JPH0666185B2 - Magnetic field generator for magnetic resonance imaging apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to nuclear magnetic resonance (NMR).
The present invention relates to a magnetic field generator using a permanent magnet used in a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”) that obtains a tomographic image of an inspection region of a subject by utilizing a phenomenon, and particularly, a magnetizing process and an assembly adjustment of the permanent magnet. The present invention relates to a magnetic field generator for an MRI apparatus that can easily achieve high magnetic field uniformity.

[0002]

2. Description of the Related Art An MRI apparatus utilizes an NMR phenomenon to obtain a nuclear spin density distribution at a desired inspection site in an object,
The relaxation time distribution and the like are measured, the measurement signal is arithmetically processed, and an image is displayed as a tomographic image of the inspection site. Here, when measuring a spatially wide range such as the human body, a static magnetic field of about 0.05 to 2 T (Tesla; 1 Tesla is 10,000 Gauss) is applied in a measurement space consisting of a spherical space with a diameter of 30 to 50 cm. It is necessary to have a magnetic field generator that can generate a uniformity of several tens of ppm or less. Conventionally, three types of magnetic field generators have been used: a normal conducting magnet, a superconducting magnet, and a permanent magnet.

A conventional magnetic field generator for an MRI apparatus using a permanent magnet, as described in Japanese Patent Laid-Open No. 62-177903, has a yoke formed in a polygonal tubular shape and this yoke. Main magnets composed of a pair of permanent magnets that are fixed in parallel to each other on the inner wall surfaces of the iron so as to face each other in parallel, and generate a main magnetic flux in the vertical direction on the parallel surfaces, and another inner wall surface of the yoke. And a sub-magnet composed of a plurality of permanent magnets, each of which is fixed to the main magnet to correct the uniformity of the magnetic field lines by the main magnet,
A void in which a subject can enter is formed in the central portion surrounded by the main magnet and the sub magnet, and a uniform static magnetic field is generated in this void.

This will be described with reference to FIGS. 5 and 6.
In the figure, a yoke 1 is a member that forms a magnetic circuit together with main magnets 2a and 2b and sub magnets 3, 4, 5 and 6 which will be described later, and is formed of a soft magnetic material into a rectangular tubular shape.
Main magnets 2a and 2b are fixed to the surfaces of the inner wall surface of the yoke 1 which are vertically positioned and face each other in parallel. The main magnets 2a and 2b generate a main magnetic flux in a direction perpendicular to the inner wall surface, and are composed of, for example, a pair of permanent magnets formed in a plate shape having a trapezoidal cross section. Sub magnets 3, 4, 5, 6 are fixed to the surfaces of the inner wall of the yoke 1 that face each other on the left and right. These sub magnets 3 to
Reference numeral 6 is for correcting the homogeneity of the lines of magnetic force by the main magnets 2a and 2b, and each is composed of, for example, a plate-shaped permanent magnet having an isosceles triangular cross section. And
For example, the surfaces of the sub magnets 3 to 6 on the short side are the main magnets 2a, 2
In the state of being joined to each of the trapezoidal inclined surfaces of b, a void C in which a subject can enter is formed in the central portion surrounded by the main magnets 2a and 2b and the sub magnets 3 to 6. Here, the respective permanent magnets forming the main magnets 2a and 2b and the sub magnets 3 to 6 are
It is magnetized uniformly in the direction perpendicular to the boundary surface with the void C, that is, in the direction indicated by each arrow in FIG. With the above configuration, a uniform static magnetic field can be generated in the space C in the direction indicated by the arrow A.

[0005]

However, in such a conventional magnetic field generator for an MRI apparatus, the main magnet 2 is used.
Since a and 2b are made of a pair of permanent magnets formed in a plate shape having a trapezoidal cross section, and the sub-magnets 3 to 6 are made of plate-shaped permanent magnets having an isosceles triangular cross section, respectively. It was difficult to improve the accuracy of the magnetic characteristics and reduce the variation in the magnetization of the permanent magnet having the cross-sectional shape. That is, it is difficult to form the shape of the trapezoidal section correctly, and to uniformly magnetize in the direction perpendicular to the surface in its thickness direction. Also, in the shape of the inequilateral triangular section, the angles of the respective sides are formed correctly and the gap C It was difficult to magnetize uniformly in the direction perpendicular to the middle side located at the boundary surface. Therefore, even after the magnetic member having the trapezoidal cross section or the isosceles triangular cross-section is manufactured, the magnetic member may be formed before or after the magnetization of the trapezoidal trapezoidal surface or the isosceles triangle in order to accurately obtain the angle of each side. Had to be cut off as appropriate. Further, when the main magnets 2a, 2b and the sub magnets 3 to 6 are fixed to the inner wall surface of the yoke 1, it is difficult to accurately join the trapezoidal inclined surface and each side of the isosceles triangle. At the same time, the subsequent fine adjustment is difficult, and it takes time to assemble and adjust the magnetic field generator. For these reasons, it is difficult to manufacture the permanent magnets that form the main magnets 2a and 2b and the auxiliary magnets 3 to 6, the yield is reduced, and it is difficult to achieve high magnetic field uniformity in assembly.

Therefore, the present invention addresses such problems and provides a magnetic field generator for an MRI apparatus capable of easily magnetizing the permanent magnets, assembling and adjusting them, and achieving high magnetic field uniformity. With the goal.

[0007]

In order to achieve the above object, a magnetic field generator for an MRI apparatus according to the present invention is arranged such that a yoke formed in a polygonal cylindrical shape and an inner wall surface of the yoke are parallel to each other. A main magnet composed of a pair of permanent magnets that are fixed to face the opposite surfaces and generate a main magnetic flux in the vertical direction on the parallel surface, and the main magnet that is fixed to the other inner wall surface of the yoke. A sub magnet including a plurality of permanent magnets for correcting the homogeneity of the magnetic force lines is provided, and a void is formed in the central portion surrounded by the main magnet and the sub magnet, through which a subject can enter, and the inside of this void is uniform. In the magnetic field generator of a magnetic resonance imaging apparatus for generating a static magnetic field, the main magnet and the sub-magnet are all formed in a rectangular parallelepiped shape having a rectangular cross section, and the magnetization direction is relative to one surface of the rectangular cross section. To be vertical One in which the.

Further, it is effective that the yoke is formed in a hexagonal tubular shape.

Further, the sub magnets are fixed to the slanted inner wall surface of the yoke via a block-shaped yoke column formed to have a predetermined length, and the gaps between the plurality of sub magnets on each inner wall surface. The end portions on the side may be arranged on the same plane.

[0010]

The magnetic field generating device thus constructed is a main magnet which is formed in a rectangular parallelepiped shape having a rectangular cross section and whose magnetization direction is perpendicular to one surface of the rectangular cross section. With the auxiliary magnet, the magnetizing process and the assembling adjustment of the permanent magnets forming the main magnet and the auxiliary magnet can be facilitated, and high magnetic field homogeneity can be achieved.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a front view showing an embodiment of a magnetic field generator of an MRI apparatus according to the present invention. This magnetic field generator uses a permanent magnet to generate a uniform static magnetic field in a space into which a subject is inserted. As shown in the figure, the yoke 1, the main magnets 10a and 10b, and the auxiliary magnets are used. 11a-11
d, 12a to 12d, 13a to 13d, 14a to 14d
And.

The yoke 1 is composed of main magnets 10a, 10 which will be described later.
b and the sub magnets 11a to 11d, 12a to 12d, 13a.
.About.13d and 14a to 14d, which are members forming a magnetic circuit, are formed of a soft magnetic material into, for example, a quadrangular cylinder. Main magnets 10a and 10b are fixed to the surfaces of the inner wall surface of the yoke 1 which are vertically positioned and face each other in parallel. These main magnets 10a and 10b generate a main magnetic flux in the vertical direction with respect to the upper and lower inner wall surfaces of the yoke 1, and are composed of a pair of permanent magnets formed in a rectangular parallelepiped shape having a rectangular cross section. In addition, the magnetization direction is set to be perpendicular to one surface of the rectangular cross section of the rectangular parallelepiped permanent magnet forming each of the main magnets 10a and 10b. In this embodiment, each main magnet 10a,
The magnetization direction of 10b is a vertical direction from the bottom to the top while being fixed to the inner wall surface of the yoke 1.

On the inner wall surface of the yoke 1 that faces the left and right, the sub magnets 11a to 11d, 12a to 12d,
13a to 13d and 14a to 14d are fixed. These sub magnets 11a to 14d are the main magnets 10a and 1d.
This is for correcting the uniformity of the magnetic field lines due to 0b, and is composed of a plurality of permanent magnets each formed in a rectangular parallelepiped shape having a rectangular cross section. In addition, each magnetization direction is made perpendicular to one surface of the rectangular cross section of the rectangular parallelepiped permanent magnet forming each of the sub magnets 11a to 14d.
Further, in this embodiment, the sub magnets are groups (11a to 11d) in the upper left inner wall surface, groups (12a to 12d) in the lower left inner wall surface, and groups (13) in the upper right inner wall surface.
a to 13d), a group of lower right inner wall surface (14a to 14d)
d) are provided in four groups, and each group is composed of, for example, four permanent magnets, each of which is a main magnet 10a.
Alternatively, the permanent magnet closer to 10b has a larger rectangular cross section, and the rectangular cross section becomes smaller as it goes away from the main magnet.

Further, each of the sub magnets 11a to 14d in each of the above groups has yoke columns 15a to 15d and 16a to 16 similarly formed in a rectangular parallelepiped shape having a rectangular cross section.
It is horizontally fixed to the left and right inner wall surfaces of the yoke 1 via d, 17a to 17d and 18a to 18d. These yoke columns 15a to 18d correspond to the sub magnets 11a to 14d.
The magnet 1 and the yoke 1 are magnetically coupled to each other. The one closer to the main magnet 10a or 10b has a larger rectangular cross section, and the rectangular cross section becomes smaller as the distance from the main magnet increases. . Therefore, each of the sub magnets 11a
14d are located on the central side of the yoke 1,
They are arranged in a sloping manner as they move away from the main magnet. In this embodiment, the magnetization directions of the sub magnets 11a to 14d are the first group 11a to 11d and the third group 13a to 13d.
Is the direction perpendicular to the left and right inner wall surfaces of the yoke 1 and outward,
Second group 12a-12d and fourth group 14
a to 14d are perpendicular to the left and right inner wall surfaces of the yoke 1 and directed inward.

In the above state, the main magnet 10a,
10b and sub magnets 11a to 11d, 12a to 12d, 1
A void C in which a subject can enter is formed in a central portion surrounded by 3a to 13d and 14a to 14d. Then, as a combination of the magnetization directions of the main magnets 10a and 10b and the sub magnets 11a to 14d, a uniform static magnetic field is generated in the space C in the direction indicated by the arrow A. The direction A of the static magnetic field coincides with the direction of the main magnetic flux generated by the main magnets 10a and 10b. When the static magnetic field is generated in the downward direction opposite to the arrow A, the main magnet 10a,
The magnetization directions of 10b and the sub magnets 11a to 14d may be set in the directions completely opposite to those in FIG. Further, in FIG. 1, the yoke columns 15a to 18d are formed by dividing each of the sub magnets into a rectangular parallelepiped shape, but not limited to this.
Each group 11a to 11d, 12a to 12 of the sub magnet
Alternatively, units of d, 13a to 13d, and 14a to 14d may be integrated to form a stepwise inclined member.

FIG. 2 is a front view showing a second embodiment of the present invention. In this embodiment, the yoke 1'is formed into a hexagonal tubular shape. In this case, the four corners of the quadrangular tubular yoke 1 shown in FIG. 1 are formed in a slanted shape, and the amount of the members constituting the hexagonal tubular yoke 1'can be reduced. Moreover, since each yoke column 15a-18d is located in the horizontal direction on the sloped inner wall surface of the said yoke 1 ',
The protruding length is shorter than in the case of Fig. 1, and the yoke column 1
It is also possible to reduce the usage amount of the members forming 5a to 18d. Therefore, according to the second embodiment, the weight of the magnetic field generator can be reduced and the external dimensions can be reduced. Further, as compared with the case of FIG. 1, the member length of the yoke 1'through which the magnetic flux passes is reduced, so that the magnetic resistance is reduced and the magnetic field generation efficiency can be improved.

In the embodiment shown in FIGS. 1 and 2,
Although the auxiliary magnets 11a to 14d are fixed to the inner wall surface of the yoke 1 or 1'via the yoke columns 15a to 18d, the present invention is not limited to this, and the yoke columns 15a to 18d may be omitted to omit the auxiliary magnets 11a to. 14d may be directly fixed to the inner wall surface of the yoke 1 or 1 '. In this case, the dimensions of the sub magnets 11a to 14d may be appropriately determined in consideration of the lengths of the yoke columns 15a to 18d shown in FIG. 1 or 2.

FIG. 3 is a front view showing a third embodiment of the present invention. In this embodiment, the yoke 1'is formed into a hexagonal tubular shape, and the sub magnets 11a to 11 of the first to fourth groups are formed.
d, 12a to 12d, 13a to 13d, 14a to 14d
Are fixed directly to the slanted inner wall surface of the yoke 1 '. In this case, since the yoke columns 15a to 18d shown in FIG. 2 are omitted, it is possible to further reduce the number of used members and reduce the weight of the magnetic field generator as compared with the embodiment of FIG. Further, since the respective sub-magnets 11a to 14d are fixed vertically to the slanted inner wall surface of the yoke 1 ', the respective sub-magnets 11a to 14d are fixed.
The magnetization direction of d will include an upward component, as shown in FIG. Thus, each of the sub magnets 11a to 14 is
Since the magnetization direction of d includes a component along the direction of the upward uniform static magnetic field indicated by the arrow A, the magnetic field generation efficiency is improved, and the dimensions of the respective sub-magnets 11a to 14d can be reduced.

FIG. 4 is a front view showing a fourth embodiment of the present invention. In this embodiment, the yoke 1'is formed into a hexagonal tubular shape, and the sub magnets 11a to 11 of the first to fourth groups are formed.
d, 12a to 12d, 13a to 13d, 14a to 14d
The block-shaped yoke columns 15b to 15 formed to have a predetermined length with respect to the slanted inner wall surface of the yoke 1 ', respectively.
d, 16b to 16d, 17b to 17d, 18b to 18d
And the ends of the plurality of sub-magnets on the inner wall of the inclined surface on the side of the void C are aligned and arranged on the same plane. In the embodiment of FIG. 4, the length of the yoke column to which the first sub magnets 11a, 12a, 13a, 14a of each group are fixed is shown as zero. In this case, the sub magnets 11a to 11d and 12a of the first to fourth groups are
Since the ends of 12d, 13a to 13d, and 14a to 14d on the side of the air gap C are aligned on the same plane for each group, the magnetic field distribution in the vicinity of the surface of each sub-magnet becomes smooth and uniform. The magnetic field space can be expanded. Therefore, a better MRI image can be obtained.

In the embodiments shown in FIGS. 1 to 4, the number of sub-magnets is shown to be four, one group consisting of four groups, for a total of 16 groups, but the present invention is not limited to this. The appropriate number may be determined in consideration of the balance between the degree of magnetic field homogeneity to be achieved and the number of steps required for manufacturing the magnetic field generator. 2 to 4, the yoke 1'is shown as being formed in a hexagonal tubular shape, but the present invention is not limited to this and may be formed in any other polygonal tubular shape.

[0021]

Since the present invention is constructed as described above,
The main magnet and the sub-magnets are all formed in a rectangular parallelepiped shape having a rectangular cross section, and the magnetization direction is perpendicular to one surface of the rectangular cross section, thereby forming the main magnet and the sub-magnet. The permanent magnet can be magnetized easily and accurately. Further, when assembling the magnetic circuit, the permanent magnets are easily handled, so that the magnetic circuit can be assembled with high accuracy and the yield can be improved. Further, the adjustment for achieving the magnetic field homogeneity becomes easy, and the high magnetic field homogeneity can be achieved.

Further, according to the embodiment shown in FIG. 2, since the yoke 1'is formed in a hexagonal tubular shape, the weight of the magnetic field generator can be reduced and the external dimensions can be reduced. Further, since the member length of the yoke 1'through which the magnetic flux passes is shortened, the magnetic resistance is reduced and the magnetic field generation efficiency can be improved.

Further, according to the embodiment shown in FIG. 4, the gap side ends of the plurality of sub-magnets fixed to the slanted inner wall surface of the yoke 1'are arranged on the same plane. , The magnetic field distribution in the vicinity of the surface of each sub magnet becomes smooth, and the space of the uniform static magnetic field can be expanded. Therefore, a better MRI image can be obtained.

[Brief description of drawings]

FIG. 1 is a front view showing an embodiment of a magnetic field generator of an MRI apparatus according to the present invention,

FIG. 2 is a front view showing a second embodiment of the present invention,

FIG. 3 is a front view showing a third embodiment of the present invention,

FIG. 4 is a front view showing a fourth embodiment of the present invention,

FIG. 5 is a front view showing a magnetic field generator of an MRI apparatus according to a conventional example,

FIG. 6 is a perspective view showing the conventional magnetic field generator.

[Explanation of symbols]

1, 1 '... Yoke, 10a, 10b ... Main magnet, 11a
-11d, 12a-12d, 13a-13d, 14a-
14d ... Sub magnets, 15a to 15d, 16a to 16d,
17a to 17d, 18a to 18d ... Yoke column, C ... Air gap, A ... Direction of uniform static magnetic field.

Claims (3)

[Claims] 1. A yoke having a polygonal cylindrical shape, and an inner wall surface of the yoke fixed to face parallel surfaces facing each other in parallel to generate a main magnetic flux in a vertical direction. A pair of permanent magnets, and a sub-magnet, which is fixed to the other inner wall surface of the yoke and corrects the uniformity of the lines of magnetic force of the main magnet. In the magnetic field generator of the magnetic resonance imaging apparatus for forming a void into which a subject can enter in a central portion surrounded by the magnet and the sub magnet, and generating a uniform static magnetic field in the void, the main magnet and the sub magnet are A magnetic field generator for a magnetic resonance imaging apparatus, characterized in that it is formed in a rectangular parallelepiped shape having a rectangular cross section, and its magnetization direction is perpendicular to one surface of the rectangular cross section. 2. The magnetic field generator for a magnetic resonance imaging apparatus according to claim 1, wherein the yoke is formed in a hexagonal tubular shape. 3. The sub-magnet is fixed to a slanted inner wall surface of a yoke through a block-shaped yoke column formed to have a predetermined length, and the plurality of sub-magnet gaps on each inner wall surface. The magnetic field generator of the magnetic resonance imaging apparatus according to claim 2, wherein the end portions on the side are arranged on the same plane.