JPH0244485Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial field This invention is a permanent magnet used in medical nuclear magnetic resonance tomography (hereinafter referred to as NMR-CT), which can obtain cross-sectional images of objects and depict the properties of tissues. Regarding the magnetic field generator used,
This invention relates to a magnetic field generator that generates a strong, highly accurate, and uniform static magnetic field within a large air gap.

BACKGROUND ART
In order to obtain the desired tomographic image by inserting the device into an air gap that generates a strong magnetic field of 10 KG, this magnetic field must be strong, uniform, and stable with an accuracy of 10 ^-4 or less. Known magnetic field generators include normal conducting magnets made of conductive wire made of copper or aluminum wrapped in a cylindrical shape, and superconducting magnets that use special conducting wire cooled to a temperature close to zero insulation. .

The former is structurally inexpensive, but requires a huge amount of electricity and cooling water, and has high running costs.On the other hand, the latter superconducting magnet requires the use of expensive liquid helium as a cooling medium. There is a problem in that not only the so-called initial cost but also the running cost is extremely high.

The applicant has previously developed an air gap-forming magnetic field generating device that has a permanent magnet circuit that has a magnetic field strength equal to or higher than that of the above-mentioned normal conduction magnet, consumes no power, is small and lightweight, and can obtain a highly accurate uniform magnetic field. A magnetic field generating device for magnetically coupling magnetic pole pieces facing each other and at least one permanent magnet with a yoke to generate a magnetic field in the gap, characterized in that an annular protrusion is provided on each opposing surface of the magnetic pole pieces. proposed a magnetic field generator (Japanese Unexamined Patent Publication No. 88407/1988).

With this magnetic field generator, it was possible to improve the uniformity and accuracy of the magnetic field generated in the air gap. However, for practical use, a uniform magnetic field with even higher precision is required, so fine adjustment of the magnetic field is required.

In addition, as a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field, a pair of square plate-shaped yokes that are magnetically connected by using four cylindrical yokes as spacing members to form an air gap, and facing each other, are used. A magnetic field generator in which a permanent magnet is attached to the opposing surface, a magnetic pole piece having an annular projection is provided on the opposing surface of each permanent magnet, the distance between the plate yokes can be finely adjusted, and a magnetic field is generated in the gap. was proposed (Special Application No. 113713-1981).

The magnetic field generating device described above can conveniently enter the air gap of the object to be diagnosed in any direction on all four sides, and because it uses a square plate-shaped yoke, it has the advantage of high magnetic field uniformity on the horizontal plane. For example, when diagnosing the entire human body, the size of the plate yoke is at least 2000 mm on one side, and even when only the head is used for diagnosis, the size of the plate yoke is approximately 1500 mm on each side, so it cannot be used inside an existing building. Due to demands such as ease of transport and reduction of installation space, it has been desired to further reduce size and weight while maintaining excellent magnetic field uniformity.

Purpose of the invention In view of the current situation, this invention has been developed in a magnetic field generator using permanent magnets that can generate a strong magnetic field. It is intended as a generator.

Structure and effect of the invention This invention was developed as a result of various studies aimed at creating a compact magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field.
By providing an annular protrusion on the magnetic pole piece and appropriately attaching small pieces of magnetic material to the annular protrusion, it is possible to finely adjust the magnetic field within the air gap, and by adjusting the magnetic field, a highly accurate uniform magnetic field can be obtained. It has been discovered that it is possible to make the magnetic circuit smaller and lighter, and that the uniformity of the magnetic field within the magnetic circuit air gap is significantly improved, making it possible to expand the highly accurate and uniform magnetic field area.

That is, this invention provides a magnetic field generating device in which a pair of magnetic pole pieces facing each other with an air gap and at least one permanent magnet are magnetically coupled with a yoke to generate a magnetic field in the air gap. This magnetic field generating device is characterized in that an annular protrusion is provided on the opposing surface, and a magnetic field adjustment piece made of one or more magnetic materials is attached to the landing position of the annular protrusion.

Preferred configuration of the invention The magnetic circuit can use any configuration as long as at least one permanent magnet is used and magnetic pole pieces facing each other with an air gap and provided with annular protrusions on the opposing surfaces are magnetically coupled with a yoke. Depending on the size of the object to be diagnosed, the installation space of the device, etc., the magnetic properties and shape of the permanent magnet, the shape and size of the yoke, the required air gap size, the shape of the magnetic pole piece, the annular protrusion, etc. should be selected as appropriate. is desirable.

In addition, the shape of the yoke determines the overall shape and installation space of the magnetic field generator, so for example,
A structure that uses a rectangular plate-shaped yoke and a cylindrical yoke as shown in the figure, or a configuration that is excellent in space-saving properties such as the cylindrical yoke shown in Fig. 3 is preferable, and in particular, it is preferable to use the magnetic field adjustment piece according to this invention. As a result, a highly accurate uniform magnetic field can be obtained, which has the advantage of making it easy to set, change, and modify the yoke shape etc. according to the installation space.

In addition, the annular protrusion can have various shapes such as a triangular or trapezoidal cross-sectional protrusion on the periphery of the magnetic pole surface plane or any other location, and the inner peripheral surface shape of the annular protrusion is an inclined surface that expands upward. is preferred.

The magnetic field adjustment pieces made of one or more magnetic materials that are attached to desired locations on the annular protrusion may be of any shape, size, and number, and their attachment positions may be placed on the magnetic pole piece 13 as shown in FIG. Protruding annular projection 1
5 may be on the upper surface a, the inner inclined surface b, or the outer circumferential surface c, and the dimensions, shape, and mounting position may be determined by the magnetic circuit, the magnetic properties of the permanent magnet, the shape and size of the magnetic pole piece, the shape and size of the annular protrusion, etc. It is necessary to select it appropriately in order to obtain the required uniform magnetic field. Further, the magnetic field adjustment piece may be provided as appropriate after assembling the magnetic circuit, or may be provided integrally with the annular protrusion.

In addition, in order to improve the uniformity of the magnetic field in the direction in which the magnetic pole piece faces, a convex protrusion may be provided at the center of the annular protrusion on the magnetic pole piece, which may be continuous or discontinuous with the annular protrusion, separately from the magnetic field adjusting piece. Good too.

The permanent magnet used in the magnetic field generator of this invention is
Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but Fe-B-R permanent magnets, which the applicant previously proposed as new high-performance permanent magnets that do not contain expensive Sm or Co, are the most effective. Not only is the energy product large, but the temperature coefficient of residual magnetic flux density (Br) is 0.07%/°C to 0.15%/°C.
By applying this permanent magnet to the above-mentioned NMR-CT, it is possible to downsize the device and obtain excellent performance. By cooling and using the magnet, a significantly higher maximum energy product can be obtained and the magnet can be used more effectively.

The above Fe-BR-based permanent magnet has R (R is Nd,
At least one of Pr, Dy, Ho, Tb or further La, Ce, Sm, Gd, Er, Eu, Tm,
At least one of Yb, Lu, Y) 8 atomic % ~
30 atomic%, B2 atomic% ~ 28 atomic%, Fe42 atomic% ~
It is a permanent magnet whose main component is 90 atomic% and whose main phase is a tetragonal phase.R is a resource-rich light rare earth mainly Nd and Pr, and B and Fe are the main components. It is an excellent permanent magnet that exhibits an extremely high energy product.

Disclosure of the invention based on the drawings FIG. 1 is a cross-sectional top view and a front explanatory view showing a magnetic field generating device according to the invention. FIG. 2 is an explanatory diagram showing details of the annular protrusion of the magnetic pole piece. FIG. 3 is a longitudinal sectional perspective view of another magnetic field generating device according to this invention.

In the magnetic circuit shown in FIG. 1, four cylindrical yokes 10 are used as spacing members, and these are arranged at the four corners of a plate-shaped yoke 11 having a rectangular main surface, and a pair of plate-shaped yokes 11
A pair of plate-shaped yokes 11 with a gap 14 formed therebetween.
Further, a permanent magnet 12 is attached to the opposing surface of each plate-shaped yoke 11, and a disk-shaped magnetic pole piece 13 having an annular projection 15 is provided to the opposing surface of each permanent magnet 12. This configuration generates a magnetic field in the air gap 14.

The magnetic pole piece 13 has an annular protrusion 1 having a trapezoidal cross section on its periphery.
5 is provided, and a planar circular projection is also provided at the center of the annular projection 15.

Magnetic field adjustment pieces 16 are respectively attached to the inner inclined surfaces of the annular projections 15 at positions opposite to the short sides of the plate-shaped yoke 11.

In addition, a rectangular magnetic flux shunt material 17 is attached to the center of the long side of the pair of plate yokes 11, and the magnetic pole piece 1
3, and is configured to bolt to the plate-shaped yoke through the elongated hole of the magnetic flux shunt material 17, making it movable in the vertical direction and making it possible to vary the shunt amount of magnetic flux from the magnetic pole piece 13. There is.

In the above configuration, the effect of improving the magnetic field uniformity by attaching the annular protrusion 15 can be further enhanced by adjusting the uniformity of the magnetic field in the required space of the air gap 14 by the magnetic field adjustment piece 16, and by adjusting the magnetic field uniformity in the magnetic pole piece 13 by the magnetic flux shunting material 17. The effect of shunting a part of the magnetic flux from the pole piece 1, due to the synergistic effect of these
The magnetic field distribution around 3 is made equivalent to the case where the plate-shaped yoke 11 is square, and extremely uniform magnetic field can be obtained.

This magnetic flux shunt material 17 has an L-shaped configuration in which its tip is arranged between the facing surface of the magnetic pole piece 13 and the permanent magnet 12 mounting surface of the plate-shaped yoke 11, and the tip is flat. In order to obtain a uniform magnetic field together with the magnetic field adjusting piece 16, the shunt material 17 may be placed outside the magnetic circuit or placed directly on the plate-shaped yoke 11 on the side end surface of the permanent magnet 12. The shape is selected appropriately.

In the above magnetic circuit, the plate-shaped magnetic pole piece has a ratio L/W of the short side (L) and the long side (W) of the plate-shaped yoke.
is selected as appropriate depending on the size of the object to be diagnosed, the dimension (D) of the magnetic pole piece in the short side direction, the shape and dimensions of the yoke that magnetically connects the plate yoke, but usually
Approximately 1.5 to 2.5 is desirable.

Furthermore, if the ratio of the short side (L) of the plate-shaped yoke to the dimension (D) of the magnetic pole piece in the short side direction is less than 0.6, the magnetic field uniformity on the horizontal plane in the air gap will decrease slightly, but the magnetic It is not possible to miniaturize the circuit, and 1.2
If the ratio D/
L is set to 0.6 to 1.2, but it can be adjusted as appropriate depending on the magnetic properties of the permanent magnet, the shape and dimensions of the plate yoke and the yoke that magnetically connects the plate yoke, and the size of the required air gap. It is desirable to select

The magnetic circuit shown in Fig. 3 consists of a pair of Fe-B-R
A magnetic pole piece 21 is fixed to one end of each of the system permanent magnets 20 to face each other, and the other end of the permanent magnet 20 is connected to a yoke 22 which is made of a cylindrical body and has a part of its circumference open. It is configured to generate a static magnetic field in the air gap 23 between the magnetic pole pieces 21, and the pair of magnetic pole pieces 21
An annular protrusion 24 with a triangular cross section having a predetermined inner diameter and height is protruded from the periphery of the opposing surface of the Fe.
-BR permanent magnets 20 are magnetized in the same direction.

Three magnetic field adjusting pieces 25 are installed at opposing positions on the inner inclined surface of this annular protrusion 24, that is, at opposing positions perpendicular to the advancing and retreating direction of the object to be diagnosed from the opening of the yoke 22. . Thereby, the uniformity of the magnetic field within the required space of the air gap 23 can be adjusted and improved.

Example In the configuration shown in Figure 1, Fe-B-R plate permanent magnets having a maximum energy product of 35 MGOe are used, and the opposing distance of the magnetic pole pieces is 450 mm.
Plate magnetic pole piece dimensions: 1000mmL x 1800mmW
x 120 mm T and a magnetic pole piece outer diameter D of 950 mm. After assembly, a magnetic field adjustment piece was attached to the inner inclined surface of the annular projection.

The magnetic field adjustment piece is 13mmφ x 8mm within a 45° range around the central axis in the long side direction of the plate when viewed from the center of the magnetic pole piece.
By installing seven φ x 5 mm iron magnetic field adjustment pieces, the magnetic field uniformity in the short and long sides of the plate yoke on a horizontal plane with a radius of 100 mm from the center of the gap was changed from 170 ppm to 70 ppm. .

Furthermore, when we attached a magnetic flux shunt material with dimensions of 400 mm length x 250 mm width x 15 mm thickness and adjusted it by moving it up and down, an excellent value of less than 10 ppm was obtained for the magnetic field uniformity on the horizontal plane. Ta.

In addition, in the case of the magnetic field generator of the present invention having the above configuration, the installation space is about 80% of that of a comparative magnetic field generator with a square plate yoke when compared with an equivalent uniform magnetic field. It can be seen that it is excellent in size and weight reduction.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional top view and a front explanatory view showing a magnetic field generating device according to this invention. FIG. 2 is an explanatory diagram showing details of the annular protrusion of the magnetic pole piece. FIG. 3 is a longitudinal sectional perspective view of another magnetic field generating device according to this invention. 10...Cylindrical yoke, 11...Plate yoke, 1
2,20...Fe-BR-based permanent magnet, 13,2
1...Magnetic pole piece, 14, 23...Air gap, 15, 24
...Annular projection, 16, 25...Magnetic field adjustment piece, 17
...Magnetic flux shunt material, 22...Yoke.

Claims (1)

[Scope of utility model registration request] A magnetic field generating device that magnetically couples a pair of magnetic pole pieces facing each other with an air gap and at least one permanent magnet using a yoke to generate a magnetic field in the air gap,
1. A magnetic field generating device characterized in that an annular projection is provided on each opposing surface of a magnetic pole piece, and magnetic field adjusting pieces made of one or more magnetic materials are attached at required locations on the annular projection.