JPH07194572A - Magnetic field generating device for magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To generate a uniform magnetostatic field in a measuring space by arranging permanent magnets formed by combining first permanent magnets arranged at intervals from the measuring space and second permanent magnets fixed by projecting to the measuring space side in a peripheral part on the side facing the measuring space of these permanent magnets with each other, around a specimen. CONSTITUTION:In this magnetic field generating device in which upper and lower yokes 1a and 1b are provided, and permanent magnets 12a and 12b are arranged around a specimen to be put in the cavity 4, and a uniform magnetostatic field is generated in a measuring space 7 of a central part in the cavity 4, the permanent magnets 12a and 12b are constituted by combining the first permanent magnets 13a and 13b which are arranged separately by a specific distance from the measuring space 7 and are low in the maximum energy product and the second permanent magnets 14a and 14b which are fixed by projecting to the measuring space 7 side in a peripheral part on the side facing the measuring space 7 of these permanent magnets and are high in the maximum energy product with each other. Thereby, uniformity of a magnetic field in a peripheral part 7' of the measuring space 7 is heightened, and uniformity of a magnetic field in the measuring space 7 is improved as a whole.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to nuclear magnetic resonance (NMR).
The present invention relates to a static magnetic field generator using a permanent magnet used in a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”) that obtains a tomographic image of an examination region of a subject by utilizing a phenomenon.
In particular, the present invention relates to a magnetic field generator of an MRI apparatus capable of improving the magnetic field homogeneity of the measurement space at the center of the void into which the subject is inserted and reducing the amount of permanent magnet material used.

[0002]

2. Description of the Related Art An MRI apparatus uses a NMR phenomenon to perform arithmetic processing on a signal measured at a desired examination site of an object to obtain a tomographic image of the density distribution and relaxation time distribution of nuclear spins at the examination site. Is displayed as an image.
Then, in order to generate the above-mentioned NMR phenomenon, a static magnetic field having a spatially and temporally uniform strength and direction is required. For example, when measuring a spatially wide range such as the human body, 0.05 to 2T (Tesla; 1 Tesla is 10,000 in a measurement space consisting of a spherical space with a diameter of 30 to 50 cm).
A magnetic field generator that can generate a static magnetic field of about Gauss) with a uniformity of several tens of ppm or less is required. As such a magnetic field generator, three types of conventional methods have been used: a normal conducting magnet, a superconducting magnet, and a permanent magnet.

A conventional magnetic field generator for an MRI apparatus using a permanent magnet is roughly classified into, for example, as disclosed in JP-A-60-88407 and JP-A-2-83903. An opposed type magnetic field generator in which permanent magnets arranged to face each other with the measurement space of the subject interposed therebetween generate a static magnetic field in the measurement space, and a plurality of permanent magnets annularly arranged around the measurement space of the subject. There is an annular magnetic field generator in which a magnet block generates a static magnetic field in the measurement space. These will be described with reference to FIGS. 8 and 9.

First, FIG. 8 is a partially sectional front view showing a conventional opposed type magnetic field generator disclosed in Japanese Patent Laid-Open No. 60-88407. In FIG. 8, a pair of flat plate-shaped yokes 1a,
1b is a permanent magnet 2a, 2b and columns 3, 3, which will be described later.
And a magnetic circuit, which is made of a soft magnetic material. The yokes 1a, 1b are supported at their four corners by four columns 3, 3, ... And held facing each other with a predetermined distance. The pair of permanent magnets 2 are provided on the surfaces of the inner wall surfaces of the yokes 1a and 1b which face each other vertically.
a and 2b are arranged so as to face each other with forming a space 4 between them for allowing a subject to enter. These permanent magnets 2a, 2b
Is for generating a main magnetic flux in the vertical direction with respect to the inner wall surface as shown in the figure, and is formed in a disc shape, for example.
The polarities of the surfaces facing each other are different. Further, magnetic pole pieces 5a and 5b for improving the magnetic field homogeneity in the air gap 4 are attached to the opposing surfaces of the permanent magnets 2a and 2b. Furthermore, the opposing pole pieces 5a, 5
At the peripheral edge of b, the upper and lower annular projections 6a having the same shape,
6b is formed so as to suppress the leakage of magnetic flux to the outside of the periphery and generate a uniform static magnetic field in the measurement space 7 at the center of the void 4.

Next, FIG. 9 is a perspective view showing a conventional annular magnetic field generator described in Japanese Patent Laid-Open No. 2-83903.
In FIG. 9, an annular permanent magnet group 8 arranged in the central portion
And a group of similarly annular permanent magnets 9 and 10 arranged on both sides thereof to form a tubular body having a polygonal cross section as a whole. The outer peripheral portions of these permanent magnet groups 8, 9 and 10 are There is no yoke to cover. Then, the thickness of the permanent magnet groups 9 and 10 arranged on both sides is made larger than the thickness of the central permanent magnet group 8 so that the subject can enter the center of the tubular body. This is intended to improve the magnetic field homogeneity of the measurement space in the created void 4. In this example, the block magnets constituting each of the permanent magnet groups 8, 9, and 10 are made of the same material, and their magnetization directions are as shown by the arrows in FIG. Each is oriented in a direction perpendicular to the central axis 11.

In any of the above-mentioned conventional examples, many permanent magnets are arranged around the gap 4 in order to generate a uniform static magnetic field in the measurement space in the gap 4 into which the subject is inserted. With so many permanent magnets, MRI
Since the entire apparatus is large and heavy, and the permanent magnet material is very expensive, the cost is high. Therefore, it is necessary to reduce the amount of use as much as possible. Then, in order to reduce the usage amount of the magnet, the magnetic field generation efficiency as the magnetic field generation device may be improved.

[0007]

However, in the conventional opposed type magnetic field generator shown in FIG. 8, the upper and lower permanent magnets 2 are provided.
a, 2b are magnetically connected to the yokes 1a, 1b and the four columns 3, 3, ... To form a magnetic circuit, but a magnetic flux is generated from between the opposing pole pieces 5a, 5b. It may leak out, as shown by the chain line curve C ₁ in FIG.
The magnetic field uniformity in the peripheral portion of the measurement space 7 was reduced. In order to compensate for this leakage of magnetic flux and improve the magnetic field uniformity of the measurement space 7, the permanent magnets 2a, 2
It is necessary to increase the size of b, which lowers the magnetic field generation efficiency, and uses a large number of permanent magnet materials, resulting in a large size, a large weight, and a high cost.

Further, in the conventional annular magnetic field generator shown in FIG. 9, magnetic flux may leak from the openings at both ends of the tubular body. Similarly, as shown by a dashed line curve C ₁ in FIG. The magnetic field homogeneity in the peripheral portion of the measurement space inside the void 4 was reduced. In order to compensate for this leakage of magnetic flux and improve the magnetic field homogeneity in the measurement space,
It is necessary to make the thickness of the permanent magnet groups 9 and 10 at both ends thicker than the thickness of the permanent magnet group 8 at the central portion, or to make the entire length in the depth direction longer, which also lowers the magnetic field generation efficiency. However, a large number of permanent magnet materials were used, resulting in a large size, a large weight, and a high cost.

Therefore, the present invention can cope with such a problem and improve the magnetic field homogeneity of the measurement space in the central portion of the void into which the subject is inserted and reduce the amount of the permanent magnet material used. An object of the present invention is to provide a magnetic field generator for an MRI apparatus that can be used.

[0010]

In order to achieve the above object, a magnetic field generator of an MRI apparatus according to the present invention forms a void into which a subject can enter and a permanent magnet is provided around the subject. In the magnetic field generator of the magnetic resonance imaging apparatus, which is arranged to generate a uniform static magnetic field in the measurement space at the center of the air gap, the permanent magnet is arranged at a constant distance from the measurement space and has a relative maximum. A first permanent magnet having a low energy product, and a second permanent magnet having a relatively high maximum energy product, which is fixed and protrudes toward the outer measurement space at the peripheral portion of the first permanent magnet facing the measurement space. It is configured by combining with a permanent magnet.

Further, at a position near the center of the measurement space in the peripheral portion of the first permanent magnet facing the measurement space, the second permanent magnet projects and is fixed to the measurement space. In addition, a third permanent magnet having magnetic characteristics equivalent to that of the first permanent magnet may be fixed to the outer end of the peripheral portion of the first permanent magnet facing the measurement space. Good.

Further, the first permanent magnet and the third permanent magnet use ferrite magnets having a low maximum energy product, and the second permanent magnets use ferrite magnets or rare earth magnets having a high maximum energy product. Is.

Furthermore, it is effective to dispose a fourth permanent magnet having a large coercive force between the second permanent magnet and the third permanent magnet.

Further, one type of the magnetic field generator of the present invention is configured such that the permanent magnets arranged around the subject are opposed to each other with the measurement space of the subject interposed therebetween.

Another type of magnetic field generator according to the present invention is a cylinder having a polygonal cross section, wherein a permanent magnet is arranged around the subject, and a plurality of permanent magnet blocks are provided around the measurement space of the subject. It is arranged so as to form a shape.

[0016]

The magnetic field generator of the MRI apparatus configured as described above is arranged at a predetermined distance from the measurement space at the center of the cavity into which the subject is inserted and has a relatively low maximum energy product. A combination of a magnet and a second permanent magnet having a relatively high maximum energy product, which is fixed and protrudes toward the measurement space at a peripheral portion of the first permanent magnet facing the measurement space, is formed. A permanent magnet is arranged around the subject and operates so as to generate a uniform static magnetic field in the measurement space. As a result, the magnetic field homogeneity of the measurement space can be improved and the amount of the permanent magnet material used can be reduced.

[0017]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 1 is a half longitudinal sectional view showing an embodiment of a magnetic field generator for an MRI apparatus according to the present invention, and the overall shape of the apparatus is the same as that of the conventional opposed magnetic field generator shown in FIG. . This magnetic field generator uses a permanent magnet to generate a uniform static magnetic field in a space into which a subject is inserted. As shown in FIG. 1, upper and lower yokes 1a and 1b are provided.
, And upper and lower permanent magnets 12a and 12b, and four columns 3, 3, ...

The yokes 1a and 1b are permanent magnets 1 described later.
The members 2a, 12b and the columns 3, 3, ... Form a magnetic circuit, and are formed of a soft magnetic material such as an iron plate or a silicon steel plate into a rectangular flat plate shape, and are provided in a pair in the upper and lower sides. The yokes 1a, 1b are supported at their four corners by four columns 3, 3, ... And held facing each other with a predetermined distance. Then, on the inner wall surfaces of the yokes 1a, 1b, which face vertically, a pair of permanent magnets 12a, 1b is formed.
2b are arranged so as to face each other, forming a space 4 between the two, into which a subject can enter. These permanent magnets 12a, 12b
Is for generating a main magnetic flux in the vertical direction with respect to the inner wall surface as shown in the figure, and is formed in a disc shape, for example.
The polarities of the surfaces facing each other are different. With the above configuration, for example, a uniform static magnetic field from the bottom to the top is generated in the measurement space 7 at the center of the void 4.

Here, in the present invention, the permanent magnets 12a and 12b are, as shown in FIG. 1, arranged apart from the measurement space 7 by a certain distance and having a relatively low maximum energy product. 13a and 13b, and the measurement space 7 at a position near the center of the measurement space 7 in the peripheral portion of the first permanent magnets 13a and 13b facing the measurement space 7.
The second permanent magnets 14a and 14b that are protruded and fixed to the side and have a relatively high maximum energy product, and the first permanent magnet 1 described above.
Adjacent to the second permanent magnets 14a, 14b at the outer ends of the peripheral portions of 3a, 13b facing the measurement space 7, the first permanent magnets 13a, 13b have a relative maximum distance. It is configured by combining with third permanent magnets 15a and 15b having a low energy product. As the respective magnet materials, the first permanent magnets 13a and 13b and the third permanent magnets 15a and 15b are ferrite magnets having a low maximum energy product, and the second permanent magnets 14a and 14b.
Is a rare earth magnet such as a ferrite magnet having a high maximum energy product or a neodymium magnet or a rare earth cobalt magnet. The magnetization directions of the second permanent magnets 14a and 14b and the third permanent magnets 15a and 15b are the same as the magnetization directions of the first permanent magnets 13a and 13b which are the main magnets.

Next, as described above, the first permanent magnet 13
The reason why the second permanent magnets 14a and 14b and the third permanent magnets 15a and 15b are combined with a and 13b will be described. In the opposed magnetic field generator shown in FIG. 1, as described in the description of the conventional example shown in FIG. 8, structurally, the magnetic field strength of the peripheral portion 7 ′ in the radius R direction of the measurement space 7 is equal to that of the measurement space 7. It was smaller than the inner side, and the magnetic field homogeneity of the measurement space 7 sometimes deteriorated. Therefore, in the present invention, the second permanent magnets 14a, 1 having a relatively high maximum energy product are provided at a position near the peripheral portion 7'of the measurement space 7.
4b is arranged so that the magnetic field strength of the peripheral portion 7'is made larger than the other positions in advance.

Therefore, in order to investigate the position where the magnetic field homogeneity in the measurement space 7 can be effectively improved by the second permanent magnets 14a, 14b having a relatively high maximum energy product as described above, The second permanent magnet 14a,
The result of the simulation of the change in the magnetic field homogeneity in the measurement space 7 when the position of 14b is changed will be described with reference to FIGS. 2 and 3. FIG. 2 is a quarter of the permanent magnet 12a showing a simulation state in a simple manner.
FIG. In the drawing, the measurement points P _{1 to} P ₆ are located on the outer circumference of the measurement space 7 and serve as a guide for determining the magnetic field homogeneity in the air gap 4. Further, the second permanent magnet 14a having a relatively high maximum energy product has the same magnetization direction as the first permanent magnet 13a and the third permanent magnet 15a, and is closer to the center side of the measurement space 7 from the side A , B, C,
It was placed at the position D and the amount of change in the magnetic field strength at each of the measurement points P _{1 to} P ₆ was examined.

FIG. 3 is a graph showing the state of changes in the magnetic field strength with respect to the positions of the respective measurement points P _{1 to} P ₆ , where the horizontal axis represents the positions of the respective measurement points P _{1 to} P ₆ and the vertical axis represents the above respective points. Measurement points P _{1 to} P ₆
Shows the amount of change in the relative magnetic field strength at. A change in magnetic field strength when the second permanent magnet 14a is arranged at the position A in FIG. 2 is represented by a solid line 16a in FIG. 3, and a change in magnetic field strength when the second permanent magnet 14a is arranged at the position B is indicated by a broken line 16a.
The change in the magnetic field strength when placed at position C is represented by a dashed line 16c, and the change in the magnetic field strength when placed at position D is represented by a two-dot chain line 16d. As can be seen from this graph, the second permanent magnet 14 generally shown in FIG.
The peak-shaped measurement points near the positions A, B, C, and D where a is arranged indicate peaks, and changes in the magnetic field strength of the mountain shape are shown. However, the measurement points P ₅ and P ₆ corresponding to the peripheral portion 7 ′ of the measurement space 7 shown in FIG.
It can be seen that the amount of change in the magnetic field strength is smaller than that at the other measurement points P _{1 to} P ₄ .

[0023] Therefore, in order to obtain a better magnetic field uniformity in the measurement space 7, it is necessary to vary the magnetic field strength of only around the measuring point P ₅ and P _6, of the measurement points P _5, P ₆ It is necessary to locally dispose the second permanent magnet 14a at a position that greatly affects the magnetic field strength, that is, near the position D shown in FIG. As a result, as shown in FIG. 1, at a position closer to the center side of the measurement space 7 than the third permanent magnets 15a and 15b, the second permanent magnet 14a is adjacent to the third permanent magnets 15a and 15b. , 14b are arranged. Then, in FIG. 1, by disposing a shimming magnet (not shown) having a relatively low maximum energy product inside the second permanent magnets 14a, 14b having a high maximum energy product, the periphery of the measurement space 7 is arranged. The magnetic field strength at positions other than the portion 7'can be partially adjusted, and the magnetic field homogeneity of the entire measurement space 7 can be improved.

Explaining this with reference to FIG. 10, the magnetic field homogeneity of the measurement space 7 in the conventional opposed magnetic field generator shown in FIG. 8 is lowered in the peripheral portion as shown by a dashed line curve C _1. However, by adding the second permanent magnets 14a and 14b to the first permanent magnets 13a and 13b in the present invention shown in FIG. 1, a two-dot chain curve C ₂ (shown in FIG. 3) in FIG. The change in the magnetic field strength indicated by the two-dot chain line 16d) is newly added. Therefore, in the magnetic field generator of the present invention, the magnetic field strength changes as shown by the solid curve C _{3 in} which the curves C ₁ and C ₂ are added in FIG. 10, and the peripheral portion of the measurement space 7 shown in FIG. The magnetic field uniformity in 7'can be improved. In this case, the second permanent magnets 14a and 14b having a relatively high maximum energy product are replaced by the first permanent magnets 13a and 13b.
By arranging the first permanent magnets 13a and 13b in the peripheral portion of b, the size of the first permanent magnets 13a and 13b in the radius R direction, which basically affects the magnetic field uniformity of the peripheral portion 7'of the measurement space 7, can be reduced. . Therefore, the amount of permanent magnet material used can be reduced, and the size and weight can be reduced.

In FIG. 1, the first permanent magnet 1
The second permanent magnet 14 having a high maximum energy product is adjacent to the insides of the third permanent magnets 15a and 15b provided at the outer ends of the peripheral portions of the sides 3a and 13b facing the measurement space 7.
Although the example in which a and 14b are provided is shown, the present invention is not limited to this, and the second permanent magnets 14a and 14b are separated from the third permanent magnets 15a and 15b in the measurement space 7 It may be arranged at a position where the magnetic field homogeneity can be most improved. Further, the thickness of the second permanent magnets 14a, 14b does not have to be the same as that of the third permanent magnets 15a, 15b, and even if they have different thicknesses, the magnetic field uniformity in the measurement space 7 still has the highest value. The thickness may be improved. further,
In FIG. 1, the second permanent magnets 14a, 14a are provided on the peripheral portion of the first permanent magnets 13a, 13b on the side facing the measurement space 7.
An example in which 14b and the third permanent magnets 15a and 15b are combined and fixed is shown, but the present invention is not limited to this, and if it is necessary to improve the magnetic field homogeneity in the measurement space 7 most, Only the second permanent magnets 14a and 14b may be provided with an appropriate size.

FIG. 4 is a half longitudinal sectional view showing a second embodiment of the present invention. In this embodiment, the fourth permanent magnets 17a and 17b having a large coercive force are provided between the second permanent magnets 14a and 14b and the third permanent magnets 15a and 15b shown in FIG.
Is arranged. This is because the second permanent magnets 14a, 14 having a relatively high maximum energy product are adjacent to the third permanent magnets 15a, 15b having a relatively low maximum energy product.
When b is arranged, the magnetic flux density of the portions of the third permanent magnets 15a and 15b adjacent to the second permanent magnets 14a and 14b becomes small, and the third permanent magnets 15a and 15b have a coercive force (iHc). This is for preventing the portion adjacent to the second permanent magnets 14a and 14b from being irreversibly demagnetized in the case of a small magnet material.
That is, the second permanent magnet 14 having a high maximum energy product
Third permanent magnets 15a, 15 generated by a, 14b
The fourth permanent magnets 17a, 17 having a coercive force (iHc) that does not cause irreversible demagnetization due to the decrease in magnetic flux density in b
b is arranged between the two. This allows
It is possible to prevent the third permanent magnets 15a and 15b from being irreversibly demagnetized and maintain the magnetic field uniformity.

FIG. 5 is a front view showing a third embodiment of the present invention. This embodiment shows an example in which the present invention is applied to a polygonal tubular magnetic field generator similar to the conventional annular magnetic field generator shown in FIG. In FIG. 5, this embodiment is arranged so as to form a yoke 18 formed in a polygonal tubular shape and a void 4 fixed to each of a plurality of inner wall surfaces of the yoke 18 and into which a subject is inserted. Permanent magnet 19a
.About.19f. Then, the permanent magnet 1
9a to 19f are arranged so as to generate a uniform static magnetic field in the measurement space 7 at the center of the void 4. Here, the permanent magnets 19a located above and below the measurement space 7,
19b is configured similarly to the upper and lower permanent magnets 12a and 12b shown in FIG. 1, and as shown in the 1/4 vertical sectional view of FIG. 6, a first permanent magnet 20a having a relatively low maximum energy product,
Second permanent magnet 21a having a relatively high maximum energy product
And a third permanent magnet 22 having a relatively low maximum energy product
and a. The other permanent magnets 19a to 19f are made of a magnetic material having a relatively low maximum energy product. Further, as shown in FIG. 6, the magnetization directions of the second permanent magnet 21a and the third permanent magnet 22a are the same as the magnetization directions of the first permanent magnet 20a which is the main magnet. According to the present embodiment as described above, it is possible to improve the magnetic field homogeneity in a state where the length in the Y direction, which influences the magnetic field homogeneity in the Y axis direction of the polygonal cylindrical magnetic field generator, is shortened, and the magnet is used. The amount can be reduced and the size and weight can be reduced.

FIG. 7 shows a modification of the embodiment shown in FIG.
4 is a vertical cross-sectional view of FIG. 4, in which a fourth permanent magnet 23a having a large coercive force is arranged between the second permanent magnet 21a and the third permanent magnet 22a, as in the embodiment of FIG. is there.
Thereby, the third permanent magnet 22a can be prevented from being irreversibly demagnetized due to the influence of the second permanent magnet 21a having a relatively high maximum energy product, and the magnetic field uniformity can be maintained. it can.

Although not shown, the present invention is not shown in FIG.
~ Like the embodiment applied to the polygonal cylindrical magnetic field generator shown in Fig. 7, it can be applied to the conventional annular magnetic field generator shown in Fig. 9.

[0030]

Since the present invention is constructed as described above,
A first permanent magnet having a relatively low maximum energy product which is arranged at a fixed distance from the measurement space in the center of the void into which the subject is inserted, and the side of the first permanent magnet facing the measurement space. By arranging a permanent magnet, which is formed by combining a second permanent magnet having a relatively high maximum energy product, which is fixedly protruded to the measurement space side in the peripheral portion of the periphery of the subject, A uniform static magnetic field can be generated inside. In this case, by arranging the second permanent magnet having a relatively high maximum energy product in the peripheral portion of the first permanent magnet, it is possible to improve the magnetic field uniformity in the peripheral portion of the measurement space, and as a whole. The magnetic field homogeneity in the measurement space can be improved. Further, from the above, it is possible to reduce the radial size of the first permanent magnet that basically affects the magnetic field homogeneity in the peripheral portion of the measurement space, thereby reducing the amount of permanent magnet material used. As a result, the size and weight can be reduced. Further, it is possible to reduce the cost.

Further, in the case where the fourth permanent magnet having a large coercive force is arranged between the second permanent magnet and the third permanent magnet, the third permanent magnet having a relatively low maximum energy product. Can prevent irreversible demagnetization due to the influence of the second permanent magnet having a relatively high maximum energy product, and maintain the magnetic field homogeneity.

[Brief description of drawings]

FIG. 1 is a half longitudinal sectional view showing an embodiment of a magnetic field generator of an MRI apparatus according to the present invention.

FIG. 2 is a 1/4 vertical cross-sectional view of the entire permanent magnet, briefly showing a state in which a simulation is performed on a change in magnetic field homogeneity in a measurement space when the position of a second permanent magnet is changed.

FIG. 3 is a graph showing changes in magnetic field strength with respect to positions of measurement points in the simulation.

FIG. 4 is a half longitudinal sectional view showing a second embodiment of the present invention.

FIG. 5 is a front view showing a third embodiment of the present invention.

FIG. 6 is a 1/4 vertical cross-sectional view of the entire permanent magnet for explaining the third embodiment.

7 is a 1/4 vertical sectional view of the entire permanent magnet showing a modification of the embodiment shown in FIG.

FIG. 8 is a partially sectional front view showing a conventional opposed magnetic field generator.

FIG. 9 is a perspective view showing a conventional annular magnetic field generator.

FIG. 10 is a graph showing a state of change in magnetic field strength in the peripheral portion of the measurement space, comparing the conventional example with the present invention.

[Explanation of symbols]

 1a, 1b, 18 ... Yoke 3 ... Column 4 ... Void 7 ... Measuring space 7 '... Peripheral part 12a, 12b, 19a-19f ... Permanent magnet 13a, 13b, 20a ... First permanent magnet 14a, 14b , 21a ... Second permanent magnet 15a, 15b, 22a ... Third permanent magnet 17a, 17b, 23a ... Fourth permanent magnet

Claims (6)

[Claims] 1. Magnetic resonance imaging in which a void is formed in the center of the subject and a permanent magnet is arranged around the subject to generate a uniform static magnetic field in the measurement space in the center of the void. In the magnetic field generator of the device, the permanent magnet is a first permanent magnet which is arranged at a certain distance from the measurement space and has a relatively low maximum energy product, and a surface of the first permanent magnet in the measurement space. A magnetic field generating apparatus for a magnetic resonance imaging apparatus, characterized in that the magnetic field generating apparatus is configured by combining with a second permanent magnet having a relatively high maximum energy product, the second permanent magnet protruding and fixed to the outer measurement space side at the peripheral portion on the side of the above. 2. The position near the center of the measurement space in the peripheral portion of the first permanent magnet facing the measurement space,
The second permanent magnet is projected and fixed to the measurement space side, and at the peripheral end on the side facing the measurement space of the first permanent magnet, the outer end portion is equivalent to the first permanent magnet. The magnetic field generator for a magnetic resonance imaging apparatus according to claim 1, wherein a third permanent magnet having magnetic characteristics is fixed. 3. The first permanent magnet and the third permanent magnet are ferrite magnets having a low maximum energy product, and the second permanent magnets are ferrite magnets or rare earth magnets having a high maximum energy product. The magnetic field generator of the magnetic resonance imaging apparatus according to claim 1 or 2. 4. A magnetic resonance imaging apparatus according to claim 2, wherein a fourth permanent magnet having a large coercive force is arranged between the second permanent magnet and the third permanent magnet. Magnetic field generator. 5. The magnetic resonance imaging apparatus according to claim 1, wherein the permanent magnets arranged around the subject are arranged to face each other with a measurement space of the subject interposed therebetween. Magnetic field generator. 6. The permanent magnet arranged around the subject is constituted by arranging a plurality of permanent magnet blocks around a measurement space of the subject so as to form a cylindrical body having a polygonal cross section. The magnetic field generator of the magnetic resonance imaging apparatus according to any one of claims 1 to 4.