Magnetic resonance imaging apparatus

Abstract

Provided is an MRI device having a magnetic shield structure that suppresses magnetic flux leakage. The MRI device includes a pair of static magnetic field magnets disposed in opposition about a subject space, and a pair of gradient magnetic field coils disposed in opposition about the subject space. The static magnetic field magnets include a disc-shaped magnetic pole (201) and a ring-shaped magnetic pole (202). The gradient magnetic field coils include a first coil (204) that generates a gradient magnetic field in the Z-axis direction in the subject space, and a laminate (301) that shields magnetic flux generated by the first coil from the disc-shaped magnetic pole. The laminate is thinner in the Z-axis direction on the ring-shaped magnetic pole side than in the center of the subject space. The lowest point of the laminate end portion on the ring-shaped magnetic pole side of the laminate is located higher than the lowest point of the laminate central portion on the subject space side of the laminate.

Description

Technical Field

[0001] The present invention relates to a magnetic shielding structure for suppressing leakage flux formed by tilted magnetic field coils in a magnetic resonance imaging (MRI) device. Background Technology

[0002] An MRI device is a device that applies a high-frequency magnetic field to the subject, causing hydrogen atoms in the body to resonate. A receiving coil captures the electromagnetic waves generated during this resonance and extracts these waves as an image. An MRI device mainly consists of the following components: a static magnetic field magnet for creating a uniform static magnetic field; a tilting magnetic field coil that generates a linear magnetic field in space and adds positional information to the signals obtained from the MRI; and the aforementioned receiving coil for receiving the electromagnetic waves generated from the hydrogen atoms.

[0003] Because the tilting magnetic field coils circulate pulsed current waveforms to generate the desired spatial magnetic field, they produce a fluctuating magnetic field through eddy currents corresponding to these current changes. This eddy currents then induce eddy currents in the metal components within the MRI device. This fluctuating magnetic field can degrade the uniformity of the static magnetic field or affect the spatial distribution of the tilting magnetic field, thus contributing to image degradation. In particular, the magnets used to generate the static magnetic field are mostly composed of iron poles, which may also affect the magnetization of these pole materials.

[0004] Therefore, in recent MRI devices, image degradation has been suppressed by adding a magnetic field with the opposite direction of the eddy current generated by an active shielding coil.

[0005] However, because the distance between the magnet and the camera space increases due to the area occupied by the active shield, the magnetic reluctance becomes higher, thus requiring a larger magnetomotive force and magnetic poles to generate the necessary static magnetic field. Although the increase in magnetomotive force can be adjusted by adjusting the coil current and the number of turns, it may lead to an increase in magnetic energy due to the increased coil current and a higher cost due to the larger size of the coil and magnetic poles.

[0006] Based on this background, the following countermeasure method is disclosed: In an MRI device with a low to medium magnetic field, a magnetic pole piece, such as a soft ferrite or silicon steel plate, is provided between the tilted magnetic field coil and the metal structure to serve as a magnetic flux path and to suppress the generation of eddy currents (see Patent Document 1).

[0007] Furthermore, in Patent Document 2, the following structure is known: In order to suppress eddy currents, a laminate consisting of a surface portion made of silicon steel plates with reduced diameter and a deep portion made of silicon steel plates with increased diameter is provided between the tilted magnetic field coil and the magnetic pole of the magnetic body to suppress eddy currents and prevent the reduction of apparent permeability caused by reducing the diameter of the silicon steel plates.

[0008] Furthermore, in Patent Document 3, a slit structure is added to the magnetic pole to reduce the area through which eddy currents flow and accelerate the decay time constant. By setting the structure in this way, leakage of magnetic flux to the magnetic pole is suppressed when the current of the tilted magnetic field coil changes over time.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: JP Japanese Patent Application Publication No. 05-182821

[0012] Patent Document 2: JP 2004-65714

[0013] Patent Document 3: JP 2016-96829

[0014] However, for example, in Patent Document 1, because a soft ferrite with low saturation magnetization is used, it loses its magnetic shielding effect under high magnetic fields, such as a static magnetic field of 1.5T, making it difficult to suppress eddy currents. Furthermore, in the structure described in Patent Document 2, various sizes of laminates need to be prepared, potentially increasing the manufacturing cost and time of the laminates. Moreover, in the structure described in Patent Document 3, because slits are added to the magnetic poles, additional reinforcing components may be required due to a decrease in the structural strength of the magnetic poles. Furthermore, because the amount of iron material reduces the number of slits, the apparent permeability decreases, potentially leading to higher costs due to an increase in the required magnetomotive force. Summary of the Invention

[0015] The object of the present invention is to solve the above-mentioned problems and provide an MRI device having a magnetic shielding structure that can suppress leakage flux.

[0016] To achieve the above objectives, the present invention provides a magnetic resonance imaging apparatus with the following structure: a pair of static magnetic field magnets arranged opposite each other with the imaging space as the center; and a pair of tilted magnetic field coils arranged opposite each other with the imaging space as the center. The static magnetic field magnets have disk-shaped magnetic body poles and ring-shaped magnetic body poles. The tilted magnetic field coils have: a first coil that provides a tilted magnetic field in the Z-axis direction in the imaging region; and a laminate that shields the magnetic flux generated from the first coil from the disk-shaped magnetic body poles. The thickness of the laminate in the Z-axis direction on the ring-shaped magnetic body pole side is thinner than that at the center of the imaging space, and the lowest vertical part of the laminate end on the ring-shaped magnetic body pole side is located higher than the lowest part of the laminate center on the imaging region side.

[0017] Furthermore, to achieve the above-mentioned objectives, the present invention provides a magnetic resonance imaging apparatus with the following structure: a pair of static magnetic field magnets arranged opposite each other with the imaging space as the center; and a pair of tilted magnetic field coils arranged opposite each other with the imaging space as the center. The static magnetic field magnets have disk-shaped magnetic body poles and ring-shaped magnetic body poles. The tilted magnetic field coils have: a first coil that provides a tilted magnetic field in the Z-axis direction in the imaging region; and a laminate that shields the magnetic flux generated from the first coil from the disk-shaped magnetic body poles. The thickness of the laminate in the Z-axis direction on the side of the ring-shaped magnetic body poles is thicker than that at the center of the imaging space. The lowest vertical part of the laminate end on the side of the ring-shaped magnetic body poles is positioned lower than the lowest part of the center of the laminate.

[0018] Invention Effects

[0019] According to the present invention, an MRI device can be provided that, without changing the diameter of the stacked body according to the position, can suppress leakage to the magnetic poles of the magnet constituting the static magnetic field, prevent image quality deterioration, and reduce eddy currents without increasing the manufacturing cost of the stacked body. Attached Figure Description

[0020] Figure 1 This is a diagram illustrating the structure of an open-type MRI device having the laminated body described in Embodiment 1.

[0021] Figure 2 This is a diagram illustrating the flow of magnetic flux formed by the laminate, in which the ends of the laminate involved in Example 1 are arranged relative to the Z-axis of the center of the laminate, the magnetic poles of the magnetic body, and the tilted magnetic field coil.

[0022] Figure 3 This is a diagram illustrating the flow of magnetic flux formed by the stacked body with the end of the stacked body arranged on the upper side relative to the Z-axis of the center of the stacked body, the magnetic pole of the magnetic body, and the tilted magnetic field coil, as described in Example 1.

[0023] Figure 4 This is a diagram showing a structure of the laminate involved in Example 1.

[0024] Figure 5 This is a diagram illustrating a structure of the insulator and laminate involved in Example 1.

[0025] Figure 6 This is a diagram illustrating other structures of the insulator and laminate involved in Example 2.

[0026] Figure 7 This is a diagram illustrating the structure of an MRI device having the laminated body described in Embodiment 2.

[0027] Explanation of reference numerals in the attached figures

[0028] 1: Open-type MRI device

[0029] 101: Camera Space

[0030] 201: Magnetic poles of a disk-shaped magnetic body

[0031] 202: Circular magnetic body poles

[0032] 204: Inclined magnetic field coil

[0033] 301: Laminated body

[0034] 301a: End of laminate

[0035] 301b: Central part of the laminate

[0036] 301c: The end portion of the laminate having a thickness greater than that of the central portion 301b of the laminate.

[0037] 401: Radial

[0038] 401a: Radial positive component

[0039] 401b: Radial negative component

[0040] 402: Height Direction

[0041] 501: Thin film body

[0042] 502: Insulating material

[0043] 601: Magnetic Flux

[0044] 602: Magnetic flux. Detailed Implementation

[0045] The following description uses the accompanying drawings to illustrate various embodiments of the present invention. Furthermore, in the drawings, the same parts are assigned the same numbers.

[0046]

Example 1

[0047] Example 1 is an embodiment of an MRI device with the following structure: a pair of static magnetic field magnets arranged opposite each other with the imaging space as the center; and a pair of tilted magnetic field coils arranged opposite each other with the imaging space as the center. The static magnetic field magnets have disk-shaped magnetic body poles and ring-shaped magnetic body poles. The tilted magnetic field coils have: a first coil that provides a tilted magnetic field in the Z-axis direction in the imaging region; and a laminate that shields the magnetic flux generated from the first coil from the disk-shaped magnetic body poles. The thickness of the laminate in the Z-axis direction on the side of the ring-shaped magnetic body poles is thinner than that at the center of the imaging space. The lowest vertical part of the laminate end on the side of the ring-shaped magnetic body poles is located higher than the lowest part of the center of the laminate on the imaging region side of the laminate.

[0048] Figure 1 This illustrates a schematic configuration of the open-type MRI apparatus of Embodiment 1. The MRI apparatus 1 generates a static magnetic field strength using a pair of static magnetic field magnets consisting of a disk-shaped magnetic pole 201 and a ring-shaped magnetic pole 202 arranged around the imaging space 101, and a ring-shaped coil utilizing a superconducting material or the like. Furthermore, to reduce the magnetic field outside the imaging area, a shielding coil is constructed that generates a magnetic field facing in the opposite direction to the ring-shaped coil.

[0049] When the toroidal coil used to generate the static magnetic field is made of a superconducting material, the coil is housed in a vacuum container, radiation shield, liquid helium, or similar container used for vacuum configuration, and maintained at an extremely low temperature. Furthermore, the tilted magnetic field coil 204, positioned opposite each other at the center of the imaging space, outputs a pulsed waveform with a magnetic field whose strength corresponds to the distance from the imaging center.

[0050] Figure 2 This is a conceptual diagram illustrating the path of the magnetic flux 601 formed by the tilted magnetic field coil 204 flowing through the laminated body 301, the disk-shaped magnetic pole 201, and the annular magnetic pole 202. The tilted magnetic field coil 204 consists of a coil 204a that generates a positive magnetic field along the Z-axis 402 and a coil 204b that generates a negative, downward-facing magnetic field along the Z-axis 402, forming a magnetic circuit via the laminated body and the static magnetic field magnet. The tilted magnetic field coil 204 is positioned inside the diameter of the disk-shaped magnetic pole 201.

[0051] The magnetic flux 601 formed by the tilted magnetic field coil 204 tends to flow through components with high permeability, such as iron, and with short magnetic reluctance paths. However, it may also flow through magnetic paths with high magnetic reluctance, such as air. For example, magnetic flux 602 flows from the laminate 301 through air into the magnetic pole 202 of the annular magnetic body, thereby causing eddy currents to flow through that pole. In particular, due to the imaging area side of the laminate 301 ( Figure 1 The distance between the inclined magnetic field coil 204 and the Z-axis 402 (negative direction) is physically short, resulting in low magnetic reluctance. Therefore, the leakage magnetic field flowing to the toroidal magnetic pole 202 increases due to this low magnetic reluctance. To suppress this increase in leakage magnetic field, the physical distance between the toroidal magnetic pole and the stack is lengthened by shortening the laminate 301 relative to the R-axis (radial) 401, thus increasing the magnetic reluctance. However, this also increases the inflow of magnetic flux from the inclined magnetic field coil 204 to the disk-shaped magnetic pole 201, leading to an increase in eddy currents.

[0052] In contrast, because the distance between the positive Z-axis 402 of the laminate and the tilted magnetic field coil is long, the magnetic reluctance is high. Therefore, compared with the aforementioned case where it is arranged in the negative Z-axis 402 direction, the leakage magnetic field leaking to the magnetic poles of the toroidal magnetic body is smaller. Therefore, the end 301a of the laminate is arranged in the center relative to the central portion 301b of the laminate (which is a silicon steel plate) in the Z-axis direction to reduce the leakage magnetic flux leaking to the magnetic poles of the toroidal magnetic body. That is, the end of the laminate is arranged in the center relative to the central portion of the laminate in the Z-axis direction.

[0053] In addition, Figure 2 In the middle, relative to the central part 301b of the laminate, the end part 301a of the laminate is arranged in the center relative to the Z-axis 402 direction, but it can also be as follows: Figure 3 As shown, it is configured relative to the positive Z-axis 402. Furthermore, Figure 2 The magnetic flux flow shown represents the inclined magnetic field coils called XGC and YGC that generate inclined magnetic fields relative to the X and Y axes in the radial direction. The magnetic flux flow is different with respect to ZGC, which generates an inclined magnetic field relative to the Z axis.

[0054] Figure 4 This diagram shows a schematic representation of the laminate 301 in this embodiment. The laminate is a structure in which multiple thin film bodies 501 are stacked. The thin film bodies 501 are, for example, made of electromagnetic steel sheets with excellent electrical and magnetic properties. The thin film body can also be made of other materials, as long as it has high resistance and high magnetic permeability, like an electromagnetic steel sheet.

[0055] Under high magnetic fields, such as a static magnetic field of 1.5T, the thickness of the central portion 301b of the laminate is arbitrary depending on the required performance of the MRI device. However, when using silicon steel plates as described above and considering the shielding of magnetic flux reaching the disk-shaped magnetic poles 201, a thickness of at least 20 mm is desirable. Furthermore, the thickness of the end portion 301a of the laminate, which serves as an insulating block, is desirable to be at least 10 mm. Moreover, the size of the thin film 501 is desirable to be 100 mm × 100 mm or less. That is, preferably, the laminate is constructed with a thickness of 20 mm or more by stacking thin film bodies with a diameter of 100 mm or less, and the shapes of the thin film bodies are preferably all approximately identical.

[0056] Figure 5 Indicates for Figure 4 The laminate 301 shown is a structure used to maintain the uniformity of its thickness. In the laminate of this embodiment, electromagnetic forces are generated, which, in the worst case, may cause equipment failure due to displacement of the laminate structure. Therefore, it is desirable to fix the laminate with a fixing member that holds the components consisting of the laminate and the insulator 502 together.

[0057] Furthermore, in this embodiment, if the radial length of the central portion 301b of the laminate is located closer to the imaging area than the tilted magnetic field coil 204, the magnetic flux generated by the tilted magnetic field coil may leak directly to the magnetic pole of the disk-shaped magnetic body. Therefore, it is desirable that the radial length of the laminate is longer than the position of the tilted magnetic field coil 204. That is, preferably, the thickness of the laminate in the Z-axis direction on the magnetic pole side of the annular magnetic body is thinner than that in the center of the imaging space, and the lowest vertical part of the laminate end on the magnetic pole side of the annular magnetic body is located higher than the lowest part of the central portion of the laminate on the imaging area side.

[0058]

Example 2

[0059] Example 2 is an embodiment of a magnetic resonance imaging device with the following structure: a pair of static magnetic field magnets arranged opposite each other with the imaging space as the center; and a pair of tilted magnetic field coils arranged opposite each other with the imaging space as the center. The static magnetic field magnets have disk-shaped magnetic body poles and ring-shaped magnetic body poles. The tilted magnetic field coils have: a first coil that provides a tilted magnetic field in the Z-axis direction in the imaging region; and a stack that shields the magnetic flux generated from the first coil from the disk-shaped magnetic body poles. The thickness of the stack in the Z-axis direction on the side of the ring-shaped magnetic body poles is thicker than that at the center of the imaging space. The lowest vertical part of the stack end on the side of the ring-shaped magnetic body poles is positioned lower than the lowest part of the center of the stack.

[0060] In Embodiment 1, the shape of the laminate 301 being thicker at the end 301a than at the center 301b was described. Figure 6 , Figure 7 As shown, the end portion 301c of the laminate is thicker than the central portion 301b of the laminate, and the lowest part of the end portion 301c is located at a lower position than the lowest part of the central portion 301b of the laminate. This allows the leakage magnetic flux that leaks to the magnetic poles of the annular magnetic body to be discharged vertically downward, thereby reducing eddy currents and suppressing image quality degradation.

[0061] In this embodiment, the laminate is constructed by stacking thin film bodies with a thickness of 20 mm or more and a diameter of 100 mm or less, and the shapes of the thin film bodies are preferably all approximately the same. Furthermore, the tilted magnetic field coil is preferably positioned inside the diameter of the static magnetic field magnet.

[0062] Furthermore, the present invention is not limited to the embodiments described above, and may further include various modifications. For example, the foregoing embodiments have been described in detail for ease of understanding of the present invention, but are not necessarily limited to having all the described structures. In addition, a part of the structure of one embodiment can be replaced with the structure of another embodiment, and the structure of another embodiment can be added to the structure of one embodiment.

Claims

1. A magnetic resonance imaging device, characterized in that, have: A pair of static magnetic field magnets positioned opposite each other with the camera space at the center; and A pair of tilted magnetic field coils are arranged opposite each other with the camera space as the center. The static magnetic field magnet has disk-shaped magnetic poles and ring-shaped magnetic poles. The tilted magnetic field coil has: The first coil provides an inclined magnetic field in the Z-axis direction within the imaging area; and The stacked body that shields the magnetic flux generated from the first coil from the magnetic poles of the disk-shaped magnetic body. The laminate has a central portion sandwiched between the first coil and the magnetic pole of the disk-shaped magnetic body, and an end portion facing the magnetic pole of the annular magnetic body through air. The thickness of the end portion of the laminate in the Z-axis direction is thinner than the thickness of the central portion of the laminate in the Z-axis direction. The lowest vertical part of the end of the annular magnetic pole side of the laminate is located higher than the lowest part of the central part of the imaging area side of the laminate.

2. The magnetic resonance imaging device according to claim 1, characterized in that, The laminate has a thickness of 20 mm or more.

3. The magnetic resonance imaging device according to claim 2, characterized in that, The laminate is constructed by stacking thin films with a diameter of less than 100 mm, all of which have the same shape.

4. The magnetic resonance imaging device according to claim 1, characterized in that, The tilted magnetic field coil is positioned inside the diameter of the static magnetic field magnet.

5. The magnetic resonance imaging device according to claim 1, characterized in that, The end of the laminate is positioned in the center relative to the central portion of the laminate along the Z-axis.

6. A magnetic resonance imaging device, characterized in that, have: A pair of static magnetic field magnets positioned opposite each other with the camera space at the center; and A pair of tilted magnetic field coils are arranged opposite each other with the camera space as the center. The static magnetic field magnet has disk-shaped magnetic poles and ring-shaped magnetic poles. The tilted magnetic field coil has: The first coil provides an inclined magnetic field in the Z-axis direction within the imaging area; and The stacked body that shields the magnetic flux generated from the first coil from the magnetic poles of the disk-shaped magnetic body. The laminate has a central portion sandwiched between the first coil and the magnetic pole of the disk-shaped magnetic body, and an end portion facing the magnetic pole of the annular magnetic body through air. The thickness of the end portion of the laminate in the Z-axis direction is greater than the thickness of the central portion of the laminate in the Z-axis direction. The lowest vertical part of the end of the annular magnetic pole side of the laminate is positioned lower than the lowest part of the central part of the laminate.

7. The magnetic resonance imaging apparatus according to claim 6, characterized in that, The laminate has a thickness of 20 mm or more.

8. The magnetic resonance imaging apparatus according to claim 7, characterized in that, The laminate is constructed by stacking thin films with a diameter of less than 100 mm, all of which have the same shape.

9. The magnetic resonance imaging apparatus according to claim 6, characterized in that, The tilted magnetic field coil is positioned inside the diameter of the static magnetic field magnet.

Images