JPH04132539A - Even magnetic field magnet for magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To obtain an inexpensive uniform magnetic field magnet with higher magnetic moment and magnetic field intensity as well as less magnetic flux leakage by arranging a heat insulating vacuum container surrounding the outer circumferential side of a pair of pole pieces and a superconductive excitation coil housed therein each for one pair of pole pieces. CONSTITUTION:A pair of whole pieces 7A and 7B or the like 7 supported by a yoke 9 is each provided with a magnetic field intensity equalizing means 10 comprising a ring-shaped projecting part 10A projected to the side of a space part 6 and a recessed part 10B formed concentric on the inner circumferential side thereof. A superconductive excitation coil 20 formed in a ring is provided on each outer circumference of the pair of whole pieces 7A and 7B surrounding it. Moreover, it is so arranged that an even magnetic field 6A with a flux density B0 is generated in an area as wide as possible of the space part 6 between the opposed whole pieces. This allows a superconductive coil 11 with a small section to secure a sufficient distance with respect to the yoke 9 and the whole piece 7 easily to achieve a higher magnetic moment while the existence of the yoke results in a less flux leakage.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a core-type uniform magnetic field magnet 7) having a superconducting coil used in a (nuclear) magnetic resonance imaging system W (hereinafter abbreviated as MRr). .

[Conventional technology]

In the MHI, a magnetic field strength of 0.1 T to 2 T is applied within a spherical region with a diameter of approximately 50 cm approximately at the center of a space that accommodates a human body.

A uniform magnetic field magnet is required that can generate a highly uniform static magnetic field with a uniformity of magnetic field strength of 10 pp- or less.

Among this type of uniform magnetic field magnet, the magnetic field strength is 0.2
For magnets smaller than T, type 1 magnets are generally used. Furthermore, superconducting magnets are used as magnets with high magnetic field strengths exceeding 0.5 T.

FIG. 7 is a simplified cross-sectional view of a conventional superconducting uniform magnetic field magnet. The superconducting coil 1 formed as a plurality of pairs of Herl-5-Holtz coils is stored as a refrigerant in a liquid helium container 2C of an insulated vacuum container 2. It is stored immersed in liquid helium 3. The adiabatic vacuum volume 252 has a vacuum between the liquid helium container 2C and the vacuum container 2A, and a radiation shield 2B.

The superconducting coil 1 is insulated by a multi-layer heat insulating layer (not shown), etc., and the superconducting coil 1 is injected from the service port 4A to cool the superconducting coil 1 to the liquid helium temperature.
By supplying an excitation current to the space 6, a static magnetic field BO passing through the space 6 in the axial direction is generated, and a uniform magnetic field 6A is formed approximately at the center of the space 6. Note that the refrigeration unit 115 is connected to the radiation shield 2B, etc. within the service boat 4B, and its insulation performance is maintained at a high level.

On the other hand, normal-conducting uniform magnetic field magnets are broadly classified into air-core magnets consisting of multiple pairs of Helmholtz coils similar to the above-mentioned superconducting uniform-field magnets, and core-type uniform magnetic field magnets.

8 and 9 are a partially fragmented side view and a plan view showing a core-type normal S-conducting homogeneous magnetic field magnet, in which a pair of circles are arranged parallel and concentrically to each other while holding a gap 6. Plate-shaped pole piece 7A.

7B, etc. 7 are supported by a yoke 9, and the yoke 9 is provided with a pair of electric barrel-shaped excitation coils 8A, 8B, etc. 8, and when excited, a uniform magnetic field 6A is generated approximately in the center of the space 6. It is formed. le is a ring-shaped protrusion 10A formed on the outer periphery of the pole piece 7 so as to protrude toward the space 6, and a recess 10B concentrically formed on the inner periphery of the ring-shaped protrusion 10A. The leakage magnetic flux that bulges outward from the outer edge of the disc-shaped pole piece 7 and the disturbance of the magnetic lines of force (non-uniformity of the magnetic field strength) that occurs in the uniform magnetic field 6A due to this leakage flux are absorbed by the convex portion 10A. This is corrected by the strengthened part of the magnetic field strength formed by the recess 10B and the reduced part of the magnetic field strength formed by the recess 10B, which straightens the magnetic flux lines in the uniform magnetic field 6A part and improves the uniformity of the magnetic field strength of the uniform magnetic field. can be improved.

[Problem to be solved by the invention]

The above-mentioned N conductivity type uniform magnetic field magnet can obtain a uniform magnetic field magnet with low power consumption and high magnetic field strength by taking advantage of the low loss property of the superconducting coil, but it is possible to obtain a uniform magnetic field magnet with high magnetic field strength. Since the dimensions are determined by the Helmholtz coil, advanced processing technology is required, resulting in an expensive insulated vacuum container that is large and has a magnetic field strength of 0.
Magnets of about 3 to 0.5 T have a large economic disadvantage, and the amount of leakage magnetic flux distributed outside the magnet is extremely large, so a large amount of iron material is required for magnetic shielding to avoid magnetic effects on surrounding objects. Since this requires a large amount of construction cost, the economic disadvantages are becoming more and more problematic, and this is also a cause of hindering the spread of MRl.

On the other hand, core-type homogeneous magnetic field magnets are mainly composed of inexpensive iron-based magnetic materials and steel materials, and their processing is low, so they have the advantage of being economically advantageous in producing uniform magnetic field magnets with low magnetic field strength. In addition, since the yoke suppresses leakage magnetic flux, magnetic shielding, etc. can be simplified, and it is therefore expected to contribute to the spread of magnetic resonance imaging devices. However, since the normally conducting coil is used, the excitation loss is large (and the excitation coil 7 is surrounded on three sides by the yoke 9 and the pole piece 7, and the excitation coil 7 is surrounded by the yoke 9 and the pole piece 7 on three sides, and the local magnetic resistance is lower than that in the space 6). Since the magnetic circuit 9L is formed and this reduces the magnetic efficiency of the magnet, a large excitation power source and excitation coil are required, and for example, there is a problem that it is not easy to create a uniform magnetic field magnet with a magnetic field strength of about 0.4T. .

An object of the present invention is to obtain an inexpensive uniform magnetic field magnet that uses a small superconducting excitation coil, has low leakage magnetic flux, and has high magnetic efficiency and magnetic field strength.

[tlIII! Means for Solving I] In order to solve the above problems, according to the present invention, a pair of upper and lower disc-shaped discs are supported by a yoke part so as to face each other across a space in which a subject is to be housed. A magnetic field strength equalizing means including a ring-shaped protrusion formed on the outer circumference of the pole piece and protruding toward the space, and a recess formed concentrically on the inner diameter side of the pole piece, and excitation. A device that generates a uniform magnetic field in the center of the space by exciting a coil,
A superconducting excitation coil is provided for each of the pair of pole pieces, the superconducting excitation coil consisting of a ring-shaped insulating vacuum container surrounding the outer periphery of the pair of pole pieces, and a superconducting coil housed inside the vacuum container along with a refrigerant liquid. Or, a superconducting excitation coil consisting of a ring-shaped insulated vacuum container surrounding the outer periphery of a pair of pole pieces, and a superconducting coil housed inside the container along with a refrigerant.
Of the pair of upper and lower pole pieces, only the lower ball piece entry side is provided.

Further, the insulating vacuum container is formed into a ring shape having a rectangular cross section, and the vacuum container of the insulating vacuum container is made of a non-magnetic metal.
This includes a type in which only the inverted plate that closely surrounds the pole piece is made of a ferromagnetic material, and a type in which a pair of upper and lower pole pieces are formed asymmetrically with respect to each other.

Furthermore, at least two oblique circles* are supported concentrically and rotatably at intervals on the outer peripheral portion of the pole piece so as to form a convex portion inclined with respect to the opposing surfaces of the pair of pole pieces. , =, a recess formed between the oblique cylinders, and is provided with a magnetic field strength equalizing means for shimming out unevenness in magnetic field strength by rotating the oblique cylinders.

[Effect]

In the configuration of the present invention, a superconducting excitation coil consisting of a ring-shaped insulating vacuum container surrounding the outer periphery of the pair of pole pieces and a superconducting coil housed inside the container along with a refrigerant liquid is connected to the pair of pole pieces. By providing each piece, the presence of the core frees the superconducting coil from the geometric constraints imposed by Helmholck coils, allowing the expensive insulated vacuum container to be downsized into a donut shape, and significantly reducing the amount of leakage magnetic flux. It is possible to obtain at which increases the magnetic resistance between the core and the core and improves the magnetic efficiency.

Furthermore, if the superconducting excitation coil is provided only on the lower pole piece side of the pair of upper and lower ball heaths, only one expensive superconducting excitation coil is required.
In addition, the magnetic attraction force acting between the pair of superconducting coils can be eliminated, and the support structure of the excitation coil can be simplified, resulting in the advantage that the configuration of the uniform magnetic field magnet can be simplified by -N.

Furthermore, the insulating vacuum container is formed into a ring shape having a rectangular cross section, and the vacuum container is made of a non-magnetic metal.

If only the side plate that closely surrounds the pole piece is made of ferromagnetic material, the leakage magnetic flux on the 1st coil side will be woven, and therefore act on the coil when the excitation current is switched on and off. The superconducting coil can be mechanically stabilized by reducing the magneto-mechanical force, and the positioning accuracy relative to the pole piece can be relaxed.

In addition, if the superelectric S excitation coil is provided only on the lower pole piece, it is possible to form them asymmetrically, for example by providing a means for equalizing the magnetic field strength only on the lower pole piece. The structure of the pole piece can be simplified without adversely affecting the uniformity of the pole piece.

Furthermore, at least two oblique cylinders are concentrically and rotatably supported at intervals on the outer circumferential portion of the pole pieces so as to form convex portions that are inclined with respect to opposing surfaces of the pair of pole pieces. , and a recess formed between the oblique cylinders, and in the case of a structure including a magnetic field strength equalizing means for shimming uneven magnetic field strength by rotation of the oblique cylinder, the convex part and the recess Shimming finely corrects the unevenness of the magnetic field strength in the angular direction by rotating two oblique cylinders whose protruding dimensions change in the circumferential direction and changing the positional relationship of the protruding dimensions. Therefore, a uniform magnetic field magnet for a magnetic resonance imaging apparatus with excellent uniformity of magnetic field strength can be obtained.

〔Example〕

Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a simplified cross-sectional view of a uniform magnetic field magnet of a magnetic resonance imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a schematic diagram showing the magnetic flux distribution in the embodiment. In Fig. 0, in which detailed explanations are omitted by using the same reference numerals for portions, the pair of pole pieces 7A, 7B, etc. 7 supported by the yoke 9 are each a ring protruding toward the space 6 side. The magnetic field strength equalizing means 10 is comprised of a shaped convex portion 10A and a recess 10B concentrically formed on the inner circumferential side of the convex portion 10A. Furthermore, a ring-shaped superconducting excitation coil 20 surrounding the pair of pole pieces 7A and 7B is provided on the outer periphery of each pole piece. The superconducting excitation coil 20 is stored in a liquid helium container 12C in an insulated vacuum container 12 together with liquid helium 3.
The space between the liquid helium container 12c and the vacuum container 12A is heat shielded by a radiation shield 12B, a multilayer heat insulating layer (not shown), etc. The yoke 9 is made of soft iron with a high relative magnetic permeability, and the pair of upper and lower pole pieces 7 are made of a powder iron core with a smaller relative permeability than this, so that the magnetic flux within the yoke 9 is made of soft iron with a high relative magnetic permeability. It is configured to generate a uniform magnetic field 6A having a magnetic flux density BO that spreads evenly over the entire piece and further covers as wide a region as possible in the space 6 between opposing pole pieces. In addition, the leakage magnetic flux that expands and is distributed outside the space 6 sandwiched between the pole pieces and the distortion of the magnetic flux lines of the uniform magnetic field caused by this are caused by the convex portion of the magnetic field strength equalizing means 10 in FIG. 10A and the high magnetic field part BH formed by I! The low magnetic field portion BL formed by the one location 10B corrects it to be straight.

In the core-shaped superconducting homogeneous magnetic field magnet configured in this way, the uniformity of the magnetic field strength is determined by the shape of the pole piece, and the superconducting excitation coil has the desired magnetic field strength (for example, 0.4T).
It is sufficient to satisfy a sufficient amount of rotation to generate an amount of magnetic flux corresponding to the amount of magnetic flux (degree), and the insulating vacuum container can be miniaturized into a ring shape with a small cross-sectional area. In addition, since the superconducting coil 11 with a small cross section can easily secure a sufficient distance from the yoke 9 and the pole piece 7, it prevents the formation of a local magnetic path (corresponding to 9L in FIG. 8). It has high magnetic efficiency and low loss due to the combination of features such as high magnetic efficiency, low leakage magnetic flux due to the presence of the yoke, and near zero power loss of the superconducting coil. Furthermore, it is possible to provide a uniform magnetic field magnet for a magnetic resonance imaging apparatus in which leakage magnetic flux has less adverse effects on surrounding objects.

FIG. 3 is a sectional view showing a different embodiment of the invention,
B conductive excitation coil 20 to lower pole piece 7B! ! This is different from the previous embodiment in that it is provided only in the first embodiment. Since the pair of ball pieces A and 7'B have the function of forming a uniform magnetic field, the excitation coil 2 can be
It is possible to reduce the number of zeros to one, which provides significant economic benefits. In addition, by reducing the number of superconducting excitation coils to one, the service port 4, current leads, etc. can be simplified, and the magnetic attraction force that acts between the two excitation coils is also eliminated. Il so that it stands still at the position with the best magnetic efficiency.
f: Therefore, the electromagnetic mechanical force acting on the excitation coil is only the radial force generated when the current suddenly changes, and the support structure of the superconducting excitation coil 20 can be greatly simplified.

FIG. 4 is a cross-sectional view of main parts showing still another embodiment of the present invention. Among the vacuum container 12A, the radiation shield 12B, and the liquid helium container 12C, all of which are made of non-magnetic metal of the insulating vacuum container 12, This embodiment differs from the previous embodiments in that only the side wall 22A surrounding the outer periphery of the pole piece 7 of the vacuum vessel 12A is made of a ferromagnetic plate. In this case, by placing a ferromagnetic material on the inner diameter side of the superconducting coil 11, the leakage magnetic flux density near the superconducting coil can be reduced, and the radial force acting on the superconducting coil due to sudden changes in current can be reduced, resulting in mechanical stability. In addition, the ferromagnetic material functions to prevent the radial force acting on the superconducting coil from reaching the pole piece, which improves the dimensional accuracy for positioning the excitation coil concentrically with respect to the pole piece. This has the advantage that the support structure of the superconducting coil can be further simplified.

FIG. 5 is a sectional view showing another embodiment of the present invention, in which the magnetic field strength equalization means 10 is formed only on the lower pole piece 7B provided with the superconducting excitation coil 20, and the upper pole piece is flat. Pole piece 2 in the shape of a flat disk having a magnetic pole surface
7 is different from the above-mentioned embodiments, and in this case as well, a uniform magnetic field can be generated as in the above-mentioned embodiments, and the advantage that the machining of the pole piece can be simplified can be obtained.

FIG. 6 is a sectional view of a main part showing another embodiment of the present invention, showing two slopes 1137A on which a pole piece 37 is rotatably supported on its outer circumferential side.

37B (one end of a short cylinder cut off diagonally), each forming a convex portion whose protrusion length changes in the circumferential direction, and a ring-shaped recess 3 between the oblique cylinders.
7C, the magnetic field strength equalization means 40
This embodiment differs from the previous embodiments in that it is configured as follows.

When a superconducting uniform magnetic field magnet is constructed using a pair or one of the pole pieces 37 formed in this way, by rotating each of the two oblique cylinders, the combination of the protruding lengths of the convex portions can be adjusted at each position in the circumferential direction. Since it can be adjusted fairly arbitrarily, it is possible to obtain a shimming function that finely adjusts the non-uniformity of the magnetic field strength of the uniform magnetic field and its angular variation at the same time, thereby further increasing the uniformity of the magnetic field strength. can. The magnetic field strength equalization means 40 shown in FIG. 6 can be applied not only to the above-mentioned superconducting uniform magnetic field magnet but also to a normal tS core type uniform magnetic field magnet to obtain similar functions and effects.

〔Effect of the invention〕

As described above, the present invention is configured such that a ring-shaped superconducting excitation coil surrounding each of a pair of upper and lower pole pieces supported by a yoke and having a 1 Mi field strength equalization means is provided. As a result, by utilizing the magnetic field strength equalization effect of the pair of pole pieces, the superconducting excitation coil can be significantly downsized compared to conventional superconducting uniform magnetic field magnets, which provides great economic benefits. High magnetic field strength, low magnetic flux loss, and high magnetic flux performance that cannot be obtained with normal conducting homogeneous magnetic field magnets are obtained, and the amount of leakage magnetic flux is small, so there is little negative impact on surrounding objects (because magnetic shielding It is possible to provide a uniform magnetic field magnet for a magnetic resonance imaging system that has a number of features, such as a simplified system and a significant reduction in construction costs.

In addition, if the superconducting excitation coil is provided only on one pole piece side, even greater economic benefits can be obtained, and the spread of magnetic resonance imaging equipment, which has been hindered due to its high price, can be promoted, and the magnetic attraction force can be increased. By reducing the
11 support structures for excitation coils! You can get the advantage of being able to use IT.

Furthermore, if part of the flattened plate of the vacuum vessel is made of ferromagnetic material, the radial force acting on the superconducting coil can be reduced, making it super! Since the coil support structure can be further simplified and restrictions on positioning accuracy with respect to the pole piece can be relaxed, a uniform magnetic field magnet with excellent mechanical stability can be provided.

Furthermore, if the configuration is such that a superconducting excitation coil is provided on one ball base side, it is possible to make the pole piece asymmetric, such as by providing a means for equalizing the magnetic field strength on the pole piece on the coil side. This has the advantage of saving labor in machining the pole pieces.

On the other hand, if the magnetic field strength equalization means is composed of at least two oblique cylinders rotatably supported by the pole piece, a noming function can be obtained to finely adjust the angular unevenness of the magnetic field strength. Since it is possible to provide a uniform magnetic field magnet with excellent uniformity, in addition to the effect of improving the magnetic field strength by the superconducting excitation coil, it contributes to economically advantageous provision of high-performance magnetic resonance imaging equipment with high magnetic field strength. can.

[Brief explanation of drawings]

Fig. 1 is a simplified cross-sectional view of a uniform magnetic field magnet of a magnetic resonance imaging apparatus according to an embodiment of the present invention, Fig. 2 is a magnetic flux distribution diagram in the embodiment, and Fig. 3 is a different embodiment of the invention. 4 is a sectional view of a main part showing a further different embodiment of the invention, FIG. 5 is a sectional view showing another embodiment of the invention, and FIG. 6 is a sectional view of another embodiment of the invention. Figure 7 is a cross-sectional view of a conventional superconducting uniform magnetic field magnet, and Figures 8 and 9 are a side view and a plan view of a conventional normal-conducting, core-shaped uniform magnetic field magnet. . 1.11...Superconducting coil, 2.12...Insulated vacuum vessel, 3...Liquid helium, 6...Space, 6A...uniform magnetic field, ? , 7A, 7B, 27.37~・Pole piece, 9
・Yoke, 10.40・Magnetic field strength equalization means, IOA
...Protrusion, IOB. 7C recess, 20... superconducting excitation coil, 2 2... side plate (ferromagnetic material), 37A, 37B... oblique cylinder. Representative patent attorney Yamaguchi 1 insect. Yu2 figure tZ'! Figure I34

Claims (1)

[Scope of Claims] 1) A pair of upper and lower disc-shaped pole pieces supported by a yoke so as to face each other across a space in which a subject is to be housed, and the space formed on the outer periphery of the pole pieces. It is equipped with a ring-shaped convex part protruding to the side, and a magnetic field strength equalization means including a recess formed concentrically on the inner diameter side thereof, and by exciting an excitation coil, the magnetic field strength is uniformly distributed in the central part of the space part. In the device that generates a magnetic field, a superconducting excitation coil consisting of a ring-shaped insulating vacuum container surrounding the outer periphery of the pair of pole pieces, and a superconducting coil housed inside the vacuum container together with a refrigerant, is connected to the pair of pole pieces. A uniform magnetic field magnet for a magnetic resonance imaging apparatus, characterized in that each pole piece is provided with a uniform magnetic field magnet. 2) A pair of upper and lower disc-shaped pole pieces supported by a yoke so as to face each other across a space in which the subject is to be housed, and a ring-shaped pole piece formed on the outer periphery of the pole pieces and protruding toward the space. and a magnetic field strength equalization means including a concave portion concentrically formed on the inner diameter side of the convex portion, and generates a uniform magnetic field in the center of the space by exciting an excitation coil. , a superconducting excitation coil consisting of a ring-shaped insulating vacuum container surrounding the outer periphery of the pair of pole pieces, and a superconducting coil housed inside the container along with a refrigerant liquid, is placed between the pair of upper and lower pole pieces. A uniform magnetic field magnet for a magnetic resonance imaging apparatus, characterized in that it is provided only on the lower pole piece side. 3) The insulating vacuum vessel is formed into a ring shape with a rectangular cross section, and only the side plate of the vacuum vessel of the insulating vacuum vessel made of a non-magnetic metal, which closely surrounds the pole piece, is made of a ferromagnetic material. A uniform magnetic field magnet for a magnetic resonance imaging apparatus according to claim 1 or claim 2. 4) A uniform magnetic field magnet for a magnetic resonance imaging apparatus according to claim 2, wherein the pair of upper and lower pole pieces are formed asymmetrically with respect to each other. 5) A pair of upper and lower disc-shaped pole pieces supported by a yoke so as to face each other across a space in which the subject is to be stored, and a ring-shaped pole piece formed on the outer periphery of the pole pieces and protruding toward the space. and a magnetic field strength equalization means including a concave portion concentrically formed on the inner diameter side of the convex portion, and generates a uniform magnetic field in the center of the space by exciting an excitation coil. , at least two oblique cylinders rotatably and concentrically supported at intervals on the outer circumferential portion of the pole piece so as to form a convex portion inclined with respect to opposing surfaces of the pair of pole pieces; A uniform magnetic field of a magnetic resonance imaging apparatus, characterized in that the magnetic resonance imaging apparatus comprises a recess formed between the oblique cylinders, and includes a magnetic field strength equalizing means for shimming uneven magnetic field intensity by rotating the oblique cylinders. magnet.