JP3369653B2 - Permanent magnet facing 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 permanent magnet facing type magnetic field generator for generating a magnetic field having good uniformity. INDUSTRIAL APPLICABILITY The permanent magnet facing magnetic field generator according to the present invention is suitable for use in a magnetic resonance tomography device for medical use and a magnetic resonance tomography device for food inspection that obtain an image of an object by using magnetic resonance, and a nuclear magnetic resonance device. is there.

[0002]

2. Description of the Related Art The sharpness of the image of a magnetic resonance tomography apparatus is greatly affected by the homogeneity of the magnetic field generated by the magnetic field generator. Therefore, there is a demand for a magnetic field generator capable of generating a magnetic field with high homogeneity, and at present, the permanent magnet facing type is most mass-produced.

FIG. 2 shows an example of a permanent magnet facing type magnetic field generator. 2A is a cross-sectional view when the device is vertically cut, and FIG. 2B is a cross-sectional view taken along the cutting line AA ′ in FIG. The plate yokes 10a, 10b are connected by columnar yokes 12a, 12b, 12c, 12d, and the disk magnets 14a, 14b are provided inside them, and the magnetic shunting plates 16a, 16b having annular protrusions on the outer peripheral portion. Is provided.

In the permanent magnet facing type magnetic field generator of FIG. 2, the magnetic force lines emitted from the permanent magnet 14b pass through the magnetic compensating plates 16b, pass through the space between the magnetic compensating plates, and enter the permanent magnet 14a via the magnetic compensating plate 16a. . This line of magnetic force further forms a circuit that returns to the permanent magnet 14b through the plate yoke and the cylindrical yoke. As a result, a uniform magnetic field is created in the space between the opposed magnetic shunt plates 16a and 16b.

Further, a coil is provided for changing the magnetic field strength in each of the X, Y and Z directions to form a gradient magnetic field. These coils are usually sandwiched between non-magnetic discs and installed at predetermined locations. In FIG. 2, only the coils 18a and 18b for the gradient magnetic field in the Z-axis direction are shown. The position information of the image can be obtained by the gradient magnetic field formed by these gradient magnetic field coils.

By the magnetic resonance tomography apparatus using the magnetic field generator of the permanent magnet facing type as described above, it is possible to obtain a tomographic image of a human body, food or the like.

The provision of the magnetic shunting plates 16a and 16b as shown in FIG. 2 is for generating a uniform magnetic field in a wide range in a magnetic field generator for generating a magnetic field of 0.2 T or less, such as for a magnetic resonance tomography device. This is one of the most effective means. As the material of the magnetic shunt plate, low carbon steel having a small amount of carbon (SS41, S10C, SM400B, SUY, etc.) is used.

However, since the magnetic shunting plates 16a and 16b are provided, a part of the lines of magnetic force (for example, the lines of magnetic force 19) do not contribute to the generation of the uniform magnetic field. This is because a part of the magnetic lines of force passing through the permanent magnet 14a (14b) is part of the magnetic shunt plate 16a.
The plate-like yoke 10a passes through the side surface of (16b), and passes through the cylindrical yoke 12a (or 12b, 12c, 12d).
This is because a closed circuit that passes through (10b) and returns to the permanent magnet 14a (14b) is created. It can be seen that the magnetic lines of force 19 in FIG. 2 (a) form such a closed circuit. That is, since the magnetic shunting plate is provided, there is a problem that the ratio of the magnetic force lines emitted from the permanent magnet to the magnetic field formation between the magnetic shunting plates decreases.

On the other hand, since the permanent magnet is expensive,
Further, there is a demand for reducing the weight of the permanent magnet in order to reduce the size and weight of the magnetic field generator.

Therefore, it is necessary to increase the efficiency of use of the magnetic field lines emitted from the permanent magnet so that a uniform magnetic field can be generated in a necessary region with a smaller weight.

Due to the above-mentioned need, a magnetic field generator without a magnetic shunt has been proposed. By not providing the magnetic shunt plate, the efficiency of use of the lines of magnetic force is improved, and the weight of the permanent magnet can be reduced. Further, the space between the permanent magnets can be narrowed by the elimination of the magnetic shunt, and the weight of the cylindrical yoke can be reduced. Therefore, there is an advantage that the entire magnetic field generator can be reduced in size and weight.

However, since it is difficult to integrally mold a large magnet for a magnetic field generator, the divided magnets are attached to each other. Therefore, when the magnets are attached to each other, the magnetic characteristics of the individual divided magnets vary, and The overall dimensional accuracy becomes poor, and the uniformity of the generated magnetic field becomes low. That is, even if the magnet is designed to generate a magnetic field with sufficiently high homogeneity, in order to fabricate the divided magnets by bonding,
There is a problem that the magnetic field homogeneity decreases.

Further, in the magnetic field generator having no magnetic shunt plate, there is a problem that an eddy current is generated in the plate yoke because the gradient magnetic field passes through the plate yoke. Therefore, there is an invention which proposes to form all or a part of the plate yoke by laminating a plurality of non-oriented silicon steel plates. Although this can suppress the eddy current generated in the plate yoke, such plate yoke has low mechanical strength, and
There is a problem of being expensive.

[0014]

SUMMARY OF THE INVENTION It is an object of the present invention to increase the efficiency of use of permanent magnets in a permanent magnet facing type magnetic field generator to reduce the size and weight of the entire device.

[0015]

A pair of permanent magnets that are magnetized in the thickness direction and have annular protrusions on the outer periphery of the facing surface are provided facing each other inside a yoke, and a magnetic field is provided in the space between the permanent magnets. In a permanent magnet facing magnetic field generator that generates
A laminated silicon steel sheet having a thickness of 5 mm or less is fixed to the facing surface of the permanent magnet. Further, the permanent magnet and the laminated silicon steel plate fixed to the facing surface of the permanent magnet are covered with a non-magnetic cover, and the space generated in the non-magnetic cover is filled with a molding agent to be solidified.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to improve the efficiency of use of permanent magnets and reduce the size and weight of the entire apparatus, the present inventor considered a permanent magnet facing type magnetic field generator without a magnetic compensating plate. However, the permanent magnet facing type magnetic field generator without a rectifying plate has a big problem that the homogeneity of the generated magnetic field is low as described in the section of the conventional technique. The present invention improves on this point.

An embodiment of the present invention is shown in FIG. FIG. 1 shows a cut surface when the device is cut vertically. The same parts as those in FIG. 2 are designated by the same reference numerals.

Permanent magnet 2 in the magnetic field generator of the present invention
Nos. 0a and 20b are provided with annular protrusions on their outer peripheral portions so that a magnetic field having a sufficiently high degree of uniformity can be generated without a magnetic shunt plate.

Gradient coils (for Z-axis direction) 18a, 18b
Are provided in the recesses of the permanent magnets 20a, 20b (gradient coils for the X-axis and Y-axis directions are omitted as in FIG. 2).

Although the permanent magnets 20a and 20b of the present invention are designed to generate a magnetic field having a sufficiently high homogeneity, they are designed to be manufactured by laminating divided magnets as described in the section of the prior art. It is difficult to get the accuracy of the street. Therefore, the thickness of the permanent magnets 20a and 20b is 5
The thin plates 22a and 22b made of laminated silicon steel plates of mm or less were fixed to improve the magnetic field uniformity. This is a great feature of the present invention.

By sticking a silicon steel plate of 5 mm or less to the surface of the permanent magnet, variations in the magnetic characteristics of the individual split magnets forming the permanent magnets 20a and 20b, and
It is possible to alleviate the dimensional variation and significantly improve the homogeneity of the magnetic field.

In the present invention, even if the silicon steel plates 22a and 22b are thinned to 5 mm or less, a highly uniform magnetic field can be obtained because the permanent magnets have already been designed so that sufficient magnetic field homogeneity can be obtained only by the permanent magnets. Because there is. That is,
This is because the silicon steel plates 22a and 22b need only level out the variation in magnetic characteristics and the variation in dimensions due to the lamination of the divided magnets.

Further, since the silicon steel plates 22a and 22b are provided on the surfaces of the permanent magnets 20a and 20b, the gradient magnetic fields generated by the gradient coils 18a and 18b pass through the silicon steel plates and the plate yokes 10a and 10b. Absent. Therefore, there is an advantage that eddy current is not generated in the plate yoke and it is not necessary to manufacture all or part of the plate yoke with the laminated silicon steel plate as in the conventional invention.

On the other hand, the silicon steel plates 22a, 2 on the surface of the permanent magnet
The eddy current due to the gradient magnetic field passing through 2b
It can be suppressed by making a and 22b into a laminated structure and further dividing the radial direction into several parts. Therefore,
Compared with the conventional magnetic shunt plate made of bulk iron, there is an advantage that it is easier to remove the adverse effect of eddy current.

Furthermore, the silicon steel sheet is characterized in that the change in magnetic characteristics due to machining is small. Moreover, the silicon steel plate provided on the surface of the permanent magnet is different from the iron material compensating plate,
Since the divided pieces are manufactured separately, it is possible to easily heat-treat each divided piece after machining, so that the change in the magnetic characteristics can be almost restored. Therefore, the silicon steel sheet has an advantage that it does not disturb the homogeneity of the generated magnetic field even if it is machined.

As explained above, the silicon steel sheet according to the present invention can fully function at 5 mm or less, unlike the conventional iron material compensating plate. Therefore, the space between the permanent magnets can be narrowed, and the device can be reduced in size and weight.

By assembling the magnetic field generator by fixing the silicon steel plate to the surface of the permanent magnet and then subjecting the surface of the silicon steel plate or the surface of the gradient coil to step processing, the uniformity of the generated magnetic field can be further increased.

However, in the present invention, the permanent magnet and the plate yoke, and the silicon steel plate and the permanent magnet are mechanically fixed only by the magnetic attraction between them and the force of the adhesive. Is not fixed, there is a risk that it will come off. For this reason, the permanent magnet and the silicon steel plate on the surface thereof are covered with a non-magnetic cover, the cover is mechanically fixed to the plate yoke, and the gap formed between the silicon steel plate and the cover is filled with a non-magnetic molding material. It is desirable to solidify it. As a result, it is possible to prevent the permanent magnet and the silicon steel plate from scattering due to vibration and deterioration of the adhesive.

[0029]

The magnetic field generator of the present invention does not need to be provided with a thick magnetic compensating plate as in the conventional case. Therefore, the use efficiency of the permanent magnet is improved, and the weight of the permanent magnet can be reduced. Further, the distance between the surfaces of the permanent magnets is reduced, and the weight of the columnar yoke can be reduced. That is, as compared with the conventional permanent magnet facing type magnetic field generator, it is possible to reduce the weight of the permanent magnet, the magnetic shunt plate, and the yoke, and thus it is possible to reduce the size and weight of the entire device. is there. Further, unlike the conventional magnetic field generator without a magnetic shunt plate, since the gradient magnetic field does not pass through the yoke, there is an advantage that the yoke can be made of an inexpensive iron material having high mechanical strength.

[Brief description of drawings]

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

FIG. 2 is an example of a conventional permanent magnet facing type magnetic field generator.

Front page continuation (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01F 7/02 A61B 5/055

Claims (3)

(57) [Claims] 1. A permanent magnet facing type magnetic field generator for providing a pair of permanent magnets magnetized in the thickness direction so as to face each other inside a yoke and to generate a magnetic field in a space between the permanent magnets. Of the permanent magnets, an annular projection is provided on the outer peripheral portions of the facing surfaces of the permanent magnets, and a laminated silicon steel plate having a thickness of 5 mm or less is fixed only to the facing surfaces of the pair of permanent magnets. Magnet facing type magnetic field generator. 2. The non-magnetic cover covers the permanent magnet and the laminated silicon steel plate fixed to the facing surface of the permanent magnet.
The permanent magnet facing type magnetic field generator according to claim 1, wherein a space generated in the non-magnetic cover is filled with a molding agent to be solidified. 3. A permanent magnet facing type magnetic field generator for providing a pair of permanent magnets magnetized in the thickness direction so as to face each other inside a yoke and to generate a magnetic field in a space between the permanent magnets. An annular protrusion is provided on the outer peripheral portion of the facing surface of the above, and a laminated silicon steel plate having a thickness of 5 mm or less is fixed to the facing surface of the permanent magnet, and the laminated silicon steel plate is covered with a non-magnetic cover. A permanent magnet facing type magnetic field generator characterized in that a non-magnetic molding agent is filled in the generated space and solidified.