JP3445303B2 - Magnetic field generator for MRI - Google Patents

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 and the like, and has a pair of magnetic pole pieces opposed to each other with a gap formed therebetween, and particularly in a peripheral portion thereof. At least the surface layer of the protrusions provided is made of laminated silicon steel plate or soft ferrite to reduce the eddy current and remanence phenomenon in the pole piece by the gradient magnetic field coil without impairing the magnetic field uniformity in the air gap. M that enabled imaging
The present invention relates to a magnetic field generator for RI.

[0002]

2. Description of the Related Art A magnetic resonance tomography apparatus for medical use (hereinafter referred to as MR
(I) is a device capable of inserting a part or all of a subject into a void of a magnetic field generator that forms a strong magnetic field, obtaining a tomographic image of an object, and drawing out even the nature of the tissue. Is. In the above-mentioned magnetic field generator for MRI, the space needs to be wide enough to allow a part or the whole of the subject to be inserted, and in order to obtain a clear tomographic image, normally, the imaging field of view in the space is , 0.02-2.0
It is required to form a stable and strong uniform magnetic field with T and an accuracy of 1 × 10 ⁻⁴ or less.

As a magnetic field generator used for MRI, magnetic pole pieces are fixed to one end of each of a pair of permanent magnet constructions using an R-Fe-B system magnet as a magnetic field generation source to face each other, and the other end is connected. A configuration is known in which a static magnetic field is generated in a gap between magnetic pole pieces by connecting with iron. In order to improve the homogeneity of the magnetic field distribution in the air gap, the pole pieces are provided with annular protrusions on the periphery, and are usually a plate-shaped bulk (integrated) made by cutting out magnetic materials such as electromagnetic soft iron and pure iron. (Japanese Patent Laid-Open No. 60-88407).

[0004]

In MRI, a gradient magnetic field coil (GC), which is usually composed of three coil groups corresponding to three directions of X, Y, and Z, is used to obtain position information in the air gap. A gradient magnetic field in a desired direction which changes in a trapezoidal wave shape with time can be generated by applying a pulse current to the gradient magnetic field coil which is arranged in the vicinity of the one piece.

However, the arrangement of the magnetic pole pieces has problems of eddy current and residual magnetism in relation to the gradient magnetic field coil. That is, the magnetic field generator for MRI gives position information to a signal of nuclear magnetic resonance by applying a gradient magnetic field to a normally uniform magnetic field. It is necessary to apply a large number of pulsed gradient magnetic fields to obtain one image. This gradient magnetic field is generated by applying a pulse current to the gradient magnetic field coil, but if a conductor with high conductivity such as iron is present in the vicinity of the gradient magnetic field coil, an eddy current is generated and the gradient magnetic field is hard to rise sharply.

For example, in the FSE method (fast spin echo method) for high-speed imaging, a pulse sequence for switching a gradient magnetic field at a high speed is used, so that a magnetic field generator that generates very little eddy current is required.

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, and reduces the generation of eddy currents without decreasing the magnetic field homogeneity in the air gap and reduces the gradient magnetic field in a short time. For the purpose of providing a magnetic pole piece having a structure capable of obtaining a clear image by increasing the strength of the magnetic pole, and further for providing a magnetic pole piece having a structure that is easy to process and manufacture and has excellent assembling workability. There is.

[0008]

SUMMARY OF THE INVENTION The present invention is variously studied for the purpose of an MRI magnetic field generator having a structure capable of reducing the generation of eddy currents and reducing the residual magnetism phenomenon and enabling high-speed imaging such as the FSE method. As a result, by forming the air gaps to form the surface layer portions of the pair of pole pieces facing each other with soft ferrite, and further forming the surface layer of the protrusions provided on the peripheral portions of the pole pieces with soft ferrite or laminated silicon steel sheet. 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 that the processing and manufacturing are easy.

That is, the present invention has a magnetic field generation source and a pair of magnetic pole pieces which are arranged so as to face each other with a gap therebetween and each of which has an annular protrusion formed on the peripheral edge of the facing surface. To generate a predetermined uniform magnetic field and cut the gradient magnetic field at high speed.
In the magnetic field generator for MRI using the pulse sequence, the inner peripheral surface of the annular protrusion and the opposing surfaces of the annular protrusions are made of laminated silicon steel plate or soft ferrite eddy current reducing material. The magnetic field generator for MRI is characterized in that it is covered with a layer. Further, the present invention is the magnetic field generating apparatus for MRI according to the above-mentioned structure, wherein each of the magnetic pole pieces has a bottom surface in the recess of the annular protrusion covered with a layer of an eddy current reducing material. . According to the present invention, the peripheral protrusion of the pole piece and the surface layer in the vicinity thereof are formed of an eddy current reducing material such as a laminated silicon steel plate or soft ferrite, a central protrusion is provided in the center, or a peripheral protrusion other than the peripheral protrusion is formed. It is characterized in that the surface layer portion including the central protrusion is formed of soft ferrite,
It is possible to adopt a configuration in which the main body of the magnetic pole pieces including the peripheral protrusions is composed of a magnetic material base such as a soft iron material, and a laminated silicon steel plate or soft ferrite is laminated thereon, that is, a structure of covering with an eddy current reducing material layer.

In the present invention, in the magnetic circuit, two yoke plates are arranged so as to face each other with four columnar yokes, and the pole pieces are attached to the facing surfaces of the yoke plates. In the case of a tubular yoke, various configurations can be adopted, such as arranging a pair of permanent magnet constituents inside each other and arranging a plurality of auxiliary permanent magnet constituents on the inner peripheral surface where the pair of permanent magnet constituents are not arranged. Although a soft magnetic material such as iron is used, the effect of reducing the eddy current is increased by using a yoke made of a laminated body of iron, a silicon steel plate or the like.

In the case of a tubular yoke, the above-mentioned pair of permanent magnet constituents are the main magnetic field generating sources that contribute to the formation of the magnetic field in the air gap, and normally the air gap opposing surfaces are arranged in the direction orthogonal to the magnetic field direction in the air gap. Is forming. Due to the influence of the gradient magnetic field coils arranged along the inner peripheral surfaces of the pair of permanent magnet constituents, an eddy current is generated inside the permanent magnet constituents in the direction parallel to the air gap facing surface. In order to reduce the generation of this eddy current, for example, it is important that the permanent magnet structure is electrically insulated in the direction orthogonal to the magnetization direction of each permanent magnet block. The resistance increases and eddy current can be suppressed. The magnetizing direction of the permanent magnet structure is variously selected according to the structure of the magnetic circuit and the location on the yoke, but any direction can be used as long as it is arranged so that a uniform static magnetic field can be obtained in the required air gap. Good. 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. Particularly, R is a light rare earth element rich in resources centered on Nd and Pr, 30 with B and Fe as main components
Fe-, which has an extremely high energy product over MGOe
By using the BR permanent magnet, the size can be significantly reduced.

In the present invention, the laminated silicon steel sheet of the eddy current reducing material is laminated in the laminating direction in the facing direction of the magnetic pole pieces, or in the direction orthogonal to the facing direction of the magnetic pole pieces. Any of the above may be used, and if necessary, the stacking direction may be changed to form a plurality of layers. Alternatively, the pole piece member may have various shapes, and the members may be combined into a desired shape. Furthermore, the directionality of the easy axis of magnetization of the silicon steel sheet used is arbitrary,
When it is made of non-oriented silicon steel sheet (JISC2552 etc.), it shows a remarkable effect in reducing the residual magnetism phenomenon. The silicon steel sheet may have any thickness, but since a silicon steel sheet that is generally easily available is as thin as about 0.35 mm, it is possible to obtain a plurality of pieces having a predetermined size once as a configuration that has extremely good stacking and assembling workability. A plurality of rectangular non-oriented silicon steel sheets were laminated in a predetermined number in a direction orthogonal to the facing direction of the pole pieces to prepare a plurality of block pole piece members, and the plurality of block pole piece members were directly magnetized. Stick to the structure or
We propose a structure such as fixing onto a magnet structure via a plate-shaped magnetic material base.

In the present invention, the material of the soft eddy current reducing material is Mn-Zn ferrite powder, Ni-
Made of various soft ferrite materials such as Zn ferrite powder, a large block made of soft ferrite processed into the required shape, or a small block assembled into the required shape with an adhesive can be used. For the purpose of improving once, an annular protrusion with various cross-sectional shapes is provided on the periphery of the air gap side, a circular convex part or a protrusion with a trapezoidal cross-section is provided in the center part, and the magnetic field is A magnetic field adjusting piece made of a magnetic material or a magnet may be attached for the purpose of adjusting the uniformity. 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, then sintered, and further for improving the density,
HP, HIP (Hot Isostatic Pres
Sing method and the like may be used together, and the obtained small blocks may be bonded with an adhesive such as an epoxy resin to assemble them into a desired shape. Among the soft ferrite materials, for example, Mn-Zn based soft ferrite is
It 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 for eddy currents and a low coercive force (several A / s).
It has the characteristics of m). In the present invention, it is preferable that the soft ferrite has Bs of 0.4 T or more in order to efficiently cause the magnetic flux generated from the magnet constituent to 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 preferably 0.4T or more, preferably 0.5T or more, and more preferably 0.5T.
It is 5T or more. Further, if 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. To reduce the eddy current, the specific resistance ρ is 10 ⁻⁵ Ω · m or more, and more preferably 1
0 ^-3 Ω · m or more.

[0014]

The structure and operation of the magnetic field generator for MRI according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective explanatory view showing an embodiment of a magnetic field generator for MRI in which the present invention is applied to a magnetic circuit using plate-like and columnar yokes, where A is the whole and B is the pole piece. 2 is a perspective explanatory view showing an embodiment of a magnetic field generator for MRI in which the present invention is applied to a magnetic circuit using a cylindrical yoke, where A is the whole and B is the pole piece. In the magnetic field generator shown in FIG. 1, two yoke plates 1 and 1 are arranged so as to face each other with four columnar yokes 2, and a disk-shaped permanent magnet structure 3, is provided at the center of the facing surface of each yoke plate 1, 1. 3 is provided, and magnetic pole pieces 4 and 4 are further provided on the permanent magnet constructing bodies 3 and 3. The magnetic pole piece 4 has a disc-shaped base material 5 such as iron as shown in FIG. 1B. The protrusions 6 are arranged on the peripheral edge of the.
By forming the projection 6 and the surface layer portion 7 in the vicinity thereof with a silicon steel plate or soft ferrite, the eddy current generated near the peripheral projection portion is reduced even when the gradient magnetic field coil is energized with a GC pulse. Further, the central portion of the pole piece 4 is provided with the central protrusion portion 8 made of soft ferrite, which is mostly a flat protrusion, so that the magnetic field of the air gap 9 is made uniform, and the effect of reducing the eddy current by the gradient magnetic field coil is It has an effect of reducing the residual magnetism generated by the GC pulse.

The magnetic field generator shown in FIG. 2 has a structure in which a permanent magnet structure is arranged on the inner peripheral surface of a hexagonal tubular yoke 10, and a soft magnetic material such as iron is used for the yoke. , Especially iron,
By using a yoke made of a laminated body such as a silicon steel plate, the effect of reducing the eddy current is increased. A pair of permanent magnet constructions 1 which are main magnetic field generation sources contributing to the formation of a magnetic field in the air gap.
1a and 11d usually have a magnetization direction in the magnetic field direction Y in the void 12 and form void facing surfaces. The magnetization directions of the other permanent magnet components 11b, 11c, 11e, 11f are selected variously according to the configuration of the magnetic circuit and the location on the yoke, etc., so that a uniform static magnetic field can be obtained in the required air gap. Any orientation can be used as long as it is located at. The permanent magnet structure may have any shape and structure, and in particular, reference numerals 11b, 11c, 11e, 11 in the figure in which pole pieces are not installed.
By configuring the permanent magnet structure of f by a plurality of electrically insulated permanent magnet blocks, the eddy current generated in the permanent magnet structure can be reduced. The gradient magnetic field coil is installed along the inner circumferential surface of the permanent magnet constituting body which is arranged in combination in a polygonal cylindrical shape.

The pole piece 13 is installed on the surfaces of the pair of permanent magnet components 11a and 11d that form the main magnetic field, facing each other,
It is fixed to the yoke 10 by penetrating the magnet with a bolt or the like. The shape of the pole piece is selected variously according to the configuration of the magnetic circuit and the location of the permanent magnet structure, etc., but is selected so that a uniform static magnetic field can be obtained in the required air gap within the above-mentioned shape of the pole piece. do it. As the material of the base material 14 of the pole piece 13, a soft magnetic material such as iron can be appropriately selected.
Protrusions (shims) 15 are arranged at both ends of the opening facing the air gap. By forming the projection 15 and the surface layer portion 16 in the vicinity thereof with a silicon steel plate or soft ferrite, the eddy current generated near the peripheral projection portion is reduced even when the gradient magnetic field coil is energized with a GC pulse. Further, a central protrusion 17 is also provided in the center of the pole piece 13, and the magnetic field in the air gap is made uniform by forming the central protrusion 17 with soft ferrite, and the effect of reducing the eddy current by the gradient magnetic field coil is obtained. At the same time, there is an effect that the residual magnetism generated by the GC pulse is reduced.

In the present invention, the silicon steel sheet used for the protrusion of the pole piece has a high saturation magnetic flux density Bs, makes it easy to achieve a uniform magnetic field in the air gap, and is electrically insulated with a small coercive force Hc and hysteresis loss. Since a plurality of thin plates are laminated, the eddy current generated in the vicinity of the peripheral protrusion is reduced even when the gradient magnetic field coil is energized with the GC pulse, and the residual magnetism can be reduced. Further, the magnetic field of the air gap is made uniform by the soft ferrite layer on the surface of the pole piece, and the effect of reducing the eddy current by the gradient magnetic field coil and the effect of reducing the residual magnetism caused by the GC pulse are obtained, and a clear tomographic image can be obtained. .

[0018]

EXAMPLE A magnetic field generator similar to that of FIG.
Using Nd-Fe-B based permanent magnet with ax35MGOe, the protrusions are arranged on the periphery of the base material of pure iron, the protrusions and the surface layer portion in the vicinity thereof are formed of silicon steel plate, and the central portion is mostly flat. A central protrusion made of soft ferrite was provided. In the structure of the present invention, the distance between the pole piece air gap facing surfaces was set to 500 mm, a pulse current was applied to the gradient magnetic field coil to apply a large number of pulsed gradient magnetic fields, and the rising characteristics and image characteristics of the gradient magnetic field were measured. The non-oriented silicon steel sheet has Hc = 40 A / m and Bs = 1.7.
T, ρ = 45 × 10 ⁻⁸ Ω · m. Soft ferrite is Mn-Zn type ferrite, Hc = 6.0 A / m, Bs
= 0.58T and ρ = 0.2Ω · m. Pure iron is Hc
= 80 A / m, Bs = 2.0 T, ρ = 1 × 10 ⁻⁷ Ω · m
Is.

For comparison, magnetic pole pieces of the same size and shape, all of which were made of pure iron, were arranged in the magnetic field generator of Example 1, and the rising characteristics of the gradient magnetic field and the image characteristics were similarly measured. The rising characteristic of the gradient magnetic field of the magnetic field generator according to the present invention was significantly improved as compared with the comparative example indicated by the one-dot chain line, as shown in practice in FIG. 3, and an excellent image was obtained.

[0020]

In the magnetic field generator for MRI according to the present invention, the main body of the magnetic pole piece is composed of a magnetic material base such as a soft iron material, and the surface layer of the peripheral projection of the magnetic pole piece is laminated on the laminated silicon steel plate or soft ferrite. Or the surface layer portion other than the vicinity of the peripheral protrusion is formed of soft ferrite. The silicon steel sheet has a high saturation magnetic flux density Bs, and it is easy to achieve a uniform magnetic field in the air gap and a coercive force Hc.
Also, since a plurality of electrically insulated thin plates having a small hysteresis loss are laminated, the eddy current generated in the vicinity of the peripheral protrusion is reduced even when a GC pulse is applied to the gradient magnetic field coil, and the remaining It is also possible to reduce the magnetic phenomenon. Further, the magnetic pole pieces can be easily processed and manufactured and the assembling workability is excellent.

[Brief description of drawings]

FIG. 1 is a perspective explanatory view showing an embodiment of a magnetic field generator for MRI in which the present invention is applied to a magnetic circuit using plate-like and columnar yokes, where A is the whole and B is a pole piece.

FIG. 2 is a perspective explanatory view showing an embodiment of a magnetic field generator for MRI in which the present invention is applied to a magnetic circuit using a tubular yoke, where A is the whole and B is the pole piece.

FIG. 3 is a graph showing a relationship between a time showing a rising rate of a gradient magnetic field and a GC magnetic field strength.

[Explanation of symbols]

1 Yoke Iron Plate 2 Column Yoke 3, 3, 11a, 11b, 11c, 11d, 11e, 11f
Permanent magnet structure 4,13 Pole piece 9,12 Void 10 Cylindrical yoke 5,14 Base material 6,15 Protrusion 7,16 Surface layer portion 8,17 Central protrusion portion

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masaaki Aoki               2-15-17 Egawa, Shimamoto-cho, Mishima-gun, Osaka Prefecture               Sumitomo Special Metals Co., Ltd. Yamazaki Works (72) Inventor Hiroyuki Takeuchi               1-1-14 Kanda, Uchida, Chiyoda-ku, Tokyo               Hitachi Medical Co., Ltd. (72) Inventor Hirotaka Takeshima               1-1-14 Kanda, Uchida, Chiyoda-ku, Tokyo               Hitachi Medical Co., Ltd.                (56) References JP-A-4-138131 (JP, A)                 JP-A-4-138132 (JP, A)                 JP-A-4-23411 (JP, A)

Claims (2)

(57) [Claims] 1. A magnetic field generation source and a pair of magnetic pole pieces, which are arranged so as to face each other with a gap therebetween, and each of which has an annular projection formed on a peripheral edge portion of the facing surface. A pulse sequence that generates a magnetic field and switches the gradient magnetic field at high speed
In each of the magnetic pole pieces, the inner peripheral surface of the annular protrusion and the opposing surfaces of the annular protrusions are covered with a layer of an eddy current reducing material. Magnetic field generator for MRI. 2. The magnetic field generator for MRI according to claim 1, wherein each of the magnetic pole pieces has a bottom surface inside the concave portion of the annular projection portion covered with a layer of an eddy current reducing material.