JPH05291026A - Magnetic field generating device for mri - Google Patents

Abstract

PURPOSE:To decrease the leakage magnetic flux of an aperture part, to improve the uniform magnetic field of the prescribed part, especially the magnetic-field distribution in Z-direction of a magnetic-field generating device is made uniform, to reduce the weight of a magnet, and to form the title device of small size and light weight. CONSTITUTION:The intensity of magnetic field in Z-axis direction is uniformed by arranging pole pieces 4 and 4, being shorter than permanent magnet constituent bodies 2a to 2f in Z-axis direction, on the surface facing to the void of a pair of permanent magnets by which the main magnetic field is produced, and besides, protrusions 5 and 6 are provided on the center part and both end parts of the pole pieces 4 and 4. As a result, the leakage of magnetic flux of the aperture part of the device can be prevented, the length of the device in axial direction can be made short, the pole pieces 4 and 4 can be fixed to a yoke 1 by bolts, and a small-sized and weight-reduced device can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a magnetic field generator used in a magnetic resonance tomography apparatus for medical use (hereinafter referred to as MRI) and the like, and a permanent magnet structure is provided on an inner peripheral surface of a polygonal cylindrical yoke. The present invention relates to a magnetic field generator for MRI in which the magnetic flux pieces in a specific shape are arranged so as to face each other to reduce the leakage magnetic flux in the opening, improve the uniform magnetic field in a required portion, and reduce the amount of permanent magnets used.

[0002]

2. Description of the Related Art In a conventional MRI magnetic field generator, a magnetic circuit is known in which a plurality of rod-shaped permanent magnet components are annularly combined and arranged on the inner peripheral portion of a yoke made of polygonal tubular iron or the like. This configuration has less magnetic leakage than a magnetic circuit in which a pair of permanent magnet constructions facing each other with a gap formed between them are magnetically coupled by a yoke and integrated, and the magnets can be used efficiently. It is adopted as a possible configuration.

However, even in the above structure, magnetic leakage from the permanent magnet structure at the opening is not prevented, and it is necessary to keep the uniform magnetic field space away from the end of the magnetic circuit for the purpose of improving the magnetic field homogeneity of the required space. The entire length of the magnetic field generation device is increased, which leads to an increase in size of the magnet and the device. As a method for improving this problem, a method has been proposed (Japanese Patent Laid-Open No. 63-43304) in which the thickness of the magnet at the end of the magnet opening is partially increased to reduce the leakage flux at the opening of the apparatus.

According to this structure, a plate-shaped permanent magnet having a trapezoidal cross section and an isosceles triangular cross section is fixed to the inner wall surface of the core formed in a polygonal cylindrical shape so as to cover the inner wall surface, and is perpendicular to the interface with the hollow portion. Along the both ends of the permanent magnet magnetized in the direction of the axis Z, as shown in FIG. 7 showing a configuration example of the permanent magnet of the conventional magnetic field generator for MRI, as shown in FIG. The present invention is characterized by having a permanent magnet piece 10 for correcting a non-uniform magnetic field magnetized in a direction perpendicular to the interface with the portion. The magnetic flux generated by the permanent magnet piece 10 for correcting the nonuniform magnetic field compensates for the decrease in the magnetic field strength at both ends of the magnetic circuit, which leads to the correction of a slight decrease in the magnetic field strength in the uniform magnetic field region, and the uniform magnetic field. Since it is not necessary to extend the axial length of the magnetic field generation device to hold it once, it is possible to prevent an increase in size and weight of the device.

[0005]

However, in the above example, since the thickness of the device opening of the permanent magnet is increased, an expensive high-performance permanent magnet is additionally used, resulting in an increase in cost and an increase in weight of the magnet. Was a problem. Further, since the permanent magnet is exposed to the hollow portion, there is a problem that the nonuniformity of the magnetization characteristics of the permanent magnet material directly affects the magnetic field uniformity of the hollow portion. In addition, from the viewpoint of safety, it is necessary to fix the permanent magnet structure to the yoke, but in the structure where the permanent magnet is exposed, a space for the fixing plate is required, which equivalently increases the air gap length and reduces the weight of the permanent magnet. And the cost was increased.

According to the present invention, in a structure in which a rod-shaped permanent magnet structure is annularly arranged on a surface of a cylindrical yoke inner peripheral portion facing a space, a leakage magnetic flux at an opening is reduced and a uniform magnetic field at a required portion is improved. In particular, it is an object of the present invention to provide a magnetic field generator for MRI in which the magnetic field distribution in the Z-axis direction of the magnetic field generator is made uniform, the weight of the magnet is reduced, and the device can be reduced in size and weight.

[0007]

SUMMARY OF THE INVENTION According to the present invention, a pair of permanent magnet constructions for forming a main magnetic field are arranged opposite to each other on a cylindrical yoke inner peripheral surface, and a plurality of permanent magnets are arranged on the yoke inner peripheral surface. A magnetic field generating apparatus for MRI, wherein constituents are arranged and magnetically coupled to each other, wherein magnetic pole pieces are installed on a pair of permanent magnet constituent body gap facing surfaces that form a main magnetic field. is there.

Further, the present invention is the magnetic field generator for MRI characterized in that, in the above-mentioned structure, the magnetic pole pieces have protrusions at both ends and / or in the center of the air gap facing surface.

In the present invention, the magnetic circuit is arranged by combining permanent magnet components having a triangular cross section, a trapezoidal shape, or a rectangular shape in a polygonal cylindrical shape along the inner peripheral surface of the cylindrical yoke, for example, a polygonal cylindrical yoke. It is an integrated one. With this configuration, there is little magnetic leakage, and the magnet can be used efficiently.

FIG. 1 shows an embodiment of a magnetic circuit according to the present invention. The structure of the present invention will be described in detail below with reference to the drawings.
In the present invention, the cylindrical yoke 1 having a polygonal cylindrical shape or the like on which the permanent magnet structure is arranged on the inner peripheral surface is made of a soft magnetic material such as iron, and is particularly composed of a laminated body such as iron and a silicon steel plate. The use of the yoke increases the effect of reducing the eddy current. The silicon steel sheets to be laminated have a thickness of 0.35 mm and a thickness of 0.
A commonly used one such as 5 mm may be used. Further, the entire yoke need not be a silicon steel plate, and various configurations such as the position and shape of arranging the silicon steel plate or a combination with other materials are selected in consideration of the permanent magnet structure and device described later. It

The permanent magnet structure may have any shape and structure, and in particular, reference numeral 2b in the drawing in which no pole piece is installed,
By configuring the permanent magnet constructions 2c, 2e, 2f by a plurality of electrically insulated permanent magnet blocks, it is possible to reduce the eddy current generated in the permanent magnet constructions. The pair of permanent magnet constituents 2a and 2d, which are the main magnetic field generation sources that contribute to the formation of the magnetic field in the air gap, usually have a magnetization direction (indicated by an arrow M in the figure) in the magnetic field direction Y in the air gap 3,
It forms a space facing surface. Other permanent magnet structure 2
The magnetization directions of b, 2c, 2e, and 2f are variously selected according to the configuration of the magnetic circuit, the location on the yoke, etc., but if they are arranged so that a uniform static magnetic field can be obtained in the required air gap, It can be in any direction.

Alnico, ferrite magnets, permanent magnets such as Sm-Co are used for the permanent magnet structure, and especially R-Fe- having a maximum energy product of 30 MGOe or more.
By using a B-system permanent magnet, a stable uniform magnetic field can be obtained and the device can be downsized. In the present invention, the gradient magnetic field coils are installed along the inner circumferential surface of the permanent magnet constituting body arranged in combination in a polygonal cylindrical shape.

In the present invention, as shown in FIGS. 1 and 2, the magnetic pole pieces 4 and 4 are installed on the surfaces of the pair of permanent magnet constructing bodies 2a and 2d which form the main magnetic field, facing each other with a gap, and the magnets are fixed by bolts or the like. It penetrates and is fixed to yoke 1. Further, as shown in FIG. 3, the magnetic pole pieces 4 and 4 are not only on the air gap facing surfaces of the permanent magnet constituents 2a and 2d that form the main magnetic field, but are also arranged horizontally as shown in FIG. It may be arranged to abut on 2b. The shape of the pole piece is selected variously depending on the configuration of the magnetic circuit and the location on the permanent magnet structure, etc., but within the range of the above-mentioned pole piece shape, a uniform static magnetic field can be obtained in the required air gap. Should be selected. As the material of the pole piece, a soft magnetic material such as iron, soft ferrite or silicon steel plate is used. The thickness T of the pole piece is determined by the magnetic field strength of the air gap, the material of the pole piece, and the size of the uniform magnetic field space, and is preferably about 20 to 80 mm, more preferably about 50 mm. The length l in the axial direction (Z-axis direction) of the pole piece is preferably shorter than the length L of the entire device in order to prevent magnetic flux leakage, and is preferably about 1 = 0.8L.

When the pole piece is not installed or when the flat pole piece is installed, the leakage magnetic flux in the air gap of the magnetic field generator flows toward the opening, but as shown in FIGS. Protrusions (shims) on both ends of the surface facing the air gap of 4
By arranging 5, the magnetic flux can be pushed inside the device and the magnetic flux leakage at the opening can be reduced. No need for expansion means, downsizing of the device,
Weight reduction can be achieved. Further, by providing the central protruding portion 6 also in the center of the magnetic pole piece 4, the magnetic field uniformity in the gap 3 can be further improved, so that the magnet weight can be further reduced.

In the present invention, magnetic pole pieces shorter than the length of the pair of permanent magnet constituents forming the main magnetic field in the Z-axis direction are installed on the air gap facing surfaces of the permanent magnet constituents. Permanent magnet shims having a magnetization direction inclined by 30 ° to 60 ° with respect to the Z-axis direction so as to open toward the magnetic circuit opening are provided on the exposed magnet surfaces of the non-pole piece installation portions formed at both ends of the magnetic pole. As a result, the magnetic field distribution can be improved and the magnet weight can be reduced as compared with the conventional device.

[0016]

The present invention provides a magnetic field generator for MRI,
A magnetic pole piece having a shorter length in the Z-axis direction than that of the permanent magnet structure is arranged on the surfaces of the pair of permanent magnets forming the main magnetic field, which face each other to make the magnetic field strength uniform in the Z-axis direction. By providing protrusions at the center and / or both ends of the magnet, magnetic flux leakage at the device opening can be prevented and the length of the device in the axial direction can be shortened. Furthermore, by fixing the pole pieces to the yoke with bolts, etc. Can be fixed, and the size and weight of the device can be reduced.

[0017]

【Example】

Example 1 Similar to the configuration of FIG. 1, a thickness of 90 mm and a Z-axis direction length of 13
A permanent magnet structure composed of a pair of trapezoidal Nd-Fe-B-based magnets having a pair of magnetizing directions in the same direction and generating a main magnetic field on the surfaces of the yokes made of a hexagonal iron bulk body of 00 mm facing the air gap. The bodies were arranged to face each other, and a permanent magnet constituting body made of a Nd-Fe-B system magnet having a triangular cross section was arranged on the other facing surface of the void of the yoke to be magnetically integrated.

Further, magnetic pole pieces having a thickness of 40 mm, a width of 520 mm, and a length of 1050 mm in the Z-axis direction, which are made of bulk pure iron and soft ferrite, are fixed to the air gap facing surfaces of the permanent magnet structure for generating a pair of main magnetic fields, A magnetic pole piece having a protrusion having a thickness of 10 mm and a length of 600 mm in the Z-axis direction at the center of the magnetic pole piece, and a protrusion having a thickness of 60 mm and a length of 80 mm in the Z-axis direction at both ends of the magnetic pole piece as shown in FIG. Was established. FIG. 6A shows the result of measurement of the magnetic field distributions corresponding to the X axis, Y axis, and Z axis generated in the gap between the pole piece gap facing surfaces in the configuration of the present invention.

COMPARATIVE EXAMPLE 1 With the same structure as in Example 1, no pole pieces were installed, and a pair of main magnetic fields for generating a pair of main magnetic fields were formed at both ends of the air gap facing surface, as shown in FIG. 5, with a thickness of 100 mm and a width of 100 mm. 70 mm, Z
A magnetic field generator of a comparative example was prepared in which a permanent magnet shim 10 having a length of 410 mm in the axial direction and having a magnetization direction in the same direction as the permanent magnet structure was installed. In the structure of this comparative example, the X-axis and the Y-axis generated in the gap between the pole piece gap facing surfaces.
The result of measuring the magnetic field distribution corresponding to the Z axis and the Z axis is shown in B of FIG.
Shown in.

As shown in FIG. 6, the magnetic field distribution in the Z-axis direction is uniform in the structure of Example 1 in comparison with the structure of Comparative Example 1 not only in the center of the air gap but also in the periphery of the opening of the permanent magnet structure. I understand that The weight of the permanent magnet was 3,696 kg in the case where a uniform magnetic field was obtained in the configuration of the comparative example 1 in which the permanent magnet shim was installed in the opening of the air bearing surface of the permanent magnet structure for generating the pair of main magnetic fields. On the other hand, in the structure in which the pole pieces are attached to the air bearing surface of the permanent magnet structure of Example 1 and the projections are provided at the central portion and both end portions of the pole piece, the weight of the permanent magnet can be significantly reduced to 3,432 kg. did it.

[0021]

As is apparent from the embodiments of the present invention, a rod-shaped permanent magnet structure is annularly arranged on the inner surface of the polygonal cylindrical yoke facing the air gap, and the main magnetic field of the permanent magnet structure. Magnetic pole pieces having a shorter length in the Z-axis direction than the permanent magnet structure are installed on the air gap facing surfaces of the pair of permanent magnet structures that generate
Providing protrusions at both ends and the center of the air gap facing surface of the magnetic pole piece makes it possible to make the magnetic field distribution of the magnetic field generator uniform in the Z-axis direction, reduce the magnet weight, and reduce the size and weight of the device. ..

[Brief description of drawings]

FIG. 1 is a perspective explanatory view showing a configuration of a magnetic field generator for MRI according to the present invention.

FIG. 2 is an explanatory view of a main part of a magnetic field generator for MRI showing a configuration of a magnetic pole piece according to the present invention.

FIG. 3 is an MRI showing another configuration of the pole piece according to the present invention.
It is a principal part explanatory view of the magnetic field generator for use.

FIG. 4 is a schematic explanatory view showing a configuration of magnetic pole pieces of the magnetic field generator for MRI according to the present invention.

FIG. 5 is a schematic explanatory view showing a configuration of a permanent magnet shim of a conventional magnetic field generator for MRI.

FIG. 6 is a graph showing the relationship between the magnetic field strength and the magnetic field strength, which shows the magnetic field distribution corresponding to the X-axis, the Y-axis, and the Z-axis, which are generated in the space between the facing surfaces of the permanent magnet structure body. In the case of the invention, B indicates the case of the comparative example.

FIG. 7 is a vertical cross-sectional explanatory view showing a configuration example of a permanent magnet of a conventional magnetic field generator for MRI.

[Explanation of symbols]

 1 Yoke 2a, 2b, 2c, 2d, 2e, 2f Permanent magnet structure 3 Air gap 4 Magnetic pole piece 5 Protrusion (shim) 6 Central protrusion

Claims (2)

[Claims] 1. A pair of permanent magnet constructs that form a main magnetic field are arranged to face each other on a cylindrical yoke inner peripheral surface, and a plurality of permanent magnet constructs are magnetically arranged on the yoke inner peripheral surface. Combined MR
In the magnetic field generator for I, the magnetic pole generator for MRI is characterized in that pole pieces are installed on a pair of permanent magnet constituent body gap facing surfaces that form a main magnetic field. 2. The M according to claim 1, wherein the pole piece has protrusions at both ends and / or a central portion of the air gap facing surface.
Magnetic field generator for RI.