JPH01714A - magnetic field compensator - Google Patents

Abstract

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

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a magnetic field compensator for compensating for a radial error magnetic field component included in a generated magnetic field in a main coil of a magnet device for a magnetic resonance apparatus, and in particular, the present invention relates to a magnetic field compensator for compensating for a radial error magnetic field component included in a generated magnetic field. The present invention relates to an inexpensive and highly accurate magnetic field compensator.

[Prior Art] Generally, in a superelectromagnetic groove magnetic resonance apparatus, one of the important characteristics is the spatial magnetic field homogeneity in a region where a subject is placed. Therefore, in order to make the magnetic field highly uniform, various improvements have been made to the shape, current components, etc. of the main coil for generating the output magnetic field of the magnet device used in the magnetic resonance apparatus.

However, a highly uniform magnetic field is easily disturbed by internal conditions of the magnet, such as manufacturing precision and temperature conditions, or external ambient conditions, such as the placement of a ferromagnetic material near the magnet. has a built-in magnetic field compensation device to compensate for error magnetic fields.

The component B (X, Y, Z) of the error magnetic field can be expressed by the Macrolin expansion of the magnetic field output at the center point of the magnetic field, and B (X,
Y, Z> = 8. +B,X+B2Y+B,Z +B,X2 +BsY2+l16Z2+BtXY+Bs
YZ+BqZX...■. Here, Bo is the required uniform magnetic field component, BOX.

BAY and B12 indicate first-order error magnetic field components in each of the X, Y, and Z directions, and hereinafter sequentially indicate higher-order error magnetic field components.

When the main magnetic field generated by the main coil is oriented in the Z direction, in equation (2), error magnetic field components including X and Y (for example, BIX, B2Y, B, X2.B5Y2.B,
), YZ, B, ZX.

) is called the radial error magnetic field component.

Conventionally, compensation for the radial error magnetic field component is performed for each component. Therefore, for example, when using coils for compensation, the X-compensation coil (X-shim coil), the Y-compensation coil (Y-shim coil),
-Shim coil), XY compensation coil (XY-Shim coil)
, etc., independent coil systems corresponding to each component are arranged.

Fig. 3 is a block diagram showing the X-compensation coil of a conventional magnetic field compensation device. In Fig. 0, (11) to (14) are four saddle-shaped coils installed in a cylindrical shape. H is distributed along the line. Moreover, these saddle-shaped coils (11) to (1
4) constitute one unit of coil system, which are connected in series and excited by one power source (not shown). Arrow i indicates each saddle-shaped coil (11) to (14).
), a is the current flowing in each saddle-shaped coil (11) to (14)
The radius of the circular arc portion and θ indicate the opening angle of the circular arc portion.

In an actual magnetic field compensator, in order to compensate for the other radial error magnetic field components (for example, Z2X component, X3 component, etc.) mentioned above, another cylindrical compensation coil (see FIG. It has a multilayer structure in which layers (not shown) are stacked one on top of the other. Each coil system for compensating for high-order error magnetic field components such as the z'x component and the X3 component is equipped with 6 to 8 saddle-shaped coils. typically requires 18 or more saddle-shaped coils.

FIG. 4 shows an example of a superconducting magnet in which the X compensation coil shown in FIG. 3 is not actually arranged together with other compensation coils (Y, Z, ZX, FIG. 5 is a sectional view taken along line AA in FIG. 4.

In the figure, (30) is the opening of the superconducting magnet, (
31) is a superconducting main coil, (32) is a compensation coil manufactured as a superconducting coil and provided concentrically with the superconducting main coil (31), and (33) is a superconducting main coil (31).
and a liquid helium tank with a built-in compensating coil (32) to serve as a heat shield, (35) a vacuum tank covering the liquid helium tank (34), and (36) a normally conducting coil and an opening (30). A compensation coil located within.

Note that the compensation coils (32) and (36) may be installed together, or only one of them may be installed.

Next, the error magnetic field compensation operation in two directions by the X compensation coil shown in FIG. 3 will be explained. The magnetic field component that compensates for the error magnetic field in the Z direction is generated only by the circular arc portion of the circular arc portions and straight portions forming each of the saddle-shaped coils (11) to (14). The formula expressing the magnetic field output Bz (x, y + z) at this time is: Bz (X, y, Z) = θB, /θx + (x3/8) θ3BJtlx” + (
xy”/2)83Bzlaxay2+(xz2/2)θ
3B2/θx+? z2+(x'/120)J'B-/θ
xs"(x3y2/12)a'Bz/Jx3Jy2+(
xy'/24) J5Bz/θ×θy4"(x'z2/1
2) a'Bz/θx″θz2+(xy2z2/4)J'
Bz/JxJy2#z""(xz'/24)85Bzl
It is expressed as axaz'+=-...■. Equation (2) represents the first, third, and fifth order outputs, and ignores higher order outputs.

In the case of an X-compensating coil, the first term of the first order in the equation (2) is an effective term, and the third order and fifth order or higher order become compensation errors. Also, ■
Specifically, the partial differential in the formula can be expressed by the radius a of the arc portion, the opening angle θ, and the position in the Z direction where the arc portion is arranged. Therefore, in the X compensation coil, the Z direction position and opening angle θ are selected to appropriate values, and each saddle-shaped coil (1
As the total sum of the eight circular arc parts 1) to (14), the third-order term (or fifth-order or higher) of equation (2) is eliminated.

Therefore, in order to reduce the other components, the position in the Z direction is limited, and the opening angle θ is also limited and cannot be made large, resulting in a small compensation magnetic field component per ampere turn.

[Problems to be Solved by the Invention] As described above, the conventional magnetic field compensator compensates the radial error magnetic field for only one component (for example, the X component), so the other components are reduced to zero or sufficiently small. In order to do this, the Z-direction position and opening angle θ of the arc portion of the compensation coil are limited, so the compensation magnetic field component per ampere turn becomes small and accuracy deteriorates, and the compensation for the high-order error component of the main coil is limited. Regarding coils, there are problems in that the number of saddle-shaped coils and the number of calendars increase, making the device larger and uneconomical.

This invention was made to solve the above-mentioned problems, and it is possible to reduce the number of saddle-shaped coils and the number of layers to make it smaller and cheaper, and to generate a predetermined compensation magnetic field component with fewer ampere turns. The purpose of this study is to obtain a highly accurate magnetic field compensation device that can

[Means for Solving the Problems] The magnetic resonance apparatus according to the present invention includes a plurality of compensation coils each comprising a plurality of series coils arranged concentrically along a main magnetic field and symmetrically with respect to a plane perpendicular to the main magnetic field. and a plurality of power supplies for individually supplying current to these compensation units.

[Operation] In the present invention, each compensation unit includes a plurality of components of the radial error magnetic field, and these components are simultaneously compensated as a whole in the plurality of compensation units, and each current is adjusted according to the configuration of each compensation unit. is determined and current adjustment is made for each compensation unit.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a power supply structure for the coil system of FIG. 1.

The structure of the superconducting magnet in which the coil system shown in FIG. 1 is arranged is as shown in FIGS. 4 and 5. Also, here, a case is shown in which X, Z2x, and X3 components among the main coil error magnetic field components are compensated.

In the figure, (101) to (112) are 12 saddle-shaped coils arranged along the Z direction, and (1'
01) to (104) are the first compensation unit U1, (105
) to (108) are the second compensation unit U2, (109)
~(112) constitutes the third compensation unit U3,
Each compensation unit U1 to U3 is connected in series to form three series coils.

Saddle-shaped coils (105) to (10) of the second compensation unit U2
8) is a saddle-shaped coil (101) of the first compensation unit u1.
- (104) are arranged in pairs on both sides. Also, each saddle-shaped coil (109) to (112) of the third compensation unit υ3 shown in the lower row for convenience! i, in fact, the first compensation unit Ulf7) each saddle-shaped coil (101) to (10
4), they are arranged concentrically overlapping each other so that the X, Y, and Z directions coincide.

Therefore, the saddle-shaped coils (101) to (112) are arranged symmetrically with respect to the XY plane perpendicular to the Z (main magnetic field) direction for each compensation unit U1 to U3, respectively.

Arrows 11 to i3 indicate currents flowing through the saddle-shaped coils (101) to (112) in each compensation unit u1 to u3, and ri21) to
(23) are three power supplies for individually supplying current to each compensation unit u1 to u3.

Next, the operation of the embodiment of the present invention shown in FIGS. 1 and 2 will be explained.

Saddle-shaped coil (101) ~ by power supplies (21) ~ (23)
(112) is excited, each compensation unit υ1 to u3
A magnetic field output is generated. The axially generated magnetic field output consisting of the sum of these magnetic field outputs is three compensation magnetic field components B, (X component), B, (Z2X component), and B2 (X3 equation), and from the above equation 0, B, (X component) = x CaBz/Jx) +(^Tl)+x(+?B
z/Jx)2(Ar2)+x(JBz#x)s(Ar3
) B2 (Z"X component) = (z"x/2) (J'Bt/JxJz2), (^T,
)+(z2x/2)(#'B,/JxJz"), (Ar
2)+(z”x/2)(#3B-/JxJz”)z(A
r3) B, (X3 equation) −(x3/6)<a”Bzlaxko), (^Tl)+(
x'/6) (#ko82/JX')2 (Ar1)"(x
3/6) (a'Bz/ax3>*(8T3). Here, subscripts 1 to 3 represent each compensation unit U1 to
Indicates the component for each u3, and 8T1 to ΔT3 are compensation units U
Putting together these formulas that indicate the ampere turns of the saddle-shaped coils in 1 to u3, the required magnetic field is expressed as the product of the matrix determined by the structure of the coil system and the matrix determined by the ampere turns. It will be done.

By taking the inverse of the matrix of the first term on the right side of equation 0, 3
Each compensation power supply (2
Current values 1) to (23) can be set.

In this way, a method is adopted in which one compensation unit compensates for multiple main coil magnetic field error components, and together with other compensation units, it compensates for a predetermined main coil magnetic field error, and the current value of the entire compensation unit is By making adjustments, the Z-direction position and opening angle θ of the arc portion can be optimally set, and structural restrictions are relaxed.

Therefore, the configuration of the present invention has the following advantages (i) to (iii>) compared to the conventional system.

(i>With only 12 saddle-shaped coils (101) to (112), magnetic field compensation for the above three equations can be achieved, and the number of saddle-shaped coils and the number of layers can be reduced.

<1i) Since the conditions for the Z direction position and the opening angle θ are relaxed, it is possible to manufacture a saddle-shaped coil with high magnetic field output sensitivity, and it is possible to reduce the weight of the device or the voltage and current of the power source.

(iii) Since the magnetic field is corrected by all of the multiple compensation units 01 to u3, the magnetic field characteristics are improved, and the compensation unit 01 to u3
A device with high output accuracy can be obtained by the combination method of ~u3.

In the above embodiment, the coil system that generates the X component, the Z2X component, and the X3 equation was explained as an example, but it goes without saying that the same configuration can be used for the Y component, the Z2Y component, and the Y33 equation.

Further, it may be combined with other magnetic field components depending on the method of combining the coil system and the compensation unit.

Further, although the third compensation unit U3 is a separate layer and has a two-layer structure, a one-layer structure may be obtained by extending the layer including the first compensation unit U1 in the Z direction and forming the same layer.

[Effects of the Invention] As described above, according to the present invention, there are provided a plurality of compensation units each comprising a plurality of series coils arranged concentrically along a main magnetic field and symmetrically with respect to a plane perpendicular to the main magnetic field; and a plurality of power supplies for individually supplying current to the compensation units, each compensation unit generates a plurality of error magnetic field components to obtain a predetermined compensation magnetic field across the plurality of compensation units, and each compensation unit Since each current is determined according to the configuration, the number of coils and layers can be reduced, and a predetermined compensation magnetic field component can be generated with a small number of ampere turns. This has the effect of providing a high magnetic field compensator.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing a power supply configuration for the coil system in Fig. 1, Fig. 3 is a block diagram showing a conventional magnetic field compensator, and Fig. 4 5 is a side sectional view showing the structure of a general superconducting magnet, and FIG. 5 is a sectional view taken along line A--A in FIG. 4. (21) to (23)...Power supply (101) to (112)...Saddle-shaped coils i1 to i3.
. . . Current Z . . . Main magnetic field U 1 .

Claims (5)

[Claims] (1) In a magnetic field compensator that compensates for the radial error magnetic field of a magnet device used in a magnetic resonance apparatus, a plurality of series components each arranged concentrically along a main magnetic field and symmetrically with respect to a plane perpendicular to the main magnetic field a plurality of compensation units each comprising a coil; and a plurality of power supplies for individually supplying current to the compensation units; each compensation unit containing a plurality of components of the radial error magnetic field; A magnetic field compensator characterized in that the plurality of components are simultaneously compensated for by the entire compensation unit, and each of the currents is determined according to the configuration of each compensation unit. (2) The plurality of compensation units include a first compensation unit consisting of four saddle-shaped coils arranged in a cylindrical shape, and a pair of four saddle-shaped coils arranged in a cylindrical shape on each side of the first compensation unit. The present invention is characterized in that it is comprised of a second compensation unit consisting of a coil, and a third compensation unit consisting of four saddle-shaped coils arranged concentrically over each of the saddle-shaped coils of the first compensation unit. A magnetic field compensator according to claim 1. (3) The plurality of compensation units include a first compensation unit consisting of four saddle-shaped coils arranged in a cylindrical shape, and a pair of four saddle-shaped coils arranged in a cylindrical shape on each side of the first compensation unit. 2. The magnetic field compensator according to claim 1, comprising a second compensation unit and a third compensation unit each comprising a coil. (4) The magnetic field compensator according to any one of claims 1 to 3, wherein the coil in each compensation unit is a superconducting coil. (5) The magnetic field compensator according to any one of claims 1 to 3, wherein the coil in each compensation unit is a normally conducting coil.