JPS5949681B2 - Pole piece for magnet device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnet device for a nuclear magnetic resonance apparatus that requires high magnetic field uniformity, and in particular is equipped with a magnetic field distribution correction device such as a current shim, and has a magnetic field uniformity higher than that of a single magnet. This relates to the shape of a pole piece in a magnet device in which uniformity is to be achieved.

In a nuclear magnetic resonance apparatus, a high degree of uniformity is required for the static magnetic field in which a sample is placed. FIG. 1 shows an outline of a magnet device used in a magnetic resonance apparatus. In Fig. 1, 1 is a yoke, 2, 2' is an excitation coil, 3, 3,
is a truncated conical core, 4 and 4' are spacers made of pure iron that are also truncated conical, 5 and 5' are pole pieces attached to the tips of the magnetic poles, and 6 and 6' are magnetic field distribution correction devices called current shims. A correction magnetic field is generated by combining a large number of coils wound in a circular or rectangular shape, and is used for the purpose of correcting the magnetic field distribution and improving uniformity. Since the shape of the pole piece has a great effect on the magnetic field distribution, many studies have been conducted regarding the shape, and various shapes have been proposed.

At present, the shape that is widely used in practical use as a magnet for nuclear magnetic resonance apparatus is a logarithmic curved surface given by the following equation proposed by Huber and Primus. -(+)g2Zl-
9R1-Rr-, Itn()- to (Z1-Z2)π-t
an to 2Z2-9 This logarithmically curved surface means that the cross-sectional shape of the outer circumferential surface b connected to the tip plane a of the pole piece is a curved surface expressed by the logarithmic function of the above equation.

In the above equation, (Z,, R1) and (Z2, R,) are coordinates on the curved surface, g is the gap length, and is a negative value such that tan-"1 is between o and π, The optimum value is determined through experiments.

By the way, as shown in Figure 1, the quality of the magnetic field uniformity has traditionally been measured by measuring the magnetic field in two orthogonal directions (x(5y)) on the air gap center plane (plane equidistant from the two pole pieces) Z=0. Regarding distribution, when the difference between the magnetic field strength at the center of the magnetic field and the magnetic field strength at a point a certain distance from the center, or the change in the magnetic field outward from the center, is expressed by a quadratic curve, it is expressed by the coefficient of the quadratic curve. However, in reality, the uniformity of the magnetic field is required in a spatially spread region, so it is not only the distribution on the center plane, but also the distribution from the center plane to the ball piece surface. We must also consider the magnetic field distribution in a plane that is somewhat close to .
In addition, in a magnet that corrects the magnetic field distribution using a current shim and aims to obtain a higher degree of magnetic field uniformity than that obtained with a single magnet, it is necessary to use not only the second-order coefficient described above but also the fourth-order coefficient. The degree to which this has increased has a great deal to do with the ease of correction.

This fourth-order coefficient refers to the fourth-order coefficient when the change in the magnetic field from the center to the outside is represented by a fourth-order curve. Table 1 was created based on the above formula. The second-order and fourth-order magnetic field inhomogeneity components (coefficients) measured for three types of pole pieces are Z=0, Z=51jE1! and Z=101n.

Here, as the pole piece, permendur (C
O-49%, Fe-49%, V-2%) is used, and the magnetic field strength BO is 2.0T.

Also, the conditions given to determine the shape are the gap length g
is 32U, and the tip diameter Df is 1401n1B and C in example A.
So 110? , the shape factor k is -0.11 for A and C, B
Then, -0.05, and the angle θf connecting the logarithmic surface b and the pole piece tip plane a is 4 for A, 1 for B, and 1 for C.
is assumed to be 2, and x10-6 is omitted for each coefficient. From the table above, the quadratic coefficient on the central plane is negative for A and B, and positive for C, but both are 10
It shows a small value on the order of -6.

However, the magnitude of the second-order coefficient decreases as it approaches the pole piece surface, while the sign remains the same in the case of A. On the other hand, in case of B, the sign is inverted and becomes a large value, and in case of C, it is a positive value from the beginning and the value is increased. Therefore, when trying to obtain a high degree of uniformity as a whole within a certain expanse of space, if A has a negative value on the center plane and decreases as it approaches the pole piece surface, then the pole piece like 6C It turns out that this is more advantageous than increasing as you get closer to the piece. In the case of B, the sign is reversed in the middle, and the quadratic coefficient is small as a whole, so when using the magnet alone, that is, without using a current shim, the highest uniformity can be achieved. I can say that I am giving.

It can be considered that this condition coincides with what has been conventionally recommended even when using current shims. However, in the case of B, the quadratic coefficient is located between the center (Z=0) and Z
=10? 13×10 −6 CIL−2 also changes, which means that the fourth-order magnetic field inhomogeneity expressed by (X2+Y2)Z2 is strongly included.

For A and C, this value is small. Also, the fourth-order coefficient ((X
2+Y2)2 coefficient) is -0.8 for B.
It shows a large change from -7.9×10-6, and C
The case is also not small.

On the other hand, in A, not only the fourth-order coefficient itself is small, but also the change in the coefficient when z is changed from O to 10n is also very small.
In the case of a magnet device that is based on magnetic field distribution correction using the aforementioned current shim, it is desirable that the magnetic field distribution be expressed only by low-order terms as much as possible, and the more high-order terms are included, the more difficult the correction becomes. Moreover, even if it is the same fourth-order term, there is already a shim that corrects the term expressed as simple X4, y4, but the above-mentioned (X2+Y2)Z2
Since no shim has ever been created to correct this term, it cannot be corrected at all. Therefore, the desirable characteristics of a magnet that is premised on correction by current shims are that the quadratic and quartic coefficients themselves are small, but even if the coefficients themselves are somewhat large, the change in the coefficient due to Z is small, that is, Z
It is extremely important that dependence is small.

From this point of view, the pole piece B whose quadratic and quartic coefficients change significantly between Z=0 and Z=10n is not desirable in a magnet that assumes correction by current shims, and the pole piece A whose changes are extremely small. It can be said that a pole piece of is desirable. The inventors next investigated under what conditions the pole piece should be made to always obtain a preferable magnetic field distribution as in case A. The table shows θf as 1
is constant, Df = 1101 constant, k is -0.33, -
It shows the changes in the second-order and fourth-order coefficients when the coefficients are changed to three types: 0.05 and -0.11.

Magnetic field strength B. is 2.0T, and XlO-6 is omitted from each coefficient. In the table above, looking at the quadratic coefficient at Z=0, it is positive only when k=0.03, and negative when k=-0.05 and -0.11.

Considering also that when θf = 2°, the quadratic coefficient is positive even at k = -0.11 (Table I), at least when θf is smaller than 2°, Z = 0. It seems that the second-order coefficient does not depend much on k and takes a value in the range of +3 to -3×10 Hd. In these cases, the quadratic coefficient goes in a positive direction near the pole piece, but nothing showing the desirable tendency shown above is observed. That is, in the magnetic field before the saturation of the pole piece (for example, 2.2 kilogauss or less when using a FeCO pole piece), the magnetic field distribution does not depend much on k, and it is difficult to find an appropriate magnetic field distribution by selecting k. Can not. The table shows k as -0
．． 11 and shows the second-order and fourth-order coefficients and their Z dependence when the cutting angle θf is changed between 1° and 4°.

The magnetic field strength BO is 2.0T, and XIOI is omitted from each coefficient. It is clear from this table that when θf is greater than 3°, the changes in the second-order and fourth-order coefficients at Z = 5n and 10n take the desired form described above.

or,. When θf is 3° or 4°, the second-order coefficient at z=o always shows a negative value, except under special conditions where strong strain remains in the pole piece. It was extremely stable.

From the above points, the cutting angle θ at the tip of the pole piece
It can be seen that by increasing f, the values of the second-order and fourth-order coefficients and the Z dependence of each coefficient can be reduced.

The inventor conducted further experimental studies, and found that the lower limit of θf is positive at around 2°, which is undesirable, but around 2.5°, it becomes a negative value and can be used satisfactorily. .

The upper limit of θf is relatively magnetic field strength B.

When BO is small, there is a region where each coefficient is relatively small even if it exceeds 4°, but when BO is large (for example, about 2.35T), both the second and fourth order coefficients become considerably large, making it impractical. At the same time, the angle becomes too large and exceeds the level of a curved pole piece, so it is appropriate to set it around 4 degrees. There are two ways to take the angle of θf, one of which is
The first method is to cut off the curved surface part b at the point where the angle formed with the tip plane a of the pole piece 5 or 5' reaches a predetermined value θf, as shown in FIG. 2a, and form the plane a from there. It is.

The other method is to cut off the curved surface part b at the point where the angle formed with the tip plane a reaches a predetermined angle θf, as shown in FIG. 2b,
This is a method of connecting this to plane a with a straight line (actually a conical surface) c that forms an angle of θf with plane a. As detailed above, in a pole piece whose outer peripheral surface is a logarithmically curved surface (a similar curved surface is also acceptable), the angle θf formed by the end of the logarithmically curved surface and the tip plane is set in the range of approximately 2.5 degrees to 4 degrees. By this, the second-order and fourth-order coefficients of the magnetic field distribution can be reduced, and IC. Since the Z dependence of each coefficient can be made extremely small, it is possible to obtain a magnetic field that is extremely easy to correct using a magnetic field distribution correction device such as a current shim, and as a result, it is possible to obtain a magnet device for a nuclear magnetic resonance apparatus with extremely high magnetic field uniformity. can.

[Brief explanation of drawings]

Figure 1 is a diagram showing the outline of an electromagnet for nuclear magnetic resonance equipment, Figure 2
The figure is a diagram for explaining how to take θf. 1...Yo Kazuhisa 2, 2'...Excitation coil, 3,3'...Core 4,4'...
・Pure iron spacer, 5,...Pole piece, 6
, 6'... Current shim.

Claims (1)

[Claims] 1. A magnet device for a nuclear magnetic resonance apparatus comprising magnetic poles arranged to face each other, a pole piece attached to the magnetic poles, and a magnetic field distribution correction device arranged between the pole pieces,
The outer circumferential surface of the pole piece near the plane of the tip is processed into a logarithmically curved surface or a similar shape, and the angle θ_f formed by the end of the logarithmically curved surface and the plane is set in the range of approximately 2.5 degrees to 4 degrees. A magnet device for a nuclear magnetic resonance apparatus characterized by: