JP3059596B2 - Magnetic field generator for MRI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a magnetic field generator used for a medical magnetic resonance tomography apparatus (hereinafter referred to as "MRI") and the like, in which a permanent magnet structure is provided on the inner peripheral surface of a polygonal yoke. A magnetic pole piece of a specific shape is arranged opposite to the magnetic pole piece, and projections made of permanent magnet pieces are provided at both ends of the opening of the permanent magnet structure in which the magnetic pole piece is not arranged, thereby reducing the leakage magnetic flux of the opening and reducing the required space. Field of the Invention The present invention relates to a magnetic field generator for MRI in which the uniformity of the magnetic field is improved and the amount of permanent magnet used is reduced.

[0002]

2. Description of the Related Art In a conventional MRI magnetic field generator, there is known a magnetic circuit in which a plurality of rod-shaped permanent magnet structures are annularly combined and arranged around the inner periphery of a yoke made of polygonal cylindrical iron or the like. In this configuration, compared to a magnetic circuit in which a pair of permanent magnet structures opposed to each other with a gap formed therebetween are magnetically coupled with a yoke and integrated, there is less magnetic leakage and the magnet can be used efficiently. It has been adopted as a possible configuration.

However, even in the above configuration, the magnetic leakage from the permanent magnet structure in the opening cannot be prevented, and it is necessary to keep the uniform magnetic field space away from the end of the magnetic circuit in order to increase the uniformity of the magnetic field in the required space. The entire length of the magnetic field generator has been increased, which has led to an increase in the size of the magnet and the device. As a method for solving this problem, there has been proposed a method of partially increasing the thickness of the magnet at the end of the magnet opening to reduce the leakage magnetic flux at the opening of the apparatus (Japanese Patent Laid-Open No. 63-43304).

In such a configuration, a plate-shaped permanent magnet having a trapezoidal cross section and a scalene triangular cross section is fixed to an inner wall surface so as to cover the inner wall surface of a core formed in a polygonal cylindrical shape, and is perpendicular to an interface with a hollow portion. FIG. 7 shows a configuration example of the permanent magnet of the conventional magnetic field generator for MRI along both ends in the direction of the axis Z of the permanent magnet magnetized in FIG. It is characterized by having a permanent magnet piece 10 for correcting a non-uniform magnetic field, which is magnetized in a direction perpendicular to the interface with the part. The magnetic flux generated by the non-uniform magnetic field correcting permanent magnet piece 10 compensates for the decrease in the magnetic field strength at both ends in the direction of the axis Z of the magnetic circuit, and leads to the correction of the decrease in the magnetic field strength due to the leakage magnetic flux in this portion. It is not necessary to extend the length of the magnetic field generator in the axial direction in order to maintain the uniformity of the magnetic field, and it is possible to prevent the device from becoming large and heavy.

[0005]

However, since the permanent magnet is exposed to the hollow portion, the non-uniformity of the magnetization characteristics of the permanent magnet material particularly in the central portion in the direction of the axis Z directly affects the magnetic field uniformity of the hollow portion. There was a problem giving. Further, not only the opening of the apparatus but also the magnetic field strength near the direction of 45 degrees from the X axis on the axis XZ plane is low, so that there is a problem that the variation of the magnetic field distribution in the required space is large. Also,
Although it is necessary to fix the permanent magnet structure to the yoke from the viewpoint of safety, the structure in which the permanent magnet is exposed requires space for the fixing plate, equivalently increasing the air gap length and increasing the weight of the permanent magnet. , Causing an increase in cost.

According to the present invention, in a configuration in which a bar-shaped permanent magnet structure is annularly arranged on a gap-facing surface of a cylindrical yoke inner peripheral portion, a leakage magnetic flux at an opening is reduced, and a uniform magnetic field at a required portion is improved. In particular, a magnetic field generator for MRI that has a configuration in which the magnetic field distribution near the direction of 45 degrees from the X axis on the Z axis direction and the axis XZ plane of the magnetic field generator is made uniform, the magnet weight is reduced, and the apparatus can be made smaller and lighter. It is intended to provide equipment.

[0007]

According to the present invention, a pair of permanent magnet structures for forming a main magnetic field are opposed to each other on a cylindrical inner surface of a yoke, and a plurality of permanent magnets are provided on the inner surface of the yoke. In a magnetic field generator for MRI in which components are arranged and magnetically coupled, a pair of permanent magnet components forming a main magnetic field is provided with a pole piece on an air gap facing surface and a permanent magnet component in which no pole piece is disposed. A magnetic field generator for MRI, characterized in that protrusions of permanent magnet pieces magnetized in the same direction as the permanent magnet structure are provided on end faces of the gaps at both ends of the opening.

The present invention is also an MRI magnetic field generating apparatus according to the above configuration, wherein the pole piece has protrusions at both ends and / or the center of the gap facing surface.

In the present invention, the magnetic circuit is formed by combining permanent magnet structures having a triangular, trapezoidal or rectangular cross section along the inner peripheral surface of a cylindrical yoke, for example, a polygonal cylindrical yoke. And integrated. With this configuration, magnetic leakage is small and the magnet can be used efficiently.

FIG. 1 shows an embodiment of a magnetic circuit according to the present invention. The configuration of the present invention will be described in detail below with reference to the drawings.
In the present invention, a soft yoke material such as iron is used for the cylindrical yoke 1 such as a polygonal cylindrical shape in which the permanent magnet structure is disposed on the inner peripheral surface. By using the yoke, the effect of reducing the eddy current is increased. The silicon steel sheet to be laminated has a thickness of 0.35 mm or 0.1 mm.
A commonly used one such as 5 mm may be used. In addition, the entire yoke may not be a silicon steel plate, and various configurations such as a position and a shape in which the silicon steel plate is arranged or a combination with other materials are selected in consideration of a permanent magnet structure or device described later. You.

The permanent magnet structure may have any shape and configuration, and in particular, reference numerals 2b, 2b,
By configuring the permanent magnet components 2c, 2e, and 2f with a plurality of electrically insulated permanent magnet blocks, eddy currents generated in the permanent magnet components can be reduced. A pair of permanent magnet structures 2a and 2d, which are main magnetic field sources contributing to the formation of a 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.
A void facing surface is formed. 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, and the like. However, if they are arranged so as to obtain a uniform static magnetic field in the required gap, In any direction. Usually, the permanent magnet structure is formed by laminating and integrating a plurality of permanent magnet blocks. In particular, the permanent magnet structure is arranged such that the magnetization direction of the permanent magnet structure is perpendicular to the yoke contact surface. By doing so, in assembling the permanent magnet block, the working efficiency is improved, the use efficiency of the magnet is improved, and the device can be reduced in size and weight. The shape is desirably substantially triangular in cross section as shown in the figure in order to obtain a uniform static magnetic field.

As the permanent magnet structure, a permanent magnet such as an alnico, ferrite magnet, or Sm-Co is used. In particular, R-Fe- having a maximum energy product of 30 MGOe or more is used.
By using a B-based permanent magnet, a stable uniform magnetic field can be obtained and the size of the apparatus can be reduced. In the present invention, the gradient magnetic field coils are installed along the inner peripheral surface of the permanent magnet structure inside the polygonal cylinder.

In the present invention, as shown in FIGS. 1 and 2, the pole pieces 4, 4 are provided on the surfaces of the pair of permanent magnets 2a, 2d facing the gap, and form magnets with bolts or the like. It penetrates and is fixed to yoke 1. Further, as shown in FIG. 2, the pole pieces 4, 4 are not only on the gap-facing surface of the permanent magnet structure 2a that forms the main magnetic field, but also on the permanent magnet structure 2b arranged sideways as shown in FIG. An abutting arrangement may be used. The shape of the pole piece is variously selected depending on the configuration of the magnetic circuit, the location on the permanent magnet structure, etc., but within the range of the pole piece shape described above, a uniform static magnetic field can be obtained in the required gap. Should be selected.

If no pole piece is provided and if a flat pole piece is provided, the leakage magnetic flux in the magnetic field generator gap flows toward the opening.
As shown in FIGS. 1 and 2, by disposing protrusions (shims) 5 at both ends of the opening of the pole piece 4 opposed to the opening, it has a function of forcing the magnetic flux into the inside of the device, thereby reducing the leakage of the magnetic flux from the opening. This eliminates the need for a means for expanding the apparatus in the Z-axis direction in order to equalize the magnetic field distribution in the Z-axis direction, which is conventionally required, and can achieve a reduction in size and weight of the apparatus. Further, by providing the central projection 6 also at the center of the pole piece 4, the uniformity of the magnetic field in the gap 3 can be further improved, so that the weight of the magnet can be further reduced. As the material of the pole piece, a soft magnetic material such as iron, soft ferrite, or a silicon steel plate is used. The thickness T of the pole piece is determined by the magnetic field strength of the air gap,
It is determined by the material of the pole piece and the size of the uniform magnetic field space, but is preferably about 20 to 80 mm, and more preferably about 50 mm. The length l of the pole piece in the axial direction (Z-axis direction) is desirably shorter than the length L of the entire apparatus in order to prevent magnetic flux leakage, and is preferably about l = 0.8 L.

According to the present invention, a pole piece shorter than the length in the Z-axis direction of the pair of permanent magnet members forming the main magnetic field is provided on the surface of the permanent magnet member facing the gap. A permanent magnet shim having a magnetization direction inclined by about 30 ° to 60 ° with respect to the Z-axis direction is installed on the magnet exposed surfaces of the non-magnetic pole piece installation portions formed at both ends of the magnetic pole piece so as to open toward the magnetic circuit opening. Thereby, the magnetic field distribution can be improved as compared with the conventional device, and the weight of the magnet can be reduced.

A permanent magnet structure 2b without a pole piece is provided.
2c, 2e, 2f, the cross-sectional shape in the direction perpendicular to the Z axis is Z
In the case where they are the same in the axial direction (depth direction), the axis XZ
Since the magnetic field intensity near the direction of 45 degrees from the X axis on the plane is low, the magnetic field distribution in a required space varies, which causes the magnetic field uniformity to deteriorate. Since the uniform magnetic field space required in the gap 3 is a sphere or an ellipsoidal sphere, in order to obtain a uniform magnetic field over the entire set space,
Permanent magnet constructions 2b, 2c, 2e, without pole pieces
It is necessary to arrange projections (shims) 7 of permanent magnet pieces magnetized in the same direction as the permanent magnet structure on both end faces of the opening 2f facing the gap 3. The size of the protrusion 7 is determined by the magnetic field strength of the air gap and the required size of the uniform magnetic field space. As shown in FIG. 8, the length W in the axial direction (Z-axis direction) is 0.2L to 0.2L. About 4L, protrusion height t is 1-10m
m is preferable. The protrusions (shims) 7 of the permanent magnet pieces may be formed by partially increasing the thickness of both ends of the permanent magnet members 2b, 2c, 2e, 2f. It may be installed on the facing surface, and its installation method and shape may be appropriately selected.

[0017]

The present invention relates to an MRI magnetic field generator,
A magnetic pole piece having a shorter length in the Z-axis direction than the permanent magnet structure is arranged on the gap-facing surfaces of the pair of permanent magnets forming the main magnetic field to make the magnetic field strength in the axial direction Z uniform, and furthermore, the magnetic pole piece Are provided at the center and / or both ends of the permanent magnet assembly, and the projections of the permanent magnet pieces magnetized in the same direction as the permanent magnet assembly are provided at the gap-facing end surfaces at both ends of the opening of the permanent magnet assembly where no pole pieces are arranged. Thereby, the uniformity of the magnetic field is improved over the entire uniform magnetic field space of the required spherical or elliptical sphere of the magnetic field generator. Furthermore, it is possible to reduce the length of the device in the axial direction by preventing magnetic flux leakage at the opening of the device, and to fix the magnet by fixing the pole piece to the yoke with bolts or the like, thereby enabling the device to be reduced in size and weight. Things.

[0018]

【Example】

Example 1 As in the configuration of FIG.
Permanent magnet structure composed of a pair of square Nd-Fe-B magnets having a magnetizing direction in the same direction and generating a main magnetic field on the gap facing surface of a yoke made of a 00 mm hexagonal cylindrical iron bulk body The magnets were arranged opposite to each other, and a permanent magnet structure composed of an Nd-Fe-B-based magnet having a substantially triangular cross section was arranged on the other surface of the yoke opposite to the gap to be magnetically integrated.

On the facing surfaces of the permanent magnet structures 2a and 2d for generating a pair of main magnetic fields, soft ferrite is laminated on bulk pure iron of a required size and has a thickness of 40 mm and a width of 520 m.
m, Z-axis direction length fixed pole pieces 4 of 1050 mm, thickness 10mm in the central portion of the magnetic pole pieces, Z-axis direction length 600mm central protrusion 6,
And 60mm thick at both ends of the pole piece at the opening of the device, length 8 in Z-axis direction
A pole piece having a protrusion 5 as shown in FIG.
4 provided. FIG. 4 is a cross-sectional view taken along the line AA ′ in FIG. 1 having a length in the Z-axis direction of 320 mm and a height of 5 mm on both end surfaces of the permanent magnet structures 2b, 2c, 2e, and 2f facing the gap 3 of the opening. A projection 7 as shown in B of FIG. In the configuration of the present invention, the X-axis, Y-axis, Z-axis generated in the gap between the opposing surfaces of the pole piece gap, the results of measuring the magnetic field distribution corresponding to the 45 ° direction from the X-axis on the XZ plane are shown in FIG. Shown in

COMPARATIVE EXAMPLE 1 The same configuration as in Example 1 was used, except that no protrusion consisting of a pole piece and a permanent magnet piece was provided. 70m thick as shown
A magnetic field generator of a comparative example was prepared in which a permanent magnet piece ( shim ) 10 having a size of m, a width of 520 mm, and a length of 205 mm in the Z-axis direction and having the same magnetization direction as the permanent magnet structure was installed. In the configuration of this comparative example, the magnetic flux was generated in the gap between the pole piece gap facing surfaces.
FIG. 6B shows a result of measuring a magnetic field distribution corresponding to a direction at 45 ° from the X axis on the X axis, the Y axis, the Z axis, and the XZ plane. From FIG.
It can be seen that the configuration of Example 1 has a uniform magnetic field distribution over a wide range as compared with the configuration of Comparative Example 1.

[0021]

According to the present invention, as shown in the embodiments, a rod-shaped permanent magnet structure is annularly arranged on the inner cylindrical portion of the polygonal yoke facing the gap, and the main magnetic field of the permanent magnet structure is A magnetic pole piece having a shorter length in the Z-axis direction than the permanent magnet structure is installed on the gap facing surfaces of the pair of permanent magnet structures that generate
Protrusions are provided at both ends and a central portion of the gap facing surface of the pole piece, and the permanent magnet pieces magnetized in the same direction as the permanent magnet structure are provided on the gap facing surfaces at both ends of the opening of the permanent magnet structure where no pole piece is arranged. By providing the projections, the magnetic field distribution can be made uniform over a wide range.

[Brief description of the drawings]

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

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

FIG. 4A is a schematic explanatory view showing a configuration of a magnetic pole piece of an MRI magnetic field generator according to the present invention, and FIG. 4B is a schematic explanatory view showing a configuration of a permanent magnet structure in which no magnetic pole piece is arranged.

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 a magnetic field distribution corresponding to a direction of 45 degrees from the X axis on the X axis, the Y axis, the Z axis, and the axis XZ plane generated in the gap between the opposing surfaces of the permanent magnet structure gap; A shows the case of the present invention, and B shows the case of the comparative example.

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

FIG. 8 is a schematic explanatory view showing a configuration of a projection of a permanent magnet structure without a pole piece according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Yoke 2a, 2b, 2c, 2d, 2e, 2f Permanent magnet structure 3 Air gap 4 Magnetic pole piece 5, 7 Projection 6 Central projection 10 Permanent magnet piece

Claims (2)

(57) [Claims] 1. A pair of permanent magnet structures forming a main magnetic field are disposed opposite to each other on a cylindrical inner surface of a yoke, and a plurality of permanent magnet structures are arranged on the inner surface of the yoke. Combined MR
In the magnetic field generator for I, a pole piece is disposed on a gap-facing surface of a pair of permanent magnet structures forming a main magnetic field, and the permanent magnet structure is formed on a gap-facing end surface at both ends of an opening of a permanent magnet structure on which no pole piece is arranged. A magnetic field generator for MRI, comprising a projection of a permanent magnet piece magnetized in the same direction as the body. 2. A magnetic pole piece according to claim 1, wherein the pole piece has protrusions at both ends and / or the center of the gap facing surface.
Magnetic field generator for RI.