JPH02246927A - Magnetic field generator for magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To reduce the generation of an eddy current and to clear an MR image by making a magnetic pole piece into a laminated structure in plural layers with a specific constitution. CONSTITUTION:The magnetic field generator of an MRI device is equipped with a pair of permanent magnets 1a and 1b to be opposedly arranged in forming a void to house a subject B, yokes 2a and 2b to support and, simultaneously, to magnetically couple the permanent magnets and magnetic pole pieces 5a and 5b to be respectively fixed to the opposed surface at a void A side of a pair of permanent magnets 1a and 1b and to composed of disk magnetic members, and the magnetic field is generate in the void A. Here, the magnetic pole piece 5a and 5b are made into the laminated structure in plural layers in the thickness direction, a magnetic pole piece member 8 faced to the permanent magnets sides 1a and 1b is composed of a thick plate magnetic disk in an integral article, a magnetic pole piece member 9 faced at the void side A is composed of a molding magnetic member 9 molded by compounding a magnetic needle material 12 and an electric insulating material 13 and applying pressure, they are integrally coupled and fixed, and the formation is executed. In such a way, since, for the magnetic pole piece member faced to the void side, the resistance value is made large, the eddy current generated to the magnetic pole piece can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic resonance imaging apparatus (1) for obtaining a tomographic image of an examination region of a subject by utilizing the nuclear magnetic resonance (NMR) phenomenon.
The present invention relates to a magnetic field generating device using a permanent magnet used in an rMRI device (hereinafter referred to as “rMRI device”), and particularly relates to a magnetic field generating device for an MHI device that can reduce the generation of eddy currents in magnetic pole pieces due to the drive of gradient magnetic field coils.

[Conventional technology]

An MRI device uses NMR phenomena to measure nuclear spin density distribution, relaxation time distribution, etc. at a desired inspection site in a subject, processes the measurement signals, and creates an image as a tomographic image of the inspection site. It is to be displayed. Here, when measuring a spatially wide range such as the human body, 0
．． A magnetic field generating device is required that generates a static magnetic field of about 0.5 to 2 T (tesla; 1 tesla is 10,000 gauss) with a uniformity of several tens of ppm or less. Conventionally, three types of magnetic field generators have been used: normal conducting magnets, superconducting magnets, and permanent magnets.

As shown in FIGS. 15 and 16, the magnetic field generating device of the conventional MHI apparatus using permanent magnets includes a pair of permanent magnets 1a, which are arranged opposite to each other and form a gap A into which the subject can enter. lb, yokes 2a, 2b, and 3 that support and magnetically couple these permanent magnets 1a and lb, and disk-shaped magnetic yokes that are fixed to the opposing surfaces of the pair of permanent magnets 1a and lb on the air gap A side, respectively. The member has magnetic pole pieces 5a and 5b having an annular protrusion 4 formed on the periphery of the member, and is designed to generate a magnetic field within the gap A.

Furthermore, in addition to the static magnetic field generating means using the permanent magnets 1a and 1b and the magnetic pole pieces 5a and 5b, the MRI apparatus also generates a gradient magnetic field in order to obtain position information in the measurement space B set in the air gap A. It is equipped with gradient magnetic field coil groups 6a and 6b. These are X and Y installed near each magnetic pole piece 5a, 5b.

It consists of three coil pairs corresponding to the three Z directions.

In addition, in FIGS. 15 and 16.

In order to make the figure easier to read, the three coil pairs are not shown separately. A magnetic field with a gradient is generated within the measurement space B in a short time by passing a pulse current through the gradient magnetic field coil groups 6a and 6b from a gradient magnetic field coil drive power source (not shown). Here, for reference,
FIG. 17(a) shows a conceptual diagram of a gradient magnetic field coil in the Z direction as an example of a gradient magnetic field coil. In order to obtain a gradient magnetic field in the Z direction, as shown in FIG. A current i x2 is applied. By doing this, the coil pair 7a is formed as shown in FIG. 17(b).

At IFf'ltI in 7b, a magnetic field with a strength of O and a gradient whose absolute value becomes larger as it advances in either of the two directions is generated.

However, in the magnetic field generator of such an MRI apparatus, the magnetic field generated at the rising and falling times when a pulse current is passed through the gradient magnetic field coil groups 6a and 6b shown in FIG. 5b caused an eddy current. Since this eddy current forms a magnetic field in the opposite direction to that produced by the gradient magnetic field coil groups 6a and 6b in the measurement space B, it is difficult for the gradient magnetic field generated in the measurement space B to reach a predetermined strength. This would require a lot of time. When considering the time required to take a tomographic image of a subject, it is desirable to reach a predetermined gradient magnetic field strength in the shortest possible time.In order to shorten this time, it is necessary to There is a way to increase the current flowing through the gradient magnetic field coil, but this increases the burden on the gradient magnetic field coil drive power supply and is not a good idea considering the cost and installation area of the device. Since it is proportional to the ratio, the earlier the rise time of the gradient magnetic field is, the greater the ratio of the influence of the eddy current on the rise delay. Therefore, in order to shorten the time it takes for the gradient magnetic field to reach a predetermined strength, as described in Japanese Patent Laid-Open No. 63-25907, the magnetic pole pieces 5a and 5b are electrically insulated from the magnetic powder. It has been proposed to reduce the eddy current generated in the magnetic pole pieces 5a and 5b by constructing a magnetic composite member formed by molding a magnetic material.

[Problem to be solved by the invention]

However, the measures described in the above publication did not take into account the influence of the use of magnetic powder on the static magnetic field and the actual manufacturing. In other words, when considering each magnetic powder, it has a magnetic pole opposite to that of the whole. This is generally called a demagnetizing field, but
When powdered, the influence of the demagnetizing field increases, resulting in a decrease in magnetic permeability, etc., and magnetic field uniformity cannot be achieved unless the thickness of the magnetic pole pieces 5a, 5b is increased.
Since a certain height of the air gap is required, permanent magnets 1a, l
It becomes necessary to increase the gap between the spaces between b and b, which is not a good idea in terms of increased cost, increased size of the device, and installation space. Furthermore, if one attempts to manufacture the magnetic pole pieces 5a and 5b using only the magnetic composite material described above, the strength cannot be maintained unless the ratio of the magnetic powder in the composite is lowered, making it impossible to manufacture the pole pieces. There is a manufacturing method that applies pressure to achieve this, but a 50,000 ton class press machine is required to manufacture the magnetic pole pieces 5a, 5b as one piece.

However, such a press machine does not currently exist and cannot be manufactured.

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic field generator for an MRI apparatus that can solve these problems and reduce the generation of eddy currents in magnetic pole pieces caused by driving gradient magnetic field coils.

[Means to solve the problem]

In order to achieve the above object, a magnetic field generating device for an MR device according to the present invention includes a pair of permanent magnets facing each other forming a gap into which a subject can enter, a pair of permanent magnets that support these permanent magnets, and a magnetic field generator that supports these permanent magnets and A magnetic field generating device for a magnetic resonance imaging apparatus, comprising: a yoke coupled to a yoke; and magnetic pole pieces each made of a disc-shaped magnetic member fixed to opposing surfaces of the pair of permanent magnets on the gap side, and generating a magnetic field within the gap. In,
The magnetic pole piece has a laminated structure of multiple layers in its thickness direction,
The magnetic pole piece member facing the permanent magnet side is composed of a one-piece thick magnetic disc, and the magnetic pole piece member facing the air gap side is made of a composite of magnetic acicular material and electrically insulating material and molded by applying pressure. It is composed of molded magnetic members, which are integrally bonded and fixed.

Note that the magnetic pole piece member facing the air gap side may have an annular protrusion on its peripheral edge.

Furthermore, it is effective to form the above-mentioned molded magnetic member so that the thickness direction of the member is perpendicular to the direction of pressure applied for molding.

Furthermore, the molded magnetic member contains at least 60% or more of the magnetic acicular material per volume, and has a specific resistance of 0.01 Ωcm or more in a direction perpendicular to the plate thickness.

The magnetic pole piece has a laminated structure consisting of three members, the magnetic pole piece member facing the permanent magnet side is made of an integral thick magnetic disk, and the magnetic pole piece member facing the air gap side is made of a molded magnetic member. The annular protrusion portion on the peripheral edge is made of a donut-shaped thick magnetic plate, which may be integrally bonded and fixed.

The pole piece member made of the molded magnetic member facing the air gap side may be divided into a plurality of blocks.

[For production]

The magnetic field generating device of the MHI device configured in this manner has a magnetic pole piece made of a disc-shaped magnetic member having a multi-layer laminated structure in the thickness direction, and the magnetic pole piece member facing the permanent magnet side is a one-piece thick plate. It is composed of a magnetic disc, and the magnetic pole piece member facing the air gap side is composed of a molded magnetic member made of a composite of magnetic needle-like material and electrically insulating material and molded by applying pressure, and these are integrally bonded and fixed. As a result, the resistance value of the magnetic pole piece member facing the air gap side is increased. Therefore, eddy currents having the same pattern as the gradient magnetic field coil are not formed in the magnetic pole piece member facing the air gap side, and the eddy current generated in the magnetic pole piece can be reduced. From this, a gradient magnetic field with good rise and fall characteristics can be obtained.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a plan view showing an embodiment of a magnetic pole piece, which is a characteristic part of the magnetic field generating device for an MRI apparatus according to the present invention, and FIG. 2 is a cross-sectional view at the center of the magnetic field generating device according to the present invention. The overall configuration is similar to the conventional example shown in FIGS. 15 and 16.

That is, in FIGS. 15 and 16, a pair of permanent magnets 1a and lb are arranged vertically to face each other, forming a gap A between them into which a subject can enter. These permanent magnets 1
a and lb are for generating a static magnetic field in the air gap A, and are formed into, for example, a disk shape, and are supported by upper and lower yokes 2a and 2b, respectively. These yokes 2a, 2b have the permanent magnets 1a, lb and magnetic pole pieces 5a, 5b (described later) facing each other at a predetermined interval and form a magnetic path.For example, the depth is larger than the width. It is formed into a short rectangle.

The upper and lower yokes 2a, 2b are composed of a plurality of vertical yokes 3.3.
It is supported by... These vertical yokes 3,3. . . . are the upper and lower yoke 2a.

2b are arranged facing each other at a predetermined interval and close the magnetic path caused by the permanent magnets 1a and lb, and are made of a material that allows magnetic flux to easily pass through the inside, for example, the upper and lower yokes 2a
, 2. A total of four poles are installed, one at each of the four corners of the space B, and each of them forms a return circuit for the magnetic flux passing through the measurement space B set within the air gap A. Note that the pair of upper and lower permanent magnets 1a.

1b is magnetized to N and S poles in the directions shown in FIG. 16, so that magnetic flux passes through the measurement space B from top to bottom as shown by the arrow.

Magnetic pole pieces 5a and 5b are magnetically and mechanically fixed to opposing surfaces of the pair of permanent magnets 1a and lb on the air gap A side, respectively. These pole pieces 5a.

5b is set in a predetermined region within the above-mentioned air gap A, and is intended to improve the uniformity of the static magnetic field in the measurement space B, which is a spherical space with a diameter of 30 to 50 a*, and in which the test part of the subject enters. , for example, is made of a disc-shaped magnetic member, and has an annular protrusion 4 provided on its peripheral edge. Further, a gradient magnetic field coil group 6a is provided near each of the magnetic pole pieces 5a, 5b.

6b is provided. These gradient magnetic field coil group 6a
, 6b are for generating gradient magnetic fields in order to obtain position information in the measurement space B set in the air gap A, and for example, three pairs of coils corresponding to the three directions x, y, and z are used. It is of the Anderson type or Golay type as described in the specification and drawings of Japanese Patent Application No. 62-221924.

By passing a pulse current through these gradient magnetic field coil groups 6a and 6b from a gradient magnetic field coil drive power source (not shown), a magnetic field with a gradient is generated in the measurement space B in a short time. .

Eddy currents are generated in the magnetic pole pieces 5a and 5b due to the 6B field generated at the rising and falling times when a pulse current is passed through the gradient magnetic field coil groups 6a and 6b. Since the magnetic pole piece configuration as described above is adopted, the generation of this eddy current can be reduced.

That is, in one aspect of the present invention, the above-mentioned magnetic pole piece 5a.

5b has a two-layer laminated structure in its thickness direction, as shown in FIG. It is formed by integrally bonding and fixing a second pole piece member 9 facing the air gap A side shown in FIG.

The first magnetic pole piece member 8 is permanently attached to one side thereof when a plurality of divided second magnetic pole piece members 9 are integrally bonded and fixed to form the magnetic pole piece 5a or 5b. Since an attractive force of approximately 1 kg/d is applied from the magnet 1a or 1b, the deformation caused by this attractive force can be suppressed within a desired range to assemble the magnetic pole pieces 5a, 5b, and the static magnetic field caused by the permanent magnets 1a, 1b can be assembled. It is made of a one-piece thick magnetic disk with a predetermined diameter, and its material is, for example, electromagnetic soft iron with high magnetic permeability.

The second magnetic pole piece member 9 faces the air gap A side shown in FIG. In combination with 8, this improves the uniformity of the static magnetic field in the measurement space B, and is made by molding a composite of magnetic needle-like material and electrically insulating material by applying pressure, and has annular protrusions 4 around the periphery. It is made of magnetic material. As shown in FIG. 3, this molded magnetic member is made of magnetic needle-like materials 12, 12. It is made of a magnetic composite member containing 90% by volume occupancy of ..., 10% of an electrically insulating material such as epoxy resin 13,13°, etc., and hardened by applying pressure to this composite. To manufacture this magnetic composite member, for example, hot press or press is used.

CI P (Cold l5ostatic Pres
molding using the HI P (Hot
However, in the HIP method, epoxy resin cannot be used as the electrically insulating material 13 due to its heat resistance, so an inorganic material such as silicon powder may be used. In each of the above methods, pressure is applied to mold the composite material, and the pressure needs to be about 5 tons/01. Generally, even large press machines have a capacity of 8000 tons.
Since it is a bracelet, the size that can be manufactured is 8000÷5
= 1600cd, 40C for a rectangular block
It becomes 1l×40al.

Further, its thickness is limited to 30c11. here.

Since the general size of the magnetic pole pieces 5a and 5b is approximately 10 mm in diameter, the second magnetic pole piece member 9 must be constructed by dividing into a plurality of blocks of appropriate size. In the embodiment shown in the figure, for example, 30 am
Slice a block of X 30 ex
3 as one block 9', these blocks 9'
, 9' . . . to form the second magnetic pole piece member 9.
is configured.

In addition, each block 9.9' of the second magnetic pole piece member 9
. . . As shown in Fig. 3, the pressure P applied to compression mold each block 9' is manufactured with the direction perpendicular to the direction of P as the direction of the plate thickness a of the member. ing. If you do it like this.

Magnetic acicular material 12.12. The pressure P is in the longitudinal direction.

They are arranged in a direction perpendicular to the direction of P and parallel to the direction of the plate thickness of the block 9'. For this reason, the above block 9
The direction of the plate thickness Ω of ' has high magnetic permeability.

The specific resistance of the magnetic composite member (block 9' shown in FIG. 3) manufactured as described above in a direction perpendicular to the plate thickness is, for example, 0.05 Ωl.

Further, the magnetic acicular material 12 is not limited to one made of pure iron, and may be any other acicular material as long as it is made of a material with high magnetic permeability and large saturation magnetization.

The volume occupancy of the magnetic acicular material 12 and the electrically insulating material 13 is not limited to the above-mentioned 90% and 10%.
The content of the electrically insulating material 13 is at most 40% or less, and may be selected as appropriate within this range.

Further, the magnetic acicular material 12 is not limited to a columnar shape as shown in FIG. 3, but may be formed, for example, in a hexagonal columnar shape or other columnar shape as shown in FIG. Good too.

A second magnetic pole piece member 9 is attached to one side of the first magnetic pole piece member 8, as shown in FIG.

Set screw 10, 10. ...and are integrally bonded and fixed with adhesive. At this time, the set screw 1 is attached to the surface of the first magnetic pole piece member 8 that is in contact with the permanent magnets 1a and lb.
A recess is provided to prevent the head of the 0 from protruding.

Further, the length of the tip of the set screw 1o is determined so that it does not protrude from the surface of the second magnetic pole piece member 9. moreover.

Each block 9.9' of the second pole piece member 9 shown in FIG.
...The gap d is made as small as possible and 1
For example, it is set to 1- or less.

In this state, the overall dimensions of the magnetic pole pieces 5a and 5b are as shown in FIG.
For example, 41,000 m, D
, 4900m5 D, valve 750m, t480 t tz
It is said to be about 20 liters long and 20 liters long.

Then, the thickness of the central part of the magnetic pole pieces 5a and 5b (thickness t of the first magnetic pole piece member 8, thickness tx of the tenth and second magnetic pole piece member 9)
To achieve uniformity of static magnetic field.

Approximately 40 mm is necessary to absorb variations in the magnetic properties of the permanent magnets 1a and 1b used and to use them without saturating the internal magnetic flux density. In this example, the thickness of about 40 layers is t1'':2o■9t1420■tt:
Although tl=1:1, the ratio is not limited to this and may be selected as appropriate.

However, the thickness t2 of the second magnetic pole piece member 9 is determined based on the relationship between its magnetic permeability, specific resistance, and plate thickness, and considering practical magnetic permeability and specific resistance.

It is necessary to have five or more cabinets. In addition, when considering actual assembly, in order to suppress deformation due to the attractive force received from the permanent magnets 1a and lb within a desired range, it is necessary that tU≧20 m.

The magnetic field generating device configured in this way includes magnetic pole pieces 5a, 5
b has a two-layer laminated structure in the thickness direction, and the second pole piece member 9 facing the gap A side is formed by combining a magnetic needle-like material 12 and an electrically insulating material 13 and molding the composite by applying pressure. Since it is made of a magnetic member, the resistance value of this second pole piece member 9 is large, and eddy currents with the same pattern as those of the gradient magnetic field coil groups 6a, 6b are not formed, and the magnetic pole pieces 5a, 5 are
The eddy current generated at b becomes smaller.

In the embodiments shown in FIGS. 1 and 2, the annular protrusion 4 is formed on the peripheral edge of the second pole piece member 9 facing the air gap, but in the present invention, the annular protrusion 4 is 4 is not essential and may be omitted. However, the annular protrusion 4 is an effective means for improving the uniformity of the magnetic field formed between the air gap A, and it can meet the required uniformity of the magnetic field. It is desirable to appropriately select whether or not the annular protrusion 4 is to be formed, its shape, dimensions, etc., taking into consideration the degree of power and the size of the magnetic circuit.

5 and 6 show a magnetic pole piece 5a according to the present invention.

FIG. 5b is a plan view and a central cross-sectional view showing a second embodiment of 5b. In this embodiment, the magnetic pole pieces 5a and 5b have a laminated structure consisting of three members, and the permanent magnets 1a and 1a shown in FIG.
The first pole piece member 8 facing the lb side is made of a one-piece thick magnetic disc, and the second pole piece member 9 facing the air gap A side shown in FIG. 16 is made of the aforementioned magnetic needle material. 12 and an electrically insulating material 13, and the third magnetic pole piece member 11 forming the annular protrusion 4 on the peripheral edge is made of the same material as the first magnetic pole piece member 8. These three members 8, 9 are made of donut-shaped thick magnetic plates.
．． 11 are integrally connected and fixed with setscrews 10 and 10' and adhesive. In this case, the second pole piece member 9 made of an expensive molded magnetic member can be manufactured to be thinner at the annular protrusion 4 than in the example shown in FIG. 2, and thus can be manufactured at a lower cost.

7 and 8 show a magnetic pole piece 5a according to the present invention.

5b is a plan view and a central cross-sectional view showing a third embodiment of the present invention. In this embodiment, like the second embodiment, the magnetic pole pieces 5a and 5b have a laminated structure consisting of three members, and most of the annular protrusion 4 is a third magnetic pole piece made of a doughnut-shaped thick magnetic plate. It consists of a member 11', which is made slightly thick so that its bottom surface directly contacts the first magnetic pole piece member 8, and the second magnetic pole piece member 9 is fitted inside the inner circumference thereof. It was formed. In this case, the second pole piece member 9 made of an expensive molded magnetic member can be manufactured with a smaller outer diameter than the example shown in FIG. 2, and can be manufactured at a lower cost.

FIG. 9 and FIG. 10 show magnetic pole pieces 5a, 5b according to the present invention.
FIG. 7 is a plan view, a diagram, and a central cross-sectional view showing a fourth embodiment of the present invention. In this embodiment, like the second embodiment, the magnetic pole pieces 5a and 5b have a laminated structure consisting of three members, and the third magnetic pole piece member 11' is made of a donut-shaped thick magnetic plate. It is made slightly thicker so that its bottom surface directly contacts the first magnetic pole piece member 8, and the second magnetic pole piece member 9 is fitted inside a part of the tapered inner circumference thereof. . In this case, the second pole piece member 9 made of an expensive molded magnetic member can be manufactured with a smaller outer diameter than the example shown in FIG. 6, and can be manufactured at a lower cost than any of the above examples. can.

FIGS. 11 to 14 are explanatory diagrams showing modified examples of the second pole piece member 9. FIG. 1 to 8, the second pole piece member 9 has one block 9', for example 30 a
Formed into a block of a x 30 am x 30 cs and sliced in the direction perpendicular to the plate thickness, 30a IIX 30
A square with a size of These blocks 9', 9', etc. may be formed into a quarter circular shape, and four of these blocks 9', 9', etc. may be connected and fixed in parallel in a circular shape. A modification corresponding to the example shown in the figure is shown, and FIGS. 13 and 14 show modifications corresponding to the example shown in FIGS. 9 and 10. The shape of the two blocks 9' is not limited to a quarter circle, but may be a fifth circle, a tenth circle, a large circle, or the like.

〔Effect of the invention〕

Since the present invention is constructed as described above, the magnetic pole pieces 5a and 5b made of disc-shaped magnetic members have a multi-layer laminated structure in the thickness direction, and the first magnetic pole piece faces the permanent magnets 1a and la side. The member 8 is composed of a one-piece thick magnetic disc, and the second magnetic pole piece member 9 facing the air gap A side is made of a composite of a magnetic acicular material 12 and an electrically insulating material 13 and is molded by applying pressure. Since the second magnetic pole piece member 9 facing the air gap A side is made of molded magnetic members and is formed by bonding and fixing them into one body, the resistance value of the second pole piece member 9 facing the above-mentioned air gap A side is increased. Therefore, eddy currents having the same pattern as those of the gradient magnetic field coils are not formed in the second pole piece member 9 facing the air gap A side, and the eddy currents generated in the pole pieces 5a and 5b can be reduced. From this,
A gradient magnetic field with good rise and fall characteristics can be obtained, and both MR images can be made clear. Furthermore, the load on the gradient magnetic field coil drive power source can be reduced. Furthermore, in the laminated structure of multiple layers, although the magnetic permeability of the second pole piece member 9 on the air gap A side is slightly lower, the molded magnetic member forming the second pole piece member 9 is By forming the perpendicular direction as the thickness direction of the member, the longitudinal direction of the magnetic needle-like material 12 can be arranged parallel to the thickness direction of the magnetic needle-like material 12, and high magnetic permeability can be achieved in the thickness direction. The first magnetic pole piece member 8 on the side of the permanent magnets 1a and 1a is made of a thick magnetic disk, which is a high permeability member.
As a whole, the required accuracy can be achieved without deteriorating the uniformity of the static magnetic field. In addition, in the case where the second magnetic pole piece member 9 is divided into a plurality of blocks, the magnetic pole piece 5a
, 5b can be easily manufactured.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment of a magnetic pole piece which is a characteristic part of a magnetic field generating device for an MHI device according to the present invention. FIG. 2 is a central cross-sectional view thereof, FIGS. 3 and 4 are explanatory diagrams showing the material structure of the molded magnetic member constituting the second magnetic piece member, and FIGS. 5 and 6 are magnetic poles according to the present invention. A plan view showing the second embodiment of the piece and its central cross-sectional view, FIGS. 7 and 8
9 and 10 are plan views and central cross-sectional views showing a third embodiment of the magnetic pole piece according to the present invention, and FIGS. FIGS. 11 and 12 are plan views showing modified examples of the second pole piece member, and FIGS. 13 and 14 are plan views showing modified examples of the second pole piece member. FIG. 15 is a perspective view showing the overall structure of the magnetic field generating device according to the present invention and the conventional example, and FIG. 16 is a partial view showing the overall structure of the magnetic field generating device. The cross-sectional front view, FIG. 17, is an explanatory diagram showing the concept of a gradient magnetic field coil in the Z direction. la, lb-permanent magnet, 2a, 2b, 3--yoke,
4... Annular projection, 5a, 5b... Magnetic pole piece, 6a
, 6b... Gradient magnetic field coil group, 8... First magnetic pole piece member, 9... Second magnetic pole piece member, 9'... Block of second magnetic pole piece member, 10.10' ...Set screw, 11.11'11'...Third magnetic pole piece member, 12...Magnetic acicular material, 13...Electrical insulating material, A...Gap, B... measurement space.

Claims (6)

[Claims] (1) A pair of permanent magnets facing each other forming a gap into which the subject can enter, a yoke that supports and magnetically couples these permanent magnets, and a yoke that faces the pair of permanent magnets on the gap side. In a magnetic field generating device for a magnetic resonance imaging apparatus, the magnetic field generating device includes magnetic pole pieces each made of a disc-shaped magnetic member fixed to each surface and generates a magnetic field in the air gap, wherein the magnetic pole piece has a laminated structure of multiple layers in the thickness direction. The magnetic pole piece member facing the permanent magnet side is composed of a one-piece thick magnetic disc, and the magnetic pole piece member facing the air gap side is formed by combining a magnetic needle-like material and an electrically insulating material and applying pressure. 1. A magnetic field generating device for a magnetic resonance imaging apparatus, characterized in that the magnetic field generating device is composed of molded magnetic members, and is formed by integrally bonding and fixing these members. (2) The magnetic field generating device for a magnetic resonance imaging apparatus according to claim 1, wherein the magnetic pole piece member facing the air gap side has an annular projection on its peripheral edge. (3) The magnetic field generating device for a magnetic resonance imaging apparatus according to claim 1 or 2, wherein the molded magnetic member is formed so that the thickness direction of the member is perpendicular to the direction of pressure applied for molding. (4) The molded magnetic member contains at least 60% or more of the magnetic acicular material in terms of volume occupancy, and has a specific resistance of 0.01 Ωcm or more in a direction perpendicular to the plate thickness.
4. A magnetic field generating device for a magnetic resonance imaging apparatus according to 2 or 3. (5) The magnetic pole piece has a laminated structure consisting of three members,
The magnetic pole piece member facing the permanent magnet side is made of a one-piece thick plate magnetic disc, the magnetic pole piece member facing the air gap side is made of a molded magnetic member, and the annular protrusion portion on the periphery is made of a donut-shaped thick plate magnetic disc. 5. A magnetic field generating device for a magnetic resonance imaging apparatus according to claim 2, wherein the magnetic field generating device is made of plates, which are integrally bonded and fixed. (6) The magnetic field generating device for a magnetic resonance imaging apparatus according to claim 1, 2 or 5, wherein the magnetic pole piece member made of a molded magnetic member facing the air gap side is divided into a plurality of blocks.