JPH0787141B2 - 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 NMR-CT, it is necessary to generate a uniform magnetic field with a magnetic field strength of several tens of PPm in a uniform magnetic field space with a diameter of about 400 mm in the center of the space surrounded by a uniform magnetic field magnet, and a permanent magnet is used. By doing so, the enormous power consumption in the method using the normal conducting uniform magnetic field coil and the power consumption for cooling in the superconducting magnet method are eliminated, so that there is an advantage that a uniform magnetic field magnet with a low operating cost can be obtained, and it is attracting attention. . However,
When comparing the prices of permanent magnets and copper wire rods, ferrite magnets are several times more expensive and rare earth magnets are 100 times more expensive, so the amount of permanent magnets used is small, and the permanent magnet type is small and lightweight. The appearance of a uniform magnetic field magnet is required.

FIG. 6 is a perspective view of a permanent magnet type uniform magnetic field magnet already proposed by the inventors, and FIG. 7 is a front view of FIG. In the figure, 1 is a rectangular cylindrical core made of a soft magnetic material such as an iron plate and a silicon steel plate, and the magnetic flux lines in the uniform magnetic field direction.
On the inner wall surfaces 2A and 2B perpendicular to 100, plate-shaped permanent magnets 2 and 12 having a pair of trapezoidal cross sections that generate the main magnetic flux, and on the inner wall surfaces 2C and 2D parallel to the magnetic flux lines 100 in the uniform magnetic field direction are provided. Is four plate-shaped permanent magnets 3, 13, 4, 14 having an isosceles triangular cross section.
However, the surface on the long side is inscribed in the inner wall surfaces 2C, 2D, and the surface on the short side is joined to the inclined surfaces of the plate-shaped permanent magnets 2, 12 having a trapezoidal cross section to form the boundary line 27 of the inner wall surface. The length of the hollow portion 5 is increased by forming the joining surface 28 passing therethrough and by uniformly magnetizing each permanent magnet in the direction perpendicular to the interface with the hollow portion 5 and in the polar direction indicated by N ← S in the figure. It is configured so that a uniform magnetic field 15 spreading over the entire rectangular hollow portion 5 can be generated in the central portion in the vertical direction. FIG. 8 is a principle explanatory view of determining the shape of a plate-shaped permanent magnet having an isosceles triangular cross section and replacing it with a current sheet, and the upper right portion of FIG. 7 will be described as an example. . In the figure, the long side AB has a length equivalent to half the width of the inner wall surface 2D, and the scalene triangular section ABC
The position of the medial side BC of the plate-shaped permanent magnet 13 formed on the circle is centered on the apex A (the boundary line 27 of the inner wall surface) and the radius is the thickness d of the plate-shaped permanent magnet 2 having a trapezoidal cross section. It is determined so as to coincide with the tangent line BD connecting the peripheral surface 29 and the apex B, and the position of the apex C is determined at the position where the tangent line BD and the hollow base side of the plate-shaped permanent magnet having a trapezoidal cross section intersect. As a result, a common joint surface 28 (side AC) is formed between both permanent magnets.

The plate-shaped permanent magnet 13 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 H _c has opposite directions on the short side AC and the long side AB. assuming thin lines leading the current I while arranged at equal densities on the length d becomes AD direction, i.e. it can be considered replaced car rent sheet, car rent sheets each ampere turns coercivity H _c and the side It is given by the product of AD length d.
Also, the inclined surface AC of the plate-shaped permanent magnet 2 having a trapezoidal cross section can be considered by replacing the thin wires passing the current I with a current sheet uniformly arranged in the thickness d direction. Since the sheets have the same size and the current flows in the opposite direction, the carrent sheets cancel each other out to zero, and only the carrent sheet disposed on the long side AB side of the plate-shaped permanent magnet 13 having the inequilateral triangular cross section is used. Is left.

FIG. 9 is an explanatory view showing a 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. 10 shows the magnetic flux distribution having the above-described structure, which is equivalently obtained by the arrangement of the current sheet shown in FIG. As you can see from the figure, carrent sheet 30C, 30
Since the leakage flux crossing the inner wall surfaces 2C and 2D can be eliminated by D, the magnetic flux lines 110 are linear and evenly spaced, and it is shown that the uniform magnetic field space 15 can be generated in the entire hollow portion of the core 1 including the permanent magnet. There is.

In the permanent magnet type uniform magnetic field magnet configured as described above, the core 1 serving as the circulation passage of the magnetic flux lines 100 can be made of an inexpensive steel material, and the uniform magnetic field is provided over the entire hollow portion 5 including the interface with the permanent magnet. Since the hollow portion 5 can be formed, the peripheral length of the hollow portion 5 can be small, and thus the amount of expensive permanent magnet material used can be reduced.

FIG. 10 is a magnetic flux distribution diagram of the magnet configured as described above, where 0 is the center point of the core 1, 201 is the axis of the cylindrical core 1, 202 is the center point 0, and symmetry is perpendicular to the axis 201. Of the core 1 and the permanent magnet 2 in the hollow portion 5 including the cross section.
The magnetic flux distribution in the 1/4 region is shown. In the figure, 1
00 is a magnetic flux line distributed in the hollow portion 5, and in the vicinity of a spherical uniform magnetic field space 15 having a diameter of about 400 mm shown by a broken line in the figure, the lines are linear with equal intervals, and the magnetic field uniformity is excellent. It shows that a uniform magnetic field is formed. However, the hollow portion 5 is in a state in which both ends in the direction of the axis 201 (Z direction) are opened for carrying in and out of a patient, and a leakage magnetic flux line 101 spreading toward the outside of the magnet is generated. There is a problem that the magnetic flux lines in the hollow portion 5 are distorted. In the figure, the magnetic flux lines 100 appear to be sufficiently uniform, but NMR-
In CT magnets, magnetic field disturbances that do not appear in the figure pose a problem.

FIG. 11 is a characteristic diagram showing the distribution of magnetic field strength (Bg) on the axis 201. In the figure, the strength Bg of the magnetic field sharply decreases at the end portion of the core 1, but due to the influence thereof, the value gradually decreases toward the end portion even in the uniform magnetic field space 15. And this change will reduce the uniformity of the magnetic field in the uniform magnetic field space. In order to avoid this, it is necessary to move the end away from the uniform magnetic field space, that is, to lengthen the length of the magnet in the axial direction, which causes a drawback that not only the permanent magnet but also the whole weight and size increase. .

[Object of the Invention]

The present invention has been made in view of the above-mentioned circumstances, and it is possible to improve the homogeneity of a magnetic field without increasing the length of the magnet in the axial direction. Therefore, a compact, lightweight, and inexpensive permanent magnet type uniform magnetic field magnet is provided. The purpose is to provide.

[Main points of the invention]

The present invention provides a core formed in the shape of a hollow polygonal cylinder having a plurality of inner wall surfaces parallel to each other, and a plurality of permanent magnets fixed so as to cover the inner wall surfaces, so that the hollow portion inside the core has an axis line. A unit for generating a uniform magnetic field in one vertical direction, wherein the long side surfaces of two plate-like parallel surfaces having a trapezoidal cross section are fixed to the inner wall surface of the core perpendicular to the uniform magnetic field. A plate-shaped permanent magnet having a trapezoidal cross section that is magnetized in a direction perpendicular to, and a plate-shaped permanent magnet that has a trapezoidal cross-section and that has a trapezoidal cross-section. Fixed to the inner wall surface in the direction, so that the surface on the short side forms a common joint surface that passes through the boundary line with the adjacent inner wall surface and is joined to the inclined surface of the permanent magnet having the adjoining trapezoidal cross section, The hollow side end of the core of the joint surface is an isosceles triangular cross section It is located on a tangent line connecting the intersections of the long side and the middle side of the plate-shaped permanent magnet having and the circumference having 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 magnetized in a direction perpendicular to the surface of the medial side in contact with the hollow portion, and the hollow portion having a protrusion on the hollow portion side. A permanent magnet piece for inhomogeneous magnetic field correction magnetized to be in a direction perpendicular to the interface with
The plate-shaped permanent magnets having the trapezoidal cross section and the plate-shaped permanent magnets having the isosceles triangular cross section are provided along both ends in the axial direction. As a result, the magnetic flux generated by the permanent magnet pieces increases the strength of the magnetic field in the hollow part at both ends in the axial direction, and it is possible to compensate for the decrease in the magnetic field strength due to the leakage flux in this part. It is intended to increase the homogeneity of the magnetic field in.

Example of Invention

 The present invention will be described below based on examples.

FIG. 1 is a partial crushing side sectional view showing an apparatus according to an embodiment of the present invention,
FIG. 2 is a cross-sectional view taken along the line AA in FIG. 1, and the same parts as those of the prior art are designated by the same reference numerals and detailed description thereof will be omitted. In the figure, the axis 201 of the rectangular cylindrical core 1
The plate-shaped permanent magnets 2 and 12 each having a trapezoidal cross section formed to be slightly shorter than the length in the direction, and the permanent magnets 3 and 13 each having an unequal equilateral triangular cross section. The part has a trapezoidal cross section having a portion protruding toward the hollow part 5 for correcting the non-uniform magnetic field.
1, and permanent magnet pieces 22 having a triangular cross section are arranged respectively, and the permanent magnet pieces 21 and 22 are plate-shaped permanent magnets 2,
Similar to 3 and the like, it is configured to be magnetized in the same direction as the surface surrounding the hollow portion 5 in the same direction, and to generate an extra magnetic flux by the thickness of the permanent magnet piece portion in the magnetizing direction.

FIG. 3 is a magnetic flux distribution diagram in the example magnet,
The non-uniform magnetic field correcting permanent magnet piece, for example, 21 arranged at the end of the plate-shaped permanent magnet 2 is thicker than the permanent magnet 2,
Although the amount of leakage flux lines 101 also increases due to the higher generated magnetic flux density of the permanent magnet pieces, magnetic flux lines 103 having a curved component are generated on the hollow portion 5 side, and the magnetic flux lines inside the hollow portion 5 are generated. By acting to prevent 100 distortion,
A non-uniform component of the magnetic field in the uniform magnetic field space 15 is reduced, and a permanent magnet type uniform magnetic field magnet having high homogeneity can be formed without increasing the length in the axial direction.

FIG. 4 is a characteristic diagram showing the strength of the magnetic field on the axis of the example magnet. The increase in the magnetic flux generated by the permanent magnet pieces is added to the magnetic field strength (curve 51) generated by the magnetic flux generated by the plate-shaped permanent magnet. By adding (curve 52), the overall magnetic field strength compensates for the magnetic field strength at the end where the permanent magnet pieces are arranged, as shown by the curve 50, which results in a uniform magnetic field space 15
It is shown that a permanent magnet type uniform magnetic field magnet that spreads to the strength of the magnetic field in the vicinity and is almost horizontal as compared with the curve 51, that is, the magnetic field strength is uniform.

FIG. 5 is a side sectional view showing a different embodiment of the present invention,
The point is that the permanent magnet piece 31 protruding toward the hollow portion 5 is fixed to the inside of a plate-shaped permanent magnet, for example 2, which is formed to have the same length as the core 1 in the direction of the axis 201. In the permanent magnet type uniform magnetic field magnet thus formed, the magnetizing direction of the plate-like permanent magnets 3 and 13 having an isosceles triangular cross section and the magnetizing direction of the permanent magnet piece 31 are different. However, a magnetomotive force is generated at the interface between both permanent magnets, which deviates from the ideal arrangement, but by adding a premagnetized permanent magnet piece to the conventional device,
The advantage that the non-uniform component of the uniform magnetic field can be corrected is obtained.

The polygonal tubular core is not limited to the rectangular tubular shape, and a permanent magnet type uniform magnetic field magnet can be formed by using cores of various shapes such as a hexagonal tubular shape and a rhombic tubular shape. In addition, the protrusion of the permanent magnet piece toward the hollow portion may be formed in a stepwise width in the axial direction. By doing so, the leakage flux is small and the permanent magnet type with high correction performance for non-uniform magnetic field is formed. A uniform magnetic field magnet can be obtained.

〔The invention's effect〕

As described above, the present invention provides a plate-shaped permanent magnet having a trapezoidal cross section on the inner wall surface of the hollow part of the core formed in a polygonal cylinder shape and perpendicular to the uniform magnetic field direction. Plate-shaped permanent magnets having an isosceles triangular cross section are arranged on the inner wall surface so that the long side surfaces cover the inner wall surface, and each permanent magnet is magnetized in a direction perpendicular to the interface with the hollow portion,
The cross-sectional shape of a plate-shaped permanent magnet having an isosceles triangular cross-section and the joint surface of adjacent permanent magnets including the surface on the short side thereof are determined so that the curent sheet remains only on the long-side surface, and Along with both ends in the axial direction of each permanent magnet, there is a protrusion on the hollow side, and a permanent magnet piece for correcting the non-uniform magnetic field magnetized in a direction perpendicular to the interface with the hollow is provided. did. As a result, the magnetic flux generated by the permanent magnet pieces compensates for the decrease in the magnetic field strength at both ends of the magnet, and this also affects the correction of a slight decrease in the magnetic field strength in the uniform magnetic field region. It is possible to provide a permanent magnet type uniform magnetic field magnet having one time. Further, in order to maintain the homogeneity of the magnetic field, it is necessary to extend the length in the axial direction, thus eliminating the problem of the prior art that the magnet becomes large and heavy, and it is small, lightweight and inexpensive. A magnet type uniform magnetic field magnet can be provided.

[Brief description of drawings]

FIG. 1 is a partial crushing side sectional view showing an embodiment of the present invention,
1 is a sectional view taken along the line AA in FIG. 1, FIG. 3 is a magnetic flux distribution diagram in the example magnet, FIG. 4 is a magnetic field strength characteristic diagram on the axis in the example magnet, and FIG. 5 is different. Fig. 6 is a side sectional view showing an embodiment, Fig. 6 is a perspective view of a conventional device, Fig. 7 is a front view of Fig. 6, and Fig. 8 is a shape determination of a plate-like permanent magnet having an isosceles triangular cross section and curling. FIG. 9 is an explanatory view showing the principle of replacement with a sheet, FIG. 9 is an explanatory view showing a state of a current sheet of the uniform magnetic field magnet shown in FIG. 7, and FIG. 10 is a magnetic flux distribution chart having the configuration shown in FIG. 7 and FIG. FIG. 12 is an axial magnetic flux distribution diagram of the conventional device, and FIG. 12 is an axial magnetic field strength characteristic diagram of the conventional device. 1 ... Core, 2,12 ... Permanent magnet (trapezoidal cross section), 3,4,1
3,14 …… Permanent magnet (triangular unequal cross section), 5 …… Hollow part, 15 …… Uniform magnetic field space, 21,31 …… Permanent magnet piece (trapezoidal cross section), 22 …… Permanent magnet piece (unequal side) Triangular section), 10
0,103 …… Magnetic flux line, 101 …… Leakage magnetic flux line, 201 …… Axis line, 2
02 …… Symmetrical plane.

Claims (3)

[Claims] 1. A core formed in the shape of a hollow polygonal cylinder having a plurality of inner wall surfaces that are parallel to each other, and a plurality of permanent magnets fixed so as to cover the inner wall surfaces, whereby an axis line is formed in a hollow portion inside the core. For generating a uniform magnetic field in one direction perpendicular to the two sides of the plate-like parallelepiped having a trapezoidal cross section on the long side, and fixed to the inner wall surface of the core perpendicular to the uniform magnetic field. Different from a plate-shaped permanent magnet having a trapezoidal cross section magnetized in a direction perpendicular to the plane, and a plate-shaped permanent magnet having a trapezoidal cross section with a long side surface formed in a plate shape having an isosceles triangular cross section. It is fixed to the inner wall surface in the angle direction, and the surface on the short side forms a common joint surface that passes through the boundary line with the adjacent inner wall surface so as to join with the inclined surface of the permanent magnet having the trapezoidal cross section. , The end of the joint surface on the hollow part side of the core has an unequal triangle Positioned on the tangent line connecting the intersections of the long side and the middle side of the plate-shaped permanent magnet having the and the circumference having the radius of the thickness of the plate-shaped permanent magnet having the trapezoidal cross section with the boundary line as the center. And a plate-shaped permanent magnet having an isosceles triangular cross section magnetized in a direction perpendicular to the surface of the medial side in contact with the hollow portion, and having a protruding portion on the hollow portion side and a hollow portion. A non-uniform magnetic field correcting permanent magnet piece magnetized in a direction perpendicular to the interface with the plate portion, a plate-like permanent magnet having the trapezoidal section and a plate-like permanent magnet having the unequal triangle section. And a permanent magnet type uniform magnetic field magnet, which are provided along both ends in the axial direction of each of. 2. The permanent magnet piece according to claim 1, wherein the permanent magnet pieces provided at both ends of the permanent magnet for correcting a non-uniform magnetic field partially increase the thickness of the permanent magnet in the magnetizing direction. A permanent magnet type uniform magnetic field magnet characterized by being formed by the above. 3. The structure according to claim 1, wherein permanent magnet pieces for correcting the non-uniform magnetic field provided at both ends of the permanent magnet are fixed to the interface of the permanent magnet on the hollow side. Permanent magnet type uniform magnetic field magnet.