JPH07313490A - Gradient magnetic field generator for magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To prevent the generation of eddy currents in electric conductor having planar spiral shapes constituting a gradient magnetic field generator for an MRI apparatus by providing these electric conductors with nonconductive parts so as to divide current conducting paths to a plurality without cutting these paths. CONSTITUTION:The gradient magnetic field coil 30 of the gradient magnetic field generator consists of the electric conductors 31. The shapes of the electric conductors 31 exclusive of an outer peripheral part are regulated by spacings 33 between the electric conductors 31 to form the current conducting paths from the points near the center on the left side through the respective spirals on the right side and the left side to the points near the center on the right side. The broad parts of the electric conductors 31 are provided with the non-conducting paths 34 so as to divide the current conducting paths to two without cutting the current conducting paths. As a result, the areas of the electric conductors have no more wide parts and the effective area with respect to generation of the eddy currents are nearly halved. The generation of the eddy currents is thus prevented. Cutting of the currents intrinsic to the gradient magnetic field coil does not arise and the coil patters do not substantially change and, therefore, the coil characteristics do not change.

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 generator for a magnetic resonance imaging apparatus (hereinafter referred to as "MRI apparatus"), and in particular, it is possible to suppress the generation of eddy currents and obtain an excellent MRI image. A gradient magnetic field generator for a device.

[0002]

PRIOR ART AND PROBLEMS TO BE SOLVED BY THE INVENTION MRI
The apparatus measures a signal obtained from a nuclear magnetic resonance (hereinafter referred to as “NMR”) phenomenon of a subject placed in a magnetic field and performs arithmetic processing to obtain a density distribution of nuclear spins in the subject,
The relaxation time distribution and the like are displayed as a tomographic image, which is used for various diagnoses with the human body as the subject.

In order to obtain a signal from the NMR phenomenon, the subject is placed in a static magnetic field having a spatially and temporally uniform intensity and direction, and the subject is irradiated with electromagnetic waves in a pulse by a high frequency coil. The NMR signal generated thereby is received by the high frequency coil. Further, a gradient magnetic field is superimposed on the static magnetic field in order to give position information to the NMR signal. Therefore, in addition to the static magnetic field generator, the MRI apparatus includes coils for gradient magnetic field generation orthogonal to the three-axis directions, and a gradient magnetic field is generated by passing a current through these gradient magnetic field coils in a predetermined pulse sequence.

The static magnetic field generator for such an MRI apparatus can be roughly classified into three types, one using a permanent magnet, one using a superconducting coil, and one using a normal conducting coil. FIG. 6 is a front sectional view of a portion of a static magnetic field generator and a gradient magnetic field generator of a general MRI apparatus using a magnet, and FIG. 7 is a perspective view thereof. In the apparatus shown in FIGS. 6 and 7, the measurement space 50 into which the subject 60 is inserted
A pair of permanent magnets 2a and 2b having different polarities are provided with the pole sandwiched therebetween. These permanent magnets 2a and 2b are held in contact with a pair of upper and lower rectangular yoke plates 3a and 3b, and the yoke plates 3a and 3b are supported by a yoke rod 4 and the permanent magnet 2
A measurement space 50 between a and 2b is secured. Further, the pole pieces 10 are provided on the surfaces of the permanent magnets 2a and 2b on the measurement space side.
The a and 10b are abutted and held in close contact with each other, and annular projections 12a and 12b are provided on the peripheral portions of the surfaces of the magnetic pole pieces 10a and 10b which face each other. In the apparatus of FIGS. 6 and 7, a static magnetic field generator is configured by the above-mentioned elements, and magnetic flux is generated from the permanent magnets 2a and 2b to generate a static magnetic field in the measurement space 50 via the pole pieces 10a and 10b. Let The yoke plates 3a and 3b and the yoke bar 4 mechanically constitute the static magnetic field generator, and at the same time, the pole piece 10
a, 10b and the permanent magnets 2a, 2b are magnetically coupled to form a magnetic flux path. Further, the annular protrusions 12a and 12b prevent the magnetic flux from leaking to the peripheral portion of the measurement space 50, and this function can improve the homogeneity of the magnetic field inside the measurement space 50.

Then, gradient magnetic field coils 20a, 2 which are gradient magnetic field generators for giving position information to the NMR signal.
0b is composed of a coil made of an electric conductor in the form of a flat spiral so as not to narrow the space for inserting the subject 60, and is designed to be accommodated in the space surrounded by the annular protrusions 12a, 12b. There is. As described above, in the gradient coil, three independent coil groups corresponding to the directions of the three axes (the directions of the X, Y, and Z axes shown in FIG. 7) are arranged above and below the measurement space. 7, the respective coils are not shown, and the coil groups arranged on the upper and lower portions are collectively represented as the gradient magnetic field coils 20a and 20b. The gradient magnetic field coil is composed of a pair of parallel coils or four coils for each direction, and these structures are described in, for example, Japanese Patent Laid-Open No. 63-65848.

The electric conductor in the form of a flat spiral that constitutes the above-mentioned gradient magnetic field coil has, for example, a gradient that gives a gradient magnetic field in the X-axis direction (horizontal direction) or the Y-axis direction (direction perpendicular to the X-axis and horizontal). In the case of a magnetic field coil, a coil having a pattern as shown in FIG. 5 is generally used. In the gradient magnetic field coil 20 shown in FIG. 5, the electric conductor 21 starts from a point 21a near the center on the left side of the drawing, expands spirally to the outside, and moves to the right side, and the left side with respect to the center of the circle on the center line 22. The vortex spirals in the opposite direction in a pattern symmetrical with and ends at a point 21b near the center on the right side. The shape other than the outer peripheral portion of the electric conductor 21 has a gap 23 between the electric conductors.
No current flows through the gap 23.

According to the pattern shown in FIG. 5, two coils are formed on the left side and the right side. These coils are provided at positions facing the measurement space 50 (for example, 20a), and those having the same shape are also provided on opposite sides of the measurement space 50 (20b), whereby four coils are provided as X coils. A gradient magnetic field coil for generating a gradient magnetic field about one axis in the axial direction or the Y-axis direction is configured.
5, assuming that the horizontal direction of the drawing is the X-axis direction and the vertical direction is the Y-axis direction, what is shown is a part of the gradient magnetic field coil that applies a gradient magnetic field in the X direction. The shape of the electric conductor forming the coil as shown in FIG. 5 is not a simple spiral shape, and is mainly used for ensuring the linearity of the change in the gradient magnetic field strength.

Such gradient magnetic field coils 20a, 20b
For example, by applying a pulse current that changes with time as shown in the graph of FIG. 8, a magnetic field that changes correspondingly can be formed (for example, a magnetic field strength change as shown by a broken line in the graph of FIG. 9). Gradient magnetic field).

When a gradient magnetic field that changes with time is generated as described above, if an electric conductor exists in the magnetic field of the gradient magnetic field, an induction current, that is, an eddy current is generated in the electric conductor by electromagnetic induction. This eddy current is generated by the gradient coil 20.
A magnetic field generated in the measurement space 50 is generated in the opposite direction to the magnetic field formed in the measurement space 50 by a and 20b. Therefore, the time (rise time) until the gradient magnetic field in the measurement space 50 reaches a predetermined constant strength is extended. Has an adverse effect. That is, as described above, where the change in magnetic field strength as shown by the broken line in the graph of FIG. 9 should be shown, the change in magnetic field strength as shown by the solid line in FIG. 9 results in the time until the desired constant magnetic field strength (gradient The rise time of the magnetic field) is extended.

In the conventional MRI apparatus, the influence method is as follows:
Since a relatively long imaging time was required, even if the rising time of the gradient magnetic field as described above takes about 1 ms, there is no great influence, and the effect of the eddy current, that is, the rising of the gradient magnetic field as shown in FIG. The effect of the delay on the image was almost negligible.

However, like the ultra-high-speed imaging method (for example, echo planar method) which has been developed for practical use in recent years,
If one image is taken in an extremely short time of several tens of ms, the rise time of the gradient magnetic field must also be on the order of 0.1 ms. The state of spin changes and it becomes impossible to obtain an accurate signal. Therefore, in order to improve the image quality, it is preferable that the gradient magnetic field waveform is not influenced by the eddy current and has a short rise time, and the delay of the rise time due to the generation of the eddy current cannot be ignored.

As the electric conductor existing in the magnetic field of the gradient magnetic field and capable of generating the eddy current as described above, for example, in the device shown in FIG. 6, the magnetic pole pieces 10a and 10b can be considered. 2-218343, JP-A-2-246927
As described in Japanese Patent Publication No.
It is possible to eliminate the influence on the rise time of the gradient magnetic field by making b substantially composed of a poor electrical conductor so that eddy current does not substantially flow through the pole pieces.

Further, in the MRI apparatus employing the static magnetic field generator using the superconducting coil as shown in FIG. 10, the superconducting coil 40 for generating the static magnetic field needs to be kept at a very low temperature. It is housed in a liquid helium container 44, and the outside thereof is a cylindrical heat shield 42.
Although it is covered with a and 42b, this heat shield 42a,
42b also generates an eddy current. That is, the heat shield 42
Since a and 42b are generally formed of a metal having a high thermal conductivity such as aluminum for the purpose of effectively shielding heat, and therefore are also made of a material having a high electrical conductivity, an eddy current is generated by the gradient magnetic field and the rising of the gradient magnetic field occurs. It causes a delay in time.

For this, for example, Japanese Unexamined Patent Publication No. 62-194842
It is effective to provide an active shield coil as described in No. This active shield coil has a cylindrical main coil 25 and a cylindrical shield coil 2 having the same pattern as the main coil that is coaxial with the main coil 25 and is provided outside so as to cover the main coil.
It consists of 6. As described above, the gradient magnetic field generator is composed of the main coil 25 and the active shield coil 26, and the magnetic field is generated outside the active shield coil by optimizing the balance of the gradient magnetic fields generated by these two coils. To prevent the generation of eddy currents in the external heat shield, for example.

Further, in the gradient magnetic field coil having a planar shape as shown in FIG. 5, an eddy current is also generated in the electric conductor itself forming the coil due to the magnetic field generated by the gradient magnetic field coil. FIG. 11 shows the result of computer simulation of how the rising of the gradient magnetic field waveform changes due to the effect of the eddy current generated in the electric conductor itself of the gradient magnetic field coil. The current has a trapezoidal shape that starts flowing from time 0 and reaches a constant value in 1 ms, and the gradient magnetic field strength (relative value) is plotted against the time after the current is applied. ○ indicates that there is an effect of eddy current, and □ indicates that there is no eddy current. In this way, eddy currents are also generated by the gradient magnetic field coil itself, and this is the same in the main coil and the shield coil of the active shield system, and in particular, the width of the electric conductor that constitutes the coil is wide and the area of the electric conductor is large. Larger eddy currents are generated in the portion where there is, and the above-mentioned problems are more likely to occur. And since the gradient coil must be formed of an electrical conductor, the solution as proposed for the pole pieces described above cannot be adopted.

Therefore, the present invention can effectively prevent the generation of eddy currents in the planar spiral electric conductor used in the gradient magnetic field generator for MRI apparatus.
An object is to provide a gradient magnetic field generator for a device.

[0017]

The gradient magnetic field generator for an MRI apparatus of the present invention which achieves the above object is a gradient magnetic field generator for a magnetic resonance imaging apparatus, which is composed of an electric conductor in the form of a planar spiral. In the above, the electric conductor is provided with a non-conductive portion so as not to cut the current conducting path and to divide the electric conducting path into a plurality of portions.

In a preferred embodiment of the gradient magnetic field generator for an MRI apparatus of the present invention, the non-conductive portion is selectively provided only in a portion where the width of the electric conductor in the current conducting direction is relatively wide.

In a preferred embodiment of the gradient magnetic field generator for an MRI apparatus of the present invention, a non-conductive portion is further provided so as to divide the current conducting path into three or more.

In one aspect of the gradient magnetic field generator for an MRI apparatus of the present invention, the non-conductive portion is provided in the main coil of the gradient magnetic field generator using the active shield system.

In one aspect of the gradient magnetic field generator for an MRI apparatus of the present invention, the non-conductive portion is provided in the shield coil of the gradient magnetic field generator using the active shield coil system.

[0022]

When a current is passed through the gradient magnetic field coil, a magnetic field is generated around it. An eddy current is also induced in the electric conductor itself of the gradient coil by the dielectric action of this gradient magnetic field. The degree of induced eddy current depends on the area of the electrical conductor,
The larger the area, the larger the eddy current will flow for a long time. Therefore, in order to reduce the influence of the eddy current, it is effective to make the area of the electric conductor as small as possible. It should be noted that the term "wide area" used here does not mean the area of the entire electric conductor, but means that the effective area for generating eddy currents is relatively large. In the gradient magnetic field coil No. 5, the width of the electric conductor 21 in the current conducting direction is larger than that of the other portions.

On the other hand, it is desirable to reduce the resistance value of the electric conductor as much as possible in order to prevent heat generation in the gradient magnetic field coil, and it is necessary to increase the cross-sectional area of the conductor. However, since the gradient magnetic field coil is formed in a planar shape, it is manufactured by etching a printed circuit board or cutting a gap in a copper plate by water jet processing (cutting processing with high-pressure water). Therefore, increasing the thickness of the electric conductor has a limit in terms of manufacturing technology, and in order to reduce the resistance, it is necessary to reduce the width of the gap and use a wide electric conductor pattern. This is contrary to the prevention of the eddy current.

Further, the pattern of the gradient magnetic field coil is adopted to secure the linearity of the change of the gradient magnetic field strength as described above, and the characteristic of the gradient magnetic field coil is changed by this pattern. It is impossible to modify the pattern itself.

Therefore, in the present invention, a planar spiral electric conductor that constitutes the gradient magnetic field generator for an MRI apparatus is provided with:
In particular, a non-conductive portion is provided so as not to cut the current conducting path and to divide it into a plurality in a portion where the width is wide, so that the characteristics of the coil can be changed significantly, such as changing the resistance value of the coil. In addition, since there can be no relatively large area of the electric conductor, it is possible to prevent the generation of eddy currents in the gradient magnetic field coil and the like, and thus to increase the rising time of the gradient magnetic field. , The image quality of the image of the MRI apparatus can be improved.

[0026]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a plan view of a gradient magnetic field generating coil 30 of a gradient magnetic field generating apparatus of the present invention having a coil pattern similar to the gradient magnetic field generating coil used in the MRI apparatus using the permanent magnet shown in FIG. The basic structure of the gradient magnetic field coil shown in FIG. 1 is the same as that of the conventional gradient magnetic field coil shown in FIG. 5, and is composed of an electric conductor 31 for passing an electric current. Conductor 3
A current conducting path is defined by the gap 33 between the first and the second, and extends from a point near the center on the left side to a point near the center on the right side through each spiral on the right side and the left side. Similar to FIG. 5, what is shown is a part of a gradient magnetic field coil that provides a gradient magnetic field in the X direction.

In the gradient magnetic field coil of FIG. 1, a non-conductive portion 34 is provided in a wide portion of the electric conductor 31 so as not to cut the current conducting path and divide the current conducting path into two. There is. As a result, the large area of the electric conductor is eliminated, the effective area for eddy current generation is almost halved, the generation of eddy current can be prevented, and the original current of the gradient coil is not cut off. Since the coil pattern does not change substantially, the coil characteristics do not change substantially.

That is, in the portion where the non-conductive portion 34 is provided, the original current of the gradient magnetic field coil is divided into two and flows in parallel to the respective electric conductors, but it does not change the coil pattern itself. While the coil characteristics are not substantially changed, the electric conductor is divided into two, and the effective area for eddy current generation is reduced to almost half, so the magnitude of the eddy current generated is greatly reduced. Is something that can be done.

The non-conductive portion 34 preferably has an elongated shape as shown in FIG. 1 and is formed on the electric conductor 31 substantially parallel to the current conducting direction of the electric conductor 31. If the width of the non-conductive portion 34 is made sufficiently smaller than the width of the electric conductor, the resistance value hardly changes. Further, from the viewpoint of reducing the effective area of the electric conductor 31 for generating the eddy current, the non-conductive portion 34 may be provided only in a relatively wide portion of the electric conductor 31. Since the non-conductive portion 34 is not provided in a portion having a small width and a small amount of eddy current generation, the electric conductor 31
It is possible to suppress an increase in the resistance value of, and to reduce the number of manufacturing steps. On the contrary, in a portion where the electric conductor 31 has a wider width, two or more non-conductive portions 34 may be provided and the current conducting path may be divided into three or more to further reduce the area of the electric conductor.

The gradient magnetic field coil is manufactured by etching the printed circuit board as described above or cutting out the gap 33 in the copper plate by water jet processing (cutting processing with high pressure water). Non-conductive portion 34 according to the present invention
Can be easily manufactured by the manufacturing process of such a conventionally used gradient magnetic field coil. That is, in the case of the etching process, the slit can be formed only by previously inserting the pattern of the non-conductive portion in the mask that defines the etching pattern. However, the width of the non-conductive part is
If the thickness of the electric conductor is too small compared to the thickness of the electric conductor, the electric conductor cannot be sufficiently removed by etching, and an eddy current will flow because it becomes conductive, so care must be taken. Further, in the case of water jet processing, since the processing pattern is usually controlled by a computer, it can be produced by simply inputting the shape of the non-conductive portion in the processing pattern program in advance.

The electric conductor 31 is generally formed of a soft material such as thin copper as described above. Therefore, although not explicitly shown in FIG. 1, the electric conductor is fixed on the base, and the shape and mechanical strength of the electric conductor are held by the base. Such a base material can be selected from various materials such as an epoxy resin material and a bakelite material according to design conditions such as mechanical strength, workability and price.

Although FIG. 1 shows a gradient magnetic field coil in the X direction, the gradient magnetic field coil in the Y direction has a shape obtained by rotating the pattern of FIG. 1 by 90 ° with respect to the center point of the coil, and is generally The gradient magnetic field coils in the X and Y directions are provided on one base. The gradient magnetic field coil in the Z-axis direction is different from the one in the X- and Y-directions, and is generally manufactured by winding a rectangular wire rod of about several mm in one spiral shape, and is formed by two coils in the Z-axis direction. A magnetic field coil is constructed.
Therefore, in the case of the gradient magnetic field coil in the Z-axis direction, there is usually no portion where the electric conductor has a large area, and the amount of eddy current generated is small, so that the above-mentioned generation of eddy current does not pose a problem. However, even in the case of the gradient magnetic field coil in the Z-axis direction, as shown in FIG.
Needless to say, the present invention can be applied regardless of the pattern when an electric conductor having a wide portion similar to the pattern as shown in FIG.

In the above description, the present invention has been described with respect to the MRI apparatus using a permanent magnet in the static magnetic field generator, but the problem of the eddy current generated in the gradient magnetic field coil has a large area in the gradient magnetic field coil. As long as an electric conductor having a planar spiral shape is used, it is generated regardless of the type of magnet used in the static magnetic field generator. For example, a superconducting coil was used as a static magnetic field generator as shown in FIG.
The same applies to the gradient magnetic field coil in the MRI apparatus. The structure of the main coil 25 constituting this gradient magnetic field generator is shown in FIGS. 12 and 13. As the shield coil for active shield, a coil in which the number of turns of the spiral is smaller than that of the main coil and the width of the electric conductor forming the spiral is wider than that of the main coil is used. It is almost the same.

These coils have a cylindrical shape as shown in FIG. 12, and are composed of respective coil groups for applying a gradient magnetic field in the directions of the X and Y axes. Figure 1
The main coil 25 for giving a gradient magnetic field in the Y-axis direction shown in FIG. 2 is composed of four planar spiral coils, and has a radius R and a length 2L as shown in FIG. When a line segment m having a length of 2L and defined by the horizontal plane passing through the center line 1 and the cylindrical surface is developed as a cutting line on the plane, its plan view is shown in FIG. These planar coils have a planar spiral shape like the gradient magnetic field coil of the MRI apparatus using the above-mentioned permanent magnet in the static magnetic field generator, and have a large area of the electric conductor,
As with the above, the generation of eddy currents can be a problem. Therefore, M using such a superconducting coil in a static magnetic field generator
Also in the gradient magnetic field generator of the RI apparatus, the problem of eddy current can be solved by applying the present invention and providing a non-conductive portion in the electric conductor so as not to cut the current conducting path and to divide the electric conducting path into a plurality. it can.

FIG. 2 shows the expanded coil shown in FIG. 13 provided with a non-conductive portion so as not to cut the current conducting path and to divide it into a plurality of pieces according to the present invention. FIG. 2 shows one of the four coils shown in FIG. In the coil 35 shown in FIG. 2, as in the coil shown in FIG. 1, the shape of the electric conductor 31 is defined by the gap 33 except for the outer peripheral portion, and the electric current is applied to the wide portion of the electric conductor 31. The non-conductive portion 34 is provided so as not to cut the passage and divide the passage into two.

Further, the shield coil provided outside the main coil is also generally a coil having the same planar spiral shape as the main coil, and the present invention can be applied. The shield coil generally has the same pattern as the main coil, but a coil in which the number of spiral turns is smaller than that of the main coil and the width of the electric conductor forming the spiral is wider than that of the main coil is used. Therefore, the electric conductor of the shield coil has a portion of the electric conductor having a larger area than that of the main coil, and the amount of eddy current generated is larger, so that the effect obtained by applying the present invention is more effective. It will be remarkable. Main coil 3 shown in FIG.
FIG. 3 shows a shield coil 36 corresponding to No. 5 provided with a non-conductive portion 34 which does not cut the current conducting path and divides it into two in a wide portion of the electric conductor 31 according to the present invention.

Further, in the case where the electric conductor has a wider area like this shield coil, as described above, not only one non-conductive portion is provided in the same portion, but two or more non-conductive portions are not provided. By providing the conductive portions at the same location, the effective area where the eddy current is generated can be further narrowed and the effect of suppressing the eddy current can be enhanced.
In this case, the current conducting path is divided into three or more. FIG. 4 shows an example of a coil in which two or more non-conductive portions 34 are provided at the same position in the same pattern as the coil 36 shown in FIG.

The case where the present invention is applied to each of the main coil and the shield coil of the gradient magnetic field generator of the MRI apparatus using the active shield coil system has been described above. However, the present invention is applied only to both coils. However, it also includes the case where it is applied to either one.
As described above, the present invention can be applied regardless of the shape of the gradient magnetic field coil depending on the type of the static magnetic field generator of the MRI apparatus as long as it uses a planar spiral electric conductor for the gradient magnetic field coil. it can.

[0039]

As is apparent from the above description, in the gradient magnetic field generator for an MRI apparatus of the present invention, a current conducting path is cut in a wide spiral portion of a planar spiral electric conductor. The non-conductive part is provided so as to be divided into a plurality of parts, and thus it is possible to prevent the part of the electric conductor having a relatively large area from being changed without significantly changing the characteristics of the coil. It is possible to prevent the generation of eddy currents in the gradient magnetic field coil and the like. Therefore, it is possible to solve the problem of extending the rising time of the gradient magnetic field due to the generation of the eddy current, and to improve the image quality of the image of the MRI apparatus even in the imaging that requires high-speed switching of the gradient magnetic field such as ultra-high-speed imaging. Can be improved. Further, the present invention provides a non-conductive portion in an electric conductor without substantially changing the pattern of the gradient magnetic field coil, and therefore, the eddy current of the eddy current is substantially affected without substantially affecting the original characteristics of the gradient magnetic field coil. Since it prevents the generation, it can be applied regardless of the pattern of the gradient magnetic field generating coil.

[Brief description of drawings]

FIG. 1 is a plan view of an embodiment of a gradient magnetic field coil of a gradient magnetic field generator of the present invention used in an MRI apparatus using a permanent magnet in a static magnetic field generator.

FIG. 2 is a partial plan development view of an embodiment in which a main coil of a gradient magnetic field generator using an active shield coil system, which is used in an MRI apparatus using a superconducting magnet as a static magnetic field generator, is configured according to the present invention.

FIG. 3 is a partial plan development view of an embodiment in which a shield coil of a gradient magnetic field generator using an active shield coil system, which is used in an MRI apparatus using a superconducting magnet as a static magnetic field generator, is constructed according to the present invention.

FIG. 4 is a partial plan development view of an embodiment in which a shield coil of a gradient magnetic field generator using an active shield coil system, which is used in an MRI apparatus using a superconducting magnet as a static magnetic field generator, is constructed according to the present invention.

FIG. 5 is a plan view of a gradient magnetic field coil of a conventional gradient magnetic field generator used in an MRI apparatus using a permanent magnet in a static magnetic field generator.

FIG. 6 is a schematic sectional view of an MRI apparatus using a permanent magnet in a static magnetic field generator.

FIG. 7 is a schematic perspective view of an MRI apparatus using a permanent magnet in a static magnetic field generator.

FIG. 8 is a graph showing the change over time of the current passed through the gradient magnetic field coil.

FIG. 9 is a graph showing changes in gradient magnetic field strength with and without the influence of eddy currents.

FIG. 10: MRI using a superconducting magnet in a static magnetic field generator
The schematic sectional drawing of an apparatus.

FIG. 11 is a graph showing a temporal change in gradient magnetic field strength due to an eddy current generated in a gradient magnetic field coil.

FIG. 12: MRI using a superconducting magnet in a static magnetic field generator
The perspective view which shows the shape of the coil used for the gradient magnetic field generator of an apparatus.

FIG. 13: MRI using a superconducting magnet in a static magnetic field generator
FIG. 4 is a plan development view of a conventional coil used in the gradient magnetic field generator of the apparatus.

[Explanation of symbols]

 30 Gradient magnetic field coil 31 Electric conductor 33 Gap 34 Non-conductive part 35 Main coil 36 Shield coil

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location H01F 5/00 C 4231-5E G01N 24/06 510 Y 8203-2G G01R 33/22

Claims (5)

[Claims] 1. A gradient magnetic field generator for a magnetic resonance imaging apparatus comprising a planar spiral electrical conductor, wherein the electrical conductor is non-conductive so as not to cut a current conducting path and to divide the current conducting path into a plurality of sections. A gradient magnetic field generator for a magnetic resonance imaging apparatus, characterized in that a magnetic field is provided. 2. The gradient magnetic field for a magnetic resonance imaging apparatus according to claim 1, wherein the non-conductive portion is selectively provided only in a portion where the width of the electric conductor with respect to the current conducting direction is relatively wide. Generator. 3. The gradient magnetic field generator for a magnetic resonance imaging apparatus according to claim 1, wherein the non-conductive portion is provided so as to divide the current conducting path into three or more. 4. A magnetic field according to claim 1, wherein the planar spiral electric conductor is a main coil of a gradient magnetic field generator using an active shield coil system. Gradient magnetic field generator for resonance imaging. 5. The planar spiral-shaped electric conductor is a shield coil of a gradient magnetic field generator using an active shield coil system.
A gradient magnetic field generator for a magnetic resonance imaging apparatus according to any one of 1.