JPS6262301B2 - - 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 can be used to design the conductor spacing. I can't. That is, this is because such a relational expression presupposes that the conductor is in close contact with the pole piece. The present inventor has already proposed a magnetic field device that can obtain a uniform magnetic field gradient in the direction perpendicular to the main magnetic field direction even when the conductor is separated from the pole piece surface if the conductor arrangement satisfies a specific relationship. That is, as shown in Fig. 1, this device has four straight conductors parallel to each other between opposing electromagnetic pole pieces 1 and 2 for applying a DC magnetic field as the main magnetic field, without contacting the pole pieces. 3a-3
d, 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. , and a DC current of the same direction and magnitude is supplied to each of the conductors.More specifically, in the above device, the center of the rectangle 6 is located at the opposing center 0 of the pole piece surfaces 4 and 5. At the same time, the distances between each side of the rectangle 6 parallel and perpendicular to the pole pieces 4, 5 and the above center 0 are respectively d and a, and the distance between the pole piece surfaces 4, 5 and the above center 0 is l. Then, these d, a and l are Re[(cosec ² α-2/3)cosec ² α]=0 (1) where α=π/2(d/l-ia/l) i: complex Unit Re: The value that satisfies the real part in the following [ ]. Next, a method for deriving the above equation (1) will be explained. The four straight conductors 3a to 3d are placed between the pole piece surfaces 4 and 5 with a magnetic pole spacing of 2l, and are placed at each vertex of a 2d x 2d virtual rectangle, and a current J is passed through one straight conductor. The magnetic field strength in the Z direction when
According to Savard's law, Hz=-J/2πRe[1/a+id-ξ]...(i) ξ=y+iz Re: Represented by the real component of the following [ ]. In equation (i) above, pole piece surfaces 4 and 5
Adding the contributions from all video currents to include the influence from becomes. This equation (ii) is the origin (pole piece surfaces 4, 5
When we perform series expansion on ξ at the center between 0), we get becomes. Here, C ₂ 〓 ₊₁ is a function only of a/l and d/l. Here, if C ₃ = 0, the above equation (iii) can be approximated as Hz=J/lC ₁ Re(πξ/2l)∝y (iv) where C ₁ =Re(cosec ² α) C ₃ =Re [(cosec ² α−2/3)cosec ² α] α=π/2(d/l−ia/l). Therefore, the condition for C ₃ =0 is as shown in equation (1) above. In FIG. 2, the correlation between d/l and a/l that satisfies the above relational expression is shown by a solid line. In the above device, as shown in FIG.
If the direction perpendicular to 5 is the z-axis and the center 0 is the coordinate origin, the magnetic field strength H near the origin where y/l < 0.5 and z/l < 0.5 is given by equation (iv) above, H =Ho+Hz=Ho+Gy・y (2) Furthermore, 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 Gy is the gradient in the y-axis direction. Gy〓πJ/2l ² Re〔cosec ² α〕 (3) (J 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 y-axis direction orthogonal to the main magnetic field direction. Now, in the above device, there was only one set of four conductors, but since the principle of superposition holds true for magnetic fields, it is possible to obtain the desired strength by arranging multiple sets in parallel as long as each one satisfies the above formula (1). Can be done. FIG. 3 shows this state, in which the conductors 8, 8, . . . forming each set are arranged substantially in a straight line. Note that this figure shows only the first quadrant of the coordinate axes, and the same applies to the other quadrants. However, in such a conductor arrangement, a small set of d comes in, and it has been found that as d becomes smaller, the uniform gradient magnetic field region becomes narrower. Figures 4a and 4b are obtained by calculating the magnetic field distribution from the position of the conductor using the Biot-Savart law, and connecting equal percentage positions with a curved line based on the slope of the origin. b indicates the gradient distribution corresponding to each point a and b in FIG. 2, respectively. As can be seen by comparing Figure 4 a and b, in the case of a large set of d that satisfies the above formula (1) (see Figure 4 b), both the ±0.5% line and the ±5% line are located almost isotropically. The uniform region is almost isotropic, whereas the ±0.5% line is almost isotropically located for the small set of d (see Figure 4a), but for the set of large d In comparison, it is isotropically narrower, and the ±5% line approaches the center 0 especially in the Z direction, and the uniform region becomes narrower in the Z direction. In addition, the radius of the dotted circle in FIGS. 4a and 4b is 1/3. Therefore, the present invention aims to increase the magnetic field strength without narrowing the uniform gradient magnetic field region. The present invention is based on the new finding that in the case of a set in which the value of a is slightly smaller than the above conditional expression, the uniform region becomes narrower in the y direction and wider in the z direction. The curve shown by the dotted line in FIG. 2 indicates a condition where the value of a is slightly small, and the gradient distribution corresponding to point c on the curved line is shown in FIG. 4 c. As is clear from a comparison between the same figure and FIG. 4b, for substantially the same value of d/l, if a becomes slightly smaller, the uniform region becomes narrower in the y direction and wider in the z direction. That is, the present invention corresponds to each value of d, a set of straight conductors having values of d and a that satisfy the above formula (1), and a set of straight conductors having approximately the same value of d and a slightly smaller value of a. It is characterized by the combination of
In principle, the present invention consists of a pair of sets.
It is most effective to combine a set of straight conductors with a value of a and another set with a value of d greater than the d of the set. In this case, the magnetic field strength distribution in Figure 4a and the magnetic field strength distribution in Figure 4c will be superimposed.For example, if you overlap a -5% area and a -5% area, you will get a -5% area. remains, -5
If you overlap the % area and 0% area, -2.5%
According to the principle of superposition, the narrow distribution in the Z-axis direction in Figure 4 a is corrected by the distribution in Figure 4 c, which is narrow in the Y-axis direction, expanding the uniform area and making it almost isotropically uniform. The result is a magnetic field. Furthermore, the fourth
The set of straight conductors at point b in Figure 2, which has the distribution shown in Figure b, itself forms a more uniform magnetic field than the straight conductor at point a in Figure 2, so there is no need for correction, and the straight line at points a and c is directly It is possible to add it to a conductor. However, in practice, more pairs are required to obtain sufficient magnetic field strength, so in such a case, it is necessary to arrange multiple pairs that satisfy equation (1) and to reduce the deterioration in the z direction due to this arrangement. is a slightly smaller than a that satisfies equation (1).
The correction is performed using a plurality of sets of arrays having . Figure 5 shows the third conductor arrangement in the embodiment of the present invention.
Similar to the figure, the first quadrant is representatively shown. This array consists of outer array conductors aligned in a nearly straight line.
It consists of A ₁ , A ₂ ... A ₉ and inner array conductors B ₁ , B ₂ ... B ₈ , and the outer array is along the solid line in Figure 2, that is, the relationship that satisfies the above formula (1). The inner arrangement is along the dotted line in FIG. 2, that is, the case where a is slightly smaller. The table below shows specific values for each of the above conductor arrangements. Note that l=30 mm.

[Table] FIG. 6a shows a gradient distribution similar to that in FIG. 4 in the above embodiment, and as is clear from the figure, the region of radius 1/3 is a uniform gradient magnetic field region of ±5% or less.
For comparison, the gradient distribution when the same number of conductors are arranged in the arrangement shown in FIG. 3 is shown in FIG. 6b. The table below shows specific values for each conductor arrangement in this case. Note that l=30 mm.

[Table] From the above comparison, the improvement effect of the present invention is clear. The value of each a of the conductor array used for correction is set to be slightly smaller than the value that satisfies the above formula (1), but the difference is preferably within a range of less than 0.05l, and the value satisfying the above formula (1) is set.
The range of d in the conductor arrangement satisfying (1) is preferably d/l≧0.7, and if a and d deviate from these ranges, correction becomes difficult. As is clear from the above explanation, according to the present invention, when arranging a conductor for imparting a magnetic field gradient in a direction perpendicular to the main magnetic field direction between opposing pole pieces, there is no need for the conductor to be in close contact with the pole piece surface. Since it can be spaced apart from the pole piece surface, a desired magnitude of gradient can be easily obtained, and furthermore, the magnetic field strength can be increased without narrowing the uniform gradient magnetic field region.

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views and curve diagrams for explaining the present invention, Figure 3 is a cross-sectional view showing an example of conductor arrangement, Figure 4 is a magnetic field gradient distribution diagram, and Figure 5 is a diagram showing the implementation of the present invention. Cross-sectional view showing an example conductor arrangement, Figures 6a and b
is a magnetic field gradient distribution diagram in the same example and comparative example. 1, 2...Pole piece surface, _A1 to _A9 , _B1 to _B8 ...
conductor.

Claims (1)

[Claims] 1 A plurality of sets of four straight conductors parallel to each other without contacting the pole pieces are provided between opposing pole pieces for applying a DC magnetic field, and each conductor of the set is connected to the above pole. A magnetic field device arranged to pass through each vertex of a virtual rectangle in an arbitrary plane perpendicular to the facing surface of the piece, and in which current is supplied in the same direction and the same magnitude to each of the conductors. , the center of the rectangle is located at the opposing center of the pole piece surface, and the distances between each side of the rectangle parallel and perpendicular to the pole piece surface and the center are d and a, respectively, and the pole piece surface ^If the distance ^between /l) i: Complex unit Re: Continued A magnetic field characterized by a combination of the above set of straight conductors that satisfies the real part in [ ] and another set that has a slightly smaller than a that satisfies the above formula. Device.