Magnetic microparticle imaging device

Abstract

The alternating-current magnetic field applying coil (3) generates an alternating-current magnetic field that changes the magnetism of the magnetic microparticles (1). The winding frame (6) holds the alternating-current magnetic field applying coil (3). The DC magnetic field applicator generates a region having a low magnetic field strength such that only the magnetism of the magnetic microparticles (1) in an arbitrary region of the object to be inspected (2) is changed. Measurement coils (4a, 4b) detect magnetic changes in the magnetic microparticles (1). At least one of the thickness of the hollow cylinder of the winding frame (6) in the radial direction and the thickness of the flange of the winding frame (6) is greater than or equal to the first skin thickness and less than or equal to the second skin thickness. The first skin thickness is determined by the frequency of the third harmonic of the AC magnetic field excited by the AC magnetic field application coil (3) and the conductivity and magnetic permeability of the winding frame (6). The second skin thickness is determined by the frequency of the fundamental wave of the AC magnetic field excited by the AC magnetic field application coil (3) and the conductivity and magnetic permeability of the winding frame (6).

Description

Technical Field

[0001] The present disclosure relates to a magnetic microparticle imaging device. Background Art

[0002] A magnetic particle detection device having an AC magnetic field applying coil and a DC magnetic field applying unit is known (for example, refer to Patent Document 1). Only the magnetic particles existing around the zero magnetic field area formed by the DC magnetic field applying unit undergo magnetization changes due to the excitation magnetic field, so by scanning the relative position of the zero magnetic field area and the magnetic particles, the distribution information of the magnetic particles can be obtained. The distribution of the magnetic particles contained in the inspected object is obtained by scanning the zero magnetic field area, thereby performing imaging. The magnetic particle detection device of Patent Document 1 suppresses the fundamental component of the signal output from the measuring coil for detecting the magnetic field, and only extracts the higher harmonic components.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2003-199767 Summary of the invention

[0006] In the magnetic particle imaging device described in Patent Document 1, in the AC magnetic field applying coil, due to the characteristics of the connected AC power supply, not only the fundamental current but also the harmonic current flows. As a result, the AC magnetic field of the excitation also contains harmonics. Therefore, the measuring coil will simultaneously measure the harmonics output from the magnetic particles and the harmonics excited by the AC magnetic field applying coil. The measuring coil cannot measure only the harmonics output from the magnetic particles that are originally intended to be measured, so the detection accuracy of the magnetic particles is deteriorated.

[0007] Therefore, an object of the present disclosure is to provide a magnetic microparticle imaging device having higher detection accuracy than before.

[0008] The magnetic particle imaging device involved in the present disclosure is a magnetic particle imaging device for determining the spatial distribution of magnetic particles in an object to be inspected in an inspection area, wherein the device comprises: an AC magnetic field applying coil for generating an AC magnetic field that changes the magnetism of magnetic particles; a winding frame for holding the AC magnetic field applying coil; a DC magnetic field applicator for generating an area with a low magnetic field intensity in a manner that changes the magnetism of magnetic particles in only an arbitrary area of ​​the object to be inspected; and a measuring coil for detecting the change in magnetism of the magnetic particles. At least one of the radial thickness of the hollow cylinder of the winding frame and the thickness of the flange of the winding frame is greater than the first epidermal thickness and less than the second epidermal thickness. The first epidermal thickness is determined by the frequency of the third harmonic of the AC magnetic field excited by the AC magnetic field applying coil and the conductivity and permeability of the winding frame. The second epidermal thickness is determined by the frequency of the fundamental wave of the AC magnetic field excited by the AC magnetic field applying coil and the conductivity and permeability of the winding frame.

[0009] The magnetic microparticle imaging device disclosed in the present invention has higher detection accuracy than ever before because at least one of the radial thickness of the hollow cylinder of the winding frame and the thickness of the flange of the winding frame is greater than the first skin thickness and less than the second skin thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is an XZ cross-sectional view of the AC magnetic field applying coil 3, winding frame 6, measuring coils 4a and 4b, and the inspection object 2 included in the magnetic microparticle imaging device of the first embodiment.

[0011] Figure 2 This is a diagram showing the reel 6 and the measuring coils 4a and 4b included in the magnetic microparticle imaging device according to the first embodiment as viewed from the X-axis direction.

[0012] Figure 3 This is a diagram of the reel 6 and the AC magnetic field applying coil 3 included in the magnetic microparticle imaging device of the first embodiment as viewed from the Y-axis direction.

[0013] Figure 4 This is a diagram schematically showing the structure of the reel 6 included in the magnetic microparticle imaging apparatus according to the first embodiment.

[0014] Figure 5 It is a diagram for explaining the process of detecting the magnetic field from the magnetic fine particles 1 .

[0015] Figure 6 This is an XZ cross-sectional view of the AC magnetic field applying coil 3, winding frame 6, measuring coils 4a and 4b, shield 7, and inspection object 2 included in the magnetic microparticle imaging device of the second embodiment.

[0016] Figure 7This is a diagram showing the reel 6, the shield 7, and the measuring coils 4a and 4b included in the magnetic microparticle imaging device according to the second embodiment, as viewed from the X-axis direction.

[0017] Figure 8 This is an XZ cross-sectional view of the reel 6, the shield 7, and the AC magnetic field applying coil 3 included in the magnetic microparticle imaging device of the second embodiment.

[0018] Fig. 9 This is a diagram showing the reel 6, the heat sink 8, and the measuring coils 4a and 4b included in the magnetic microparticle imaging device of the third embodiment as viewed from the X-axis direction. DETAILED DESCRIPTION

[0019] The embodiments will be described with reference to the drawings.

[0020] Implementation method 1.

[0021] Figure 1 This is an XZ cross-sectional view of the AC magnetic field applying coil 3, winding frame 6, measuring coils 4a and 4b, and the inspection object 2 included in the magnetic microparticle imaging device of the first embodiment. Figure 2 This is a diagram showing the reel 6 and the measuring coils 4a and 4b included in the magnetic microparticle imaging device according to the first embodiment, as viewed from the X-axis direction. Figure 3 This is a diagram showing the reel 6 and the AC magnetic field applying coil 3 included in the magnetic microparticle imaging device according to the first embodiment as viewed from the Y-axis direction. Figure 4 This is a diagram schematically showing the structure of the reel 6 included in the magnetic microparticle imaging apparatus according to the first embodiment.

[0022] The magnetic particle imaging device determines the spatial distribution of magnetic particles 1 in the inspection object 2 in the inspection region. The magnetic particle imaging device includes an AC magnetic field applying coil 3, a winding frame 6, and measuring coils 4a and 4b.

[0023] The DC magnetic field applicators 5a and 5b generate a non-magnetic field region having a low magnetic field intensity so as to change the magnetic properties of magnetic particles in only an arbitrary region of the inspection object 2.

[0024] The DC magnetic field applicators 5a and 5b generate a linear zero approximate magnetic field area FFL (Field Free Line) in a manner that changes the magnetic properties of the magnetic particles 1 contained in the object to be inspected 2. The DC magnetic field applicators 5a and 5b form a linear zero approximate magnetic field area FFL in the imaging area where the object to be inspected 2 is placed. Specifically, the DC magnetic field applicators 5a and 5b are composed of two permanent magnets that are arranged opposite to each other with opposite magnetization directions. Alternatively, the DC magnetic field applicators 5a and 5b can also be permanent magnets with two magnetic yokes that are magnetized opposite to each other through a magnetic yoke, or electromagnets. The direction of the line of the linear zero approximate magnetic field area FFL formed by the DC magnetic field applicators 5a and 5b is the Y-axis direction.

[0025] The measuring coils 4a and 4b measure the magnetic change of the magnetic particles. The direction of the central axis C1 of the measuring coil 4a and the direction of the central axis C2 of the measuring coil 4b are the X-axis direction. The measuring coils 4a and 4b are arranged so as to sandwich the magnetic particles 1 and the object 2 to be inspected containing the magnetic particles 1.

[0026] The AC magnetic field applying coil 3 generates an AC magnetic field that changes the magnetism of the magnetic microparticle 1. The AC magnetic field applying coil 3 is configured so as to sandwich the magnetic microparticle 1, the object to be inspected 2, and the measuring coils 4a and 4b. The AC magnetic field applying coil 3 excites the AC magnetic field in the imaging area where the object to be inspected 2 is placed. An AC power source (not shown) is connected to the AC magnetic field applying coil 3. The direction of the central axis C3 of the AC magnetic field applying coil 3 is the X-axis direction. The central axis C3 coincides with the central axis C1 and the central axis C2.

[0027] The winding frame 6 holds the AC magnetic field applying coil 3. The winding frame 6 is composed of a winding tube. The winding frame 6 has flanges 21a, 21b and a hollow cylinder 22. The AC magnetic field applying coil 3 is wound around the hollow cylinder 22. The central axis of the hollow cylinder 22 is consistent with the central axis C3 of the AC magnetic field applying coil 3. Flanges 21a, 21b are installed at both ends of the hollow cylinder 22.

[0028] The thickness of the flanges 21a and 21b is d2. The radial thickness of the hollow cylinder 22 is d1. That is, the difference between the outer diameter and the inner diameter of the hollow cylinder 22 is d1.

[0029] The magnetic particle imaging is performed by changing the relative position of the linear zero approximate magnetic field region FFL formed by the DC magnetic field applicators 5a and 5b relative to the object 2 and scanning the measurement position. The relative position can be changed by mechanically moving the positions of the DC magnetic field applicators 5a and 5b, the AC magnetic field application coil 3, and the measurement coils 4a and 4b.

[0030] Figure 5 It is a diagram for explaining the process of detecting the magnetic field from the magnetic fine particles 1 .

[0031] The AC magnetic field applying coil 3 generates an AC magnetic field that changes the magnetization of the magnetic particles 1. When the main frequency of the AC magnetic field is set to the fundamental frequency f1 (Hz), the magnetic particles 1 change their magnetization according to the magnetic field magnetization curve due to the AC magnetic field of the fundamental frequency f1 (Hz).

[0032] The magnetic field magnetization curve of the magnetic fine particle 1 shows nonlinear behavior if it includes magnetic saturation due to the saturation magnetic field. When superparamagnetic magnetic fine particles with little hysteresis are selected as the magnetic fine particles 1, the magnetization change includes many odd-order harmonics.

[0033] The signal processing device connected to the measuring coils 4a and 4b extracts odd-numbered harmonics other than the fundamental wave of the fundamental frequency (f1) by filtering processing, etc. The frequencies of the odd-numbered harmonics are f3, f5, f7, and f9 represented by the following formula. As a result, the signal caused by the fundamental frequency f1 of the AC magnetic field from the AC magnetic field applying coil 3 is not affected, and only the signal caused by the magnetic microparticles 1 can be extracted.

[0034] f3=3×f1…(1a)

[0035] f5=5×f1…(1b)

[0036] f7=7×f1…(1c)

[0037] f9=9×f1…(1d)

[0038] However, when the AC power supply that supplies current to the AC magnetic field applying coil 3 is, for example, a switching power supply, a harmonic component of the fundamental frequency f1 (Hz) flows through the AC magnetic field applying coil 3 due to the characteristics of the switching element. When evaluating a signal from a tiny magnetic particle 1 in a living body, the AC magnetic field X of the harmonic component caused by the AC power supply overlaps the AC magnetic field Y caused by the magnetization change of the magnetic particle 1. As a result, the SNR (Signal to Noise Ratio) deteriorates, and the frequency characteristics of the AC magnetic field X and the AC magnetic field Y match, so it is difficult to extract only the signal from the magnetic particle 1.

[0039] As described above, due to the AC power supply, the AC magnetic field applying coil 3 radiates not only the magnetic field of the fundamental frequency f1 but also the magnetic field of odd-numbered harmonics of the third harmonic and above.

[0040] In order to prevent the measuring coils 4a and 4b from measuring the magnetic field of the higher harmonic components generated by the AC magnetic field applying coil 3, the material and thickness of the winding frame 6 for holding the AC magnetic field applying coil 3 are specified. Specifically, the eddy current generated in the winding frame 6 absorbs the higher harmonic components of the AC magnetic field, thereby attenuating the components of the higher harmonic components of the AC magnetic field that are linked to the measuring coils 4a and 4b.

[0041] It is known that eddy current is generated in a region of skin thickness δ determined by the following equation depending on the frequency f of the alternating magnetic field, the electrical conductivity σ of the object, and the magnetic permeability μ. μr is the relative magnetic permeability of the object, and μ0 is the magnetic permeability of a vacuum.

[0042] μ=μr×μ0…(2a)

[0043] [Formula 1]

[0044]

[0045] Generally speaking, a magnetic shield for shielding noise can obtain a sufficient magnetic shielding effect by setting it thicker than the skin thickness δ of formula (2b). When the magnetic shield is thinner than the skin thickness δ of formula (2b), the shielding due to eddy current is insufficient and the unshielded component passes through the magnetic shield.

[0046] In this embodiment, in order to produce a magnetization change in the magnetic particles 1 , an AC magnetic field of a fundamental frequency f1 is passed through the winding frame 6 , and the winding frame 6 is used to shield the AC magnetic field of odd-order harmonics above the third harmonic required for evaluating the magnetization change of the magnetic particles 1 .

[0047] The first skin thickness δ3a is determined by the frequency f3 of the third harmonic of the AC magnetic field excited by the AC magnetic field applying coil 3, the conductivity σa of the winding frame 6, and the magnetic permeability μa of the winding frame 6. The second skin thickness δ1a is determined by the frequency f1 of the fundamental wave of the AC magnetic field excited by the AC magnetic field applying coil 3, the conductivity σa of the winding frame 6, and the magnetic permeability μa of the winding frame 6. Specifically, the second skin thickness δ1a and the first skin thickness δ3a are expressed by the following formulas. uar is the relative magnetic permeability of the winding frame 6, and u0 is the magnetic permeability of vacuum.

[0048] μa=μar×μ0…(3a)

[0049] [Formula 2]

[0050]

[0051] The thicknesses d1 and d2 of the bead frame 6 are designed so as to satisfy the following formula.

[0052] δ3a≤d1≤δ1a…(4)

[0053] δ3a≤d2≤δ1a…(5)

[0054] Thus, among the AC magnetic field components from the AC magnetic field applying coil 3, the component of the fundamental frequency f1 can be transmitted through the winding frame 6, and the magnetic field components above the third harmonic can be shielded. The component of the fundamental frequency f1 is incident on the measuring coils 4a, 4b and the magnetic particles 1.

[0055] Due to the alternating magnetic field, a signal caused by the magnetization magnetic field curve is generated depending on the material of the winding frame 6. Therefore, in addition to the conditions of equations (4) and (5), a non-magnetic material such as aluminum or copper may be used as the material of the winding frame 6.

[0056] Furthermore, at least one of the equation (4) and the equation (5) may be satisfied. Even when only one of the equation (4) and the equation (5) is satisfied, the detection accuracy of magnetic fine particles is improved compared to the conventional method.

[0057] Implementation method 2.

[0058] In Embodiment 1, the thickness and material of the winding frame 6 are selected according to the fundamental frequency f1 of the AC magnetic field excited by the AC magnetic field applying coil 3 and the frequency f3 of the third harmonic, so that the harmonic components caused by the AC power supply can be shielded and the SNR can be improved. In this embodiment, a magnetic shielding member is provided on the entire surface in a manner that not only surrounds the winding frame 6 but also surrounds the AC magnetic field applying coil 3.

[0059] Figure 6 This is an XZ cross-sectional view of the AC magnetic field applying coil 3, winding frame 6, measuring coils 4a and 4b, shield 7, and inspection object 2 included in the magnetic microparticle imaging device of the second embodiment. Figure 7 This is a diagram showing the reel 6, the shield 7, and the measuring coils 4a and 4b included in the magnetic microparticle imaging device according to the second embodiment, as viewed from the X-axis direction. Figure 8 This is an XZ cross-sectional view of the reel 6, the shield 7, and the AC magnetic field applying coil 3 included in the magnetic microparticle imaging device of the second embodiment.

[0060] The outer periphery of the AC magnetic field applying coil 3 cannot be covered by the winding frame 6 due to the manufacturing process of the winding. Figure 6 to Figure 8 As shown, a shield 7 is provided on the outer peripheral portion of the AC magnetic field applying coil 3 which cannot be covered by the winding frame 6 .

[0061] The shielding member 7 is joined to the flange of the winding frame 6 to seal the AC magnetic field applying coil 3. The AC magnetic field applying coil 3 is arranged in a sealed space formed by the winding frame 6 and the shielding member 7. The shielding member 7 has a hollow cylindrical shape. The central axis of the shielding member 7 is consistent with the central axis C3 of the AC magnetic field applying coil 3.

[0062] The third skin thickness δ3b is determined by the frequency f3 of the third harmonic of the AC magnetic field excited by the AC magnetic field applying coil 3, the conductivity σb of the shield 7, and the magnetic permeability μb of the shield. The fourth skin thickness δ1b is determined by the frequency f1 of the fundamental wave of the AC magnetic field excited by the AC magnetic field applying coil 3, the conductivity σb of the shield 7, and the magnetic permeability μb of the shield 7. Specifically, the fourth skin thickness δ1b and the third skin thickness δ3b are expressed by the following formulas. ubr is the relative magnetic permeability of the shield 7, and u0 is the magnetic permeability of vacuum.

[0063] μb=μbr×μ0…(6a)

[0064] [Formula 3]

[0065]

[0066] The thickness d3 of the shield 7 is designed so as to satisfy the following expression.

[0067] δ3b≤d3≤δ1b…(7)

[0068] Thus, among the components of the AC magnetic field from the AC magnetic field applying coil 3 , the shield 7 allows the component of the fundamental frequency f1 to be transmitted, while shielding the magnetic field components of the third harmonic and higher harmonics.

[0069] Furthermore, in terms of manufacturing management and management of the influence of magnetism, the material of the shield 7 can be made the same as that of the bobbin 6 .

[0070] In this embodiment, by providing a shield, the detection accuracy of magnetic fine particles can be further improved compared with the first embodiment.

[0071] Implementation method 3.

[0072] The AC magnetic field applying coil 3 heats up due to AC loss and resistance heating caused by the AC magnetic field. When the electrical characteristics of the AC magnetic field applying coil 3 change due to temperature changes, disturbances are generated in the excitation field, so disturbances are also generated in the signal of magnetic particle imaging. The heat generated by the AC magnetic field applying coil 3 is also transmitted to the measuring coils 4a, 4b and the inspected object 2 arranged inside the measuring coils 4a, 4b. As a result, the detection accuracy of the magnetic particles 1 deteriorates.

[0073] The winding frame 6 is made of a material with high electrical conductivity for magnetic shielding, so it also has high thermal conductivity. By using the winding frame 6 as a heat sink, it can help dissipate heat, so it is possible to suppress heat rise and reduce noise. However, if you want to improve the heat dissipation efficiency, you need to change the thickness of the structure by extending the heat sink through thermal design, etc., and compromise with the size specified by the skin thickness described in the above embodiment.

[0074] Fig. 9 This is a diagram showing the reel 6, the heat sink 8, and the measuring coils 4a and 4b included in the magnetic microparticle imaging device of the third embodiment as viewed from the X-axis direction.

[0075] The magnetic microparticle imaging apparatus of the present embodiment includes a heat sink 8 connected to the inner surface of the hollow cylinder 22 of the reel 6 .

[0076] The heat sink 8 has grooves extending along the axial direction (X direction) of the AC magnetic field applying coil 3 .

[0077] The length direction (X-axis direction) of the heat sink 8 and the power-on direction of the AC magnetic field applying coil 3 are perpendicularly intersected. Eddy current flows in parallel with the current direction of the coil, so the heat sink 8 can be used to sharply reduce the influence of the eddy current. As a result, the above-mentioned trade-off can be eliminated. In addition, by providing a groove in the axial direction (X-direction) of the AC magnetic field applying coil 3, a passage for a cooling solvent, such as air, passing through the heat sink 8 from the outside can also be ensured, so a sufficient cooling effect can be achieved.

[0078] The embodiments disclosed this time should be considered to be illustrative in all aspects and not restrictive. The scope of the present disclosure is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0079] (Explanation of symbols)

[0080] 1: magnetic particles; 2: object to be inspected; 3: AC magnetic field applying coil; 4a, 4b: measuring coils; 5a, 5b: DC magnetic field applicators; 6: winding frame; 7: shielding; 8: heat sink; 21a, 21b: flanges; 22: hollow cylinder.

Claims

1. A magnetic microparticle imaging device for determining the spatial distribution of magnetic microparticles in an object to be inspected in an inspection area, the magnetic microparticle imaging device comprising: an AC magnetic field applying coil for generating an AC magnetic field that changes the magnetic properties of the magnetic particles; A winding frame, used for holding the AC magnetic field applying coil; a DC magnetic field applicator for generating an area with a low magnetic field intensity in such a manner as to change the magnetic properties of the magnetic particles in only an arbitrary area of ​​the object to be inspected; as well as The measuring coil detects the magnetic change of the magnetic particles. At least one of the radial thickness of the hollow cylinder of the winding frame and the thickness of the flange of the winding frame is greater than or equal to the first skin thickness and less than or equal to the second skin thickness, The first skin thickness is determined by the frequency of the third harmonic of the AC magnetic field excited by the AC magnetic field applying coil and the electrical conductivity and magnetic permeability of the winding frame. The second skin thickness is determined by the frequency of the fundamental wave of the AC magnetic field excited by the AC magnetic field applying coil and the electrical conductivity and magnetic permeability of the winding frame.

2. The magnetic microparticle imaging device according to claim 1, wherein: Both the radial thickness of the hollow cylinder of the winding frame and the thickness of the flange of the winding frame are greater than or equal to the first skin thickness and less than or equal to the second skin thickness.

3. The magnetic microparticle imaging device according to claim 1 or 2, wherein: The winding frame is made of non-magnetic material.

4. The magnetic microparticle imaging device according to any one of claims 1 to 3, wherein: The magnetic particle imaging device further comprises a shielding member joined to the flange of the winding frame, the shielding member being used to seal the AC magnetic field applying coil. The thickness of the shield is greater than or equal to the third skin thickness and less than or equal to the fourth skin thickness, The third skin thickness is determined by the frequency of the third harmonic of the AC magnetic field and the electrical conductivity and magnetic permeability of the shielding member. The fourth skin thickness is determined by the frequency of the fundamental wave of the AC magnetic field and the electrical conductivity and magnetic permeability of the shield.

5. The magnetic microparticle imaging device according to claim 4, wherein: The shield has a hollow cylindrical shape, and the central axis of the shield coincides with the central axis of the AC magnetic field applying coil 3 .

6. The magnetic microparticle imaging device according to claim 4 or 5, wherein: The material of the roll frame is the same as that of the shielding member.

7. The magnetic microparticle imaging device according to any one of claims 1 to 6, wherein: The magnetic microparticle imaging device further includes a heat sink having a groove extending along the axial direction of the AC magnetic field applying coil.

8. The magnetic microparticle imaging device according to claim 7, wherein: The heat sink is connected to the inner surface of the hollow cylinder of the winding frame.

9. The magnetic microparticle imaging device according to claim 7 or 8, wherein: The length direction of the heat sink and the current-carrying direction of the AC magnetic field applying coil intersect each other perpendicularly.

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