MAGNETIC PARTICLE IMAGING DEVICE

Abstract

An alternating current magnetic field coil (3) generates an alternating current magnetic field that changes the magnetism of magnetic particles (1). A winding frame (6) holds the alternating current magnetic field coil (3). A direct current magnetic field applicator creates a region with a low magnetic field strength to change only the magnetism of the magnetic particles (1) in any region of an object (2). A measuring coil (4a, 4b) detects a change in the magnetization of the magnetic particles (1). At least one of the radial thicknesses of a hollow cylinder of the winding frame (6) and one of the thicknesses of a flange of the winding frame (6) is greater than or equal to a first skin depth and less than or equal to a second skin depth.The first skin depth is determined by the frequency of a third harmonic of the alternating current magnetic field excited by the coil (3) applying the alternating current magnetic field, and by the electrical conductivity and magnetic permeability of the winding frame (6). The second skin depth is determined by the frequency of a fundamental harmonic of the alternating current magnetic field excited by the coil (3) applying the alternating current magnetic field, and by the electrical conductivity and magnetic permeability of the winding frame (6).

Description

Magnetic particle imaging device

[0001] The present disclosure relates to a magnetic particle imaging device.

[0002] A magnetic particle detector equipped with an AC magnetic field application coil and a DC magnetic field application means is known (see, for example, Patent Document 1). Only magnetic particles present around the zero magnetic field region created by the DC magnetic field application means undergo magnetization fluctuations due to the excitation magnetic field, so distribution information of the magnetic particles can be obtained by scanning the relative positions of the zero magnetic field region and the magnetic particles. Imaging is performed by obtaining the distribution of magnetic particles contained in the test object by scanning the zero magnetic field region. The magnetic particle detector of Patent Document 1 suppresses the fundamental wave component of the signal output from the measurement coil that detects the magnetic field, and extracts only the harmonic components.

[0003] Japanese Patent Application Laid-Open No. 2003-199767

[0004] In the magnetic particle imaging device described in Patent Document 1, not only fundamental current but also harmonic current flows through the AC magnetic field application coil due to the characteristics of the AC power supply connected to it. As a result, the excited AC magnetic field also contains harmonics. Therefore, the measurement coil simultaneously measures the harmonics output from the magnetic particles and the harmonics excited by the AC magnetic field application coil. Since the measurement coil cannot measure only the harmonics output from the magnetic particles that are the intended measurement, the detection accuracy of the magnetic particles deteriorates.

[0005] Therefore, an object of the present disclosure is to provide a magnetic particle imaging device having higher detection accuracy than conventional devices.

[0006] The magnetic particle imaging device disclosed herein is a magnetic particle imaging device for determining the spatial distribution of magnetic particles within an inspection area of ​​an object under test, and includes an AC magnetic field application coil for generating an AC magnetic field that changes the magnetism of the magnetic particles, a bobbin for holding the AC magnetic field application coil, a DC magnetic field applicator that generates a region of low magnetic field strength so as to change the magnetism of only magnetic particles in a given region of the object under test, and a measurement coil for detecting the change in magnetism of the magnetic particles. At least one of the radial thickness of the hollow cylinder of the bobbin and the thickness of the flange of the bobbin is equal to or greater than a first skin depth and equal to or less than a second skin depth. The first skin depth is determined by the frequency of the third harmonic of the AC magnetic field excited by the AC magnetic field application coil and the conductivity and permeability of the bobbin. The second skin depth is determined by the frequency of the fundamental wave of the AC magnetic field excited by the AC magnetic field application coil and the conductivity and permeability of the bobbin.

[0007] The magnetic particle imaging device of the present disclosure has higher detection accuracy than conventional devices because at least one of the radial thickness of the hollow cylinder of the reel and the thickness of the flange of the reel is greater than or equal to the first skin depth and less than or equal to the second skin depth.

[0008] FIG. 1 is an XZ cross-sectional view of the AC magnetic field application coil 3, spool 6, measurement coils 4a and 4b, and object to be inspected 2 included in the magnetic particle imaging apparatus of embodiment 1. FIG. 2 is a view of the spool 6 and measurement coils 4a and 4b included in the magnetic particle imaging apparatus of embodiment 1, viewed from the X-axis direction. FIG. 3 is a view of the spool 6 and AC magnetic field application coil 3 included in the magnetic particle imaging apparatus of embodiment 1, viewed from the Y-axis direction. FIG. 4 is a view schematically showing the structure of the spool 6 included in the magnetic particle imaging apparatus of embodiment 1. FIG. 5 is a view for explaining the process of detecting a magnetic field from a magnetic particle 1. FIG. 6 is an XZ cross-sectional view of the AC magnetic field application coil 3, spool 6, measurement coils 4a and 4b, shield 7, and object to be inspected 2 included in the magnetic particle imaging apparatus of embodiment 2. FIG. 7 is a view of the spool 6, shield 7, and measurement coils 4a and 4b included in the magnetic particle imaging apparatus of embodiment 2, viewed from the X-axis direction. FIG. 8 is an XZ cross-sectional view of the spool 6, shield 7, and AC magnetic field application coil 3 included in the magnetic particle imaging apparatus of embodiment 2. 11 is a view of the bobbin 6, the heat sink 8, and the measurement coils 4a and 4b included in the magnetic particle imaging device of the third embodiment, as viewed from the X-axis direction. FIG.

[0009] The embodiments will be described with reference to the drawings. Embodiment 1. Fig. 1 is an XZ cross-sectional view of the AC magnetic field applying coil 3, spool 6, measurement coils 4a and 4b, and inspection object 2 included in the magnetic particle imaging apparatus of embodiment 1. Fig. 2 is a view of the spool 6 and measurement coils 4a and 4b included in the magnetic particle imaging apparatus of embodiment 1, viewed from the X-axis direction. Fig. 3 is a view of the spool 6 and AC magnetic field applying coil 3 included in the magnetic particle imaging apparatus of embodiment 1, viewed from the Y-axis direction. Fig. 4 is a diagram schematically showing the structure of the spool 6 included in the magnetic particle imaging apparatus of embodiment 1.

[0010] The magnetic particle imaging device determines the spatial distribution of magnetic particles 1 in an inspection area within an object 2. The magnetic particle imaging device comprises an AC magnetic field applying coil 3, a bobbin 6, and measurement coils 4a and 4b.

[0011] The DC magnetic field applicators 5a and 5b generate a magnetic field-free region having a low magnetic field strength so as to change only the magnetism of magnetic nanoparticles in a given region of the object 2 to be inspected.

[0012] The DC magnetic field applicators 5a and 5b generate a linear zero approximation magnetic field region FFL (Field Free Line) so as to change the magnetism of the magnetic particles 1 contained in the object 2 to be inspected. The DC magnetic field applicators 5a and 5b form the linear zero approximation magnetic field region FFL in the imaging region where the object 2 to be inspected is placed. Specifically, the DC magnetic field applicators 5a and 5b are composed of two permanent magnets arranged facing each other with their magnetization directions opposite each other. Alternatively, the DC magnetic field applicators 5a and 5b may be two yoke-equipped permanent magnets or electromagnets whose magnetizations are opposed by the yokes. The direction of the line of the linear zero approximation magnetic field region FFL formed by the DC magnetic field applicators 5a and 5b is the Y-axis direction.

[0013] The measurement coils 4a and 4b measure the magnetic change of the magnetic particles. The direction of the central axis C1 of the measurement coil 4a and the direction of the central axis C2 of the measurement coil 4b are the X-axis direction. The measurement coils 4a and 4b are arranged so as to sandwich the magnetic particles 1 and the test object 2 containing the magnetic particles 1.

[0014] The AC magnetic field application coil 3 generates an AC magnetic field that changes the magnetism of the magnetic particles 1. The AC magnetic field application coil 3 is arranged so as to sandwich the magnetic particles 1, the object to be inspected 2, and the measurement coils 4a and 4b. The AC magnetic field application coil 3 excites an AC magnetic field in the imaging region where the object to be inspected 2 is placed. An AC power supply (not shown) is connected to the AC magnetic field application coil 3. The direction of the central axis C3 of the AC magnetic field application coil 3 is the X-axis direction. The central axis C3 coincides with the central axes C1 and C2.

[0015] The winding frame 6 holds the AC magnetic field application coil 3. The winding frame 6 is formed of a bobbin. The winding frame 6 has flanges 21a, 21b and a hollow cylinder 22. The AC magnetic field application coil 3 is wound around the hollow cylinder 22. The central axis of the hollow cylinder 22 coincides with the central axis C3 of the AC magnetic field application coil 3. The flanges 21a, 21b are attached to both ends of the hollow cylinder 22.

[0016] The flanges 21a and 21b have a thickness d2. The hollow cylinder 22 has a radial thickness d1. In other words, the difference between the outer diameter and the inner diameter of the hollow cylinder 22 is d1.

[0017] Magnetic particle imaging is performed by scanning the measurement position by changing the relative position of the linear zero approximation magnetic field region FFL formed by the DC magnetic field applicators 5a and 5b with respect to the object 2. The relative positions can be changed by mechanically moving the DC magnetic field applicators 5a and 5b, the AC magnetic field applying coil 3, and the measurement coils 4a and 4b.

[0018] 5 is a diagram for explaining the process of detecting a magnetic field from a magnetic particle 1. The AC magnetic field applying coil 3 generates an AC magnetic field that causes a change in magnetization of the magnetic particle 1. When the main frequency of the AC magnetic field is set to a fundamental frequency f1 (Hz), the magnetic particle 1 causes a change in magnetization based on a magnetic field magnetization curve due to the AC magnetic field of the fundamental frequency f1 (Hz).

[0019] The magnetic field magnetization curve of the magnetic particle 1 exhibits nonlinear behavior when magnetic saturation due to the saturation magnetic field is included. When superparamagnetic magnetic particles with little hysteresis are selected as the magnetic particle 1, the magnetization change contains many odd-order harmonics.

[0020] The signal processing device to which the measurement coils 4a and 4b are connected extracts odd harmonics other than the fundamental wave of the fundamental frequency (f1) by filtering or the like. The frequencies of the odd harmonics are f3, f5, f7, and f9, which are expressed by the following equations. This makes it possible to extract only the signal caused by the magnetic nanoparticles 1 without being affected by the signal caused by the fundamental frequency f1 of the AC magnetic field from the AC magnetic field application coil 3.

[0021] f3=3×f1 (1a) f5=5×f1 (1b) f7=7×f1 (1c) f9=9×f1 (1d) However, if the AC power supply supplying current to the AC magnetic field applying coil 3 is, for example, a switching power supply, harmonic components of the fundamental frequency f1 (Hz) will flow into the AC magnetic field applying coil 3 due to the characteristics of the switching element, etc. When evaluating signals from minute magnetic particles 1 in a living body, the AC magnetic field X of harmonic components caused by the AC power supply will be superimposed on the AC magnetic field Y caused by magnetization changes in the magnetic particles 1. As a result, the SNR (Signal to Noise Ratio) will deteriorate and the frequency characteristics of the AC magnetic field X and the AC magnetic field Y will match, making it difficult to extract only the signal from the magnetic particles 1.

[0022] As described above, due to the AC power supply, the AC magnetic field applying coil 3 radiates not only a magnetic field of the fundamental frequency f1 but also magnetic fields of odd harmonics of the third harmonic or higher.

[0023] The material and thickness of the bobbin 6 for holding the AC magnetic field application coil 3 are specified so that the measurement coils 4a, 4b do not measure the magnetic field of the harmonic components generated by the AC magnetic field application coil 3. Specifically, the harmonic components of the AC magnetic field are absorbed by eddy currents generated in the bobbin 6, thereby attenuating the components of the harmonic components of the AC magnetic field that interlink with the measurement coils 4a, 4b.

[0024] It is known that eddy currents occur in a region of skin depth δ, which is determined by the following formula based on the frequency f of an AC magnetic field, the electrical conductivity σ of an object, and the magnetic permeability μ: μr is the relative magnetic permeability of the object, and μ0 is the magnetic permeability of a vacuum.

[0025] μ=μr×μ0...(2a)

[0026]

[0027] Generally, a magnetic shield for noise shielding can achieve a sufficient magnetic shielding effect by making it thicker than the skin depth δ in equation (2b). If the magnetic shield is thinner than the skin depth δ in equation (2b), the shielding by eddy currents is insufficient and the components that could not be shielded pass through the magnetic shield.

[0028] In this embodiment, the alternating magnetic field of fundamental frequency f1 is transmitted through the reel 6 in order to cause a change in the magnetization of the magnetic particles 1, while the alternating magnetic field of odd harmonics higher than the third harmonic, which is necessary to evaluate the change in the magnetization of the magnetic particles 1, is shielded by the reel 6.

[0029] The first skin depth δ3a is determined by the frequency f3 of the third harmonic of the AC magnetic field excited by the AC magnetic field application coil 3, the conductivity σa of the bobbin 6, and the magnetic permeability μa of the bobbin 6. The second skin depth δ1a is determined by the frequency f1 of the fundamental wave of the AC magnetic field excited by the AC magnetic field application coil 3, the conductivity σa of the bobbin 6, and the magnetic permeability μa of the bobbin 6. Specifically, the second skin depth δ1a and the first skin depth δ3a are expressed by the following equations, where uar is the relative magnetic permeability of the bobbin 6, and u0 is the magnetic permeability of a vacuum.

[0030] μa=μar×μ0...(3a)

[0031]

[0032] The thicknesses d1 and d2 of the bobbin 6 are designed to satisfy the following equations: δ3a≦d1≦δ1a (4) δ3a≦d2≦δ1a (5) This makes it possible for the bobbin 6 to transmit the fundamental frequency f1 component of the AC magnetic field from the AC magnetic field application coil 3 and to shield magnetic field components of third harmonic and higher. The fundamental frequency f1 component is incident on the measurement coils 4a, 4b and the magnetic particle 1.

[0033] The AC magnetic field generates a signal due to the magnetization field curve, depending on the material of the spool 6. Therefore, in addition to the conditions of equations (4) and (5), the spool 6 may be made of a non-magnetic material such as aluminum or copper.

[0034] At least one of the formulas (4) and (5) may be satisfied. Even if only one of the formulas (4) and (5) is satisfied, the detection accuracy of magnetic fine particles is improved compared to the conventional method.

[0035] Embodiment 2 In embodiment 1, the harmonic components caused by the AC power supply are shielded and the SNR is improved by selecting the thickness and material of the bobbin 6 based on the fundamental frequency f1 and the frequency f3 of the third harmonic of the AC magnetic field excited by the AC magnetic field applying coil 3. In this embodiment, a magnetic shield is provided on the entire surface so as to surround not only the bobbin 6 but also the AC magnetic field applying coil 3.

[0036] Fig. 6 is an XZ cross-sectional view of the AC magnetic field applying coil 3, spool 6, measurement coils 4a and 4b, shield 7, and inspection object 2 included in the magnetic particle imaging apparatus of embodiment 2. Fig. 7 is a view of the spool 6, shield 7, and measurement coils 4a and 4b included in the magnetic particle imaging apparatus of embodiment 2, viewed from the X-axis direction. Fig. 8 is an XZ cross-sectional view of the spool 6, shield 7, and AC magnetic field applying coil 3 included in the magnetic particle imaging apparatus of embodiment 2.

[0037] Due to factors in the winding manufacturing process, the outer periphery of the AC magnetic field applying coil 3 cannot be covered with the bobbin 6. As shown in Figures 6 to 8, a shield 7 is provided on the outer periphery of the AC magnetic field applying coil 3 that cannot be covered with the bobbin 6.

[0038] The shield 7 is joined to the flange of the bobbin 6 and is provided to seal the AC magnetic field application coil 3. The AC magnetic field application coil 3 is placed in a sealed space formed by the bobbin 6 and the shield 7. The shield 7 has a hollow cylindrical shape. The central axis of the shield 7 coincides with the central axis C3 of the AC magnetic field application coil 3.

[0039] The third skin depth δ3b is determined by the frequency f3 of the third harmonic of the AC magnetic field excited by the AC magnetic field application coil 3, the conductivity σb of the shield 7, and the magnetic permeability μb of the shield. The fourth skin depth δ1b is determined by the frequency f1 of the fundamental wave of the AC magnetic field excited by the AC magnetic field application coil 3, the conductivity σb of the shield 7, and the magnetic permeability μb of the shield 7. Specifically, the fourth skin depth δ1b and the third skin depth δ3b are expressed by the following equations, where ubr is the relative magnetic permeability of the shield 7, and u0 is the magnetic permeability of a vacuum.

[0040] μb=μbr×μ0...(6a)

[0041]

[0042] The thickness d3 of the shield 7 is designed to satisfy the condition of the following formula: δ3b≦d3≦δ1b (7) This makes it possible for the shield 7 to transmit the component of the fundamental frequency f1 of the AC magnetic field from the AC magnetic field applying coil 3, while blocking magnetic field components of the third harmonic and above.

[0043] Furthermore, in order to control manufacturing and magnetic influence, the material of the shield 7 can be the same as that of the bobbin 6 .

[0044] In this embodiment, by providing a shield, it is possible to further improve the detection accuracy of magnetic particles compared to the first embodiment.

[0045] Embodiment 3 The AC magnetic field applying coil 3 heats up due to AC loss and resistance heat caused by the AC magnetic field. When the electrical characteristics of the AC magnetic field applying coil 3 change due to temperature changes, perturbations occur in the excitation magnetic field, and therefore in the magnetic particle imaging signal. The heat generated by the AC magnetic field applying coil 3 is also transmitted to the measurement coils 4a and 4b and to the specimen 2 placed inside the measurement coils 4a and 4b. As a result, the detection accuracy of the magnetic particles 1 deteriorates.

[0046] The bobbin 6 is made of a material with high electrical conductivity for magnetic shielding, and therefore has high thermal conductivity. By using the bobbin 6 as a heat sink, heat dissipation can be aided, suppressing heat rise and reducing noise. However, to improve heat dissipation efficiency, it becomes necessary to change the thickness of the heat sink structure through thermal design, for example by lengthening the heat dissipation grooves, which results in a trade-off with the dimensions determined by the skin depth described in the above embodiment.

[0047] FIG. 9 is a view of the bobbin 6, heat sink 8, and measurement coils 4a and 4b included in the magnetic particle imaging apparatus of the third embodiment, as viewed from the X-axis direction.

[0048] The magnetic particle imaging device of this embodiment has a heat sink 8 connected to the inner surface of the hollow cylinder 22 of the bobbin 6 .

[0049] The heat sink 8 has a groove extending along the axial direction (X direction) of the AC magnetic field applying coil 3 .

[0050] The longitudinal direction (X-axis direction) of the heat sink 8 and the current direction of the AC magnetic field applying coil 3 intersect perpendicularly. Because eddy currents flow parallel to the current direction in the coil, the heat sink 8 can significantly reduce the effects of eddy currents. As a result, the above trade-off can be resolved. Furthermore, by providing grooves in the axial direction (X-axis direction) of the AC magnetic field applying coil 3, a passage for a cooling medium, such as air, passing through the heat sink 8 from the outside can be secured, thereby achieving a sufficient cooling effect.

[0051] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0052] 1 magnetic fine particle, 2 test object, 3 AC magnetic field application coil, 4a, 4b measurement coil, 5a, 5b DC magnetic field application device, 6 bobbin, 7 shield, 8 heat sink, 21a, 21b flange, 22 hollow cylinder

Claims

1. A magnetic particle imaging device for determining the spatial distribution of magnetic particles within an object under test in an inspection area, comprising: an AC magnetic field application coil for generating an AC magnetic field that changes the magnetism of the magnetic particles; a bobbin for holding the AC magnetic field application coil; a DC magnetic field applicator that generates a region with a low magnetic field strength so as to change the magnetism of only the magnetic particles in a given region of the object under test; and a measurement coil that detects the change in magnetism of the magnetic particles, wherein at least one of the radial thickness of the hollow cylinder of the bobbin and the thickness of the flange of the bobbin is equal to or greater than a first skin depth and equal to or less than a second skin depth, wherein the first skin depth is determined by the frequency of the third harmonic of the AC magnetic field excited by the AC magnetic field application coil and the conductivity and permeability of the bobbin, and the second skin depth is determined by the frequency of the fundamental wave of the AC magnetic field excited by the AC magnetic field application coil and the conductivity and permeability of the bobbin.

2. A magnetic particle imaging device according to claim 1, wherein both the radial thickness of the hollow cylinder of said spool and the thickness of the flange of said spool are equal to or greater than said first skin depth and equal to or less than said second skin depth.

3. A magnetic particle imaging device according to claim 1 or 2, wherein said bobbin is made of a non-magnetic material.

4. A magnetic particle imaging device according to any one of claims 1 to 3, further comprising a shield joined to the flange of the spool for sealing the AC magnetic field applying coil, wherein the thickness of the shield is equal to or greater than a third skin depth and equal to or less than a fourth skin depth, wherein the third skin depth is determined by the frequency of the third harmonic of the AC magnetic field and the conductivity and permeability of the shield, and wherein the fourth skin depth is determined by the frequency of the fundamental wave of the AC magnetic field and the conductivity and permeability of the shield.

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

6. A magnetic particle imaging device according to claim 4 or 5, wherein the material of said bobbin and the material of said shield are the same.

7. A magnetic particle imaging device according to any one of claims 1 to 6, further comprising a heat sink having a groove extending along the axial direction of said AC magnetic field applying coil.

8. A magnetic particle imaging device according to claim 7, wherein said heat sink is connected to the inner surface of said hollow cylinder of said bobbin.

9. A magnetic particle imaging device according to claim 7 or 8, wherein the longitudinal direction of said heat sink and the direction of current flow through said AC magnetic field applying coil intersect perpendicularly.

Images