JPH079845B2 - Permanent magnet type uniform magnetic field magnet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear magnetic resonance computed tomography apparatus (hereinafter
The present invention relates to a permanent magnet type uniform magnetic field magnet used for NMR-CT).

[Prior art and its problems]

In the NMR-CT, a uniform magnetic field space having a diameter of 400 to 500 mm in the central portion of the space surrounded by the uniform magnetic field magnets,
It is necessary to generate a uniform magnetic field with a homogeneity of the magnetic field strength of several tens of PPM, and by using a permanent magnet, huge power consumption in the system using the normal conducting uniform magnetic field coil and cooling power in the superconducting magnet system are achieved. It has attracted attention because it has the advantage that a uniform magnetic field magnet can be obtained that consumes less and thus has a lower operating cost. However, when comparing the prices of permanent magnets and copper wire rods, the permanent magnets are effective enough to reach several times in ferrite magnets and 100 times in rare earth magnets. Appearance of a magnet type uniform magnetic field magnet is required.

FIG. 11 is a front view of the main part showing the prior art. Magnetization is performed in the same direction as the uniform magnetic field direction 100 on the inner wall surfaces 2A, 2B of the rectangular cylindrical core 1 which are perpendicular to the uniform magnetic field direction 100. The permanent magnets 3A, 3B and the magnetic pole pieces 4A, 4B made of a soft magnetic material such as an iron plate having the annular projection 5 for preventing the leakage magnetic flux are provided, and the center of the gap 6 between the pair of magnetic pole pieces 4A, 4B is provided. A uniform magnetic field space 10 having a diameter of about 400 to 500 mm is formed in the portion. However, since the magnetic resistance of the air gap 6 between the magnetic pole pieces 4A and 4B is high, it is impossible to completely prevent the leakage magnetic flux 101 leaking from the outer circumference of the magnetic pole piece to the core 1 side. Even if the distance S is increased to reduce the leakage flux 101, the homogeneity of the magnetic field of the uniform magnetic field 10 is lowered due to the distortion of the magnetic flux in the air gap 6 due to the leakage flux. Therefore, in order to maintain the homogeneity of the magnetic field of the uniform magnetic field 10, the diameter of the permanent magnets 3A, 3B becomes large and a large amount of expensive permanent magnet material is required, which increases the total amount of magnetic flux generated. There is a problem that the cross-sectional area of the core 1 that recirculates the magnetic field increases, and the magnetic field length of the core 1 for maintaining the distance S increases, and the weight of the uniform magnetic field magnet becomes high. In some cases, there are drawbacks that result in greater economic disadvantage.

FIG. 12 is a front view of an essential part showing the improved prior art, in which eight trapezoidal rod-shaped permanent magnets 13 are combined in an octagonal tube shape to form an octagonal uniform magnetic field space 16 therein. The magnetizing direction 102 of each permanent magnet shown by the solid line arrow in the figure is set to be an angle direction twice the angle θ formed by each permanent magnet with respect to the direction 100 of the uniform magnetic field. It can be confined in an octagonal tube-shaped uniform magnetic field magnet, the amount of leakage flux 101 can be reduced without using a core, and the total amount of magnetic flux generated can be reduced by utilizing the hollow part of the uniform magnetic field magnet as a wide uniform magnetic field space. It has the advantages that the outer diameter can be reduced. However, since the permanent magnet 13 is used for the circulation passage of the total generated magnetic flux, the dimension of the trapezoid in the height direction becomes large, and it is necessary to use a large amount of expensive permanent magnet material, which causes an economic disadvantage. At the same time, since the magnetization directions of the permanent magnets 13 are different, it is considered that the magnetizing process and the assembling process are difficult.

[Object of the Invention] The present invention has been made in view of the above-mentioned situation, and the total amount of magnetic flux generated is small because the hollow portion of the core can be widely used as a uniform magnetic field, and the permanent magnet material is used in a small amount. An object is to provide a uniform magnetic field magnet.

[Main points of the invention]

According to the present invention, the relative magnetic permeability of the permanent magnet material does not disturb the magnetic field in the hollow portion close to that of air, and a predetermined current is applied to a specific surface by the combination of the cross-sectional shape of the plate-shaped permanent magnet and the direction of its magnetization. This was done by paying attention to the fact that it can be considered as an ideal conductor (called a current sheet) that has a zero cross-sectional area that uniformly flows in the length direction of the plate. A core made of a magnetic material is formed into a polygonal cylindrical shape, and a plate shape having a trapezoidal cross section is formed on an inner side surface of the inner side surface perpendicular to the uniform magnetic field direction and magnetized in a direction perpendicular to two parallel surfaces. The plate-shaped permanent magnet having a trapezoidal cross section is arranged so that the wide bottom surface closely covers the inner wall surface, and the inner wall surface in an angle direction different from the inner wall surface has a cross section of long side, short side, and middle side. Formed in an isosceles triangle with sides A plate-shaped permanent magnet having an isosceles triangular cross-section magnetized in a direction perpendicular to the direction is arranged so that the surface on the long side closely covers the inner side surface, and, for example, a plate-shaped permanent magnet having an isosceles triangular cross section. The end of the common joint surface that passes through the boundary line of the inner wall surface, that is, the inclined side of the plate-shaped permanent magnet having the trapezoidal section and the surface on the short side, that is, the plate-shaped permanent section having the unequal triangle section. By configuring one of the vertices of the magnet to coincide with a tangential line on the circumference having the thickness of the plate-shaped permanent magnet having the trapezoidal cross section as the center, the hollow portion and the permanent magnet The current sheet at the interface of is erased by the condition that the magnetization direction of the permanent magnet is orthogonal to this surface, and the disturbance of the magnetic field on this surface is eliminated. Rank By eliminating the disturbance of the magnetic field at the joint by adapting the above condition to the above condition, and as a result, the curent sheet is provided only at the boundary between the plate-shaped permanent magnet having an isosceles triangular cross section and the inner wall surface of the core. Since it is possible to make the state equivalent to the arrangement and prevent leakage flux in a direction different from the uniform magnetic field, the polar direction of each plate-shaped permanent magnet corresponds to the angle formed by the inner wall surface with respect to the uniform magnetic field direction. By making the polarity suitable for the generation of a uniform magnetic field, a uniform magnetic field can be generated in the entire hollow portion excluding the plate-shaped permanent magnet inside the core.

Example of Invention

 The present invention will be described below based on examples.

FIG. 1 is a schematic perspective view of an apparatus according to an embodiment of the present invention, and FIG. 2 is a front view of FIG. In the figure, 21 is a rectangular cylindrical core made of a soft magnetic material such as an iron plate and a silicon steel plate, and has a pair of trapezoidal cross sections that generate a main magnetic flux on inner wall surfaces 2A and 2B perpendicular to the uniform magnetic field direction 100. Plate-shaped permanent magnets 23A, 23B
However, on the inner wall surface 2C, 2D side parallel to the uniform magnetic field direction 100,
Four plate-shaped permanent magnets 24A, 24 having an isosceles triangular cross section
B, 25A, 25C are the inner wall surfaces where the long side faces are in close contact with the inner wall faces 2C, 2D and the short side faces are joined to the inclined surfaces of the plate-like permanent magnets 23A, 23B having a trapezoidal cross section. A joint surface passing through the boundary line 27 of
28 and the permanent magnets are uniformly magnetized in the direction perpendicular to the interface with the hollow portion 26 and in the polar direction indicated by N ← S in the figure, the longitudinal direction of the hollow portion 26 is reduced. It is configured so that a uniform magnetic field 10 that spreads over the entire rectangular hollow portion 26 can be generated in the central portion.

FIG. 3 is a principle explanatory view of determining the shape of a plate-shaped permanent magnet having an isosceles triangular cross section and replacing the permanent magnet with a current sheet, and the upper right portion of FIG. 2 will be described as an example. . In the figure, the position of the middle side BC of the plate-shaped permanent magnet 24A formed in the scalene triangular cross section ABC with the long side AB having a length corresponding to half the width of the inner wall surface 2D is the vertex A (the boundary of the inner wall surface). It is determined so as to coincide with a tangent line BD connecting a vertex B with a circumferential surface 29 having a thickness d of a plate-shaped permanent magnet 23A having a trapezoidal cross section with a line 27) as the center, and the position of the vertex C is the above tangent line. A common joint surface 28 (side AC) is formed between the permanent magnets by determining the position where BD and the bottom side of the plate-shaped permanent magnet having a trapezoidal cross section on the hollow portion side intersect.

The plate-shaped permanent magnet 24A having an isosceles triangular cross-section whose shape is determined as described above and which is magnetized in the direction perpendicular to the medial side BC with the coercive force Hc has opposite currents on the short side AC and the long side AB. A thin wire passing through I is assumed to be arranged at a uniform density in the AD direction of length d, that is, it can be replaced by a carrent sheet, and the ampere-turn of each carrent sheet can be considered as coercive force Hc and side AD. It is given by the product of length d. Also, the inclined surface A of the plate-shaped permanent magnet 23A having a trapezoidal cross section
Regarding C, it is possible to consider by replacing the thin wires that pass the current I with a curent sheet in which the currents are evenly arranged in the thickness d direction. The carrent sheets cancel each other out to zero, and only the carrent sheet arranged on the long side AB side of the plate-shaped permanent magnet 24A having the unequal triangle cross section is left.

FIG. 4 is an explanatory view showing the state of the current sheet of the uniform magnetic field magnet formed as shown in FIG.
The uniform magnetic field magnets can be equivalently replaced by the carrent sheets 30C and 30D which are arranged on the inner wall surfaces 2C and 2D of which the direction of the current is indicated by cross dot symbols.

FIG. 5 is a magnetic flux distribution chart in the above-mentioned embodiment,
It is equivalently determined by the carrent sheet arrangement shown in the figure. As can be seen from the figure, since the leakage magnetic flux intersecting the inner wall surfaces 2C, 2D can be completely eliminated by the carrent sheets 30C, 30D, the magnetic flux lines 110 are perfectly straight and evenly spaced, and the core including the permanent magnets. It is shown that the uniform magnetic field space 10 can be generated over the entire hollow portion of 21. A permanent magnet
If the permanent magnet material of 23A, 24A is made of barium ferrite magnet or rare earth magnet with relative permeability of 1.05 or less, and if the magnetization curve of the permanent magnet is linearized by demagnetization treatment, etc., the curent sheet is replaced with the permanent magnet. Even 5th
A permanent magnet type uniform magnetic field magnet that generates a uniform magnetic field similar to that shown in the figure can be obtained.

FIG. 6 is a magnetic flux distribution diagram showing a comparative example of the above-mentioned embodiment, in which the magnetization direction N ← S of the plate-shaped permanent magnet 24A having an isosceles triangular cross section is measured on the long side AB side close to the inner wall surface 2D. It is different from the above-mentioned embodiment in that the direction perpendicular to the plane was adopted, and in this case, the equivalent of arranging the current sheet at the interface with the hollow portion on the side BC of the middle side and the joint surface 28 on the side AC of the short side. Therefore, the magnetic flux lines 110 in the plate-shaped permanent magnet 24A having an isosceles triangular cross section are significantly disturbed, and the leakage magnetic flux 102 perpendicular to the inner wall surface 2D is generated.
Occurs. The turbulence of the magnetic flux lines also affects the homogeneity of the magnetic field in the uniform magnetic field space 10, so that the homogeneity of the magnetic field is significantly reduced as compared with the above-described embodiment.

As is clear by comparing FIGS. 5 and 6 described above, it is not necessary to simply join a plate-shaped permanent magnet having a trapezoidal cross section and a plate-shaped permanent magnet having an isosceles triangular cross section in a frame shape. It is impossible to generate a uniform magnetic field, and the curent sheet at the interface is eliminated by magnetizing both permanent magnets in the direction perpendicular to the interface with the hollow part, and a plate-shaped permanent magnet with an isosceles triangular cross section is eliminated. By specifying the shape of the magnet, the curent sheet on the joint surface of both permanent magnets is eliminated, and the curent sheet is left only on the inner wall side interface of the plate-shaped permanent magnet having an isosceles triangular cross section. By setting the polarity of the magnetization of the permanent magnets so as to repel the leakage flux orthogonal to the inner wall surfaces 2C and 2D as shown in FIG. 2, the leakage flux 110 orthogonal to the inner wall surfaces 2C and 2D is completely eliminated, It is possible to generate a uniform magnetic field 10 that spreads over the entire hollow portion inside the core 21. Therefore, since the thickness d of the plate-shaped permanent magnets 23A and 23B having a trapezoidal cross section is the minimum weight required to generate a uniform magnetic field in the uniform magnetic field space 10, it is possible to reduce the weight of the permanent magnets and the uniform magnetic field space. Since the utilization rate of the hollow part is high, the leakage flux is eliminated and the total amount of generated magnetic flux is reduced.
The weight can be reduced to the minimum necessary.

FIG. 7 is a front view showing a different embodiment of the present invention, and FIG. 8 is an explanatory view of a state of a current sheet, in which a plate-shaped permanent magnet 33A having a trapezoidal cross section is formed inside a rectangular tubular core 31. ,
It differs from the above-mentioned embodiment in that it has an asymmetrical structure in which only a pair of plate-shaped permanent magnets 34A, 35A having a pair of isosceles triangular sections joined to this are arranged, but the current sheet is
As shown in the figure, it is the same as the above-mentioned embodiment, except that the total cross-sectional area of the plate-like permanent magnet having an isosceles triangular cross section is twice that of the above-mentioned embodiment. Similar performance can be obtained.

FIG. 9 is a front view showing a further different embodiment of the present invention,
FIG. 10 is an explanatory view of the state of the carrent sheet. In FIG. 9, a core 41 is formed in a hexagonal tube shape, and an inner wall surface thereof is composed of a plate-shaped permanent magnet 43A having a trapezoidal cross section and plate-shaped permanent magnets 44A, 45B having an isosceles triangular cross section. A magnet and a permanent magnet group consisting of this permanent magnet group and 43B, 44B, 45B symmetrically arranged with this permanent magnet group are arranged, and each permanent magnet is arranged in a direction perpendicular to the interface with the uniform magnetic field space 10. It is configured to generate a uniform magnetic field indicated by the uniform magnetic field direction 100 in the uniform magnetic field space 10 by being magnetized to the polarity shown by the arrow in the figure. The shape of the plate-shaped permanent magnet having an isosceles triangular cross section and the position of the joint surface are determined in the same manner as in the above-described embodiment described with reference to FIG. The amount of change in the thickness of the permanent magnet along the inner wall surface in the direction of magnetization is proportional to the cosine of the angle θ formed by the inner wall surface with respect to the uniform magnetic field direction 100, and decreases as θ increases. The uniform magnetic field magnet configured as shown in FIG. 9 can be replaced with the current sheet arrangement shown in FIG. Further, as is clear from FIGS. 9 and 10, the plate-shaped permanent magnet having an isosceles triangular cross section not only prevents the deformation of the main magnetic flux generated in the plate-shaped permanent magnet having the trapezoidal cross section, It can be seen that it has a function of generating a part of the main magnetic flux.

In each of the above-mentioned embodiments, in the permanent magnet type uniform magnetic field magnet in which the relative magnetic permeability of the permanent magnet material is slightly larger than 1, the magnetization of the plate-shaped permanent magnet having an isosceles triangular cross section By slightly shifting the direction from the direction perpendicular to the interface of the uniform magnetic field based on a predetermined formula, it is possible to correct the disturbance of the magnetic field in the hollow portion.

〔The invention's effect〕

As described above, the present invention focuses on the fact that the relative magnetic permeability of the permanent magnet material is close to 1, and the core made of the soft magnetic material is formed into a polygonal cylindrical shape, and the inner wall surface of the hollow part thereof perpendicular to the uniform magnetic field direction. Is a plate-shaped permanent magnet with a trapezoidal cross section, and a plate-shaped permanent magnet with an isosceles triangular cross section on the inner wall surface in an angle direction different from the inner wall surface so that the long side surface covers the inner wall surface. Adjacent permanent magnets that are arranged in close contact with each other and magnetize each permanent magnet in a direction perpendicular to the interface with the hollow portion, and that include the cross-sectional shape of a plate-shaped permanent magnet having an isosceles triangular cross section and the surface on the short side thereof. The joining surface of was determined so that the condition that the curent sheet remained only on the long side was satisfied. As a result, it is possible to almost completely eliminate the leakage magnetic flux intersecting the inner wall surface of the plate-shaped permanent magnet having an isosceles triangular cross section and the disturbance of the magnetic field inside the permanent magnet due to the leakage magnetic flux, and thus the entire polygonal hollow portion. It is possible to generate a uniform magnetic field that spreads over a wide area, and in the prior art using a pair of disk-shaped permanent magnets, the increase in the diameter of the permanent magnet required to avoid the disturbance of the magnetic field due to the leakage flux, and the total generated magnetic flux based on it The problem of increasing the amount of permanent magnets used and increasing the weight of a uniform magnetic field magnet, such as an increase in the volume, cross-sectional area of the core and magnetic path length, was eliminated, and the utilization rate of the hollow part as a uniform magnetic field space was eliminated. It is possible to provide a compact and lightweight permanent magnet type uniform magnetic field magnet economically and advantageously because it is high, the amount of permanent magnet material used and the total amount of magnetic flux generated are small. In addition, the problem of increasing the amount of permanent magnet material used by utilizing the permanent magnet in the circulation passage of the generated magnetic flux, which has been a problem in the conventional permanent magnet type uniform magnetic field magnet without a core, is small and inexpensive. There is an advantage that can be eliminated by using a polygonal tubular core. Further, the polygonal tubular core has an advantage that a shape suitable for accommodating a human body can be selected from various shapes such as a rectangular tubular shape and a hexagonal tubular shape.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention, FIG. 2 is a front view of FIG. 1, FIG. 3 is a principle explanatory view of the embodiment, and FIG. FIG. 5 is a magnetic flux distribution diagram in the embodiment, FIG. 6 is a magnetic flux distribution diagram in the comparative example, FIG. 7 is a front view showing a different embodiment, and FIG. FIG. 9 is a front view showing a further different embodiment, FIG. 10 is an explanatory view of the arrangement of a carrent seat in FIG. 9, FIG. 11 is a front view showing a conventional technique, and FIG. 12 is an improved conventional technique. It is a front view shown. 1,21,31,41 ... Polygonal cylindrical core, 2A, 2B ... Inner wall surface perpendicular to the uniform magnetic field, 2C, 2D ... Inner wall surface in different angle directions, 23A, 23B, 33
A, 43A, 43B ... Plate-shaped permanent magnets having a trapezoidal cross section, 24A, 24
B, 25A, 25B, 34A, 35A ... Plate-shaped permanent magnet having an isosceles triangular cross section, 26 ... Hollow part, 10 ... Uniform magnetic field space, 30C, 30D ...
Carrent sheet, 100 ... Direction of uniform magnetic field, S → N ... Direction of magnetization, d ... Thickness of plate-shaped permanent magnet having trapezoidal trapezoidal surface,
27 ... Boundary of inner wall surface, 28 ... Bonding surface, 101, 102 ... Leakage magnetic flux line, 110 ... Main magnetic flux line.

Claims (1)

[Claims] 1. A core made of a soft magnetic material formed in a polygonal cylinder shape having a plurality of pairs of inner wall surfaces parallel to each other, and a plurality of permanent magnets fixed so as to cover the plurality of pairs of inner wall surfaces. A uniform magnetic field in one direction perpendicular to the axis is generated in the hollow portion inside the core, the two parallel long sides of a plate having a trapezoidal cross section having a uniform magnetic field of the core. A plate-shaped permanent magnet having a trapezoidal section magnetized in a direction perpendicular to the two surfaces and fixed to an inner wall surface perpendicular to the two sides, and a plate having a trapezoidal section having an isosceles triangular section, and the long side surface having the trapezoidal section. Has a trapezoidal cross section which is fixed to an inner wall surface in an angle direction different from that of a plate-shaped permanent magnet having, and has a adjoining trapezoidal section whose short side faces form a common joint surface that passes through a boundary line with an adjacent inner wall surface. The joint surface of the permanent magnet should be joined so that it joins the inclined surface of the permanent magnet. The end of the hollow side of the is the intersection of the long side and the middle side of the plate-shaped permanent magnet having an isosceles triangular cross section, and the radius of the thickness of the plate-shaped permanent magnet having a trapezoidal cross section with the boundary line as the center. And a plate-shaped permanent magnet having an isosceles triangular cross-section, which is magnetized in a direction perpendicular to the surface on the medial side in contact with the hollow portion. A permanent magnet type uniform magnetic field magnet characterized by the above.