JPS6321155B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic field device used, for example, in nuclear magnetic resonance.

When measuring molecular diffusion in a liquid or forming two-dimensional or three-dimensional images of spin density by nuclear magnetic resonance, it is necessary to create a gradient in the magnetic field strength in a predetermined direction in a main magnetic field in a fixed direction.

Conventionally, in order to form such a magnetic field, a plurality of mutually parallel linear conductors are closely arranged on the opposing pole piece surfaces of the main magnetic field generating DC electromagnet, and the spacing between the conductors is set according to a specific relational expression. It is known that current can be passed in a predetermined direction through these conductors.

However, in the above conventional method, in order to obtain a larger magnetic field gradient, it is necessary to move the conductor away from the pole piece and closer to the opposing center of the pole piece, but in this case, the above conventional relational expression is used to design the conductor spacing. It is not possible. That is, this is because such a relational expression presupposes that the conductor is in close contact with the pole piece surface.

For example, Journal of the Institute of Electronics and Communication Engineers '78/6, VoL.J61
-C No. 6, (1978), pages 396 to 397, a coil is placed directly within the magnetic pole gap 2Z ₀ , and when a current in the same direction is passed through this straight coil, the center of the magnetic pole gap The optimum position of the straight conductor is determined by determining the value of η=Y ₀ /Z ₀ under the condition that the _distance Y 0 of the straight coil from is calculated. However, if it is desired to create a magnetic field gradient parallel to the main magnetic field direction by passing current in different directions through the linear coil, the optimum position of the linear coil cannot be calculated using the above calculation method. Moreover, although the straight conductor according to the above calculation method may create a uniform magnetic field along the Y axis, there is a problem in that it cannot necessarily be said that a uniform magnetic field can be created near the origin.

The present inventor has discovered that even when the conductor is separated from the pole piece surface, a uniform magnetic field gradient in the direction parallel to the main magnetic field direction can be obtained if the conductor arrangement satisfies a specific relationship.

That is, as shown in FIG. 1, the present invention provides four linear conductors 3a that are parallel to each other and do not come into contact with the pole pieces between the opposing electromagnet pole pieces 1 and 2 for applying a DC magnetic field as the main magnetic field. ~3d, and each conductor of the set is arranged to pass through each vertex of a virtual rectangle 6 in an arbitrary plane perpendicular to the opposing flat surfaces 4 and 5 of the pole pieces 1 and 2. A magnetic field device in which direct current is supplied in the same direction to each conductor on the same diagonal of the rectangle 6, and direct current in a different direction is supplied to each conductor on the same line parallel to the pole piece surfaces 4 and 5. It provides: That is, in the example shown in FIG. 1, an upward current with respect to the plane of the paper is passed through the conductors 3a and 3c, and a downward current is passed through the conductors 3b and 3d, respectively.

More specifically, the present invention provides the above-mentioned apparatus comprising:
The center of the rectangle 6 is located at the opposing center 7 of the pole piece surfaces 4, 5, and the pole piece surfaces 4, 5
The distances between each side of the rectangle 6 parallel and perpendicular to the center 7 are d and a, respectively, and the pole piece surfaces 4 and 5 are
If the distance between the center 7 and the center 7 is ^l , these d, a, and l are d/l-ia/l) i: Complex unit Im: A value that satisfies the imaginary part in the following brackets [ ]. The above conditional expression is obtained as follows. That is, from the Biot-Savart law, the magnetic field strength in the Z-axis direction formed by one straight conductor is Hz = -J/2πRe [1/a + id - ξ]... (i) ξ = y + iz Re: Continued [ ] Real part i: Complex unit J: Represented by the value of the current flowing through the conductor. Adding the contribution from the image current to the above equation (i) to include the influence of the pole piece, Hz=-J/2πRe〔 _∞ 〓 〓 ^m=-∞ (1/a+i(4ml+d)-ξ+1/a+i (4ml
+2l-d)-ξ)]...(ii). Furthermore, when formula (ii) is expanded into a series with respect to ξ, Hz=-J/2πRe〔 _∞ 〓 ^m=-∞ _∞ 〓 〓 〓 ⁼⁰ ξ〓(1/{a+i(4ml+d)}〓 ⁺¹ +1/{
a+i(4ml+2l-d)}〓 ⁺¹ 〕...(iii) From equation (iii) above, we can calculate the magnetic field strength formed by four parallel conductors with current flowing in different directions as shown in Figure 1: Hz=J/l _∞ 〓 〓 ⁼⁰ C ₂ 〓 ₊₁ Re〔 (iπ/2 ξ/l) ² 〓 ⁺¹ 〕 …(iv) C ₁ = −Im [cot α・cosecα] C ₃ = −Im [1/6−cosec ² α) cotα・cosecα] However, α= π/2(d/l-ia/l) 〓 〓 Im: The imaginary part in the following [ ] is obtained. For small ξ (near center 7),
If C ₃ = 0, the contribution of higher order terms can be ignored and formula (iv)
becomes Hz∝Z. That is, the optimal condition for forming a uniform gradient magnetic field in the direction parallel to the Z-axis direction near the origin (center 7) is C ₃ =-Im [(1/6-cosec ² α) cotα・cosecα] = 0... ...(1) α=π/2(d/l−ia/l). FIG. 2 shows the correlation between d/l and a/l that satisfies the above relational expression.

In the above device, as shown in FIG. If the direction perpendicular to is the z-axis, and the center 7 is the coordinate origin, the magnetic field H near the origin where y/l < 0.5 and z/l < 0.5 is: H=Hz=Ho+Gz・Z (2 ) becomes. In addition, Hz is the magnetic field strength in the Z-axis direction, Ho is the main magnetic field strength due to pole pieces 1 and ² , and Gz is the gradient in the Z-axis direction. is the current value flowing through one conductor, and all conductors 3a
It is the same value for ~3d. ) Also, the size of the pole piece surfaces 4 and 5 is set such that the opposing length thereof is sufficiently larger than l, and is practically three times or more, preferably five times or more.

Here, as is clear from the above equation (2), the obtained magnetic field has a uniform magnetic field gradient in the Z-axis direction parallel to the main magnetic field direction.

Next, to explain an example, l=30mm, d=23.1
When setting mm and a=25.5mm, these can be calculated using the above formula (1)
However, the gradient Gz of the magnetic field distribution due only to each conductor 3a to 3d is a plane perpendicular to the x axis (x = 0)
In the area within 10mm radius from center 7 -
It became uniform at 0.10 Gauss/mm・A. still,
The radius of the pole piece surfaces 4 and 5 is 150 mm.

In the above embodiment, there was only one set of four conductors, but since the principle of superposition holds true for magnetic fields, multiple sets can be arranged side by side as long as each one satisfies the above formula (1) to obtain the desired strength and gradient. Obtainable.

As is clear from the above explanation, according to the present invention, when arranging conductors carrying currents in different directions between opposing pole pieces in order to impart a magnetic field gradient in a direction parallel to the main magnetic field direction, such conductors are Since it does not need to be in close contact with the pole piece surface and can be spaced apart from the pole piece surface, a desired magnitude of gradient can be easily obtained, and the range of applications of this type of magnetic field device is expanded.

[Brief explanation of the drawing]

1 and 2 are a sectional view and a curved view for explaining the present invention. 4, 5... Pole piece surface, 3a to 3d... Conductor.

Claims (1)

[Claims] 1. At least one set of four straight conductors parallel to each other without contacting the pole pieces is provided between opposing pole pieces for applying a DC magnetic field, and each conductor of the set has the above-mentioned conductors. A current flows in the same direction to each conductor that is arranged to pass through each vertex of a virtual rectangle in an arbitrary plane perpendicular to the facing surface of the pole piece, and is located on the same diagonal of the rectangle, and also to the pole piece surface. In a magnetic field device in which currents in different directions are supplied to conductors on the same parallel line, the centers of the rectangles are located at opposing centers of the pole piece surfaces, and the rectangles are parallel and perpendicular to the pole piece surfaces. The distance between each side of and the above center is d
and a, and the distance between the pole piece surface and the center is l, then these d, a and l are Im[(cosec ² α-1/6)cot α・cosec α]=0, where α= π/2(d/l-ia/l) i: Complex unit Im: A magnetic field device characterized in that it is a value that satisfies the imaginary part in the following [ ].