On the other hand, the amplitude of the noise component included in the secondary AC detection magnetic field is H4 (<H3). For the component with an amplitude of H4 in the secondary AC detection magnetic field, since the magnetosensing elements 21 to 24 operate in the non-saturation region, the voltage V of the secondary detection signal S2 responds linearly between voltage v3 and voltage v4.

[0032] Here, in order to make the magnetosensing elements 21 to 24 respond non-linearly to the detection signal component and linearly to the noise component, a preliminary magnetic measurement operation is performed in a state where the magnetic body P does not exist in the measurement region A, that is, in a state where the detection signal component is not included. The gain and filter characteristics of the amplifier circuit 15 can be adjusted so that the magnetosensing elements 21 to 24 respond linearly to the noise component caused by the AC excitation magnetic field.

[0033] Thereby, the detection signal component included in the secondary AC detection magnetic field is converted into a non-sine wave component of the secondary detection signal S2, and the noise component included in the secondary AC detection magnetic field is converted into a sine wave component of the secondary detection signal S2. That is, although both the detection signal component and the noise component included in the primary detection signal S1 are sine waves, they are converted back into a magnetic field using the magnetic field generating coil C2, and further reconverted into the secondary detection signal S2 using the magnetic sensor 16, so that the detection signal component and the noise component are separated into non-sine wave components and sine wave components.

[0034] The secondary detection signal S2 generated in this manner is supplied to a signal processing circuit 18 via an amplifier 17. The signal processing circuit 18 generates a tertiary detection signal S3 by extracting harmonic components contained in the secondary detection signal S2. As described above, the detection signal components contained in the secondary detection signal S2 are composed of non-sinusoidal components, resulting in the generation of harmonics. In contrast, the noise components contained in the secondary detection signal S2 are composed of sinusoidal components, resulting in almost no harmonics. Therefore, by detecting the harmonic components contained in the secondary detection signal S2, it is possible to selectively extract the detection signal components.

[0035] 6 is a graph showing the waveforms of each signal, with (a) showing the waveform of the AC excitation current i1, (b) showing the waveform of the primary detection signal S1, (c) showing the waveform of the secondary detection signal S2, and (d) to (h) showing the waveforms of the third, fifth, seventh, ninth, and eleventh harmonics included in the secondary detection signal S2. In addition, in FIGS. 6(b) to (h), the solid lines show the detection signal components, and the dashed lines show the noise components.

[0036] As shown in FIG. 6(a), the AC excitation current i1 is a sine wave. In this embodiment, the AC excitation magnetic field causes the magnetization M of the magnetic body P to respond linearly. Therefore, as shown in FIG. 6(b), the detection signal component and noise component included in the primary detection signal S1 are both sine waves. However, in this embodiment, the magnetic field generating coil C2 and the magnetic sensor 16 are used to change the detection signal component into a non-sine wave. Therefore, as shown in FIG. 6(c), the detection signal component included in the secondary detection signal S2 is non-sine wave, and the noise component included in the secondary detection signal S2 is sine wave. As a result, as shown in FIGS. 6(d) to 6(h), large harmonics appear in the detection signal component, while almost no harmonics appear in the noise component. As an example, if the ratio (SN ratio) of the detection signal component to the noise component contained in the primary detection signal S1 is 9.6 dB, the SN ratios of the third harmonic, fifth harmonic, seventh harmonic, ninth harmonic, and eleventh harmonic contained in the secondary detection signal S2 are 11.1 dB, 15.3 dB, 14.7 dB, 13.3 dB, and 9.0 dB, respectively.

[0037] Therefore, by extracting predetermined harmonic components from the secondary detection signal S2 using the signal processing circuit 18, it becomes possible to extract the detection signal components resulting from the magnetization change of the magnetic body P. The detection signal components extracted in this way are output to the outside as the tertiary detection signal S3.

[0038] As described above, the magnetic measurement device 1 according to this embodiment linearly responds to the magnetization M of the magnetic body P using an AC excitation magnetic field, while selectively converting the detection signal component into a non-sinusoidal wave using the magnetic field generating coil C2 and the magnetic sensor 16. This not only makes it possible to significantly reduce the amount of AC excitation current i1, but also ensures a sufficient signal-to-noise ratio even when the frequency of AC excitation current i1 is reduced to approximately 10 kHz. Moreover, because the primary detection signal S1 is converted into the secondary detection signal S2 using a physical device, there is no delay, unlike when the primary detection signal S1 is directly processed. This enables magnetic particle imaging of a measurement object that is relatively large, such as a human body.

[0039] The above describes a preferred embodiment of the present invention, but the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the present invention, and it goes without saying that these modifications are also included within the scope of the present invention. [Explanation of symbols]

[0040] 1. Magnetic measurement device 11,12 Magnets 13 Excitation circuit 14 Compensation circuit 15 Amplifier circuit 16 Magnetic Sensor 17 Amplifier 18 Signal Processing Circuit 21~24 Magnetic sensing element A Measurement area C0 detection coil C1 Excitation coil C2 magnetic field generating coil C3 Cancellation coil P magnetic material S1 Primary detection signal S2 Secondary detection signal S3 Third detection signal i1 AC excitation current i2 AC detection current i3 Cancellation current φ Gradient DC magnetic field

Claims

[Claim 1] a first coil that applies an AC excitation magnetic field to a measurement object including a magnetic body, the AC excitation magnetic field having an intensity at which the magnetization of the magnetic body changes in a non-saturated region, thereby causing a linear response of the magnetization change of the magnetic body; a first magnetic sensor that detects a primary AC detection magnetic field generated by a change in magnetization of the magnetic body, thereby generating a primary detection signal; an amplifier circuit that generates an AC detection current based on the primary detection signal; a second coil receiving the AC detection current and generating a secondary AC detection magnetic field; a second magnetic sensor that detects the secondary AC detection magnetic field to generate a secondary detection signal including a non-sinusoidal component and a sinusoidal component; a signal processing circuit for detecting the secondary detection signal; a third coil that receives a cancellation current supplied from the compensation circuit and cancels the AC excitation magnetic field applied to the first magnetic sensor; the primary detection signal includes a detection signal component caused by the primary AC detection magnetic field and a noise component caused by the AC excitation magnetic field that is smaller than the detection signal component and not completely canceled by the third coil, the secondary AC detection magnetic field includes a detection signal component corresponding to the detection signal component included in the primary detection signal, and a noise component corresponding to the noise component included in the primary detection signal, a gain of the amplifier circuit is adjusted so that the detection signal component included in the secondary AC detection magnetic field is converted into the non-sine wave component of the secondary detection signal by the second magnetic sensor operating in a saturated region, and the noise component included in the secondary AC detection magnetic field is converted into the sine wave component of the secondary detection signal by the second magnetic sensor operating in a non-saturated region; The signal processing circuit detects harmonic components of the non-sinusoidal components contained in the secondary detection signal, thereby generating a tertiary detection signal resulting from a change in magnetization of the magnetic body.

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