JPH05182821A - Magnetic field generator for mri - Google Patents

Abstract

PURPOSE:To provide a pole piece composed of constitution, in which the generation of eddy currents is lowered and a graded magnetic field can be increased to specified intensity in a short time without reducing magnetic field uniformity in the air gap of a magnetic field generator for an MRI and a retentivity phenomenon is diminished and a distinct image can be obtained with high sensitivity. CONSTITUTION:A pole piece 10 consists of a laminated silicon steel plate layer 11 formed by members for a plurality of block-shaped pole pieces laminated by using non-oriented silicon steel plates, a rectangular sectional soft-iron made magnetic material ring 12, which is provided around the peripheral section of the laminated silicon steel plate 11, and soft ferrite layers 13, in which members for a large number of block-shaped pole pieces constituted by compression-molding soft ferrite powder in a rectangular plate shape are combined in a discoidal shape with adhesives and laid on the top face of the laminated silicon steel plate 11. Accordingly, the magnetic field uniformity of an air gap is easily attained, eddy currents generated in a magnetic pole are lowered even when GC pulses are applied to a graded magnetic field coil, and a retentivity phenomenon is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a magnetic field generator used in a magnetic resonance tomography apparatus for medical use (hereinafter referred to as MRI) and the like. It is composed of two layers of a plurality of block-shaped magnetic pole piece members, which are formed by laminating silicon steel sheets in a required direction and are integrated, and a soft ferrite layered thereon, without impairing the magnetic field uniformity in the air gap. The present invention relates to a magnetic field generator for MRI in which an eddy current in a pole piece and a residual magnetism phenomenon are reduced by a gradient magnetic field coil.

[0002]

2. Description of the Related Art In MRI, a part or all of a subject is inserted into the space of a magnetic field generator that forms a strong magnetic field, and a tomographic image of the subject is obtained to delineate the nature of the tissue. It is a device that can

In the above-mentioned magnetic field generator for MRI, the space needs to be large enough to allow a part or all of the subject to be inserted, and in order to obtain a clear tomographic image, the field of view of the imaging in the space is usually used. Inside, it is required to form a stable and strong uniform magnetic field having an accuracy of 0.02 to 2.0 T and 1 × 10 ⁻⁴ or less.

FIG. 6 shows a magnetic field generator used for MRI.
As shown in FIG. 3, magnetic pole pieces 2 and 2 are fixed to one end of each of a pair of permanent magnet constructing bodies 1 using an R-Fe-B system magnet as a magnetic field generation source so as to face each other, and a yoke 3 is provided at the other end. It is known that a static magnetic field is generated in the space 4 between the magnetic pole pieces 2 and 2 by connecting with each other.

In order to improve the homogeneity of the magnetic field distribution in the air gap 4, the magnetic pole pieces 2 and 2 have annular projections 5 on their peripheral portions.
Is usually provided and is constituted by a plate-shaped bulk (integral body) obtained by cutting out a magnetic material such as electromagnetic soft iron or pure iron (JP-A-60-88407).

The gradient magnetic field coil 6 arranged near each magnetic pole piece 2 and 2 is usually composed of three coil groups corresponding to three directions of X, Y and Z in order to obtain position information in the air gap 4. However, they are simply shown in the drawings. By applying a pulse current to the gradient magnetic field coil 6, it is possible to generate a gradient magnetic field in a desired direction that changes in a trapezoidal wave shape with time.

[0007]

When a pulse current is passed through the gradient magnetic field coil 6, the magnetic pole piece 2 is made of a plate-like bulk as described above, so that the magnetic field changes rapidly when the current rises and falls. Then, an eddy current is generated in the magnetic pole pieces 2 and 2. Since this eddy current forms a magnetic field in the direction opposite to the magnetic field formed by the gradient magnetic field coil 6, it takes a long time for the gradient magnetic field to reach a predetermined strength.

As means for solving the above-mentioned problems, a flat plate-like laminated body in which soft magnetic thin films such as a permalloy steel plate and an amorphous steel plate are laminated as a magnetic pole piece so that the laminated surface is perpendicular to the magnetic pole surface is formed. A magnetic field generator using a structure in which two layers are arranged and integrated so that the directions are different from each other by about 90 degrees (Japanese Patent Laid-Open No. 61-203605), a magnetic field generator using magnetic powder having a high specific resistance (Japanese Patent Laid-Open No. Sho 61-203605). 63-2590
7) is proposed.

However, even in the above-described structure in which the eddy current is reduced, the magnetic pole piece is magnetized by the magnetic field formed by the gradient magnetic field coil (GC), and the GC pulse is stopped by the magnetic hysteresis phenomenon (residual magnetization phenomenon). However, there is a problem that the homogeneity of the magnetic field in the air gap is disturbed by the residual magnetism phenomenon. In addition, the applicant has arranged a magnetic field generating device (Japanese Patent Application No. 2-1295) to arrange soft ferrite on the surface of soft iron to reduce the residual magnetism phenomenon.
No. 03) was proposed. However, the GC pulse intensity tends to increase as the function of the device increases, and the soft ferrite portion needs to be thickened in order to suppress a significant increase in the residual magnetism due to the increase in the intensity. Since soft ferrite has a small saturation magnetization, leakage magnetic flux and magnetomotive force loss increase, and there is a problem that the device becomes large.

The present invention is proposed in view of the above-mentioned current situation in the magnetic pole piece of the magnetic field generator for MRI. The gradient magnetic field is reduced in a short time by reducing the generation of eddy current without lowering the magnetic field homogeneity in the air gap. For the purpose of providing a magnetic pole piece having a structure capable of increasing to a predetermined strength, and for providing a magnetic pole piece having a structure capable of obtaining a highly sensitive and clear image by reducing a residual magnetism phenomenon, and It is an object of the present invention to provide a magnetic pole piece that is easy to process and manufacture, has high mechanical strength, and is excellent in assembling workability.

[0011]

SUMMARY OF THE INVENTION The present invention has variously studied to achieve the above-mentioned object in a magnetic field generator for MRI. As a result, a pair of magnetic pole pieces facing each other with a gap formed are
By forming two layers of soft ferrite laminated on a laminated silicon steel sheet layer formed by a plurality of block-shaped pole piece members that are formed by laminating a plurality of silicon steel sheets in a desired direction, respectively, It has been found that the eddy current due to the gradient magnetic field coil and the residual magnetism phenomenon can be reduced without lowering the magnetic field strength and the magnetic field homogeneity, and further, the processing and manufacturing are easy.

According to the present invention, in a magnetic field generator for MRI, which has a pair of magnetic pole pieces facing each other with a gap formed therebetween, the magnetic pole piece is made of soft ferrite or laminated silicon steel plate from the gap side. It is a magnetic field generator for MRI characterized by comprising two layers.

Further, according to the present invention, there is provided an MR having a pair of magnetic pole pieces facing each other with a gap formed therebetween, and generating a magnetic field in the gap.
In the magnetic field generator for I, the magnetic pole piece is composed of three layers of a soft ferrite, a laminated silicon steel plate, and a magnetic material base from the air gap side, which is a magnetic field generator for MRI.

Further, according to the present invention, in the above-mentioned structure, the magnetic field homogeneity is further improved by disposing the annular protrusion made of a magnetic material ring provided with one or more diametrical slits on the side of the pole piece facing the air gap. To do.

The magnetic field generator for MRI which is the object of the present invention is limited to the embodiments described later as long as it has a pair of magnetic pole pieces facing each other and forms a magnetic field in the gap. Without any configuration.
That is, the magnet constructing body serving as a magnetic field generation source is not limited to the permanent magnet, and an electromagnet or the like can be adopted, and the magnetic pole pieces may not be arranged directly on the magnet constructing body. Furthermore, the shape of the yoke for forming a magnetic path that magnetically connects these magnet components and a pair of magnetic pole pieces to generate a magnetic field in the air gap, etc. are required. It may be appropriately selected according to various characteristics such as uniformity.

Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used as the permanent magnets of the magnet structure used in such a magnetic circuit. In particular, R is Nd or Pr.
Resource-rich light rare earths such as B, Fe
By using an Fe-B-R based permanent magnet that has an extremely high energy product of 30 MGOe or more containing as a main component, the size can be significantly reduced.

In the present invention, the laminated silicon steel sheets may be laminated either in the direction in which the magnetic pole pieces face each other or in the direction orthogonal to the direction in which the magnetic pole pieces face each other. If necessary, the stacking direction may be changed to form a plurality of layers, or a pole piece member having various shapes may be formed and combined to obtain a desired shape. Further, the orientation of the easy-magnetization axis direction of the silicon steel sheet used is arbitrary, but when it is made of a non-oriented silicon steel sheet (JIS C2552 etc.), it shows a remarkable effect in reducing the residual magnetism phenomenon.

Although the silicon steel sheet may have any thickness,
Since a silicon steel plate which is generally easily available is as thin as about 0.35 mm, the entire flat plate-shaped laminate constituting the pole pieces as shown in the conventional example (Japanese Patent Laid-Open No. 61-203605) has the silicon steel plate in one direction. If the structure is simply laminated, the work of laminating and integrating becomes extremely complicated. Therefore, the present inventor has determined that stacking and assembling workability is extremely good by once stacking a predetermined number of rectangular non-oriented silicon steel plates having a predetermined size in a direction orthogonal to the facing direction of the pole pieces. A plurality of block-shaped magnetic pole piece members are prepared, and these plural block-shaped magnetic pole piece members are directly fixed to the magnet structure, or fixed to the magnet structure via a plate-shaped magnetic material base. Propose the configuration of.

When a laminated silicon steel plate is used as a block-shaped member for magnetic pole pieces, the laminating direction is laminated in the above specific direction, and the laminating direction is selected at random by using small pieces of various shapes. It can also be assembled into a predetermined block shape. More specifically, circular silicon steel plates are laminated in the thickness direction to form a disk shape, which is divided into 8 or 16 parts in the vertical and horizontal directions, or 8 or 16 parts in the diametrical direction. It is made by stacking silicon steel plates in a direction orthogonal to the facing direction of the pole pieces to form a disk shape, dividing this vertically or horizontally or in the diameter direction, and further dividing the disk into 8 or 16 parts in the diameter direction. , Various block-shaped pole piece members can be used, such as each block assembled by randomly selecting the stacking direction using small pieces of various shapes.

In the present invention, the material of soft ferrite is made of various soft ferrite materials such as Mn-Zn ferrite powder and Ni-Zn ferrite powder, and a large block made of soft ferrite is processed into a required shape or a small size. It is possible to use a block assembled with an adhesive in the required shape, etc., and further, in order to improve the uniformity of the magnetic field, annular projections of various cross-sections are provided in the periphery of the gap side, circular convex parts or A protrusion having a trapezoidal cross section may be provided, or a magnetic field adjusting piece made of a magnetic material or a magnet may be attached to a required position of the magnetic pole piece for the purpose of adjusting the uniformity of the magnetic field.

In order to manufacture the above-mentioned small blocks of soft ferrite, for example, Mn-Zn ferrite powder or the like is compression-molded into a required shape and then sintered, and further HP and HIP (Hot Isostatic Pr) are used to improve the density.
It is also possible to use a means such as an essing) method together, and the obtained small blocks may be bonded with an adhesive such as an epoxy resin to assemble them into a required shape.

Among the soft ferrite materials, for example, Mn
-Zn-based soft ferrite has a high magnetic permeability and a high saturation magnetic flux density Bs required as a magnetic field equalizing means, and has a sufficiently high specific resistance as a countermeasure against eddy currents and a low retention that can prevent a residual magnetism phenomenon. It has a characteristic of magnetic force (several A / m).

In the present invention, it is preferable that the soft ferrite has Bs of 0.4 T or more in order to allow the magnetic flux generated from the magnet construct to efficiently act on the air gap. That is, the amount of magnetic flux passing through the soft ferrite is determined by its Bs, and if its value is small, it will inevitably saturate and the magnetic field strength will decrease. To prevent this, it is necessary to enlarge the magnet, This leads to an increase in the size of the device. Therefore, Bs is desirably 0.4 T or more, preferably 0.5 T or more, and more preferably 0.55 T or more.

When Hc of the soft ferrite is too large, a residual magnetism phenomenon occurs. Therefore, Hc is preferably 50 A / m or less, preferably 20 A / m or less, and more preferably 10 A / m or less. Further, in order to reduce the eddy current, it is desirable that the specific resistance ρ is 10 ⁻⁵ Ω · m or more, and more preferably 10 ⁻³ Ω · m or more.

According to the present invention, the magnetic pole pieces are two layers of soft ferrite and laminated silicon steel sheet from the air gap side, or three layers of a magnetic material base such as soft iron material laid on the magnet construction side, or a soft iron bulk portion on the peripheral protrusion. The magnetic field strength required for the pole piece is optimized by optimizing the thickness ratio of the soft ferrite layer and the laminated silicon steel sheet layer in the pole piece thickness, or the thickness ratio including the magnetic material base thickness. And the effect of preventing eddy currents and residual magnetism are maximized, and the mechanical strength of the pole pieces can be appropriately selected. Note that the thinner the base thickness of the soft iron material, the better if the adverse effect of the GC pulse can be reduced only by selecting the thickness ratio between the soft ferrite layer and the laminated silicon steel sheet layer.

Alternatively, it is possible that only the central portion of the air gap facing surface of the pole piece is made of soft ferrite and the annular projection provided at the peripheral portion is made of a magnetic material ring, which is made of soft iron, low carbon steel or the like. The magnetic material base or the ring-shaped support frame can be mounted on the periphery of the magnetic material base, or can be directly mounted on the upper surface of the block-shaped magnetic pole piece member. In any configuration, it is desirable to divide the annular projection by providing one or more slits for the purpose of reducing the influence of eddy current. Furthermore, the magnetic material base or between the ring-shaped support frame and the annular projection, and the magnetic material base. Alternatively, it is desirable to electrically insulate between the ring-shaped support frame and the block-shaped pole piece member.

Disclosure Based on the Drawings FIGS. 1A and 1B are a cross sectional view and a top view showing an embodiment of a magnetic pole piece of a magnetic field generator according to the present invention. 2A and 2B are perspective views showing an embodiment of a block-shaped magnetic pole piece member constituting the magnetic pole piece of the present invention. The pole piece 10 shown in FIG.
A laminated silicon steel sheet layer 11 formed by a plurality of block-shaped magnetic pole piece members laminated by using non-oriented silicon steel sheets, and a soft iron steel sheet having a rectangular cross section provided around the laminated silicon steel sheet 11 Magnetic material ring 12 and a large number of block-shaped magnetic pole piece members formed by compression molding soft ferrite powder into a rectangular plate shape are combined into a disk shape with an adhesive, and a soft ferrite layer laid on the upper surface of a laminated silicon steel plate 11. 13 and 13. Each block-shaped pole piece member is usually fixed to the laminated silicon steel sheet layer 11 with a synthetic epoxy resin adhesive.

The soft ferrite layer 13 is assembled into a disk shape as described above. However, since the circular convex portion 14 having a required diameter is formed in the central portion, the compression molded soft ferrite block-shaped pole piece member is formed. Since those having different thicknesses are used, the circular convex portion can improve the magnetic field uniformity. Further, a soft iron core portion 15 is provided in the center of the soft ferrite layer 13, which constitutes a base for mounting the gradient magnetic field coil.

A magnetic material ring 12 made of soft iron and having a rectangular cross section is provided around the peripheral portion of the laminated silicon steel sheet layer 11 to form a magnetic pole piece 10.
Of the insulating material between the laminated silicon steel plate layer 11 and the outer peripheral portion side of the laminated silicon steel plate layer 11 to form a ring-shaped protrusion for concentrating the magnetic flux in a required gap and improving the uniformity. The magnetic material ring 12 is divided into eight in the circumferential direction in order to provide the slit 16 in the diametrical direction and reduce the influence of the eddy current.

The block-shaped pole piece member 11 shown in FIG. 2A.
A shows the case where a grain-oriented silicon steel sheet is used, and a plurality of grain-oriented silicon steel sheets that are oriented in the same direction in advance are used.
Small blocks 11a11b laminated and integrated in the thickness direction
(The arrow in the figure indicates the easy magnetization axis direction), and thereafter, in order to improve the magnetic field homogeneity, the easy magnetization axis directions are different from each other by 90 ° and laminated so as to have a predetermined thickness. Can be configured.

The block-shaped pole piece member 11 shown in FIG. 2B.
B shows the case where a non-oriented silicon steel plate is used. Due to the non-directionality, a plurality of silicon steel plates are simply laminated in the thickness direction and integrated to form a block having a predetermined thickness. The pole piece member 11B can be configured.

[0032]

The magnetic pole piece having the above structure according to the present invention is MR
When used in a magnetic field generator for I, a silicon steel sheet has a high saturation magnetic flux density Bs, it is easy to achieve a uniform magnetic field in the air gap, and a plurality of electrically insulated thin plates with a small coercive force Hc and hysteresis loss are used. Due to the stacked structure, even when the GC pulse is applied to the gradient magnetic field coil, the eddy current generated in the magnetic pole is reduced, and the residual magnetism phenomenon can be reduced. Further, the magnetic field in the air gap is made uniform by the soft ferrite layer, and there is an effect of reducing the eddy current by the gradient magnetic field coil and an effect of reducing the residual magnetism caused by the GC pulse. Therefore, by appropriately selecting the thickness ratio of the laminated silicon steel sheet layer and the soft ferrite layer, a synergistic effect of both is effective in reducing the eddy current and the residual magnetism phenomenon.

Further, when the magnetic material base has a three-layer structure in which the magnetic material base is added to the above two layers, the magnetic material base can secure the strength required for the magnetic pole piece, and the synergistic effect of the above two layers can be obtained. It does not significantly alienate, has good manufacturability, and is an extremely useful industrial structure.

In addition to the selection of the thickness ratio of the laminated silicon steel sheet layer and the soft ferrite layer, the eddy current and the residual magnetism phenomenon are caused by the laminated state of the laminated silicon steel sheet layer, the individual block shape and the assembled state (divided state). There is a difference in the reduction effect, and it is necessary to select it appropriately according to the required characteristics and applications.

[0035]

EXAMPLE 1 A magnetic field generator having the same structure as that shown in FIG.
A laminated silicon steel sheet layer in which a disk is vertically and horizontally divided by a block-shaped pole piece member configured as shown in FIG. 2B using a Nd-Fe-B system permanent magnet having Oe and a non-oriented silicon steel sheet having the following properties. In addition, a soft ferrite layer is laminated on top of it with a block-shaped pole piece member of the same shape to form a pole piece. The slits of the annular protrusion made of soft iron are provided at four locations. Was set to 500 mm.
At this time, the pole pieces were manufactured by varying the thicknesses of the soft ferrite layer and the laminated silicon steel sheet layer while keeping the thickness of the pole pieces constant at 50 mm, and the residual magnetic fields generated by the respective GC pulses were measured. The measurement results are shown in FIG. 3, where the laminated silicon steel sheet layer thickness is shown as t. Therefore, the case of the plot at the right end of the figure is the case where all of the pole pieces are laminated silicon steel sheets (Comparative Example 1). In addition, □ indicates a pulse width of 100 ms,
A mark indicates that the pulse width is 10 ms, and a mark indicates that the pulse width is 1 ms.

Example 2 In Example 1, the laminated magnetic steel sheet layer was replaced with a structure in which strip-shaped silicon steel sheets were laminated in the direction orthogonal to the pole piece opposing direction, and the residual magnetic field generated by the GC pulse was measured and measured. The results are shown in Fig. 4.

Comparative Example 2 In the magnetic field generator of Example 1, a pair of magnetic pole pieces each having a soft ferrite portion of the same quality as that of Example 1 provided on a base portion made of soft iron having the following properties and an annular protrusion made of soft iron was provided. The facing distance was set to 500 mm. At this time, the thickness of the magnetic pole pieces was kept constant, the thicknesses of the soft iron base and the soft ferrite portion were variously changed, and the magnetic pole pieces were produced, and the residual magnetic fields generated by the respective GC pulses were measured. The measurement results are shown in FIG. 5, where the soft iron base thickness is shown as t. Therefore, the plot at the right end of the figure is the case where all the pole pieces are soft iron (conventional example).

As a result, Examples 1 and 2, Comparative Examples 1 and 2,
Also in the conventional example, the measured value is in a measurement space within a radius of 200 mm from the center of the air gap, magnetic field homogeneity: 30 ppm, magnetic field strength: 0.
I got 2T. The eddy currents generated by the gradient magnetic field coils were reduced to 1/3 or less in the case of Examples 1 and 2 according to the present invention as compared with the conventional example. The residual magnetism generated by the GC pulse (1 to 10 ms) is 1 / maximum in Comparative Examples 1 and 2 as compared with the conventional example.
It was reduced to 3 or less, and in the cases of Examples 1 and 2 according to the present invention, the maximum was reduced to 1/4 or less of Comparative Examples 1 and 2.
Further, the residual magnetic field due to the difference in pulse width is
In the case of, the same reduction effect is obtained for Comparative Examples 1 and 2 regardless of the pulse width.

Furthermore, in Examples 1 and 2, a soft iron base was added to form a three-layer structure, and the pole piece thickness was 50 m.
Magnetic pole pieces were produced by varying the thickness of the soft ferrite layer and the laminated silicon steel sheet layer while keeping the value constant at m, and the residual magnetic field generated by each GC pulse was measured. It was confirmed that the residual magnetic field reducing effect has the same tendency as in Examples 1 and 2.

In the structures of Examples 1 and 2 and Comparative Examples 1 and 2, the magnetic material base had an outer diameter of 1050 mm.
The thickness of various pole pieces was 50 mm. The thickness of the non-oriented silicon steel sheet was 0.35 mm. Non-oriented silicon steel sheet has Hc = 40 A / m, Bs = 1.7 T, ρ = 4
It is 5 × 10 ⁻⁸ Ω · m. Soft ferrite is Mn-Z
n-type ferrite, Hc = 6.0 A / m, Bs = 0.58
T, ρ = 0.2Ω · m. Pure iron has Hc = 80A /
m, Bs = 2.0T, ρ = 1 × 10 ⁻⁷ Ω · m.

[0041]

The magnetic pole piece laminated with the soft ferrite layer and the laminated silicon steel sheet layer from the void side according to the present invention is subjected to MRI.
When used in a magnetic field generator for use in a magnetic field, it is easy to achieve a uniform magnetic field in the air gap, and even if a GC pulse is applied to the gradient magnetic field coil, the eddy current generated in the magnetic poles is reduced and the residual magnetism phenomenon can be reduced. Furthermore, in addition to selecting the thickness ratio of the laminated silicon steel sheet layer and the soft ferrite layer, the eddy current and residual magnetism phenomenon are reduced by selecting the laminated state of the laminated silicon steel sheet layer and the individual block shape and assembly state (divided state). Can be planned.

[Brief description of drawings]

1A and 1B are a cross-sectional view and a top view showing an embodiment of a magnetic pole piece of a magnetic field generator according to the present invention.

2A and 2B are perspective views showing an embodiment of a block-shaped magnetic pole piece member constituting the magnetic pole piece of the present invention.

FIG. 3 is a graph showing the relationship between the laminated silicon steel sheet layer thickness of the magnetic pole piece of the present invention (Example 1) and the residual magnetic field.

FIG. 4 is a graph showing the relationship between the laminated silicon steel sheet layer thickness of the magnetic pole piece of the present invention (Example 2) and the residual magnetic field.

FIG. 5 is a graph showing the relationship between the magnetic material base thickness of the pole piece of Comparative Example 2 and the residual magnetic field.

6A and 6B are a longitudinal sectional view and a lateral sectional view of a conventional magnetic field generator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Permanent magnet constituent 2,10 Magnetic pole piece 3 Yoke 4 Air gap 5 Annular protrusion 6 Gradient magnetic field coil 11 Laminated silicon steel sheet layer 11A, 11B Small block 12 Magnetic material ring 13 Soft ferrite layer 14 Circular convex part 15 Core part 16 slit

Claims (2)

[Claims] 1. A magnetic field generator for MRI, comprising a pair of magnetic pole pieces facing each other with a gap formed therebetween, wherein the magnetic pole pieces are two layers of soft ferrite and laminated silicon steel plate from the gap side. A magnetic field generator for MRI, comprising: 2. An MRI magnetic field generator having a pair of magnetic pole pieces facing each other with a gap formed, wherein the magnetic pole pieces are soft ferrite, laminated silicon steel sheet, and magnetic material from the gap side. A magnetic field generator for MRI, which comprises three layers of a base.