JPH0516678B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field to which the Invention Pertains] The present invention is directed to a method for preventing leakage magnetic fields generated by a uniform magnetic field magnet used in a nuclear magnetic resonance computer tomography system (hereinafter abbreviated as NMR-CT) from leaking outside the installation room. The present invention relates to a magnetic shielding device that is installed on a partition wall of a main part of an installation room in order to prevent this.

[Prior art and its problems]

A homogeneous magnetic field generator for NMR-CT generally uses an air-core homogeneous magnetic field magnet, which generates a strong DC uniform magnetic field of 0.05 to 0.3 T in a normal conductive type and 0.3 to 0.5 T in a superconducting type in a hollow part. In particular, in order to insert the human body into the uniform magnetic field, the hollow part has an opening communicating with the installation space, through which magnetic flux leaks into the installation chamber. Some uniform magnetic field magnets themselves are equipped with a magnetic shield (yoke) to reduce this leakage flux, but even with this configuration, it is difficult to prevent leakage flux from the opening. Furthermore, in a uniform magnetic field coil that does not have a magnetic shield, a large leakage magnetic field is generated that is close to the total amount of generated magnetic flux. on the other hand
In many cases, various examination equipment and patient rooms are installed in rooms adjacent to the top, side, or bottom of the NMR-CT installation room, and in order to avoid the influence of leakage magnetic fields, It is required to suppress the leakage magnetic flux density normally to 5 Gauss or less, and strictly to 1 Gauss or less. Therefore, NMR
- A magnetic shield made of ferromagnetic material is placed on the partition wall of the CT installation chamber to form a circulation path for the leakage magnetic field of the uniform magnetic field magnet, and measures are taken to prevent the magnetic field from leaking outside the installation room.

FIG. 9 is a side sectional view of a magnetic shielding device in an NMR-CT installation room showing the conventional technology, and FIG. 10 is a side sectional view of FIG. 9 taken in the direction A-A. , two sets of side walls 12A parallel to each other,
Flat magnetic shields 1, 2A, 2B, 3A, 3 made of iron plates (ordinary steel plates) with a predetermined thickness to cover the inner wall surfaces of each of 12B, 13A, and 13B.
B. Reference numeral 10 denotes a uniform magnetic field magnet formed in a cylindrical shape, and is formed to generate an axial magnetic flux in its hollow portion 10A to generate a uniform magnetic field, but most of the total generated magnetic flux is a leakage magnetic field. Distributed both indoors and outdoors. Dashed lines 101 and 102 are leakage magnetic flux lines, and the magnetic flux generated in the region near the center of the uniform magnetic field magnet 10 mainly becomes leakage magnetic flux lines 101 and intersects with the magnetic shields 2A and 2B perpendicular to the axis, causing magnetic flux. Magnetic shields 1 and 3 connecting between shields 2A and 2B
By forming a circulation path passing through A and 3B, leakage to the outside of the installation chamber 5 is prevented. Further, the magnetic flux generated near the periphery of the hollow part 10A of the uniform magnetic field magnet 10 becomes a leakage magnetic flux line 102 that surrounds the outer periphery of the magnet 10 and flows back into the hollow part.
The circumferential magnetic flux component 102A is the magnetic shield 3A
102B interlinks with the magnetic shield 3B, and the upper component 102H of 102 interlinks with the magnetic shield 1 and circulates through each magnetic shield, thereby preventing leakage to the outside of the installation chamber 5. In the case of the figure, since no magnetic shield is installed on the floor 14 side, the leakage magnetic flux line 102C distributed below the magnet 10 leaks below the floor 14, but the amount of magnetic flux leaking from this part to the adjacent chamber is In particular, the influence on the adjacent upper room 6 via the upper partition wall 11 is extremely small.

By the way, the flat magnetic shields 1, 2A, 2
The thickness of B, 3A, 3B, etc. is conventionally set to a uniform thickness determined by the maximum magnetic flux density in the magnetic shield, and the position in the plane direction where the maximum magnetic flux density occurs in the magnetic shield is The position directly above or to the side of the uniform magnetic field magnet 10, that is, the vertically projected portion of the uniform magnetic field magnet in each magnetic shield, means that the magnetic flux lines entering and exiting each magnetic shield pass through this portion in the creeping direction. It is obvious from the fact that the magnetic flux density at the periphery of a magnetic shield formed into a rectangular plate shape with uniform thickness is low, and the utilization rate of the magnetic shield as a circulation path for leakage magnetic flux (average magnetic flux This results in a problem that the magnetic shield has a low density (density) and the weight of the magnetic shield increases, resulting in an economic disadvantage. In addition, when the leakage magnetic flux density outside the magnetic shield, that is, on the side of the room next to the installation room 5 is B _l , the relative magnetic permeability of the steel plate constituting the magnetic shield is μ _s , and the magnetic flux density inside the magnetic shield is B _s , B There is a relationship of _l ≒ B _s / μ _s , and the leakage magnetic flux density in adjacent chambers
When trying to suppress B _l to a low level such as the aforementioned 5 Gauss or 1 Gauss, it is necessary to suppress the maximum value of the magnetic flux density B _s in the magnetic shield, and the thickness of the magnetic shield increases. (for example
The disadvantage is that it affects the strength of the building that supports it, and improvements are needed.

[Purpose of the invention]

The present invention was made in view of the above-mentioned situation, and
It is an object of the present invention to provide a magnetic shielding device for a room in which a uniform magnetic field magnet is installed, which can reduce the weight of the magnetic shield and reduce the leakage magnetic flux density in an adjacent room without increasing the weight of the magnetic shield.

[Summary of the Invention] The present invention provides a first magnetic shield plate formed in a rectangular plate shape so as to cover a partition wall surface of a uniform magnetic field magnet installation chamber mainly from the inner wall side, and a first magnetic shield plate provided with the first magnetic shield plate. A plurality of shield plates having a rectangular shape and a stepwise reduction in area, which are almost similar to these partition walls, are placed on each of the partition walls, and the shield plates overlap each other at positions in the plane direction facing the uniform magnetic field magnet. By arranging the magnetic shield in multiple layers, the cross-sectional area of each part of the magnetic shield can be adjusted in accordance with the distribution of leakage magnetic flux passing through the inside of the magnetic shield in the longitudinal direction, and the magnetic flux density of each part of the magnetic shield can be equalized. The utilization efficiency of the leakage magnetic flux as a circulation path is increased, and the weight can be reduced.In addition, by forming gaps of uniform thickness at key points between the magnetic shield plates, the magnetic resistance of the gaps can be utilized to distribute the magnetic shields toward the bulkhead. For example, by configuring the structure to reduce the magnetic flux density on the first shield plate side, leakage magnetic flux density in the adjacent chamber can be further reduced.

[Embodiments of the invention]

The present invention will be explained below based on examples.

1 and 2 are side sectional views taken in directions 90° different from each other, showing an embodiment of the present invention, and show an upper partition wall of an installation chamber 5 in which a hollow cylindrical uniform magnetic field magnet 10 is disposed approximately in the center. 11 and four side walls 12A, 12B, 13A, 13B, respectively, magnetic shields 20 and 30A, 30B, 40A,
This shows an example in which 40B is arranged. In the figure, the magnetic shield 20 provided on the lower surface side of the upper partition wall 11 includes a first shield plate 21 made of a common steel plate formed in a rectangular plate shape x ₁ and y ₁ that covers the wall surface, and a first shield plate 21 on the lower surface side. The area is x ₂・y ₂ ,
First shield 21 reduced in steps x ₃ y ₃
It is composed of a plurality of rectangular plate-shaped shield plates 22 and 23 that are almost similar to each other, and each shield plate 2
1 , 22 , and 23 are closely stacked and stacked so as to overlap each other at a position directly above the uniform magnetic field coil 10 and are connected and supported by the partition wall 11 . In addition, side walls 12A, 12B, 13A, 14
Magnetic shield 3 provided on the inner wall surface of A
0A, 30B, 40A, 40B are magnetic shields 2
0, the distance between the uniform magnetic field coil 10 is large and the amount of magnetic flux passing through is small.
1B and a plurality of shield plates 32A, 32B, 42A, and 42B made of one plate-shaped body, and the plurality of shield plates are arranged in layers centered on the position facing the uniform magnetic field magnet 10. It is configured like this.

FIG. 3 is a side sectional view of the main part of the above-mentioned embodiment, and the action of the magnetic shield 20 disposed on the lower surface side of the upper partition wall 11 will be explained using the figure. In the figure, the first shield plate 21 has a leakage magnetic flux 1 that enters and exits the magnetic shields 30A and 30B on the side perpendicular to the axial direction of the uniform magnetic field coil 10 in FIG.
01 and the leakage magnetic flux 111 that directly enters and exits from the periphery of the first shield plate 21 that does not overlap with the shield plate 22 mainly circulates, and the leakage flux 111 that directly enters and exits from the periphery of the first shield plate 21 that does not overlap with the shield plate 23 circulates. The leakage magnetic flux 112 mainly flows back into the shield plate 23, and the leakage magnetic flux 113 that directly enters and exits from the surface of the shield plate 23 flows back. Therefore, the thickness and area of each shield plate are determined by taking into account the above-mentioned amount of circulating magnetic flux and the magnetization characteristics of the steel plate, and the above-mentioned B _l =
B _s /μ _s are each set so as to satisfy a predetermined condition. Magnetic shield 20 configured as described above
In this case, the magnetic flux densities of the shield plates 21, 22, and 23 can be made almost equal to each other, and the shield plates are densely arranged in layers in the areas where the shield plates overlap each other.
The magnetic resistance between the shield plates is low, and it can be expected to have the effect of alleviating the uneven magnetic flux density between the shield plates, making it possible to reduce the weight of the steel plate by improving its utilization as a leakage flux path.
It is possible to obtain a magnetic shield device that is approximately 30 to 40% lighter than the weight of a conventional magnetic shield having a uniform thickness. Note that the same applies to the side wall side magnetic shields 30A, 30B, 40A, and 40B, so a detailed explanation will be omitted. In addition,
In the above description of the embodiment, an example was described in which magnetic shields were provided on five walls excluding the floor surface, but if the leakage magnetic flux density B _l of an adjacent room is severe, a magnetic shield may also be provided on the floor surface. You can also use B _l
If the restriction is limited to an adjacent room in a specific direction, the magnetic shield on the side opposite to the adjacent room may be omitted.

In addition, it is often difficult to form a shield plate that covers a wall with a single steel plate, but in such cases, it is necessary to butt weld multiple steel plates or to form a shield plate made of multiple thin plates. The structure may be such that the abutting portions in each layer are shifted in position so that they do not overlap with each other. Furthermore, there are no particular limitations on the method of coupling between the shield plates and the method of coupling between the magnetic shield and the partition wall.

FIG. 4 is a side sectional view of a main part showing a different embodiment of the present invention, in which a gap is regulated to a uniform dimension by a spacing piece 53 between the layers of the first shield plate 21 and the shield plate 22. The portion 51 is connected to the shield plates 22 and 2.
3, and many leakage magnetic flux lines 1 facing the uniform magnetic field coil 10.
This embodiment differs from the previous embodiment in that shield plates 23 and 24 are placed in close contact with each other in a layered manner on the portion receiving the shield plate 13. In this way, a gap 5 with high magnetic resistance is formed between the layers of the important parts of the shield plates.
1 and 52, the magnetic flux 51 flowing in and out between the shield plates through the gaps 51 and 52
A, 52A can be reduced, so the shield plate 21,
It becomes possible to set the magnetic flux densities B _s of the leakage flux paths consisting of the laminates 23 and 23 and 24 to different values. That is, in the laminated portion of the shield plates 23 and 24 where many leakage magnetic flux lines 113 enter and exit, the magnetic flux density B _s is set to the saturation magnetic flux density of ordinary steel (for example, about 1.5 T).
When the magnetic flux density is set in this way, the relative magnetic permeability μ _s of the steel material decreases significantly, and the leakage magnetic flux 51A that transfers to the shield plate 22 through the gap 52 increases, so that the magnetic flux density of the shield plate 22 becomes the leakage magnetic flux. 52A
and 112, it is set to a value lower than the saturation magnetic flux density (for example, about 1 T). By doing this, the gap 51 from the shield plate 22
The magnetic flux 5 that transfers to the first shield plate 21 via
1A is considerably less, and the first shield plate 21
The leakage magnetic flux passing through is mainly caused by the magnetic flux lines 101 circulating from the magnetic shield 30A to the side wall 12A side,
The magnetic flux lines 101 are applied not only to the shield plate 21 but also to the second
By spreading and distributing to the first shield plates 41A, 41B, etc. in the figure, the shield plate 21
The amount of magnetic flux passing through becomes smaller. Therefore, by setting only the magnetic flux density of the first shield plate 21 based on the conditional expression B _l ≒ B _s / μ _s so that the magnetic flux density B _l of the adjacent room is below the regulation value, the leakage magnetic flux density of the adjacent room can be suppressed. In addition, the first shield plate 21 can be formed using a thin and lightweight steel plate, and the magnetic flux density of the shield plates 22, 23, 24, etc. can be greatly increased compared to the above-mentioned embodiments. Since the weight can be reduced, it is possible to obtain a magnetic shielding device 50 that is even lighter in weight than the previous embodiment.

FIG. 5 is a side sectional view of a main part showing still another embodiment of the present invention, in which the first shield plate 61 is formed of a high magnetic permeability steel plate, the plurality of shield plates 22 and 23 are formed of ordinary steel plates, and , is different from the previous embodiments in that a gap 51 is provided between the first shield plate 61 and the adjacent shield plate 22, and the carbon content is 10 ^-3 % as a high magnetic permeability steel plate. Ultra-low carbon steel plates below the order of magnitude, silicon steel plates for electrical use, and in rare cases permalloy steel plates can be used, and as ordinary steel plates, iron plates can be used when the carbon content exceeds the order of 10 ^-2 %.

Figure 6 is a magnetization characteristic diagram (B-H characteristic diagram) of an ordinary steel plate and a high magnetic permeability steel plate (ultra-low carbon steel plate), and the magnetic flux density B _s of the high magnetic permeability steel plate (curve 63) is the magnetizing force H In the _H1 region where the magnetizing force H is low, _B1 is high, whereas in the ordinary steel plate (curve 62), _B2 is low, and in the region _H2 where the magnetizing force H is high, the magnetic flux densities of both are almost equal, _B3
level.

FIG. 7 is a diagram showing the relative magnetic permeability versus magnetic flux density characteristic diagram of ordinary steel sheets and high permeability steel sheets (ultra-low carbon steel sheets).
The magnetic flux density B at which the relative magnetic permeability μ of the high magnetic permeability steel plate (curve 65) peaks exhibits characteristics shifted to the low magnetic flux density region side compared to that of the ordinary steel plate (curve 64),
By suppressing the magnetic flux density of a high permeability steel plate to below the apex of curve 65, the leakage magnetic flux density B _l , expressed as B _l ≒ B _s / μ _s , is lower than that of a normal steel plate due to the thin shield plate. level (eg, below 1 Gauss).

Magnetic shield 6 configured as shown in FIG.
0, the shield plates 22 and 23 with a large amount of passing magnetic flux such as the leakage magnetic flux 112 and 113 are made of ordinary steel, and the magnetic flux density B _s is set near the saturation value B ₃ of the magnetic flux density in Fig. 6. In addition to reducing the weight of the shield plates 22 and 23, the gap portion 51 reduces the transfer of magnetic flux from the shield plate 22 to the first shield plate 61, and the amount of magnetic flux passing through the first shield plate is reduced by the magnetic flux on the side wall side. Shield 30A,
The wraparound magnetic flux from 30B is reduced to nearly 101. A high magnetic permeability steel plate is used for the first shield plate 61, and its magnetic flux density B _s is determined by the curve 65 in FIG.
Set to an area lower than the apex of. At this time, the amount of magnetic flux passing through the shield plate 61 itself is suppressed, so even if the magnetic flux density B _s is suppressed to a low value (for example, about 3000 Gauss), the thickness of the shield plate 61 is
Since it is only about mm, the amount of expensive high magnetic permeability steel plate used can be suppressed to a small amount compared to the case where the entire magnetic shield is formed of high magnetic permeability steel plate. Moreover, when the magnetic flux density B _s is suppressed to about 3000 Gauss,
It is possible to make the relative magnetic permeability μ _s more than 5000,
It becomes possible to reduce the leakage magnetic flux density in the upper adjacent chamber 6 to 1 Gauss or less, which is approximately determined by B _l ≈B _s /μ _s .

FIG. 8 is a side sectional view of a main part showing a modification of still another embodiment shown in FIG. By providing a plurality of shield plates 22 and 23 made of ordinary steel plates on the chamber 6 side and on the chamber 5 side where the uniform magnetic field magnet 10 is installed,
This is different from the previous embodiment in that the partition wall 11 is used as a gap 71, and the space occupied by the gap 52 in FIG. The first magnetic shield 61 can absorb the magnetic flux leaking to the partition wall through the gap 72 between them, so that the leakage magnetic flux density B _l in the adjacent room 6 can be absorbed by the first magnetic shield 61.
This has the advantage of further reducing the

In each of the above-mentioned embodiments, an example was mainly explained in which magnetic shielding devices were provided on the walls in five directions surrounding the installation room, but the installation room for the uniform magnetic field coil was a basement, and the If leakage magnetic flux density in an adjacent room is a problem, it may be sufficient to simply install a magnetic shield device on the upper wall of the installation room.The direction in which the magnetic shield device should be installed and the number of walls will depend on the requirements and environment. It is preferable to decide on a case-by-case basis, taking into account the conditions. Also, the first
Although it is preferable that there be no large gaps between the shield plates, if the construction becomes extremely difficult due to mutual coupling of the shield plates, a gap may be provided between the magnetic shields.

〔Effect of the invention〕

As described above, the present invention includes a magnetic shield that is arranged on the wall of the installation chamber to reduce the leakage magnetic flux density generated by the uniform magnetic field magnet, and is arranged on the wall of the installation chamber to reduce the leakage magnetic flux density in the adjacent chamber. A first shield plate formed in the shape of a rectangular plate that covers almost the entire area of It consists of a plurality of shield plates in the shape of a reduced rectangular plate, and each shield plate has a small area and is arranged on the side of the uniform magnetic field magnet so that each shield plate overlaps with the other at a position directly above or directly across from the uniform magnetic field magnet. It was configured to be arranged in layers. As a result, by arranging each shield plate closely to each other, it is possible to maintain the magnetic flux density inside the magnetic shield almost equally and at a high level within the range that can maintain the leakage magnetic flux density in adjacent chambers below a predetermined value. This makes it possible to reduce the utilization rate (average magnetic flux density) of the shield plate as a leakage flux path compared to conventional technology that uses a magnetic shield with uniform thickness.
Therefore, it is possible to provide an economically advantageous magnetic shielding device for a uniform magnetic field magnet installation room, which is lightweight, has little influence on building strength, and is easy to construct. In addition, a gap is provided between the layers of the important parts of each shield plate to increase the magnetic resistance between the shield plates, and the amount of magnetic flux passing through each shield plate and the magnetic flux density are set to different values to improve the magnetic shielding device. It is also possible to further reduce the weight, and in addition, only the first shield plate of the magnetic shielding device equipped with an air gap is made of high magnetic permeability steel plate, and a small amount of high magnetic permeability steel plate reduces the leakage magnetic flux of the adjacent chamber. Local improvements in the configuration, such as the ability to reduce the density to even lower levels, offer the advantage of further improving economic efficiency and magnetic shielding performance.

[Brief explanation of the drawing]

Figures 1 and 2 are side sectional views of an embodiment of the present invention as seen from directions 90 degrees different from each other, Figure 3 is a side sectional view of the main parts of the embodiment, and Figure 4 shows a different embodiment. Fig. 5 is a side sectional view of the main part showing a further different embodiment; Fig. 6 is a side sectional view of the main part;
FIG. 7 is a diagram of the magnetization characteristics of the steel plate in the example shown in FIG. 5. FIG. FIG. 9 is a side sectional view of the main part showing a modified example, FIG. 9 is a side sectional view showing the conventional technique, and FIG.
It is a side cross-sectional view of the AA direction in a figure. 1, 2A, 2B, 3A, 3B... Conventional magnetic shield, 5... Installation room, 6... Upper adjacent room, 1
0...Uniform magnetic field magnet, 11, 12A, 12
B, 13A, 13B, 14... Bulkhead, 20, 30
A, 30B, 40A, 40B, 50, 60, 70
...Magnetic shield of invention, 21, 31A, 31
B, 41A, 41B, 21...first magnetic shield plate, 22, 23, 32A, 32B, 42A, 4
2B... Multiple magnetic shield plates with different areas, 5
1, 52... Gap portion, 61... Magnetic shield plate made of high magnetic permeability steel plate, 71... Partition wall also serving as gap portion, 101, 102, 111, 112, 113...
...Leakage flux lines.

Claims (1)

[Scope of Claims] 1. A magnet that generates a uniform magnetic field in a hollow part is supported by a partition wall so as to mainly cover the inner wall surface of the partition wall of the installation chamber to prevent the leakage magnetic field generated by the air-core uniform magnetic field magnet from leaking to the outside of the installation room. A first shield plate made of a ferromagnetic material formed in a rectangular shape so as to cover almost the entire surface of the partition wall of the main part, and a partition wall surface on which this first shield plate is provided. Each of them is provided with a plurality of plate-shaped shield plates formed in a rectangular shape that is almost similar to the partition wall surface and whose area gradually decreases, and each of the shield plates is mutually connected to each other at a position in the plane direction facing the uniform magnetic field magnet. 1. A magnetic shielding device for a uniform magnetic field magnet installation room, characterized in that the first shield plates are arranged in layers with the first shield plates facing the wall so that the first shield plates overlap each other. 2. In what is stated in claim 1,
1. A magnetic shielding device for a uniform magnetic field magnet installation room, characterized in that each shield is arranged in layers so as to maintain an air space of uniform thickness between the layers of the important parts thereof. 3. In the item described in claim 1 or 2, the first shield plate is made of a high magnetic permeability steel plate, the plurality of shield plates are each made of a common steel plate, and the first shield plate and the plurality of shields are made of a steel plate. A magnetic shielding device for a uniform magnetic field magnet installation room, characterized in that a space is formed between the layer and the plate. 4 In what is stated in claim 3,
A uniform magnetic field magnet installation chamber characterized in that a first shield plate is arranged on the outer wall side of the installation chamber, a plurality of shield plates are arranged on the inner wall side of the installation chamber, and the partition wall also serves as a cavity. shield device.