JPH04307707A - Magnetic-field generation device - Google Patents

Abstract

PURPOSE:To reduce the amount of a leakage flux from a yoke for a magnetic circuit and to enhance the intensity of a magnetic field in the central part of an air gap at an inner magnet-type magnetic-field generation device, by the permanent magnet system, for MRI apparatus use. CONSTITUTION:A yoke for a magnetic circuit is constituted of the following: a sheetlike part used to mount and install a permanent magnet 1; and protrusions 32 used to couple supports 4 on the side face of the sheetlike part. Thereby, the magnetic circuit which reduces a leakage flux from the yoke 31 and whose efficiency is good is constituted.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention provides a method for producing a high-intensity, highly accurate, and uniform static magnetic field within a wide gap used in a nuclear magnetic resonance imaging device (hereinafter referred to as an MRI device) that takes tomographic images of a subject. The present invention relates to a magnetic field generating device that generates.

0002

2. Description of the Related Art There are three types of magnetic field generating means in an MRI apparatus: a permanent magnet type, a normal conducting magnet type, and a superconducting magnet type. Among these, the permanent magnet method is the most economical because it does not involve the consumption of electric power or helium, and has the advantages of having relatively little leakage magnetic flux and being compact, making it easy to install. Furthermore, in recent years, with the advent of rare earth magnets with strong magnetic force and improvements in the performance of signal detection devices and imaging technology, permanent magnet MRI devices have rapidly become popular.

[0003] In an MRI apparatus, the strength and uniformity of the static magnetic field affect the quality of the captured image, so a high-intensity, uniform magnetic field with an accuracy of 10-4 or less is required near the center of the gap where the subject is inserted. Ru.

Conventional permanent magnet magnetic field generators can be classified into three types based on their structure: internal magnetic type, external magnetic type, and ring type. Among these, the internal magnetic type and ring type magnetic circuits have already been put into practical use, and the internal magnetic type magnetic circuit in particular is relatively easy to manufacture and has little magnetic flux leakage, so it is attracting the most attention as a type that will become popular in the future. . As shown in FIG. 2, a conventional internal magnet type magnetic circuit is constructed by magnetically coupling a pair of permanent magnets and a pole piece using a flat yoke and four pillars.

[0005]

[Problems to be Solved by the Invention] However, in the above-mentioned conventional technology, since the yoke is flat, the magnetic flux generated from the permanent magnet does not flow efficiently to the pillar, and the amount of magnetic flux leaks from the side of the yoke to the outside of the magnetic circuit. Since it is large, it has a large influence on the magnetic material outside the magnetic circuit, and as a result, there is a problem in that the strength of the static magnetic field at the center of the air gap cannot be increased.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to propose a structure of an efficient magnetic field generating device that reduces leakage magnetic flux to the outside of the magnetic circuit and allows the magnetic flux to flow more toward the center of the air gap.

[0007]

[Means for Solving the Problems] The magnetic field generating device of the present invention includes a pair of permanent magnets that face each other with a gap formed therein, and a pole piece is attached to each of the gap-opposing surfaces of the pair, and the permanent magnet pair is made of a magnetic material. In the magnetic field generating device, a yoke and a plurality of columns are magnetically coupled to form a magnetic circuit and generate a magnetic field in the air gap, the yoke includes a plate-shaped portion for attaching a permanent magnet, and a plate-shaped portion of the plate-shaped portion. It is characterized by having a structure having a convex portion on the side surface for connecting the support column.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a longitudinal cross-sectional view and a cross-sectional view showing a magnetic circuit used in a magnetic field generating device according to an example of the present invention. The magnetic circuit includes a pair of pole pieces 2 that face each other with a gap 6 formed therebetween, and a permanent magnet 1 is attached to each side opposite to the gap, and these are connected by yokes 31 and 32 made of magnetic material and four pillars 4. Forms a magnetically coupled structure. As shown in FIG. 1, the yokes 31 and 32 are comprised of a plate-shaped portion 31 on which the permanent magnet 1 is attached and a convex portion 32 for connecting the support column 4. The plate-shaped portion 31 and the convex portion 32 of the yoke may be molded separately and then attached, or they may be molded as a single piece, but in this embodiment they are molded as one piece. The permanent magnet used here is an Nd-Fe-B system with a maximum energy product (BH) max of 25 Mega Gauss Oersted (MGOe) or more in order to avoid an increase in the weight of the magnetic circuit and obtain a strong central magnetic flux density. Alternatively, rare earth magnets such as Pr-Fe-B are preferable. Also,
As shown by the arrows in the figure, the magnetization directions of the permanent magnets are vertically the same in both the upper and lower directions. In this example, the basic composition is Pr
17 at%, Fe76.5 at%, B5.0 at%, C
A 1.4 ton rare earth magnet manufactured by hot/rolling with u1.5 at% and a maximum energy product of 28.4 MGOe was used. The materials used for the yokes 31, 32 and the pillars 4 are required to be inexpensive and have a high saturation magnetization.
00G S15C was used. The material used for the pole piece 2 is required to have high magnetic permeability and high saturation magnetization, so in this example, the saturation magnetization is 18000G.
, electromagnetic soft iron having magnetic properties with a maximum permeability of 10,000 was used. The distance between the pole pieces of this magnetic circuit is 520m
When the magnetic flux density at the center of the air gap was measured with the setting set to m, it was found that 1
It was 850G. Next, the surface of a sphere with a diameter of 300 mm centered at the center of the air gap was measured to evaluate the uniformity of the magnetic field in the space. (r, θ, φ) represent spherical coordinates; in this embodiment, r is 150 mm, and θ and φ take values every 30°. In this way, the surface of the space is divided into five cross sections at a pitch of 30 degrees, and the magnetic flux density is measured at a pitch of 30 degrees on each divided surface. Uniformity was evaluated at a total of 63 points, including 60 points measured by this method, the center 6 of the void, and the top and bottom points of the sphere. As a result, the spatial magnetic field uniformity was 40 ppm.

FIG. 2 is an explanatory diagram showing the configuration of a conventional magnetic circuit. A permanent magnet 1 is installed on the anti-gap side of each of a pair of pole pieces 2 facing each other with a gap 6 formed therebetween, and these are magnetically coupled by a yoke 3 made of a magnetic material and four pillars 4. to accomplish. However, the yoke 3 has a simple rectangular plate shape. This magnetic circuit uses materials with the same magnetic properties as the magnet, yoke, strut, and pole piece used in the magnetic circuit shown in Figure 1, and the effective gap length L is 520 m.
m, the magnetic flux density at the center of the gap was 1760 G, and the uniformity was 40 ppm.

Comparing this example with the conventional example, the magnetic flux density of the air gap was improved by about 100G. This is because the structure reduces the magnetic flux that conventionally leaked from the side surface of the yoke.

[0011]

As described above, according to the present invention, a pole piece is attached to each of the gap-opposing surfaces of a pair of permanent magnets facing each other with a gap formed therebetween, and the permanent magnet pair is made of a magnetic material. In a magnetic field generating device that forms a magnetic circuit by magnetically coupling a yoke and a plurality of columns to generate a magnetic field in the air gap, the yoke includes a plate-shaped part for attaching a permanent magnet, and a side surface of the plate-shaped part. By constructing a convex portion for connecting the pillars of the yoke, leakage magnetic flux from the side surface of the yoke is reduced, and a magnetic circuit structure with high magnetic efficiency is realized.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view and a cross-sectional view showing the basic structure of a magnetic circuit of a magnetic field generating device in an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view and a cross-sectional view showing the basic structure of a conventional internal magnet type magnetic circuit.

[Explanation of symbols]

1 Permanent magnet part 2 Pole piece 3 York 31 Yoke center plate part 32 Yoke peripheral protrusion ４　　　　pillar 5 Adjustment bolt 6 Void

Claims (1)

[Claims] 1. A pair of permanent magnets facing each other with a gap formed therein, each having a pole piece attached to the gap-opposing surfaces thereof, and the permanent magnet pair being magnetically coupled to a yoke made of a magnetic material by a plurality of supports. In the magnetic field generating device that constitutes a magnetic circuit and generates a magnetic field in the air gap, the yoke has a plate-shaped part for attaching a permanent magnet and a convex part for coupling a support to a side surface of the plate-shaped part. A magnetic field generating device characterized by forming a structure.