JPH0246003Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial field of application 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 a magnetic field generator using
This invention relates to a magnetic field generating device 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 absolute zero. .

The former is structurally inexpensive, but it requires a huge amount of electricity and cooling water to generate a sufficiently strong magnetic field, resulting in high running costs, and the leakage magnetic field created by the coil can be a negative factor depending on the application. On the other hand, the latter type of superconducting magnet has the advantage of consuming less power and being small and generating a strong magnetic field, but it requires the use of expensive liquid helium or the like as a coolant. There is a problem that the running cost as well as the so-called initial cost is extremely high, so it has not been widely used.

The present applicant has previously proposed a magnetic circuit for NMR-CT that uses permanent magnets with a magnetic field strength equal to or higher than that of the above-mentioned normal conduction magnets, consumes no power, and has a small leakage magnetic field. A magnetic field generating device for magnetically coupling a magnetic pole piece and at least one permanent magnet with a yoke to generate a magnetic field in the air gap, characterized in that an annular protrusion is provided on each opposing surface of the magnetic pole piece. Proposed a generator (patent application 1982-
No. 196785).

With the above magnetic field generating device, it was possible to significantly improve the uniformity accuracy of the magnetic field generated in the air gap. However, when designing the above magnetic field generator,
The most efficient magnetic circuit is designed through analysis based on the magnetic properties of permanent magnets and the shapes of various component parts, but due to variations in magnetic properties and dimensional accuracy of each component, it is difficult to increase the magnetic field as designed. It is difficult to obtain accuracy, and in terms of maintenance management,
There is a problem in that it is difficult to maintain the magnetic field as designed, and it is necessary to adjust the magnetic field during or after assembly of the magnetic field generator.

Purpose of the invention In view of the current situation, this invention has been developed to obtain a highly accurate, uniform and stable magnetic field in the air gap of a magnetic field generator using permanent magnets that can generate a strong magnetic field, and to provide a uniform and stable magnetic field even during maintenance management. In order to obtain a magnetic field, the object is a magnetic field generating device having a magnetic circuit capable of adjusting the magnetic field.

Structure and effect of the invention This invention was developed as a result of various studies aimed at creating a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field and that can also adjust the degree of magnetic field uniformity. By slightly moving the magnetic pole pieces in a direction parallel to the opposing plane, the magnetic field uniformity within the magnetic circuit air gap is significantly improved.
It was discovered that a highly accurate and uniform magnetic field area is expanded in the direction parallel to the opposing plane of the magnetic pole pieces.

That is, this invention provides 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, and generates a magnetic field in the air gap. This is a magnetic field generating device characterized by having a magnetic field adjusting mechanism that can relatively move the magnetic pole pieces in parallel in a direction parallel to opposing surfaces.

Any configuration can be used for the magnetic circuit as long as at least one permanent magnet is used and magnetic pole pieces that are opposed to each other with an air gap are magnetically coupled with a yoke. It is desirable to appropriately select the shape of the magnetic pole piece depending on the size and size of the required air gap.

It is also good to have a magnetic circuit with an annular protrusion on one side of the magnetic poles facing each other with an air gap in between.Protrusions of various shapes, such as protrusions with a triangular or trapezoidal cross section, can be used on the periphery of the plane of the magnetic pole face or at any desired location. It is also possible to provide convex protrusions in the center of the annular protrusion, either continuously or discontinuously.

The magnetic field adjustment mechanism relatively moves the magnetic pole pieces in parallel, and as shown in the example, a plurality of non-magnetic bolts are attached to the side of the yoke that magnetically couples the opposing magnetic pole pieces and holds them by forming a gap. Various adjustment mechanisms can be adopted depending on the magnetic circuit configuration, such as a configuration in which the amount of movement changes depending on the degree of penetration of the bolt, but the influence on each magnetic pole piece in the magnetic circuit incorporating the adjustment mechanism , are preferably magnetically equivalent, and are desirably selected appropriately depending on the magnetic properties and dimensions of the permanent magnet, the dimensions of the yoke, the size of the air gap, etc.

In addition, if the amount of relative parallel movement of the magnetic pole pieces becomes larger than necessary, it will not only make the adjustment mechanism larger and more complicated, but also cause various problems such as causing a loss of magnetic balance. The amount of parallel movement is preferably 2% or less with respect to the diameter.

The magnetic field generating device according to this invention is a device that can generate a strong magnetic field of 1 to 5 kg by using the following permanent magnet, and weighs less than 10 tons.
100ton of magnetic field generator using ferrite magnet
Compared to the above, a significant reduction in size and weight can be achieved.

Permanent magnet used in the invention 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 the applicant has previously proposed a new high-performance permanent magnet that does not contain expensive Sm or Co. At least one kind of rare earth element) Permanent magnet (Special application)
57-145072) not only has a large maximum energy product, but also has a temperature coefficient of residual magnetic flux density (Br) of 0.07%/°C to 0.15%/°C. The above NMR-CT
By applying this permanent magnet, it is possible to downsize the device and obtain excellent performance.Furthermore, the magnetic properties of this permanent magnet make it possible to achieve a significantly high maximum energy product by cooling it to below 0°C. You can effectively utilize the properties you can obtain.

The above Fe-BR-based permanent magnet has R (where R is at least one kind of rare earth elements including Y) 8 at% to 30 at%, B2 at% to 28 at%, Fe42 at% to 90 at%. It is a permanent magnet whose main phase is a tetragonal phase, and R is a light rare earth that is abundant in resources, mainly Nd and Pr.
It is an excellent permanent magnet that has extremely high energy product of over 25MGOe as its main component.

Disclosure of the invention based on drawings Figures 1 and 2 are NMR-CT based on this invention.
FIG. 2 is a vertical front view and a cross-sectional side view of a magnetic circuit used in the present invention.

The magnetic circuit shown in Figures 1 and 2 consists of a pair of Fe
- A magnetic pole piece 2 is fixed to one end of each of the B-R permanent magnets 1, 1, and the other end of the permanent magnet 1 is made of a cylindrical body with a part of its circumference open. They are connected by a yoke 3 and are configured to generate a static magnetic field in the air gap 4 between the magnetic pole pieces 2.The pair of magnetic pole pieces 2, 2 have a predetermined inner diameter and height on the inside of the periphery of their opposing surfaces. Fe-B-R permanent magnet 1,
1 are magnetized in the same direction.

Furthermore, on the yoke 3 located opposite the outer periphery of each magnetic pole piece 2, there are non-magnetic movement adjustment bolts screwed in three places at 120° intervals from the outer periphery of the yoke 3 toward the center. 6 are arranged, and each magnetic pole piece 2 is configured to be able to move relatively in parallel in the parallel direction of the pair of opposing surfaces of the magnetic pole pieces by screwing the movement adjustment bolts 6 back and forth. It has been done.

In the above configuration, for example, the position of one magnetic pole piece 2 is fixed, and the position of the other magnetic pole piece 2 is moved by screwing the three movement adjustment bolts 6 back and forth,
By slightly shifting the opposed center lines of the pair of magnetic pole pieces 2, a static magnetic field having extremely high uniformity in the horizontal direction can be obtained within the air gap 4.

By the way, the configuration shown in Figures 1 and 2
An Fe-B-R permanent magnet with a maximum energy product of 35 MGOe was used in the NMR-CT, and the opposing distance of the magnetic pole pieces was set to 600 mm, and the magnetic field was adjusted using the travel adjustment bolt. As a result, when compared with a magnetic circuit having the same configuration as the NMR-CT described above but without the magnetic field adjustment mechanism described above, the range of a uniform magnetic field of 150 ppm or less in three axial directions was expanded by 15%.

[Brief explanation of the drawing]

1 and 2 are a vertical front view and a cross-sectional side view of a magnetic circuit used in a magnetic field generating device according to this invention. 1... Fe-BR-based permanent magnet, 2... Magnetic pole piece,
3...Yoke, 4...Gap, 5...Annular protrusion, 6...
...Travel adjustment bolt.

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,
A magnetic field generating device characterized by having a magnetic field adjustment mechanism capable of relatively moving a magnetic pole piece in parallel in a direction parallel to a pair of opposing surfaces of the magnetic pole pieces.