JPH0244484Y2 - - 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. However, there is a problem in that not only the 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 piece. proposed a magnetic field generator (Japanese Unexamined Patent Publication No. 88407/1988).

In addition, as a result of various studies aimed at creating a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field, we found that four cylindrical yokes 1 are used as spacing members to form gaps 5, as shown in Permanent magnets 3 are attached to opposing surfaces of a pair of square plate yokes 2 connected to and facing each other, and a magnetic pole piece 4 having an annular projection is provided on the opposing surface of each permanent magnet. proposed a magnetic field generating device (Japanese Patent Application No. 113,713/1983) that could finely adjust the distance between the gaps and generate a magnetic field in the gap.

The magnetic field generating device described above has the advantage that the object to be diagnosed can enter the air gap 5 in any direction in any four directions, and because it uses the square plate-shaped yoke 2, the magnetic field uniformity on the horizontal plane is high. However, for example, when diagnosing the entire human body, the size of the plate-shaped yoke 2 is
At least one side is approximately 2,000 mm, and even if only the head is used for diagnosis, a side of approximately 1,500 mm is required. Therefore, excellent magnetic field uniformity is required due to requirements such as ease of transport into existing buildings and reduction of installation space. It was desired to further reduce the size and weight while maintaining the same characteristics.

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 As a result of various studies aimed at creating a compact magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field, this invention was developed by making the plate yoke rectangular in the magnetic circuit shown in Fig. 4 above. It has been discovered that by arranging magnetic thin plates at least on the long sides, it is possible to reduce the size and weight of the magnetic circuit, significantly improve the magnetic field uniformity within the magnetic circuit gap, and expand the highly accurate and uniform magnetic field area. It is.

That is, in this invention, permanent magnets are attached to the opposing surfaces of a pair of plate-shaped yokes that are magnetically connected by using columnar yokes as spacing members to form an air gap, and magnetic poles are attached to the opposing surfaces of each permanent magnet. In a magnetic field generating device that generates a magnetic field in the air gap by providing a pair of plate yokes, the short side (L) of the plate yoke and the dimension (D) of the magnetic pole piece in the direction of the short side are ) and the ratio D/L is 0.6
~1.2, at least on the long side of the plate yoke,
This is a magnetic field generating device characterized in that a magnetic thin plate is arranged in contact with or in close proximity to a plate-shaped yoke.

Preferred configuration of the invention In the plate-shaped magnetic pole piece, the ratio L/W of the short side (L) and the long side (W) of the plate-shaped yoke is determined by the size of the object to be diagnosed;
It is appropriately selected depending on the dimension (D) of the magnetic pole piece in the direction of the short side, the shape and dimensions of the yoke to which the plate yoke is magnetically connected, and it is usually desirably about 1.5 to 2.5.

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

Furthermore, the dimensional ratio between the short side of the plate-shaped yoke and the permanent magnet in this direction is preferably about the same as that of the magnetic pole piece, although it depends on the shape of the permanent magnet.

In this invention, the magnetic thin plate shunts part of the magnetic flux from the magnetic pole piece, making the magnetic field distribution around the magnetic pole piece equivalent to that of a square plate yoke, and has a magnetic shielding effect. The magnetic circuit is placed at least on the long side of the plate yoke, and preferably has a width of 2/3W or more, or a size equivalent to the length of the magnetic pole piece in the long side direction. Any installation method can be used, such as placing it outside.

In addition, the thickness t of the magnetic thin plate is preferably t/T = 0.005 to 0.05 in the case of the plate-like yoke thickness T. If it is too thick, the magnetic field strength in the gap will decrease, and if it is too thin, the above-mentioned shunt effect will occur. In addition, since the magnetic shielding effect decreases, it is necessary to select an appropriate amount according to the magnetic properties of the permanent magnet, the shape and size of the magnetic pole pieces, the gap size, etc.

In addition, the magnetic pole piece can have various shapes, and it is good to have an annular protrusion on the magnetic pole face, or a protrusion with a triangular or trapezoidal cross section on the periphery of the plane of the magnetic pole face or at any other location. It is also possible to provide convex projections in the center of the annular projection in a continuous or discontinuous manner.

Further, as the columnar yokes that magnetically connect the plate yokes, four columnar yokes that can be opened on all sides, or a pair of plate or square columnar yokes that have an opening in one direction can be appropriately selected.

This idea uses a magnetic flux shunt material that has the same purpose as the magnetic thin plate and shunts part of the magnetic flux from the magnetic pole piece to make the magnetic field distribution around the magnetic pole piece equivalent to that when the plate-shaped yoke is square. It may also be placed close to or in contact with the magnetic pole piece, and its shape and dimensions should be selected appropriately depending on the position close to or in contact with the magnetic pole piece, the magnetic properties of the permanent magnet, etc.
Furthermore, the arrangement method is such that the distance and positional relationship to the magnetic pole piece can be varied, or it can be placed outside the magnetic circuit, or directly,
Various means can be adopted, such as installing a plate-like yoke.

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 important. 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. FIGS. 2 and 3 are longitudinal cross-sectional views showing the arrangement of magnetic flux shunt materials according to this invention.

In the magnetic circuit of the embodiment, four cylindrical yokes 1 are used as spacing members, and these are arranged at the four corners of a plate-shaped yoke 2 having a rectangular main surface, thereby creating a gap between the pair of plate-shaped yokes 2. 5, and the pair of plate yokes 2 are magnetically connected, and furthermore, a permanent magnet 3 is attached to the opposing surface of each plate yoke 2, and an annular magnet is attached to the opposing surface of each permanent magnet 3. It has a configuration in which a disk-shaped magnetic pole piece 4 having a protrusion 6 is provided to generate a magnetic field in the gap 5.

The magnetic pole piece 4 has 6 annular protrusions with a trapezoidal cross section on its periphery.
Furthermore, a planar circular projection is provided at the center of the annular projection 6.

A magnetic thin plate 7 is attached so as to come into contact with the long sides of the pair of plate-shaped yokes 2 and completely cover the area between them. This magnetic thin plate 7 has an opening 8 to the air gap in the center thereof, through which the object to be diagnosed is allowed to enter.

In addition, a rectangular magnetic flux shunt material 9 is attached to the center portion of the long side of the pair of plate-shaped yokes 2 by cutting out a portion of the magnetic thin plate 7, and is placed close to the magnetic pole piece 4. By bolting the shunt material 9 to the plate-like yoke through the elongated holes, it can be moved vertically and the amount of shunt can be varied.

With this magnetic thin plate 7 and magnetic flux shunt material 9,
A portion of the magnetic flux from the pole piece 4 is shunted to the pole piece 4.
The surrounding magnetic field distribution can be made equivalent to the case where the plate-shaped yoke 2 is square.

The magnetic thin plate 7 and the magnetic flux shunt material 9 can be arranged independently, and the magnetic thin plate 7 can also be attached to the short sides of the pair of plate yokes 2 to connect to the magnetic circuit. As shown in FIG. 2, it can be placed outside the magnetic circuit so as to enclose the magnetic circuit, and its placement and shape are appropriately selected in order to obtain a uniform magnetic field.

Similarly, the magnetic flux shunt material 9 can be selected from various shapes and arrangements, and its tip is usually placed between the opposing surface of the magnetic pole piece 4 and the mounting surface of the plate-shaped yoke 2, between H in FIG. In addition to the L-shaped configuration with flat parts,
As shown in FIG. 3, the arrangement and shape of the magnetic flux shunting material 9 are appropriately selected in order to obtain a uniform magnetic field, such as by arranging the magnetic flux shunting material 9 together with the magnetic thin plate 7 outside the magnetic circuit.

Example In the configuration shown in Fig. 1, an Fe-B-R permanent magnet having a maximum energy product of 35 MGOe is used, the facing distance of the magnetic pole pieces is set to 450 mm, and the plate yoke dimensions are 1000 mm L x 1800mmW×120
mmT, the outer diameter D of the magnetic pole piece is 950 mm, and a magnetic thin plate with a thickness of 2.5 mm is attached to the long side of a pair of plate yokes. Excellent magnetic field uniformity of less than 50 ppm was obtained, and in addition, 400 mm length x 250 mm width x 15
When a magnetic flux shunt material with a thickness of mm was installed and adjusted by moving it up and down, an extremely excellent magnetic field uniformity of less than 20 ppm was obtained within a radius of 100 mm on the horizontal plane at the center of the gap.

In addition, in the case of the magnetic field generator of the embodiment with the above configuration, the installation space is 80% of that of the comparative magnetic field generator shown in Fig. 4 when compared with an equivalent uniform magnetic field.
It can be seen that the size and weight of the device are excellent.

[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. FIGS. 2 and 3 are longitudinal cross-sectional views showing the arrangement of magnetic flux shunt materials according to this invention. FIG. 4 is a cross-sectional top view and a front explanatory view showing the magnetic field generating device. 1...Cylindrical yoke, 2...Plate yoke, 3...
Fe-BR-based permanent magnet, 4... magnetic pole piece, 5...
Air gap, 6... Annular projection, 7... Magnetic thin plate, 8...
Opening, 9...Magnetic flux shunt material.

Claims (1)

[Scope of utility model registration request] Permanent magnets are attached to the opposing surfaces of a pair of plate-shaped yokes that are magnetically connected with the columnar yokes used as spacing members to form a gap, and magnetic pole pieces are provided on the opposing surfaces of each permanent magnet. In a magnetic field generating device that generates a magnetic field in an air gap, a pair of plate yokes are rectangular, and the ratio of the short side (L) of the plate yoke to the dimension (D) of the magnetic pole piece in the short side direction is D/ A magnetic field generating device characterized in that L is set to 0.6 to 1.2, and a magnetic thin plate is arranged at least on the long side of the plate-shaped yoke in contact with or close to the plate-shaped yoke.