Magnetic resonance imaging device

Abstract

To provide a MRI device equipped with a magnetic shield structure for suppressing a leakage magnetic flux.SOLUTION: A magnetic resonance imaging device includes a laminate 301 for blocking a magnetic flux generated from a first coil 204 for a disk-like magnetic material magnetic pole 201. The laminate includes a central part sandwiched between the first coil and the disk-like magnetic material magnetic pole and an end part opposed to an annular magnetic material magnetic pole 202 through air. A thickness in a Z-axis direction of the end part of the laminate is larger than a thickness in the Z-axis direction of the central part of the laminate. A position along the Z-axis direction of a surface on an imaging space side of the end part of the laminate is positioned closer to the first coil than a position along the Z-axis direction of a surface on an imaging space side of the central part of the laminate.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a magnetic shield structure that suppresses leakage magnetic flux generated by a gradient magnetic field coil in a magnetic resonance imaging (hereinafter, MRI) apparatus. [Background technology]

[0002] An MRI device is a device that applies a high-frequency magnetic field to a subject, causes resonance in hydrogen atoms in the human body, and captures the radio waves generated during the resonance with a receiving coil, which can extract the radio waves obtained as an image. An MRI device is mainly composed of a static magnetic field magnet for forming a uniform static magnetic field, a gradient magnetic field coil for generating a spatially linear magnetic field and adding position information to the signal obtained from the MRI, and the above-mentioned receiving coil for receiving the radio waves generated from the hydrogen atoms.

[0003] Gradient coils pass a pulsed current waveform to generate a desired spatial magnetic field, which generates a fluctuating magnetic field due to eddy currents in response to the current changes, and these eddy currents generate eddy currents in the metal parts of the MRI device. This fluctuating magnetic field can worsen the uniformity of the static magnetic field and affect the spatial distribution of the gradient magnetic field, causing image degradation. In particular, magnets that generate static magnetic fields are often made of iron poles, and this can affect the magnetization of the pole material.

[0004] For this reason, recent MRI devices have been designed to suppress image degradation by adding a function called an active shield coil, which cancels out the opposing magnetic field that generates eddy currents.

[0005] However, the area occupied by this active shield increases the distance between the magnet and the imaging space, and the magnetic resistance increases, so the magnetomotive force and magnetic poles required to generate the required static magnetic field become larger. The increase in magnetomotive force can be adjusted by the coil current and number of turns, but this can lead to an increase in magnetic energy due to the increase in coil current, and to higher costs due to the larger coils and magnetic poles.

[0006] Against this background, a countermeasure has been disclosed for medium-to-low magnetic field MRI devices, in which magnetic pole pieces such as soft ferrite or silicon steel plate are provided between the gradient magnetic field coil and the metal structure, which act as a path for magnetic flux and suppress the generation of eddy currents (see Patent Document 1).

[0007] Furthermore, in Patent Document 2, in order to suppress eddy currents, a laminate is provided between the gradient magnetic field coil and the magnetic pole, which is divided into a surface layer made of silicon steel plate with a smaller diameter and a deep layer made of silicon steel plate with a larger diameter, thereby suppressing eddy currents and preventing a decrease in apparent permeability due to the reduction in the diameter of the silicon steel plate.

[0008] Furthermore, in Patent Document 3, by introducing a slit structure into the magnetic pole, the area through which eddy currents flow is reduced and the damping time constant is increased, thereby suppressing leakage of magnetic flux into the magnetic pole when the current in the gradient magnetic field coil changes over time. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 05-182821 [Patent Document 2] JP 2004-65714 A [Patent Document 3] JP 2016-96829 A Summary of the Invention [Problem to be solved by the invention]

[0010] However, for example, in Patent Document 1, soft ferrite with low saturation magnetization is used, so that in a high magnetic field, for example, a static magnetic field of 1.5 T, the magnetic shielding effect is lost, and it is difficult to suppress eddy currents. In addition, in the configuration described in Patent Document 2, it is necessary to prepare a plurality of sizes of laminates, which may lead to an increase in the manufacturing cost of the laminates and a prolonged manufacturing time. Furthermore, in the configuration described in Patent Document 3, the structural strength of the magnetic poles is deteriorated by inserting slits in the magnetic poles, which may require the addition of reinforcing members. In addition, the amount of iron material is reduced by the slits, and the apparent magnetic permeability is reduced, which may lead to an increase in the required magnetomotive force, which may lead to high costs.

[0011] An object of the present invention is to solve the above problems and provide an MRI apparatus equipped with a magnetic shield structure that suppresses leakage magnetic flux. [Means for solving the problem]

[0012] In order to achieve the above object, the present invention provides a magnetic resonance imaging device comprising: a pair of static magnetic field magnets arranged opposite each other with respect to the imaging space; and a pair of gradient magnetic field coils arranged opposite each other with respect to the imaging space; the static magnetic field magnets have disk-shaped magnetic poles and annular magnetic poles; the gradient magnetic field coils have a first coil arranged inside the diameter of the disk-shaped magnetic poles and applying a magnetic field in the Z-axis direction in which the pair of static magnetic field magnets face each other in the imaging space; and the device further comprises a laminate that shields the magnetic flux generated from the first coil from the disk-shaped magnetic poles; the laminate has a central portion sandwiched between the first coil and the disk-shaped magnetic poles and an end portion facing the annular magnetic poles via air; the thickness of the end portion of the laminate in the Z-axis direction is thicker than the thickness of the central portion of the laminate in the Z-axis direction; and the position of the end portion of the laminate along the Z-axis direction on the imaging space side is located closer to the first coil than the position along the Z-axis direction on the imaging space side of the central portion of the laminate. Effect of the Invention

[0013] According to the present invention, an MRI apparatus can be provided in which eddy currents are reduced, leakage to the magnetic poles that constitute the static magnetic field magnet is suppressed without changing the diameter of the laminate depending on the position, deterioration of image quality is prevented, and eddy currents are reduced without increasing the manufacturing costs of the laminate. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing one configuration of an open-type MRI apparatus including a laminate according to a first embodiment. [Diagram 2] 1 is a diagram showing the flow of magnetic flux produced by a laminate in which an end portion of the laminate according to Example 1 is disposed in the center with respect to the Z axis of the center of the laminate, a magnetic pole, and a gradient magnetic field coil. FIG. [Diagram 3] 1 is a diagram showing the flow of magnetic flux generated by a laminate in which an end portion of the laminate according to Example 1 is disposed above the center of the laminate with respect to the Z axis, a magnetic pole, and a gradient magnetic field coil. FIG. [Figure 4] FIG. 2 is a diagram showing one configuration of a laminate according to Example 1. [Diagram 5] FIG. 2 is a diagram showing one configuration of an insulator and a laminate according to the first embodiment. [Figure 6] 13 is a diagram showing another configuration of the insulator and the laminate according to the second embodiment. FIG. [Figure 7] FIG. 11 is a diagram showing one configuration of an MRI apparatus including a laminate according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Various embodiments of the present invention will now be described with reference to the drawings, in which the same parts are given the same reference numerals. EXAMPLES

[0016] Example 1 is an example of an MRI apparatus including a pair of static magnetic field magnets arranged opposite to each other with the imaging space at the center, and a pair of gradient magnetic field coils arranged opposite to each other with the imaging space at the center, the static magnetic field magnets having disk-shaped magnetic poles and annular magnetic poles, the gradient magnetic field coils having a first coil arranged inside the diameter of the disk-shaped magnetic poles and applying a magnetic field in the Z-axis direction in which the pair of static magnetic field magnets face each other in the imaging space, and further including a laminate that shields the disk-shaped magnetic poles from magnetic flux generated from the first coil, the laminate having a central portion sandwiched between the first coil and the disk-shaped magnetic poles and an end portion facing the annular magnetic pole, the thickness of the end portion of the laminate in the Z-axis direction being thinner than the thickness of the central portion of the laminate in the Z-axis direction, and the position along the Z-axis direction of the surface of the central portion of the laminate facing the imaging space is located closer to the first coil than the position along the Z-axis direction of the surface of the laminate facing the imaging space.

[0017] 1 shows a schematic configuration of an open-type MRI apparatus according to Example 1. The MRI apparatus 1 generates a static magnetic field strength by a pair of static magnetic field magnets composed of a disk-shaped magnetic pole 201 and an annular magnetic pole 202 arranged around an imaging space 101, and an annular coil made of a superconducting material or the like. Furthermore, in order to reduce the magnetic field outside the imaging area, a shield coil is provided to generate a magnetic field in the opposite direction to the annular coil.

[0018] When the annular coil for forming the static magnetic field is made of a superconducting material, it is housed in a vacuum vessel for arranging the coil in a vacuum, a radiation shield, a vessel containing liquid helium, etc., and is maintained at an extremely low temperature. In addition, the gradient magnetic field coil 204, which is the first coil arranged facing each other with the imaging space at the center, is designed to output a pulse-like waveform that makes the magnetic field strength in the space correspond to the distance from the imaging center.

[0019] 2 shows a conceptual diagram of the path of magnetic flux 601 that flows in the laminate 301, the disk-shaped magnetic pole 201, and the annular magnetic pole 202, created by the gradient magnetic field coil 204. The gradient magnetic field coil 204 is composed of a coil 204a that generates a magnetic field in the positive direction of the Z axis 402, and a coil 204b that generates a magnetic field that is directed downward in the negative direction of the Z axis 402, and a magnetic path is formed via the laminate and the static magnetic field magnet. The gradient magnetic field coil 204 is disposed inside the diameter of the disk-shaped magnetic pole 201.

[0020] The magnetic flux 601 generated by the gradient magnetic field coil 204 tends to flow through materials with high magnetic permeability such as iron materials and magnetic resistance with a short path length, but there is a possibility that the magnetic flux 602 flows into the annular magnetic pole 202 through a magnetic path through air with high magnetic resistance, for example, from the laminate 301 through the air, and an eddy current flows in the magnetic pole. In particular, the imaging region side (negative direction of the Z axis 402 in FIG. 1) of the laminate 301 is physically short in distance from the gradient magnetic field coil 204 and has small magnetic resistance, so that the leakage magnetic field to the annular magnetic pole 202 increases through the small magnetic resistance. In order to suppress the increase in the leakage magnetic field to the annular magnetic pole 202, the laminate 301 is shortened with respect to the R axis (radial direction) 401, which increases the physical distance between the annular magnetic pole and the laminate, thereby increasing the magnetic resistance. However, the amount of magnetic flux flowing from the gradient magnetic field coil 204 to the disk-shaped magnetic pole 201 increases, and eddy currents increase.

[0021] On the other hand, in the positive direction of the Z axis 402 of the laminate, the distance from the gradient magnetic field coil is long and the magnetic resistance is large, so the leakage magnetic field to the annular magnetic pole is smaller than when it is arranged in the negative direction of the Z axis 402. Therefore, the end 301a of the laminate is arranged in the center with respect to the Z axis 402 direction of the central part 301b of the laminate, which is a silicon steel plate, to reduce the leakage magnetic flux to the annular magnetic pole. In other words, the end of the laminate is arranged in the center with respect to the Z axis direction of the central part of the laminate.

[0022] Although the laminate end portion 301a is disposed in the center with respect to the Z-axis 402 direction with respect to the laminate center portion 301b in Fig. 2, it may be disposed in the positive direction of the Z-axis 402 as shown in Fig. 3. The flow of magnetic flux shown in Fig. 2 indicates gradient magnetic field coils called XGC and YGC that generate gradient magnetic fields with respect to the X-axis and Y-axis in the radial direction, and the flow of magnetic flux is different for ZGC that generates a gradient magnetic field with respect to the Z-axis direction.

[0023] 4 shows a schematic diagram of the laminate 301 of this embodiment, in which the laminate is configured by laminating a plurality of thin films 501, and for example, an electromagnetic steel sheet having excellent electrical and magnetic properties is used as the thin film 501. The thin film may be made of other materials as long as it has high electrical resistance and high magnetic permeability like the electromagnetic steel sheet.

[0024] Under a high magnetic field, for example, a static magnetic field of 1.5 T, the thickness of the laminated central portion 301b is arbitrary depending on the required performance of the MRI apparatus, but as described above, when silicon steel plate is used and the shielding of the magnetic flux reaching the disk-shaped magnetic pole 201 is taken into consideration, the thickness is preferably at least 20 mm or more. Also, the thickness of the laminated end portion 301b, which is an insulating block, is preferably at least 10 mm or more. Furthermore, the size of the thin film 501 is preferably 100 mm x 100 mm or less. That is, preferably, the laminated body is formed by stacking thin films having a thickness of 20 mm or more and a diameter of 100 mm or less, and the shapes of all the thin films are approximately the same.

[0025] Fig. 5 shows a configuration for maintaining the uniformity of the thickness of the laminate 301 shown in Fig. 4. In the laminate of this embodiment, electromagnetic force is generated, and in the worst case, the laminate structure may become misaligned, resulting in equipment failure. For this reason, it is desirable to fix the laminate with a fixing member, and the fixing member should hold the parts constituted by the laminate and the insulator 502 together.

[0026] Furthermore, when the radial length of the laminate central portion 301b described in this embodiment is located closer to the imaging region than the gradient magnetic field coil 204, there is a possibility that the magnetic flux generated by the gradient magnetic field coil may leak directly into the disk-shaped magnetic material, and therefore it is desirable that the radial length of the laminate is longer than the position of the gradient magnetic field coil 204. That is, it is preferable that the Z-axis direction thickness of the laminate on the annular magnetic material pole side is thinner than that of the center of the imaging space, and that the vertical bottom of the laminate end on the annular magnetic material pole side of the laminate is located at a higher position than the bottom of the laminate central portion on the imaging region side of the laminate. EXAMPLES

[0027] Example 2 is an example of a magnetic resonance imaging device including a pair of static magnetic field magnets arranged opposite to each other with the imaging space at the center, and a pair of gradient magnetic field coils arranged opposite to each other with the imaging space at the center, the static magnetic field magnets having disk-shaped magnetic poles and annular magnetic poles, the gradient magnetic field coils having a first coil arranged inside the diameter of the disk-shaped magnetic poles and applying a magnetic field in the Z-axis direction in which the pair of static magnetic field magnets face each other in the imaging space, and further including a laminate that shields the disk-shaped magnetic poles from magnetic flux generated from the first coil, the laminate having a central portion sandwiched between the first coil and the disk-shaped magnetic poles and an end portion facing the annular magnetic poles via air, the thickness of the end portion of the laminate in the Z-axis direction being thicker than the thickness of the central portion of the laminate in the Z-axis direction, and the position of the end portion of the laminate along the Z-axis direction on the imaging space side is located closer to the first coil than the position along the Z-axis direction on the imaging space side of the central portion of the laminate.

[0028] In the first embodiment, the laminate 301 is described as having a shape in which the thickness of the laminate end portion 301a is greater than the thickness of the laminate central portion 301b. However, as shown in Figures 6 and 7, if the laminate end portion 301c is thicker than the laminate central portion 301b and the bottom of the laminate end portion 301c is located lower than the bottom of the laminate central portion 301b, the leakage magnetic flux to the ring-shaped magnetic pole can be guided vertically downward, thereby reducing eddy currents and suppressing deterioration in image quality.

[0029] In this embodiment, the laminate is also formed by laminating thin films having a thickness of 20 mm or more and a diameter of 100 mm or less, and it is preferable that all the thin films have substantially the same shape. In addition, it is preferable that the gradient magnetic field coil is disposed inside the diameter of the static magnetic field magnet.

[0030] The present invention is not limited to the above-described embodiments, and various modifications are included. For example, the above-described embodiments are described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the configurations described. In addition, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. [Explanation of symbols]

[0031] 1: Open MRI device 101: Imaging space 201: Disk-shaped magnetic pole 202: Circular magnetic pole 204: Gradient magnetic field coil 301: Laminate 301a: laminate end 301b: Center of laminate 301c: laminate end portion having a thickness greater than that of the laminate central portion 301b 401: Radial direction 401a: Radial positive component 401b: Radial negative component 402: Height direction 501: Thin film 502: Insulating materials 601: Magnetic flux 602: Magnetic flux

Claims

1. A pair of static magnetic field magnets arranged to face each other with an imaging space at the center, and a pair of gradient magnetic field coils arranged to face each other with the imaging space at the center, The static magnetic field magnet has a disk-shaped magnetic pole and an annular magnetic pole, The gradient magnetic field coil has a first coil arranged inside the diameter of the disk-shaped magnetic pole and applying a magnetic field in a Z-axis direction in which the pair of static magnetic field magnets face each other in the imaging space, a laminate that shields the disk-shaped magnetic pole from magnetic flux generated from the first coil; the laminate has a central portion sandwiched between the first coil and the disk-shaped magnetic pole and an end portion facing the annular magnetic pole through air, the end portion of the laminate having a thickness in the Z-axis direction being greater than a thickness in the Z-axis direction of the central portion of the laminate; a position along the Z-axis direction of a surface of the end portion of the laminate on the imaging space side is closer to the first coil than a position along the Z-axis direction of a surface of the central portion of the laminate on the imaging space side; A magnetic resonance imaging apparatus comprising:

2. 2. The magnetic resonance imaging apparatus according to claim 1, The thickness of the central portion of the laminate in the Z-axis direction is 20 mm or more. A magnetic resonance imaging apparatus comprising:

3. 2. The magnetic resonance imaging apparatus according to claim 1, The laminate is formed by laminating thin films, and the thin films are all substantially the same in shape. A magnetic resonance imaging apparatus comprising:

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