JP3016544B2 - Permanent magnet magnetic circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic resonance tomography apparatus for medical diagnosis (hereinafter referred to as an MRI apparatus).
This is a permanent magnet magnetic circuit for a nuclear magnetic resonance tomography apparatus useful for generating a bias magnetic field in which an eddy current is less generated due to application of a gradient magnetic field and a gradient magnetic flux easily passes.

[0002]

2. Description of the Related Art As a magnet used in a magnetic resonance tomography apparatus, a magnet facing type permanent magnet magnetic circuit is well known. For example, WHOldendorf; WO 84/00611 (PCT / US83 / 0
1175), Jitsuhei 2-44483, Jitsuhei 2-44484, Jitsuhei 2-
No. 44485, Jitsuho 2-44486, each of the gazettes of video information, volume 15 p.3
79 (1983) and Pathophysiology, Vol. 4, p. 91 (1985).
In the magnetic circuit, sufficient magnetic field uniformity cannot be obtained at the center of the air gap only by making the magnet face the air gap. For this reason, the intensity of the magnetic field generated in the gap slightly decreases, but as shown in FIG. 6, an iron plate called a magnetic shunt is attached to the magnet surface facing the gap to improve the magnetic field uniformity. Various measures have been taken to improve the uniformity of the magnetic field in the shape of the magnetic shunt plate. For example, providing an annular protrusion on the outer periphery of the magnetic shunt plate improves the magnetic field in the center of the gap (magnetic field uniformity is evaluated by the magnetic field distribution in a sphere space virtually provided in the center of the gap), and the pole and the equator Alleviates the inhomogeneity of the magnetic field in the part. Further, an annular step smaller than the annular protrusion is also provided on the surface of the magnetic shunt plate, and as a magnetic field uniformity within a uniform sphere (for example, a sphere space having a diameter of 40 cm), for example, a magnetic field of 50 ppm or less for a generated magnetic field of 2000 G Uniformity is obtained. In this way, the magnetic shunt plate is an indispensable member for improving the magnetic field uniformity in the permanent magnet opposed magnetic circuit.

On the other hand, in the MRI apparatus, a gradient magnetic field is generated in the magnetic circuit space by energizing a solenoid coil in order to obtain positional information. The gradient magnetic field is superimposed on the bias magnetic field generated by the permanent magnet, and changes the nuclear magnetic resonance (NMR) frequency of the hydrogen nucleus depending on the location. M
The role of the gradient magnetic field in the RI apparatus is not limited to this, but it is essentially necessary to apply a rectangular pulse magnetic field to the time axis. In a magnet facing type magnetic circuit, a gradient coil is generally installed on the surface of a magnetic shunt as shown in FIG. When a pulse rectangular wave gradient magnetic field is applied to the gradient coil, an eddy current flows through the magnetic shunt plate in a direction to cancel the gradient magnetic field because the gradient coil and the magnetic shunt plate are close to each other. The eddy current acts to slow down the rise of the pulse gradient magnetic field waveform (rectangular wave) and reduce the tail of the fall (see FIG. 5). This is very harmful in obtaining an image in an MRI apparatus, and it is desirable to minimize the generation of eddy current in the magnetic shunt plate when a gradient magnetic field is applied.

In order to reduce the eddy current, various types of magnetic shunt plates having a hybrid structure have been devised, and a combination of a base iron plate and a soft magnetic layer is generally used. For example, JP-A-63-241905, JP-A-1-304709, JP-A-2-603,
JP-A-2-184002, JP-A-2-218343, JP-A-3-203203
No., JP-A-4-82536, JP-A-4-138131, JP-A-5-18
Each gazette such as 2821 is disclosed. The characteristics required for the material of the surface of the magnetic shunt plate facing the gradient coil are that the magnetic permeability is high, the coercive force is low, the soft magnetism is high, and the electric resistance is high, and this is structurally retained. Therefore, a bulk iron plate is used as a base. Among the characteristics required for the material of the surface of the magnetic shunt plate, the former is necessary to obtain a uniform magnetic field, and the latter is necessary to suppress eddy current. Soft magnetic materials satisfying such requirements include soft magnetic ferrites (MnZn ferrite, NiZn ferrite, etc.), amorphous ribbons, silicon steel sheets, permalloy,
There is pure iron. In the latter four, the electric resistance is not high in the bulk state, so that it is necessary to laminate thin plate shapes. Further, it is desirable to perform insulation treatment between the thin plates in order to increase the electric resistance as much as possible. In addition, resin iron obtained by compacting iron powder with resin can also be used because it has high electric resistance and soft magnetic characteristics. Among these soft magnetic materials, ferrite having very high electric resistance in bulk state and high magnetic permeability is preferable in terms of magnetic properties. However, ferrite is inherently brittle because it is ceramic and is placed in a bias magnetic field, so that it is always subjected to a force such that the longitudinal direction of the ferrite is oriented in the direction of the magnetic field. For this reason, it is difficult to fix the magnet to the magnetic shunt plate base, and there is a risk that cracks and cracks may occur. In addition, since it is a baked product manufactured by the powder sintering method, it is difficult to manufacture a large block in terms of dimensional accuracy and processing. Pure iron laminate and resin iron are not as good as other materials in terms of soft magnetic properties. Further, although permalloy has very excellent soft magnetic properties, it is very expensive and is difficult to use because its magnetic properties are easily deteriorated by stress or the like.

On the other hand, silicon steel sheets have good soft magnetic properties, are easy to process large thin sheets by punching and the like, and are inexpensive because they are used in large quantities in motors and the like. Are suitable. There are two types of silicon steel sheets: non-oriented and oriented, and oriented silicon steel sheets have better magnetic properties but are difficult to use. The gradient magnetic field is X,
Since the voltage is applied in three directions of Y and Z, the orientation of the directional silicon steel sheet may be arranged in the directional silicon steel sheet so as to satisfy all the directions in which the magnetic flux flows in the three directions of the X, Y and Z gradient magnetic fields. Have difficulty. It is also proposed to stack oriented silicon steel sheets so that the easy axis is in two directions (see JP-A-4-138131).
However, a non-oriented silicon steel sheet is generally laminated instead of a laminated structure that satisfies the flow of gradient magnetic fluxes in the X, Y, and Z directions.

[0006]

Accordingly, as a magnetic shunt plate of a permanent magnet magnetic circuit opposed to a magnet, a magnetic shunt plate structure which generates less eddy current, allows a gradient magnetic flux to flow easily, and is easy to manufacture as compared with the conventional magnetic shunt plate. Is desired. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has an object to improve the image quality of an MRI apparatus by providing a magnetic shunt plate structure in which gradient magnetic fluxes of X, Y, and Z easily flow, and in which eddy current is less generated.

[0007]

Means for Solving the Problems The present inventors have developed a magnetic shunt plate having a hybrid structure of a silicon steel plate and an iron plate, which is a magnet facing type permanent magnet magnetic circuit, in which a gradient magnetic flux easily flows and an eddy current is less generated, The present invention was completed by establishing various conditions,
The gist is that in a permanent magnet facing magnetic circuit in which a pair of magnets are opposed to each other and a magnetic shunt plate is provided on the surface of the magnet gap, and these are joined by a yoke to form a closed magnetic circuit, the magnetic shunt plate and the iron plate are combined. A laminated steel sheet composed of a silicon steel sheet, wherein the silicon steel sheet is disposed on the surface of the magnetic shunt plate facing the gap, and the silicon steel sheet has a radially oriented silicon steel sheet and a non-oriented silicon steel sheet whose orientation directions are radially oriented. Or a permanent magnet magnetic circuit characterized in that the radially oriented silicon steel sheet and a circumferentially oriented silicon steel sheet whose orientation direction is oriented in the circumferential direction are laminated so as to be mixed in the thickness direction. More specifically, the laminated silicon steel sheet has a thickness sufficient to pass a gradient magnetic flux, and
Each silicon steel sheet is divided into two or more parts in a radial direction or more, and each silicon steel sheet is in a permanent magnet magnetic circuit that is electrically insulated from an adjacent silicon steel sheet.

Hereinafter, the present invention will be described in detail. As described above, the lamination of the grain-oriented silicon steel sheet in the radial direction is excellent in the soft magnetic property with respect to the Z gradient magnetic field,
There is a problem that it is difficult to pass a magnetic flux to the X and Y gradient magnetic fields. On the other hand, the lamination of non-oriented silicon steel sheets is X, Y,
Although it can cope with a Z gradient magnetic field, its soft magnetic properties are inferior to those of a grain oriented silicon steel sheet. Therefore, in order to improve the hybrid magnetic shunt plate composed of an iron plate and a silicon steel plate, a directional silicon steel plate having a radial orientation with respect to a Z gradient magnetic field is adopted, and a non-directional silicon steel plate or a circumferentially oriented silicon steel plate with respect to an X, Y gradient magnetic field is employed. Oriented silicon steel sheet was adopted. The arrangement of the silicon steel sheet corresponding to the Z gradient magnetic field is divided in the radial direction as shown in FIG. 2 (a), and each silicon steel sheet segment is formed into a disc shape by combining those oriented in the pseudo radial direction, and is easily magnetized. The direction was radial orientation.
The Z-gradient magnetic flux generated by the gradient coil passes through the gap-side bottom surface of the magnetic shunt plate and returns to the original state through the annular projection on the outer periphery. For this reason, since the Z gradient magnetic flux at the bottom surface of the magnetic shunt plate flows in the radial direction toward the annular protrusion, a radially oriented silicon steel plate is employed. As a result, magnetic flux easily flows in the radial direction.
The generation of eddy current due to the application of the gradient magnetic field and the change in residual magnetization due to the magnetic shunt plate are reduced. On the other hand, the division of the non-oriented silicon steel sheet or the circumferentially oriented silicon steel sheet corresponding to the X, Y gradient magnetic field is the same as that of the radial silicon steel sheet and is made into a disk shape. For example, the X gradient coil has a shape as shown in FIG. 4A, and a gradient magnetic flux flows in the X direction as shown in FIG. 4B. X
A directional silicon steel sheet is arranged in the circumferential direction in which the gradient magnetic flux flows, or a non-oriented silicon steel sheet is used. In the latter case, there is no need to consider the orientation. The directional silicon steel sheet having the circumferential orientation and the non-oriented silicon steel sheet have the same ease of passing the gradient magnetic flux. This is because the circumferential orientation does not always coincide with the optimal magnetic guiding direction of the X gradient magnetic flux. Therefore, either may be adopted for the X and Y gradient magnetic fields.

[0009] Directional and non-oriented silicon steel sheets may be laminated alternately one by one, alternately by a plurality of the same number of sheets, or alternately by a different number of sheets, but in the thickness direction of the magnetic shunt plate. It is preferable that they are mixed, and at least 4 or more, preferably 6 or more are good. Since the gradient magnetic flux flows through a place where the magnetic flux on the surface of the magnetic shunt plate easily flows (a place with high magnetic permeability), for example, a set of alternating ones in which the upper half is directional and the lower half is non-directional (or vice versa). Lamination is not preferred. This is because in such a stack, the X and Y gradient magnetic fluxes may flow through the annular protrusion, and the linearity of the gradient magnetic field is disturbed. The thickness of the silicon steel sheet used in this case may be between 0.1 mm and 1 mm.

In order to suppress the eddy current, the silicon steel sheet needs to be divided into a radial direction and a circumferential direction. Four or more divisions are necessary in the circumferential direction, and preferably six or more divisions (see FIG. 3A). In the radial direction, two or more divisions are preferable in order to further reduce the eddy current. The surface of each silicon steel sheet segment in the laminating direction (thickness direction) is insulated, so that even if an eddy current flows, it is limited to the plane of the magnetic shunt plate. Also, it is desirable that the insulation treatment is performed between adjacent segments, and eddy current flowing in the inner circumferential direction of the magnetic shunt plate can be suppressed. By the surface insulation treatment and division of the silicon steel sheet, the intensity of the eddy current can be significantly reduced.

In the present invention, conventionally known techniques can be applied to the design conditions of the annular projection of the magnetic shunt plate and the gradient coil.

[0012]

EXAMPLES Hereinafter, embodiments of the present invention will be described specifically with reference to Examples, but the present invention is not limited thereto. (Example) A magnet facing magnetic circuit having a center magnetic field of 2,000 G,
A magnetic shunt having a diameter of 1,000 mm was used. The inner surface layer 30 mm of the magnetic shunt plate 60 mm thick is a silicon steel plate, and the remaining 30 mm is a base iron plate.
The surface layer silicon steel sheet was divided into eight parts in the circumferential direction and four parts in the radial direction. 5 oriented silicon steel sheets (2.5mmt) and 5 non-oriented silicon steel sheets (2.5mmt)
t) were alternately laminated in the thickness direction to form a surface layer in six sets (30 mmt). A rectangular wave-shaped gradient magnetic field (1 msec rising, pulse width 5 msec, magnetic field in the same direction to the upper and lower gradient coils) corresponding to 1 G / cm in the X direction and the Z direction is applied to the magnetic shunt plate, and the magnetic field in the uniform magnetic field space is applied. When the change was measured, the rate of change was 20 ppm at the center.

(Comparative Example) When the same magnetic field as in the example was applied to a magnetic shunt plate having a surface layer formed only of a non-oriented silicon steel sheet, the change rate was 56 ppm. From the above results, by using a grain-oriented silicon steel sheet and a non-oriented silicon steel sheet together, the eddy current was suppressed and the magnetic flux became easier to pass,
It was found that the change of the magnetic field in the uniform space became small.

[0014]

According to the present invention, it is possible to achieve both the soft magnetic characteristics of the surface of the magnetic shunt plate against the gradient magnetic field and the reduction of the eddy current, and to reduce the residual magnetization of the magnetic shunt plate due to the gradient magnetic field. The circuit can be realized, which is effective for improving the image quality of the nuclear magnetic resonance tomography apparatus.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an arrangement of a magnetic shunt plate and a gradient coil in a magnet facing magnetic circuit of the present invention.

FIG. 2A is a plan view showing an example of a magnetic shunt plate of the present invention;
(B) It is a longitudinal cross-sectional view.

FIG. 3A is a plan view showing another example of the magnetic shunt plate of the present invention;
(B) It is a longitudinal cross-sectional view.

4A is a plan view showing a gradient coil shape and FIG. 4B is a longitudinal sectional view showing a flow of a gradient magnetic flux in the magnetic shunt plate of the present invention. The coil was omitted).

FIG. 5 is an explanatory diagram showing a relationship between a gradient magnetic field and an eddy current.

FIG. 6 is a perspective view showing a conventional magnet-facing magnetic circuit.

[Description of Signs] 1 Permanent magnet 2 Magnetic shunt plate 3 Iron yoke 4 Magnetic field space 5 Annular protrusion 6 Gradient coil 7 Non-directional silicon steel sheet 8 Oriented silicon steel sheet 9 Radially oriented directional silicon steel sheet 10 Circularly oriented directional silicon steel sheet 11 X gradient coil → direction of magnetic flux orientation

Claims (2)

(57) [Claims] 1. A permanent magnet facing magnetic circuit in which a pair of magnets are opposed to each other and a magnetic shunt plate is provided on the surface of the magnet gap, and these are joined by a yoke to form a closed magnetic circuit. And a silicon steel sheet, wherein the silicon steel sheet is disposed on the surface of the magnetic shunt plate facing the gap, and the silicon steel sheet has a radially oriented silicon steel sheet and a non-oriented silicon steel sheet whose orientation directions are oriented in the radial direction. Or a permanent magnet magnetic circuit, wherein the radially oriented silicon steel sheet and the circumferentially oriented silicon steel sheet whose orientation is oriented in the circumferential direction are laminated so as to be mixed in the thickness direction. 2. The laminated silicon steel sheet according to claim 1 has a thickness sufficient to allow a gradient magnetic flux to pass through, and is divided into four or more in the circumferential direction and two or more in the radial direction. A permanent magnet magnetic circuit which is electrically insulated from a silicon steel sheet.