JP2764458B2 - Magnetic field generator for MRI - Google Patents

Description

The present invention relates to a medical magnetic resonance tomography apparatus (hereinafter referred to as MRI).
In order to improve the magnetic field generator using a permanent magnet used for such a purpose, the magnetic pole piece attached to the permanent magnet is made of soft ferrite, or a soft iron base is laid on the magnet structure side to form a gap. Magnetic Resonance (MRI) for Equalizing Magnetic Fields and Reducing Eddy Currents and Remanent Phenomena Using Gradient Coil
Field of the Invention

BACKGROUND ART Medical magnetic resonance tomography (hereinafter referred to as MRI)
This is a device capable of inserting a part or all of a subject into a gap of a magnetic field generating device that forms a strong magnetic field, obtaining a tomographic image of an object, and delineating the properties of the tissue.

In the magnetic field generator for MRI, the gap needs to be large enough to allow a part or all of the subject to be inserted.
In order to obtain a clear tomographic image, it is generally required to form a stable strong uniform magnetic field having an accuracy of 0.05 to 2.0 T and an accuracy of 1 × 10 ^-4 or less in the imaging visual field in the gap. You.

As shown in FIG. 2, a pair of permanent magnet structures (1) and (1) each using a Fe—BR system magnet are provided at one end of each of magnetic pole pieces (2) and (2) as a magnetic field generator used for MRI. ) Is fixed and opposed to each other, and the other end is connected by an iron break (3) to generate a static magnetic field in a gap (4) between the pole pieces (2) and (2).

The pole pieces (2) and (2) are provided with annular projections (5) at the periphery thereof to improve the uniformity of the magnetic field distribution in the air gap (4). It is composed of a plate-shaped bulk (integral) formed by cutting out a magnetic material (Japanese Patent Application Laid-Open No. 60-88407).

A gradient magnetic field coil (6) arranged near each pole piece (2) (2) is used to obtain position information in the air gap (4).
Usually, it is composed of three sets of coil groups corresponding to three directions of X, Y, and Z, but is simply shown in the figure.

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 which changes with time in a trapezoidal waveform.

Problems with the prior art When a pulse current is applied to the gradient magnetic field coil (6), the pole piece (2) is composed of a plate-shaped bulk as described above. Due to the change, eddy currents are generated in the pole pieces (2) (2).

Since the eddy current forms a magnetic field in the opposite direction to the magnetic field formed by the gradient magnetic field coil (6), it takes much time for the gradient magnetic field to reach a predetermined intensity.

As means for solving the above-mentioned problem, a plate-shaped laminated body obtained by laminating soft magnetic thin plates in one direction as a pole piece is arranged and integrated in two layers such that the laminating directions are different from each other by approximately 90 degrees. Magnetic field generator using a magnetic field generator (JP-A-61-20)
No. 3605), and a magnetic field generator using a magnetic powder having a high specific resistance (JP-A-63-25907) has been proposed.

However, even in the above-described configuration 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 after the GC pulse is stopped due to the magnetic hysteresis phenomenon, the residual magnetism causes the uniformity in the air gap. Once there is a problem.

Object of the Invention The present invention proposes in view of the above-mentioned current situation, processing,
The object of the present invention is to provide a magnetic field generator for MRI in which a magnetic pole piece that is easy to manufacture and has a pole piece that can reduce the eddy current caused by the gradient magnetic field coil and the residual phenomenon without reducing the uniformity of the magnetic field in the air gap. is there.

SUMMARY OF THE INVENTION The present invention relates to an MRI magnetic field generator,
As a result of various studies aimed at magnetic pole pieces that can reduce the remanence phenomenon, a soft iron base was laid on the magnet structure side, and the magnetic pole piece facing the air gap was made of soft ferrite, and the soft iron base thickness and soft By optimizing the thickness ratio of the ferrite pole pieces, it was found that the eddy current caused by the gradient coil and the remanence phenomenon could be reduced without reducing the magnetic field strength and the magnetic field uniformity, and completed the present invention. .

That is, according to the present invention, a pair of permanent magnet components facing each other with a gap formed therebetween is magnetically coupled with a yoke, and a magnetic pole piece having an annular protrusion in a peripheral portion is fixed to the gap facing surface of each permanent magnet component. An MRI magnetic field generator for generating a magnetic field in the gap, wherein a soft ferrite having a Bs (saturation magnetic flux density) of 0.4 T or more is used for the pole piece member. .

Also, the present invention, in the above configuration, a soft iron material base is laid on the magnet structure side, or a soft iron bulk portion is further provided on the peripheral protrusion, and a magnetic pole piece made of soft ferrite is arranged on the base. This is a magnetic field generator for MRI.

In the present invention, the magnetic circuit forms a gap and magnetically couples a pair of opposing permanent magnet components with a yoke, and a magnetic pole piece made of soft ferrite is provided on the gap facing surface of each permanent magnet component. Any structure may be used as long as the structure is fixed.For example, the structure of the pole piece is appropriately selected according to the magnetic characteristics and shape and size of the permanent magnet, the shape and size of the yoke, and the size of the required gap. It is desirable to do.

Ferrite magnets, alnico-based magnets, rare-earth cobalt-based magnets can be used as permanent magnets of the magnet structure used in such a magnetic circuit, but in particular, R is a resource-rich light rare earth, mainly Nd or Pr, 39 with B and Fe as main components
Fe-BR, which shows an extremely high energy product higher than MGOe
By using the system permanent magnet, the size can be significantly reduced.

In the present invention, the material of the pole piece is made of various soft ferrite materials such as Mn-Zn ferrite powder and Ni-Zn ferrite powder, and is obtained by processing a large block made of soft ferrite into a required shape or a small block. It is possible to use a shape assembled with an adhesive, etc. In addition, for the purpose of improving the uniformity of the magnetic field, annular protrusions with various cross-sections are provided around the gap side, a circular convex portion or a trapezoidal cross-section at the center Or a magnetic field adjusting piece made of a magnetic material or a magnet may be attached to a desired position of the magnetic pole piece for the purpose of adjusting the uniformity of the magnetic field.

Further, in order to manufacture the above-mentioned small block forming the pole piece, for example, after compressing Mn-Zn ferrite powder or the like into a required shape, sintering, and further improving the density, HP, HI
Means such as the P (Hot Isostatic Pressing) method may be used in combination, and the obtained small blocks may be bonded to each other with an adhesive such as an epoxy resin to assemble them into a required shape.

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

In the present invention, the reason for limiting to soft ferrite having Bs of 0.4 T or more is that if Bs is less than 0.4 T, the magnetic flux generated from the magnet structure cannot efficiently act on the air gap. That is, the amount of magnetic flux passing through the pole piece is determined by the Bs of the magnetic local piece. If the value is small, the magnetic flux piece will necessarily be saturated, and to prevent this, the pole piece needs to be thickened. This will increase the size of the piece.

Therefore, Bs is preferably 0.4T or more, preferably 0.5T
It is more preferably 0.55T or more.

Further, if the Hc of the soft ferrite is too large, a remanence phenomenon occurs. Therefore, the Hc is desirably 50 A / m or less, preferably 20 A / m or less.

Further, in order to reduce the eddy current, the specific resistance p is desirably 10 ⁻⁵ Ω · m or more.

Further, the present invention is characterized in that a soft iron material base is laid on the magnet structure side, or a soft iron bulk portion is further provided on the peripheral projection, and a magnetic pole piece made of soft ferrite is arranged on the base, By optimizing the ratio of the base thickness of the material to the thickness of the soft ferrite pole piece,
The effect of equalizing the magnetic field strength required for the pole piece and preventing eddy currents and remanent phenomena is maximized, and the mechanical strength of the pole piece made of soft ferrite can be reinforced.

The base of the soft iron material is preferably made of pure iron, low carbon steel, or the like, and a magnetic pole piece made of soft ferrite having an annular protrusion provided on the periphery of the gap side is mounted on a single base having a required thickness. By doing so, the magnetic field can be made uniform.

Alternatively, only the magnetic pole piece can be made of soft ferrite, and the annular projection formed on the periphery of the base can be made of soft iron material. At this time, one or more slits are formed in the annular projection to reduce the influence of eddy current. It is desirable to provide and divide it, and furthermore, it is desirable to electrically border between the base and the annular projection and between the base and the pole piece.

In the combination of the soft ferrite of the pole piece and the base of the soft iron material according to the present invention, various conditions such as the pole piece thickness, the gap size, the required magnetic field strength in the gap and its uniformity, the shape of each part, the mechanical There are restrictions depending on the target strength, etc.
As long as the adverse effect of the GC pulse can be reduced only by selecting the thickness of the soft ferrite, the thinner the base thickness of the soft iron material, the better.

However, the thinner the base thickness of the soft iron material, the higher the passing magnetic flux density and the greater the magnetic hysteresis of the base itself, so that the necessary eddy current and remanence phenomena are prevented, the magnetic field strength in the air gap, and its uniformity. It is necessary to optimize the thickness ratio between the soft ferrite and the base in the pole piece once or the like.

A preferable range of the soft ferrite base thickness to soft ferrite thickness ratio is base thickness / soft ferrite thickness = 0.25 to 2.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a and FIG. 1b are a top view and a cross-sectional view showing one embodiment of a pole piece of a magnetic field generator according to the present invention.

A pole piece (10) shown in FIG. 1 includes a base portion (11) made of a disc-shaped soft iron, and a soft iron ring portion (12) having a rectangular cross section provided around a periphery of the base portion (11). And a soft ferrite part (13) laid on the upper surface of the base part (11) by combining a large number of small pieces formed by compressing and molding a soft ferrite powder into a rectangular plate shape with an adhesive.

A soft iron ring (12) with a rectangular cross section provided around the periphery of the base (11) raises the height of the outer peripheral side of the pole piece (10) higher than the other, and transfers the magnetic flux to the required air. It is for forming an annular projection that concentrates and improves uniformity,
Insulation material is interposed between the base part (11) and bolts, and the ring part (12) is divided in the circumferential direction to provide slits to reduce the influence of eddy current. .

The soft ferrite part (13) is assembled in a disk shape as described above. However, in order to form a circular convex part (14) with a required diameter in the center part, it is necessary to use a soft ferrite part with a different thickness for compression molding. The circular convex portion can improve the magnetic field uniformity.

A soft iron core (15) is provided at the center of the soft ferrite (13), which constitutes a base for mounting a gradient magnetic field coil.

When the magnetic pole piece (10) having such a configuration is used in a magnetic field generator for MRI, the magnetic field in the air gap is made uniform, and the effect of reducing the eddy current by the gradient coil and the effect of reducing the remanence generated by the GC pulse. There is.

Example A (BH) max35MGO
Using a Fe-BR-based permanent magnet having e, a soft ferrite part is provided on a base part made of soft iron having the following properties, and the facing distance between a pair of pole pieces provided with an annular protrusion made of soft iron is set to 500 mm. Then, the magnetic field in the air gap was measured.

As a result, a magnetic field uniformity of 30 ppm and a magnetic field strength of 2.0 kG were obtained in a measurement value in a measurement space within a radius of 200 mm from the center of the gap.

The remanence generated by the GC pulse is reduced to 1/3 or less in the case of the pole piece according to the present invention, compared to a conventional pole piece having the same size and shape as the base part and the same soft iron bulk material. Was.

Base part Pure iron Hc = 80A / m Bs = 2.0T p = 1 × 10 ^-4 Ω · m Soft ferrite part Mn-Zn ferrite Hc = 6.0A / m Bs = 0.58T p = 0.2Ω · m

[Brief description of the drawings]

FIGS. 1a and 1b are a top view and a cross-sectional view showing one embodiment of a pole piece of a magnetic field generator according to the present invention. FIG. 2 is a longitudinal sectional view of a conventional magnetic field generator. 1 ... permanent magnet structure, 2,10 ... pole piece, 3 ... yoke,
4 ... air gap, 5 ... annular projection, 6 ... gradient magnetic field coil,
11 ... base part, 12 ... ring part, 13 ... soft ferrite part, 14 ... circular convex part, 15 ... core part.

Continuing from the front page (72) Inventor Hirotaka Takeshima 2-1 Shinjyuyo, Kashiwa City, Chiba Prefecture Inside the Hitachi Medical Research Laboratory Co., Ltd. (72) Inventor Shigeru Sato 2-1 Shinjuyoichi Kashiwa City, Chiba Prefecture Hitachi Medical Technology Co., Ltd. In the laboratory (58) Field surveyed (Int.Cl. ⁶ , DB name) H01F 7/02

Claims (2)

(57) [Claims] 1. A pair of permanent magnet components facing each other with a gap formed therebetween are magnetically coupled with a yoke, and a magnetic pole piece having an annular projection on a peripheral portion is fixed to a surface of each permanent magnet component facing the gap, An MRI magnetic field generator for generating a magnetic field in the gap, wherein a soft ferrite having a Bs (saturated magnetic flux density) of 0.4 T or more is used for the pole piece member. 2. A base made of a soft iron material is laid on the magnet structure side,
2. The magnetic field generator for MRI according to claim 1, wherein a soft iron bulk portion is further provided on the peripheral projection, and a pole piece made of soft ferrite is arranged on the base.