JPS63109846A - Magnetic field correction apparatus - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a magnetic field correction device used in an imaging device using nuclear magnetic resonance (NMR).

(Prior Art) An imaging apparatus using NMR requires a highly uniform magnetic field. Therefore, the coil (main coil) that generates the base magnetic field is designed to generate a highly uniform magnetic field.

However, when the main coil is actually excited, the magnetic field is disturbed by the influence of magnetic materials such as iron around the main coil, resulting in an error component. A device for eliminating this error component and obtaining a highly uniform magnetic field is a magnetic field correction device. As magnetic field correction devices, there are devices that use magnetic field correction coils (shim coils) and devices that use magnetic materials. A magnetic field correction device (iron shim) using a magnetic material places a magnetic material in the magnetic field (main magnetic field) generated by a main coil, magnetizes the magnetic material, and uses the magnetic field generated by the magnetic material for magnetic field correction.

As an example of using a magnetic material for magnetic field correction, see the document "5UPE"
RCONDUCTING MAGNETS FORMR
I” IEEETransactions on N
uclear 5c1ence, Vol, N5-3
1°No, 4. At+guSt 1984. An example is shown in FIG.

The figure shows an example of correcting even-order error magnetic field components regarding the Z coordinate included in the main magnetic field, with two axes of a rectangular coordinate system in the direction of the main magnetic field. (a) shows correction in the positive direction. (b) shows an example of an iron shim that generates a negative correction magnetic field. That is, the cylindrical magnetic bodies 13, 13' or 14 are arranged symmetrically with respect to the Z-0 plane to generate an even-order correction magnetic field regarding the Z-axis. The cylindrical magnetic bodies 13, 13' and 14 used here are made using large sheet-shaped magnetic bodies. The reason why such a large cylindrical magnetic body is used is that a small ring-shaped magnetic body tends to generate a locally changing magnetic field.

(Problems to be Solved by the Invention) However, as described above, since a large sheet-like magnetic material is conventionally used, there is a problem in that it is very difficult to handle it.

An object of the present invention is to provide a magnetic field correction device that can efficiently correct even-order error magnetic field components in the main magnetic field direction using a small magnetic body.

[Structure of the invention]

(Means for solving the problem) The present invention takes the Z-axis of a rectangular coordinate system in the direction of the main magnetic field, and
In a magnetic field correction device in which one or more pairs of magnetic bodies are arranged symmetrically with respect to the zero plane, on the circumference with the Z axis as the central axis,
Moreover, by placing the magnetic bodies in positions where each magnetic body pair does not generate a 2n-order (n is an integer greater than or equal to 0 determined for each magnetic body pair) correction magnetic field component with respect to the Z coordinate, small magnetic bodies can be This is used to efficiently eliminate even-order error magnetic field components regarding the Z coordinate.

(for production) The present invention will be explained in detail with reference to the principle diagram shown in FIG.

The origin is set at the center point 0 of the main magnetic field formed in the main coil l, and Z
Consider a rectangular coordinate system with an axial direction. Z- of this coordinate system
A pair of magnetic bodies 2 and 2' are placed symmetrically with respect to the
Arrange on the circumference with the axis as the central axis. That is, the Z coordinates are 2 and -Z.

A ring-shaped magnetic body 2°2' is placed on the circumference of a circle with radius R.

When the Z-axis direction component B of the magnetic field B at the measurement point P in the main coil 1 is expanded by a spherical harmonic function, B (r, θ)mΣa fist r −P (co
sθ). It is generally expressed as J n n (1). Here, r and θ are the coordinates of the measurement point P expressed in a spherical shape with the center point 0 as the origin. Using this equation (1) to express the magnetic field B2 (only the Z-axis direction component) generated by the above-mentioned magnetic body 2゜2' near Z-0, only the even-order terms are obtained, and B = a +a f +-+a2nf2n+-
・・・・・・・・・・・・(2) It becomes. Here, n is an integer greater than or equal to 0, f2n is a 2n-order function regarding the Z coordinate expressed by "2n-P2n(CO8θ)" in equation (1), and a2n is a coefficient of the function [2n. The formula is for a ring-shaped magnetic body as shown in the figure, but even if small magnetic bodies are arranged on the circumference and configured like a ring, it can be approximately expressed by formula (2). The coefficient a is the position where the magnetic material is placed (Z,
CC determined by R).

The main magnetic field Bl generated by the main coil 1 can also be expressed in the same manner as in equation (2). The main magnetic field component that the present invention attempts to correct is an even-order magnetic field component with respect to the Z coordinate, and when only this component is written, B - A + A
f +-...+A2nf2n+...
...(3) becomes. In this equation (3), Ao is the base magnetic field, and A2f2 and below are the error magnetic fields to be erased by the magnetic material.

If the error magnetic field component to be corrected is up to the 2nth order, then n pairs of magnetic bodies as described above are provided, that is, 10...
It is sufficient to establish the following relationship (4). Here, a
21. j is the coefficient of the 21st order term included in the magnetic field generated by the first pair of magnetic bodies, and ■ is a quantity related to the size of the magnetic body placed at each position, and is the standard size of magnetism. When the magnitude of the magnetic field generated by the body is 1,
This means arranging a magnetic body of a size such that the magnitude of the generated magnetic field is V.

Now, when solving equation (4) and determining vl, the solution is easier to obtain as the 7-trix (a 2., j) contains more zero components. In particular, if the matrix (a21.j) is a unit matrix, the solution can be obtained most easily.

This means that a magnetic pair placed at a certain position generates a magnetic field of only one order of terms. However, it is impossible to generate such a magnetic field with a pair of magnetic bodies, and in order to do so, it is necessary to arrange the magnetic bodies at a plurality of positions with respect to Z and combine them. In other words, in order to realize a magnetic system that forms a unit matrix, a very large number of magnetic bodies would be required, which is not realistic.

Therefore, in the present invention, each magnetic material pair is arranged at the 2nth order with respect to the Z coordinate (n is an integer of 0 or more determined for each magnetic material pair).
Place it in a position that does not generate magnetic field components. In this way, it is possible to prevent the occurrence of terms of only one order using a pair of magnetic bodies, and it is extremely easy to determine the placement position of such a pair of magnetic bodies. That is, in FIG. 3, the opening angle seen from the center point 0 of the magnetic pair 2.2' is θ.

(θ - jan' (Z /R) ), then the position where this CCC magnetic pair 2.2' does not generate the 2nth order term regarding the Z coordinate (the position where a2n becomes 0) is determined only by the opening angle θ .

For example, ao, a2, a4. a a
Table 1 shows the values of the opening angle θ, where each of is O.

Table 1 If each magnetic material pair is arranged at a position with such an opening angle θ, at least the same number of components as the number of magnetic material pairs in the matrix (a21.j) of equation (4) will be zero.

Therefore, the solution (vl
) can be obtained.

As described above, according to the present invention, when performing magnetic field correction using a relatively small number of magnetic bodies, the arrangement position and size (Vl) of each magnetic body can be relatively easily determined.
An efficient and practical magnetic field correction device can be provided.

(Example) The present invention will be explained below with reference to Examples.

FIG. 1 shows the arrangement position of the magnetic body in one embodiment of the present invention. Note that the positional relationship between the coordinate system and the main coil (not shown) is the same as that in FIG. 3. This example assumes that the error magnetic field components to be corrected in the main magnetic field are up to 6th order, and three pairs of rings are arranged symmetrically with respect to the z-0 plane, each with the Z-axis as the center axis. shaped magnetic material 3.3', 4.4'
, 5.5' is used. and a pair of magnetic bodies 3.
3' is at the opening angle θ8-±19.9° where aZ=O in Table 1, and another pair 4.4' is at the opening angle θ-±41.4° where aZ=O, and further Another pair of 5.5
' is the opening angle θ which is assumed to be a6-0. -±10.5°.

In such a configuration, equation (4) becomes 10
・・・・・・・・・(
5). Therefore, since there are three O components in the matrix (a21.j), the sizes Vt and V2 + Vs of each magnetic body can be determined relatively easily.

In this way, according to this embodiment, two pairs of magnetic bodies are used.
．． The 4.6th order magnetic field can be erased relatively easily.

FIG. 2 shows the arrangement of magnetic bodies in another embodiment of the present invention. Also in this figure, the positional relationship between the main coil (not shown) and the coordinate system is the same as in FIG. 3. In this embodiment, the error magnetic field component to be corrected is up to fourth order, and two pairs of ring-shaped magnetic bodies 6
．． 6', 7. 7' is the opening angle θ −±1 that does not generate the 6th order error magnetic field on the circumference with the Z axis as the central axis.
It is arranged at 0.5', ±31.7'. In addition, the opening angle θ
We selected two of the smallest opening angles, but other opening angles θ −±
You may also choose 52.8° or ±73.8°. Further, although the distances of each pair of magnetic bodies from the center point 0 are different from each other in this embodiment, they may be equal.

In such a configuration, equation (4) is...
...(B) becomes. In this way, since the sixth-order error magnetic field component is not generated, it is possible to efficiently correct the second-order and fourth-order error magnetic field components.

As described above, the same effect can be obtained not only with a ring-shaped magnetic material but also with a magnetic material in which small magnetic materials are arranged on a circumference with the Z-axis as the central axis.

〔Effect of the invention〕

As explained above, according to the present invention, one or more pairs of magnetic bodies arranged symmetrically with respect to the Z-0 plane are arranged on the circumference with the Z-axis as the central axis, and 2n regarding two coordinates. Since it is arranged at a position where the next magnetic field component is not generated, magnetic field correction can be performed relatively efficiently with a relatively small number of magnetic bodies.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing the magnetic material arrangement in one embodiment of the present invention, FIG. 2 is a vertical cross-sectional view showing the magnetic material arrangement in another embodiment of the present invention, and FIG. 3 is a detailed view of the present invention. FIG. 4 is a longitudinal cross-sectional view for explanation, and a perspective view showing an outline of a conventional magnetic field correction device. 1... Main coil, 3.3', 4.4', 5.5'. 6.6', 7.7'...magnetic material. Applicant's agent Yuo Sato 1 Figure 2

Claims (1)

[Claims] 1. One or more pairs of magnetic bodies are arranged symmetrically with respect to the Z=O plane of a rectangular coordinate system with the origin at the center of the main magnetic field and the Z-axis direction in the direction of the main magnetic field. In the apparatus for correcting even-order error magnetic field components related to the Z coordinate included in the main magnetic field, A magnetic field correction device, characterized in that it is arranged at a position such that the spherical harmonic expansion of the correction magnetic field does not have a 2nth order term (n is an integer of 0 or more determined for each pair of magnetic bodies) regarding the Z coordinate. 2. A magnetic field correction device according to claim 1, wherein the magnetic body is a ring-shaped magnetic body arranged with the Z-axis as the central axis. 3. A magnetic field correction device according to claim 1, wherein the magnetic body is composed of a plurality of small magnetic bodies arranged on a circumference with the Z-axis as the central axis. 4. A magnetic field correction device according to claim 1, characterized in that three pairs of magnetic bodies are arranged. 5. In the device according to claim 4, the pair of magnetic bodies is arranged at a position where the expansion does not have a quadratic term.
The pair is added at a position where the expansion does not have a fourth-order term.
A magnetic field correction device characterized in that the pairs are respectively arranged at positions where the expansion does not have a sixth-order term. 6. A magnetic field correction device according to claim 1, characterized in that two pairs of magnetic bodies are arranged. 7. A magnetic field correction device according to claim 6, characterized in that the two pairs of magnetic bodies are both arranged at positions where the expansion does not have a sixth-order term.