JPH0464338A - Magnet for magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To enable the carrying into an existing hospital building and installation therein with ease with a compact shape, a less weight and a low magnetic field leakage while achieving a highly uniform magnetic field by a method wherein a disc-shaped end lid is made up of a magnetic disc with a center hole and a non-magnetic ring joined inside the center hole and the magnetic disc is joined to a cylindrical shell while the non-magnetic ring joined to a normal temperature bore cylinder to form a vacuum container as a whole. CONSTITUTION:A container comprising a cylindrical shell 3, a disc-shaped end lid 4 and a normal-temperature bore cylinder 91 acts as vacuum container of a magnet 2 as in the past. On the other hand, as regard to a magnetic shield, a portion comprising the cylindrical shell 3 and a magnetic disc 14 functions as yoke magnetic shield. Under a situation with a length of a superconductive coil 12 and a shape of a cylindrical shell 3 determined, a width of the non- magnetic ring is adjusted, that is, an open diameter of the magnetic disc 14 is adjusted to be optimized. This enables the lowering of a magnetic field leakage in a magnetic circuit comprising the superconductive coil 12, the cylindrical shell 3 and the magnetic disc 14 while the formation of a highly uniform magnetic field.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to magnetic resonance (MR)
The so-called computed tomography (CT) is based on the density distribution of specific nuclear spins in a specific cross section of a living subject using the e5onance phenomenon.
CT image (Comp
ted tomogram) as an image (I mag
iB) A magnet for a magnetic resonance imaging device.

(Prior Art) Conventional magnets for magnetic resonance imaging devices are described in detail in, for example, Japanese Patent Application Laid-Open No. 61-252613 MR Magnet Shield, so a brief explanation will be provided here using FIG. 2. .

In FIG. 2, the iron yoke magnetic shield 1 includes a cylindrical shell 3 surrounding a static field magnet 2, two disc-shaped end caps 4 and legs 5 attached to both ends of the cylindrical shell. It is composed of. The iron yaw magnetic shield 1 is made of a magnetic material such as electromagnetic soft iron.

The static magnetic field magnet 2 is housed inside the iron yoke magnetic shield 1, and the magnetic center 6 of the static magnetic field magnet and the vertical axis ml of the cylindrical shell 3! 7 are arranged coaxially.

Next, the effects of the iron yoke magnetic shield configured as described above will be described. The static field magnet 2 generates a magnetic flux 8 as shown. The magnetic flux 8 exiting the normal temperature bore 9 of the static field magnet is absorbed by the iron yoke magnetic shield 1 made of a magnetic material such as iron.

The amount of magnetic flux that an iron magnetic shield can absorb is limited by the magnetic saturation properties of the magnetic material. The magnetic flux that cannot be absorbed leaks to the outside of the magnetic shield and becomes a so-called leakage magnetic field.

When an iron yoke magnetic shield is attached to a static field magnet, the above-mentioned leakage magnetic field is significantly reduced compared to a static field magnet alone. Figure 3 shows this situation.

FIG. 3 shows the 5 Gauss leakage magnetic field distribution in the case of only a 5000 Gauss static magnetic field magnet and in the case of an iron yoke magnetic shield. As is clear from this figure, when the iron yoke magnetic shield is attached, the 5 Gauss leakage magnetic field area is halved.

However, the iron yoke magnetic shield type magnet, which has the above-mentioned advantages, has a major drawback. In other words, in order to reduce leakage magnetic fields, large amounts of magnetic materials such as iron are used, which increases the weight and external dimensions, making it difficult to install in existing hospital buildings in many cases. I'll come.

Therefore, a magnetic vacuum container type magnet was proposed in which the vacuum container of the static magnetic field magnet 2 and the iron yoke magnetic shield were integrated. Regarding this type of conventional technology, for example, Japanese Patent Application Laid-open No. 6
1-159714 r Superconducting Magnet”. Here, the outline will be explained using FIG. 4.

In FIG. 4, the vacuum container of the magnet 2 is made of a magnetic material such as electromagnetic soft iron, and also serves as a yoke magnetic shield. Hereinafter, this will be referred to as the magnetic vacuum container 10.

The magnetic vacuum container 10 includes a magnetic cylindrical shell 3 and two magnetic cylindrical shells 4 and F attached to both ends of the cylindrical shell.
It consists of a room-temperature bore hole made of non-magnetic material such as RP (fiber reinforced plastic).

The structure between these three components is such that a vacuum state can be maintained inside the container by means such as welding or a vacuum O-ring.

The internal configuration of this magnetic vacuum container 10 is the same as the internal configuration of a conventional magnet. That is, the superconducting coil I2 is wound around the inner cylinder, ie, the bobbin, of the liquid helium container 11 filled with liquid helium. In order to maintain the liquid helium container at an extremely low temperature, the liquid helium container 11 is surrounded by a double radiation shield plate 13 for shielding heat from entering from the outside.

These, liquid helium container 11, radiation shield plate 13,
The space between the magnetic vacuum containers 10 is in a high vacuum state and is filled with a multilayer insulation material, which blocks heat from the outside and maintains the liquid helium container at an extremely low temperature.

Next, the functions and effects of the conventional magnetic vacuum container magnet configured as described above will be described.

The magnetic flux generated from the superconducting coil 12 is transferred to the magnetic vacuum vessel 10.
The action and effect of significantly reducing the leakage magnetic field absorbed by the magnetic field are the same as those explained in the prior art with reference to FIGS. 2 and 3.

The advantage of the magnetic vacuum container 10 is that since the magnetic vacuum container and the iron yoke magnetic shield are integrated, the external dimensions are reduced to make it more compact and the weight is reduced. This will facilitate installation in existing hospital buildings and accelerate the spread of magnetic resonance imaging devices.

However, the conventional magnet for magnetic resonance imaging apparatus configured in this manner has the following problems.

The essence of a magnet for a magnetic resonance imaging device is the high uniformity of its magnetic field.

Adding a yoke magnetic shield to a magnet reduces the leakage magnetic field, but because the magnetic flux distribution is disturbed, it is more difficult to achieve a highly uniform magnetic field than with a magnet without a magnetic shield.

Therefore, in the past, many studies have been conducted on superconducting coils and yoke magnetic shield arrangements to achieve a highly uniform magnetic field.

(Problems to be Solved by the Invention) Generally, in order to achieve a highly uniform magnetic field in a situation where a superconducting coil and a yoke magnetic shield are combined, it is necessary to: ■ Minimize the magnetic interaction between the coil and the yoke magnetic shield as much as possible. Place them apart.

■ Expand the opening diameter II D ++ of the disc-shaped end cap as much as possible.

etc. are required.

By the way, when the magnet and yoke magnetic shield shown in FIG. 2 are separated, the arrangement and shape of the superconducting coil and yoke magnetic shield can be easily optimized so as to satisfy the above conditions (1) and (2).

On the other hand, in the case of the magnetic vacuum vessel shown in Fig. 4, the arrangement of the superconducting coil and the magnetic vacuum vessel is determined by the thermal design of the magnet.
Usually, the superconducting coil and the magnetic vacuum container are placed close to each other.

Furthermore, since the disc-shaped end cap forms a part of the vacuum vessel, its diameter must necessarily be the same as the normal temperature bore diameter, and it is impossible to enlarge the opening diameter. .

For the above reasons, the conventional magnetic vacuum container magnet shown in FIG. 4 has a fatal drawback for magnetic resonance imaging apparatuses, in that the magnetic field uniformity deteriorates.

Therefore, an object of the present invention is to eliminate the drawbacks of conventional magnetic vacuum container magnets, achieve a highly uniform magnetic field, and at the same time make the magnet compact, lightweight, and have a low leakage magnetic field, so that it can be easily transported into existing hospital buildings. An object of the present invention is to provide a magnet for a magnetic resonance imaging device that can be installed.

[Structure of the invention]

(Means for Solving the Problems) The magnet for magnetic resonance imaging apparatus according to the present invention is configured as follows in order to solve the above problems and achieve the objects. That is, as shown in FIG. 1, the magnetic vacuum container 10 that integrates the vacuum container of the magnet 2 and the yoke magnetic shield consists of a magnetic cylindrical shell 3 and two disks attached to both ends of this cylindrical shell. The disc-shaped end cover 4 is composed of a normal-temperature bore cylinder 91 made of a non-magnetic material such as M4 and FRP, and the disc-shaped end cover 4 includes a magnetic disc 14 having a center hole and a non-magnetic pipe bonded to the inside of the center hole. The magnetic disc 14 is connected to the cylindrical shell 3 and the non-magnetic ring 1 by means of welding or the like.
5 are each joined to a normal temperature bore tube 91, forming a vacuum container as a whole.

(Function) With the above configuration, the portion corresponding to the yoke magnetic shield of the magnetic vacuum container consists of the cylindrical shell 3 and the magnetic disk 14, so the width of the non-magnetic ring 15 can be set arbitrarily. By doing so, the diameter of the opening of the disc-shaped end cap can be made substantially larger, and De can also be arbitrarily adjusted to match the shape of the superconducting coil and the shape of the cylindrical shell to obtain a highly uniform magnetic field. You can choose.

Therefore, a highly uniform magnetic field can be achieved while maintaining the compactness that characterizes magnetic vacuum container magnets.

(Example) An example of the magnet for magnetic resonance imaging apparatus of the present invention will be described below with reference to FIG.

(Configuration of Example) FIG. 1 is a diagram showing the configuration of this embodiment.

The magnetic vacuum container 10, which integrates the function of a vacuum container and the function of a yoke magnetic shield, has a cylindrical shell 3 made of electromagnetic soft iron and two disk-shaped ends attached to both ends of this cylindrical shell. It is composed of a lid 4 and a normal temperature bore cylinder 91 made of FRP.

The disc-shaped end cap 4 consists of a magnetic disc 14 made of an electromagnetic soft iron layer and a non-magnetic ring 15 made of stainless steel, and these two members are welded at a joint 16.

The magnetic disk 14 is joined to the cylindrical shell 3 by welding. On the other hand, the non-magnetic ring 5 is joined to a normal temperature bore cylinder 91 made of FRP via a vacuum O-ring (not shown).

A vacuum container is formed by welding these members and forming a ring structure. The internal structure of the magnetic vacuum container 10 is the same as a conventional magnet configuration. That is, a superconducting coil 12 is wound around the inner cylinder or bobbin of a liquid helium container 11 filled with liquid helium, and in order to maintain this liquid helium container 11 at an extremely low temperature, it is shielded from heat entering from the outside. A double radiation shield plate 13 and a multilayer heat insulating material (not shown) are arranged.

(Operation of Example) Next, the operation of the magnet for a magnetic resonance imaging apparatus of this example configured as described above will be explained.

Cylindrical shell 3, disc-shaped end cap 4, and normal temperature bore tube 91
The container conventionally serves as a vacuum container for the magnet 2.

On the other hand, regarding the magnetic shield, the portion consisting of the cylindrical shell 3 and the magnetic disk 14 functions as a yoke magnetic shield. By adjusting the width of the non-magnetic ring under the conditions where the length of the superconducting coil 12 and the shape of the cylindrical shell 3 are determined, in other words, by adjusting and optimizing the opening diameter of the magnetic disk 14, superconductivity can be achieved. coil 12, cylindrical shell 3,
In the magnetic circuit formed by the magnetic disk 14, leakage magnetic fields can be reduced and a highly uniform magnetic field can be formed.

(Effects of Example) As explained above, according to this example, the characteristics of compactness, weight reduction, and low leakage magnetic field of the magnetic vacuum container are maintained, while the high uniformity required for magnetic resonance imaging equipment is maintained. A magnet that can generate a magnetic field can be provided.

(Other examples) Next, another embodiment of the present invention will be described.

In FIG. 1, instead of the room-temperature bore cylinder 91 made of FRP, a room-temperature bore made of stainless steel, which is the same material as the non-magnetic ring 15, is used. Thereby, the room-temperature bore tube 91 and the non-magnetic ring 15 can be welded together, which facilitates manufacturing. The functions and effects of this embodiment are the same as those of the first embodiment.

In addition, in FIG. 1, the magnetic disk 14 and the non-magnetic ring 15 are
uses a welded configuration, but any bonding method that maintains a vacuum can be applied.

Furthermore, the coil may be normal conductive instead of superconducting.

〔Effect of the invention〕

According to the present invention, it is possible to eliminate the deterioration of magnetic field uniformity, which is a drawback of conventional magnetic vacuum container magnets, and achieve a highly uniform magnetic field. It is possible to provide a magnet for a magnetic resonance imaging device that can be easily transported and installed in an existing hospital building by taking advantage of leakage magnetic field.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an embodiment of the magnet for magnetic resonance imaging apparatus of the present invention, Fig. 2 is a perspective view showing a conventional yoke magnetic shield type magnet, Fig. 3 is a leakage magnetic field distribution diagram, and Fig. 4 is a diagram illustrating the leakage magnetic field distribution. FIG. 2 is a configuration diagram showing a conventional magnetic vacuum container magnet. 1... Iron yoke magnetic shield, 2... Static magnetic field magnet, 3... Cylindrical shell, 4...
disc-shaped end cap, 5... leg portion, 6... magnetic center,
7... Vertical axis line, 8... Magnetic flux,
9... Room temperature bore 10... Magnetic vacuum container, 11
...Liquid helium container, 12...Superconducting coil, 1
3... Radiation shield plate, 14... Magnetic plate,
15...Nonmagnetic ring, 16...Joint part. 91...Normal temperature bore tube. Agent: Patent Attorney Noriyuki Chika

Claims (4)

[Claims] (1) A cylindrical shell made of a magnetic material, a magnetic disk connected to both ends of this shell and having a center hole, a nonmagnetic ring bonded to the center hole of this disk, and A magnet for a magnetic resonance imaging apparatus, characterized in that a coil for generating a magnetic field is housed in an annular space formed by a room-temperature bore tube attached to the inside. (2) Claim (1) characterized in that the shell and disc are made of iron, the ring is made of stainless steel, and the room temperature bore tube is made of FRP.
The magnet for the magnetic resonance imaging device described above. (3) The magnet for a magnetic resonance imaging apparatus according to claim (1), wherein the shell and the disc are made of iron, the ring is made of stainless steel, and the room-temperature bore tube is made of stainless steel. (4) The magnet for magnetic resonance imaging apparatus according to claim (2) or (3), wherein the disk and the ring are joined by welding.