JP4392910B2 - Open magnet system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an open-type magnet device, and more particularly to an open-type magnet device suitable for obtaining a highly uniform static magnetic field.
[0002]
[Prior art]
In a horizontal magnetic field type magnet device or a vertical magnetic field type magnet device using a pole piece, a magnetic shim is used to increase the uniformity of a static magnetic field generated by the magnet device.
[0003]
However, in particular, a vertical magnetic field type magnet device using a pole piece employs a rose ring structure in which the outer periphery of the pole piece protrudes in order to improve the uniformity of the static magnetic field generated by the magnet device. On the other hand, in order to secure a wide measurement space, a structure is adopted in which the gradient magnetic field coil is disposed in the space in the protruding portion of the pole piece. For this reason, in the conventional example, the tray for attaching the magnetic material shim is formed in a disk shape in accordance with the inner peripheral surface of the pole piece outer peripheral projection and is set before attaching the gradient coil.
Therefore, after the gradient magnetic field coil is attached, access to the magnetic material shim becomes impossible and the uniformity cannot be adjusted. On the other hand, even after the magnetic field homogeneity adjustment is performed in the factory, since the magnetic field environment generally changes when it is brought into the field, adjustment is usually required again. However, installation of a large and heavy gradient magnetic field coil on site is often limited due to work space and the like.
[0004]
On the other hand, in a vertical magnetic field type magnet device that does not use a pole piece, the uniformity of the static magnetic field generated by the magnet device is exclusively enhanced by an adjustment coil introduced into the magnet device.
Japanese Patent Laid-Open No. 7-250819, which discloses a horizontal magnetic field type magnet device, discloses a technique for arranging a magnetic shim along the wall surface of the central hole of a pair of magnet assemblies constituting the magnet device. ing.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a magnetic shim mounting structure that can achieve even higher magnetic field uniformity and can easily attach and detach a magnetic shim for adjusting the magnetic field uniformity while the gradient coil is attached. It is providing a type | mold magnet apparatus.
[0006]
[Means for Solving the Problems]
A pair of superconducting magnet assemblies that are arranged opposite to each other and generate a uniform static magnetic field space region that covers the portion to be examined between them, each of the pair of superconducting magnet assemblies being A main superconducting coil for generating a magnetic field space region, a tuning superconducting coil for adjusting the magnetic field uniformity of the uniform static magnetic field space region, and the main superconducting coil and the adjusting superconducting coil for accommodating and maintaining the superconducting state A cooling container is provided, and a plurality of magnetic field adjusting means mounting trays are provided on the opposing surfaces of the respective cooling containers, in which magnetic field adjusting means for further adjusting the magnetic field uniformity of the uniform static magnetic field space region is detachably mounted. Each of the magnetic field adjusting means mounting trays can be inserted along the opposing surface of the cooling container, and thus the gradient magnetic field coil is attached. It made it possible to implement a magnetic field homogeneity adjusting work up.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described.
As shown in FIGS. 1 and 2, the open magnet device 10 has a pair of upper and lower magnet assemblies 12 and 14 that face each other, and the pair of upper and lower magnet assemblies 12 and 14 each have a uniform static magnetic field therebetween. The main superconducting coil 18 for generating the space region 16, the adjusting superconducting coil 20 for adjusting the uniformity of the static magnetic field generated by the main superconducting coil 18, the main superconducting coil 18 and the adjusting superconducting coil 20 are made to be below the superconducting temperature. A cooling container comprising a refrigerant container 22 that encloses the refrigerant to be cooled and accommodates these superconducting coils, and a vacuum chamber 24 that encloses the refrigerant container 22 with a nonmagnetic material such as aluminum or stainless steel to prevent convection of heat; And a ferromagnetic plate 26 that surrounds these outer sides and suppresses the magnetic flux leakage from the superconducting coil. The refrigerant containers 22 and the vacuum chambers 24 of the upper and lower magnet assemblies 12 and 14 are connected by a connecting tube 28, and the upper and lower ferromagnetic plates 26 and 26 are supported by a yoke 30 and magnetically. Is bound to.
[0008]
Paying attention to the fact that the magnetic shim can be attached from the side because the pole piece does not exist even when the gradient coil 34 is attached, unlike the vertical magnetic field type magnet that uses the pole piece. By attaching the body shim piece 38 and inserting / removing the tray 36, the magnetic field uniformity can be adjusted even when the gradient coil 34 is attached.
[0009]
A specific shape of the tray 36 is shown in FIG. In this embodiment, fan-shaped trays 36 are arranged radially, and tray guides 40 for positioning the trays 36 are arranged between the trays 36. For this reason, since some space is generated between the trays 36, this space portion can be used to attach the gradient coil 34 to the magnet.
The tray guide 40 is made of a material such as stainless steel or aluminum. Since the tray guide 40 can be fixed to the surface of the vacuum chamber 24 serving as a cooling container by welding, screws, adhesion, or the like, the tray guide 40 can also serve as a reinforcing material for the cooling container by assuming a role as a beam. In order to increase the vertical distance of the magnet opening, it is desirable to make the wall thickness of the cooling vessel as thin as possible, which is also useful in this respect.
Further, a magnetic shim piece 38 is fixed to each tray 36 by using means such as adhesion and screws. This point will be described in detail later with reference to FIGS.
In the case of the tray arrangement according to this embodiment, the magnetic shim pieces 38 can basically be arranged substantially concentrically. On the other hand, as an evaluation function generally used for evaluating the magnetic field distribution, an axially symmetric function is employed. Therefore, in the present embodiment in which the tray 36 can be inserted and removed from the radial direction, the arrangement of the magnetic shims determined based on the evaluation function is easily realized as it is.
[0010]
It should be noted that the radial direction length of the tray 36 ′ where the connecting pipe 28 is provided is shortened. The insertion / removal of the tray 36 'having a short radial dimension is performed by first moving the adjacent tray 36 having a long dimension.
Finally, when the arrangement of the magnetic shim pieces 38 to be mounted on the trays 36 and 36 'is determined, the trays 36 and 36' are in a vacuum chamber through a plurality of fixing bolt holes 42 provided respectively. It is fixed to the bottom surface of 24 with bolts.
[0011]
In this embodiment, since the connecting pipe 28 is in the way, it is difficult to insert and remove the tray 36 ′ with the gradient coil 34 attached. Therefore, when a magnetic material shim necessary for on-site adjustment can be predicted in advance, only this portion can be omitted.
[0012]
FIG. 4 shows a second embodiment of the present invention. In this embodiment, in order to make the shim mounting structure more versatile, rectangular trays 36 are arranged in parallel and parallel. In this case, even when the gradient coil 34 is attached, all trays can be easily inserted and removed.
[0013]
In particular, when the width of the tray 36 is narrowed, the tray 36 can be disposed near the outer periphery of the cooling vessel or the peripheral portion of the connecting pipe 28, and the restriction on the magnetic field distribution generated by the magnetic shim piece 38 is reduced. It is possible and effective. In this case, the number of magnetic shim pieces 38 attached to one tray 36 is reduced. This is advantageous when shimming is performed with a static magnetic field applied. That is, when there is a static magnetic field, an electromagnetic force acts on each magnetic shim piece 38, but if the number of shim pieces per tray decreases, this force acting on the entire tray is weakened. Therefore, the tray 36 can be easily taken in and out even when there is a static magnetic field.
In order to facilitate loading and unloading, the number of shims attached to one tray is determined by the static magnetic field strength, the total number of magnetic shim pieces 38 required for shimming, and the like. In order to reduce the length and weight of the tray attached to the central portion, a structure in which the tray is divided into two at the central portion and inserted from both sides as shown in FIG.
[0014]
More detailed structural examples of the tray and the magnetic shim piece are shown in FIGS. Although the tray used in the embodiment of FIG. 4 is shown here, the same applies to other embodiments. The tray 36 has an elongated plate shape, and its both sides are positioned for insertion by a tray guide 40 attached to the cooling container. The tray guide 40 does not need to be disposed over the entire length of the tray 36, and may be disposed at a critical point so that the tray 36 can be set according to the required positional accuracy. Further, the inserted tray 36 is fixed to the cooling container 24 with screws, bolts or the like through fixing bolt holes 42 provided at several appropriate places as shown in FIG. It goes without saying that fixing bolt holes may be provided in the configuration of FIG.
On the other hand, the magnetic shim piece 38 can be attached to the tray 36 by means such as a fixing screw 44. Further, in place of the fixing screw, it can be attached by adhesion. In this case, in order to obtain the accuracy of the mounting position of the magnetic shim piece 38 with respect to the tray 36, a marking indicating the mounting position is provided on the tray 36 side. This method is effective.
In this embodiment, only one row of shim pieces 38 of different thicknesses is provided on the tray 36. However, how many and how many shim pieces are attached per tray can be arbitrarily determined as necessary.
[0015]
FIGS. 8 and 9 show a modification in which the tray 36 shown in FIGS. That is, the protrusions 46 are provided on the tray 36 to reduce the contact area between the bottom of the tray 36 and the surface of the cooling container, thereby reducing the frictional resistance. Further, a protrusion may be provided on the cooling container, or a groove may be provided on the tray 36 or the cooling container.
As another means, a means for coating the tray surface with a silicon-based lubricant or the like can also be applied.
[0016]
【The invention's effect】
According to the present invention, there are provided a plurality of magnetic field adjustment means mounting trays that detachably mount magnetic field adjustment means on the mutually opposing cooling vessel facing surfaces of a pair of superconducting magnet assemblies arranged vertically opposite to each other. Thus, adjustment by the magnetic field adjustment means is possible with the gradient magnetic field coil still attached. That is, since the uniform magnetic field adjustment can be performed by freely inserting and removing from the side surface along the cooling container facing surface, the efficiency of the shimming operation can be achieved.
[Brief description of the drawings]
FIG. 1 is a schematic external perspective view of an open magnet device to which the present invention is applied.
FIG. 2 is a longitudinal sectional view of FIG.
FIG. 3 is a plan view showing the arrangement of fan-shaped trays in the first embodiment of the present invention.
FIG. 4 is a plan view showing the arrangement of rectangular trays in a second embodiment of the present invention.
FIG. 5 is a plan view in which rectangular trays according to a second embodiment of the present invention are divided and arranged.
6 is a partial perspective view of the tray in FIG. 4;
7 is a cross-sectional view of the magnetic shim and tray viewed from the arrow side of FIG. 6;
FIG. 8 is a view showing a modified example of the tray in FIGS.
9 is a perspective view showing the tray of FIG. 8 upside down. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Open type magnet apparatus 12 Upper magnet assembly 14 Lower magnet assembly 16 Uniform static magnetic field space area 18 Main superconducting coil 20 Adjustment superconducting coil 22 Refrigerant container 24 Vacuum tank, cooling container 26 Ferromagnetic plate 28 Connecting pipe 30 Yoke 32 Magnetic material shim arrangement area 34 Gradient magnetic field coil 36 Tray 38 Magnetic material shim piece 40 Tray guide 42 Fixing bolt hole 44 Fixing screw 46 Projection

Claims (2)

A pair of superconducting magnet assemblies that are arranged opposite to each other and generate a uniform static magnetic field space region that covers a portion to be examined of the subject, and each of the pair of superconducting magnet assemblies includes the uniform static magnetic assembly. A main superconducting coil for generating a magnetic field space region, an adjusting superconducting coil for adjusting the magnetic field uniformity of the uniform static magnetic field space region, and the main superconducting coil and the adjusting superconducting coil for accommodating and maintaining a superconducting state A cooling container is provided, and a plurality of magnetic field adjustment means mounting trays that detachably mount magnetic field adjustment means for further adjusting the magnetic field uniformity of the uniform static magnetic field space region are provided on the opposing surfaces of the respective cooling containers. is and, each of the magnetic field adjustment means mounted tray comprising an open magnet system, characterized in that it is removably mounted along opposite surfaces of the cooling vessel
2. The open type magnet apparatus according to claim 1, wherein the cooling container is formed in a circular shape with opposing surfaces, and the magnetic field adjusting means mounting tray is formed in a fan shape. 2. The open type magnet apparatus according to claim 1, wherein the magnetic field adjusting means mounting tray is inserted and mounted from the outer periphery of the circular opposing surface of the cooling container toward the center thereof.

Images