JP3151129B2 - Permanent magnet permanent magnet magnetic circuit and its magnetic field adjustment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic circuit used in a magnetic field generator for NMR or ESR.
In particular, the present invention relates to a permanent magnet magnetic circuit of a magnet facing type of a permanent magnet type MRI apparatus.

[0002]

2. Description of the Related Art In a permanent magnet type MRI apparatus, several magnetic circuits have been proposed and put into practical use. 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 rotates twice during one round. Since dipole magnetic charges of N and S appear on the inner circumference of the ring, it is called a dipole ring type, and if it has a sufficient axial length, a uniform magnetic field distribution directed in one axis radial direction inside the ring can get. For example, K. Halbach, Nuclear In
A magnetic circuit has been reported in Instruments and Methods 169 (1980), p1, and as an MRI apparatus, see JP-A-61-385.
No. 54 (US Pat. No. 4,800,098), Japanese Patent Application Laid-Open No. 62-104011, and the like. Since the magnetic circuit does not need to use a yoke at the outer periphery to pass the magnetic flux, the weight of the magnetic circuit can be reduced and the magnetic circuit can be made compact. Also, there is an advantage that the upper limit of the generated magnetic field can be set higher than 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. The magnetic field adjustment is performed 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 permanent magnet type MRI magnets currently in practical use are of this type. For example
WO84 / 00611 (PCT / US83 / 01175), Jiro 2-44483
Japanese Patent Publication No. 2-44484, Japanese Utility Model Publication 2-44
No. 485, Japanese Utility Model Publication No. 2-44486, image information 15 (1983), 379, and pathophysiology 4 (1985), 91.

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

The magnetic field uniformity of the magnetic circuit is evaluated based on the magnetic field distribution in a spherical space or an elliptical space (hereinafter referred to as an evaluation space) virtually provided at the center of the gap. When the magnetic shunt plate has a simple disk shape, the equatorial magnetic field intensity in the space becomes lower than that of the pole. When a magnetic shunt plate provided with an annular projection is used, the physical distance between the spatial equator and the annular projection is short,
Since the magnetic field strength in the equatorial region is improved, the magnetic field uniformity in the entire evaluation space is improved. In order to further improve the uniformity of the magnetic field, it has been devised to provide a plurality of small circumferential protrusions (those having a smaller step than the outer circumferential annular protrusion) on the bottom of the magnetic shunt plate 162. As described above, the shape of the magnetic shunt plate is very important for improving the magnetic field uniformity in the evaluation space.

Also, in the magnet-facing magnetic circuit, a yoke column-shaped yoke is required to allow the magnetic flux generated by the upper and lower magnets to flow through the iron yoke to form a closed magnetic path. Although the shape and the number of columnar yokes differ depending on the design, 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 drawn by the yoke columnar yoke. In order to improve this, it has been devised to provide a shunt iron yoke for magnetic flux short-circuiting in the opening direction to reduce the magnetic field strength in the opening direction.

[0007] A magnetic circuit capable of generating a uniform magnetic field in the evaluation space is designed by combining the above-described elements and performing numerical analysis or the like. However, even if a magnetic circuit capable of generating a uniform magnetic field can be manufactured, it is rare that a magnetic circuit that is actually assembled and completed will achieve magnetic field uniformity as designed. In general, variations in magnet characteristics, processing errors, assembly errors, and the like are superimposed, and the magnetic field uniformity is generally significantly lower than design specifications. Therefore, to achieve the required magnetic field uniformity,
After assembly, it is necessary to adjust the magnetic field.

[0008]

In the magnetic field adjustment, mechanical shimming corresponding to coarse adjustment and magnetic material shimming corresponding to fine adjustment are performed. Mechanical shimming includes tilt adjustment of the back yoke, movement of the magnetic shunt, adjustment of the shunt yoke, and adjustment of insertion of the magnetic material into the back yoke. Magnetic material shimming improves magnetic field uniformity by fixing magnetic materials of various volumes to various places on a magnetic shunt plate or on a shim plate provided independently of the magnetic shunt plate. .

As the magnetic material, a soft magnetic material such as soft magnetic iron or an iron-based alloy, Ni or a Ni-based alloy, or amorphous, or a magnetic material such as a hard magnetic ferrite magnet, a rare earth magnet, or a bonded magnet thereof is used. Can be As the shape, a chip shape or a thin plate shape is used. Soft magnetic material shimming, when laminating a plurality, between the number and the adjustment magnetic field amount,
Since a linear relationship does not hold, adjustment using a soft magnetic material is difficult, and this is a problem particularly when the adjustment amount is large.
However, a thin plate or a small chip can be easily manufactured, and precise fine adjustment is possible. On the other hand, in the case of the magnet shimming, the adjustment is easy because a linear relationship is established between the number of shimming and the amount of the adjustment magnetic field. However, it is not easy to fabricate a fine chip or a thin plate of a magnet as a soft magnetic material, so that it is not suitable for precise fine adjustment.

[0010] In order to effectively utilize the characteristics of the two types of shimming materials, it is conceivable to use both types in combination. However, when the magnet material and the 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. For this reason, in the conventional shimming adjustment, it was not possible to use both together. The above point is a major problem in the operation 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 magnetic shunt is provided on the surface of the magnet gap, and these are connected with a yoke to form a closed magnetic circuit. In a magnet facing type permanent magnet magnetic circuit provided with a gradient coil and a shim plate, a magnet or a magnetic material for adjusting the magnetic field is fixed to the surface of the magnetic shunt plate, and the shim plate is arranged on the gap side than the gradient coil,
Further, the present invention provides a magnet facing type permanent magnet magnetic circuit wherein a magnet or a magnetic material for adjusting a magnetic field is fixed on the shim plate. Further, in the magnetic circuit, the amount and position of the magnetic material or the magnet material for adjusting the magnetic field are determined by a linear programming method, and the magnetic field in the void space is adjusted, and the magnetic field adjusting method for the permanent magnet magnetic circuit of the magnet facing type is characterized in that It is the gist. This will be described in more detail below.

[0012]

FIG. 1 shows an embodiment of a magnetic shunt plate according to the present invention. Conventionally, the shimming adjustment of the magnetic field has been performed on the bottom 162 of the magnetic shunt plate. In FIG. 1, the magnetic shunt plate 16 is shown.
Has an annular projection 161 (first shim), and the bottom 162 does not particularly show a circumferential step (higher order shim), but can be provided if necessary. The concave portion of the magnetic shunt plate accommodates the gradient coil 18 and the shim plate 19, which are fixed to the magnetic shunt plate, respectively. The present inventors placed a magnet or a magnetic material (hereinafter, referred to as a magnetic material) for adjusting the magnetic field on the magnetic shunt plate bottom 162 and the shim plate 19.
They are referred to as shim material 20 and shim material 21. Has been found to perform shimming adjustment both on the magnetic shunt plate and on the shim plate. That is, in the present invention, first, the nonmagnetic shim plate 19 is provided on the gradient coil 18, the shim material 21 is further fixed on the shim plate (or below the shim plate), and the magnetic The shim material 20 is fixed to the bottom part 162, and fine adjustment of the magnetic field is performed by shimming adjustment. Determining the amounts and positions of the shims 20 and 21 may be performed empirically, but is adjusted mainly with the support of numerical calculations.

Since the bottom of the magnetic shunt plate 162 is relatively far away from the evaluation space, the influence of the fixation of the shim material 20 on the magnetic field distribution is reduced, but the influence extends to a relatively wide gap space. On the other hand, since the shim plate 19 is provided closer to the gap space (above 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, fixing of a large soft magnetic material or fixing of a magnet material) is performed on the magnetic shunt plate bottom 162, and the magnetic field distribution in a large area of the evaluation space can be improved. Next, on the shim plate 19, precise fine adjustment can be performed by using a small shim material 21 (adhesion of a minute soft magnetic material or a magnet material).

The shims 20 and 21 are soft magnetic materials or magnet materials. However, in the case of shimming on the magnetic shunt plate bottom 162, if a magnet material is used, fine adjustment is performed to improve linearity during shimming adjustment. Since it can be secured, the magnetic field adjustment becomes easy. Of course, soft magnetic material shimming may be used, but it is necessary to secure linearity of superposition when a plurality of soft magnetic materials are fixed. On the other hand, a fine soft magnetic material is desirable on the shim plate 19 because precise fine adjustment is required. Since the amount of shimming on the shim plate is small, even if a soft magnetic member is used, the deviation from linearity is small.

Here, the magnet material (shim material 20) on the magnetic shunt plate
It is feared that the linearity of the magnet material is disturbed due to the magnetic coupling between the magnetic material and the magnetic material (the shim material 21) on the shim plate.
Numeral 9 is disposed on the gap side from the bottom of the magnetic shunt plate and is spatially separated from the magnet material (shim material 20) on the magnetic shunt plate. Found that it was not disturbed. Therefore, the adjustment can be performed precisely by using the relative relationship between the evaluation space and the shimming position. However, if they are mixed on the same plane, the above-described non-linearity occurs, which is not desirable.

The magnet material on the magnetic shunt plate may be fixed only by its own attraction force. However, since there is a possibility of displacement due to vibration or the like, it is preferable to fix the magnet material with an adhesive or a screw. Since the soft magnetic material on the shim plate may be similarly displaced due to vibration or the like, it is essential to fix the soft magnetic material with an adhesive or a screw. As the shape of the soft magnetic material, a material having a perforated thin plate shape that can be screwed is preferable. A plurality of types having different shapes may be prepared, but it is easier to adjust the size by the number of magnetic materials in the minimum unit. The lamination may be performed by laminating flat ones in the thickness direction, but it is preferable that the lamination is performed vertically so that the magnetic field direction by the permanent magnet and the longitudinal direction of the magnetic material coincide. Thereby, the linearity when a plurality of sheets are stacked is improved.

As the soft magnetic material, iron (or iron alloy), Ni (or Ni alloy), or an 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 thin member with a small thickness. For example, Ni or a Ni-based alloy (eg, Permalloy)
It is suitable for use as a soft magnetic member for shimming because it has a saturation magnetization of 7000 G or less and can easily produce a thin plate by rolling or the like. Co-based amorphous ribbons can also be used because they have relatively low saturation magnetization and can provide ribbons of 20 to 30 μm. Further, the flat magnet material may be either a ferrite-based magnet or a rare-earth magnet, and may be either a sintered magnet or a bonded magnet. In magnetic material shimming, the amount of magnetic field adjustment is often large, so it is sufficient to change the magnitude according to the amount of magnetic field to be adjusted, and prepare multiple types of magnet materials so that adjustment can be made in several stages Is valid. Note that the shim plate 19 may be made of a resin material (for example, polyvinyl chloride, bakelite, or the like). The metal plate is not preferable because an eddy current flows when a gradient magnetic field is applied.

The shim adjustment in the magnetic member or the like in the present invention corresponds to the fine adjustment of the magnetic field. When the global magnetic field uniformity of the evaluation space (for example, φ400 sphere) is improved by mechanical adjustment before shim adjustment, uniformity of about 200 ppm to 400 ppm is usually obtained. The shimming position and the amount of the magnet material or the magnetic material for obtaining a good magnetic field uniformity can be improved to some extent even if it is empirically performed. However, the required magnetic field uniformity specification is, for example, 50 ppm or less at 2000 G, preferably 10 ppm or less. This is an absolute value
Since it is 0.1 G or less, which is less than a fraction of the terrestrial magnetism, it is difficult to achieve this without systematic magnetic field adjustment with computer assistance. For computer-assisted adjustments,
Methods such as mathematical programming, linear programming, and 7-plane method can be used. Above all, in the linear programming method, if the difference between the initial value and the target value is not so large (for example, a difference of two digits), the optimal solution can be obtained with a relatively small amount of calculation. I have.

[0019]

According to the present invention, it is possible to realize a magnet for a magnet facing type MRI which can easily realize a uniform magnetic field and can easily adjust the magnetic field.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram of a conventional magnet-facing permanent magnet magnetic circuit. (A) A schematic longitudinal section. (B) The cross-sectional schematic diagram by the AA 'plane of (a).

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

[Explanation of symbols]

REFERENCE SIGNS LIST 10 back yoke 12 yoke 14 magnet 16 magnetic shunt plate 161 magnetic shunt plate annular projection 162 magnetic shunt plate bottom 18 gradient coil 19 shim plate 20 shim material on the magnetic shunt plate 21 shim material on the shim plate

Continued on the front page (72) Inventor Yuji Yoneda 2-5-1-5 Kitafu, Takefu-shi, Fukui Shin-Etsu Chemical Industry Co., Ltd. Magnetic Materials Research Laboratories Yokogawa Medical System Co., Ltd. (56) References JP-A-9-75324 (JP, A) JP-A-61-276309 (JP, A) JP-A-1-164356 (JP, A) JP-A-4- 186703 (JP, A) Japanese Utility Model Application Hei 3-83903 (JP, U) Japanese Patent Publication No. 5-87962 (JP, B2) US Patent 5,864,275 (US, A) US Patent 5,631,117 (US, A) European Patent Application 760484 ( EP, A 2) EP 985934 (EP, A 1) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 H01F 7/02

Claims (3)

(57) [Claims] 1. A pair of magnets are opposed to each other, a magnetic shunt plate is provided on the surface of the magnet gap, these are connected by a yoke to form a closed magnetic circuit, and a gradient coil and a shim plate are provided in the magnetic shunt plate recess. In the magnet facing type permanent magnet magnetic circuit, a magnet or a magnetic material for adjusting a magnetic field is fixed to the surface of the magnetic shunt plate, and a shim plate is arranged on the gap side of the gradient coil, and the magnetic field is further adjusted on the shim plate. Permanent magnet magnetic circuit, wherein a permanent magnet or magnetic material is fixed. 2. A method for adjusting a magnetic field of a magnet-facing permanent magnet magnetic circuit, comprising: adjusting a magnetic field in an air gap by the magnet-facing permanent magnet magnetic circuit according to claim 1. 3. The magnetic field adjusting method according to claim 2, wherein the amount and the position of the magnetic member or the magnetic member for adjusting the magnetic field are determined and adjusted by a linear programming method.