JPH0956692A - Magnet opposition type permanent magnet magnetic circuit and method of regulating its magnetic field - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily realize an uniform magnetic field, and the regulation of magnetic field by fixing a magnetic field regulating magnet or magnetic material to a magnetic shunt plate surface, arranging a shim plate closer to the cavity side from a gradient coil, and fixing a magnetic field regulating magnet or magnetic material onto the shim plate. SOLUTION: A magnetic shunt plate 16 has an annular projection 161 (first shim), and a circumferential stepped part (higher shim), which is not shown, can be particularly formed on a bottom part 162 as occasion demands. A gradient coil 18 and a shim plate 19 are housed in the recessed part of the magnetic shunt plate, and fixed to the magnetic shunt plate 16, respectively. A magnetic field regulating shim materials 20 and 21 are fixed onto the magnetic shunt plate bottom part 162 and the shim plate 19, and shimming regulation is performed on both the magnetic shunt plate and the shim plate. The nonmagnetic shim plate 19 is formed on the gradient coil 18, the shim material 21 is further fixed on (or under) the shim plate, the shim material 20 is fixed to the magnetic shunt plate bottom part 162 on the magnetic shunt plate 16, and the magnetic fine control is performed by shimming regulation.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a magnetic circuit used in a magnetic field generator for NMR and ESR,
In particular, the present invention relates to a magnet facing type permanent magnet magnetic circuit of a permanent magnet type MRI apparatus.

[0002]

2. Description of the Related Art Several magnetic circuits have been proposed and put to practical use in a permanent magnet type MRI apparatus. For example,
In the dipole ring type magnetic circuit as shown in FIG. 3, the magnetization direction of the magnet gradually changes in the ring circumferential direction, and the magnetization makes two rotations during one round. It is called a dipole ring type because it has N and S bipolar magnetic charges coming out on the inner circumference of the ring, and if it has a sufficient axial length, a uniform magnetic field distribution in the uniaxial radial direction inside the ring is obtained. can get. For example, K. Halbach, Nuclear In
struments and Methods 169 (1980), p1, a magnetic circuit is reported, and as an MRI apparatus, it is disclosed in JP-A-61-385.
54 (USP4580098), JP-A-62-104011 and the like. Since it is not necessary to use a yoke on the outer peripheral portion of the magnetic circuit to pass magnetic flux, the weight of the magnetic circuit can be reduced and the magnetic circuit can be made compact. Further, it has an advantage that the upper limit of the generated magnetic field can be set higher than that of a magnetic circuit using a yoke. However, in the region of the generated magnetic field of about 3000 G or less, the weight of the magnet used is larger than that of other permanent magnet magnetic circuits, so there are not many practical examples. The magnetic field is adjusted by moving each segment magnet in the radial direction and changing the tilt.

On the other hand, a magnet facing type magnetic circuit as shown in FIG. 2 is well known, and most of the permanent magnet type MRI magnets currently put into practical use are of this type. For example
WO84 / 00611 (PCT / US83 / 01175), Jikken 2-44483
Publication No. 2-44484, Publication No. 2-44
No. 485, Japanese Utility Model Publication No. 2-44486, video information 15 (1983), 379, pathophysiology 4 (1985), 91 and the like.

A basic magnet facing type magnetic circuit will be described with reference to FIG. A magnet 14 magnetized in the height direction is fixed to the back yoke 10, and a magnetic material called a magnetic compensator 16 is arranged on the surface of the magnet 14 on the side of the air gap in order to achieve high magnetic field uniformity. . The shape of the magnetic shunt is axially symmetric, and various measures have been taken to improve the uniformity. A well-known magnetic shunt plate shape has an annular protrusion 161 called a rose shim (or annular protrusion) provided on the outer peripheral portion of the disk. A pair of vertically symmetrical back yoke, magnet, and magnetic shunt plate are magnetically coupled by a yoke 12. In FIG. 2, the yoke 12 has four pillars and magnetically forms a closed magnetic circuit. The gradient coil 18 is fixed in the concave portion of the magnetic shunt plate 16, and the height of the gap side coil is substantially the same as the height of the annular protrusion 161 (first shim) on the outer peripheral portion of the magnetic shunt plate.

The magnetic field homogeneity of the magnetic circuit is evaluated by a magnetic field distribution in a spherical space or an elliptical space (hereinafter referred to as an evaluation space), which is virtually provided at the center of the void. When the magnetic shunt plate has a simple disk shape, the magnetic field strength in the equatorial part of the space is lower than that in the pole part. When using a magnetic shunt plate provided with an annular protrusion, the physical distance between the space equator and the annular protrusion becomes short,
Since the magnetic field strength in the equator is improved, the magnetic field homogeneity in the entire evaluation space is improved. Further, in order to further improve the magnetic field homogeneity, it is also devised to provide a plurality of circumferentially small protrusions (having smaller steps than the outer peripheral annular protrusion) on the magnetic shunt plate bottom 162. As described above, the shape of the shunt plate is very important because it improves the magnetic field homogeneity in the evaluation space.

Further, in the magnet facing type magnetic circuit, a yoke yoke is required in order to flow the magnetic flux generated by the upper and lower magnets through the iron yoke to form a closed magnetic circuit. The shape and number of the columnar yokes differ depending on the design, but there are four columns, two columns, two plates, and the like. The magnetic field strength in the column direction of the evaluation space tends to be low because the magnetic flux is attracted by the yoke columnar yoke. In order to improve this, it has been proposed to provide a shunt iron yoke for magnetic flux short circuit in the opening direction to reduce the magnetic field strength in the opening direction.

A magnetic circuit capable of generating a uniform magnetic field in the evaluation space is designed by a numerical analysis or the like by combining the above elements. However, even if a magnetic circuit capable of generating a uniform magnetic field can be produced, it is rare that the magnetic circuit completed by actually assembling can obtain the magnetic field homogeneity as designed. Generally, the magnetic field homogeneity is significantly lower than the design specification due to variations in magnet characteristics, processing errors, and assembly errors. Therefore, to achieve the required magnetic field homogeneity,
After assembly, it is necessary to adjust the magnetic field.

[0008]

In magnetic field adjustment, mechanical shimming, which is a rough adjustment, and magnetic material shimming, which is a fine adjustment, are performed. Mechanical shimming includes tilt adjustment of the back yoke, movement of the rectifying plate, shunt yoke adjustment, and magnetic material insertion adjustment to the back yoke. The magnetic material shimming improves magnetic field homogeneity by fixing magnetic materials of various volumes to various places on a magnetic shunt plate or a shim plate provided independently of the magnetic shunt plate. .

As the magnetic material, a soft magnetic material such as soft magnetic iron or iron-based alloy, Ni or Ni-based alloy, amorphous, or magnetic material such as hard magnetic ferrite magnet / rare earth magnet or bonded magnet thereof is used. To be As the shape, a chip shape or a thin plate shape is used. When laminating a plurality of soft magnetic material shimming, between the number and the adjustment magnetic field amount,
Since a linear relationship does not hold, it is difficult to make adjustments using a soft magnetic material, and this is a problem especially when the adjustment amount is large.
However, it is easy to manufacture thin plates and microchips, and precise fine adjustment is possible. On the other hand, in the magnet material shimming, a linear relationship is established between the number of shimmings and the adjustment magnetic field amount, so that the adjustment is easy. However, it is not as easy to fabricate a micro chip or a thin plate of a magnet as a soft magnetic material, and thus is not suitable for precise fine adjustment.

In order to effectively utilize such characteristics of the two types of shimming materials, it is conceivable to mix and use both of them. However, when a magnet material and a soft magnetic material are mixed and used for shimming, the magnetic flux generated by the magnet material affects the soft magnetic material, and the linearity of the magnet is impaired. Therefore, the conventional shimming adjustment cannot be used as a mixture of both. The above-mentioned point has been a major problem in the work of adjusting the uniform magnetic field of the magnetic circuit.

[0011]

According to the present invention, a pair of magnets are opposed to each other, a magnetizing plate is provided on the surface of the magnet gap, and a magnetizing plate is connected to form a closed magnetic circuit. In a magnet facing permanent magnet magnetic circuit provided with a gradient coil and a shim plate, a magnet or magnetic material for magnetic field adjustment is fixed to the surface of the rectifying plate, and the shim plate is arranged on the gap side of the gradient coil.
Further, it is a gist of a magnet facing permanent magnet magnetic circuit characterized in that a magnet or a magnetic material for magnetic field adjustment is fixed on the shim plate. Further, in the magnetic circuit, the amount and position of the magnetic material or the magnetic material for magnetic field adjustment are determined by a linear programming method to adjust the magnetic field in the void space, and the magnetic field adjustment method for a magnet facing permanent magnet magnetic circuit. Is the gist. This will be described in more detail below.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of an embodiment of a magnetic shunt plate of the present invention. Conventionally, the shimming adjustment of the magnetic field has been performed on the bottom portion 162 of the magnetic shunt plate. In FIG. 1, the magnetic shunt 16
Has an annular protrusion 161 (first shim) and does not show a circumferential step (higher shim) on the bottom 162, but it can be provided if necessary. The gradient coil 18 and the shim plate 19 are housed in the recesses of the magnetic shunt plate, and are fixed to the magnetic shunt plate, respectively. The present inventors have found that a magnet or magnetic material for magnetic field adjustment (hereinafter referred to as “magnetic compensator plate bottom portion 162” and shim plate 19)
They are called shim material 20 and shim material 21. It was found that the shimming adjustment is performed both on the magnetic shunt plate and on the shim plate by fixing). That is, according to the present invention, first, the non-magnetic shim plate 19 is provided on the gradient coil 18, the shim material 21 is further fixed on the shim plate (or under the shim plate), and the magnetic compensating plate 16 is mainly arranged. The shim member 20 is fixed to the bottom portion 162, and the magnetic field is finely adjusted by the shimming adjustment. The amount and position of the shim members 20 and 21 may be determined empirically, but adjustment is mainly performed by numerical calculation support.

Since the bottom of the magnetic shunt plate 162 is relatively distant from the evaluation space, the influence of the fixation of the shim member 20 on the magnetic field distribution can be reduced, but the influence extends to a relatively wide void space. On the other hand, since the shim plate 19 is provided closer to the void space (on the gradient coil 18) than the bottom of the magnetic shunt plate, the shim material 21 on the shim plate 19 easily affects the magnetic field distribution in the evaluation space. . In addition, since it is close to the evaluation space, the influence on the magnetic field is local. Therefore, coarse fine adjustment (that is, fixation of a large soft magnetic material or fixation of a magnetic material) can be performed on the magnetic shunt plate bottom 162 to improve the magnetic field distribution in a large area of the evaluation space. Next, on the shim plate 19, it is possible to perform precise fine adjustment with a small shim material 21 (adhesion of minute soft magnetic material or magnet material).

Although the shim members 20 and 21 are soft magnetic materials or magnet materials, in the case of shimming on the bottom part 162 of the magnetic shunting plate, if a magnet material is used, the linearity at the time of shimming adjustment can be obtained by fine adjustment. Since it can be secured, the magnetic field adjustment becomes easy. Of course, soft magnetic material shimming may be used, but when a plurality of soft magnetic materials are fixed, it is necessary to ensure the linearity of superposition. On the other hand, a fine soft magnetic material is desirable on the shim plate 19 because precise fine adjustment is required. Since the shimming amount on the shim plate is small, the deviation from the linearity is small even if the soft magnetic member is used.

Here, the magnet material (shim material 20) on the magnetic shunt plate
There is concern that the linearity of the magnet material may be disturbed due to the magnetic coupling between the magnetic material on the shim plate (the shim material 21).
Since 9 is arranged on the gap side from the bottom of the magnetic compensator and is spatially separated from the magnet material (shim material 20) on the magnetic compensator plate, the linearity of the magnet material (shim material 20) on the magnetic compensator plate is shown. Found out not to be disturbed. Therefore, the adjustment can be performed precisely by using the relative relationship between the evaluation space and the shimming position. However, if the two are mixed on the same plane, the nonlinearity as described above occurs, which is not desirable.

The magnet material on the magnetic compensating plate may be fixed only by its own attracting force, but since it may be displaced due to vibration or the like, adhesion or fixing with screws or the like is desirable. Similarly, the soft magnetic material on the shim plate may be displaced due to vibration or the like, and therefore it is necessary to bond or fix the soft magnetic material with screws. The shape of the soft magnetic material is preferably a thin plate shape with holes and capable of being screwed. A plurality of types having different sizes may be prepared, but it is easier to adjust the number of magnetic materials in the minimum unit. As for the lamination, flat ones may be laminated in the thickness direction, but it is desirable to laminate them vertically so that the magnetic field direction of the permanent magnet and the longitudinal direction of the magnetic material coincide with each other. This improves the linearity when a plurality of sheets are stacked.

As the soft magnetic material, iron (or iron alloy), Ni (or Ni alloy), or amorphous soft magnetic material as described above is used. In order to perform precise fine adjustment, it is desirable to use a magnetic material that has a small saturation magnetization and is easy to manufacture a small member with a thin plate. For example, Ni or a Ni-based alloy (eg, Permalloy) is
It is suitable as a soft magnetic member for shimming because it has a saturation magnetization of 7,000 G or less and can be easily manufactured into a thin plate by rolling. Also, Co-based amorphous ribbons and the like can be used because they have a relatively low saturation magnetization and a ribbon of 20 to 30 μm can be obtained. The flat magnet material may be either a ferrite magnet or a rare earth magnet, and may be either a sintered magnet or a bond magnet. In magnet material shimming, the amount of magnetic field adjustment is often large, so the size can be changed according to the amount of magnetic field to be adjusted. Prepare multiple types of magnet materials so that adjustment can be performed in several steps. Is effective. The shim plate 19 may be made of a resin material (for example, polyvinyl chloride, bakelite, etc.). Metal plates are not preferable because eddy currents flow when a gradient magnetic field is applied.

The shim adjustment of the magnetic member or the like in the present invention corresponds to the fine adjustment of the magnetic field. If the global magnetic field homogeneity of the evaluation space (for example, φ400 sphere) is improved by mechanical adjustment before shim adjustment, the homogeneity is usually about 200 ppm to 400 ppm. The shimming positions and amounts of the magnet material and the magnetic material for obtaining good magnetic field homogeneity can be improved to some extent even by empirical experiments. However, the required magnetic field homogeneity specifications are, for example, 50 ppm or less at 2000 G, preferably 10 ppm or less. This is an absolute value
Since it is less than 0.1 G, which is less than a fraction of the earth's magnetism, it will be difficult to realize without systematic magnetic field adjustment with computer support. For computer assisted adjustments,
Techniques such as mathematical programming, linear programming, and 7-plane method can be used. Among them, the linear programming method is suitable for the shim adjustment of the present invention because an optimal solution can be obtained with a relatively small amount of calculation unless the difference between the initial value and the target value is large (for example, a difference of two digits). There is.

[0019]

As described above, according to the present invention, it is possible to realize a magnet facing type MRI magnet, which can easily realize a uniform magnetic field and can easily adjust the magnetic field.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view of a magnetic shunt plate portion of a magnet facing permanent magnet magnetic circuit according to the present invention.

FIG. 2 is a schematic diagram of a conventional magnet facing permanent magnet magnetic circuit. (A) A vertical cross-sectional schematic diagram. (B) A schematic cross-sectional view taken along the line AA ′ in (a).

FIG. 3 is a perspective view of a conventional dipole ring type magnetic circuit.

[Explanation of symbols]

10 back yoke 12 yoke 14 magnet 16 magnetizing plate 161 magnetizing plate annular protrusion 162 bottom part of magnetizing plate 18 gradient coil 19 shim plate 20 shim material on magnetizing plate 21 shim material on shim plate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuji Yoneda 2-5, Kitafu, Takefu City, Fukui Prefecture Shin-Etsu Kagaku Kogyo Co., Ltd. Magnetic Materials Research Center (72) Inventor Yuji Inoue 4-chome, Asahigaoka, Hino-shi, Tokyo Address 127 GE Yokogawa Medical System Co., Ltd.

Claims (3)

[Claims] 1. A pair of magnets are opposed to each other, a magnetizing plate is provided on the surface of the magnet gap, and a magnetic flux is connected to form a closed magnetic circuit, and a gradient coil and a shim plate are provided in the magnetizing plate recess. In a magnet facing type permanent magnet magnetic circuit, a magnet or magnetic material for magnetic field adjustment is fixed to the surface of the rectifying plate, and the shim plate is arranged closer to the gap than the gradient coil, and the magnetic field is adjusted on the shim plate. A magnet facing type permanent magnet magnetic circuit, characterized in that a magnet or a magnetic material for a vehicle is fixed. 2. A magnetic field adjusting method for a magnet facing permanent magnet magnetic circuit, comprising: adjusting a magnetic field in a void space by the magnet facing permanent magnet magnetic circuit according to claim 1. 3. The magnetic field adjusting method for a magnet facing permanent magnet magnetic circuit according to claim 2, wherein the amount and position of the magnetic member or the magnetic member for adjusting the magnetic field is determined and adjusted by a linear programming method.