JPH02184003A - Magnetic field generator for mri - Google Patents

Abstract

PURPOSE:To reduce leakage magnetic fluxes so as to prevent the lowering of magnetic field uniformity in a space by providing a pair of magnets for magnetic field adjustment in specific constitution which are opposed to each other with a space between on the short sides of plate-shaped yokes. CONSTITUTION:Four columnar yokes 3b of specific lengths are erected at the corners of rectangular plate-shaped yokes 3a so as to form a necessary space, and another plate-shaped yoke 3a is fixed to the end parts of the four columnar yokes 3b, thus a yoke 3 is constituted. A pair of permanent magnet constituents 1 and 1 are fixed to the opposing faces of the plate-shaped yokes 3a and 3a of the yoke 3, and further magnetic pole pieces 2 and 2 are fixed and opposed to respective magnetic pole faces so as to make them magnetically couple to generate a static magnetic field inside the space 4 between the magnetic pole pieces 2 and 2. And magnets 6 and 6 for magnetic field adjustment consist of bar-shaped united substances, and are arranged at the center between opposing magnetic pole pieces 2 and 2 and on each short side of the plate-shaped yokes 3a, which are positioned outside the circular projections 5 and 5 of the magnetic pole pieces 2 and 2, within the space 4. These bar-shaped magnets 6 and 6 for magnetic field adjustment are so provided that the magnetizing direction may run parallel with the magnetic field generated in the space 4 and it may be reverse to that of a pair of permanent magnet constituents 1 and 1.

Description

[Detailed Description of the Invention] Field of Application This invention relates to a permanent magnet used in medical nuclear magnetic resonance tomography equipment (hereinafter referred to as MRI), etc., which can obtain cross-sectional images of objects and depict the properties of tissues. Regarding the magnetic field generator used, in a configuration where a pair of plate yokes supported by multiple columnar yokes are rectangular, the amount of leakage magnetic flux is different between the long side and short side of the plate yoke. The present invention relates to a magnetic field generation device for MRI in which a pair of magnetic field adjustment magnets are disposed on the short side facing each other with a gap interposed therebetween to prevent the magnetic field uniformity within the gap from decreasing.

BACKGROUND ART As shown in FIG. 4, as a magnetic field generating device used in MHI, in order to achieve miniaturization of the device, Fe-B-R is usually used.
A magnetic pole piece (2X2) is fixed to one end of each of a pair of permanent magnet structures (IXI) using system magnets, facing each other, and the other end is connected with a yoke (3) described later to form a magnetic pole piece ( 2X2)
A configuration is known in which a static magnetic field is generated in the air gap (4) between the two.

The magnetic pole piece (2X2) is provided with an annular protrusion (5) at its periphery in order to improve the uniformity of the magnetic field distribution within the air gap (4).

The yoke (3) that magnetically couples the permanent magnet structure (IXI) has a wide opening and a narrow depth to reduce the feeling of pressure on the subject. In addition, in order to facilitate transportation to existing hospitals, etc., it consists of a pair of rectangular plate yokes (3a x 3a) connected by four columnar yokes (3b) of a predetermined length that form the required gap. .

In the magnetic field generator for MRI (hereinafter referred to as the magnetic field generator) having the above configuration, the gap (4) must be wide enough to allow part or all of the subject to be inserted, and in order to obtain a clear tomographic image. Generally, it is required to form a stable and strong uniform magnetic field in the air gap (4) with a precision of 0.5 to 10 kG and less than 1×1O−'.

Problems with the prior art The magnetic field generator shown in Fig. 4 is a plate-shaped yoke (3a x 3a).
Since it is rectangular, the long side and short side of the magnetic pole piece (2
) (2) and the permanent magnet structure (IXI) are different, that is, there is a large amount of leakage magnetic flux toward the short side, so there was a problem that the uniformity of the magnetic field in the air gap (4) decreased. .

Therefore, the magnetic pole pieces (2X2) and the permanent magnet structure (IXI
), one of the applicants has already proposed a means of arranging leakage flux repelling magnets on the inner surface of the yoke (Utility Model Application No. 61-88).
No. 210), means for arranging a magnetic flux shunt plate on the long side of a rectangular plate-shaped yoke in close proximity to a magnetic pole piece (Utility Model Application No. 62-
101205), means for placing a magnetic thin plate in contact with the long side of a rectangular plate-shaped yoke (Utility Model Application No. 62-101206)
(Japanese Utility Model Publication No. 112106/1983), a means for attaching a magnetic field adjustment piece to a desired position of an annular protrusion of a magnetic pole piece (Japanese Utility Model Publication No. 112106/1983)
, Means for arranging permanent magnets that generate a large amount of magnetic flux on the opposing surfaces of the short sides of rectangular plate-shaped yoke (Utility Model Application No. 62-112107)
(No. Publication).

However, the above-mentioned technology alone cannot sufficiently satisfy the various conditions currently required of magnetic field generating devices, such as improving the clarity of tomographic images and reducing the size and weight of devices.

Purpose of the Invention The present invention is proposed in view of the above-mentioned current situation, and solves the problem that the amount of leakage magnetic flux is different between the long side and the short side of a plate-shaped yoke, and the uniformity of the magnetic field in the air gap decreases. The object of the present invention is to provide a magnetic field generator having a configuration that makes it possible to improve the clarity of tomographic images and to reduce the size and weight of the device by taking advantage of the advantages of a magnetic field generator using a rectangular plate-shaped yoke.

Summary of the Invention As a result of various studies aimed at eliminating the difference in the amount of leakage magnetic flux between the long side and the short side of a plate-shaped yoke in a magnetic field generation device, the present invention has been developed. By providing a pair of magnetic field adjustment magnets with a specific configuration facing each other with an air gap,
This invention was completed based on the discovery that leakage magnetic flux could be reduced.

That is, the present invention includes: a pair of rectangular plate yokes that are magnetically connected and arranged to face each other, with a plurality of columnar yokes forming a gap as spacing members;
An MRI device comprising a permanent magnet structure attached to each opposing surface of a plate-shaped yoke, and a magnetic pole piece having an annular projection on the periphery provided on each opposing surface of the magnet structure, and generating a magnetic field in the air gap. In the magnetic field generating device for magnetic field generation, a pair of magnetic field adjustment magnets are installed on each short side of the rectangular plate-shaped yoke at the center between the opposing magnetic pole pieces and outside the annular protrusion of the magnetic pole piece. This is a magnetic field generating device for MRI, characterized in that it is provided in a direction opposite to that of a permanent magnet.

Structure of the Invention In the present invention, a magnetic circuit includes a pair of rectangular plate-shaped yokes that are magnetically connected and arranged to face each other, with a plurality of columnar yokes forming a gap as spacing members, and each of the plate-shaped yokes. The structure consists of a permanent magnet structure attached to one facing surface, and a magnetic pole piece having an annular projection on the peripheral part provided on each opposing surface of the permanent magnet structure, and the magnetic properties and shape and dimensions of the permanent magnet are It is desirable to appropriately select the construction means, their position, etc., depending on the shape and dimensions of the yoke, the size of the required gap, etc.

In this invention, the magnetic pole piece is made of a known high permeability magnetic material such as disc-shaped electromagnetic soft iron or pure iron. It is possible to appropriately select means for concentrating the magnetic flux in a required gap and improving uniformity, such as attaching a magnetic field adjusting piece at a required position.

As the permanent magnet of the magnet structure used in such a magnetic circuit, ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but in particular, as R, light rare earths, which are rich in resources, mainly Nd and Pr, are used. By using a Fe--B--R permanent magnet, which is mainly composed of B and Fe and exhibits an extremely high energy product of 30 MGOe or more, it is possible to significantly reduce the size.

Furthermore, in this invention, the yoke is composed of a rectangular plate-shaped yoke and a columnar yoke, but the short side (L) of the rectangular plate-shaped yoke
The ratio W/L between the long side (W) and the long side (W) is usually 1.5 to 1.5, taking into account the size of the patient, the size of the installation location, the workability of transportation, etc.
Set it to 2.5. Further, the ratio D/L of the pole piece diameter (D) to the short side (L) is usually set to 0.6 to 1.0 in consideration of the size of the subject, magnetic field uniformity, etc.

The columnar yokes are selected in consideration of the effective cross-sectional area necessary for forming the magnetic path, mechanical strength, ease of assembly, magnetic balance, etc., and four pieces are usually preferred.

The magnetic field adjusting magnet, which is a feature of this invention, is provided with a pair of magnetic field adjusting magnets on each short side of a rectangular plate-shaped yoke in the center between the opposing magnetic pole pieces and outside the annular protrusion of the magnetic pole piece. Although it is provided so that the magnetization direction is opposite to that of the pair of permanent magnet components, the shape is not limited to the rod shape shown in the example, and consideration is given to the required magnetic field uniformity, workability, and low installation cost. It is also possible to select a curved or bent shape along the magnetic pole piece so that the adjusting magnetic field acts uniformly around the magnetic pole part, and it does not have to be constructed as a single piece.

Specifically, when a magnetic field adjusting magnet is configured with a plurality of magnets, a plate-shaped magnetic pole piece can be attached to the magnetic pole surface to equalize the magnetic flux and improve mechanical strength. Furthermore, it is also possible to separate and arrange a plurality of magnets by supporting them with plate-shaped magnetic pole pieces with gaps between them.

In addition, in order to obtain the same effect as a curved or bent magnetic field adjusting magnet along a magnetic pole piece, with a bar-shaped magnetic field adjusting magnet,
Among the plurality of magnets, it is possible to adopt measures such as increasing the size of the adjusting magnet at a position far from the magnetic pole piece, or using a magnet with higher magnetic force.

The width (dimension in the magnetization direction), thickness (dimension parallel to the long side of the rectangular plate yoke), and length (dimension parallel to the short side of the rectangular plate yoke) of the magnetic field adjustment magnet are determined by the magnetic field. Select the adjusting magnet by considering its magnetic properties, air gap size, magnetic field strength, etc.

The material of the magnetic field adjustment magnet does not need to be the same as the material of the permanent magnet component that is the magnetic field generation source; any known magnet can be used, taking into account the required magnetic field strength, magnetic field uniformity, etc. be.

Preferred Embodiment of the Invention FIG. 1a is a longitudinal sectional view of a magnetic field generating device showing an embodiment of the invention, and FIG. 1b is a horizontal sectional view.

As shown in Fig. 1, the magnetic field generator according to the present invention has four columnar yokes (3b) of a predetermined length erected at the four corners of a rectangular plate-shaped yoke (3a) in order to form the required air gap. In addition, another plate-like yoke (3a) is penetrated and fixed to the tips of the four columnar yokes (3b) to form a yoke (3).

A permanent magnet structure (1) (1) using a Fe-B-R magnet of one ta τj is attached to the plate-shaped yoke (3a) (
3a), and a magnetic pole piece (
2
A static magnetic field is generated in the air gap (4) between 2×2).

In addition, the magnetic pole piece (2X2) has an annular projection (
5) is provided.

The magnetic field adjusting magnet (6x6) used in the example of Fig. 1 is as follows:
A plate-shaped yoke (consisting of a rod-shaped integral body) located in the center between the opposing magnetic pole pieces (2X2) in the gap (4) and outside the annular projections (5) (5) of the magnetic pole pieces (2X2). 3a) are arranged on each short side.

This bar-shaped magnetic field adjustment magnet (6x6) is provided so that its magnetization direction is parallel to the magnetic field generated in the air gap (4) and opposite to the magnetization direction of the pair of permanent magnet structures (IXI). ing.

Effects of the Invention By arranging the magnetic field adjusting magnet (6×6) in the magnetic field generator having the above configuration, the following effects can be obtained.

■ Due to the arrangement of the magnetic field adjustment magnets (6x6), the air gap (4)
The magnetic flux lines formed between the pair of magnetic pole pieces (2X2) are suppressed in the central part by suppressing the magnetic flux lines that curve in the outer circumferential direction, in other words, in the horizontal direction of the air gap (4), and in the horizontal direction in the figure. Most can be made straight.

■ Magnetic field adjustment magnets (6x6) are attached to each rectangular plate yoke (3a
) is arranged on the short side of the plate-shaped yoke (3a) to prevent a decrease in magnetic field uniformity caused by the shape of the plate-shaped yoke (3a).

(2) Since the axial magnetic field uniformity and the magnetic field strength within the air gap (4) are improved, it is possible to improve the clarity of the tomographic image and to make the magnetic field generator smaller and lighter.

Embodiment A magnetic field generator having the same configuration as in FIG.
Using Fe-B-R permanent magnets with 5 MGOe, bar-shaped magnetic field adjustment magnets were placed on the short side of each rectangular plate-shaped yoke, and the diameter of the magnetic pole pieces with annular protrusions on the periphery was 1100 mm.
mm, and the distance between a pair of magnetic pole pieces (air gap) was set to 500 mm, and the magnetic field within the air gap was measured.

The magnetic field uniformity and magnetic field strength within a radius of 175 mm from the center of the air gap are: Magnetic field uniformity: 40 ppm See Figure 2 Magnetic field strength: 2.
It was OkG.

In addition, as a result of measuring the magnetic field in the air gap of a conventional device under the same conditions as the device of the present invention except that no magnetic field adjustment magnet is used, the following results were obtained: Magnetic field uniformity: 60 ppm See Figure 3. Magnetic field strength: 2.
It was OkG.

The configuration of the yoke and the magnetic field adjustment magnet is as follows.

Short side (L): 1200mm, long side (W): 1950, countersunk thickness 5mT); 130mm Material: Fe-B-R magnet, magnetic properties: ( BH) max 35MGOe convergence (1);
30 dishes mm Width (w): 1200mm, Thickness (t): 20mm, Horizontal opposing distance: 1200mm

[Brief explanation of the drawing]

FIG. 1a is a longitudinal cross-sectional view of a magnetic field generating device showing an embodiment of the present invention, and FIG. 1b is a cross-sectional view. FIGS. 2 and 3 are graphs showing the magnetic field uniformity at the center of the air gap of the magnetic field generator, and FIG.
FIG. 3 shows the case of a conventional device. FIG. 4 is a front view showing a conventional magnetic field generating device. DESCRIPTION OF SYMBOLS 1... Permanent magnet structure, 2... Magnetic pole piece, 3... Yoke, 3a... Plate yoke, 3b... Column yoke, 4...
- Gap, 5... Annular protrusion, 6... Magnetic field adjustment magnet.

Claims (1)

[Scope of Claims] 1. A pair of rectangular plate-shaped yokes that form a gap using a plurality of columnar yokes as spacing members, and are magnetically connected and arranged to face each other;
Consists of a permanent magnet structure attached to each opposing surface of a plate-shaped yoke, and a magnetic pole piece having an annular protrusion on the periphery provided on each opposing surface of the magnet structure, and generates a magnetic field in the air gap. In a magnetic field generation device for MRI, a pair of magnetic field adjustment magnets are installed on each short side of a rectangular plate-shaped yoke at the center between opposing magnetic pole pieces and outside the annular protrusion of the magnetic pole pieces, and the magnetization direction is set to the pair of magnetic field adjustment magnets. 1. A magnetic field generator for MRI, characterized in that the magnetic field generator is provided in a direction opposite to that of a permanent magnet.