JPH0845729A - Magnetic field generating device for mri - Google Patents

Abstract

PURPOSE:To provide a magnetic field generating device capable of increasing the operating efficiency of a permanent magnet, while relieving the oppressed feeling of a patient. CONSTITUTION:A so-called longitudinal C-sectional yoke is composed by connection- supporting one end of a pair of sheetlike yokes 1a, 1b opposite arranged forming a cavity 2 supporting yoke 3 so that subpermanent 5a, 5b may be arranged on the cavity opposite surface of the supporting yoke 3 on the main permanent magnets 4a, 4b arranged on respective cavity opposite surfaces of the sheetlike yokes 1a, 1b through the intermediary of gaps 2b, 2c while magnet pole pieces 6a, 6b whereon protrusions 7a, 7b are formed on the periphry excluding the arrangement side periphery of the subpermanent magnets 5a, 5b arranged on the cavity opposite surface of the main permanent magnets 4a, 4b. Accordingly, the cavity 2 central part wherein the patient is placed is opened in three sides excluding the supporting yoke 3 side. Furthermore, the leakage of magnetic flux from the main permanent magnets 4a, 4b and the magnet pole pieces 6a, 6b to the supporting yoke 3 can be reduced by the arrangement of the subpermanent magnets 5a, 5b in a specific shape on the cavity opposite surfaces of the supporting yoke 3 thereby enabling the magnetic field intensity in the cavity 2 and the uniformity of magnetic field to be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a magnetic field generator used in a magnetic resonance tomography apparatus for medical use (hereinafter referred to as MRI) and the like, and particularly, by using a so-called C-type yoke as a yoke. While reducing the pressure on the person,
The present invention relates to a magnetic field generator for MRI in which the use efficiency of permanent magnets is significantly improved by devising the arrangement and the like of permanent magnets as magnetic field generation sources and the configuration of pole pieces.

[0002]

2. Description of the Related Art In a conventional magnetic field generator for MRI,
As a configuration in which a permanent magnet is used as a magnetic field generation source, a configuration using a so-called four-column type yoke as shown in FIGS. 5A and 5B is most often used. Note that FIG. 5B is a sectional view taken along line BB of FIG. That is, in the magnetic field generator for MRI shown in FIGS. 5A and 5B, four cylindrical yokes are arranged so that a pair of plate yokes 11a and 11b are arranged facing each other with a predetermined gap 12 formed therebetween. 13a, 13b, 13
c and 13d are connected and supported, and pole pieces 16a and 16b are arranged together with the permanent magnets 14a and 14b on the air gap facing surfaces of the pair of plate yokes 11a and 11b (actually, fairness). 2-44483).

Further, as a magnetic field generator for MRI which makes it possible to improve the efficiency of use of permanent magnets and to downsize the apparatus, the so-called cylindrical (tunnel type) shown in FIG. 6 is used.
A structure using a yoke is proposed (Japanese Patent Laid-Open No. 5-32).
6252). That is, the magnetic field generator for MRI shown in FIG. 6 has a pair of main permanent magnets 24a, 24b and a plurality of auxiliary permanent magnets 25a, on each inner peripheral surface of the hexagonal tubular yoke 21.
25b, 25c, 25d are arranged in a ring shape, and magnetic pole pieces 26a, 26b (26a is not shown) are arranged on the surfaces of the pair of main permanent magnets 24a, 24b facing the air gap 22.

Furthermore, as a magnetic field generator for MRI that can reduce the feeling of pressure given to a subject at the time of diagnosis, a structure using a so-called C-shaped yoke as shown in FIG. 7 has been proposed (design). 849825). That is, the magnetic field generator for MRI shown in FIG. 7 has a pair of plate yokes 31 a, 31.
One end of the pair of plate yokes 31a and 31b is connected and supported by a support yoke 33 so that b is arranged to face each other with a predetermined gap 32.
The permanent magnets 34a, 34b are provided on the surfaces facing the air gaps of a, 31b
(34a is not shown) and magnetic pole pieces 36a and 36b are arranged.

[0005]

Among the above-mentioned magnetic field generators for MRI having various configurations, FIGS. 5 (a) and 5 (b) are shown.
In the magnetic field generator for MRI shown in FIG. 4, four cylindrical yokes 13a, 1 are provided around the void 12 into which the subject is inserted.
Since only 3b, 13c, and 13d are arranged, the subject has an advantage that there is a feeling of openness and a feeling of pressure at the time of diagnosis is relatively reduced. However, from the viewpoint of the efficiency of the magnetic circuit, there is a drawback that the leakage of magnetic flux to the outside of the air gap 12 is large and the use efficiency of the permanent magnet is poor.

On the other hand, the magnetic field generator for MRI shown in FIG. 6 has the advantages that the leakage of the magnetic flux to the outside of the air gap 22 is small and the use efficiency of the permanent magnet is high, but the subject is inserted. A plurality of permanent magnets 24a, 24b having voids 22
And 25a, 25b, 25c, 25d, there is a drawback that the subject feels a great sense of oppression.

Further, the magnetic field generator for MRI shown in FIG. 7 can obtain a feeling of openness equal to or higher than that of the magnetic field generator for MRI shown in FIGS. 5 (a) and 5 (b). The arrangement configuration of the pieces is substantially the same as the configuration shown in FIGS. 5A and 5B, the use efficiency of the permanent magnet is poor, and it is not possible to reduce the size and weight of the device.

An object of the present invention is to provide a magnetic field generator for MRI which is constructed so as to reduce the feeling of pressure on the subject.
In particular, it is an object of the present invention to provide a magnetic field generator for MRI in which the use efficiency of permanent magnets is significantly improved by devising the arrangement and the like of the permanent magnets as the magnetic field generation source and the configuration of the pole pieces. The magnetic field generator for MRI having the various configurations proposed in 1. is positively utilized.

[0009]

According to the present invention, as a result of repeating various experiments to achieve the above-mentioned object, the magnetic field generator for MRI shown in FIG. 7 reduces a feeling of oppression to a subject. Although the above configuration is preferable, the permanent magnets 34a, 34b and the magnetic pole pieces 36a, 36b are located close to the support yoke 33 for connecting and supporting the pair of plate yokes 31a, 31b from the viewpoint of reduction in size and weight of the entire apparatus. It is arranged, and in particular, it is confirmed that the magnetic flux leaks to the supporting yoke 33 so much that the use efficiency of the permanent magnet is reduced, and further, the magnetic flux leaks to the supporting yoke 33. By arranging the auxiliary permanent magnet, it was found that as a result, it is possible to improve both the magnetic field strength and the magnetic field homogeneity in the air gap 32, and completed the invention.

That is, the present invention has a pair of plate-like yokes that face each other and form a predetermined gap, and a supporting yoke that connects and supports one end of the pair of plate-like yokes. Main permanent magnets are arranged on the air gap facing surfaces of the pair of plate yokes, respectively, and a pair of auxiliary permanent magnets are arranged adjacent to the main permanent magnets on the air gap facing surfaces of the supporting yoke, and the pair of main magnets are arranged. In the magnetic field generator for MRI, magnetic pole pieces are arranged on the surfaces of the permanent magnets facing the air gap.

Further, in the above structure, the MRI is characterized in that the protrusions are formed on the peripheral portions of the pair of magnetic pole pieces other than the peripheral edge portion on the side where the auxiliary permanent magnets are disposed on the air gap facing surfaces.
A magnetic field generator for use is proposed as a particularly preferable configuration. Further, in the above configuration, the main permanent magnet has a substantially rectangular cross section, the auxiliary permanent magnet has a substantially triangular cross section, and the main permanent magnet and the auxiliary permanent magnet are rare earth permanent magnets or ferrite permanent magnets. A magnetic field generator for MRI, further characterized in that the main permanent magnet is made of a rare-earth permanent magnet having a substantially trapezoidal cross section, and the auxiliary permanent magnet is made of a ferrite permanent magnet having a substantially triangular cross section. We also propose a magnetic field generator for MRI.

In the present invention, the main permanent magnet and the auxiliary permanent magnet to be used are required magnetic field strength, device size,
It is desirable to select the material according to economic efficiency, especially rare earth permanent magnets such as rare earth cobalt-based permanent magnets and rare earth / iron / boron permanent magnets, and ferrite permanent magnets such as strontium ferrite permanent magnets. Further, it is effective to use them in combination, but when these permanent magnets are used, it is desirable to adopt the above-mentioned cross-sectional shape and arrangement configuration. In addition, it is desirable to select various configurations for the shape of the pair of plate yokes and the supporting yokes according to the shapes and dimensions of the main permanent magnet and the auxiliary permanent magnet, and only from the viewpoint of forming the magnetic path. Instead, it is desirable to select it in consideration of mechanical strength and weight reduction.

[0013]

The operation of the present invention will be described in detail with reference to the following embodiments. However, the present invention is not limited to the configuration of the embodiment. 1A and 1B show an embodiment of a magnetic field generator for MRI according to the present invention. FIG. 1A is a longitudinal sectional explanatory view, and FIG. 1B is an AA sectional explanatory view of FIG. It is a figure. In the figure, 1a and 1b are a pair of plate-like yokes that are arranged to face each other with a predetermined gap 2, and one end (the left end in the figure) of each is connected and supported by a support yoke 3 and the other end thereof. The vertical cross-section whose right end is open constitutes a so-called C-shaped yoke. In the figure, 4a and 4b are main permanent magnets arranged on the surfaces of the pair of plate yokes 1a and 1b facing the air gap 2, and 5a and 5b are main magnets on the air facing surfaces of the support yoke 3 respectively. It is an auxiliary permanent magnet arranged adjacent to the permanent magnets 4a and 4b.

In the configuration shown in the figure, the main permanent magnets 4a and 4b are provided.
Is a rare earth-based permanent magnet having a substantially rectangular cross section, and the auxiliary permanent magnets 5a and 5b are rare earth-based permanent magnets having a substantially triangular cross section. A pair of main permanent magnets 4a, 4
As shown by the arrows in the figure, the magnetization directions of b and the pair of auxiliary permanent magnets 5a, 5b are both perpendicular to the contact surface between the plate yokes 1a, 1b and the supporting yoke 3. Further, in the figure, the air gap facing surfaces of the main permanent magnet 4a and the auxiliary permanent magnet 5a are S poles, and the air gap facing surfaces of the main permanent magnet 4b and the auxiliary permanent magnet 5b are magnetized to N poles. .

When both the main permanent magnets 4a and 4b and the auxiliary permanent magnets 5a and 5b are made of ferrite permanent magnets, it is preferable to adopt substantially the same sectional shape and magnetization direction as the above. The main permanent magnet 4a,
4b and auxiliary permanent magnets 5a, 5b, it is desirable to select a preferable shape, size, etc. of the pair of plate yokes 1a, 1b and supporting yoke 3 depending on the shape, size, etc. For example, in the illustrated configuration, the main permanent magnets 4a, 4b and the plate yoke 1a,
The planar shape of 1b is substantially the same. That is, each of the main permanent magnets 4a, 4b and the plate yokes 1a, 1b which are orthogonal to the perpendicular from the center point O of the spherical space 2a having a radius r formed of a highly uniform magnetic field formed in the substantially central portion of the air gap 2. Semicircle with radius R centered on the point O'on the facing surface, and the supporting yoke 3
It has a configuration in which a rectangular portion extending to the arrangement side is integrated.

The sectional shape of the plate yokes 1a and 1b is
The main permanent magnets 4a and 4b may have a substantially rectangular cross section similar to that of the main permanent magnets 4a and 4b, but the thickness of the semicircular portion is gradually reduced toward the open end according to the magnetic flux density in the plate yokes 1a and 1b as illustrated. With the shape, weight reduction can be achieved without hindering the formation of the magnetic path. The supporting yoke 3 includes auxiliary permanent magnets 5a, 5
It is necessary to select it according to the shape of b. According to an experiment by the present inventor, the auxiliary permanent magnets 5a and 5b can be removed from the main permanent magnets 4a and 4b and a pole piece described later without reducing the uniformity of the magnetic field formed by the main permanent magnets 4a and 4b. In order to reduce the leakage of magnetic flux to the support yoke 3, it is desirable that the cross-sectional shape as shown in the figure be substantially triangular, and accordingly the cross-sectional shape of the support yoke 3 should be a vertical cross-section as shown in the figure. Is desirable. Therefore, gaps 2b and 2c are formed between the main permanent magnets 4a and 4b and the auxiliary permanent magnets 5a and 5b, respectively.

That is, the magnetizing direction of the auxiliary permanent magnets 5a and 5b is determined from the above-mentioned reasons (usually, a plurality of block-shaped permanent magnets are laminated and fixed in the magnetizing direction and integrated so as to have a predetermined sectional shape). As described above, when the direction is perpendicular to the contact surface of the supporting yoke 3, the optimum angle θ ₁ between the air bearing surfaces of the auxiliary permanent magnets 5a and 5b and the horizontal plane including the center point O of the spherical space 2a, together with the auxiliary angle. The optimum angle θ ₂ between the contact surface of the permanent magnets 5a and 5b with the support yoke 3 and the horizontal plane including the center point O of the spherical space 2a is determined, and as a result, the air gap facing surface side of the support yoke 3 is determined. The shape is determined and the cross section becomes a substantially V shape. Note that, according to an experiment by the present inventor, θ ₁ is usually 3
It is desirable to select 0 to 60 degrees and θ ₂ in the range of 45 to 90 degrees.

Further, according to the magnetic flux density in the supporting yoke 3,
The thickness can be gradually reduced toward the connection with the plate yokes 1a and 1b, and the weight can be reduced without hindering the formation of the magnetic path. From the viewpoint of mechanical strength, the lower part of the support yoke 3 (the part located below the horizontal plane including the center point O of the spherical space 2a) is thickened toward the connection part with the plate yoke 1b. It is also possible to adopt. That is, the support yoke 3 has a function of arranging the auxiliary permanent magnets 5a and 5b at predetermined positions, and the plate yoke 1a, the main permanent magnet 4a, and the magnetic pole piece 6 arranged above the magnetic circuit.
Since it is required to have a function of safely supporting these structural bodies, which are heavy objects of a, and a function of ensuring stable magnetic path formation, the auxiliary permanent magnet 5a, as described above, It is desirable to select the optimum conditions according to the shape and size of 5b.

In the figure, reference numerals 6a and 6b denote magnetic pole pieces arranged on the air gap facing surfaces of the pair of main permanent magnets 4a and 4b. In the illustrated configuration, auxiliary poles on the air gap facing surfaces of the magnetic pole pieces 6a and 6b are provided. Protrusions 7a and 7b are formed on the peripheral edge portion other than the peripheral edge portions on the arrangement side of the permanent magnets 5a and 5b. That is, the planar shape of the magnetic pole pieces 6a, 6b is also a shape in which the semicircular portion and the rectangular portion are integrated like the main permanent magnets 4a, 4b and the plate yokes 1a, 1b. 7b
The plane shape of is also a so-called horseshoe shape because it is formed on the peripheral edge other than the peripheral edge on the side where the auxiliary permanent magnets 5a and 5b are arranged. By forming the protrusions 7a and 7b, it is possible to improve the magnetic field uniformity in the void 2. Furthermore, FIG.
In the magnetic field generator for MRI configured as shown in FIG. 2, for example, protrusions 8 and 8 having a predetermined width as shown in FIG. 2 are provided at both ends in the width direction of the gap facing surface side slopes of the auxiliary permanent magnets 5a and 5b. By forming it, it becomes possible to further improve the magnetic field homogeneity in the void 2.

Further, as shown in FIG. 3, the magnetic pole pieces 6a and 6b are in contact with the main permanent magnets 4a and 4b, and the projection 7 is formed.
Is composed of a magnetic material having a high magnetic permeability such as pure iron or electromagnetic soft iron, and a plurality of blocks 10 made of a laminated body of soft ferrite or a non-oriented silicon steel sheet having a high electric resistance and a high magnetic permeability on the surface facing the air gap. The eddy current is generated in the magnetic pole pieces 6a, 6b due to the pulse current applied to the gradient magnetic field coil (not shown) arranged near the magnetic pole pieces 6a, 6b. And the residual magnetism phenomenon can be reduced.

A magnetic field generator for MRI shown in FIG. 4 is a cross-sectional explanatory view showing another embodiment of the present invention. In particular, a rare earth permanent magnet is used as a main permanent magnet and a ferrite permanent magnet is used as an auxiliary permanent magnet. The structure which used the magnet is shown. In the figure, the plate yoke 1a and the pole piece 6a are substantially the same as the configuration of FIG. However, the main permanent magnet 4a using a rare earth permanent magnet has a substantially trapezoidal cross section, and the auxiliary permanent magnet 5a using a ferrite permanent magnet has a substantially triangular cross section.

The magnetization directions of the main permanent magnets 4a and 4b are the same as those in FIG. 1, but the magnetization directions of the auxiliary permanent magnets 5a and 5b are perpendicular to the air gap facing surfaces of the auxiliary permanent magnets 5a and 5b, respectively. It is the direction. That is, also in the configuration of FIG. 4, considering the assemblability of the auxiliary permanent magnets 5a and 5b (usually, a plurality of block-shaped permanent magnets are stacked and fixed in the magnetization direction to be integrated so as to have a predetermined cross-sectional shape) and the like. ,
It is preferable that the magnetization direction is perpendicular to the contact surface of the support yoke 3 or the perpendicular direction to the air gap facing surfaces of the auxiliary permanent magnets 5a and 5b. According to FIG. 4, in the case of the configuration of FIG. 4, as shown in the figure, the configuration in which the auxiliary permanent magnets 5a and 5b are arranged in the direction perpendicular to the air gap facing surfaces has the best effect on the assemblability and the characteristics of the permanent magnets made of different materials. It was confirmed that it can be utilized effectively and that a configuration with excellent magnetic efficiency can be provided. Further, the adjoining portions of the auxiliary permanent magnets 5a and 5b and the main permanent magnets 4a and 4b are substantially in contact with each other at their facing surfaces. Auxiliary permanent magnets 5a, 5
It is desirable that the ends of b and the magnetic pole pieces 6a and 6b are also substantially in contact with each other as shown in the drawing.

Even in the structure in which the rare earth-based permanent magnet and the ferrite-based permanent magnet are used together, the shape of the supporting yoke 3 connected to the plate-like yokes 1a and 1b is the auxiliary permanent magnets 5a and 5b made of ferrite-based permanent magnets. The shape determines the preferred shape. That is, when the magnetization directions of the auxiliary permanent magnets 5a and 5b are perpendicular to the air gap facing surfaces of the auxiliary permanent magnets 5a and 5b as described above, the auxiliary permanent magnets 5a and 5b are formed.
a and 5b facing the air gap and the center point O of the spherical space 2a
(See FIG. 1), the optimum angle θ ₁ with the horizontal plane, and the optimum angle between the contact surfaces of the auxiliary permanent magnets 5 a and 5 b with the supporting yoke 3 and the horizontal plane including the center point O of the spherical space 2 a. θ ₂ is determined, and as a result, the shape of the support yoke 3 on the side facing the air gap is determined. In the illustrated case, unlike the shape shown in FIG. 1, the shape is substantially flat (rectangular in cross section). In addition, according to the experiment by the present inventor, normally, θ ₁ is 30
It is desirable to select in the range of 60 to 60 degrees and θ ₂ in the range of 55 to 100 degrees. In this configuration, since inexpensive ferrite-based permanent magnets are used, compared to a configuration using only rare earth-based permanent magnets, the device is not made much larger,
It is possible to provide an inexpensive magnetic field generator for MRI.

In each of FIGS. 1 and 4 described above, the main permanent magnet and the plate yoke have a planar shape in which a semicircular portion and a rectangular portion are integrated. A rectangular configuration or the like can also be adopted. However, it has been confirmed that the configurations shown in FIGS. 1 and 4 are preferable in consideration of the uniformity of the magnetic field and the magnetic efficiency of the magnetic circuit. In the magnetic field generator for MRI shown in FIGS. 1 and 4, the yoke has a configuration in which only one end of the pair of plate yokes 1a and 1b is connected and supported by the support yoke 3, and The central part of the void 2 arranged is open on all three sides except the support yoke 3 side, increasing the freedom of the insertion direction of the subject in the void and reducing the feeling of pressure on the subject at the time of diagnosis. Have the effect.

Further, by arranging auxiliary permanent magnets 5a, 5b having a predetermined shape on the surface of the supporting yoke 3 facing the air gap, the main permanent magnets 4a, 4b and the magnetic pole pieces 6a, 6b are connected to the supporting yoke 3. It has become possible to reduce the leakage of magnetic flux and to improve both the magnetic field strength and the magnetic field homogeneity in the air gap 2. From the above, the use efficiency of the magnets can be improved, and the distance between the main permanent magnets 4a, 4b and the supporting yoke 3 is not unnecessarily increased. Therefore, the pair of plate yokes 1a, It is also possible to shorten the length of each of 1b, so that it is possible to realize the reduction in size and weight of the magnetic field generator for MRI as a whole.

[0026]

【Example】

Comparative Example 1 A conventional M shown in FIG. 7 using a rare earth / iron / boron permanent magnet with (BH) max = 40 MGOe as a permanent magnet.
A magnetic field generator for RI was created. And the distance L between the magnetic poles
g is 500 mm, radius r of the center of the void is 200 mm
The total magnet weight and the length L of the magnetic field generator for MRI were measured when the magnetic field strength at the center of the spherical space was 0.3 T and the magnetic field uniformity was 50 ppm.

Example 1 A magnetic field generator for MRI according to the present invention shown in FIG. 1 was prepared using a rare earth / iron / boron permanent magnet of the same material as in Comparative Example 1 as a main permanent magnet and an auxiliary permanent magnet. Then, the total magnet weight and the length L of the magnetic field generator for MRI when the magnetic pole distance Lg, the magnetic field strength and the magnetic field homogeneity in the spherical space at the center of the air gap are set to the same conditions as in Comparative Example 1, are measured, respectively. The ratio of the total magnet weight W of Comparative Example 1 to the length L of the device is shown in Table 1. However, the angle θ ₁ between the air gap facing surface of the auxiliary permanent magnet and the horizontal plane including the center point O of the spherical space is 45 degrees, and the contact surface of the auxiliary permanent magnet with the supporting yoke and the center point of the spherical space. The angle θ ₂ with the horizontal plane containing O was 55 degrees.

EXAMPLE 2 A rare earth / iron / boron permanent magnet having (BH) max = 40 MGOe is used as a main permanent magnet, and a ferrite permanent magnet having (BH) max = 4 MGOe is used as an auxiliary permanent magnet. Which is an embodiment of the present invention shown in FIG.
A magnetic field generator for use was created. Then, the distance Lg between the magnetic poles,
The total magnet weight and M when the magnetic field strength and the magnetic field homogeneity in the spherical space at the center of the gap are set under the same conditions as in Comparative Example 1.
The length L of the RI magnetic field generator was measured, and the ratio of the total magnet weight W of Comparative Example 1 to the length L of the device is shown in Table 1.
It was shown to. However, the angle θ ₁ between the air gap facing surface of the auxiliary permanent magnet and the horizontal plane including the center point O of the spherical space is 45 degrees, and the contact surface of the auxiliary permanent magnet with the supporting yoke and the center point of the spherical space. The angle θ ₂ with the horizontal plane containing O was 92 degrees.

[0029]

[Table 1]

Comparative Example 2 A conventional magnetic field generator for MRI shown in FIG. 7 using a ferrite permanent magnet having (BH) max = 4MGOe as a permanent magnet was prepared. Then, the distance Lg between the magnetic poles is set to 500
mm, the magnetic field strength of the central portion is 0.1T, and the magnetic field homogeneity is 5 in a spherical space having a radius r of 200 mm at the central portion of the void.
The total magnet weight and the length L of the magnetic field generator for MRI at 0 ppm were measured.

Example 3 A magnetic field generator for MRI according to the present invention shown in FIG. 1 was prepared using a ferrite permanent magnet of the same material as in Comparative Example 2 as the main permanent magnet and the auxiliary permanent magnet. Then, the total magnet weight and the length L of the magnetic field generator for MRI when the magnetic pole distance Lg, the magnetic field strength and the magnetic field homogeneity in the spherical space at the center of the air gap are set under the same conditions as in Comparative Example 2, are measured. The ratio of the total magnet weight W of Comparative Example 2 to the length L of the apparatus is shown in Table 1. However, the angle θ between the air bearing surface of the auxiliary permanent magnet and the horizontal plane including the center point O of the spherical space.
₁ was 45 degrees, and the angle θ ₂ between the contact surface of the auxiliary permanent magnet with the supporting yoke and the horizontal plane including the center point O of the spherical space was 56 degrees.

[0032]

[Table 2]

Comparative Example 1, Example 1 and Comparative Example 2 described above
From the comparison between Example 3 and Example 3, in the configuration according to the present invention, the central portion of the void in which the subject is arranged is open on three sides except the supporting yoke side, and the degree of freedom in the direction of insertion of the subject in the void is high. However, even if the material of the permanent magnet used is the same, the magnet efficiency can be greatly improved, and the magnetic field generator for MRI can be made smaller and lighter. I understand. Further, according to the present invention from Comparative Example 1 and Example 2, by effectively disposing the rare earth-based permanent magnets and the ferrite-based permanent magnets, the usage amount of the rare earth-based magnets can be reduced and the magnetic field generator for MRI can be remarkably used. It can be seen that it is possible to provide a highly economical device without causing an increase in size.

[0034]

As is apparent from the above-mentioned embodiments, the magnetic field generator for MRI of the present invention is inserted into the void of the subject by the construction of the yoke and the arrangement of the permanent magnets and pole pieces. It not only has the effect of reducing the sense of pressure on the subject at the time of diagnosis while also increasing the degree of freedom in direction, but it can also greatly improve the magnet efficiency, and realizes a compact and lightweight magnetic field generator for MRI. can do.

[Brief description of drawings]

1A and 1B show an embodiment of a magnetic field generator for MRI according to the present invention, in which FIG. 1A is a longitudinal sectional explanatory view, and FIG. 1B is an AA sectional explanatory view in FIG. 1A.

FIG. 2 is a perspective explanatory view showing an embodiment of an auxiliary permanent magnet of the magnetic field generator for MRI of the present invention.

FIG. 3 is a perspective explanatory view showing an embodiment of magnetic pole pieces of the magnetic field generator for MRI of the present invention.

FIG. 4 is a longitudinal cross-sectional explanatory view showing another embodiment of the magnetic field generator for MRI of the present invention.

5A and 5B show an example of a conventional magnetic field generator for MRI, in which FIG. 5A is a vertical cross-sectional explanatory view, and FIG.
It is an AA cross-section explanatory drawing of.

FIG. 6 is a perspective explanatory view showing another embodiment of the conventional magnetic field generator for MRI.

FIG. 7 is a perspective explanatory view showing another embodiment of the conventional magnetic field generator for MRI.

[Explanation of symbols]

 1a, 1b Plate yoke 2 Gap 2a Spherical space 2b, 2c Gap 3 Support yoke 4a, 4b Main permanent magnet 5, 5a, 5b Auxiliary permanent magnet 6a, 6b Magnetic pole piece 7a, 7b Projection part 8 Projection part 9 Contact Part 10 Block 11a, 11b Plate-like yoke 12, 22, 32 Gap 13a, 13b, 13c, 13d Cylindrical yoke 14a, 14b Permanent magnet 16a, 16b Magnetic pole piece 21 Hexagonal tubular yoke 24a, 24b Main permanent magnet 25a, 25b, 25c, 25d Auxiliary permanent magnets 26a, 26b Magnetic pole pieces 31a, 31b Plate yoke 33 Support yoke 34a, 34b Permanent magnets 36a, 36b Magnetic pole pieces

Claims (4)

[Claims] 1. A pair of plate-like yokes having a pair of plate-like yokes that face each other and form a predetermined gap, and a supporting yoke that connects and supports one end of the pair of plate-like yokes. A main permanent magnet is arranged on each of the gap facing surfaces of the yoke, and a pair of auxiliary permanent magnets is arranged adjacent to the main permanent magnet on the gap facing surface of the supporting yoke, and the gap of the pair of main permanent magnets is arranged. A magnetic field generator for MRI, wherein magnetic pole pieces are arranged on opposite surfaces. 2. The magnetic field generator for MRI according to claim 1, wherein a protrusion is formed on a peripheral edge portion of each of the pair of magnetic pole pieces other than the peripheral edge portion on the auxiliary permanent magnet arrangement side on the air gap facing surface. 3. The main permanent magnet has a substantially rectangular cross section, the auxiliary permanent magnet has a substantially triangular cross section, and the main permanent magnet and the auxiliary permanent magnet are rare earth permanent magnets or ferrite permanent magnets. Claim 1 characterized by
The magnetic field generator for MRI described. 4. The magnetic field generation for MRI according to claim 1, wherein the main permanent magnet is a rare earth-based permanent magnet having a substantially trapezoidal cross section, and the auxiliary permanent magnet is a ferrite-based permanent magnet having a substantially triangular cross section. apparatus.