JPH03209803A - Magnetic field generating device for mri - Google Patents

Abstract

PURPOSE:To obtain a magnetic field having the uniformity equal to or higher than the constitution using a pole piece, and also to reduce weight by a method wherein permanent magnets, having different lengths of magnetization direction, are arranged without using any pole piece. CONSTITUTION:The magnet segments on the required position of a permanent magnet constituent body 1 is arranged by changing their length in magnetization direction. For example, a pair of permanent magnet constituent bodies 1 and 1, in which an Fe-B-R magnet is used, are opposingly positioned, their other ends are connected to rectangular plate-like yokes 4 and 6 and to the four pole-like yokes 6 which are provided on the four corners, and an electrostatic magnetic field is generated in the gap 7 located between the permanent magnet constituent bodies 1 and 1. The required permanent magnets, among a plurality of permanent magnets of the permanent magnet constituent bodies 1 and 1 arranged, are arranged by changing the length of magnetizing direction in accordance with the intensity of magnetic field in the gap 7, and a uniform magnetic field is obtained. As a result, the generation of an eddy current can be reduced without using any magnetic piece, lightweightedness can be achieved, and a uniform magnetic field equal to or higher than the constitution using a magnetic piece can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Application The present invention relates to an improvement in a magnetic field generating device using permanent magnets used in medical nuclear magnetic resonance tomography devices, etc. In a permanent magnet structure consisting of a plurality of magnet segments, the magnet segments at a desired position or a desired position of the structure are arranged with different heights in the gap direction, that is, in the magnetization direction, to generate a uniform magnetic field without using magnetic pole pieces, This invention relates to a magnetic field generator that reduces eddy current generation and is lightweight.

BACKGROUND TECHNOLOGY A medical nuclear magnetic resonance tomography apparatus (hereinafter referred to as MRI) takes a tomographic image of an object by inserting part or all of the subject into the gap of a magnetic field generator that generates a strong magnetic field. It is a device that can be used to visualize the properties of the tissue.

In the above magnetic field generation device for MRI, the gap must be wide enough to allow part or all of the subject to be inserted;
In order to obtain a clear tomographic image, the field of view within the gap is normally filled with a temperature of 0.05 to 1.5T and 1xlO-
It is required to create a stable, strong, uniform magnetic field with an accuracy of 4 or less.

As shown in Fig. 9, the magnetic field generator used in MHI uses a pair of permanent magnet structures (
A magnetic pole piece (2X2) is fixed to one end of each of the IXI) and placed facing each other, and the other end is connected with a plate yoke (4X5), and these are further connected with four columnar yokes (6) to form a magnetic pole piece. (2X2)
A configuration is known in which a static magnetic field is generated in the air gap (7) between the two.

For example, in the above configuration, in order to form a more stable uniform magnetic field, an annular protrusion (3) is provided on the peripheral edge of each of the opposing surfaces of the magnetic pole pieces, or a convex protrusion (3) is provided on the center of each of the opposing surfaces of the magnetic pole pieces. A magnetic field generator equipped with protrusions (8) has been proposed (
(Japanese Utility Model Application Publication No. 166110/1983).

In such a magnetic field generator, a uniform magnetic field is obtained by providing a magnetic pole piece on the opposing surface of a permanent magnet, but this configuration has the problem of increasing leakage from the sides of the magnetic pole piece and reducing the magnetic field strength in the air gap. be.

Furthermore, since the magnetic pole piece accounts for about 10% of the total weight of the magnetic field generating device, it has also been an obstacle to reducing the weight.

On the other hand, in order to obtain positional information within the air gap, a gradient magnetic field coil is used, which generates a magnetic field tilted in a desired direction in a short time by applying a pulse current, that is, usually x, y,
Three sets of coils corresponding to the three directions of z are placed near each magnetic pole piece, but the gradient magnetic field coils generate eddy currents, and it takes a long time for the gradient magnetic field to reach a predetermined strength. There is a problem that needs to be addressed.

Purpose of the Invention The present invention is proposed in view of the above-mentioned current situation, and is an MR device that reduces the generation of eddy current and reduces weight without using magnetic pole pieces, and that can obtain a uniform magnetic field equivalent to or higher than a configuration using magnetic pole pieces.
The purpose is to provide a magnetic field generator for I.

Summary of the Invention The present invention was developed as a result of various studies aimed at creating a magnetic field generator for MRI that generates a highly accurate uniform magnetic field within a required air gap without using magnetic pole pieces. This invention was completed based on the finding that a uniform magnetic field can be obtained and the above object can be achieved by partially adjusting the length in the magnetization direction.

That is, the present invention provides an MRI system in which a pair of permanent magnet structures each consisting of a single or a plurality of magnet segments facing each other with an air gap are magnetically coupled with a yoke to generate a magnetic field in the air gap. A magnetic field generating device for MRI, characterized in that magnet segments at desired positions or desired positions of a permanent magnet structure are arranged with different lengths in their magnetization directions, and a uniform magnetic field is generated in the gap. be.

Specifically, in the magnetic field generating device for MRI having the above configuration, the present invention provides the following: When the permanent magnet structure has a single magnet segment, the length of the magnet in the magnetization direction of the magnet at a predetermined position is changed, or the magnet segment has a plurality of magnet segments. In this case, the magnet segments are arranged at predetermined positions with different lengths in the magnetization direction, and a uniform magnetic field is generated in the gap.

Furthermore, in addition to changing the length of the permanent magnet in the magnetization direction (height in the air gap direction), this invention also changes the magnet length, magnet width, magnetic properties,
The present invention is characterized in that at least one of the magnet materials is changed to generate a uniform magnetic field in the gap.

Structure of the Invention In this invention, if the magnetic circuit has a structure in which a pair of permanent magnet structures facing each other with an air gap are magnetically coupled by a yoke, the yoke, two dimensions of the magnet shape of the permanent magnet structure, Any configuration may be used for the arrangement method, and it is desirable to appropriately select the magnetic properties, shape and dimensions of the permanent magnet structure, shape and dimensions of the yoke, etc., depending on the size of the required air gap and the like.

In order to generate a uniform magnetic field in a required gap without providing a magnetic pole piece on the surface facing the gap of a permanent magnet structure, a plurality of permanent magnets are arranged with different lengths in the magnetization direction (height in the gap direction). The basic technical idea is, for example, to form a predetermined permanent magnet structure with permanent magnets of the same size and characteristics, and to increase the magnetic field strength within the required air gap. By analyzing the distribution and changing the length of the permanent magnet in the magnetization direction at the required location, that is, the height of the magnet in the air gap direction,
The purpose is to arrange the permanent magnets in such a manner that a uniform magnetic field is generated within the air gap.

Further, in the present invention, the pair of permanent magnet structures facing each other with a gap formed therebetween can have any shape as described above, and the plurality of permanent magnets forming this permanent magnet structure may be formed as described above. , in addition to changing the length of the permanent magnet in the magnetization direction, that is, the magnet height in the air gap direction, the magnet length, magnet width,
The magnetic properties and magnet material may be changed, and these magnets can be arranged in various patterns.

The present invention is characterized in that the magnet segments in a given position or in a given position of a pair of permanent magnet structures made up of a single or a plurality of magnet segments are arranged with different lengths in the magnetization direction, and a uniform magnetic field is generated in the gap. However, the method of arranging the magnets by changing their length in the magnetization direction, the shape of the permanent magnets, the arrangement pattern, etc. may vary depending on the main purpose such as weight reduction or improving the uniformity of the magnetic field, or the differences in the various uses of the device. As shown in the examples described later, an analysis method is selected for determining the magnetic field strength and uniformity within the required air gap, and the specifics of the analysis results and the position at which the magnet height is changed, the selection of the magnet segment, and the amount of change are made. Since the optical relationship and preferred configuration change, it is necessary to appropriately select the magnet segment, the length of the magnet in the magnetization direction, etc., depending on each condition.

The permanent magnet of the magnet structure consisting of one or more magnet segments used in such a magnetic circuit is a ferrite magnet,
Alnico-based magnets and rare-earth cobalt-based magnets can be used, but in particular, use resource-rich light rare earths such as Nd and Pr as R, and use 30MG magnets with B and Fe as the main components.
Fe-B- exhibits an extremely high energy product exceeding Oe.
By using R-based permanent magnets, the size can be significantly reduced.

Based on the Drawings (Disclosure of the Invention) Figures 1a and 1b are explanatory diagrams showing the permanent magnet structure (lower side) of the magnetic field generating device according to the present invention.

FIG. 2a is a longitudinal cross-sectional view of the magnetic field generator without magnetic pole pieces, and the same figure is a cross-sectional view.

FIG. 3 is a perspective explanatory view of an air gap showing a method of measuring a magnetic field within a spherical space of a magnetic field generating device.

FIGS. 4A to 4D are explanatory diagrams showing examples of changing the magnetization direction height of required segments of the permanent magnet structure (upper side) of the present invention.

8- Figures 5 and 6 are graphs showing the degree of magnetic field inhomogeneity of P1 to P7 in the spherical space of Figure 3, and Figure 5 is a graph showing the magnetic field inhomogeneity of P1 to P7 in the spherical space of Figure 3. In this case, FIG. 6 shows the case of the permanent magnet arrangement of FIGS. 4a-d.

FIG. 7 shows P1 to P1 in the spherical space in the embodiment of FIG. 1a.
It is a graph showing magnetic field non-uniformity of P7.

Figure 8 shows the optimized permanent magnet structure of this invention (upper side)
FIG. 2 is an explanatory diagram showing an example.

Device configuration Here, as shown in FIG. 2, the magnetic field generating device shown in FIG.
A pair of permanent magnet structures using B-R magnets (IXI)
facing each other, the other end is connected by a rectangular plate yoke (4x5), and these are further connected by four columnar yokes (6) arranged at the four corners, and the permanent magnet structure (1) (1) is connected. A magnetic field generating device configured to generate a static magnetic field in the air gap (7),
An example to which this invention is applied will be explained.

First, assuming that the permanent magnet structure (1) is formed into a disk shape in a predetermined pattern with a large number of permanent magnets having the same size and characteristics, or is formed into such a configuration, the magnetic field situation generated in the air gap (7) will be explained. , analyze intensity distribution, etc.

Analysis Method Various analysis methods such as the finite element method and the boundary element method can be adopted as the analysis method. Further, as an optimization method, a mathematical programming method such as linear programming is used, but here, a case where the finite element method and linear programming are used together will be explained.

The magnetic field situation in the magnetic field generator air gap (7) is investigated using the finite element method.

In addition, in the air gap (7) of the magnetic field generator having the above configuration, a spherical space with a predetermined radius r is set from the center (0), and as shown in FIG. A cross section is made on seven horizontal planes (P1 to P7), and further on the intersection circumference of each horizontal plane (Pi to P7) and the spherical space,
The magnetic field strength at a plurality of points equally divided at a required angle around the Z-axis is calculated, and the dispersion of the magnetic field strength on the circumference of each horizontal plane (p1 to P7) is investigated.

Next, the length in the magnetization direction of each of the many magnet segments constituting the permanent magnet structure (IXI) is determined in the horizontal plane (Pi to P
7) Investigate the influence on the magnetic field strength on the circumference.

As shown in Figure 2, the permanent magnet structure (IX
The horizontal plane (Pi
~P7) When measuring the magnetic field strength on the circumference, the fifth
As shown in the figure, the non-uniformity of the magnetic field strength is expressed as a deviation (%) from the magnetic field strength at the center of the air gap on the horizontal plane (Pl~
P7) The magnetic field strength is significantly non-uniform.

Therefore, as shown in Fig. 4 a to d, when the position of the magnet segment whose height in the magnetization direction is lowered is sequentially moved outward from the center of the permanent magnet structure (1), the horizontal plane (P1 to P
When the influence of 7) on the magnetic field strength on the circumference is investigated, the results shown in FIG. 6 are obtained.

In this way, the influence of the position of the magnet segment and the amount of change in height in its magnetization direction on the magnetic field strength on the circumference of the horizontal plane (P1 to P7) is investigated, and the optimal magnet segment is selected and superimposed. By this, the horizontal plane (p1~
The magnetic field strength of P7) becomes constant.

11- In the same way, it is possible to obtain a uniform magnetic field by expanding the optimization of magnet segments to the entire area and overlapping the optimal magnet segments, but since it requires a large number of steps through trial and error, we will not discuss it here. The height of each magnet segment in the magnetization direction was determined using linear programming, which is one of the optimization methods.

Based on this analysis, the required length in the magnetization direction of the permanent magnet is selected from among the many permanent magnets of the permanent magnet structure (IXI) depending on the strength of the magnetic field in the air gap (7), etc. By varying the arrangement as shown in FIG. 1a, a uniform magnetic field can be obtained as shown in FIG. 7.

The permanent magnet structure (1) shown in Fig. 1a is a plate-shaped yoke (5
), permanent magnets with different magnetization directions and lengths are arranged on the top of the plate yoke (5). Magnet structure (
The same effect can be obtained even if the opposing surface of the gap (7) in 1) is made a flat surface.

12-Furthermore, for the purpose of reducing the weight of the magnet, the height of each magnet segment in the magnetization direction was optimized using the analysis method described above.

As shown in Fig. 8, by forming an annular protrusion by elongating the magnetization direction particularly at the periphery, and by changing the magnetization direction at a predetermined position in the center and arranging it, a more highly accurate uniform magnetic field can be obtained. -The weight of the layer could be reduced.

Example: Fe-B with (BH)max of 35 MGOe at room temperature
-Using R-based permanent magnets, diameter 300 discs, height 3.0mm
Convex projection, outer diameter 1100 mm, inner diameter 900 mm, height 8
In the conventional magnetic field generator shown in FIG. 9, in which magnetic pole pieces having an annular protrusion of 0 disks are arranged and the opposing distance of the magnetic pole pieces is set to 500 disks, the magnetic field is equalized within a radius of 20 axis disks from the center of the air gap. When measured once, it was 50 ppm.

In the above-mentioned conventional magnetic field generating device, the magnetic pole piece is removed, and the length in the magnetization direction of each permanent magnet of the permanent magnet structure is optimized by applying the above-mentioned analysis method, and the facing distance of the permanent magnet structure is reduced by 500- A magnetic field generating device according to the present invention having a permanent magnet structure shown in FIG. 8 was manufactured.

Radius 2 from the center within the air gap of the magnetic field generator according to the present invention
When we measured the magnetic field uniformity within 0.00 mm, it was the same as the conventional device, and since no magnetic pole pieces were used, the weight of the magnet could be reduced by more than 10%, making it possible to reduce the weight of the entire device.

Effects of the Invention The magnetic field generating device according to the present invention can obtain a uniform magnetic field equivalent to or better than a configuration using magnetic pole pieces by arranging permanent magnets with different lengths in the magnetization direction without using magnetic pole pieces. Since no magnetic pole pieces are used, the generation of eddy currents can be suppressed, and the weight can also be reduced.

[Brief explanation of drawings]

FIGS. 1a and 1b are explanatory diagrams showing the permanent magnet structure (lower side) of the magnetic field generator according to the present invention. FIG. 2a is a longitudinal cross-sectional view of the magnetic field generator without magnetic pole pieces, and the same figure is a cross-sectional view. FIG. 3 is a perspective explanatory view of an air gap showing a method of measuring a magnetic field within a spherical space of a magnetic field generating device. FIGS. 4a to 4d are explanatory diagrams showing examples in which the heights in the magnetization direction of required segments of the permanent magnet structure (upper side) of the present invention are changed. 5 and 6 are graphs showing the degree of magnetic field inhomogeneity of P1 to P7 in the spherical space of FIG. 3, and FIG. 5 is a graph showing the magnetic field inhomogeneity of P1 to P7 in the spherical space of FIG. FIG. 6 shows the case of the permanent magnet arrangement of FIGS. 4a-d. FIG. 7 shows P1 to P1 in the spherical space in the embodiment of FIG. 1a.
It is a graph showing magnetic field non-uniformity of P7. Figure 8 shows the optimized permanent magnet structure of this invention (upper side)
FIG. 2 is an explanatory diagram showing one embodiment of the invention. FIG. 9a is a longitudinal cross-sectional view of a conventional magnetic field generator, and the same figure is a cross-sectional view. DESCRIPTION OF SYMBOLS 1... Permanent magnet structure, 2... Magnetic pole piece, 3... Annular protrusion, 4, 5... Plate-shaped yoke, 6... Column-shaped yoke,
7...Void, 8...Convex projection. So← 0) (%) (%) 8th doctor\1

Claims (1)

[Scope of Claims] 1. A pair of permanent magnet structures formed of one or more magnet segments facing each other with a gap formed therein are magnetically coupled with a yoke,
In an MRI magnetic field generation device that generates a magnetic field in the air gap, magnet segments at desired positions or positions of the permanent magnet structure are arranged with different lengths in their magnetization directions, and a uniform magnetic field is generated in the air gap. Features of magnetic field generator for MRI.