JPH01208815A - Generator for magnetostatic field - Google Patents

Abstract

PURPOSE:To lower eddy currents generated in a magnetic pole, to improve the linearity of the quantity of phase encoded and to enhance a ghost in the direction of phase of a picture image by arranging an electric good conductor generating eddy currents at the time of the change of a gradient magnetic field on the front of the magnetic pole. CONSTITUTION:A pair of magnetic poles 105 and 106 uniformly correcting the magnetostatic fields of a pair of permanent magnets 108 and 109 are attracted to the permanent magnets 108 and 109 fixed onto the opposite surfaces of a magnetic yoke. Gradient field coils 103, 104 shaping gradient magnetic fields into the magnetostatic fields and a plurality of electric good conductors 1 and 2 formed to a shape concentric and similar to these coils and disposed under mutually separate states onto the surfaces of each magnetic pole are mounted on each opposed surface side. When magnetic fields are changed on the rise and fall of the gradient magnetic fields, eddy currents are generated in the good conductors 1, 2, and the magnetic poles are shielded, thus remarkably reducing the generation of eddy currents in the magnetic poles, then preventing a ghost displayed in the direction of phase of a picture image.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a magnetic resonance imaging system fW (hereinafter referred to as MHI), and relates to the structure of a permanent magnet type static magnetic field generator that forms and supplies a main magnetic field. It is.

(b) In conventional MHI technology, position information is converted into frequency information by applying a gradient magnetic field in pulses to a space where a uniform static magnetic field is formed, and data is generated as an NMR (nuclear magnetic resonance) signal. Reconstruct the image by collecting and Fourier transforming.

Figure 10 shows the two-dimensional Fourier transform (hereinafter referred to as 2DFT).
90 atoms (e.g. 'H) in the subject, demonstrating the principle of the method.
"FID that appears after excitation with RF pulse (101)
signal < 102), first, the gradient magnetic field Gy in the Y direction is set to t
A gradient magnetic field Gx in the X direction is applied for y time and then for tx time, and the signal is observed after time tx. The information on the position in the X direction is given by the difference in frequency taX expressed as wx''r (Ho + Gx・X), and the information on the position in the Y direction is given by the difference in phase 9y of the signal observed during time tx, 9
It is given by y−y(Ho +cy・Y)・ty. Here, γ is the gyromagnetic ratio and Ho is the static magnetic field.

Therefore, we sequentially change the gradient of the gradient magnetic field cy, observe as many signals as the number of pixels in the Y direction of the image, and convert these signals into 2DFT.
Then, a two-dimensional image of the subject in the X and Y directions can be obtained.

In general, the frequency gradient (Gx) is always applied with a constant magnitude, and the phase gradient (Gy) is one in which the intensity changes linearly for each pulse. i) must vary linearly with respect to the projection.

Figure 9 shows a general permanent magnet type static magnetic field generator. In the figure, (107) is a yoke, (108) (109>
is a permanent magnet. Gradient magnetic field fill (103) (104
), eddy currents are generated in nearby metals, especially the magnetic poles (105) (106), and the rise and fall of the magnetic field formed by the gradient magnetic field coils (103) (104) is the gradient magnetic field fill (103) (104>
There was a problem in that a so-called "rounding" phenomenon was observed, in which the rise and fall of the current flowing through the circuit were delayed. Furthermore, the generated eddy current impairs the linearity of the phase encode amount, causing ghosts to appear in the phase direction of the image.

(c) Problems to be Solved by the Invention The problem to be solved by the present invention is to reduce the eddy currents generated in the magnetic poles of a permanent magnet type static magnetic field generator and eliminate ghosts that appear in the phase direction of images. .

(d) Means for solving the problem A magnetic yoke forming a closed magnetic circuit, a pair of permanent magnets fixed to opposing upper and lower surfaces of the magnetic yoke,
a pair of magnets that are attracted to each opposing surface of these magnets and uniformly correct the static magnetic field formed by the magnet; and a pair of magnets that are attached and positioned on each opposing surface of the magnetic poles to form a gradient magnetic field within the static magnetic field. The present invention is a static magnetic field generating device that includes a gradient magnetic field coil and a plurality of electrically conductive conductors formed in a similar shape concentrically with the gradient magnetic field coil and arranged in a separated state on the surface of the magnetic pole.

(e) By generating eddy currents in the electrically conductive material placed in front of the working magnetic pole, magnetic field changes at the rise and fall of the gradient magnetic field are suppressed from reaching the magnetic poles, reducing the eddy currents generated at the magnetic poles. let As a result, the linearity of the amount of phase enhancement is improved, and ghosts appearing in the phase direction of the image are improved.

(F) Example Hereinafter, the static magnetic field generator of the present invention will be described in detail with reference to an example of the drawings. Fig. 3 shows a sectional view of the main parts of a permanent magnet type static magnetic field generator.
106) and the gradient magnetic field coils (103) and (104), a vortex is generated in the electrically good conductors (<1) and (2) at the rise and fall of the gradient magnetic field. 0 For example, when the current shown in Fig. 3 (a) is passed through the gradient magnetic field coils (103) (104), when the current rises, the gradient magnetic field is Flows (3) and (4) are generated in a vortex in the opposite direction to the current direction of fill (toa>(t04). In the figure, ■ is the direction that penetrates the paper from the front to the back, and ■ is the direction that penetrates the paper from the back to the front. Due to these eddy currents (3) and (4), which represent the 'Wl flow in the penetrating direction, the magnetic i@ (105) and (106) can be shielded from changes in the magnetic field, and the vortices generated at the magnetic poles (105 and 106) The current is significantly reduced.Also, the gradient coil (10
3) When the current of (104) falls, the magnetic pole (105)
) (106) are reduced.

Next, an example of a two-gradient magnetic field coil is shown in the perspective view of FIG. 1(a) and the vertical sectional view of FIG. 1(b).
On top of 6), a good electrical conductor (21) cut into a ring shape.
~ (27) Place the board and place 2 circular films (5
) are placed. Since the eddy current generated in the electrical conductors (21) to (27) by the second coil (5) flows concentrically, the eddy current will not be reduced by cutting it into a ring shape, that is, the magnetic pole (106) The effect of shielding the magnetic field from the rise and fall of the gradient magnetic field is equivalent to that of the integrated electrical conductors (1) and (2). However, cutting into a ring shape has the effect of suppressing the eddy currents generated in the electric bipaths (21) to (27) by the X gradient magnetic field fill and the Y gradient magnetic field coil, and the two gradient magnetic field filters can be used independently. It is possible.

Also, the larger the eddy currents generated in the electrically conductive conductors (21) to (27), the greater the effect of shielding the magnetic pole (106) from gradient magnetic field changes. A good electrical conductor (
It is necessary to limit the eddy currents generated in (21) to (27). For that purpose, good electrical conductors (21) to (27
) as thin as possible, and place the ring-shaped plate on the magnetic pole (1
06) Take measures such as not placing it all over the surface, but placing it only in areas where it has a large effect. Figure 6 shows a ring-shaped plate (28)
An example in which ~(30) is partially arranged is shown.

The ring-shaped electrically conductive conductors (21) to (30) may be made of any non-magnetic electrically conductive material that does not affect the uniformity of the static magnetic field, but lightweight aluminum is preferable, and the ring width The narrower the field, the greater the effect of suppressing the generation of eddy currents caused by the X and Y gradient m-field coils, but as a result of actual application,
It has been found that a width of several G is sufficiently effective.

Furthermore, although it is desirable that the ring-shaped plates (21) to (30) be electrically insulated from each other, the effects of the present invention will not be diminished even if they are partially electrically connected.

In the above, we have explained a circular two-gradient magnetic field coil, but polygonal two-gradient magnetic field coils such as triangles and squares are also possible. In that case, the electrically conductive material is divided into rings with the same shape as the two-gradient magnetic field coil. The structure is as follows.

Next, an example will be explained with reference to the X gradient magnetic field fill. X gradient magnetic field s il (61) ~ (64), (65) = (
68) has a symmetrical shape with respect to the XY plane and the YZ plane, as shown in FIGS. 4(b) and 5(b), for example.

Eddy currents generated in a good electrical conductor have a current distribution in the same shape as the X-gradient magnetic field coil, so in the cases of Figures 4(b) and 5(b), the eddy currents in Figures 4(a) and 5(b), respectively. Figure 5 (a
) Divide the electrically conductive material like (71) to (80), (8
1) to (92), two gradient magnetic field films,
X without generating eddy currents due to Y gradient magnetic field coils.
Measures that are independent of the gradient magnetic field fill are possible. X of other shapes
Regarding gradient magnetic field coils, the present invention can also be implemented by dividing the electrically conductive material into the same shape as the X-gradient magnetic field coil.

In addition, the Y gradient magnetic field coil has a structure in which the X gradient magnetic field coil is rotated 90 degrees around two axes, and the electrical conductor has a structure in which the one for the X gradient magnetic field coil is rotated 90 times around two axes. Good.

As described above, by combining good electrical conductors with unique shapes for the x, y, and X gradient magnetic field coils, it is possible to eliminate ghosts in the phase direction of images when using arbitrary directions for phase enrichment .

In addition, by cutting each ring-shaped electrical conductor to create a structure that can be mechanically opened and shorted, the optimal ring arrangement can be easily determined according to the magnetic pole shape, and variations in the manufacturing of the static magnetic field generator can be easily determined. It is possible to absorb the influence of changes in the magnetic pole shape due to etc. A structure that can be opened and shorted, such as a switch mechanism (10), is a ring (1) as shown in Figure 7.
1), or it may be provided outside the static magnetic field generator using a lead (12) as shown in FIG.

(G) Effects of the Invention As described above, the present invention comprises a magnetic yoke forming a closed magnetic circuit, a pair of permanent magnets fixed to the opposing upper and lower surfaces of the magnetic yoke, and each of these magnets. a pair of magnetic poles that are attracted to opposing surfaces and uniformly correct the static magnetic field formed by the magnets; and a pair of magnetic poles that are attached to the opposing surfaces of the magnetic poles,
a gradient magnetic field coil that forms a gradient magnetic field within the static magnetic field;
Since the gradient magnetic field coils are formed in seven concentric similar shapes and are equipped with a plurality of electrically conductive conductors separated from each other on the surface of the magnetic poles, eddy currents generated in the magnetic poles can be reduced and phase encoded. This has the effect of improving the linearity of the quantity and reducing ghosting in the phase direction of the image.

[Brief explanation of the drawing]

Figures 1-8F5! :J is related to the present invention, and FIG. 1(a) is a perspective view showing an embodiment of a two-gradient magnetic field coil of a static magnetic field generator. : (b) is the same vertical cross-sectional view of (a), the second
The figure is a longitudinal cross-sectional view of the main part of the static magnetic field generator, Figure 3 (a) is a time characteristic diagram of the current flowing through the gradient magnetic field coil; (b) is (a)
Figure 4 (a) is a diagram showing the eddy current distribution caused by the current of
) is an overview perspective view showing one embodiment of the X-gradient magnetic field coil; (
b) is a diagram showing the arrangement of the X-gradient magnetic field coil in a static magnetic field, and FIGS. 5(a) and (b) are other embodiments of the X-gradient magnetic field coil corresponding to FIGS. 4(a) and (b). Figure 6 shows the first
A diagram showing an example of a pond of good electrical conductor corresponding to Figure (a),
FIG. 7 is a perspective view showing an example of a switch mechanism with good electrical conductivity, FIG. 8 is a view showing another example of a switch mechanism corresponding to No. 7110, and FIG. 9 is a conventional permanent magnet type static magnetic field FIG. 10, a longitudinal cross-sectional view of the generator, is a diagram explaining the principle of the 2DFT method. (107) ... Magnetic yoke, (108) (109)
...Permanent magnet, (105) (106)...Magnetic pole, (1
03) (104), (5), (61) to (68)...
Gradient magnetic field coil, (1) (2), (11), (21)
~(30). (71) to (92)... Good electrical conductor.

Claims (1)

[Claims] (1) A magnetic yoke that forms a closed magnetic circuit, a pair of permanent magnets fixed to opposing upper and lower surfaces of the magnetic yoke, and a static magnet that is attracted to each opposing surface of these magnets and formed by the magnets. a pair of magnetic poles for uniformly correcting a magnetic field; a gradient magnetic field coil mounted on each opposing surface of the magnetic poles to form a gradient magnetic field within the static magnetic field; and a gradient magnetic field coil having a similar shape concentric with the gradient magnetic field coil. a plurality of electrically conductive conductors formed on the surface of the magnetic pole and arranged in a separated state from each other.