JPH0223010B2 - - Google Patents

Description

[Detailed Description of the Invention] Field of Application The present invention relates to a magnetic field generating device using a permanent magnet used in a medical nuclear magnetic resonance tomography device, and the present invention relates to a magnetic field generating device using a permanent magnet used in a medical nuclear magnetic resonance tomography device. Partly, the present invention relates to a permanent magnet type magnetic field generator that stably generates a strong, highly accurate, and uniform magnetic field over a long period of time.

Background technology Medical nuclear magnetic resonance tomography device {NMR-CT
Or medical MRI (Magnetic Resonance)
(hereinafter referred to as medical MRI)},
Inside the sky of a magnetic field generator that creates a strong magnetic field,
This device is capable of inserting part or all of a subject into the test subject, obtaining a tomographic image of the object, and depicting the nature of its tissue.

In the medical MRI magnetic field generator described above, the space must be wide enough to allow part or all of the subject to be inserted, and in order to obtain a clear tomographic image, usually, for example, Within the imaging field of view of a spherical space with the required radius, there is a
It is required to create a stable and strong uniform magnetic field with an accuracy of 10 ^-4 or less.

As magnetic field generators for medical MRI, there are two types of magnets: normal conductive magnets made of copper or aluminum conductive wire wrapped around a cylinder, and superconducting magnets that use special conductive wire cooled to a temperature close to absolute zero. It is being

The former is structurally inexpensive, but in order to generate a sufficiently strong magnetic field, it requires a huge amount of electric power and a huge amount of cooling water to cool down the heat generated by this, resulting in high running costs and the large size generated by the coil. Leakage magnetic fields may cause problems depending on the intended use, while the latter type of superconducting magnets have the advantage of consuming less power and being able to generate strong magnetic fields in a small size. The use of expensive liquid helium or the like is essential, and there is a problem that the so-called initial cost and running cost are extremely high.

In the case of a magnetic circuit using permanent magnets, for example, if the strength of the magnetic field generated in a space large enough to fit a person's head or whole body can be obtained equal to that of the normal conducting or superconducting electromagnets mentioned above. , there is no power consumption, the leakage magnetic field is weak, and it can be said that it is ideal for the practical application of the above-mentioned applications. However, the magnetic circuit of the currently known configuration causes various problems when it is put into practical use, and its application is extremely difficult. It will be limited.

In addition, MRI uses electromagnets to generate a highly uniform magnetic field in a very small space with a distance between magnetic poles of about 4 cm.
magnetic circuits have been proposed. To be more specific, assuming a transverse plane passing through the center between magnetic poles consisting of opposing flat surfaces, the magnetic field attenuates slightly at a required distance from the center in proportion to the distance. A magnetic circuit has been proposed (Japanese Patent Publication No. 7545/1983) in which magnetic materials having a thickness sufficient for attenuation are sequentially arranged, that is, an annular auxiliary magnetic pole whose thickness is gradually increased toward the outer periphery of the magnetic pole surface. There is.

Furthermore, considering the influence of the magnetic permeability of the magnetic pole material on the magnetic poles consisting of opposing flat surfaces, we found that the isomagnetic surfaces, which are the contour lines of the magnetic field obtained on the magnetic pole surfaces, are convex spherical surfaces with the required curvature, and we compensated for this. Therefore, an MRI magnetic circuit has been proposed (Japanese Patent Publication No. 36-4664) in which the magnetic pole surface is calculated in advance based on this influence and wrapped into a concave spherical surface with a required curvature.

This proposal is based on the idea that when the magnetic pole diameter is very large and the distance between the magnetic poles is very small compared to the magnetic pole diameter, the magnetic field strength between the magnetic poles is uniform. ratio, magnetic pole diameter D, and distance between magnetic poles Lg, if D/Lg is large (D/Lg=5
~10) An electromagnetic magnetic circuit, which generates a highly uniform magnetic field in a very small space with a distance between magnetic poles of about 4 cm.In particular, an extremely high uniform magnetic field can be obtained in the central space where a small sample is placed. There is.

However, conversely, the distance between the magnetic poles with respect to the magnetic pole diameter D
A permanent magnet magnetic circuit with a large ratio of Lg, i.e.
In a magnetic field generator using permanent magnets, the distance between the magnetic poles is large enough to fit a person's head or whole body, and the magnetic material of the magnetic pole pieces is increased or decreased to form a concave spherical shape. Center cross section (x-y
Even if we apply technology to correct the magnetic field on the magnetic circuit (plane), it is a vast three-dimensional space that is significantly expanded in the direction between the magnetic pole pieces (Lg, z-axis direction), which is incomparable to the sky in the magnetic circuit. Therefore, it is completely impossible to obtain the required highly uniform magnetic field required for medical MRI.

In addition, the above proposal is an electromagnetic magnetic circuit, which cannot stably obtain the required high uniform magnetic field for a long time, and requires a huge amount of power and a huge amount of cooling water to cool down the heat generated. , there is a problem of high running costs.

Purpose of the Invention In view of the current situation where no magnetic field generating device for MRI using permanent magnets that can generate a strong magnetic field has been proposed, the present invention has been developed to generate The object of the present invention is to provide a magnetic field generating device having a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field in a space large enough to accommodate a person's head or whole body.

Summary of the Invention This invention forms a space and magnetically couples opposing permanent magnets with a yoke, attaches a magnetic pole piece to the surface of each permanent magnet facing the space, and connects the space as required in the center of the space. A permanent magnet type magnetic field generator for a medical nuclear magnetic resonance tomography apparatus that generates a highly uniform magnetic field in a space, characterized in that an annular projection satisfying the following formula is provided on each peripheral edge of the opposing surface of the magnetic pole piece. This is a permanent magnet type magnetic field generator.

D ₂ ≧1/2・D ₁ H≦1/4・Lg However, D ₁ : External diameter of the magnetic pole piece, D ₂ : Inner diameter of the annular protrusion, Lg : Distance between the magnetic pole pieces, H : Height of the annular protrusion.

Structure of the invention The permanent magnet used in the magnetic field generating device of this invention is
Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but Fe-B-R magnets (R is Y (at least one kind of rare earth element containing %/℃, so by applying this permanent magnet to the above MRI, it is possible to downsize the device.
Excellent performance can be obtained, and furthermore, by cooling this permanent magnet to 0° C. or below, it is possible to effectively utilize the property that a significantly high maximum energy product can be obtained.

The above Fe-BR-based permanent magnet is R (where R is Y
(at least one of rare earth elements containing It is an excellent permanent magnet that uses resource-rich light rare earths such as Nd and Pr as R, and has B and Fe as its main components and exhibits an extremely high energy product of 25 MGOe or more.

The magnetic circuit of the permanent magnet type magnetic field generator according to the present invention is not limited to the embodiments described below.
The present invention can be applied to any magnetic circuit as long as it has a configuration in which a pair of permanent magnets facing each other with a space in between is provided with a magnetic pole piece having annular projections on opposing surfaces.

The shape and dimensions of the annular protrusion may be appropriately selected depending on the dimensions of the permanent magnet, the magnetic properties, and the size of the cavity, but the following range is particularly effective.

D ₂ ≧1/2・D ₁ H≦1/4・Lg However, D ₁ : External diameter of the magnetic pole piece, D ₂ : Inner diameter of the annular protrusion, Lg : Distance between the magnetic pole pieces, H : Height of the annular protrusion.

Furthermore, by forming the inner diameter surface on the inner peripheral side of the annular protrusion into an inclined surface that expands upward, a more uniform magnetic field can be stably obtained.

In addition, the distance Lg between the magnetic pole pieces is, for example, in the case of FIG. 1, the measurement point of the annular protrusion inner diameter D ₂ of each magnetic pole piece,
In other words, it is the distance between the flat surface portions that are the starting points of the inclination of the inner diameter surface.

DISCLOSURE OF THE INVENTION BASED ON THE DRAWINGS FIG. 1 is a longitudinal sectional view showing a permanent magnet type magnetic field generating device according to the present invention. Figures 2 a to d are the first
FIG. 6 is a vertical cross-sectional view showing various shapes of the annular protrusions of the magnetic pole pieces in the figures.

The permanent magnet type magnetic field generator shown in FIG. 1 has a structure in which a pair of Fe-B-R permanent magnets 1 are each fixed with a magnetic pole piece 2 at one end facing each other, and the other end is connected with a yoke 3. The space 4 between the magnetic pole pieces 2 is large enough to accommodate part or all of a human body for diagnosis, and a strong magnetic field of 0.05 to 2.0 T is generated within the space 4.

Each of the pair of magnetic pole pieces 2 has an outer diameter of D ₁ , and the outer diameter of the opposing surface on the air 4 side has an inner diameter of D ₂ ,
An annular protrusion 5 having a height H dimension, a trapezoidal cross-sectional shape, and an inclined surface whose inner peripheral surface expands upward is protruded, and the pair of magnetic pole pieces 2 face each other at the center of the hollow 4. A highly accurate, uniform, and stable magnetic field can be obtained in the required space.

FIG. 2 shows longitudinal cross-sectional views of the magnetic pole piece 2. FIG. 2a shows a case where the annular projection 5 has a triangular cross section, FIG.
If the cross section is triangular and the inner slope is curved,
Figure d shows a case where the annular protrusion 5 has a trapezoidal cross section and its corners are chamfered to form a curved surface.

Each of the annular protrusions 5 forms an inclined surface such that the inner diameter surface expands upward, and if the distance between the magnetic pole pieces 2 is Lg, then D ₂ ≧1/2・D ₁ and H≦1/4・Lg are satisfied. are doing.

By the way, the magnetic field generator with the configuration shown in Figure 1 has an Fe-
Using a B-R permanent magnet, set Lg to 350mm,
Measure the magnetic field distribution in the air when a magnetic pole piece with an annular protrusion of D ₁ = 500 mm, D ₂ = 400 mm, and H = 30 mm is placed, and when a magnetic pole piece with an outer diameter of 500 mm without an annular protrusion is placed. As a result, in the case of this invention having an annular protrusion, compared to the comparative example, the uniform magnetic field was
The volume of the 10 ^-3 region expanded three times.

Further, in the case of the magnetic pole piece according to the structure of the present invention, when compared at the same magnetic field strength, the magnetic pole piece and magnet were required to have a smaller diameter than those of the above-mentioned comparative example, which was advantageous in reducing the size and weight.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing a permanent magnet type magnetic field generator according to the present invention. Figures 2 a to d are the first
FIG. 6 is a vertical cross-sectional view showing various shapes of the annular protrusions of the magnetic pole pieces in the figures. 1... Fe-BR-based permanent magnet, 2... Magnetic pole piece,
3... Yoke, 4... Sky, 5... Annular protrusion.

Claims (1)

[Scope of Claims] 1. Permanent magnets facing each other are magnetically coupled by a yoke to form a void, and a magnetic pole piece is attached to the surface of each permanent magnet facing the void, and a required amount of space is provided in the center of the void. In a permanent magnet type magnetic field generator for a medical nuclear magnetic resonance tomography apparatus that generates a highly uniform magnetic field in a space, an annular protrusion satisfying the following formula is provided on each peripheral edge of the opposing surface of the magnetic pole piece. Characteristic permanent magnet type magnetic field generator. D ₂ ≧1/2・D ₁ H≦1/4・Lg However, D ₁ : External diameter of the magnetic pole piece, D ₂ : Inner diameter of the annular protrusion, Lg : Distance between the magnetic pole pieces, H : Height of the annular protrusion.