JPH04138131A - Magnetic field generation device for mri - Google Patents

Abstract

PURPOSE:To reduce the occurrence of eddy current and remanence phenomena due to an inclined magnetic field coil by forming a pair of pole pieces opposite to each other via a gap with a plurality of block type pole piece materials of a plurality of silicon steel sheets intergrally laminated in the opposing direction of the pole pieces. CONSTITUTION:A pole piece 10 comprises a magnetic material base 11 made of disc-shaped soft iron, a soft iron magnetic material ring 12 of rectangular section laid around the magnetic material base 11, a plurality of block type pole piece materials 13 laid on the upper surface of the magnetic material base 11. The block type pole piece material 13 is fixed to the magnetic material base 11 with a synthetic resin adhesive. The soft fixing type magnetic ring 12 of rectangular section laid around the magnetic material base 11 is so formed as to have larger height at the side of the periphery of the pole piece 10 for the concentration of magnetic flux in the predetermined gap, and an annular projection for improving the uniformity of the magnetic flux. The ring 12 is bolted to the magnetic material base 11 via an insulation material. Furthermore, the magnetic material ring 12 is divided in a circumferential direction, thereby forming radial slits to reduce an eddy current effect.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a medical magnetic resonance tomography apparatus (hereinafter referred to as MHI).
In order to improve the magnetic field generation device used in applications such as 2017, a pair of magnetic pole pieces facing each other with a gap formed is integrated into multiple blocks made by laminating multiple silicon steel plates in the direction in which the magnetic pole pieces face each other. The present invention relates to a magnetic field generating device for MRI, which is composed of a shaped magnetic pole piece member and which aims to reduce eddy currents and residual magnetic phenomena in the magnetic pole piece due to gradient magnetic field coils without impairing magnetic field uniformity within the air gap.

Conventional MHI technology involves inserting part or all of the subject into the gap of a magnetic field generator that generates a strong magnetic field, and is able to obtain a tomographic image of the object and depict the nature of its tissue. It is a device.

In the above magnetic field generation device for MRI, the gap must be wide enough to allow part or all of the subject to be inserted;
In addition, in order to obtain a clear tomographic image, the imaging field of view within the void is usually 0.02 to 2.0T and 1×104
It is required to create a stable, strong, and uniform magnetic field with the following accuracy:

As shown in Fig. 4, the magnetic field generating device used in MRI uses an R-Fe-B magnet as a magnetic field generating source, and a magnetic pole piece (ν) is attached to one end of each of a pair of permanent magnet structures (IXI).
A known configuration is known in which magnetic pole pieces (2X2) are fixed and faced to each other, and the other ends are connected by a yoke (3) to generate a static magnetic field in the gap (4) between the magnetic pole pieces (2X2).

The magnetic pole piece (2X2) has an annular protrusion (5) on the periphery in order to improve the uniformity of the magnetic field distribution within the air gap (4).
It is usually constructed from a plate-shaped bulk (integral object) cut out of a magnetic material such as electromagnetic soft iron or pure iron (Japanese Patent Application Laid-open No. 88407/1983).

The gradient magnetic field coils (6) placed near each magnetic pole piece (2X2) are usually divided into three groups of coils corresponding to the three directions of x, y, and z in order to obtain positional information within the air gap (4). However, in the illustration, the description is simplified.

By applying a pulse current to this gradient magnetic field coil (6), it is possible to generate a gradient magnetic field in a desired direction that changes over time in a trapezoidal waveform.

Problem to be Solved by the Invention When a pulse current is passed through the gradient magnetic field coil (6), the magnetic pole piece (
2) is composed of a plate-shaped bulk as mentioned above, so
When the current rises and falls, the magnetic field changes rapidly and an eddy current is generated in the magnetic pole pieces (2×2).

Since this eddy current forms a magnetic field in the opposite direction to the magnetic field formed by the gradient magnetic field coil (6), it takes a long time for the gradient magnetic field to reach a predetermined strength.

As a means to solve the above-mentioned problem, a planar laminate in which soft magnetic thin plates are laminated as magnetic pole pieces so that the laminated plane is perpendicular to the magnetic pole face is used, and two layers are used so that the laminated directions differ by about 90 degrees from each other. - A magnetic field generator using a magnetized structure (Japanese Patent Laid-Open No. 61-203605), a magnetic field generator using magnetic powder with high resistivity (Japanese Patent Laid-Open No. 63-259)
07) has been proposed.

However, even in the configuration that aims to reduce eddy currents as described above, the magnetic pole pieces are magnetized by the magnetic field formed by the gradient magnetic field coil (GC), and even after the GC pulse is stopped due to the magnetic hysteresis phenomenon (electromagnetic phenomenon), the air gap is There is a problem that the uniformity within is disturbed.

This invention is proposed in view of the above-mentioned current state of the magnetic pole pieces of magnetic field generators for MRI, and it reduces the generation of eddy current without reducing the magnetic field uniformity in the air gap, so that the gradient magnetic field can be generated in a short time to a predetermined value. The purpose is to provide a magnetic pole piece with a structure that can increase the strength, and also to provide a magnetic pole piece with a structure that can reduce electromagnetic phenomena and obtain clear images with high sensitivity. The object of the present invention is to provide a magnetic pole piece having a structure that is easy to use, has high mechanical strength, and has excellent assembly workability.

Means for Solving the Problems As a result of various studies in order to achieve the above object in a magnetic field generator for MRI, the present invention has developed a method in which a pair of magnetic pole pieces facing each other with a gap formed therein are made of a plurality of silicon steel plates. By forming the required shape with a plurality of block-shaped magnetic pole piece members that are stacked and integrated in the opposite direction of the magnetic pole pieces, processing,
It has been found that it is easy to manufacture and can reduce eddy currents and electromagnetic phenomena caused by gradient magnetic field coils without reducing magnetic field strength and magnetic field uniformity.

That is, the present invention provides an MRI magnetic field generation device that has a pair of opposing magnetic pole pieces forming an air gap and generates a magnetic field in the air gap, in which a plurality of silicon steel plates are laminated in the direction in which the magnetic pole pieces face each other. This is a magnetic field generating device for MRI, characterized in that a magnetic pole piece is formed using a plurality of block-shaped magnetic pole piece members that are integrated with each other.

Further, in the above configuration, the present invention improves the mechanical strength of the entire magnetic pole piece by arranging a plurality of block-shaped magnetic pole piece members on a plate-shaped magnetic material base to form the magnetic pole piece, The magnetic pole piece can be easily handled, and the uniformity of the magnetic field is further improved by arranging an annular protrusion made of a magnetic material ring with one or more diametrical slits on the side facing the air gap of the magnetic pole piece. I found out that.

Furthermore, in each of the above configurations, the present invention was completed based on the finding that when the block-shaped magnetic pole piece member is made of a non-oriented silicon steel plate, a remarkable effect is exhibited in reducing electromagnetic phenomena.

The magnetic field generating device for MRI to which this invention is applied is not limited to the embodiments described below, as long as it has a pair of opposing magnetic pole pieces forming an air gap and generates a magnetic field in the air gap. , applicable to any configuration.

That is, the magnet structure that serves as a magnetic field generation source is not limited to permanent magnets, but may also be an electromagnet, and the magnetic pole piece does not have to be directly arranged on the magnet structure. Furthermore, the shape and dimensions of the yoke for forming a magnetic path that magnetically connects these magnet components and a pair of magnetic pole pieces to generate a magnetic field in the air gap, as well as the required size of the air gap, magnetic field strength, and magnetic field. It may be selected appropriately depending on various characteristics such as uniformity.

Function An example of a configuration using a pair of permanent magnets as a magnetic field generation source will be described below.

As the permanent magnet of the magnet component used in the permanent magnet magnetic circuit, ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but in particular, resource-rich light rare earths such as Nd and Pr can be used as R. , B, 3 with Fe as the main component
R-F exhibiting an extremely high energy product of 0 MGOe or more
By using e-B permanent magnets, the size can be significantly reduced.

Magnetic pole piece This invention is characterized in that a plurality of block-shaped magnetic pole piece members constituting a magnetic pole piece are formed by laminating and integrating a plurality of silicon steel plates in the direction in which a pair of magnetic pole pieces face each other. . The silicon steel sheet used is a so-called grain-oriented silicon steel sheet (JIS C255) in which the axis of easy magnetization is in the rolling direction.
3 etc.) and non-oriented silicon steel sheets (JIS
02552, etc.), but non-oriented silicon steel sheets are preferred, especially in order to reduce electromagnetic phenomena.

Although the block-shaped magnetic pole piece member consists of at least two pieces, the number of divisions is determined in consideration of the shape and dimensions of the magnetic pole piece, required characteristics, ease of assembly, etc.

In practical use, it is preferable to form a magnetic pole piece using about 40 to 200 blocks of block-shaped magnetic pole piece members made by laminating and integrating silicon steel plates in a predetermined direction and cutting them into square plate shapes of about 50 mm to 200 mm on a side. .

The thickness of the silicon steel plate constituting the block-shaped magnetic pole piece member may be any thickness, and the silicon steel plate that is generally easily available is 0.5mm thick.
Although it is thin at about 35 mm, since the lamination direction is in the direction in which the pair of magnetic pole pieces face each other, it can be laminated and magnetized with very good workability.

In addition, an insulating film is formed on the surface of each silicon steel plate, and they are electrically insulated from each other when laminated, and when they are made into blocks, they are vacuum-impregnated with insulating resin to integrate them. Therefore, the individual block-shaped magnetic pole piece members (13) are electrically insulated, and the effect of preventing the generation of eddy current can be obtained.

In addition, by adjusting the thickness of each block-shaped magnetic pole piece member constituting the magnetic pole piece, it is possible to create a substantially circular convex portion or a trapezoidal cross-sectional protrusion in the center of the magnetic pole piece, as shown in the examples described later. can be provided to further improve magnetic field uniformity.

These block-shaped magnetic pole piece members may be placed directly on the surface facing the air gap of the permanent magnet structure, but by placing them on a plate-shaped magnetic material base, which will be described later, the mechanical strength of the entire magnetic pole piece can be increased. This makes it easier to handle and assemble the magnetic circuit.

Furthermore, a plate-shaped magnetic material base is laid down, a magnetic pole piece is formed using a block-shaped magnetic pole piece member, and a ring-shaped projection is formed by arranging a magnetic material ring made of soft iron material or the like on the peripheral edge of the surface facing the air gap. is preferable for improving magnetic field uniformity.

By optimizing the thickness ratio between the magnetic material base thickness and the block-shaped magnetic pole piece member, the equalization of the magnetic field strength required for the magnetic pole piece and the prevention effect of eddy current and electromagnetic phenomena can be maximized. , the mechanical strength of a magnetic pole piece made of a plurality of block-shaped magnetic pole piece members can be reinforced, and it is necessary to appropriately select the thickness of the magnetic material base in order to obtain the required strength.

The magnetic material base is preferably made of pure iron, low carbon steel, or the like.

In addition, the magnetic material ring placed on the periphery of the magnetic pole piece can be made of the same material as the magnetic material base, and in addition to being placed on the periphery of the magnetic material base, it can also be placed directly on the top surface of the block-shaped pole piece member. can do.

In either configuration, it is desirable to divide the annular protrusion by providing one or more slits in order to reduce the effects of eddy currents. It is desirable to provide an electrical edge between the half members.

Disclosure Based on the Drawings Figures 1a and 1b are a top view and a cross-sectional view of an embodiment of a magnetic pole piece of a magnetic field generating device according to the invention.

FIGS. 2 and 3 are perspective views showing one embodiment of a block-shaped magnetic pole piece member constituting the magnetic pole piece of the present invention.

The magnetic pole piece (10) shown in FIG. 12)
and a plurality of block-shaped magnetic pole piece members (13) placed on the upper surface of the magnetic material base (11). The block-shaped magnetic pole piece member (13) is usually fixed to the magnetic material base (11) with a synthetic resin adhesive.

A magnetic material ring (12) made of soft iron and having a rectangular cross section is provided around the periphery of the magnetic material base (11) by making the outer peripheral side of the magnetic pole piece (10) higher than the other parts to provide the required magnetic flux. This is to form an annular protrusion that concentrates in the air gap and improves uniformity, and is bolted to the magnetic material base (11) with an insulating material interposed between it and the magnetic material ring (11).
2) in the circumferential direction (divided into eight pieces in the figure) to provide diametrical slits (16) to reduce the influence of eddy currents.

The block-shaped magnetic pole piece member (13) is constructed as shown in FIGS. 2 and 3.

That is, the block-shaped magnetic pole piece member (13
A) shows the case where grain-oriented silicon steel sheets are used. A small block (13aX
13b Consists of member (13A).

In the case of anisotropy, the coercive force in the rolling direction is extremely small, but the coercive force is large in the direction perpendicular to it, so it is better to use non-oriented silicon steel sheets, which will be described later, because of the reduction in electromagnetic phenomena and ease of assembly. It's advantageous.

The block-shaped magnetic pole piece member (13B) shown in Figure 3 shows the case where non-oriented silicon steel plates are used.Since it is non-oriented, multiple silicon steel plates are simply laminated in the thickness direction. A block-shaped magnetic pole piece member (13B) having a predetermined thickness can be constructed by simply integrating the two parts.

These plurality of block-shaped magnetic pole piece members (13) are arranged to form a substantially disk shape as shown in the figure.
In order to form a circular convex portion (14) with a required diameter in the center of the air gap facing surface of (10), a block-shaped magnetic pole piece member (14) is formed.
3) having different thicknesses is used, and the uniformity of the magnetic field can be improved by the circular convex portion.

Further, a core portion (15) made of soft iron is provided at the center of the magnetic material base (11), and this constitutes a base on which a gradient magnetic field coil is mounted.

Function When the magnetic pole piece (10) having the configuration shown above is used in a magnetic field generator for MRI, the silicon steel plate has a saturation magnetic flux density (Bs
), it is easy to achieve a uniform magnetic field in the air gap, and the structure is made by stacking multiple electrically insulated thin plates with a small coercive force (HcX hysteresis loss), so the GC pulse is applied to the gradient magnetic field coil. Even when applied, eddy currents generated in the magnetic poles are reduced, and it is also possible to reduce electromagnetic phenomena.

Examples Example 1 An R-Fe-B permanent magnet having a (BH)max of 35 MGOe was used in a magnetic field generator having the same configuration as shown in Fig. 4, and a magnetic material base with the following properties was placed on a magnetic material base made of soft iron with the following properties. A block-shaped magnetic pole piece member made of silicon steel plate and configured as shown in Fig. 2 was provided, and the distance between the opposing magnetic pole pieces was set to 500 mm with annular protrusions made of soft iron (with 4 slits). did.

Example 2 A magnetic field generating device was assembled with exactly the same configuration as in Example 1, using a non-oriented silicon steel plate having the following properties as the block-shaped magnetic pole piece member and having the configuration shown in FIG. 3.

COMPARATIVE EXAMPLE A magnetic field generating device was assembled by having exactly the same structure as in Example 1, but comprising a magnetic pole piece having the same size and shape as the base portion and the same bulk material of soft iron.

Measurement results: Magnetic field uniformity and magnetic field strength in the above three types of magnetic field generators, and the eddy current reduction effect of gradient magnetic field coils! =, and the residual magnetism caused by the GC pulse was measured.

As a result, in both Examples 1 and 2 and the comparative example, the magnetic field uniformity was measured in the measurement space within a radius of 200 mm from the center of the gap;
0 ppm and magnetic field strength of 0.2 T were obtained.

In the case of Example 1 and Example 2 according to the present invention, the eddy current due to the gradient magnetic field coil was reduced to 1/3 of that in the comparative example.
Reduced to below.

In the case of Example 1 according to the present invention, the residual magnetism caused by the GO pulse was reduced to 1/2 or less compared to the comparative example, and in the case of Example 2, it was reduced to 173 or less compared to the comparative example.

The magnetic material base had an outer diameter of 1050 mm and a thickness of 25 mm.

Further, the thickness (maximum thickness) of the various magnetic pole piece members after being placed on the magnetic material base was 25 mm. However, the thickness of the oriented and non-oriented silicon steel plates was 0.35 mm.

Magnetic material base part Pure iron He = 8OA/m Bs = 2.0T p = 1xlO'Ω・m Grain-oriented silicon steel plate Hc = 4.8A/mBs =
1.8T p=48xlO'Ω・m Non-oriented silicon steel plate Hc=40A/mBs=1.7T p=45xlO'Ω・m As is clear from the embodiments of the invention, a plurality of silicon steel plates are used as magnetic poles. When a magnetic pole piece consisting of a plurality of block-shaped pole piece members laminated and integrated in opposing directions is used in an MRI magnetic field generator, the magnetic field in the air gap is made uniform, and the magnetic field produced by the gradient magnetic field coil is In addition to the effect of reducing eddy currents, it also has the effect of reducing electromagnetic waves generated by GC pulses.6 Also, since the lamination direction of the silicon steel sheets is the same as the direction in which the magnetic pole pieces face each other, the lamination and magnetization work is extremely easy. have.

[Brief explanation of the drawing]

FIGS. 1a and 1b are a top view and a cross-sectional view showing one embodiment of a magnetic pole piece of a magnetic field generating device according to the present invention. FIG. 2 and FIG. 3 are perspective explanatory views of a block-shaped magnetic pole piece member. FIGS. 4a and 4b are a longitudinal cross-sectional view and a cross-sectional view of a conventional magnetic field generating device. DESCRIPTION OF SYMBOLS 1... Permanent magnet structure, 2, 10... Magnetic pole piece, 3.
...Yoke, 4...Gap, 5...Annular protrusion, 6...
Gradient magnetic field coil, 11...Magnetic material base, 12...
Magnetic material ring, 13. 13A, 13B... Block-shaped magnetic pole piece member, 14... Circular convex portion, 15... Core portion.

Claims (1)

[Claims] 1. In an MRI magnetic field generator that has a pair of opposing magnetic pole pieces forming an air gap and generates a magnetic field in the air gap, a plurality of silicon steel plates are laminated in the direction in which the magnetic pole pieces face each other. 1. A magnetic field generating device for MRI, characterized in that a magnetic pole piece is formed using a plurality of block-shaped magnetic pole piece members integrated together. 2. From a magnetic material ring in which a plurality of block-shaped magnetic pole piece members are arranged on a plate-shaped magnetic material base to form a magnetic pole piece, and one or more diametrical slits are provided on the side facing the air gap of the magnetic pole piece. 2. The magnetic field generating device for MRI according to claim 1, further comprising an annular protrusion. 3. M according to claim 1 or claim 2, wherein the block-shaped magnetic pole piece member is made of a non-oriented silicon steel plate.
Magnetic field generator for RI.