JP4182712B2 - Magnetic field generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic field generator, and more particularly to a permanent magnet type magnetic field generator used for a small MRI apparatus for inspecting a part of a body such as a finger, a small animal, and the contents of food.
[0002]
[Prior art]
Recently, a magnetic field generator that generates a high magnetic field of 0.8 T or more is being used as a permanent magnet type magnetic field generator used in an MRI apparatus or the like. However, since permanent magnets always generate a magnetic field, the higher the central magnetic field strength of the magnetic field generator is, the more concerned there is an accident caused by a leakage magnetic field.
[0003]
The present applicant has proposed an example of this type of magnetic field generator (see Patent Document 1).
In this prior art, a magnet having a magnetization direction parallel or perpendicular to its main surface is used as a magnet from the viewpoint of improving productivity. However, in this case, a leakage magnetic field is generated due to a sudden change in the magnetization direction, such as a change in the magnetization direction by 90 degrees in a part of the magnetic field generation device. For example, like a Halbach type magnetic field generation device However, when the magnetic field generator is used for an MRI apparatus as a medical device or a measuring device, there is a problem in use and work.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-70280
[Problems to be solved by the invention]
When such a magnetic field generator is used for an MRI apparatus, it is required that the leakage magnetic field within 1 m from the center of the apparatus main body (magnetic circuit) is 0.5 mT.
The simplest method for that purpose is to cover the apparatus main body itself with a shield member having a high magnetic permeability such as iron or permalloy.
However, the thickness of the shield member and the distance between the shield member and the apparatus main body are important in order to obtain a good central magnetic field strength and to sufficiently suppress the leakage magnetic field. If the thickness of the shield member is increased, the shielding effect is increased, but the weight of the entire magnetic field generator is increased accordingly, and the installation location is further limited due to the strength of the floor.
[0006]
Therefore, a main object of the present invention is to provide a magnetic field generator capable of suppressing the leakage magnetic field and improving the central magnetic field strength while suppressing the weight of the device itself.
[0007]
[Means for Solving the Problems]
[0009]
In order to achieve the above object, the magnetic field generator according to claim 1 uses a plurality of permanent magnets so that a magnetic field generation space having a magnetic field strength of 0.8 T or more and 2.0 T or less is provided on the inner peripheral side thereof. Device body configured in a ring shape, a shield member having a saturation magnetization of 1.5 T or more provided on the outer periphery of the device body away from the surface of the permanent magnet , and a saturation magnetization provided between the permanent magnet and the shield member of 0.2 T It comprises the following interposed member.
[0010]
The magnetic field generator according to claim 2 is the magnetic field generator according to claim 1, characterized in that the permanent magnet is magnetized parallel or perpendicular to the main surface.
The magnetic field generator according to claim 3 is the magnetic field generator according to claim 1 or 2 , wherein the permanent magnet is an R-Fe-B magnet.
[0014]
If a member having a saturation magnetization of 0.2 T or less is provided between the magnet of the apparatus main body and the shield member as in the magnetic field generator according to claim 1 , the magnetic flux is short-circuited by the apparatus main body and the shield member. And the leakage magnetic field can be suppressed.
By using a magnetic member having a saturation magnetization of 1.5 T or more as the shield member, the leakage magnetic field can be more effectively suppressed. Further, when such a magnetic member is used, the effect of substantially increasing the magnetic path is increased, so that magnetic saturation can be suppressed and the central magnetic field strength can be improved more effectively.
When a magnetic field of 0.8 T or more and 2.0 T or less is generated in the magnetic field generation space of the apparatus main body, the leakage magnetic field increases accordingly, but the magnetic field generation apparatus according to claim 1 generates such a strong magnetic field. Even in this case, the leakage magnetic field can be effectively suppressed.
[0015]
If a magnet magnetized parallel or perpendicular to the main surface is used as each magnet used in the apparatus body, a part in which the direction of the magnetic flux changes abruptly occurs in the apparatus body (magnetic circuit). appear. However, in the magnetic field generator according to claim 2 , such a leakage magnetic field can be effectively suppressed by providing a predetermined shield member at a predetermined position.
[0016]
When the magnet used is an R—Fe—B magnet, a strong magnetic field is generated and the leakage magnetic field increases accordingly. However, the magnetic field generator according to claim 3 is such a case. However, the leakage magnetic field can be effectively suppressed.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
1 to 3, a magnetic field generator 10 according to an embodiment of the present invention includes a device main body 12. The apparatus main body 12 is formed in a substantially rectangular parallelepiped shape using a plurality of permanent magnets having a rectangular parallelepiped shape or a substantially rectangular parallelepiped shape, and has an outer diameter of, for example, 470 mm × 440 mm × 423 mm. Each permanent magnet used for the apparatus main body 12 is magnetized substantially parallel or substantially perpendicular to its planar main surface (of the permanent magnet). For example, an anisotropic R-Fe-B sintered magnet ( R is a rare earth element including Y).
[0018]
The apparatus main body 12 includes a pair of rectangular parallelepiped permanent magnets 14a and 14b.
Around the permanent magnet 14a (side surfaces), rectangular parallelepiped permanent magnets 16a and 18a and substantially rectangular parallelepiped permanent magnets 20a and 22a are provided. Combined with At this time, the permanent magnets 16a and 18a are disposed opposite to each other with the permanent magnet 14a interposed therebetween, and the permanent magnets 20a and 22a are disposed to face each other with the permanent magnets 14a, 16a, and 18a interposed therebetween.
[0019]
Similarly, a rectangular parallelepiped permanent magnets 16b and 18b and substantially rectangular parallelepiped permanent magnets 20b and 22b are provided around (side surfaces) of the permanent magnet 14b. The permanent magnet 14b includes the permanent magnets 16b, 18b, 20b and 22b. Touching and magnetically coupled. At this time, the permanent magnets 16b and 18b are disposed opposite to each other with the permanent magnet 14b interposed therebetween, and the permanent magnets 20b and 22b are disposed to face each other with the permanent magnets 14b, 16b, and 18b interposed therebetween. As shown in FIG. 3B, the permanent magnet 18b is provided on the rear side surface of the ferromagnetic body 28b and below the permanent magnet 18a so as to form a pair with the permanent magnet 16b.
[0020]
A permanent magnet 24 is provided between the permanent magnets 20a and 20b, and a permanent magnet 26 is provided between the permanent magnets 22a and 22b. As a result, a gap is formed between the permanent magnets 14a and 14b.
[0021]
Thus, magnetic circuits A1 and A2 as shown in FIG. 3A are formed by arranging a plurality of permanent magnets in an annular shape. Fig.3 (a) is an illustration figure which shows the BB cross section (vertical cross section which passes along the upper surface of the permanent magnet 14a) of FIG.
The permanent magnets 14a and 14b have the same dimensions, the permanent magnets 16a, 16b, 18a and 18b have the same dimensions, the permanent magnets 20a, 20b, 22a and 22b have the same dimensions, and the permanent magnets 24 and 26 have the same dimensions. Are the same dimensions.
[0022]
Further, ferromagnetic bodies 28a and 28b are provided on the lower surface of the permanent magnet 14a and the upper surface of the permanent magnet 14b, respectively.
In this embodiment, “ferromagnetic material” refers to a member having a saturation magnetization of 1.0 T or more.
As the ferromagnetic bodies 28a and 28b, for example, electromagnetic soft iron, JIS: S15C or permendur or the like is used. The ferromagnetic bodies 28a and 28b have the same width and depth as the permanent magnets 14a and 14b in this embodiment.
The lower surface of the ferromagnetic body 28a is formed to be flush with the lower surfaces of the permanent magnets 16a, 18a, 20a, and 22a. Accordingly, the ferromagnetic body 28a is disposed in the vicinity of the magnetic field generation space 32 (described later) and embedded in the permanent magnet at a location where the magnetic flux passes. Similarly, the upper surface of the ferromagnetic body 28b is formed to be flush with the upper surfaces of the permanent magnets 16b, 18b, 20b, and 22b. Accordingly, the ferromagnetic body 28b is disposed so as to be embedded in the permanent magnet at a location near the magnetic field generation space 32 and through which the magnetic flux passes. As a result, the ferromagnetic body 28a is magnetically coupled (surface contact) with a plurality of permanent magnets 14a, 16a, 18a and 20a and 22a having different magnetization directions. The same applies to the ferromagnetic material 28b. The angle formed between the direction of the magnetic flux in the magnetic field generation space 32 and the magnetization direction of the permanent magnets 14a and 14b is less than 5 degrees and is substantially the same direction. With this configuration, the magnetic field strength of the magnetic field generation space 32 can be improved.
[0023]
Further, a magnetic pole piece 30a is provided on the lower surface of the ferromagnetic body 28a, and a magnetic pole piece 30b is provided on the upper surface of the ferromagnetic body 28b. Therefore, the ferromagnetic body 28a is sandwiched between the permanent magnet 14a and the pole piece 30a, and the ferromagnetic body 28b is sandwiched between the permanent magnet 14b and the pole piece 30b. A magnetic field generation space 32 is formed between the pole pieces 30a and 30b. In this embodiment, in the magnetic field generation space 32, the gap G1 between the pole pieces 30a and 30b is set to 90 mm. The magnetic field intensity of the magnetic field generation space 32 is 0.8T to 2.0T.
[0024]
FIG. 3B is an illustrative view showing a CC section (transverse section passing through the permanent magnets 24 and 26) of FIG. 2, and shows a state in which the pole piece 30b including an annular protrusion is arranged on the ferromagnetic body 28b. Show. The gaps G2 and G3 between the pole piece 30b (annular protrusion) and the permanent magnets 24 and 26 are set to 3 mm or more, preferably 5 mm or more, to prevent short-circuiting of magnetic flux and deterioration of uniformity. Further, as shown in FIG. 3B, the area surrounded by the outer peripheral surface of the pole piece 30b (annular protrusion) is made smaller than the area of the upper surface of the ferromagnetic body 28b, and more efficiently by the ferromagnetic body 28b. The magnetic flux can be collected. The configuration is the same on the pole piece 30a side.
[0025]
In this way, the apparatus main body 12 is configured in an annular shape so as to have the magnetic field generation space 32 using a plurality of permanent magnets.
[0026]
In the drawings, solid arrows drawn in the permanent magnet indicate the magnetization direction of the magnet. A symbol with x (cross mark) in ○ (maru) indicates that it is magnetized in a direction penetrating the permanent magnet perpendicularly from the surface where the symbol is shown. In addition, a symbol in which a circle is drawn in a circle indicates that the permanent magnet is magnetized in a direction that vertically protrudes from the surface on which the symbol is indicated.
[0027]
Further, as shown in FIG. 1, the magnetization directions of the permanent magnets 14a, 16a, 18a, 20a and 22a around the ferromagnetic body 28a are outward when viewed from the ferromagnetic body 28a, that is, the ferromagnetic body 28a side is It is formed to be the S pole. Therefore, the ferromagnetic material 28a functions as a powerful pole piece. On the other hand, the magnetization directions of the permanent magnets 14b, 16b, 18b, 20b and 22b around the ferromagnetic body 28b are formed so as to be directed toward the ferromagnetic body 28b. Therefore, the ferromagnetic material 28b functions as a powerful pole piece. The magnetic flux is generated from the magnetic pole piece 30b toward the magnetic pole piece 30a.
[0028]
Plate-shaped shield members 34a, 36a, 38a and 40a are provided on the outer periphery of the apparatus main body 12, respectively. That is, the shield member 34a is formed on the upper surface of the apparatus body 12, the shield member 36a is formed on the lower surface of the apparatus body 12, the shield member 38a is formed on one side surface of the apparatus body 12, and the shield member 40a is formed on the other side surface of the apparatus body 12. Are provided. The shield members 34a, 36a, 38a and 40a are made of a plate-like magnetic member made of, for example, iron or permalloy having a saturation magnetization of 1.5T or more. Insertion members 34b, 36b, 38b, and 40b having a saturation magnetization (4πIs) of 0.2 T or less are formed on one main surface of shield members 34a, 36a, 38a, and 40a, respectively. These insertion members are made of, for example, stainless steel, SUS304, aluminum or the like. The saturation magnetization (4πIs) of these insertion members is 0.1T or less.
[0029]
As shown in FIGS. 2 and 3A, the shield members 34a, 36a, 38a, and 40a are respectively arranged on the outer periphery of the apparatus main body 12 so that the insertion members 34b, 36b, 38b, and 40b come inside. Thereby, the apparatus main body 12 is shielded.
[0030]
At this time, the distance from the surface of the permanent magnet of the apparatus main body 12 to the surface of the shield members 34a to 40a (for example, the shield member 38a is indicated by X in FIG. 3A: equivalent to the thickness of the insertion members 34b to 40b) is 10% or less of the maximum outer dimension of the apparatus main body 12 (in this embodiment, the width dimension of 470 mm) and the thicknesses of the shield members 34a to 40a (for example, the shield member 38a is indicated by Y in FIG. 3A) It is desirable to set it to 10% or less of the maximum value of 12 outer dimensions.
In this embodiment, it is desirable that the distance from the permanent magnet surface of the apparatus main body 12 to the surface of the shield members 34a to 40a is 0 mm to 35 mm, and the thickness of the shield members 34a to 40a is 5 mm to 35 mm.
[0031]
In addition, the leakage magnetic field increases outside the magnetic field generation direction, that is, outside the permanent magnets 14a and 14b. In this region, the distance from the surface of the permanent magnet 14a to the surface of the shield member 34a, the surface of the permanent magnet 14b to the shield member The distance to the surface of 36a and the thickness of the shield members 34a and 36a are set to 10% or less of the outer dimension (corresponding to a height dimension of 423 mm in this embodiment) of the apparatus main body 12 in the magnetic field generation direction. Is more desirable.
[0032]
According to such a magnetic field generation device 10, the shield members 34a to 40a are provided to be separated from the permanent magnet surface of the device main body 12 by 0 mm or more and 35 mm or less, and the thickness of the shield members 34a to 40a is set to 5 mm or more and 35 mm or less. Accordingly, it is possible to suppress the leakage magnetic field and improve the shielding effect and the central magnetic field strength while suppressing the weight of the device itself.
[0033]
Regardless of the size of the apparatus main body 12, the distance from the permanent magnet surface of the apparatus main body 12 to the surface of the shield members 34a to 40a is 10% or less of the maximum outer dimension of the apparatus main body 12, and the thickness of the shield members 34a to 40a. Is set to 10% or less of the maximum outer dimension of the apparatus body 12, the same effect as described above can be obtained.
[0034]
Furthermore, the leakage magnetic field can be more effectively suppressed by using magnetic members having a saturation magnetization of 1.5 T or more as the shield members 34a to 40a. Further, when such a magnetic member is used, the effect of substantially increasing the magnetic path is increased, so that magnetic saturation can be suppressed and the central magnetic field strength can be improved more effectively.
[0035]
When a strong magnetic field of 0.5 T or more is generated in the magnetic field generation space 32 of the apparatus main body 12, or as each permanent magnet used in the apparatus main body 12, a magnet magnetized parallel or perpendicular to the main surface is used. Even when the R-Fe-B sintered magnet is used, the leakage magnetic field can be effectively suppressed by providing the predetermined shield members 34a to 40a at the predetermined positions.
[0036]
Further, if the insertion members 34b to 40b are provided between the permanent magnet of the apparatus main body 12 and the shield members 34a to 40a, it is possible to suppress a short circuit of magnetic flux between the apparatus main body 12 and the shield members 34a to 40a, and leakage. The magnetic field can be suppressed.
[0037]
An experimental example using such a magnetic field generator 10 will be described.
In this experimental example, iron plates were used as the shield members 34a to 40a.
In this experimental example, the distance between the shield members 34a to 40a and the apparatus main body 12 is changed, and the central magnetic field strength and the leakage magnetic field 0.5 mT line of the magnetic field generator 10 when the shield members 34a to 40a are arranged at the respective positions. Was measured. Moreover, the weight of the whole magnetic field generator 10 was measured about each case where the space | interval of the shield members 34a-40a and the apparatus main body 12 is 0 mm, 35 mm, and 40 mm. This experiment was performed at each thickness by increasing the thickness of the shield members 34a to 40a from 5 mm to 40 mm in increments of 5 mm.
[0038]
Here, the “central magnetic field strength” refers to the strength of the magnetic field in the central portion of the magnetic field generation space 32. The “leakage magnetic field 0.5 mT line” is represented by the distance from the center of the apparatus main body 12 to the position where the leakage magnetic field is 0.5 mT above the permanent magnet 14 a of the apparatus main body 12. It will be small.
Incidentally, when there was no shield member, the central magnetic field strength of the magnetic field generator was 1.053T, and the leakage magnetic field 0.5 mT line was 1.3 m.
[0039]
As can be seen from FIGS. 4 to 11, as the thickness of the shield members 34 a to 40 a increases, the central magnetic field strength improves and the leakage magnetic field can be suppressed. This is because as the shield members 34a to 40a functioning as magnetic paths become thicker (thick), magnetic flux can be easily passed and magnetic saturation can be suppressed.
[0040]
As shown in FIGS. 4 and 5, when the thickness of the shield members 34 a to 40 a is relatively small, the shield members 34 a to 40 a should be arranged slightly apart rather than too close to the permanent magnet surface of the apparatus main body 12. The central magnetic field strength increases. This is because when the shield members 34a to 40a are too close to the permanent magnet surface of the apparatus main body 12, the magnetic flux is easily short-circuited.
However, as shown in FIGS. 4 to 11, when the distance between the shield members 34 a to 40 a and the permanent magnet surface of the apparatus main body 12 exceeds 35 mm, the effect of improving the central magnetic field strength is reduced.
[0041]
Further, referring to FIG. 4, if the thickness of shield members 34a to 40a is less than 5 mm, the central magnetic field strength and the leakage magnetic field are not changed so much as compared with the case where no shield member is provided, and the shield function is not improved so much. Ineffective.
On the other hand, as shown in FIG. 11, when the thickness of the shield members 34a to 40a exceeds 35 mm, the weight of the magnetic field generator 10 itself becomes too large. Further, the shielding effect is not improved.
[0042]
Therefore, it is desirable to provide the shield members 34a to 40a at a distance of 0 mm or more and 35 mm or less from the permanent magnet surface on the outer periphery of the apparatus main body 12, and to set the thickness of the shield members 34a to 40a to 5 mm or more and 35 mm or less. While suppressing the weight of the device itself, the leakage magnetic field can be suppressed and the shielding effect and the central magnetic field strength can be improved.
[0043]
In addition, this invention is applicable also when providing a clearance gap between the shield members 34a-40a and the apparatus main body 12, without using the insertion members 34b-40b.
In the above-described embodiment, the shield member is provided so as to cover the four outer peripheral surfaces of the apparatus main body 12, but is not limited thereto, and may be provided so as to cover the entire outer periphery (six surfaces) of the apparatus main body 12. .
Examples of the annular magnetic field generator to which the present invention is applied include not only a box-type device such as the magnetic field generator 10 but also a Halbach type magnetic field generator (magnetic circuit) disclosed in Japanese Patent Application No. 2001-86098. Thus, a device in which magnets are arranged in a ring shape may be used.
[0044]
【The invention's effect】
According to the present invention, it is possible to suppress the leakage magnetic field and improve the shielding effect and the central magnetic field strength while suppressing the weight of the device itself.
[Brief description of the drawings]
FIG. 1 is an exploded perspective view showing an embodiment of the present invention.
FIG. 2 is a perspective view showing an embodiment of the present invention.
3A is an illustrative view showing a BB section of the embodiment shown in FIG. 2, and FIG. 3B is an illustrative view showing a CC section of the embodiment shown in FIG. 2;
FIG. 4A is a table showing experimental results when the thickness of the shield member is 5 mm, and FIG. 4B is a graph thereof.
FIG. 5A is a table showing experimental results when the thickness of the shield member is 10 mm, and FIG. 5B is a graph thereof.
FIG. 6A is a table showing experimental results when the shield member has a thickness of 15 mm, and FIG. 6B is a graph thereof.
7A is a table showing experimental results when the thickness of the shield member is 20 mm, and FIG. 7B is a graph thereof.
FIG. 8A is a table showing experimental results when the thickness of the shield member is 25 mm, and FIG. 8B is a graph thereof.
FIG. 9A is a table showing experimental results when the thickness of the shield member is 30 mm, and FIG. 9B is a graph thereof.
FIG. 10A is a table showing experimental results when the thickness of the shield member is 35 mm, and FIG. 10B is a graph thereof.
FIG. 11A is a table showing experimental results when the shield member has a thickness of 40 mm, and FIG. 11B is a graph thereof.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Magnetic field generator 12 Apparatus main body 14a, 14b, 16a, 16b, 18a, 18b, 20a, 20b, 22a, 22b, 24, 26 Permanent magnet 28a, 28b Ferromagnetic material 30a, 30b Magnetic pole piece 32 Magnetic field generating space 34a, 36a , 38a, 40a Shield members 34b, 36b, 38b, 40b

Claims (3)

An apparatus main body configured in a ring shape using a plurality of permanent magnets so as to have a magnetic field generation space having a magnetic field intensity of 0.8 T or more and 2.0 T or less on the inner peripheral side thereof ;
Interposed member saturation magnetization below 0.2T provided between the outer periphery to the saturation magnetization 1.5T or more shielding member provided away from the permanent magnet surface of the apparatus main body, and said permanent magnet said shield member A magnetic field generator. The magnetic field generator according to claim 1, wherein the permanent magnet is magnetized parallel or perpendicular to the main surface. The magnetic field generator according to claim 1 or 2 , wherein the permanent magnet is an R-Fe-B magnet.

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