JPH1176198A - Inclined magnetic field coil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain an eddy current, reduce a resistance value of conductors, and restrain heating at coil driving time by lengthening a cross section of the conductors to constitute an inclined magnetic field coil in the direction vertical to its electric current pattern surface. SOLUTION: An incline magnetic field coil is made by inserting electric conductors 12 and 13 into a spiral groove 11 bored on a base board 10 composed of an electric insulator. A pattern of the groove 11 is formed by end mill work so that the prescribed inclined magnetic field distribution is obtained in a prescribed photographing area. A depth of this groove is decided by considering the necessity of housing the inclined magnetic field coil in a thickness of a certain degree, but is more desirable to connect the thin conductors 12 and 13 in parallel to each other than thickening it in the pattern surface direction (in the direction perpendicular to the magnetic flux rushing-in direction 14). Generation of an eddy current is restrained by using conductors having a long cross-sectional shape in the vertical direction to a pattern surface as the electric conductors 12 and 13, and a time constant of the eddy current is shortened, and adverse influence on an image is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradient magnetic field coil suitable for a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), and more particularly, to suppressing an eddy current generated by a pulse magnetic field generated by the gradient magnetic field coil. The present invention relates to a gradient magnetic field coil capable of suppressing a rise in temperature due to energization and having ease of manufacturing.

[0002]

2. Description of the Related Art FIGS. 5 and 6 show a gradient magnetic field coil used in a superconducting magnet of a horizontal magnetic field type. The superconducting magnet generates a horizontal (Z-axis) magnetic field. Since the magnet coil uses a superconducting wire, it needs to be cooled to a predetermined temperature (for example, in the case of an alloy-based superconductor, to a liquid helium temperature (4.2 K) level). For this reason, the superconducting coil includes the vacuum container 50 and the refrigerant container 51.
(In the figure, a liquid helium container), and is held in a cooling container composed of such as. In this case, the gradient magnetic field coil 52 is formed on a set of cylinders, is housed in the hollow of the vacuum vessel 50, and generates a gradient magnetic field in three directions of X, Y, and Z along with the three-dimensional space. . This gradient magnetic field coil 52
As shown in FIG. 6, a main coil 62 made of a copper plate and a shield coil 63 are formed on a pair of insulating pobins 60 and 61. The main coil 62 generates a predetermined gradient magnetic field mainly in a uniform magnetic field region, and the shield coil generates a magnetic field in a direction opposite to that of the main coil, thereby reducing a magnetic field intensity generated outside the gradient magnetic field coil. .

FIG. 7 shows a configuration of a permanent magnet type magnetic resonance imaging apparatus, and FIG. 8 shows a part of the inside thereof. The apparatus is arranged with four yokes 70 and upper and lower sides thereof. The yoke 71 includes a yoke 71, 72 and a permanent magnet 74 having a pole piece 73 provided on the yoke. The gradient magnetic field coil 75 is housed inside the pole piece 73.
This is important for reducing the manufacturing cost of the magnetic circuit by shortening the distance between the upper and lower pole pieces as much as possible. Therefore, in this case, the gradient coil 75
Is usually a disk shape in accordance with the shape of the pole piece 73.

[0004] As one of the conventional methods for producing such a gradient magnetic field coil, it is known to obtain a necessary current pattern by processing a copper plate. Specifically, it is known to obtain a current pattern required for a gradient coil by processing a copper plate by etching or cutting the copper plate by water jet processing or the like. FIG. 9 shows an example of the etching process. In this example, conductors 91 and 92 are formed on a substrate 90 by etching.
To form a current pattern.

On the other hand, as one of the characteristics required of the gradient magnetic field coil, a conductor serving as a current path of the gradient magnetic field coil is required to have a low resistance value in order to suppress heat generation in the conductor. However, when a gradient magnetic field coil is produced by the above method, it is difficult to use a thick copper plate due to its technical limitations. Therefore, in order to reduce the resistance value, a coil having a large width is formed along the component surface of the gradient coil. As a result, an eddy current is generated in the wide coil portion by the pulse-shaped magnetic field generated by the gradient coil.
The eddy current generates an unnecessary magnetic field in the imaging space, and the unnecessary magnetic field cannot be temporally and spatially suppressed. Therefore, in order to deteriorate the required magnetic field characteristics,
There is a problem that the image is adversely affected.

As a conventional method for producing a gradient magnetic field coil, as shown in FIG. 10, a groove 103 having a predetermined pattern is formed on a substrate 101, and a conductor 102 made of a single wire or a stranded wire is inserted into the groove 103. A technique for forming a gradient magnetic field coil is known from Japanese Patent Application Laid-Open No. 1-64638.

At this time, when a stranded wire is used as the conductor, the stranded wire is flexible and therefore easy to manufacture, but the occupation ratio of the stranded wire itself is lower than that of a single wire, so that the actual conductor cross-sectional area is reduced. Is difficult to increase, and is not suitable for flowing a large current. In addition, when a single wire is used, if the conductor cross section is to be enlarged, the conductor becomes thick and hard and cannot be easily processed.

[0008]

SUMMARY OF THE INVENTION As mentioned above,
In the prior art, a coil structure that achieves both easy workability, prevention of eddy current generation, and small conductor resistance to reduce heat generation cannot be achieved. In addition to the above problems, in recent MRI apparatuses, since the gradient magnetic field pulse rises at a high speed, a skin effect in which current flows only near the conductor surface occurs, and the actual resistance is not inversely proportional to the conductor cross-sectional area. Had occurred.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gradient coil which is easy to manufacture by suppressing eddy currents and suppressing heat generation during coil driving by lowering the resistance of the conductor.

[0010]

According to the present invention, a gradient magnetic field coil used in a magnetic resonance imaging apparatus is characterized in that a cross section of a conductor constituting the gradient magnetic field coil is long in a direction perpendicular to a current pattern plane. And

Further, the present invention is characterized in that a plurality of conductors having a long cross section in a direction perpendicular to the current pattern surface are grouped together to form a set of conductors. Further, a groove corresponding to a current pattern for generating a desired gradient magnetic field is formed in a substrate made of an electric insulator, and the conductor is embedded in the groove. Furthermore, the conductor described in the above item 2 is covered with a flexible material.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows an embodiment of a gradient coil according to the present invention. FIG. 2 shows a pattern diagram of a gradient magnetic field coil in the z direction of a gradient magnetic field coil used in a facing magnetic circuit using a permanent magnet. FIG.
FIG. 3 shows a cross-sectional view of the coil conductor in FIG. 2.

The gradient magnetic field coil is formed by inserting electric conductors 12 and 13 into a groove 11 dug in a substrate 10 made of an electric insulator. The pattern of the groove 11 is formed by end milling or the like so as to obtain a predetermined gradient magnetic field distribution in a predetermined imaging region. The conductors 12 and 13 in the figure are connected in parallel at the ends. According to the present invention, the depth of the groove is not a limitation in processing as compared with the conventional example, but considering the space where the gradient magnetic field coil can be placed in the MRI apparatus, the gradient magnetic field coil has a certain thickness. At this time, in order to exhibit the effect of the present invention, it is preferable to connect thin conductors in parallel rather than increase the thickness in the pattern surface direction (the direction perpendicular to the magnetic flux entry direction 14).

In the present invention, the point of the problem in the prior art is solved as follows by using a conductor having a cross section long in a direction perpendicular to the pattern surface as the electric conductor.

One is that, as in the first prior art, the conductor does not have a wide portion in the direction perpendicular to the plane on which the gradient coil is formed, so that the eddy current generated in the conductor is reduced. Generation can be suppressed. Also, for the generated eddy current, the loop size of the eddy current is smaller than that of a single wire, so that the time constant of the eddy current is shortened and the adverse effect on the image is reduced.

Second, in the case of the shape as in the embodiment, there is no restriction on the length of the conductor in the longitudinal direction, so that by increasing the length, the conductor resistance can be sufficiently reduced. Third, if a thin plate as in this embodiment is used, the conductor is flexible in the thickness direction, so that the gradient coil can be easily manufactured. Fourth, when a conductor having an elongated cross section as in the present embodiment is used, the surface area is increased as compared with a conductor having a square or circular cross section, so that an increase in resistance due to the skin effect is unlikely to occur.

As described above, according to the present invention, the conductor having a sectional shape that is long in the direction perpendicular to the pattern surface is used as the conductor of the gradient coil, so that eddy current generated in the conductor is small and resistance is reduced. A gradient coil having a low value can be easily manufactured. As a conductor to be actually applied, it is needless to say that a material having an appropriate softness is selected in order to facilitate the work of embedding in the groove, not to mention having a low electric resistance. Examples of such a material include formal copper wire.

Another embodiment of the present invention will be described with reference to FIG. In the present embodiment, a first groove 31 and a second groove 32 corresponding to an electric pattern for generating a desired gradient magnetic field are formed on a substrate 30 made of the same electrical insulator as in the first embodiment. . In this case, the second groove 32 is the second groove 32 of the first groove 31.
It is twice as large. A set of four conductors 33 is buried in the first groove 31. The conductor 33 is covered with a flexible covering material 34. As the conductors of the conductors 33, conductors having a cross section long in the direction perpendicular to the pattern surface are used as in the first embodiment. ing. Each conductor is constituted by a bare conductor, but each may be covered with an insulating covering material. Next, a conductor 3 having the same configuration as the conductor 33 is provided in the groove 32.
5 and 36 are buried, and the conductor 33 for the groove 31 is 2
Double current will flow.

According to the present embodiment, the thickness of each conductor inserted in each groove in the plane direction can be made smaller than that of the first embodiment. That is, since the region where the eddy current is generated is subdivided, the effect of suppressing the thin current can be further enhanced.

In addition, since the thickness of each sheet is reduced,
Since the mechanical flexibility of the conductor is increased, the conductor is easier to handle than a single thick conductor, and the work of inserting the conductor into the groove becomes easier. Furthermore, since the pattern of the gradient magnetic field coil usually has a spiral shape in one direction, the length along the circumference differs between the inside and outside of the conductor. For this reason, there is a possibility that a problem such as wrinkles on the inner peripheral side of the conductor may occur. However, if the conductor is thin, the difference becomes small, and this problem can be substantially avoided.

As shown in FIG. 4, if a plurality of conductors 40 are previously covered with one bag-shaped flexible covering material 41 and integrated, it is necessary to manage the plurality of conductors individually. No handling is required. Also, when there are three or more conductors, when inserting the conductor into the groove, friction acts only between the wall of the groove and the conductor arranged at both ends, and the insertion depth of the conductor may vary. There is. Such a problem can be avoided by covering a plurality of conductors. Also, when the conductors are put in the coating, if the conductors are shifted from each other in the coating, the problem of wrinkles due to the difference in the circumferential length between the inner and outer circumferences can be avoided as described above.

The coating used here may be such that the conductors do not vary and enter the groove when inserted into the groove. For this reason, it is not necessary to completely cover the periphery of the conductor, and it is possible to integrate the conductor by putting the conductor in the one-sided covering. Further, when the conductor is put into the coating, the use of a liquid having a high viscosity, a flexible instant adhesive, or the like can further promote the integration of the conductor.

If a conductive material such as copper wire is used as the flexible covering material, the covering itself can be used as a part of the conductor of the gradient coil.
The resistance value of the gradient coil can be reduced.

On the contrary, if a material having an electric insulator is used for this coating, it can also serve as a partition between the grooves. Therefore, the partition only has to serve as a guide for determining the position of the conductor. That is, by making the partition thinner, it is possible to narrow the interval between the conductors, and it is possible to increase the number of turns. Alternatively, it is possible to reduce the height of the partition plate. In this case, a thin substrate can be used, and the number of steps for forming grooves is reduced, so that the manufacturing cost can be reduced.

In the above description, a method of inserting a conductor into a groove formed in a substrate has been described as an example. However, the gist of the present invention is to use a conductor having a large cross-sectional area in a direction perpendicular to the substantial pattern surface of the gradient magnetic field coil. Things are possible.

As an example, it is also possible to adopt a method in which a frame suitable for the pattern of the gradient magnetic field coil is divided into a plurality of frames, and a conductor is wound around the frame in such a manner as to sequentially follow the frame. is there. If the conductor surface is insulated and covered, it is not necessary to arrange a winding frame in a portion where the coil arrangement density is high in the outer peripheral portion, so that the arrangement interval of the conductors can be made very small. The above structure alone cannot withstand the Lorentz force generated when current flows, but it can be reinforced by attaching a member with excellent structural strength to the front or back of the gradient coil It is. As a matter of course, it is necessary to use a material having a high electric resistance so that no eddy current is generated in the reinforcing member, or to provide a slit so that the eddy current is hardly generated.

The above description has been made for the case where only the flat main coil is used. However, it can be easily presumed that the present invention can be similarly implemented even when a cylindrical gradient magnetic field coil or a shield coil is provided.

[0028]

According to the present invention as described above, the following effects can be obtained. That is, in the conductor of the gradient magnetic field coil, since the conductor width is narrow in the direction in which the magnetic flux enters,
The eddy current induced in the conductor when the gradient magnetic field coil is driven can be suppressed, and the generated eddy current has a smaller loop size than that of a single wire. Becomes shorter. As a result, an adverse effect on an image can be suppressed, so that good image quality can be obtained. Furthermore, since the conductor cross-sectional area can be increased as compared with the related art, the resistance value of the gradient magnetic field coil can be reduced, and the driving time of the gradient magnetic field coil can be suppressed. In addition, since the conductor surface area is larger than before, the influence of the skin effect when the gradient magnetic field is started at a high speed is reduced. And production is easy, maintaining these effects.

[Brief description of the drawings]

FIG. 1 is a partial sectional view showing a first embodiment of the present invention.

FIG. 2 is a plan view of FIG.

FIG. 3 is a partial sectional view showing a second embodiment of the present invention.

FIG. 4 is a partial perspective view showing a third embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a horizontal magnetic field type magnetic resonance imaging apparatus.

FIG. 6 is a perspective view showing the gradient coil of FIG. 5;

FIG. 7 is a perspective view showing a magnetic resonance imaging apparatus of a vertical magnetic field type.

8 is an enlarged perspective view and a cross-sectional view of a part of FIG.

FIG. 9 is a partial perspective view showing a conventional gradient magnetic field coil.

FIG. 10 is a partial cross-sectional view showing another example of a conventional gradient magnetic field coil.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Substrate 11 Groove 12 and 13 Conductor 30 Substrate 31 and 32 Groove 33 Conductor 34 and 41 Flexible covering material

Claims (4)

[Claims] 1. A gradient magnetic field coil used in a magnetic resonance imaging apparatus, comprising a conductor for forming a current pattern for generating a desired gradient magnetic field,
A gradient magnetic field coil, wherein a cross section of the conductor is long in a direction perpendicular to a current pattern surface. 2. A gradient coil according to claim 1, wherein a plurality of conductors each having a long section in a direction perpendicular to the current pattern surface are grouped together to form a set of conductors. 3. The semiconductor device according to claim 1, wherein a groove corresponding to a current pattern for generating a desired gradient magnetic field is formed in a substrate made of an electric insulator, and said conductor is buried in the groove. 2. The gradient coil according to 2. 4. A gradient coil wherein the conductor according to claim 2 is covered with a flexible material.