JP3871789B2 - Passive shield superconducting magnet - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a passive shield superconducting magnet used in a magnetic resonance imaging (hereinafter abbreviated as MRI) apparatus, and more particularly to an improvement of a refrigerator mounting structure using a passive shield superconducting magnet.
[0002]
[Prior art]
Although superconducting magnets are used in various fields, the most practical use of general-purpose superconducting magnets is currently being applied to MRI apparatuses. The MRI apparatus creates a uniform magnetic field with a constant magnetic field strength in a fixed direction in a fixed space by a magnet, and applies various coils (gradient magnetic field coil-MRI to provide a gradient magnetic field to obtain spin position information; high frequency ( RF) coil—irradiates a subject with a high frequency magnetic field, excites nuclei, and receives nuclear magnetic resonance (NMR) signals generated in the subject; etc.) from the subject inserted in the space NMR information is collected, and a cross-sectional view in an arbitrary position and an arbitrary direction of the subject is reconstructed. In general, MRI apparatuses are classified into those in the horizontal direction and those in the vertical direction according to the direction of the uniform magnetic field.
[0003]
Conventionally, in a medical MRI apparatus, a horizontal magnetic field method is generally used as a superconducting magnet. However, in recent years, an open-type permanent magnet MRI apparatus using a permanent magnet as a magnet and having a uniform magnetic field in the vertical direction has been put into practical use, and because of its openness, its value has been widely recognized all over the world. Therefore, at present, an open type (vertical magnetic field type) MRI apparatus based on a superconducting method capable of generating a high magnetic field intensity that cannot be achieved by a permanent magnet is being developed.
[0004]
In a superconducting magnet, a cooling method of a superconducting coil for generating a uniform magnetic field is an important technique. The following four types of cooling methods for superconducting magnets are generally performed. (Hideki Nakagome; Superconducting magnet using a refrigerator: 1st Superconductivity / Low Temperature Young Seminar) Title: Basic theory of superconductivity / Basics of devices and principles / applications of refrigerators, p.14-20, 1996 (July 25-27, sponsored by the Low Temperature Engineering Association Tohoku / Hokkaido Branch)
(1) Open type: A cooling method using only a refrigerant (liquid helium or the like), and cooling is performed by immersing the superconducting coil in the refrigerant in the refrigerant container.
(2) Babysitter type; the superconducting coil itself is directly cooled with a refrigerant in the same manner as in (1), and a two-layer heat shield for housing the refrigerant container among the cooling containers is set at two temperature levels (for example, a cryogenic refrigerator) , 77K and 20K).
(3) Direct cooling type: The superconducting coil itself is directly cooled with a refrigerant as in (1), but the refrigerant evaporated in the cryogenic refrigerator is liquefied again and returned to the refrigerant container.
(4) Refrigerator direct type: The temperature of the tip of the cryogenic refrigerator is made lower than the critical temperature of the superconducting coil, and the superconducting coil is cooled directly (or through a good heat conductor).
In the above cooling method, initially, the method (1) occupied the mainstream of cooling of the superconducting MRI apparatus. However, in recent years, as the performance of the cryogenic refrigerator is improved, the methods (2) to (4) Are gradually being used.
[0005]
[Problems to be solved by the invention]
In a vertical magnetic field type superconducting magnet, if an attempt is made to increase the magnetic field strength of a uniform magnetic field, the leakage of the magnetic field increases, and how to prevent the leakage of the magnetic field becomes a problem. In Japanese Patent Application No. 8-165317 and Japanese Patent Application Laid-Open No. 5-234746, as shown in FIG. 8, a ferromagnetic magnetic field generating source 1 composed of a pair of superconducting coils 2 and a cooling vessel 3 is arranged vertically. A vertical magnetic field superconducting magnet having a structure surrounded by a magnetic shield plate 4 made of a body and a yoke 5 made of a ferromagnetic material that supports the magnetic shield plate is disclosed. In the superconducting magnet having this structure, the static magnetic field generating source 1, the magnetic shield plate 4 and the yoke 5 constitute a magnetic circuit, and the magnetic field generated by the static magnetic field generating source 1 is prevented from leaking outside the superconducting magnet as much as possible. ing. Such a superconducting magnet is called a passive shield type superconducting magnet.
[0006]
When an attempt is made to construct a high magnetic field vertical magnetic field MRI apparatus using this passive shield superconducting magnet, the magnetic shield plate 4 must be thickened and the yoke 5 thickened in order to reduce the leakage magnetic field. As a result, a large amount of ferromagnetic material is required, and the apparatus becomes large. When trying to reduce the size and size of the device, the leakage magnetic field may be insufficiently reduced.
[0007]
On the other hand, a cryogenic refrigerator (hereinafter referred to as a refrigerator) used for cooling a superconducting magnet compresses gas evaporated from a refrigerant such as liquid helium on the high temperature side and expands it on the low temperature side, thereby increasing the entropy of the gas. The heat energy is changed to move the heat energy from the low temperature side to the high temperature side to cool the tip (low temperature side) of the refrigerator. The portion that compresses the gas on the high temperature side is called a compressor, and this compressor uses a piston to compress the gas. An induction motor is used to drive the piston. When the magnetic body in the induction motor is exposed to a DC leakage magnetic field, the magnetic body may be saturated, and the motor may not operate.When this magnetic body is exposed to an AC leakage magnetic field, the rotational frequency of the motor may be reduced. There is a problem that it may be disturbed, and it may cause a malfunction in installing the refrigerator at a position where the leakage magnetic field is large.
[0008]
As described above, the arrangement of the refrigerator in the superconducting magnet is an important issue, and depending on the arrangement, the performance of the refrigerator cannot be sufficiently exhibited, and the performance of the superconducting magnet may be affected. In addition, the installation position in the case where the refrigerator is installed in the vertical magnetic field type passive shield superconducting magnet which is the subject of the present invention has been hardly studied.
Therefore, an object of the present invention is to provide a passive shield superconducting magnet in which a leakage magnetic field does not adversely affect the refrigerator.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a passive shield superconducting magnet according to the present invention comprises a set of superconducting coils arranged facing each other in the vertical direction, and a set of cooling containers for storing and cooling the respective superconducting coils. It has a static magnetic field generation source, magnetic shield plates made of a ferromagnetic material are arranged on the upper and lower outer sides of the static magnetic field generation source, and the upper and lower magnetic shield plates are coupled by a yoke made of one or more ferromagnetic materials. In the passive shield type superconducting magnet, the refrigerator for cooling the cooling vessel is on the upper magnetic shield plate and from the joint portion of the upper magnetic shield plate and the yoke (hereinafter referred to as shield plate / yoke joint portion). It is attached at a remote location and is connected to the cooling vessel through the upper magnetic shield plate .
[0010]
In the passive shield type superconducting magnet, on the upper magnetic shield plate, the leakage magnetic field at the periphery of the shield plate / yoke junction increases, and the leakage magnetic field decreases at a position away from it. By mounting and arranging, the adverse effect on the refrigerator due to the leakage magnetic field can be reduced. Moreover, since it is attached to a position where the leakage magnetic field is small, the operation reliability of the refrigerator is improved and the superconducting coil is reliably cooled.
[0011]
In the passive shield type superconducting magnet of the present invention, the number of the yokes is one, and the refrigerator is attached to a central portion of the upper magnetic shield plate or a position further away from the shield plate / yoke joint portion than the central portion. is not that. In this configuration, there is one shield plate / yoke joint portion, and the leakage magnetic field in the peripheral portion becomes large. Therefore, the leakage magnetic field can be reduced by moving the refrigerator mounting position to the center of the upper magnetic shield plate or a position farther from the shield plate / yoke joint portion.
[0012]
Furthermore a passive shield type superconducting magnet of the present invention, the is a yoke 2 or more, that is attached to a substantially vertical bisector of a segment connecting the shield plate yoke junction where the refrigerator is adjacent. When there are two or more yokes, the leakage magnetic field becomes the smallest at a position approximately equidistant from the adjacent shield plate / yoke joint portion. The adverse effect on the refrigerator can be greatly reduced.
[0013]
Furthermore a passive shield type superconducting magnet of the present invention, the refrigerator is installed in a on the fixed plate made of non-magnetic material provided on said upper magnetic shield plate. In this configuration, since the fixed plate is inserted between the refrigerator and the upper magnetic shield plate, the fixed magnetic plate is separated from the superconducting coil by the thickness of the fixed plate, and the leakage magnetic field strength is reduced. It is also possible to process the fixing plate and firmly fix the refrigerator to this.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 shows a first embodiment of the passive shield superconducting magnet of the present invention. FIG. 1A is a longitudinal sectional view, and FIG. 1B is a top view. In FIG. 1A, a superconducting magnet includes a static magnetic field generation source 1 that generates a vertical magnetic field B ₀ in a uniform magnetic field region 10 and a magnetic shield plate 4 that constitutes a return path of magnetic flux lines generated by the static magnetic field generation source 1. And a yoke 5, a refrigerator 12 that cools the static magnetic field generation source 1, and a fixing portion 13 that fixes the static magnetic field generation source 1 to the magnetic shield plate 4.
[0016]
The static magnetic field generation source 1 is composed of a set of superconducting coils 2 disposed above and below a uniform magnetic field region 10 and a set of cooling containers 3 that house and cool the superconducting coils 2. The upper and lower superconducting coils 2 are each composed of one or more coils, and generate a high magnetic field with good uniformity in the uniform magnetic field region 10 by appropriately arranging the coils. The cooling container 2 includes a refrigerant container that stores a superconducting refrigerant (such as liquid helium) that holds the superconducting coil 2 in a superconducting state, a heat shield that surrounds the refrigerant container, and a vacuum container that surrounds the heat shield.
[0017]
Both the magnetic shield plate 4 and the yoke 5 serving as the return path of the magnetic flux lines 14 are made of a ferromagnetic material such as iron. The magnetic shield plates 4 are arranged substantially horizontally above and below the static magnetic field generation source 1. Hereinafter, the upper one is referred to as the upper magnetic shield plate, and the lower one is referred to as the lower magnetic shield plate. The cooling container 2 is fixed to the magnetic shield plate 4 by a fixing portion 13. The magnetic shield plate 4 and the yoke 5 are joined at the end of the yoke 5. Hereinafter, this joining portion is referred to as a shield plate / yoke joining portion. In this embodiment, the upper magnetic shield plate 4 is supported by two yokes 5, and the distance between the upper and lower magnetic shield plates 4 and the interval between the cooling containers 3 are maintained by the length of the yoke 5. By configuring the return path of the magnetic flux lines 14 in this way, most of the magnetic flux 14 generated by the superconducting coil 2 is transferred from the uniform magnetic field region 10 to the upper superconducting coil 2, the upper magnetic shield plate 4, the upper shield plate, It returns to the uniform magnetic field region 10 via the yoke joint portion 11, the yoke 5, the lower shield plate / yoke joint portion 11, the lower magnetic shield plate 4, and the lower superconducting coil 2.
[0018]
FIG. 2 shows an outline of the magnetic flux distribution in the passive shield superconducting magnet of this embodiment. According to FIG. 2, the magnetic flux 14 generated from the uniform magnetic field region 10 passes through the upper superconducting coil 2 and then splits in the left and right directions in the upper magnetic shield plate 4 and passes through the left and right shield plate / yoke joint portions 11. It can be seen that the left and right yokes 5 are entered. For this reason, the magnetic field strength is high in the peripheral portion of the shield plate / yoke joint portion 11 on the upper magnetic shield plate 4, and the central portion is a place where the magnetic field strength is low. This is because the two yokes 5 are arranged so that the magnetic flux 14 generated in the superconducting coil 2 is divided into left and right at the central portion of the upper magnetic shield plate 4 and concentrated toward the left and right shield plate / yoke joint portions 11. It is thought to go. In consideration of the distribution of the magnetic flux lines, in the present embodiment, the refrigerator 12 is disposed on the upper magnetic shield plate 4 in the center thereof.
[0019]
This will be described with reference to FIG. FIG.1 (b) is the figure which looked at the superconducting magnet of a present Example from the top. Inside the upper magnetic shield plate 4, the magnetic flux lines 14 are directed to the left and right shield plate / yoke joint portions 11 with the vertical center line 16 in the figure as a boundary. For this reason, the portion on the center line 16 on the upper magnetic shield plate 4 is a portion having the least leakage magnetic field. The center line 16 is the position farthest from the shield plate / yoke joint part 11 and corresponds to a perpendicular bisector of a line segment 15 connecting the two shield plates / yoke joint parts 11. From the above, if the refrigerator 12 is arranged on or near the center line 16 on the upper magnetic shield plate 4, the leakage magnetic field in this portion is minimized, and the adverse effect of the magnetic field on the refrigerator is minimized. be able to.
[0020]
FIG. 3 shows a second embodiment of the passive shield superconducting magnet of the present invention. FIG. 3 is a top view of the superconducting magnet. This embodiment is a superconducting magnet having two yokes 5, but the refrigerator 12 is on the perpendicular bisector 16 of the line segment 15 connecting the two shield plate / yoke joint portions 11. The shield plate 4 is arranged not at the center but at the periphery. By disposing the refrigerator 12 around the upper magnetic shield plate 4, the access to the refrigerator 12 is improved during maintenance of the refrigerator 12, and the burden on the repairer is reduced.
[0021]
FIG. 4 shows a third embodiment of the passive shield superconducting magnet of the present invention. In this embodiment, a fixed plate 15 is inserted between the upper magnetic shield plate 4 and the refrigerator 12. The material of the fixed plate 15 is a non-magnetic material, and the refrigerator 12 can be separated from the superconducting coil 2 by appropriately increasing the thickness thereof. As a result, since the leakage magnetic field at the position where the refrigerator 12 is installed can be reduced, the adverse effect of the magnetic field on the refrigerator can be further reduced. Further, since the fixing plate 15 can be processed into a structure that can easily fix the refrigerator 12, fixing the refrigerator 12 via the fixing plate 12 is more directly fixing to the upper magnetic shield plate 4. Also, the vibration resistance and the like can be improved.
[0022]
FIG. 5 shows a fourth embodiment of the passive shield superconducting magnet of the present invention. In this embodiment, one yoke 5 is used. FIG. 6 shows an outline of the magnetic flux distribution of the superconducting magnet when there is one yoke 5. According to FIG. 6, most of the magnetic flux 14 generated from the uniform magnetic field region 10 passes through the upper superconducting coil 2, and then proceeds to the left side where the shield plate / yoke joint portion 11 is located in the upper magnetic shield plate 4. The yoke 5 is entered through the plate / yoke joint portion 11. For this reason, on the upper magnetic shield plate, the leakage magnetic field is large at the periphery of the shield plate / yoke joint part 11, and the leakage magnetic field is small at the center or at a position farther from the shield plate / yoke joint part 11 than the central part. It has become. In consideration of the distribution of the magnetic flux lines, in the present embodiment, the refrigerator 12 is arranged on the upper magnetic shield plate 4 at a position farther from the shield plate / yoke joint part 11 than the central portion or the central portion. It is a thing.
[0023]
Also, in the superconducting magnet of one yoke of the fourth embodiment, the refrigerator 12 is arranged around the upper magnetic shield plate 4 as in the second embodiment, or as in the third embodiment. It is possible to insert the fixed plate 15 between the refrigerator 12 and the upper magnetic shield plate 4, and the same effect as that obtained in each embodiment can be obtained.
[0024]
In a passively shielded superconducting magnet having three or more yokes 5, the magnetic flux distribution in the upper magnetic shield plate 4 between two adjacent yokes 5 is the magnetic flux distribution of the superconducting magnet having two yokes 5. On the upper magnetic shield plate 4, the leakage magnetic field is reduced on the vertical bisector connecting the two adjacent shield plate / yoke joint portions 11. Therefore, similarly to the first embodiment, the refrigerator 12 is arranged on the upper magnetic shield plate 4 on a substantially perpendicular bisector connecting two adjacent shield plate / yoke joint portions 11. It is effective in reducing the adverse effect of the magnetic field on the refrigerator.
[0025]
FIG. 7 shows a fifth embodiment of the passive shield superconducting magnet of the present invention. In this embodiment, an example of a connection structure between the refrigerator 12 and the cooling container 2 is shown. FIG. 7 shows a cross-sectional view of the main part of the present embodiment. In FIG. 7, the refrigerator 12 is disposed on the upper magnetic shield plate 4, and the upper cooling container 3 is supported below the upper magnetic shield plate 4. A hole 25 is provided in the upper magnetic shield plate 4, and the cooling container 3 and the refrigerator 12 are connected through the hole 25. The cooling container 3 includes a refrigerant container 21 that stores a refrigerant such as liquid helium, two-layer heat shields 22 and 23 that store the refrigerant container 21, and a vacuum container 24 that stores the heat shields 22 and 23. The coil 2 is immersed in the refrigerant in the refrigerant container 21 and cooled to the superconducting state. The refrigerator 12 is a cryogenic refrigerator having cooling stages of two temperature levels, and the cooling method is a babysitter type. The refrigerator 12 includes a cooler main body portion 26, a first stage cooling portion 27, and a second stage cooling portion 28. The first stage cooling portion 27 passes through a pipe 29 to the inner heat shield 22 and the second stage cooling portion. 28 are respectively connected to the outer heat shield 23 through the pipe 30. At this time, each heat shield is cooled to a different temperature, for example, the inner heat shield 22 is cooled to 20K, and the outer heat shield 23 is cooled to 80K. The refrigerator 12 as a whole is connected to the vacuum vessel 24 through a pipe 31. By configuring the cooling method in this way, the refrigerant in the refrigerant container 21 is cooled through the cooling of the heat shields 22 and 23, and the superconducting coil 2 is maintained in the superconducting state. In addition, what is necessary is just to connect about the vacuum container 24, the heat shields 22 and 23, and the refrigerant | coolant container 21 with a connecting pipe etc. between an upper cooling container and a lower cooling container.
[0026]
The tube 31 connected to the vacuum vessel 24 and the hole 25 therethrough are securely covered with a ferromagnetic material or the like, or the hole 25 is made small so that the magnetic field does not leak onto the upper magnetic shield plate 4. The magnetic field is prevented from adversely affecting the induction motor or the like in the refrigerator main body 26.
[0027]
In the present embodiment, the case where the babysitter type cooling method is adopted as the cooling method of the cooling container 3 by the refrigerator 12 is described. However, the present invention is not limited to this, and the direct cooling type cooling method or the refrigerator direct method is used. It goes without saying that a cold type cooling method may be adopted.
[0028]
【The invention's effect】
As described above, according to the present invention, in the passive shield superconducting magnet, since the refrigerator is installed at a position where the leakage magnetic field on the upper magnetic shield plate is small, the adverse effect of the magnetic field on the refrigerator can be eliminated.
[Brief description of the drawings]
FIG. 1 shows a first embodiment of a passively shielded superconducting magnet according to the present invention.
FIG. 2 is an outline of magnetic flux distribution in the superconducting magnet of the first embodiment.
FIG. 3 shows a second embodiment of the passive shield superconducting magnet of the present invention.
FIG. 4 shows a third embodiment of the passive shield superconducting magnet of the present invention.
FIG. 5 shows a fourth embodiment of the passive shield superconducting magnet of the present invention.
FIG. 6 is an outline of magnetic flux distribution of a superconducting magnet in the case of one yoke.
FIG. 7 shows a fifth embodiment of the passive shield superconducting magnet of the present invention.
FIG. 8 is a structural example of a passive shield superconducting magnet.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Static magnetic field generation source 2 Superconducting coil 3 Cooling container 4 Magnetic shield plate 5 Yoke 10 Uniform magnetic field area 11 Shield plate / yoke joint part 12 Refrigerator 13 Fixed part 14 Magnetic flux line 15 Connecting two adjacent shield plates / yoke joints Line 16 Vertical bisector 21 Refrigerant vessel 22, 23 Heat shield 24 Vacuum vessel 25 Hole 26 Refrigerator main body 27 First stage cooling unit 28 Second stage cooling unit 29, 30, 31 Tube

Claims (4)

It has a static magnetic field generation source composed of a pair of superconducting coils arranged opposite to each other in the vertical direction and a pair of cooling containers for storing and cooling the respective superconducting coils, and above and below the static magnetic field generation source. In a passively shielded superconducting magnet in which a magnetic shield plate made of a ferromagnetic material is disposed and the upper and lower magnetic shield plates are coupled by a yoke made of one or more ferromagnetic materials,
Refrigerator for cooling the cooling container, on the upper magnetic shield plate, and the joint portion of the the upper side magnetic shield plate yoke (hereinafter, referred to as the shield plate yoke junction) is attached at a position away from the And a passively shielded superconducting magnet, which is connected to the cooling vessel through the upper magnetic shield plate .   2. The passive shield superconducting magnet according to claim 1, wherein the number of the yokes is one, and the refrigerator is located at a central portion of the upper magnetic shield plate or at a position farther from the shield plate / yoke joint portion than the central portion. A passively shielded superconducting magnet characterized by being attached.   2. The passive shield type superconducting magnet according to claim 1, wherein the number of the yokes is two or more, and the refrigerator is mounted on a substantially perpendicular bisector connecting the adjacent shield plate / yoke joint portions. A passively shielded superconducting magnet. In the passive shield type superconducting magnet according to any one of claims 1 to 3, characterized in that the refrigerator is mounted on a fixed plate made of non-magnetic material which is provided on the upper shield plate Passive Shielded superconducting magnet.

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