JP3056883B2 - 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. The present invention relates to a magnetic field generator for MRI in which a magnetic pole piece having a specific shape is arranged to reduce the leakage magnetic flux of an opening, improve a uniform magnetic field of a required portion, and reduce the amount of a permanent magnet used.

[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 of the magnetic circuit, which also leads to the correction of the slight decrease in the magnetic field strength in the uniform magnetic field region, and the uniformity of the magnetic field. It is not necessary to extend the length of the magnetic field generator in the axial direction in order to hold the device once, and it is possible to prevent the device from increasing in size and weight.

[0005]

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

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, an object of the present invention is 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 apparatus can be reduced in size and weight.

[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. A magnetic field generator for MRI comprising a structure and magnetically coupled to each other, wherein a magnetic pole piece is disposed on a pair of permanent magnet structures which form a main magnetic field and opposed to a gap facing surface. is there.

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.

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. In addition, as shown in FIG. 3, the pole pieces 4, 4 are not only on the gap-facing surfaces of the permanent magnet structures 2a, 2d forming the main magnetic field, but also as shown in FIG. 2b may be arranged. 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. 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, 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.

When no pole piece is installed or when a flat pole piece is installed, the leakage magnetic flux in the magnetic field generator gap flows toward the opening. However, as shown in FIGS. Projection (shim) at both ends of the opening facing surface of the opening 4
By arranging 5, the magnetic flux can be pushed into the inside of the device, and the leakage of the magnetic flux in the opening can be reduced. No need for expansion means, miniaturization of the device,
Weight reduction can be achieved. 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.

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.

[0016]

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 By providing projections 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. Can be fixed, and the device can be reduced in size and weight.

[0017]

【Example】

Example 1 As in the configuration of FIG.
Permanent magnet configuration composed of a pair of trapezoidal Nd-Fe-B-based magnets having a pair of same magnetization directions and generating a main magnetic field on the gap-opposing 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 made of a Nd-Fe-B-based magnet having a triangular cross section was arranged on the other surface of the yoke opposite to the gap, and magnetically integrated.

Further, a pole piece having a thickness of 40 mm, a width of 520 mm, and a length of 1050 mm in the Z-axis direction, made of bulk pure iron and soft ferrite, is fixed to the facing surfaces of the pair of permanent magnets for generating a pair of main magnetic fields, A pole piece having a thickness of 10 mm and a length of 600 mm in the Z-axis direction at the center of the pole piece, and a projection as shown in FIG. Was provided. FIG. 6A shows the result of measuring the magnetic field distribution corresponding to the X axis, Y axis, and Z axis generated in the gap between the pole piece gap opposing surfaces in the configuration of the present invention.

COMPARATIVE EXAMPLE 1 The same configuration as in Example 1 was used, except that no pole pieces were provided, and a pair of permanent magnet structures for generating a pair of main magnetic fields were formed at both ends of the air gap facing surface as shown in FIG. 70mm, Z
A magnetic field generator of a comparative example was prepared in which a permanent magnet shim 10 having a dimension of 410 mm in the axial direction and having the same magnetization direction as the permanent magnet structure was installed. In the configuration of this comparative example, the X axis and the Y axis generated in the gap between the pole piece gap opposing surfaces
The results of measuring the magnetic field distributions corresponding to the X axis and the Z axis are shown in FIG.
Shown in

FIG. 6 shows that the magnetic field distribution in the Z-axis direction of the structure of the first embodiment is more uniform than that of the first embodiment, not only at the center of the air gap but also around the opening of the permanent magnet structure. You can see that there is. The weight of the permanent magnet was 3,696 kg when a uniform magnetic field was obtained with the configuration in which the permanent magnet shim was installed in the opening of the surface facing the gap of the permanent magnet construct that generates a pair of main magnetic fields in Comparative Example 1. On the other hand, in the configuration in which the pole piece is attached to the surface facing the gap of the permanent magnet structure of Example 1 and the protrusions are provided at the center and both ends of the pole piece, the weight of the permanent magnet can be greatly reduced to 3,432 kg. did it.

[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
By providing protrusions at both ends and the center portion of the gap opposing surface of the pole piece, the magnetic field distribution in the Z-axis direction of the magnetic field generator can be made uniform, the magnet weight can be reduced, and the device can be reduced in size and weight. .

[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. 4 is a schematic explanatory view showing a configuration of a pole piece 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 position of the magnetic field showing the magnetic field distribution corresponding to the X-axis, Y-axis, and Z-axis generated in the air gap between the opposing surfaces of the permanent magnet structure air gap. In the case of the invention, 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.

[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 Projection (shim) 6 Central projection

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01F 7/02

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
1. A magnetic field generator for MRI, comprising: a magnetic field generator for I, wherein a pole piece is disposed on a surface of a pair of permanent magnet structures facing a gap forming a main magnetic field. 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.