JPS62279607A - Magnetic shield of uniform field magnet - Google Patents

Abstract

PURPOSE:To check the lowering of the uniformity of a uniform magnetic field by a construction wherein a dummy hole which is substantially analogous to a hole piercing through a tubular member of a magnetic shield is provided in an angular area dividing the tubular member equally in the circumferential direction. CONSTITUTION:A magnetic shield 15 comprises a tubular member 15B surrounding a cryostat 2 wherein a coil 1 is held, and a pair of end plates 15A which are joined to the opposite end portions in the axial direction of the tubular member 15B and have, in the central portion, a communicating hole Corresponding to a hollow member 7. A branch hole 6 and a dummy branch hole 16 are dieposed opposite in the vertical direction to each other so that they are positioned symmetrically with respect to the axis in angular areas different at an angle of 180 degrees to each other on a plane of symmetry drawn through the center O of the shield 15. While magnetic reluctance turns high in the respective space parts of both branch holes 6 and 16 and leakage flux density lowers therein, the effect of the turbulence of leakage fluxes upon the intensity B of the uniform magnetic field of a uniform magnetic field space 8 is directed reversely to each other, and consequently the effect is lessened. Thereby the lowering of the uniformity of the uniform magnetic field is checked.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Technical Field to Which the Invention Pertains] The present invention relates to a uniform magnetic field in which a hollow cylindrical uniform magnetic field coil is surrounded by an axisymmetric @ki shield. Magnets, especially nuclear magnetic resonance Konbuque tomography equipment (hereinafter referred to as M
The present invention relates to the structure of a magnetic shield for a uniform magnetic field magnet (abbreviated as MR-CT).

[Prior art and its problems]

In a uniform magnetic field magnet for NMR-CT, a normal conductivity type magnet of 0.05 to 0.3
T. In the superconducting type, it is necessary to generate a highly uniform magnetic field with a magnetic field strength of 0.3 or more and a uniformity of several 1 QPPm, and problems caused by leakage of magnetic flux outside the magnet installation room2. For example, in order to prevent malfunction of a pacemaker implanted in the body of a patient in an adjacent room, it is necessary to suppress leakage magnetic flux density to 5 Gauss or less, and leakage magnetic flux must be connected to ferromagnetic materials such as reinforcing bars in buildings. In order to eliminate problems such as the distribution of leakage magnetic fields being disturbed and the disturbance of the distribution of the leakage magnetic field reducing the uniformity of the uniform magnetic field, the uniform magnetic field coil is made of a thick iron plate or the like with axial symmetry. There is a known structure in which the leakage flux is prevented from spreading by covering it with a shield to form a return path for the leakage flux.

Fig. 8 is a schematic side sectional view of a uniform magnetic field magnet showing an example of the prior art, and Fig. 9 is a side sectional view in the axial direction at the center 0 position of Fig. 8, taking the case of a superconducting magnet as an example. This is what is shown. In the figure, 1 is a superconducting cylindrical coil I A, ! The uniform magnetic field coil 1 is a uniform magnetic field coil consisting of a ring-shaped cocoil 1B), and is housed in a cryostat 2 containing liquid helium 9. The uniform magnetic field coil 1 is centered at 0.
It is formed axially symmetrically with respect to the hollow part Z, and is configured to generate a uniform magnetic field having a magnetic flux density B in the Z direction shown by a solid arrow in a spherical uniform magnetic field 8 shown by a broken line in the hollow part Z.

The cryostat 2 also includes a cylindrical service turret 3 that protrudes in an angular region in the Z direction passing through the center 0, and a large current lead (not shown), liquid helium piping, and vacuum piping that are conductively connected from this part to the uniform magnetic field coil 1. etc. are drawn out. 5 is a magnetic shield made of iron plate or the like that surrounds the cryostat almost axially symmetrically, and consists of a cylindrical part 5B and a pair of end plates 5A having a hole corresponding to the hollow part 7 in the center. Turret 3
A tubular branch hole 6 that surrounds the service turret and projects outward is formed in a portion corresponding to the service turret.

In the uniform magnetic field magnet configured as described above, most of the magnetic flux generated in the hollow part in the direction of the arrow Z is not caused by the iron plate with high relative magnetic permeability, but by the almost axially symmetrical magnetic shield 5.
Pass through the cylindrical part 5B in the opposite direction to the arrow Z direction and enter the hollow part Z.
By circulating the magnetic flux to the side, leakage magnetic flux distributed outside the magnetic shield can be significantly reduced without disturbing the magnetic field inside the magnetic shield. However, since the service turret 6 has the branch hole 6, the magnetic shield has a locally non-axisymmetric structure in this part, and the magnetic resistance in this part increases, resulting in a low leakage magnetic flux density. This disturbance of the magnetic field also affects the magnetic flux distribution of the uniform magnetic field 8 within the hollow part Z, resulting in a disadvantage that the uniformity of the magnetic field strength decreases.

10 to 13 are characteristic line diagrams showing the disturbance of the magnetic field in the uniform magnetic field space 8 centered on the 0 point, and FIG. Figure 11 shows the distribution of the amount ΔEy, and the distribution of the amount of distortion Δ of the magnetic field in the X direction.
Figure 12 shows the distribution of Bx and the amount of distortion ΔBz of the magnetic field in the Z direction.
, Figure 16 shows the angle θ in the counterclockwise direction starting from the Z direction.
3 shows the distribution of the amount of distortion ΔBθ of the magnetic field in the angular direction with respect to the angular direction. As shown in Fig. 10, the amount of deformation ΔBy in the Z direction is smaller on the side closer to the branch hole B, and higher on the side farther away from the branch hole B, with the lower right being the distribution.
, ΔBZ exhibits a high, almost symmetrical mountain-shaped characteristic at the center 0 position directly below the branch hole 6 as shown in FIGS. 11 and 12, and as a result, the amount of deformation ΔBθ in the angle θ direction is As shown in , the distribution is lowest in the angle θ=O・ direction (Z direction) of the branch hole B, and highest in the direction of the angle θ=180′ (−Z direction) in the direction of the branch hole B. In the distribution of the amount of strain ΔBθ in the direction of the angle θ shown in FIG. 16, the component ΔBθ1, which is uniform with respect to the angle θ, has axial symmetry. There is no problem because it can be corrected, but the uniform magnetic field coil 1 cannot correct the distortion amount ΔBθ2 that is asymmetrical in the angle θ direction, and the uniformity of the magnetic field strength B of the uniform magnetic field 8 is impaired by the variation distortion amount ΔBθ2. You will be killed.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned situation, and an object of the present invention is to provide a uniform magnetic field magnet equipped with a magnetic shield that can reduce the amount of variation in the strength of a uniform magnetic field that is asymmetrical in the circumferential direction (angular direction) of the uniform magnetic field. With the goal.

[Key points of the invention]

The present invention provides a hole penetrating the cylindrical portion of the magnetic shield in a predetermined angular region of the magnetic shield surrounding a hollow cylindrical uniform magnetic field coil, and a hole penetrating the cylindrical portion in the angular region equally dividing the cylindrical portion in the circumferential direction. By providing a dummy hole that is almost similar to the hole), the dummy hole corrects the distribution of deformation amount that is out of phase in the angular direction from the distribution of deformation amount of the magnetic field that occurs in the angular direction of the uniform magnetic field. This makes it possible to reduce the asymmetrical component that cannot be corrected by the uniform magnetic field coil in the overall amount of deformation determined by the sum of both distributions of the amount of deformation.

[Embodiments of the invention]

The present invention will be explained below based on examples.

FIG. 1 is a schematic side sectional view showing an embodiment of the device of the present invention, and FIG.
The reason for this is the axial cross-sectional view passing through the center position 0 in FIG. In the figure, reference numeral 15 denotes a magnetic shield υ, which is coupled to a cylindrical portion 15B surrounding the cryostat 2 housing the winding 1 and to both axial ends of the cylindrical portion 15B.

A pair of end plates 15A each having a communication hole corresponding to the hollow part 7 in the center), and a cylindrical part 15B through which a service turret 3 penetrates in an angular region in the X direction on a plane passing through the center 0 of the uniform magnetic field coil. In addition to the branch hole 6, a dummy branch hole 16 is formed that is symmetrical to the branch hole 6 in the angle θ=180−different angle region−X direction.

In the uniform magnetic field magnet configured as described above, 1800 on the plane of symmetry passing through the center 0 of the magnetic shield 15
A branch hole 6 and a dummy branch hole 16 are arranged axially symmetrically in different angle regions.
By arranging them vertically to face each other, the magnetic resistance increases in the space of both branch holes, υ, and the leakage magnetic flux density decreases, but this disturbance in the leakage magnetic flux density causes the uniform magnetic field space 8 to Since the influences on the strength B of the uniform magnetic field are opposite to each other, the influences are attenuated and, therefore, a decrease in the uniformity of the uniform magnetic field can be suppressed.

Figures 6 and 5 are characteristic diagrams showing the amount of distortion of the uniform magnetic field in the plane of symmetry passing through the center 0 in the embodiment device, and Figure 6 shows the magnetic field strength in the angular region of the branch hole in the X direction. Figure 4 shows the distribution of the amount of strain ΔBy in the X direction.
FIG. 5 shows the amount of distortion ΔBθ in the direction of angle θ, with θ=0° in the X direction, and the horizontal axis represents the counterclockwise angle. In FIG. 6, the distortion amount curve 6y due to the branch hole B shows a characteristic of 2) (lower right), which is smaller in the X direction where the branch hole 6 is located, as in FIG. - shows the characteristics of 9 on the lower left side where there is a dummy branch hole as shown in curve 16y,
The distribution of the amount of strain ΔBy in the X direction determined by the sum of both curves is curve 15. As shown by Y, it becomes a valley-shaped curve having a lower limit value at the center 0. On the other hand, as shown in FIG. 4, the distribution of the amount of deformation ΔBx in the As shown by curve 15X, the distribution of ΔEX determined by the sum of is twice the amount of distortion as curves 6X and 16X. The result is a chevron curve as shown. Although the amount of deformation in the Z direction is not shown, it has a chevron-shaped characteristic similar to that shown in FIG. Therefore, the distribution of the amount of strain ΔBθ in the direction of angle θ, which is a combination of the distribution curves in the X direction, the X direction, and the two directions, is as shown in FIG. Since the distribution curve 16θ on the 16th side exhibits a characteristic in which the phase is shifted from each other in the 180' angle direction, ΔBθ consisting of the sum of both curves
The angular direction distribution curve 15θ has periods of curves 16θ and 6θ
It is the composite value of the non-axisymmetric component ΔBθ27 which is shortened to % of that of 9, and the axisymmetric component ΔBθ1 which is uniform in the angular direction.
Although the axisymmetric component ΔBθ1 is significantly increased compared to that of the prior art in FIG. 16, the non-axisymmetric component ΔBθ2 is significantly reduced. Therefore, if the axisymmetric component ΔBθ1 is corrected by the uniform magnetic field filter 1, the uniform magnetic field will be disturbed only by the non-axisymmetric component ΔBθ2 that cannot be corrected by the uniform magnetic field coil, and ΔBθ2 will be significantly reduced compared to the conventional technology. As a result, a decrease in the uniformity of the magnetic field strength B in the uniform magnetic field space 8 can be significantly suppressed compared to the prior art.

FIG. 6 is a side sectional view of the main part of a magnetic shield showing a different embodiment of the present invention, in which six pieces are installed in the cylindrical part of the magnetic shield 25 at positions dividing the cylindrical part into four parts with respect to the branch hole 6. This embodiment differs from the previous embodiment in that dummy branch holes 25A, 25B, and 25C are provided, and there is an advantage that the non-axisymmetric component ΔBθ2 of the amount of distortion ΔBθ of the magnetic field in the angular direction can be further reduced. Note that if two or more branch holes 6 are required, any one of the dummy branch holes 25A, 25B, and 25C can be used as the branch hole.

FIG. 7 is a schematic U cross-sectional view of a magnetic shield showing still another embodiment of the present invention, in which the cylindrical portion of the magnetic shield 35 has two holes at θ=120 with respect to the branch hole A and two at different angle regions. The difference from the previous embodiment is that the dummy branch holes 35A and 35B are provided, and the amount of distortion of the magnetic field in the angular direction can be reduced, and since the dummy branch holes do not protrude downward), the magnetic shielding There is an advantage that the height can be lower than that shown in FIG.

As described above, the application of the present invention is not limited to superconducting uniform magnetic field magnets, but for example, in a normal conducting magnet, a branch hole for drawing out a current of 1.1' By providing dummy branch holes also in pockets that protrude outward when covering protrusions, it is possible to suppress deterioration in the uniformity of the magnetic field.

Further, even when the shapes and dimensions of the branch hole and the dummy branch hole are not equal to each other, it is possible to obtain the function of suppressing a decrease in the uniformity of the magnetic field. Therefore, it is possible to avoid the economic disadvantage caused by providing a dummy branch hole by effectively utilizing the dummy branch hole not only as a means for correcting disturbances in the uniform magnetic field but also as a mechanical part with other functions. can.

〔Effect of the invention〕

As described above, the present invention provides a cylindrical portion corresponding to a branch hole that penetrates a predetermined angular region of a cylindrical portion of a magnetic shield surrounding a hollow cylindrical uniform magnetic field coil and protrudes outward. Dummy branch holes, which are substantially similar to the branch holes, are provided in angular regions that are equally divided in the circumferential direction. As a result, the dummy branch hole can generate a component that cancels out the non-axisymmetric component in the angular direction of the amount of distortion of the uniform magnetic field caused by the branch hole, and the decrease in the uniformity of the magnetic field is reduced compared to the conventional technology. It is possible to provide a magnet with a less uniform magnetic field. In addition, by utilizing the dummy branch hole as an outlet hole for the lead wires and piping of the uniform magnetic field magnet, a storage pocket for protrusions, a lead hole, etc., the economic disadvantage caused by providing the dummy branch hole can be avoided. can be avoided.

[Brief explanation of the drawing]

FIG. 1 is a schematic side sectional view showing an embodiment of the present invention, FIG. 2 is a side sectional view in the axial direction of the embodiment device, and FIG. 5 is a variable strain characteristic diagram of the magnetic field in the X direction in the embodiment. Fig. 4 is a distortion characteristic diagram in the X direction, Fig. 5 is a variation characteristic diagram in the angular direction, Fig. 6 is a sectional view of the main part showing a different embodiment, and Fig. 7 is a further different embodiment. 8 is a side sectional view of the conventional device; FIG. 9 is an axial sectional view of FIG. 8;
Figures 1 to 13 show the variation of the uniform magnetic field i in the conventional device.
It is a characteristic line diagram. 1... Uniform magnetic field coil, 2... Cryostat,
3...Service turret, 5,15.25.35...
・Magnetic shield, 5A, 15A...end plate, 5B, 1
5B... Cylindrical part, 6... Branch hole, 7... Hollow part,
8... Uniform magnetic field, 9... Liquid helium, 16,25
A, 25B, 25C, 35A, 35B...Dummy branch hole, B...Magnetic flux density of uniform magnetic field, ΔBy, ΔBx, Δ
Bz, ΔBθ...Amount of distortion of uniform magnetic field, ΔBθl...
Axisymmetric strain amount, ΔBθ2...non-axisymmetric strain amount. Figure 1 Figure 2 Figure 3 Figure 4 y -x -y x y Figure 5 Go to 25C Go to 25B Figure 6 Figure 7 Figure 8 (¥1 Figure 9 Figure 10 Figure 11 Figure 12 Figure Figure 13

Claims (1)

[Claims] 1) A uniform magnetic field coil that is formed into a hollow cylindrical shape and generates a uniform magnetic field within the hollow portion, a cylindrical portion surrounding this uniform magnetic field coil, and a cylindrical portion that is coupled to both axial ends of the cylindrical portion of the uniform magnetic field coil. a magnetic shield consisting of an end plate surrounding an axial end face and having a communication hole in the center thereof corresponding to the hollow portion; a hole penetrating the cylindrical portion in a predetermined angular region of the cylindrical portion of the magnetic shield; 1. A magnetic shield for a uniform magnetic field magnet, characterized in that a dummy hole substantially similar to the hole is provided in an angular region equally dividing the cylindrical portion in the circumferential direction with respect to the hole.