JP4214736B2 - Magnetic field generator for MRI - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an MRI magnetic field generator, and more particularly to an MRI magnetic field generator in which a magnetic pole piece composed of a plurality of layers is used.
[0002]
[Prior art]
FIG. 6 shows a pole piece 1 used in a conventional magnetic field generator for MRI.
The pole piece 1 includes a base plate 2. On one main surface of the base plate 2, an annular protrusion 3 and a laminated body 4 of silicon steel plates are formed. In the vicinity of the laminated body 4 of the pole piece 1, a gradient magnetic field coil (not shown) for obtaining positional information is provided. Be placed. When a pulse current is applied to the gradient magnetic field coil to generate a pulsed gradient magnetic field in a desired direction, an eddy current is generated in the magnetic pole piece 1 due to the influence of the gradient magnetic field.
Techniques for reducing the eddy current in the pole piece 1 have been proposed in, for example, international applications WO99-52427 and JP2561591.
[0003]
In International Publication No. WO99-52427, by using a magnetic pole piece that effectively combines a main body made of a laminate of silicon steel plates and a magnetic annular protrusion disposed on the air gap facing surface side of the main body, a gradient magnetic field is used. A technique for reducing eddy current in a magnetic pole piece generated by the influence of a pulse current flowing in a coil is disclosed.
Japanese Patent No. 2561591 discloses a technique for reducing an eddy current caused by a gradient magnetic field coil by using a pole piece in which soft ferrite and a laminated silicon steel plate are laminated from the air gap side.
[0004]
[Problems to be solved by the invention]
As shown in International Publication No. WO99-52427, if the size of each silicon steel plate constituting the main body is reduced as a whole, the current path of eddy currents is subdivided, and the apparent electrical resistance is increased as a whole. Eddy current is reduced. However, in practice, there is a problem that the number of parts increases and the manufacturing cost increases as the silicon steel sheet is made smaller. Further, if the size of the silicon steel plate is reduced, the amount of adhesive used increases, so the space factor of the silicon steel plate in the main body portion decreases, and the magnetic permeability in the direction perpendicular to the stacking direction of the silicon steel plates decreases. Therefore, the magnetic flux generated by the gradient magnetic field coil easily reaches the base plate, and an eddy current is generated in the base plate. As a result, the rising characteristic of the gradient magnetic field is deteriorated and a good image cannot be obtained. Therefore, simply subdividing the silicon steel sheet to reduce eddy current is not always a good idea.
[0005]
On the other hand, in the technique of Japanese Patent No. 2561591, the saturation magnetization of soft ferrite is relatively lower (than that of a silicon steel plate), and the magnetic flux is easily saturated in soft ferrite. Therefore, the substantial saturation magnetization in the laminated portion of the soft ferrite and the laminated silicon steel sheet is reduced, and as a result, the magnetic flux generated by the gradient magnetic field coil is easily saturated, and a magnetic base (corresponding to the base plate 2 shown in FIG. 6). Is further provided, the magnetic flux reaches the magnetic base and an eddy current is generated. Therefore, providing soft ferrite is not very effective.
[0006]
Therefore, a main object of the present invention is to provide a magnetic field generator for MRI that can reduce the eddy current generated in the pole piece and improve the rising characteristic of the gradient magnetic field without increasing the manufacturing cost. .
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the magnetic field generator for MRI according to claim 1 is an MRI magnetic field generator for generating a magnetic field in a gap having a pair of magnetic pole pieces facing each other by forming a gap. The pole piece includes a laminate including a surface layer portion having a protrusion at the center portion on the air gap side, and a deep layer portion whose surface is covered with the surface layer portion, and each of the surface layer portion and the deep layer portion includes a plurality of silicon steel plates. In addition, the size of the silicon steel plate in the surface layer portion is set to 10% or more and 50% or less of the size of the silicon steel plate in the deep layer portion.
The magnetic field generator for MRI according to claim 2 is the magnetic field generator for MRI according to claim 1, characterized in that the surface layer portion includes square-shaped silicon steel plates each having a side of approximately 50 mm or less.
[0008]
MRI magnetic field generator according to 請 Motomeko 3, in the MRI magnetic field generator according to claim 1 or 2, the thickness of the surface layer portion is set at 10% to 50% of the thickness of the laminate It is characterized by.
[0009]
In the magnetic field generator for MRI according to claim 1, the current path of the eddy current is subdivided by reducing the silicon steel plate constituting the surface layer portion provided on the air gap side of the laminate, and the apparent electrical resistance Increases, and the eddy current in the vicinity of the air gap side surface of the pole piece decreases. On the other hand, since the silicon steel plate which comprises a deep layer part is comparatively large, magnetic flux passes easily. Therefore, it is possible to suppress the gradient magnetic field from reaching the base plate, and to suppress eddy currents in the base plate. In this way, by reducing only the surface silicon steel plate and partially reducing the size of the silicon steel plate used for the pole piece, the eddy current can be reduced as a whole and the number of parts does not increase so much. And the rising characteristics of the gradient magnetic field can be improved.
[0010]
In the magnetic field generator for MRI according to claim 2, the surface layer portion of the laminate can be easily manufactured by using a square-shaped silicon steel plate having a side of approximately 50 mm or less that is easy to manufacture and obtain.
[0011]
In the magnetic field generator for MRI according to the third aspect, the eddy current in the vicinity of the air gap side surface of the magnetic pole piece is reduced by setting the electric resistance of the surface layer portion to be larger than the electric resistance of the deep layer portion. In addition, since the saturation magnetization of the surface layer portion and the deep layer portion is 1.0 T or more, magnetic saturation of the entire stacked body can be prevented. Therefore, it is possible to suppress the gradient magnetic field from reaching the base plate, and to suppress eddy currents in the base plate. Further, since it is not necessary to increase the number of parts, the manufacturing cost can be suppressed, and the rising characteristics of the gradient magnetic field can be improved.
[0012]
As described in claim 4, by setting the thickness of the surface layer portion to 10% or more and 50% or less of the thickness of the laminated body, eddy current particularly in the vicinity of the surface of the pole piece can be more effectively suppressed, and the magnetic flux Can easily pass through the laminate, and eddy currents in the base plate can be more effectively suppressed.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0014]
FIG. 1 shows an MRI magnetic field generator 10 according to an embodiment of the present invention.
The magnetic field generator for MRI 10 is an open-type magnetic field generator for MRI, and includes a pair of plate yokes 14a and 14b arranged to face each other so that the gap 12 can be formed. Permanent magnets 16a and 16b such as R-Fe-B magnets are disposed on the opposing surface sides of the plate yokes 14a and 14b, respectively, and pole pieces are provided on the opposing surface sides of the permanent magnets 16a and 16b. 18a and 18b are fixed. Near each gap 12 side (opposite surface side) of the pole pieces 18a and 18b, gradient magnetic field coils 20a and 20b are provided for giving positional information in the gap 12 to the image signal. Although the gradient magnetic field coils 20a and 20b are usually composed of three sets of coil groups corresponding to the three directions of X, Y, and Z in the gap 12, the illustration is simplified. By applying a pulse current to the gradient magnetic field coils 20a and 20b, a gradient magnetic field in a desired direction that changes in a trapezoidal waveform is generated. The strength (gradient degree) of the generated gradient magnetic field is normally set to 5 mT / m or more and 20 mT / m or less, but is preferably set to 10 mT / m or more for high-speed imaging. As the strength of the gradient magnetic field increases, eddy currents are more likely to occur.
[0015]
The plate yokes 14 a and 14 b are magnetically coupled by a single plate-like support yoke 22. That is, the support yoke 22 is positioned such that the lower end surface of the plate yoke 14a is positioned on the upper end surface of the support yoke 22 and the lower end surface of the support yoke 22 is positioned on the upper surface of the end edge side of the plate yoke 14b. Connected to the plate yokes 14a and 14b. Accordingly, the plate yokes 14a and 14b and the support yoke 22 are connected such that the connecting portion has an angle of approximately 90 degrees and is U-shaped in a side view.
[0016]
And the reinforcement member 24 is formed in the both ends of the connection part inner surface side of the plate-shaped yoke 14b and the support yoke 22, respectively. Similarly, reinforcing members 24 are formed at both ends on the inner surface side of the connecting portion between the plate-shaped yoke 14a and the support yoke 22, respectively. Therefore, in this embodiment, a total of four reinforcing members 24 are used, and the plate-like yoke 14a and the supporting yoke 22 are more firmly fixed to the plate-like yoke 14b and the supporting yoke 22 respectively. .
Further, four leg portions 26 are attached to the lower surface of the plate yoke 14b.
[0017]
The magnetic pole piece 18b will be described with reference to FIG.
The pole piece 18b includes a base plate 28 made of a bulk material such as pure iron or carbon steel and having magnetism. An annular protrusion 30 is formed on the opposite surface side of the base plate 28 in order to make the magnetic field generated between the magnetic pole pieces 18a and 18b uniform. The annular protrusion 30 is composed of a plurality of annular protrusion pieces 30a made of soft iron having a rectangular cross section. By forming the diametrical gap 32 between the annular protrusions 30a, eddy currents generated in the annular protrusion 30 can be reduced.
[0018]
A laminated body 34 is provided on the opposite surface side of the base plate 28. The stacked body 34 has a surface layer portion 34a and a deep layer portion 34b from the space 12 side. In this embodiment, the stacked body 34 has a convex protrusion at the center in order to improve the uniformity of the magnetic field distribution.
[0019]
Referring also to FIG. 3, surface layer portion 34a is formed by integrating a plurality of silicon steel plate blocks 36a with an insulating adhesive, and similarly, deep layer portion 34b is formed by integrating a plurality of silicon steel plate blocks 36b with an insulating adhesive. Formed. The size of the silicon steel plate block 36a constituting the surface layer portion 34a is set smaller than the size of the silicon steel plate block 36b constituting the deep layer portion 34b. In this embodiment, the typical size of the silicon steel plate block 36a is silicon. The steel plate block 36b is divided into four parts.
[0020]
The silicon steel plate block 36a is formed by laminating a plurality of silicon steel plates 38a in the opposing direction of the magnetic pole pieces 18a, 18b (the magnetic field generation direction in the gap 12) and integrating them with an insulating adhesive. A plurality of silicon steel plates 38b are laminated in the opposing direction of the magnetic pole pieces 18a, 18b (the magnetic field generation direction in the gap 12) and integrated with an insulating adhesive. The size of the silicon steel plate 38a constituting the silicon steel plate block 36a is set smaller than the size of the silicon steel plate 38b constituting the silicon steel plate block 36b. In this embodiment, the size of the silicon steel plate 38a is typically 38b is divided into four, and the silicon steel plates 38a and 38b are formed in a square shape. The silicon steel plate 38a is preferably formed in a square shape with sides of approximately 50 mm or less. The size of the silicon steel plate 38a is set to 10% or more and 50% or less of the size of the silicon steel plate 38b.
[0021]
The thickness of the surface layer portion 34a is preferably set to 10% or more and 50% or less of the thickness of the stacked body 34. In this case, eddy currents particularly in the vicinity of the surface of the pole pieces 18a and 18b can be effectively suppressed. Further, the magnetic flux easily passes through the laminated body 34, and the eddy current in the base plate 28 can be effectively suppressed. For example, when the thickness of the laminated body 34 is 20 mm, the thickness of the surface layer portion 34a is set to 2 mm or more and 10 mm or less. A silicon steel plate having an area larger than the silicon steel plate 38a and smaller than the silicon steel plate 38b may be provided between the silicon steel plates 38a and 38b.
The pole piece 18a is configured in the same manner as the pole piece 18b.
[0022]
According to the magnetic field generator 10 for MRI, the current path of the eddy current is subdivided by reducing the size of the silicon steel plate near the gradient magnetic field coils 20a, 20b, that is, the silicon steel plate 38a constituting the surface layer portion 34a. The apparent electric resistance increases, and the eddy current near the surface of the magnetic pole piece 18a, 18b near the gap 12 decreases.
On the other hand, since the silicon steel plate 38b is relatively large, the magnetic flux easily passes through the deep layer portion 34b, and the magnetic permeability of the entire laminate 34 can be kept high.
[0023]
Therefore, on the side of the magnetic pole piece 18b, as shown in FIG. 4A, the magnetic flux generated by the gradient magnetic field coil 20b does not flow into the base plate 28, or even if it flows, it can be kept to a small amount. Similarly, on the side of the magnetic pole piece 18a, the magnetic flux generated by the gradient coil 20a does not flow into the base plate 28, or even if it flows, it can be kept to a small amount.
[0024]
On the other hand, as shown in FIG. 4B, when the silicon steel plate constituting the laminated body 4 is made small as a whole, the magnetic flux generated by the gradient magnetic field coil easily reaches the base plate 2, so that it is larger in the base plate 2. Eddy current is generated.
[0025]
As described above, by reducing only a part of the silicon steel plate, the eddy current in the pole pieces 18a and 18b can be reduced without increasing the manufacturing cost, the rise of the gradient magnetic field can be accelerated, and the magnetic field in the gap 12 can be increased. The deterioration of uniformity can be suppressed.
[0026]
In particular, in a magnetic field generator for MRI that generates a static magnetic field of 0.25 T or more in a gap for the purpose of high-speed shooting and acquisition of a fine image, the gradient magnetic field also increases (for example, 10 mT / m or more). If the magnetic field coil is disposed inside the annular projection and close to the base plate, eddy currents are more likely to be generated. However, according to the present invention, even an MRI magnetic field generator configured as described above and generating a strong gradient magnetic field can effectively suppress eddy currents.
[0027]
Moreover, the surface layer part 34a of the laminated body 34 can be easily formed by using the square-shaped silicon steel plate 38a having a side of approximately 50 mm or less that is easy to manufacture and obtain.
[0028]
Next, in another embodiment, the surface layer portion 34a and the deep layer portion 34b may be made of different materials. In this case, the electric resistance of the material constituting the surface layer portion 34a is set larger than the electric resistance of the material constituting the deep layer portion 34b, and the saturation magnetization of the surface layer portion 34a and the deep layer portion 34b is set to 1.0 T or more.
[0029]
For example, among the materials shown in FIG. 5, a magnetic composite material (specific resistance 1 × 10 ⁻⁶ Ω · m, saturation magnetization 1.4 T) made of a sintered body of iron powder and resin is used for the surface layer portion 34a. For example, a silicon steel plate (specific resistance 5.6 × 10 ⁻⁷ Ω · m, saturation magnetization 1.97 T) or pure iron (specific resistance 1 × 10 ⁻⁷ Ω · m, saturation magnetization 2.15 T) is used for the deep layer portion 34b. Can be used. Moreover, when using a silicon steel plate for the surface layer part 34a, pure iron can be used for the deep layer part 34b.
[0030]
By using such a material for the surface layer portion 34a and the deep layer portion 34b and setting the electric resistance of the surface layer portion 34a to be larger than the electric resistance of the deep layer portion 34b, eddy currents near the surface of the magnetic pole piece 18a and 18b on the air gap 12 side Get smaller.
[0031]
In addition, by setting the saturation magnetization of the surface layer portion 34a and the deep layer portion 34b to 1.0 T or more and using a material having a saturation magnetization larger than soft ferrite as the surface layer portion 34a, magnetic saturation of the entire stacked body 34 can be prevented. Therefore, it is possible to suppress the gradient magnetic field from reaching the base plate 28 and to suppress the eddy current in the base plate 28. Furthermore, since the number of parts does not increase, the manufacturing cost can be suppressed, and the deterioration of the magnetic field uniformity in the gap 12 can be suppressed.
[0032]
【The invention's effect】
According to the present invention, the eddy current can be reduced as a whole in the magnetic field generator for MRI, the manufacturing cost can be suppressed, and the rising characteristic of the gradient magnetic field can be improved.
[Brief description of the drawings]
FIG. 1 is an illustrative view showing one embodiment of the present invention, in which (a) is a front view and (b) is a cross-sectional view.
FIG. 2 is an illustrative view showing one example of a pole piece used in the present invention, in which (a) is a cross-sectional view, (b) is a plan view, and (c) is an enlarged schematic view of a main part.
FIG. 3 is a perspective view showing an example of a silicon steel plate block.
FIG. 4 is an illustrative view showing a magnetic flux passing through a magnetic pole piece, where (a) shows a magnetic flux in the embodiment, and (b) shows a magnetic flux in the prior art.
FIG. 5 is a table showing specific resistance and saturation magnetization of various materials.
FIG. 6 is an illustrative view showing a conventional magnetic pole piece, in which (a) is a cross-sectional view, (b) is a plan view, and (c) is an enlarged schematic view of a main part.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Magnetic field generator for MRI 12 Air gap 18a, 18b Magnetic pole piece 20a, 20b Gradient magnetic field coil 28 Base plate 34 Laminated body 34a Surface layer part 34b Deep layer part 36a, 36b Silicon steel plate block 38a, 38b Silicon steel plate

Claims (3)

A magnetic field generator for MRI that has a pair of opposing magnetic pole pieces forming a gap and generates a magnetic field in the gap,
The magnetic pole piece includes a laminate including a surface layer portion having a protrusion at the air gap side central portion, and a deep layer portion whose surface on the air gap side is covered by the surface layer portion,
The surface layer portion and the deep layer portion each include a plurality of silicon steel plates, and the size of the silicon steel plate of the surface layer portion is set to 10% or more and 50% or less of the size of the silicon steel plate of the deep layer portion. Generator.   The MRI magnetic field generator according to claim 1, wherein the surface layer portion includes a square silicon steel plate having sides of approximately 50 mm or less.   The magnetic field generator for MRI according to claim 1 or 2, wherein the thickness of the surface layer portion is set to 10% or more and 50% or less of the thickness of the laminate.

Images