Magnetic Resonance Imaging System

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 for suppressing leakage magnetic flux generated by a gradient magnetic field coil in a magnetic resonance imaging (hereinafter, MRI) apparatus.

Background Art

[0002] An MRI apparatus is a device that applies a high-frequency magnetic field to a subject, generates resonance in hydrogen atoms in the human body, acquires the radio waves generated during the resonance with a receiving coil, and extracts the obtained radio waves as an image. The MRI apparatus mainly includes 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 MRI, and a receiving coil for receiving the radio waves generated from hydrogen atoms as described above.

[0003] Since the gradient magnetic field coil passes a pulsed current waveform to generate a desired spatial magnetic field, a fluctuating magnetic field is generated by eddy currents corresponding to the current change, and eddy currents are generated in metal parts in the MRI apparatus. This fluctuating magnetic field deteriorates the uniformity of the static magnetic field and affects the spatial distribution of the gradient magnetic field, which is a cause of image degradation. In particular, the magnet for generating the static magnetic field is often composed of an iron material pole, and there is a possibility of also affecting the magnetization of the pole material.

[0004] Therefore, in recent MRI apparatuses, image degradation has been suppressed by adding a function called an active shield coil that cancels out the reverse magnetic field in which eddy currents are generated.

[0005] However, due to the area occupied by this active shield, the distance between the magnet and the imaging space increases, and the magnetic resistance increases, so the magnetomotive force and poles for generating the necessary static magnetic field become larger. The increase in the magnetomotive force can be adjusted by the coil current and the number of turns, but there is a possibility of an increase in magnetic energy due to an increase in the coil current and an increase in cost due to an increase in the size of the coil and poles.

[0006] Against this backdrop, in MRI devices with medium to low magnetic fields, countermeasures have been disclosed that involve placing magnetic pole pieces, such as soft ferrite or silicon steel plates, between the gradient magnetic field coil and the metal structure, which can serve as a path for magnetic flux and suppress the generation of eddy currents (see Patent Document 1).

[0007] Furthermore, Patent Document 2 describes a configuration that suppresses eddy currents by providing a laminate between a gradient magnetic field coil and a magnetic pole of a magnetic material, which is divided into a surface layer made of silicon steel plates with a smaller diameter and a deep layer made of silicon steel plates with a larger diameter. This configuration prevents the suppression of eddy currents and the reduction in apparent permeability caused by reducing the diameter of the silicon steel plates.

[0008] Furthermore, Patent Document 3 describes a configuration in which a slit structure is incorporated into the magnetic pole to reduce the region through which eddy currents flow and to accelerate the decay time constant, thereby suppressing leakage of magnetic flux to the magnetic pole when the current of 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] Japanese Patent Publication No. 2004-65714 [Patent Document 3] Japanese Patent Publication No. 2016-96829 [Overview of the project] [Problems that the invention aims to solve]

[0010] However, in Patent Document 1, for example, soft ferrite with low saturation magnetization is used, so under high magnetic fields, for example under a static magnetic field of 1.5T, the magnetic shielding effect is lost, making it difficult to suppress eddy currents. Furthermore, the configuration described in Patent Document 2 requires the preparation of multiple sizes of laminates, which could lead to increased manufacturing costs and longer manufacturing times for the laminates. Moreover, in the configuration described in Patent Document 3, adding slits to the magnetic poles may worsen the structural strength of the magnetic poles, potentially requiring the addition of reinforcing members. In addition, the amount of iron-containing material in the slits decreases, reducing the apparent permeability, which could lead to increased costs due to the increased required magnetomotive force.

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

[0012] To achieve the above objective, the present invention comprises a pair of static magnetic field magnets arranged opposite to each other around the imaging space, and a pair of gradient magnetic field coils arranged opposite to each other around the imaging space, wherein the static magnetic field magnets have a disc-shaped magnetic pole and an annular magnetic pole, and the gradient magnetic field coils have a first coil arranged inside the diameter of the disc-shaped magnetic pole and providing a magnetic field in the Z-axis direction, which is the direction in which the pair of static magnetic field magnets face each other in the imaging space, and further comprises a laminate that shields the magnetic flux generated from the first coil to the disc-shaped magnetic pole, wherein the laminate has a central portion sandwiched between the first coil and the disc-shaped magnetic pole and an end portion facing the annular magnetic pole via air. The central part of the laminate and the edges of the laminate each have a uniform thickness. The present invention provides a magnetic resonance imaging apparatus configured such that the thickness of the end of the laminate in the Z-axis direction is greater than the thickness of the central part of the laminate in the Z-axis direction, and the position of the surface of the end of the laminate facing the imaging space along the Z-axis direction is closer to the first coil than the position of the surface of the central part of the laminate facing the imaging space along the Z-axis direction. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide an MRI apparatus that suppresses leakage to magnetic poles constituting a static magnetic field magnet, prevents deterioration of image quality, and reduces eddy currents without changing the diameter of the laminate depending on the position, and without increasing the manufacturing cost of the laminate.

Brief Description of the Drawings

[0014] [Figure 1] It is a diagram showing one configuration of an open-type MRI apparatus provided with a laminate according to Example 1. [Figure 2] It is a diagram showing the flow of magnetic flux created by a laminate, a magnetic body pole, and an inclined magnetic field coil in which the end of the laminate according to Example 1 is arranged at the center with respect to the Z-axis of the central part of the laminate. [Figure 3] It is a diagram showing the flow of magnetic flux created by a laminate, a magnetic body pole, and an inclined magnetic field coil in which the end of the laminate according to Example 1 is arranged above with respect to the Z-axis of the central part of the laminate. [Figure 4] It is a diagram showing one configuration of a laminate according to Example 1. [Figure 5] It is a diagram showing one configuration of an insulator and a laminate according to Example 1. [Figure 6] It is a diagram showing another configuration of an insulator and a laminate according to Example 2. [Figure 7] It is a diagram showing one configuration of an MRI apparatus provided with a laminate according to Example 2.

Modes for Carrying Out the Invention

[0015] Hereinafter, various examples of the present invention will be described with reference to the drawings. In each figure, the same parts are given the same numbers.

Examples

[0016] Example 1 includes a pair of static magnetic field magnets arranged opposite to each other centering on an imaging space, and a pair of gradient magnetic field coils arranged opposite to each other centering on the imaging space. The static magnetic field magnets have a disc-shaped magnetic body pole and an annular magnetic body pole. The gradient magnetic field coils are arranged inside the diameter of the disc-shaped magnetic body pole and have a first coil that provides a magnetic field in the Z-axis direction, which is the direction in which the pair of static magnetic field magnets face each other in the imaging space. The gradient magnetic field coils further include a laminate that shields the magnetic flux generated from the first coil with respect to the disc-shaped magnetic body pole. The laminate has a central portion sandwiched between the first coil and the disc-shaped magnetic body pole and an end portion facing the annular magnetic body pole. The thickness of the laminate in the Z-axis direction at the end portion is thinner than the thickness of the laminate in the Z-axis direction at the central portion. The position along the Z-axis direction of the surface on the imaging space side of the central portion of the laminate is closer to the first coil than the position along the Z-axis direction of the surface on the imaging space side of the end portion of the laminate. This is an example of an MRI apparatus having such a configuration.

[0017] FIG. 1 shows a schematic form of the open-type MRI apparatus of Example 1. The MRI apparatus 1 includes a pair of static magnetic field magnets composed of a disc-shaped magnetic body pole 201 and an annular magnetic body pole 202, which are arranged centering on the imaging space 101, and generates a static magnetic field intensity using an annular coil made of a superconducting material or the like. Further, in order to reduce the magnetic field outside the imaging region, a shield coil that generates a magnetic field in the opposite direction to the annular coil is configured.

[0018] When the annular coil for forming the static magnetic field is a superconducting material, the coil is housed in a vacuum container, a radiation shield, and a container such as liquid helium to maintain an extremely low temperature in order to arrange the coil in a vacuum. Further, the gradient magnetic field coil 204, which is a first coil arranged opposite to each other centering on the imaging space, outputs a pulsed waveform having a magnetic field according to the distance from the imaging center for the magnetic field intensity in the space.

[0019] Figure 2 shows a conceptual diagram of the path of magnetic flux 601 flowing through the laminate 301, the disc-shaped magnetic pole 201, and the annular magnetic pole 202, created by the gradient magnetic field coil 204. The gradient magnetic field coil 204 consists 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 downward magnetic field in the negative direction of the Z-axis 402, and the magnetic path is formed through the laminate and the static magnetic field magnet. The gradient magnetic field coil 204 is positioned inside the diameter of the disc-shaped magnetic pole 201.

[0020] The magnetic flux 601 created by the gradient magnetic field coil 204 tends to flow through materials with high magnetic permeability, such as iron, and through magnetic resistance with a short path length. However, if the magnetic path passes through air, which has high magnetic resistance, for example, the magnetic flux 602 may flow from the laminate 301 through the air to the annular magnetic pole 202, and eddy currents may flow in that pole. In particular, on the imaging region side of the laminate 301 (negative Z-axis 402 direction in Figure 1), the distance from the gradient magnetic field coil 204 is physically short and the magnetic resistance is small, so the leakage magnetic field to the annular magnetic pole 202 increases through this small magnetic resistance. To suppress this increase in the leakage magnetic field to the annular magnetic pole 202, shortening the R-axis (radial direction) 401 of the laminate 301 increases the physical distance between the annular magnetic pole and the laminate, thus increasing the magnetic resistance. However, the amount of magnetic flux flowing from the gradient coil 204 to the disk-shaped magnetic pole 201 increases, and the eddy currents increase.

[0021] In contrast, the positive Z-axis direction of the laminate has a longer distance from the gradient magnetic field coil and higher magnetic resistance, resulting in smaller leakage magnetic fields to the annular magnetic poles compared to the case where it is positioned in the negative Z-axis direction as described above. Therefore, the end portion 301a of the laminate is positioned in the center with respect to the Z-axis direction of the central portion 301b of the laminate, which is made of silicon steel sheet, thereby reducing leakage flux to the annular magnetic poles. In other words, the end portion of the laminate is positioned in the center with respect to the Z-axis direction of the central portion of the laminate.

[0022] In Figure 2, the end portion 301a of the laminate is positioned centrally with respect to the Z-axis 402 direction relative to the central portion 301b of the laminate, but it may also be positioned in the positive direction of the Z-axis 402, as shown in Figure 3. Furthermore, the magnetic flux flow shown in Figure 2 represents gradient coils called XGC and YGC that generate gradient magnetic fields in the radial direction, specifically along the X and Y axes, while the magnetic flux flow differs for ZGC, which generates a gradient magnetic field in the Z-axis direction.

[0023] Figure 4 shows a schematic diagram of the laminate 301 of this embodiment. The laminate is constructed by stacking multiple thin films 501, and the thin films 501 are, for example, made of electrical steel sheets with excellent electrical and magnetic properties. These thin films may be made of other materials as long as they have high electrical resistance and high magnetic permeability, like electrical steel sheets.

[0024] Under high magnetic fields, for example under a static magnetic field of 1.5T, the thickness of the central portion 301b of the laminate is arbitrary depending on the performance requirements of the MRI device. However, as mentioned above, silicon steel plates are used, and when considering the shielding of magnetic flux reaching the disc-shaped magnetic poles 201, it is desirable that the thickness be at least 20mm. Furthermore, it is desirable that the thickness of the laminate end portion 301b, which is an insulating block, be at least 10mm. In addition, it is desirable that the size of the thin film 501 be 100mm x 100mm or less. In other words, preferably, the laminate is constructed by stacking thin films with a thickness of 20mm or more and a diameter of 100mm or less, and it is preferable that the shapes of the thin films are all approximately the same.

[0025] Figure 5 shows a configuration for maintaining the uniformity of the thickness of the laminate 301 shown in Figure 4. Electromagnetic forces are generated in the laminate in this embodiment, and in the worst case, displacement of the laminate structure may occur, potentially leading to equipment failure. Therefore, it is desirable to fix the laminate with a fixing member, and for this fixing member to hold together the components composed of the laminate and the insulator 502.

[0026] Furthermore, if the radial length of the central portion 301b of the laminate described in this embodiment is on the imaging area side of the gradient magnetic field coil 204, there is a possibility that the magnetic flux created by the gradient magnetic field coil will leak directly into the disc-shaped magnetic material. Therefore, it is desirable that the radial length of the laminate be longer than the position of the gradient magnetic field coil 204. In other words, it is preferable that the thickness in the Z-axis direction of the laminate on the magnetic pole side of the annular magnetic material is thinner than that of the center of the imaging space, and that the lowest vertical position of the laminate end on the magnetic pole side of the annular magnetic material is higher than the lowest position of the central portion of the laminate on the imaging area side. [Examples]

[0027] Embodiment 2 is an embodiment of a magnetic resonance imaging apparatus comprising a pair of static magnetic field magnets arranged facing each other around the imaging space, and a pair of gradient magnetic field coils arranged facing each other around the imaging space, wherein the static magnetic field magnets have a disc-shaped magnetic pole and an annular magnetic pole, and the gradient magnetic field coils have a first coil arranged inside the diameter of the disc-shaped magnetic pole and providing a magnetic field in the Z-axis direction, which is the direction in which the pair of static magnetic field magnets face each other in the imaging space, and further comprising a laminate that shields the magnetic flux generated from the first coil from the disc-shaped magnetic pole, wherein the laminate has a central part sandwiched between the first coil and the disc-shaped magnetic pole and an end facing the annular magnetic pole via air, the thickness of the end of the laminate in the Z-axis direction is thicker than the thickness of the central part of the laminate in the Z-axis direction, and the position of the surface of the end of the laminate facing the imaging space along the Z-axis direction is closer to the first coil than the position of the surface of the central part of the laminate facing the imaging space along the Z-axis direction.

[0028] In Example 1, the laminate end portion 301a constituting the laminate 301 was described as being thicker than the central portion 301b of the laminate. However, as shown in Figures 6 and 7, if the laminate end portion 301c is thicker than the central portion 301b of the laminate, and the lowest part of the laminate end portion 301c is lower than the lowest part of the central portion 301b, the leakage magnetic flux to the annular magnetic pole can be released vertically downward, reducing eddy currents and suppressing deterioration of image quality.

[0029] In this embodiment as well, the laminate is composed of stacked thin films with a thickness of 20 mm or more and a diameter of 100 mm or less, and it is preferable that the shapes of the thin films are all approximately the same. Furthermore, it is preferable that the gradient magnetic field coil be positioned inside the diameter of the static magnetic field magnet.

[0030] It should be noted that the present invention is not limited to the embodiments described above, and includes various further modifications. For example, the embodiments described above are described in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described. Furthermore, 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-type MRI system 101: Imaging space 201: Disk-shaped magnetic poles 202: Circular magnetic poles 204: Gradient Coil 301: Laminate 301a: End of laminate 301b: Central part of the laminate 301c: End portion of the laminate with a thickness greater than the central part 301b of the laminate. 401: Radial 401a: Radial positive component 401b: Radial negative component 402: Height 501: Thin film 502: Insulating materials 601: Magnetic flux 602: Magnetic flux

Claims

1. It comprises a pair of static magnetic field magnets arranged opposite each other around the imaging space, and a pair of gradient magnetic field coils arranged opposite each other around the imaging space, The aforementioned static magnetic field magnet has a disc-shaped magnetic pole and an annular magnetic pole. The gradient magnetic field coil has a first coil that is positioned inside the diameter of the magnetic pole of the disc-shaped magnetic material and provides a magnetic field in the Z-axis direction, which is the direction in which the pair of static magnetic field magnets face each other in the imaging space. The laminate further comprises a laminate that shields the magnetic flux generated from the first coil from the magnetic pole of the disc-shaped magnetic body, The laminate has a central portion sandwiched between the first coil and the disc-shaped magnetic pole and an end portion facing the annular magnetic pole via air, the central portion and the end portion of the laminate each have a uniform thickness, and 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 position of the end of the laminate on the imaging space side along the Z-axis direction is closer to the first coil than the position of the central part of the laminate on the imaging space side along the Z-axis direction. A magnetic resonance imaging apparatus characterized by the following features.

2. A 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 characterized by the following features.

3. A magnetic resonance imaging apparatus according to claim 1, The laminate is constructed by stacking thin films, and the shapes of the thin films are all substantially the same. A magnetic resonance imaging apparatus characterized by the following features.

Images