JP3885126B2 - Magnetic field generator for magnetic resonance imaging apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic field generation apparatus for a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”), and more particularly to a magnetic field generation apparatus that reduces eddy currents generated outside a magnetic pole piece of a static magnetic field generation apparatus.
[0002]
[Prior art]
As a magnetic field generator for an MRI apparatus, an apparatus in which a magnetic circuit is configured using a permanent magnet for generating a static magnetic field is known. As shown in the perspective view of FIG. 5 (a) and the side view of FIG. 5 (b), this apparatus includes a pair of permanent magnet components 3a and 3b that generate a static magnetic field, and a permanent magnet component 3a and a permanent magnet. A pair of magnetic pole pieces 2a, 2b for keeping the static magnetic field in the space between the constituent bodies 3b uniform, a pair of yokes 4a, 4b for magnetically coupling the permanent magnet constituent bodies 3a, 3b, and the yoke 4a , 4b are opposed to each other at a predetermined distance, and columns 5a to 5d are provided, and magnet covers 8a and 8b are provided so as to surround the permanent magnet components 3a and 3b so as to protect them. The permanent magnet structure 3a and the permanent magnet structure 3b are different in polarity from each other on the facing side, and the magnetic circuit is the permanent magnet structure 3a → the magnetic pole piece 2a → the magnetic pole piece 2b → the permanent magnet structural body 3b. → The yoke 4b → the columns 5a to 5d → the yoke 4a → the permanent magnet structure 3a. Further, the peripheral portions of the opposing magnetic pole pieces 2a and 2b have annular protrusions 7 having the same shape on both the upper and lower sides in order to suppress leakage of magnetic flux to the periphery and improve the uniformity of the internal space.
[0003]
In the space between the magnetic pole piece 2a and the magnetic pole piece 2b, in addition to the subject 6, an RF irradiation coil that irradiates the subject with pulsed electromagnetic waves and excites the nuclear spin of protons in the subject (see FIG. (Not shown), an RF receiving coil (not shown) that receives the NMR signal generated thereby, and a gradient coil 1 that superimposes the gradient magnetic field on the static magnetic field is provided to give position information to the NMR signal .
[0004]
In many cases, the gradient magnetic field coil 1 is housed in a groove formed in the magnetic pole piece 2 in order to make a space for the subject 6 to enter, so that most of the magnetic flux of the gradient magnetic field is shown in FIG. As indicated by 16, an eddy current is generated in this portion through the magnetic pole piece 2. Since this eddy current acts to prevent excitation of the gradient magnetic field, it causes inaccurate position information to the NMR signal. As a result, this eddy current not only interferes with diagnosis due to the appearance of artifacts such as ghosts in the tomographic image, but also prevents the generation of a pulsed gradient magnetic field necessary for high-speed imaging.
[0005]
In order to reduce the eddy current generated in the magnetic pole piece, it has been proposed to improve the material and structure of the magnetic pole (Japanese Patent Laid-Open Nos. 63-105745 and 2-87505).
[0006]
[Problems to be solved by the invention]
However, merely reducing the eddy current generated in the pole piece cannot sufficiently eliminate the eddy current artifact. Various causes for such an artifact are conceivable, but the inventors' knowledge has revealed that one of the causes is that the magnetic flux of the gradient magnetic field leaks to the outside of the pole piece 2. That is, it was thought that the magnetic flux of the gradient magnetic field did not leak around the magnetic pole piece 2 as indicated by 16 in FIG. 6 (a). It has also reached the magnet cover 8 which extends to the outside and is attached to protect the permanent magnet structure, and it has been found that the eddy current generated there can affect the image quality.
[0007]
In general, the magnet cover 8 is made of aluminum from the viewpoint of thermal conductivity, strength, and the like, and as shown in FIG. 6 (b), it usually consists of two parts for ease of assembly. It is completely electrically connected. For this reason, when an eddy current is generated here, the image quality is deteriorated. This deterioration in image quality is particularly noticeable when there is a difference in the way in which the upper and lower eddy currents are generated.
[0008]
An object of this invention is to provide the magnetic field generator for MRI apparatuses which reduced the eddy current which generate | occur | produces around a pole piece. Another object of the present invention is to provide a magnetic field generator for an MRI apparatus that can eliminate the influence of eddy currents and obtain high-quality MR images.
[0009]
[Means for Solving the Problems]
A magnetic field generator for an MRI apparatus according to the present invention includes a pair of permanent magnet structures opposed to each other by forming a gap, a yoke for magnetically coupling these permanent magnet structures to form a magnetic circuit, and each permanent magnet structure. A magnetic pole piece for keeping the static magnetic field of the air gap uniform and a magnet cover for protecting each permanent magnet structure, the magnet cover having eddy current reducing means for reducing eddy current generated in the magnet cover It is a thing.
[0010]
In a preferred aspect of the present invention, the eddy current reducing means is a slit provided in the magnet cover. By providing a slit in the magnet cover, the area of the magnet cover through which the eddy current flows can be reduced, thereby reducing the eddy current. In another aspect of the present invention, at least a part of the magnet cover is made of a nonconductive material as the eddy current reducing means. Also in this case, the area where the eddy current flows in the magnet cover can be reduced or the eddy current can be prevented from flowing through the magnet cover, thereby reducing the eddy current.
[0011]
In yet another aspect, a non-conductive material is provided between the magnet cover and the yoke. As a result, the magnet cover and the yoke are electrically non-conductive, the non-uniform eddy current generation resulting from the non-uniform electrical continuity is eliminated, and the deterioration of the image quality is prevented.
[0012]
The magnetic field generator of the present invention may be an arbitrary combination of the above-described eddy current reducing means.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a magnetic field generator for an MRI apparatus of the present invention will be described in detail with reference to the drawings.
[0014]
FIG. 1 is a diagram showing a magnetic field generator for an MRI apparatus having a structure in which a pair of yokes that respectively support a pair of permanent magnet structures is supported by two force rams. The provided 1st Example is shown. Although the upper permanent magnet structure and the yoke are omitted in the figure, this magnetic field generator has a vertically symmetrical structure with a gap in between, as in the conventional apparatus shown in FIG. Yes.
[0015]
A pole piece 2 for improving the static magnetic field uniformity is fixed to the permanent magnet structure on the side facing the air gap. The magnetic circuit consists of the upper permanent magnet structure → the upper magnetic pole piece → the lower magnetic pole piece → the lower permanent magnet structure → the yoke → the columns 5a and 5b → the yoke → the upper permanent magnet structure. A uniform static magnetic field is formed in the gap formed between the upper and lower permanent magnet structures. Although not shown in the figure, the RF irradiation coil and the RF receiving coil of the MRI apparatus are incorporated in the gap. A gradient coil 1 for applying a gradient to the static magnetic field is disposed in the groove of the pole piece 2.
[0016]
A magnet cover 8 for protecting the magnet is provided around the permanent magnet structure. The magnet cover 8 is fixed to the yoke 4 with screws or the like. In this embodiment, the magnet cover 8 has a plurality of slits 10 as shown. As shown in FIG. 1B, the slit 10 may be one that completely separates the magnet cover 8. In this case, the magnet cover 8 is composed of a plurality of parts, and each is fixed to the yoke 4 with screws or the like. The slit 10 may be a groove-like slit left without cutting off a part of the magnet cover 8 as shown in FIG. In this case, in order to facilitate assembly, the magnet cover is preferably composed of two or more parts as in the conventional case.
[0017]
In the magnetic field generator configured as described above, when the gradient magnetic field coil 1 of the MRI apparatus is driven to generate a gradient magnetic field pulse, the magnetic flux is generated as indicated by 17 in FIG. 6A and reaches the magnet cover. . At this time, since the magnet cover 8 is divided by the plurality of slits 10 and the area through which the eddy current generated by the magnetic flux 17 flows is reduced, the eddy current generated in this portion can be reduced.
[0018]
Although the number of slits 10 is not particularly limited, generally, the greater the number of slits 10, the greater the eddy current suppression effect and the lower the strength of the magnet cover. Therefore, about several are preferable in view of the mechanical strength of the magnet cover 8.
[0019]
Next, a second embodiment of the present invention will be described.
[0020]
In this embodiment, the structure (arrangement) of the magnetic field generator is the same as the conventional one, but a non-conductive material is used as the material constituting the magnet cover. By using a non-conductive material for the entire magnet cover, the generation of eddy currents in the magnet cover can be completely eliminated, and the influence on the image quality can be eliminated.
[0021]
As such a non-conductive material, a material that is insulative and has sufficient strength as a magnet cover is used, and specifically, fiber reinforced plastic, polysiloxane, or ceramics can be used.
[0022]
When a non-conductive material is used as the eddy current reducing means, only a part of the magnet cover may be used as the non-conductive material instead of the entire magnet cover. For example, as shown in FIG. 2, a non-conductive material may be provided between the two magnet cover parts. Even in this case, the conduction between the adjacent magnet covers can be cut off, and the area through which the eddy current flows can be reduced. Therefore, the generation of eddy current can be reduced as compared with the conventional magnet cover. In this case, the non-conductive material can be the same plastic or ceramic as in the second embodiment, but the strength is relaxed compared to the case where the entire magnet cover is non-conductive material. It is possible to select the material.
[0023]
In addition, when the magnet cover is divided into a plurality of parts as shown in FIG. 1A, some of the parts may be made of the above-described non-conductive material. Further, as shown in FIG. 3A, a non-conductive material 11 may be filled between the slits 10 provided in the magnet cover 8. Also in this case, the slit 10 may be provided so as to completely separate the magnet cover (FIG. 3B) or may be provided halfway (FIG. 3C), as in the embodiment shown in FIG. . Since the strength of the magnet cover 8 can be maintained by filling the slit 10 with the nonconductive material 11, the number of slits can be increased as compared with the case where only the slits are provided, and thus the effect of reducing the eddy current can be further improved. Can do.
[0024]
Next, a third embodiment of the present invention will be described with reference to FIG.
[0025]
The magnetic field generator of this embodiment also has a configuration in which the permanent magnet structure and the pole piece 2 are fixed on the yoke 4 and the magnet cover 8 is disposed around the permanent magnet structure. However, in this embodiment, a non-conductive material 11 is interposed between the magnet cover 8 and the yoke 4 as shown in the figure. As the non-conductive material 11, the material exemplified in the second embodiment can be used, and the magnet cover 8 is fixed to the yoke 4 with screws or the like with such a material interposed.
[0026]
Thus, by interposing the non-conductive material 11 between the magnet cover 8 and the yoke 4, the current flowing from the magnet cover 8 to the yoke 4 can be suppressed and the generation of eddy currents can be reduced. In addition, the non-uniformity of eddy current generation around the upper and lower magnets can be eliminated. If the magnet cover 8 and the yoke 4 are simply fixed with screws or the like, the contact state is likely to be uneven, and eddy currents may vary in the vertical direction. By interposing the non-conductive material 11 between the yoke 4 and the yoke 4, this variation can be eliminated. As a result, it is possible to prevent image deterioration due to eddy current non-uniformity.
[0027]
In addition, although each Example mentioned above can each implement | achieve independently, it can acquire an eddy current reduction effect, respectively, but it can implement in combination and can reduce an eddy current more effectively. . For example, a magnet cover 8 provided with a plurality of slits 10 may be fixed to the yoke 4 via a non-conductive material, and a non-conductive material is interposed between adjacent magnet covers and between the magnet cover and the yoke. A non-conductive material may be interposed between them.
[0028]
Moreover, in the said Example, although the magnetic field generator of the structure which supported the yoke and the permanent magnet structure with two columns was illustrated and demonstrated, the structure and arrangement | positioning of a magnetic field generator are changed arbitrarily in a claim. be able to.
[0029]
【The invention's effect】
As described above, according to the magnetic field generator for an MRI apparatus of the present invention, an eddy current reducing means such as a slit or a non-conductive material is provided in the magnet cover portion, so that it is generated in the magnet cover attached around the pole piece. Eddy current can be reduced. As a result, image quality can be improved and high-speed imaging can be achieved. Also, according to the magnetic field generator of the present invention, the effect of suppressing eddy currents around the upper and lower magnets can be made uniform, so that deterioration of image quality due to nonuniform eddy current generation can be prevented.
[Brief description of the drawings]
FIG. 1A is a partial perspective view showing an embodiment of a magnetic field generator according to the present invention, and FIGS.
FIG. 2 is a partial perspective view showing another embodiment of the magnetic field generator of the present invention.
FIGS. 3A and 3B are partial perspective views showing another embodiment of the magnetic field generator according to the present invention, and FIGS.
FIG. 4 is a perspective view showing another embodiment of the magnetic field generator of the present invention.
FIGS. 5A and 5B illustrate a conventional magnetic field generator, in which FIG. 5A is a perspective view and FIG. 5B is a side view.
6A is a diagram illustrating the structure of a magnetic field generator and the flow of magnetic flux of a gradient magnetic field, and FIG. 6B is a diagram illustrating a part of a conventional magnetic field generator.
[Explanation of symbols]
2 ... Magnetic pole piece 3 ... Permanent magnet structure 4 ... yoke 8 ... magnet cover 10 ... slit 11 ... ... Non-conductive materials

Claims (4)

A pair of permanent magnet structures opposed to each other by forming a gap, a yoke that magnetically couples these permanent magnet structures to form a magnetic circuit, and a pole piece that keeps the static magnetic field in the gap of each permanent magnet structure uniform And a magnetic field generator for a magnetic resonance imaging apparatus comprising a magnet cover that covers the periphery of the permanent magnet structure,
The magnetic field generator according to claim 1, wherein the magnet cover is formed with one or more slits as eddy current reducing means for reducing eddy current generated in the magnet cover. The magnetic field generator according to claim 1, wherein
The magnet cover is composed of a plurality of members divided by the slit, and one or more members among the plurality of members are composed of a non-conductive material. The magnetic field generator according to claim 1, wherein
The magnetic field generator according to claim 1, wherein the slit is provided partway along a depth direction of the slit. The magnetic field generator according to any one of claims 1 to 3,
A magnetic field generator characterized in that the slit is filled with a non-conductive material selected from fiber reinforced plastic, polysiloxane and ceramics.

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