JPH05267047A - Magnetic field generator for mri - Google Patents

Abstract

PURPOSE:To reduce the generation of overcurrent of a permanent magnet and a yoke by specifying specific resistances of permanent magnet blocks and electrically and integrally insulating each permanent magnet block. CONSTITUTION:Permanent magnet components 2a and 2b which are trapezoid- shaped in the cross section and provide a magnetization orientation in a vertical direction (top and bottom) are laid out in the upper and lower opposed planes in a hexagonal cylinder-shaped yoke. Permanent magnet components 2c, 2d, 2e and 2f having a triangle-shaped cross section and a magnetization orientation in a lateral direction, are laid out in the shape of a ring which allows the permanent magnet blocks 2b, 2c, 2e and 2f to be laid out on the circumferential plane in the yoke 1. Each permanent component insulates each of R-Fe-B rare earth magnet blocks electrically and integrate them in one piece. The specific resistance of the permanent magnet blocks is specified to be 10<-3>OMEGA m and below. The yoke consists of a laminated body of silicon steel plate. This construction makes it possible to inhibit the generation of overcurrent which generates permanent magnets and yokes without dropping the magnetic field strength even if the frequency is raised.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a magnetic field generator used in a magnetic resonance tomography apparatus for medical use (hereinafter referred to as MRI) and the like, and does not use a magnetic pole piece and has a permanent magnet block magnet block. To a magnetic field generator for MRI in which eddy currents generated in a permanent magnet and a yoke are reduced by electrically insulating and integrating the magnets in a direction orthogonal to the magnetization direction.

[0002]

2. Description of the Related Art In MRI, a part or all of a subject is inserted into the space of a magnetic field generator that forms a strong magnetic field, and a tomographic image of the subject is obtained to delineate the nature of the tissue. It is a device that can The air gap of this magnetic field generator needs to be wide enough to allow a part or all of the subject to be inserted, and in order to obtain a clear tomographic image, normally, 0.02 It is required to form a stable and strong uniform magnetic field having an accuracy of ˜2.0 T and an accuracy of 1 × 10 ⁻⁴ or less. The permanent magnet structure, which serves as a magnetic field generation source of the magnetic field generator for MRI, is usually constructed by assembling a large number of permanent magnet blocks for reasons such as manufacturing problems and assembling workability.

Further, in a magnetic field generator for MRI using a conventionally used permanent magnet structure as a magnetic field source, it is necessary to dispose magnetic pole pieces in order to obtain a predetermined magnetic field homogeneity in the air gap. To obtain the position information in the void, usually X,
A gradient magnetic field coil composed of three sets of coil groups corresponding to the three directions of Y and Z is arranged in the vicinity of each magnetic pole piece, and a desired direction that changes in time like a trapezoidal wave by applying a pulse current to this gradient magnetic field coil. Can generate a gradient magnetic field.

However, the arrangement of the magnetic pole pieces has problems of eddy current and residual magnetism in relation to the gradient magnetic field coil. That is, the MRI magnetic field generator normally transmits the position information to the nuclear magnetic resonance signal by applying a gradient magnetic field to a uniform magnetic field. It is necessary to change many pulsed magnetic fields in order to obtain one image. This gradient magnetic field is generated by applying a pulse current to the gradient magnetic field coil, but if a conductor such as iron is present in the vicinity of the gradient magnetic field coil, an eddy current is generated and the gradient magnetic field is hard to rise sharply. Also,
If the residual magnetism of the magnetic material is large, the homogeneity of the magnetic field changes and the image is distorted and the sharpness of the image deteriorates.

Therefore, as a structure in which the magnetic pole pieces are not arranged, there is known a magnetic circuit in which a plurality of permanent magnet constituents are annularly combined and arranged on the inner circumference of a bulk body such as polygonal iron.

[0006]

In the structure in which the permanent magnet structure is annularly arranged in such a cylindrical yoke, the permanent magnet structure is usually made of Fe- which has excellent magnetic characteristics because of the demand for downsizing.
It is composed of a plurality of permanent magnet bars obtained by magnetizing a B-R system magnet or the like in a predetermined direction. However, Fe directly exposed to the gradient magnetic field
Since the electric resistance (specific resistance) of a permanent magnet bar such as a -BR magnet is as small as about 10 ^-6 Ω · m, an eddy current is generated in the permanent magnet with such a configuration.

Further, since the relative permeability of the Fe-BR magnet is close to 1, the magnetic flux generated from the coil passes through the magnet structure and reaches the yoke, and eddy current is also generated in the yoke. To do. Therefore, it takes a lot of time until the gradient magnetic field reaches a predetermined strength.

The same problem occurs when a magnet such as a rare earth cobalt magnet or Alnico is used in addition to the Fe-BR magnet.

It is an object of the present invention to provide a magnetic field generator for MRI of the permanent magnet type which does not use magnetic pole pieces, and which can reduce the eddy current generated in the permanent magnet and the yoke.

[0010]

SUMMARY OF THE INVENTION According to the present invention, at least a pair of permanent magnet constructions that are opposed to each other with a gap formed therein are composed of a plurality of permanent magnet blocks, and the magnet constructions are magnetically coupled by a yoke. In the magnetic field generator for MRI, which is coupled to and disposed in the above, the specific resistance of the permanent magnet block is 1
The magnetic field generator for MRI is characterized in that it is 0 ⁻³ Ω · m or less and that each permanent magnet block is electrically insulated and integrated.

Further, the present invention is characterized in that, in the above-mentioned structure, the contact portion of the yoke with at least one pair of permanent magnet constituents is made of a laminated body of silicon steel sheets, and the magnetic field generation for MRI is performed. It is a device.

In the present invention, the magnetic circuit may have any shape and arrangement as long as at least a pair of permanent magnet components are opposed to each other and magnetically coupled by a yoke. In particular, if a rod-shaped permanent magnet structure having a triangular, trapezoidal or rectangular cross section is combined and arranged in a polygonal cylindrical shape, a uniform magnetic field can be formed without the need for magnetic pole pieces, and the magnetic leakage is small, and the magnet is small. Can be used efficiently.

The pair of permanent magnet components are the main magnetic field generating sources that contribute to the formation of the magnetic field in the air gap, and usually form the air gap facing surfaces in the direction orthogonal to the magnetic field direction in the air gap. Due to the influence of the gradient magnetic field coils arranged along the inner peripheral surfaces of the pair of permanent magnet constituents, an eddy current is generated inside the permanent magnet constituents in the direction parallel to the air gap facing surface. In order to reduce the generation of this eddy current, it is necessary to increase the specific resistance in the direction parallel to the air gap facing surface of the permanent magnet structure. Therefore, when assembling the permanent magnet structure, it is desirable to completely electrically insulate each permanent magnet block, but at least the specific resistance in the direction parallel to the air gap facing surface in the permanent magnet structure after the assembly is completed. Therefore, it is necessary to consider the electrical insulation in the required direction according to the shape of the permanent magnet block so that For example, in the embodiments described below, it is important that the permanent magnet structure is electrically insulated in a direction orthogonal to the magnetization direction of each permanent magnet block (indicated by the arrow M in the figure). The apparent resistivity is increased and eddy current can be suppressed. The magnetizing direction of the permanent magnet structure is variously selected according to the structure of the magnetic circuit and the location on the yoke, but any direction can be used as long as it is arranged so that a uniform static magnetic field can be obtained in the required air gap. Good.

The insulating means of the permanent magnet block is such that at least the contact surface of each permanent magnet block is electrically insulated so that the contact surface with another permanent magnet block adjacent in the direction perpendicular to the magnetization direction of each permanent magnet block is electrically insulated. A resin coating such as an epoxy resin, an insulating coating such as electrodeposition coating, etc. can be provided in advance, and an insulating adhesive can be applied to the surface when assembling into a permanent magnet structure, and the insulating material is non-conductive. The material is not limited to the above example. The electrical insulation between the permanent magnet blocks has a specific resistance of at least 10 ^-3 Ω.
-It is necessary that the thickness is at least m, and it is desirable that the thickness of the insulating coating is approximately 10 μm or more.

The specific resistance of the permanent magnet block is 10 ⁻³ Ω · m
The reason given below is to substantially eliminate the ferrite magnet, and a permanent magnet such as a rare earth cobalt magnet or an alnico can be used for the permanent magnet block, but in particular, R having a characteristic of a maximum energy product of 30 MGOe or more is used.
By using a —Fe—B-based rare earth magnet, a stable uniform magnetic field can be obtained, and the device can be downsized. Also,
The shape and size of the permanent magnet block can be variously selected according to the form and size of the magnetic field generator, but in order to enhance the effect of reducing the eddy current, a small block whose longest side length of the permanent magnet block is 100 mm or less. Is preferred.

In the present invention, for a polygonal tubular or variously shaped yoke for arranging the permanent magnet structure on the inner peripheral surface, not only a bulk body of iron or the like but also a laminated body of silicon steel plate or iron is used. In particular, by using a silicon steel plate, it is easy to manufacture and the effect of reducing eddy current becomes large. The silicon steel sheets to be laminated may be those normally used having a thickness of 0.35 mm or 0.5 mm. Further, the entire yoke does not have to be a silicon steel plate, and various configurations such as arranging a laminated portion of silicon steel plates at a required position can be selected in consideration of a permanent magnet structure and a device. Furthermore, the configuration of the magnetic circuit is not limited to the above configuration, and in a magnetic circuit in which a pair of permanent magnet constituents facing each other through a gap are magnetically coupled with a yoke,
It is also possible to adopt a configuration in which the yokes that are in contact with the pair of permanent magnet constructions are laminated bodies such as silicon steel plates, and the other yokes are bulk materials such as iron.

Further, in the present invention, in order to prevent the change of the magnetic flux density which becomes smaller (in the Z-axis direction) away from the central portion of the permanent magnet structure due to the leakage magnetic flux from the opening,
For example, a uniform magnetic field distribution can be obtained by dividing a polygonal tubular yoke in the depth direction. Further, the gradient magnetic field coil, for example, when using a polygonal tubular yoke, is installed along the inner peripheral surface of the permanent magnet component in the polygonal tubular combination.

[0018]

In the magnetic field generator for MRI according to the present invention, the magnetic block of the permanent magnet structure is made small and electrically insulated and integrated in the direction orthogonal to the magnetizing direction without using the pole pieces, and further the yoke is used. By using a laminated body of silicon steel sheets, even when an Fe-BR system magnet having a relative magnetic permeability close to 1 is used for the permanent magnet block without using the pole pieces, the magnetic flux generated from the gradient magnetic field coil is used. It is possible to reduce the eddy current in the magnet structure and in the yoke due to the above, and to accelerate the formation of the gradient magnetic field.

[0019]

【Example】

Example 1 The magnetic field generator for MRI shown in FIG. 1 uses a hexagonal tubular yoke 1 made of a laminated body of silicon steel sheets, and six kinds of permanent magnet components 2a to 2f are annularly formed on the inner peripheral surface of the yoke 1. And a gradient magnetic field coil 4 is installed in the void 3. More specifically, in the figure, on the upper and lower facing surfaces in the hexagonal tubular yoke 1, permanent magnet components 2a and 2d having trapezoidal cross-sections having a magnetization direction in the vertical (upper and lower) direction are arranged. On the left and right facing surfaces in the yoke 1, permanent magnet constructions 2b, 2c, 2e, 2 having a triangular cross-section having a lateral magnetization direction.
By arranging f, the permanent magnet constructing bodies 2a to 2f are annularly arranged on the inner peripheral surface of the yoke 1. Each permanent magnet assembly, as shown in FIG. 2, has a plurality of R− with a maximum energy product of 35 MGOe measuring 36 mm × 40 mm × 40 mm.
The Fe-B type rare earth magnet blocks are electrically insulated from each other and integrated.

With respect to the eddy current characteristic generated in the magnetic circuit having the above structure, a sinusoidal current is caused to flow through the gradient magnetic field coil, the frequency is changed, and the generated magnetic field is generated by the electromotive force generated in the search coil installed in the air gap. It was measured and evaluated. The sinusoidal current in this case was 450 AT.

Example 2 With the same construction as in Example 1, a polygonal tubular yoke made of bulk soft iron was used as the yoke, and the magnetic field generated when a sinusoidal current was applied to the gradient coil was measured in the same manner. evaluated.

Comparative Example As in Example 2, the yoke was a polygonal tubular yoke made of bulk soft iron such as iron, and the permanent magnet blocks of the permanent magnet structure were not insulated, but the yoke of Example 1 was used. The same measurement was performed.

The above measurement results are shown in FIG. 3 as a relationship between frequency and magnetic field strength ratio. As is apparent from FIG. 3, Example 2 of the present invention in which the permanent magnet block is insulated (plotted with a mark) is compared with the comparative example in which the permanent magnet block is not insulated (plotted with a mark).
In the case of, the magnetic field strength ratio is flat up to the region where the frequency is high, which indicates that the generation of eddy current is suppressed. Further, in the case of Example 1 (plotted with a circle) in which the yoke was composed of a laminated body of silicon steel sheets, the magnetic field strength ratio did not decrease even when the frequency was increased as compared with Example 2, and the eddy current It can be seen that the occurrence is further suppressed. Also,
It was found that in the above configuration, a magnetic field homogeneity of 30 ppm was obtained in the measurement space within a radius of 200 mm from the center of the void, and there was almost no problem with residual magnetism.

[0024]

In the magnetic field generator for MRI according to the present invention, the magnetic block of the permanent magnet structure is made small and electrically insulated in the direction orthogonal to the magnetization direction and integrated without using the pole pieces. , The eddy current generated in the permanent magnet and the yoke can be reduced, and further, the use of the silicon steel sheet laminated body in the yoke can significantly reduce the eddy current as compared with the conventional device which does not use the pole pieces. it can.

[Brief description of drawings]

FIG. 1 is a perspective explanatory view showing a configuration example of a magnetic field generator for MRI according to the present invention.

FIG. 2 is a perspective view showing a permanent magnet structure according to the present invention.

FIG. 3 is a graph showing the relationship between frequency and magnetic field strength ratio.

[Explanation of symbols]

 1 Yoke 2a, 2b, 2c, 2d, 2e, 2f Permanent magnet structure 3 Gap 4 Gradient magnetic field coil

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location 8203-2G G01R 33/22 C 8203-2G D 8203-2GE (72) Inventor Hiroyuki Takeuchi Chiba Prefecture 2-11 Shinjuyo, Kashiwa-shi, Ltd. Inside the Ritsudoko Medical Research Laboratory (72) Inventor Hirotaka Takeshima 2-2-1, Shinjyu-Medicine Research Institute, Kashiwa, Chiba Prefecture

Claims (2)

[Claims] 1. An MRI system in which at least a pair of permanent magnet constructs facing each other with a gap therebetween is composed of a plurality of permanent magnet blocks, and the magnet constructs are magnetically coupled and arranged by yokes. In the magnetic field generator, the specific resistance of the permanent magnet block is 10 ⁻³ Ω · m or less, and each permanent magnet block is electrically insulated and integrated, and the magnetic field generator for MRI. .. 2. The magnetic field generator for MRI according to claim 1, wherein the portion of the yoke that contacts the at least one pair of permanent magnet components is made of a laminated body of silicon steel sheets.