JPH01164356A - Apparatus for generating uniform magnetic field - Google Patents

Abstract

PURPOSE:To improve the uniformity of the magnetic field of a yoke type magnetic field generating apparatus and to easily control said magnetic field within a short time, by mounting holding plates having permanent magnetic small pieces for controlling a magnetic field arranged and fixed thereto and arranging said plates in relation to opposed permanent magnets. CONSTITUTION:Pole pieces 2 composed of a high permeability material such as soft iron are fixed to one ends of a pair of disc-shaped permanent magnets 1 to be opposed to each other and the other ends thereof are connected by yokes 3 composed of a high permeability material such as soft iron to generate a magnetic field in the gap 4 between the pole pieces 2. A human body is introduced into said gap partially or entirely to perform diagnosis. Annular projections having an almost triangular shape composed of a predetermined inner diameter and a predetermined height are provided to the peripheral edge of the opposed surfaces thereof. Further, holding plates 7 having permanent magnet small pieces 6 arranged thereto are provided to the recessed parts of the opposed surfaces of the pole pieces. As a result, dimension can be also reduced and this apparatus can cope with not only lower order non-uniformity but also higher order non-uniformity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic field generating device, and more particularly to a magnetic field generating device used in a nuclear magnetic resonance tomography apparatus (MRI).

[Prior Art] There are permanent magnet type magnetic field generators used in MHI, and one known device is one in which a pair of permanent magnets facing each other with an air gap are magnetically coupled with a yoke. It is being This device consists of three parts: a permanent magnet to generate magnetomotive force, a magnetic pole to make the magnetic field uniform, and a yoke to form a magnetic circuit. It's called a type.

In order to be used for MHI, it is required to obtain an extremely uniform and wide magnetic field space. In order to make the magnetic field in the air gap uniform, it is necessary to carefully select the shape of the pole piece.

The simplest method is to make the area of the pole piece as large as possible. However, with this method, mN of the entire device becomes large.

Recently, attempts have begun to be made to reduce the gravity of the entire device by devising the shape of the pole piece or special adjustment means.

For example, PCT WO 84-00611 describes an adjusting means in which a number of movable magnetic screws are provided on a pole piece.

Further, Japanese Patent Application Laid-Open No. 60-257109 describes an attempt to provide a yoke with an adjustment mechanism that changes the distance between opposing surfaces of pole pieces.

Furthermore, Japanese Utility Model Application Publication No. 112106/1983 discloses an adjusting means in which a pole piece is provided with an annular projection, and one or more magnetic field adjusting pieces made of magnetic material are attached to required positions of the annular projection. Are listed.

[Problems to be Solved by the Invention] Generally, in a yoke type magnetic field generating device, sufficient magnetic field uniformity cannot be obtained unless the magnetic field is adjusted by some method after the device is assembled. There are several possible causes for the non-uniformity of the generated magnetic field, including the processing accuracy and assembly accuracy of the parts of the device. In addition, in order to be used as a magnetic field generator for MRI, the permanent magnet part must have a horizontal dimension of more than 1 m, and is composed of an aggregate of small permanent magnet pieces. A major factor is the non-uniformity of the magnetic properties of each type.

A commonly used method for expressing the non-uniformity of a magnetic field is to measure the magnetic field at many points on a spherical surface in magnetic field space and approximate the measured values as a collection of harmonic functions. In this expression, errors in machining or assembly accuracy can be expressed as a collection of relatively low-order harmonic functions (hereinafter referred to as low-order nonuniformity), whereas errors in machining or assembly accuracy are caused by nonuniformity in magnetic properties. Those that do induce relatively high-order heterogeneity.

The above-mentioned adjustment means that varies the distance between the facing surfaces of the pole pieces is suitable for adjusting low-order non-uniformity, and the method of attaching a magnetic adjustment piece to the annular protrusion is suitable for medium-wide non-uniformity. Suitable for adjusting uniformity.

On the other hand, the means using movable screws is an adjustment means that can be applied from low order to high order.

However, the magnetic field adjustment means using movable screws has the following two problems.

(1) Since there are limits to the size, movable width, and number of magnetic screws, there are limits to adjusting high-order non-uniformity.

(2) High magnetic permeability materials such as iron are used for magnetic screws, but in order to calculate the magnetic field generated by a system containing a material such as iron whose magnetic susceptibility changes significantly depending on the surrounding magnetic field. This method uses an iterative correction method such as the finite element method, and therefore requires a huge amount of time to calculate in three-dimensional space, making it difficult to optimize the position of the magnetic screw by calculation. Therefore, in practice, optimization is performed by repeating trial and error while moving the position of the magnetic screw, but such a method requires a large amount of time for adjustment, and it is difficult to sufficiently improve uniformity. That's difficult.

Therefore, an object of the present invention is to be able to adjust not only low-order non-uniformity but also high-order non-uniformity in improving the magnetic field uniformity of a yoke type magnetic field generator, and to be able to adjust it in a short time. An object of the present invention is to provide a magnetic field generating device equipped with a magnetic field adjusting means configured to be easily adjusted.

[Means for Solving the Problems] In order to achieve these objects, the present invention magnetically couples a pair of permanent magnets facing each other with an air gap formed therein, and applies a magnetic field to the air gap. A uniform magnetic field generator for generating a uniform magnetic field is characterized in that it includes a holding plate on which permanent magnet pieces for magnetic field adjustment are arranged and fixed, and the holding plate is arranged in relation to the opposing permanent magnet.

[Function] According to the magnetic field generating device of the present invention, the permanent magnet pieces are arranged as means for adjusting the magnetic field uniformity, so that the optimum arrangement of the permanent magnet pieces corresponding to the non-uniformity of the magnetic field to be adjusted can be performed with high accuracy. Required often and in a short time. Further, by selecting the dimensions, residual magnetic flux density, and mounting position of the permanent magnet pieces, it is possible to adjust the non-uniformity from low order to high order. Furthermore, since the magnetization direction of the permanent magnet pieces can be selected to be positive or negative, the size can be reduced, which is suitable for high-order non-uniformity adjustment.

[Example] The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a magnetic field generating device according to the present invention, and a second
The figure is its plane. In this embodiment, a pole piece 2 made of a high magnetic permeability material such as soft iron is fixed to one end of each of a pair of disc-shaped permanent magnets 1 and are opposed to each other, and the other end is a yoke made of a high magnetic permeability material such as soft iron. 3, a magnetic field is generated in the gap 4 between the pole pieces 2, and a part or all of the human body is inserted into this gap for diagnosis.

The pair of pole pieces 2 have an annular protrusion 5 protruding from the periphery of their opposing surfaces and having a predetermined inner diameter and height and a substantially triangular cross section.By facing each other through a gap, uniformity can be improved. It is useful for

A holding plate 7 on which a small permanent magnet piece 6 is arranged is provided in the recessed portion of the inner surface of the pole piece.

FIG. 3 is a diagram showing an example of the arrangement of the permanent magnet pieces 6. The small permanent magnet piece 6 is attached to the holding plate 7 via the pedestal 9. The permanent magnet pieces 6 have the same residual magnetic flux density and are available in multiple sizes, and the magnetization direction 8 is parallel to the magnetization direction of the permanent magnet 1, but in addition to being in the same direction (positive direction), it is also opposite. There may also be a direction (negative direction). In this way, the advantage of using the permanent magnet small piece 6 is that the magnetization direction is determined by itself, and that it can also be magnetized in the negative direction. This is a fundamentally different point.

As a result, the dimensions can be reduced and it is possible to deal with not only low-order non-uniformity but also high-order non-uniformity.

As for the placement of the permanent magnet pieces 6, it is most preferable to install them on the holding plate and then in the recesses of the pole pieces 2, as described above, in order to effectively utilize the gaps between the pole pieces. In order to avoid magnetic interaction with the iron pole piece, it is preferable to install it at a certain distance. However, in some cases, the ball screw surface may be used directly. In this case, it may be provided on the annular protrusion 5, but this portion requires less installation space and is not very effective alone.

As another embodiment of the present invention, the size and mounting position of the small permanent magnet piece 6 may be fixed and permanent magnet pieces 6 having different residual magnetic flux densities may be used, or the size of the small permanent magnet piece 6 may be fixed. It may also be possible to increase the degree of freedom in its mounting position.

Magnet materials that can be used as this permanent magnet piece 6 include S
Rare earth magnet materials such as m-Go type and Nd-Fe-8 type,
There are ferrite magnet materials or their analogs. The material of the permanent magnet piece 6 may be the same as that of the permanent magnet 1 or may be different.

FIG. 4 is a flowchart showing an example of the procedure for determining the optimum arrangement of the permanent magnet pieces 6 for magnetic field adjustment. Several types of small magnet pieces 6 having the same residual magnetic flux density and different sizes are prepared. For example, the design information regarding which size and position of the small magnet piece 6 should be fixed is calculated as follows.

First, evaluate how much the magnetic field is disturbed. It is known that it is sufficient to investigate the disturbance in the visual field space (here, a sphere) by examining the disturbance in the visual field surface. Magnetic fields ()II, H2, H3... in the direction of the main magnetic field at many (n) points on the surface
Hn) is measured.

The disturbance Cp at a certain point P is defined by the following equation.

Cp=(Hp-Ho) x 10'/Ho
(1) HO is a magnetic field centered on the baseball field. Disturbance Cp at point P
is a value with positive and negative signs, and the disturbance (non-uniformity) of the magnetic field is expressed by C at n points.

Next, adjustment information must be obtained in response to this disturbance in the magnetic field. Now, to attach the small magnet piece 6, attach a serial number to each possible position (here, a predetermined position on the holding plate 7), and set the size of the small magnet piece 6 to ΔRi for the i-th position.
It is expressed as The magnetization direction of the small magnet piece 6 is a permanent magnet! It is parallel to the magnetization direction 8 of , and has positive and negative values. When arranging the size of magnet pieces 6 arranged and fixed at each position in this numerical order for a certain disturbance, this is (△R1, △R2, △R3,
..., 68 m). This vector is called an adjustment vector or shimming vector.

To perform the adjustment, this shimming vector ΔR must be found. Consider the following equation as a basic equation.

-C=G・△R (2) n: Total number of disturbance expansion coefficients m: Total number of adjustment mechanisms Here, C is the column vector of "magnetic field disturbance" obtained from magnetic field measurement, and G is the small magnet piece at each position. It shows how the magnetic field at each measurement point changes due to minute changes in the size of (
nxm) matrix. This G is also called a sensitivity matrix.

△R is an m-th column vector that indicates the amount of change in the parameter, that is, the size of the small magnet piece 6 fixed at each position (
2) It is found as the solution of Eq. This creates a new disturbance (-〇) that cancels out the disturbance C in the magnetic field.

Solve equation (2) as follows.

G is a (nxm) rectangular matrix, - orthogonality of the set ti s,
It can be converted into the following form using T (singular value decomposition).

G=S-D-T'', (D) 1j=dij-5ij
(3) Therefore, −C=SDT+・△R(4) S” (−(:) = D −T”・△R(5) If we write this as −C”=D・△R′ (6), Equation (2) has been transformed into the form.Each element of the △R vector is △R'j = (-C:'j/di)
It can be found as (8).

If the matrix with (1/di) as diagonal elements is expressed as D-1, then (D"") ij-(1/dij) ・5ijD-'
S"(-C)-T"・△R(9)△R-TD-'S
Responsibility-C)-G+・(-C) (10). In other words, the part (TD-'S'') is the inverse matrix G+ of G. If G+ is determined in this way, the shimming vector ΔR that cancels out a certain magnetic field disturbance C can be immediately determined.

The specific procedure is shown according to FIG. First, in step S1, a possible mounting position for the small magnet piece 6 is determined. Possible positions for attachment are, for example, grid points defined at regular intervals on the holding plate 7.

Next, in step S2, information on changes in the magnetic field exerted on each measurement point on the surface of the baseball field when a unit amount of small magnet pieces 6 are placed at each position is calculated and prepared as a zero matrix. Next, in step S3, the generalized inverse matrix G+ is obtained from the 0 matrix using the method described above. After assembling the device, each measurement point (n
The magnetic field of 2) is precisely measured using an NMR Boolob, and the disturbance C of the magnetic field is determined in step S5.

The uniformity of the magnetic field is expressed in ppm by dividing the difference between the largest value and the smallest value by the absolute magnetic field (average value) based on the result of magnetic field measurement. If this value does not reach the target value (for example, 30 ppm or less is required for a unit with a central magnetic field of 3000 Gauss) in step S6, the process moves to step S7, and G+ and C are used to calculate each position using equation (10). Find the size (shimming vector) ΔR of the magnet placed at .

Finally, in step S8, permanent magnet pieces 6 are prepared according to data such as the position, size, magnetization direction, and residual magnetic flux density determined by the magnet material of the permanent magnet pieces 6 determined as described above. is placed and fixed on the holding plate 7 via a pedestal 9 made of non-magnetic material. After installation, if the permanent magnet piece 6 is misaligned, it will cause the uniformity of the magnetic field to deteriorate, so it is necessary to securely and securely fix the permanent magnet piece 6.

Then, the holding plate 7 is attached onto the pole piece 2. In order to fix it, it is attached from the pole piece 2 or from the outside using a holder made of a non-magnetic material. - An example is shown in FIG. In this example, the holding plate 7 is attached to the annular protrusion 5 of the pole piece 2 using bolts 10 made of a non-magnetic material. In other examples, it is also possible to install it within the annular projection of the pole piece 2. The retainer plate and the pole piece may be bonded together using an adhesive.

As shown in FIG. 6, the permanent magnet piece 6 is adhered onto a pedestal 9 made of a non-magnetic material, and securely fixed to a holding plate 7 made of a non-magnetic material with a non-magnetic screw 11.

After attaching the retaining plate 7, if it is desired to further improve the uniformity, the above-described operation is repeated.

In the above example, the holding plate 7 was attached perpendicularly to the magnetic flux, that is, parallel to the pole piece 2, but the present invention is not limited to this. As shown in FIG. 7, retaining plates 7 may be attached parallel to the magnetic flux so as to sandwich the air gap 4 from left and right sides. FIG. 8 is a cross-sectional view of the example of FIG. 7. The magnetization direction of the permanent magnet piece 6 is parallel to the direction of the magnetic flux, as in the example shown in FIG. 1, and has positive and negative values.

In the present invention, in addition to the adjustment means using the permanent magnet pieces 6, other adjustment means, such as adjustment means for making the distance between the facing surfaces of the pole pieces variable, or a shim coil can be used in combination.

[Effects of the Invention] As described above, according to the magnetic field generating device of the present invention, the permanent magnet pieces are arranged as means for adjusting the magnetic field uniformity, so that the magnetic field generating device according to the present invention can be adjusted to correspond to the non-uniformity of the magnetic field to be adjusted. Optimal placement of permanent magnet pieces can be determined with high precision and in a short time. Furthermore, by selecting the dimensions, residual magnetic flux density, and mounting position of the permanent magnet pieces, it is possible to adjust the non-uniformity from low order to high order. Furthermore, since the magnetization direction of the permanent magnet pieces can be selected to be positive or negative, the size can be reduced, which is suitable for high-order non-uniformity adjustment. In addition, small pieces of permanent magnets can be placed using the entire surface of the pole piece.
Effective for improving uniformity. Furthermore, the permanent magnet pieces are easy to attach. In the past, it was most suitable as a magnetic field generator for devices such as MRI.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view showing an embodiment of the uniform magnetic field generating device according to the present invention, Fig. 2 is a cross-sectional view of the magnetic field generating device shown in Fig. 1, and Fig. 3 is the arrangement of small permanent magnet pieces for magnetic field adjustment. A partial side view showing an example; FIG. 4 is a flowchart of an example of the design procedure of a small magnet piece for magnetic field adjustment; FIG. 5 is a partial side view showing an example of the specific installation of a small magnet piece for magnetic field adjustment; FIG. 6 is a cross-sectional view showing an example of fixing a permanent magnet small piece for magnetic field adjustment to a holding plate, FIG. 7 is a longitudinal cross-sectional view showing another embodiment of the magnetic field generating device according to the present invention, and FIG. FIG. 2 is a cross-sectional view of the magnetic field generator. DESCRIPTION OF SYMBOLS 1... Permanent magnet, 2... Pole piece, 3... Yoke, 4... Air gap, 5... Annular projection, 6... Permanent magnet small piece, 7... Holding plate, 8... ... Magnetization direction, 9... Pedestal, lO... Bolt, 11... Screw. d. The flow shown in row 1 on the right + - yard Figure 4 The actual installation of the small piece of permanent stone is shown in row Σ Figure 5 Figure 6 Figure 7 is a cross-sectional view of the pulling force of color A.

Claims (1)

[Claims] In a uniform magnetic field generator that magnetically couples a pair of permanent magnets facing each other with a gap formed therein, and generates a magnetic field in the gap, a holding plate on which permanent magnet small pieces for magnetic field adjustment are arranged and fixed. A device for generating a uniform magnetic field, characterized in that the holding plate is disposed in relation to the opposing permanent magnet.