JP2001149336A - Magnetic field generator for magnetic resonance imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce vibration and noise of an inclined magnetic field coil in a magnetic field generator for a magnetic resonance imaging device having a high opening characteristic and having a magnetostatic field generating source of a high magnetic field. SOLUTION: Two sets of magnetostatic field generating sources 11 are oppositely arranged, and a uniform magnetic field area 3 of a high magnetic field is formed between these. An inclined magnetic field coil 4 composed of an almost flat main coil 5 and a shield coil 6 is arranged inside the magnetostatic field generating sources 11, the main coil 5 generates an inclined magnetic field in the uniform magnetic field area 3, and the shield coil 6 shields a magnetic field generated outside the inclined magnetic field coil 4. An almost flat intermediate member 13 is arranged between the main coil 5 and the shield coil 6, and the main coil 5 and the shield coil 6 are joined to both sides of the intermediate member 13 to constitute a coil assembly 14 to increase rigidity to the inclined magnetic field coil to vibration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field generator for a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), and more particularly to a magnetic field for an MRI apparatus having a large opening and having an active shield type gradient coil. It relates to a generator.

[0002]

2. Description of the Related Art In recent years, a vertical magnetic field type MRI apparatus has been proposed in consideration of openness of a measurement space and access of an operator to a subject during an examination. Vertical magnetic field type M
In a magnetic field generator for an RI apparatus, a static magnetic field generating means such as a permanent magnet or a superconducting magnet is arranged to face each other, and forms a uniform magnetic field region on the facing surface side. Further, on the opposite surface side of the static magnetic field generating means, gradient magnetic field coils in the x, y, and z directions are arranged to face each other with a uniform magnetic field region therebetween.

For example, in a magnetic field generator using permanent magnets, two sets of permanent magnets are arranged to face each other, and the gradient magnetic field coils are almost flat, and face each other across a uniform magnetic field region formed between the two permanent magnets. It is configured to be arranged. The gradient magnetic field coil is generally housed inside the pole piece that constitutes the magnetic circuit, and the gradient magnetic field coil is driven by adopting a material having a high electric resistivity as a material of the pole piece. Techniques have been established that do not sometimes generate eddy currents.

[0004] Each of the gradient magnetic field coils is connected to a power supply device, driven at an appropriate timing and voltage in accordance with the inspection conditions of the MRI apparatus, and a pulse-like current is applied. By passing a pulse current in a static magnetic field, Lorentz force acts according to Fleming's left-hand rule. for that reason,
This electromagnetic force tried to deform the gradient coil, generating noise and vibration.

However, since the gradient coil of the magnetic field generator using the permanent magnet is an unshielded type, the generation efficiency of the gradient coil is high, and therefore, the current required to obtain the required gradient magnetic field intensity is high. Due to the low density, the force to excite the vibration is small, and the image quality deterioration and noise increase due to the vibration of the gradient coil are not particularly problematic,
In the case of a magnetic circuit using a permanent magnet, it is difficult to obtain a high static magnetic field strength in the measurement space, and the upper limit is about 0.3 terrace. The image quality of an MRI apparatus largely depends on the static magnetic field strength, and it is desirable to obtain the highest possible static magnetic field strength in order to improve the image quality.

Therefore, a high static magnetic field intensity is achieved by using a superconducting magnet as the static magnetic field generating means.

In a magnetic field generator using a superconducting magnet, two sets of superconducting magnets housed in a cooling vessel are arranged to face each other, and a uniform magnetic field region having a high magnetic field strength is formed between the two superconducting magnets. A substantially flat gradient magnetic field coil disposed opposite to the uniform magnetic field region has a main coil for mainly generating a gradient magnetic field and a main coil 5 for shielding a magnetic field generated outside the gradient magnetic field coil. And a shield coil for generating a magnetic field. In this device, high open feeling is obtained by being open on all sides, using a static magnetic field source of high magnetic field strength and a gradient magnetic field coil of active shield method,
High quality MR images can be taken.

However, in the gradient coil, a current for generating a gradient magnetic field is supplied to the main coil and the shield coil during operation. Since a current flows in the static magnetic field, Lorentz force is generated and the gradient coil is excited. In particular, in the case of a magnetic field generator using a vertical magnetic field type and a superconducting magnet, the current density is high and the exciting force is large, and since the coil shape is flat, the bending rigidity in the out-of-plane direction of the gradient magnetic field coil is weak, Out-of-plane vibration is likely to occur. Therefore, accurate position information of the MR signal cannot be obtained due to the vibration of the gradient magnetic field coil, and a high-quality MR image cannot be obtained.

Further, since a pulsed current is continuously supplied to the gradient coil, a pulse-like electromagnetic force acts on the gradient coil, and the gradient coil vibrates due to the electromagnetic force.
In particular, a continuous tapping sound is generated by the out-of-plane vibration of the gradient coil.

In a vertical magnetic field type magnetic field generator, vibration by a gradient magnetic field coil while maintaining a high static magnetic field strength,
As a method for reducing noise, there is a method disclosed in JP-A-9-308617. In a vertical magnetic field type magnetic field generating apparatus, a piezoelectric element is arranged as an element for converting electrical energy into mechanical energy on a flat holding member holding a gradient magnetic field coil conductor in three axial directions. When an exciting current is applied to the coil conductor by driving the gradient magnetic field coil, an in-plane Lorentz force is generated in the gradient magnetic field coil, thereby generating vibration in a natural vibration mode.

The piezoelectric element is arranged so as to cross the node of the vibration as described above, and by applying a predetermined voltage to the piezoelectric element at a predetermined timing based on the drive information of the gradient coil, the vibration is generated. Mode-based vibration can be canceled efficiently. As a result, vibration and noise generated from the gradient coil can be effectively suppressed.

[0012]

However, in the above method, piezoelectric elements are arranged at various positions on the holding member of the gradient magnetic field coil, and the voltage application is controlled according to the driving conditions of the gradient magnetic field coil. Therefore, there is a problem that the device is complicated and its control is also complicated.

In view of the above problems, in the present invention, by improving the structure of the plate-shaped gradient magnetic field coil, the vibration and noise of the gradient magnetic field coil can be reduced while maintaining a high static magnetic field strength with a simple configuration. MR that can be effectively suppressed
An object of the present invention is to provide a magnetic field generator for an I device.

[0014]

In order to achieve the above object, a magnetic field generator according to the present invention comprises two sets of opposed static magnetic field sources forming a uniform magnetic field region therebetween, and the static magnetic field sources. 2 which are arranged inside and facing each other with the uniform magnetic field region
A gradient coil having a substantially flat shape, wherein the gradient coil mainly shields a magnetic field generated outside the gradient coil by a main coil for generating a gradient magnetic field mainly in a uniform magnetic field region. A magnetic field generator for a magnetic resonance imaging apparatus comprising a shield coil for generating a magnetic field such that the gradient coil includes an intermediate member disposed between the main coil and the shield coil, and the intermediate member is The main coil and the shield coil have thickness or rigidity.

In this configuration, since the intermediate member is disposed between the main coil and the shield coil to constitute a coil assembly, the main coil and the shield coil of the gradient magnetic field coil are smaller than a single coil having a flat shape. In addition, the rigidity of the vibration system is increased, and the vibration of the gradient magnetic field coil can be suppressed.

In the magnetic field generator according to the present invention, the coil assembly is fixed to a stationary object via the intermediate member. In this configuration, for the vibration mode of the node diameter that vibrates with the diameter of the gradient magnetic field coil as a node, the vibration can be effectively suppressed by selecting and fixing the outer peripheral portion of the intermediate member having a large amplitude. it can.

Further, in the magnetic field generating apparatus of the present invention, the coil assembly is fixed to a stationary object via the portion of the gradient magnetic field coil. In this configuration, by appropriately selecting and fixing a position where the amplitude of vibration of the gradient magnetic field coil is large (antinode of vibration) as a fixed position,
Vibration can be effectively suppressed.

In the magnetic field generator of the present invention, the coil assembly is further fixed to a stationary object via the gradient magnetic field coil and the intermediate member. In this configuration,
Since it is fixed at both the outer peripheral part of the intermediate member and the part of the gradient magnetic field coil, by devising the arrangement of both fixing points, vibration can be reduced without providing unnecessary fixed points on the gradient magnetic field coil. It can be suppressed effectively.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the accompanying drawings.

First of the magnetic field generator for MRI apparatus of the present invention
1 is shown in FIG. In FIG. 1, two sets of static magnetic field sources 11 are arranged vertically opposite to each other with a uniform magnetic field region (measurement space) 3 therebetween, and form a uniform static magnetic field in the measurement space 3. A substantially flat gradient magnetic field coil 4 for generating a gradient magnetic field in the measurement space 3 is disposed inside the two static magnetic field generation sources 11 so as to face each other with the measurement space 3 interposed therebetween.

The gradient coil 4 mainly includes a main coil 5 for generating a gradient magnetic field and a shield coil 6 for generating a magnetic field that shields a magnetic field generated outside the gradient coil 4 by the main coil 5. It is configured. The main coil 5, the shield coil 6, and the static magnetic field source 11 are arranged substantially vertically symmetrically with respect to the center plane of the uniform magnetic field region 3 at the center of the apparatus. , And a static magnetic field generation source 11.

The gradient magnetic field coil 4 is also provided with a main coil 5 and a shield to generate a gradient magnetic field in three orthogonal directions (x, y, z-axis directions) in the same manner as in the prior art. The coil 6 is also provided with three gradient magnetic field coils. The coil conductors of these gradient magnetic field coils are generally provided with a groove corresponding to the shape of the coil conductor in a substantially flat holding member made of an insulating material such as FRP and accommodated in the groove. Alternatively, it can be formed by the same method as a printed circuit board or the like by an etching technique. Accordingly, both the main coil 5 and the shield coil 6 are formed of a substantially flat coil conductor and a holding member, and are formed substantially flat as a whole.

In order for the main coil 5 to effectively shield the magnetic field generated outside the gradient coil 4 with the magnetic field generated by the shield coil 6, the main coil 5 and the shield coil 6 are arranged at appropriate intervals. The shielding effect improves as the distance between the two is generally wider. Further, in order to suppress eddy currents generated around the conductor near the gradient magnetic field coil 4, the shield coil 6 is disposed at an appropriate distance from the conductor (in this embodiment, the container 12 of the static magnetic field generation source 11). Is arranged.

In the third conventional example, the static magnetic field generating source is a superconducting magnet, but the present invention is not limited to a superconducting magnet,
MR using normal conducting magnets and permanent magnets as static magnetic field sources
It is also applicable to the I device. For this reason, as the above-mentioned conductor, in the case of a superconducting magnet, a cooling container for accommodating a superconducting coil; In the case of a magnet, a magnetic pole (pole piece) or the like corresponds.

In this embodiment, in the magnetic field generating apparatus for an MRI apparatus, an intermediate member 13 is disposed between the main coil 5 and the shield coil 6, and the main member 5, the shield coil 6, and the intermediate member 13 are inclined. Magnetic field coil assembly (hereinafter, referred to as
The coil assembly 14 is arranged opposite to the uniform magnetic field region 3. The intermediate member 13 is a rigid body having a substantially flat plate shape as a whole. Thus, by inserting the intermediate member 13 between the main coil 5 and the shield coil 6 and joining the three members to form the coil assembly 14, the bending rigidity in the out-of-plane direction is higher than that of the coil alone. Become. The bending stiffness of the flat plate increases in proportion to the cube of the thickness of the flat plate, and the thickness can be doubled or more by forming the coil assembly 14. The rigidity of the assembly 14 is significantly improved, and the vibration of each coil in the out-of-plane direction can be significantly suppressed.

Also, the coil assembly is usually required to be as thin as possible due to space constraints. At this time, it is desirable to make the main coil and the shield coil as thin as possible and the intermediate member as thick as possible within the range where the electric resistance of each coil can be reduced to a required value or less in order to increase the rigidity.

The gradient coil 4 is connected to the coil assembly 14.
By joining one surface of the main coil 5 and the shield coil 6 to both surfaces of the intermediate member 13, the sound radiation surface due to the vibration of the main coil 5 and the shield coil 6 is reduced, so that noise can be reduced. . Also the main coil
By filling the intermediate member 13 between the coil 5 and the shield coil 6, it is possible to avoid an increase in noise due to air column resonance occurring between both coils.

Since the intermediate member 13 is a rigid body, by inserting it between the main coil 5 and the shield coil 6,
The main coil 5 and the shield coil 6 can be arranged at regular intervals. Further, if necessary, the main coil 5 and the shield coil 6 can be insulated by using an insulating material for the intermediate member 13.

The main coil 5 and the shield coil 6 are fixed to the intermediate member 13 by a method such as bonding with an adhesive or fixing with a bolt. The main coil 5 and the shield coil 6 can be arranged with high relative positional accuracy by improving the accuracy of the intermediate member 13 such as the dimension in the thickness direction.

In FIG. 1, the main coil 5, the shield coil 6, and the intermediate member 13 are individually formed to form a coil assembly 14, which is provided between the main coil 5 and the shield coil 6. May be configured such that the intermediate member 13 is disposed and molded integrally. Even if the three members are integrated, it is only necessary to maintain the configuration in which the intermediate member 13 is arranged between the main coil 5 and the shield coil 6.

The intermediate member 13 is exemplified by a solid flat plate, but is not limited to this, and has an appropriate rigidity.
What is necessary is just to be a flat plate as a whole. For example, it may be a hollow flat plate having a cooling channel inside, or a flat plate formed by arranging a plurality of beams on a plane.

Further, in this embodiment, the shape of the gradient magnetic field coil has been described as a disk shape. However, the present invention is not limited to this, and both the main coil 5 and the shield coil 6 have an elliptical plate shape or a rectangular shape. Is also good. Further, the relationship between the sizes of the main coil 5, the shield coil 6, and the intermediate member 13 is not limited to the one in which the intermediate member 13 is large as shown in the drawing.
Thirteen may be smaller, have the same dimensions, or have other relationships.

The second embodiment of the magnetic field generator for an MRI apparatus according to the present invention
2 is shown in FIG. FIG. 2 is a cross-sectional view of a main part showing an example of a fixing structure of the coil assembly. FIG. 2 shows the lower half of the magnetic field generator. In FIG. 2, the gradient coil is a main coil 5, a shield coil 6, and a main coil.
5 and an intermediate member 13 disposed between the shield coil 6 and constitute a coil assembly 14;
14 is fixed to a stationary object 15 of the magnetic field generator.

In the present embodiment, the outer diameter of the intermediate member 13 is made larger than the outer diameters of the main coil 5 and the shield coil 6, and the outer peripheral portion 16 of the intermediate member 13 (the portion where the main coil 5 and the shield coil 6 do not exist). , The coil assembly 14 is fixed to the stationary object 15 using a plurality of fixtures 17.

As the stationary object 15, the superconducting magnet 11 in FIG.
Although the case of the cooling vessel 12 is shown, there are a magnetic shunt, a magnetic shield, a floor surface on which the device is installed, and the like. Since the end of the coil assembly 14 is fixed, the circumferential portion where the amplitude becomes large is fixed against the vibration in the node diameter mode, which vibrates with the diameter of the disk-shaped gradient magnetic field coil as the node. By doing so, vibration can be effectively suppressed.

In FIG. 2, a bolt is used as a fixture 17 of the coil assembly 14 to the stationary object 15. The fixing tool 17 is not limited to a bolt, and may be any as long as it can firmly hold the coil assembly 14 to the stationary object 15 while maintaining rigidity. Further, the intermediate member 13 may be directly fixed to the cooling container 12. In the case of FIG. 2, by appropriately selecting the length of the bolt and the position of the nut, the shield coil 6 and the cooling container 12 can be arranged at an appropriate interval, and are generated around the cooling container 12 which is a conductor. Eddy current can be controlled.

Further, since a space can be secured between the shield coil 6 and the cooling vessel 12, a shim for adjusting the magnetic field for improving the uniformity of the static magnetic field can be laid in this space. Since there is no restriction on the space other than the bolt fixing portion, the magnetic field adjusting shim can be laid freely.

Further, the outer peripheral portion 16 of the intermediate member 13 is
Since the structure is fixed to 12, the coil assembly 14 can be easily installed. Since it is not necessary to provide a fixing hole in the gradient magnetic field coil, the coil can be designed without considering the fixing hole, and the degree of freedom in design is increased. Also,
Since the surface of the gradient magnetic field coil is not covered, the influence on the image signal is very small, and a high-quality MR image is easily obtained.

The third embodiment of the magnetic field generator for an MRI apparatus according to the present invention
3 is shown in FIG. FIG. 3 is a cross-sectional view of a main part showing another example of the fixing structure of the coil assembly, and shows a lower part of the magnetic field generator as in FIG. In FIG. 3, the coil assembly 14
Is composed of a main coil 5, a shield coil 6, and an intermediate member 13 arranged between the two coils, and the three members have substantially the same outer diameter. The coil assembly 14 is fixed to a stationary object (cooling vessel 12) 15 at the portion of the gradient magnetic field coil.

The fixing of the coil assembly 14
The fixture 18 which is longer than the case of FIG.
It is done in. In the fixing method, holes are formed not only in the intermediate member 13 but also in the main coil 5 and the shield coil 6 (here, holding members of both coils), and fixing members 18 such as bolts are used.
Are fixed to the cooling container 12 at a plurality of locations.

With respect to the coil assembly 14, a plurality of appropriate positions at the outer peripheral portion and the central portion of the coil assembly 14 corresponding to portions where the amplitude of the out-of-plane vibration of the gradient magnetic field coil is large (antinode of vibration) are directly fixed. By doing so, vibration can be effectively suppressed. Further, since the coil portion is directly fixed, the gradient coil can be fixed with high accuracy.

The fourth embodiment of the magnetic field generator for an MRI apparatus according to the present invention
FIG. 4 shows an embodiment of the present invention. FIG. 3 is a cross-sectional view of a principal part showing a third example of the fixing structure of the coil assembly, and shows the lower half of the magnetic field generator as in FIG. In FIG. 4, the coil assembly 14
Is composed of a main coil 5, a shield coil 6, and an intermediate member 13 disposed between both coils. The intermediate member 13 is formed to be larger than the outer diameter of both coils. This coil assembly
Reference numeral 14 is fixed to a stationary object (cooling vessel 12) 15 at an outer peripheral portion 16 of the intermediate member 13 and a portion of the gradient magnetic field coil.

To fix the coil assembly 14, the intermediate member 13 is fixed to the outer peripheral portion 16 by the fixture 17, and the gradient magnetic field coil and the intermediate member 13 are fixed to the central portion by the longer fixture 18. Fixed. In the fixing method of the present embodiment, by devising the arrangement of the fixing points of the gradient magnetic field coil and the intermediate member 13, vibration can be effectively suppressed without providing the gradient magnetic field coil with more fixing points than necessary. .

Further, since the fixing of the gradient magnetic field coil is performed at a fixing point within a necessary limit, the portion of the fixing tool 18 covering the surface of the gradient magnetic field coil can be reduced, and the influence on the MR image signal can be reduced. It is easy to obtain a quality image. Further, since the gradient magnetic field coil is directly fixed, the accuracy of the fixed position of the gradient magnetic field coil can be improved.

[0045]

As described above, according to the present invention, it is possible to take a good MR image with a high open feeling,
In addition, it is possible to provide a magnetic field generator for an MRI apparatus in which noise during image capturing is small.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a magnetic field generator according to the present invention.

FIG. 2 is a second embodiment of the magnetic field generator according to the present invention.

FIG. 3 is a third embodiment of the magnetic field generator according to the present invention.

FIG. 4 is a fourth embodiment of the magnetic field generator according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Superconducting magnet 2, 12 ... Container (cooling container) 3 ... Uniform magnetic field area (measurement space) 4 ... Gradient magnetic field coil 5 ... Main coil 6 ... Shield coil 7, 11 ... Static magnetic field generation source 8 ... Permanent magnet 9 ... Pole Piece 10… Magnetic circuit 13… Intermediate member 14… Gradient magnetic field coil assembly (coil assembly) 15… Stationary object 17,18… Fixture

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01F 27/32 G01N 24/02 540Y 24/06 510Y (72) Inventor Yasuhiro Nemoto 502, Kandachicho, Tsuchiura City, Ibaraki Prefecture Address Machinery Research Laboratory, Hitachi Ltd. (72) Inventor Takeshi Yao 1-11-1 Uchikanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Corporation (72) Inventor Takao Real Name 1-1-1, Uchikanda, Chiyoda-ku, Tokyo No. 14 Hitachi Medical Co., Ltd. (72) Inventor Hiroshi Takano 1-11-1 Uchikanda, Chiyoda-ku, Tokyo Inventor Hiroshima Takeshima Inventor Hirotaka Takeshima 1-1-1, Uchikanda, Chiyoda-ku, Tokyo No. 14 Hitachi Medical Co., Ltd. (72) Inventor Hitoshi Yoshino 1-1-14 Uchikanda, Chiyoda-ku, Tokyo Hitachi Medical Co., Ltd. The inner (72) inventor Akihiro Harada Marunouchi, Chiyoda-ku, Tokyo 2-chome No. 2 No. 3 Mitsubishi Electric Co., Ltd. in the F-term (reference) 4C096 AB47 CA16 CA67 CA70 CB19 5E044 DA01 DA07

Claims (1)

[Claims] 1. Two sets of opposing static magnetic field sources forming a uniform magnetic field region therebetween, and two sets of substantially flat surfaces inside the static magnetic field source and opposing each other across the uniform magnetic field region. A gradient magnetic field coil, wherein the gradient magnetic field coil mainly generates a gradient magnetic field in a uniform magnetic field region, and a magnetic field such that the main coil shields a magnetic field generated outside the gradient magnetic field coil. A magnetic field generating device for a magnetic resonance imaging apparatus, comprising: a shield coil to be generated; wherein the gradient magnetic field coil includes an intermediate member disposed between the main coil and the shield coil, and the intermediate member includes the main coil and the shield. A magnetic field generator for a magnetic resonance imaging apparatus characterized by having a thickness or rigidity greater than that of a coil.