JP2819221B2 - Magnetic field generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field generator for generating a magnetic field having good uniformity. The magnetic field generating apparatus according to the present invention is particularly suitable for use in a magnetic resonance tomography apparatus or the like that obtains an image of an object using magnetic resonance.

[0002]

2. Description of the Related Art The sharpness of an image of a magnetic resonance tomograph is greatly affected by the uniformity of a magnetic field generated by a magnetic field generator. Therefore, a magnetic field generator capable of generating a magnetic field with a high degree of uniformity has been demanded, and at present, the permanent magnet facing type is most mass-produced.

FIG. 3 shows a schematic view of a permanent magnet facing type magnetic field generator. FIG. 3A is a cross-sectional view when the device is cut vertically, and FIG. 3B is a cross-sectional view along a cutting line AA ′ in FIG. 3A. Plate yoke 10a, 10b are connected by columnar yoke 12a, 12b, 12c, 12d, disk-shaped magnets 14a, 14b inside them, and magnetic shunt plates 16a, 16b having annular projections on the outer periphery. Is provided.

[0006] In the permanent magnet facing type magnetic field generating apparatus shown in FIG. 3, the lines of magnetic force emitted from the permanent magnets 14b pass through the magnetic shunt plates 16b, pass through the spaces between the magnetic shunt plates, and enter the permanent magnets 14a via the magnetic shunt plates 16a. . The lines of magnetic force further create a circuit that returns to the permanent magnet 14b through the plate yoke and the columnar yoke. As a result, a uniform magnetic field is created in the space between the opposing magnetic shunt plates 16a and 16b.

Further, there is provided a coil for changing the magnetic field strength in each of the X, Y, and Z axes to form a gradient magnetic field. These coils are usually installed at a predetermined location sandwiched between nonmagnetic disks. FIG. 3 shows only the coils 18a and 18b for the gradient magnetic field in the Z-axis direction. The position information of the image can be obtained by the gradient magnetic field formed by these gradient magnetic field coils.

A tomographic image of a human body or the like can be obtained by a magnetic resonance tomography apparatus using the above-described permanent magnet opposed magnetic field generating apparatus.

The provision of the magnetic shunt plates 16a and 16b as shown in FIG. 3 is an effective means for generating a uniform magnetic field over a wide range. However, because of the provision of the magnetic shunt plate, a part of the magnetic field lines (for example, the magnetic field lines 19) does not contribute to the generation of the uniform magnetic field. This is because the permanent magnet 14a (1
Part of the magnetic field lines passing through 4b) passes through the side surface of the magnetic shunt plate 16a (16b), and forms a columnar yoke 12a (or 12b, 1b).
This is because a closed circuit that returns to the permanent magnet 14a (14b) through the plate yoke 10a (10b) through 2c, 12d) is formed. It can be seen that the magnetic field lines 19 in FIG. 3A form such a closed circuit.

As described above, the magnetic lines of force forming the closed circuit do not contribute to the magnetic field formed in the gap between the magnetic shunts.
That is, the presence of the magnetic shunt reduces the ratio of the lines of magnetic force emitted from the permanent magnets to the formation of a magnetic field between the magnetic shunts.

On the other hand, permanent magnets are expensive, and there is a demand for reducing the weight of permanent magnets from the standpoint of downsizing the magnetic field generator. Therefore,
It is necessary to increase the use efficiency of the magnetic field lines emitted from the permanent magnet.

[0010] For this reason, there has been proposed a magnetic field generator in which a magnetic shunt is not provided as shown in FIG. A method of partially changing the dimension of the magnetization direction of the magnet without providing a magnetic shunt plate (Japanese Patent Laid-Open No. 3-209803)
0b and a coil for a gradient magnetic field (a nonmagnetic disk sandwiching the coil) 18a, 18b, a method of providing a small magnetic piece or a magnet piece for adjusting the magnetic field over a wide area in the apparatus. A uniform magnetic field can be generated.

The magnetic field generator of FIG. 4 can be made smaller than the magnetic field generator of FIG. 3 by not providing a magnetic shunt plate.
For example, a similar magnet (maximum energy product 42MGOe
In the case of using an NdFeB magnet), the weight of the magnetic field generator required to generate a magnetic field of 0.2T in the gap is 2% in weight of the permanent magnet in the apparatus of FIG. 4 as compared with the apparatus of FIG.
To ３, the total weight was reduced to about ２.

In the conventional magnetic field generator (FIG. 3), a rectangular magnetic field is generated by applying a rectangular current to coils 18a and 18b for the gradient magnetic field.
Eddy currents were generated in 6a and 16b, and residual magnetization was generated. The eddy current slows down the reaction of generating and extinguishing the gradient magnetic field, and the remanent magnetization creates a local magnetic field, lowering the uniformity of the magnetic field. Therefore, there is a problem that the image of the magnetic resonance tomograph is distorted due to these.

In another conventional magnetic field generator (FIG. 4), permanent magnets 20a and 20b are in contact with each other by generating a rectangular magnetic field by applying a rectangular current to coils 18a and 18b for the gradient magnetic field. Partial plate yoke 10a, 1
An eddy current was generated within 0b, or residual magnetization was generated. Therefore, there is a problem that the image of the magnetic resonance tomography apparatus is distorted as in the conventional magnetic field generator of FIG.

[0014]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an opposed permanent magnet type magnetic field generating apparatus which uses a small amount of permanent magnets, has a small weight, and is less affected by eddy current and residual magnetization. That is.

[0015]

Means for Solving the Problems 1 magnetized in the thickness direction
In a magnetic field generator for providing a pair of permanent magnets facing each other inside a yoke and generating a magnetic field in a space between the permanent magnets, all or one of plate-like yokes in contact with the permanent magnets among the yokes The part is formed by stacking a plurality of non-oriented silicon steel sheets. Further, among the yoke, all or a part of the plate yoke in contact with the permanent magnet is constituted by a plurality of small pieces of electromagnetic soft iron that have been subjected to heat treatment and have a coercive force of 2 [Oe] or less, The small pieces are electrically insulated.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A magnetic field generating apparatus according to the present invention is characterized in that a magnetic shunt is not provided, and all or a part of a plate-like yoke is formed by laminating a plurality of non-oriented silicon steel sheets or by heat treatment. And a plurality of small pieces of soft magnetic iron having a coercive force of 2 [Oe] or less.

For example, in the conventional example shown in FIG. 4, the entire plate-like yoke 10a, 10b is formed by laminating a plurality of non-oriented silicon steel sheets, or a plurality of heat-treated coercive forces of 2 [Oe] or less. A configuration in which small pieces of electromagnetic soft iron are electrically insulated is an example of the present invention.

A plurality of non-oriented silicon steel sheets laminated,
In addition, since a plurality of pieces of heat-treated electromagnetic soft iron are configured to be electrically insulated, the electrical conductivity is small, so that generation of eddy current in the plate-like yoke can be suppressed.

Further, in the silicon steel sheet and the heat-treated electromagnetic soft iron, since the change in magnetization due to the magnetic history is small, residual magnetization hardly occurs in the plate-like yoke.

Therefore, the entire plate-like yoke 10a, 10b is formed by laminating a plurality of non-oriented silicon steel sheets or by electrically insulating a plurality of heat-treated small pieces of soft magnetic iron. In the example of the present invention, even if a rectangular magnetic field is generated by applying a rectangular current to the gradient magnetic field coils 18a and 18b, an eddy current is generated in the plate-shaped yokes 10a and 10b, or residual magnetization is generated. Things are less likely to happen. Therefore, in the case of the magnetic field generator according to the present invention using the above plate-like yoke, problems such as distortion of the image of the magnetic resonance tomography apparatus are considerably solved.

However, in the above-described example of the present invention, a plurality of non-oriented silicon steel sheets are laminated on the entire plate-like yoke, or a plurality of electromagnetic waves whose coercive force is reduced to 2 [Oe] or less by heat treatment. Since the soft iron piece is configured to be electrically insulated, there is a disadvantage that the strength of the plate-like yoke is weakened. FIG. 1 shows a compensation for this disadvantage. In FIG. 1, the same parts as those in FIG. 4 are denoted by the same reference numerals.

Of the plate yoke, portions 30a and 30b are made of iron or the like as before, and a plurality of non-oriented silicon steel sheets are laminated only on portions 34a and 34b, or the coercive force is reduced by heat treatment. A plurality of electromagnetic soft iron pieces of 2 [Oe] or less, which are electrically insulated, are used. The thickness of the plate yoke portions 34a and 34b is about half the thickness of the entire plate yoke.

According to the embodiment of the magnetic field generator of the present invention shown in FIG. 1, it is difficult to generate an eddy current or a residual magnetization in the plate yoke, and at the same time, Strength can be maintained.

An example of an experiment actually performed is shown below. FIG. 2 shows the magnetic field generator used in this experiment. FIG. 2 shows a gradient magnetic field coil (a non-magnetic disk) 18a of FIG.
A plurality of magnet pieces 40 are provided on the surface of 18b. The purpose of providing the magnet pieces 40 is to further increase the uniformity of the magnetic field by the magnetic field generator and to make the uniform magnetic field space wider. The dimensions, the number, and the installation position of the magnet pieces 40 are determined by obtaining an optimum value by numerical analysis.

The permanent magnets 20a and 20b have a residual magnetic flux density of 13.2 kG and a maximum energy product of 42 MGOe of Nd.
An FeB magnet was used. The outer diameters of the permanent magnets 20a and 20b were 1050 mm, the distance between the permanent magnets was 500 mm, and the intensity of the magnetic field generated from these permanent magnets was 0.2T.

The plate yoke portions 30a and 30b and the columnar yoke portions 12a to 12d were made of structural carbon steel (SS41). The portions 34a and 34b of the plate-like yoke were made of non-oriented silicon steel plates, each having a thickness of 0.35 mm and being laminated to 40 mm.

As a result of the experiment using the above-described magnetic field generator, a magnetic field uniformity of 50 ppm was obtained in the central portion (in a spherical space having a diameter of 300 mm) between the permanent magnets 20a and 20b.
This value is equivalent to the magnetic field uniformity of the conventional apparatus (FIG. 4) having the same standard as that of the present experimental example. That is, it was found that even when a part of the plate yoke was changed to a non-oriented silicon steel sheet as in the present invention, the magnetic field uniformity did not decrease.

On the other hand, as a result of an experiment using a magnetic field generator according to the present invention, the effect of a rectangular current flowing through a gradient magnetic field coil on the magnetic field is smaller than that of the conventional example. It was found that the influence of magnetic field uniformity due to the residual magnetization caused by the hysteresis phenomenon can be reduced to 1/3.

As can be seen from the results of the above experiments, the magnetic field generator according to the present invention can suppress the generation of eddy currents and the generation of remanent magnetization while maintaining the magnetic field uniformity as high as before. The effect on the magnetic field due to was reduced.

[0030]

According to the permanent magnet opposed type magnetic field generator of the present invention, the generation of eddy current and the generation of residual magnetization can be suppressed, and the influence on these magnetic fields is reduced. Therefore, when the magnetic field generator according to the present invention is used for a magnetic resonance tomography apparatus, there is a great advantage that a very high quality image can be obtained. Further, since the magnetic field generator according to the present invention does not include the magnetic shunt plate, the amount of the permanent magnet used can be reduced, and together with the absence of the magnetic shunt plate, the weight of the magnetic field generator is extremely low. There is also an advantage that it becomes smaller.

[Brief description of the drawings]

FIG. 1 is a sectional view of an example of a magnetic field generator according to the present invention.

FIG. 2 is a sectional view of another example of the magnetic field generator according to the present invention.

FIG. 3 shows an example of a conventional magnetic field generator.

FIG. 4 is another example of a conventional magnetic field generator.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Eiji Yoshidome 127 Yokogawa Medical System Co., Ltd., 4-7 Asahigaoka, Hino-shi, Tokyo (56) References JP-A-4-138131 (JP, A) (58) ) Surveyed field (Int.Cl. ⁶ , DB name) H01F 7/02 H01F 7/20

Claims (4)

(57) [Claims] 1. A pair of permanent magnets facing each other via a space, a gradient magnetic field forming means disposed on each space-side surface of the pair of permanent magnets, and a respective one of the pair of permanent magnets A first yoke portion disposed on a side opposite to the space, and a second yoke for supporting the pair of permanent magnets at a predetermined distance.
And a yoke portion, wherein a plate-shaped portion for suppressing generation of eddy current and residual magnetization in the first yoke portion due to the formed gradient magnetic field is provided in the first yoke portion. A magnetic field generator, comprising: 2. The magnetic field generator according to claim 1, wherein the plate portion is formed by stacking a plurality of non-oriented silicon steel plates. 3. The plate-shaped portion has a plurality of small pieces of soft magnetic iron having a coercive force of a certain value or less, and the plurality of small pieces are electrically insulated from each other. 3. The magnetic field generator according to claim 1. 4. The magnetic field generator according to claim 1, wherein a surface area of a main surface of the plate-shaped portion is larger than a surface area of a main surface of the permanent magnet.