WO1993024051A1

WO1993024051A1 - Coil for generating gradient magnetic field on opposed type magnet

Info

Publication number: WO1993024051A1
Application number: PCT/JP1993/000697
Authority: WO
Inventors: Yuji Inoue
Original assignee: Ge Yokogawa Medical Systems, Ltd.
Priority date: 1992-05-26
Filing date: 1993-05-25
Publication date: 1993-12-09
Also published as: JP2700506B2

Abstract

A coil unit for generating a gradient magnetic field on an opposed type magnet comprises two coils through which spiral currents flow in opposite directions. The two coils are disposed on opposite sides of a region where a gradient magnetic field is generated. The linearity of the gradient magnetic field is improved without increasing inductance of the coil. Each of the mutually opposed coils includes a plurality of substantially circular and substantially concentric current paths, which are connected to form a spiral coil. The coil has a current distribution in which current density is low near its center and high near its periphery. The pitch of the spiral is substantially inversely proportional to the current distribution.

Description

Specification

Gradient magnetic field coil for facing magnet

: Ά i l field

The present invention relates to a facing magnetic field gradient coil for use in an MRI (nuclear magnetic resonance imaging apparatus). Background art

MRI is a device that obtains a tomographic image of a subject by focusing on a specific nucleus using a nuclear magnetic resonance phenomenon.

The nuclei such as H, F, Na, C, and P have their own magnetic moments. When these are placed in a static magnetic field H0 whose magnetic field is in the z-axis direction, these nuclei become Perform precession. The angular frequency ωθ of this precession is given by the following equation.

ω θ = r HO

7 is called the nuclear gyromagnetic ratio and is a constant peculiar to an atom. The magnetic moment ^ points in various directions, but if the average of // is M ', M' points in the z-axis direction. In this state, when an electromagnetic field having the same angular frequency as ωθ is applied from the X-axis direction, M 'starts to fall in the y-axis direction. At this time, if the receiving coil is arranged in the y-axis direction, a high-frequency current proportional to M 'is induced in the coil. As described above, in the MRI, when an electromagnetic wave having an angular frequency ωθ is applied to a subject in a static magnetic field, a high-frequency current flows through the receiving coil, and a signal from the living body can be obtained.

In MRI, a gradient magnetic field is used to obtain a slice image of a tomographic image and obtain positional information in a cross section in order to obtain a tomographic image in a living body.

As a method for generating a gradient magnetic field in the z-axis direction, which is the direction of the static magnetic field, a gradient magnetic field coil in which two coils for passing currents in different directions are arranged in the z-axis direction is used. Conventionally, in a magnet device constituting this magnetic field, a facing magnet made of a permanent magnet shown in FIG. 8 is often used as a magnet for a static magnetic field. The figure shows the cross section of the opposed magnet, and the magnetic field H 0 is applied in the z-axis direction. In the figure, reference numeral 1 denotes a permanent magnet that forms a static magnetic field, and is provided above and below the figure so as to face each other. 2 is a permanent magnet facing up and down The yoke 1 is magnetically connected to form a magnetic circuit, and 3 is a magnetic shunt provided between the upper and lower parts of the permanent magnet 1. The subject is inserted between the upper and lower permanent magnets in a direction perpendicular to the paper.

Fig. 9 is a diagram of a pair of Maxwell-type gradient magnetic field coils used in the magnet device using the opposed permanent magnets. Reference numeral 4 denotes a coil C having a radius R0 for generating a gradient magnetic field, and reference numeral 5 denotes a coil D having a radius R0 arranged in parallel with the coil 4. These coils C 4 and D 5 are installed in parallel at a fixed distance 2 Z0, and currents in opposite directions are supplied to both coils. In these coils, the current distribution is concentrated on the circumference of R0 from the center. Therefore, if the magnetic field on the central axis formed by one virtual coil is represented by H (Z), the magnetic field Hm ( Z) is as follows.

Hm (Z) = H (Z-ZO) one H (Z + Z0)

When this Hm (Z) is Taylor-expanded at Z = 0, it becomes a power series consisting only of odd-order terms of Z. Therefore, if the coefficient of the third-order term of Z is set to 0 in order to obtain a linear gradient magnetic field with respect to Z, the following optimal relationship between the coil interval Z0 and the radius R0 is obtained.

Z0 = 0.866 R0

In such Maxwell type coils, the linearity is not good because higher-order terms of the fifth or higher order of Z remain in the process of calculation.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has as its object to increase the gradient of a gradient magnetic field coil having good linearity of the gradient magnetic field without increasing the inductance of the coil in order to avoid an increase in the capacity of the drive power supply. It is to provide a magnetic field coil. Disclosure of the invention

A first invention for solving the above-mentioned problem is a gradient magnetic field coil for opposed magnets comprising two coils which are opposed to each other with a substantially circular current flowing in opposite directions and sandwiching a gradient magnetic field generation region. Each of the opposed coils is provided with a plurality of substantially circular and substantially concentric current paths. The current paths include a region where the current density is low near the center of the coil and a region where the current density is high near the edge of the coil. Must have a current distribution that includes It is characterized by the following.

The second invention is characterized in that, in addition to the above, a plurality of substantially circular and substantially concentric current paths in each coil are arranged at a pitch substantially inversely proportional to the current distribution. is there.

The third invention is characterized in that, in addition to the above, a plurality of substantially circular and substantially concentric current paths in each coil are substantially spiral continuous current paths. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic configuration diagram of a gradient coil according to an embodiment of the present invention.

FIG. 2 is a diagram showing a pattern of a gradient magnetic field coil according to one embodiment of the present invention.

FIG. 3 is a diagram showing an outer shape for coil pattern design.

Figure 4 is a flowchart of the procedure for designing a coil pattern based on a file.

FIG. 5 is an explanatory diagram of a method of obtaining a coil winding position from a current density curve. FIG. 6 is a characteristic curve diagram showing a linearity error of a conventional coil.

FIG. 7 is a characteristic curve diagram showing a linearity error of the coil according to the present invention. FIG. 8 is a cross-sectional view showing a schematic structure of a facing magnet of MRI.

FIG. 9 is a schematic diagram of a Maxwell coil. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram of one embodiment of the present invention. In the figure, 11 is a coil A having a plurality of substantially circular and substantially concentric patterns formed on a plane perpendicular to the z-axis, and 12 is a coil A 11 on a plane parallel to the plane of the coil A 11. In the coil B having the same pattern, the centers of the concentric circles of the coil All and the coil B 12 are located on the z-axis or a straight line parallel to the z-axis. Normally, 2 Z 0 is selected as a fixed value, for example, 450 bandits, due to the relationship between facilities.

Figure 2 shows the actual detailed patterns of the coils A ll and B 12, which are composed of copper wires with a diameter of about 2 h. The leads are taken out at C l and C 2. This coil A 1 1, As shown in Fig. 5, the current distribution pattern of both current paths of B12 is such that the current distribution is as shown in Fig. 5, where the current density is low near the center of the coil whose radius is 0 to r2, and the radius is r3 to The current distribution is set to include a region with a high current density near the edge of the coil. In this current distribution, the number and position of small maxima and minima near the center, the rate of increase near the edge, and the like vary slightly depending on the magnitude relationship between Z0 and R0. Further, since the pitches of a plurality of substantially circular and substantially concentric current paths are set so as to be substantially inversely proportional to the current density, the current values flowing through the respective substantially circular current paths may be the same. Therefore, as shown in FIG. 2, a plurality of substantially circular and substantially concentric current paths can form a substantially spiral continuous pattern. Since the coil shown in Fig. 2 has a small number of turns, the local maximum of the small current density near the center is hidden by the pattern interval of the current path.However, if the number of turns is increased so that the local maximum of the current density appears clearly Accuracy improves.

Specifically, this coil is made by making a groove in the disk in the shape of the coil pattern shown in Fig. 2, and burying a copper wire in the groove.

Next, a method of designing patterns of the coils A11 and B12 (hereinafter, simply represented by the coil A11) will be described. FIG. 3 is a diagram showing the outer shape of the coil A11. The horizontal axis has r, and the radius of the coil Al1 is R0. In this coil Al1, the current distribution: 求める · (Γ) is obtained by the following equation in order to find the pattern that minimizes the error in the current distribution at each point on the coil plane.

J (r) = ∑an sin {(nττ / 2) (r / RO)} + ∑bm cos {(m) (r / RO)} n = l m = l ...手順 The procedure for this calculation is described below.

(1) Calculate the magnetic field generated when a unit current is applied to the coil having the current distribution of each term included in equation (1) according to Pio-Savart's law, and create a magnetic field data file A. For example, if the coefficients a 2 to a 6 and bl to b 6 of the terms other than a 1 are set to zero, a unit current is applied to a coil having a current distribution of J (r) = al · sin {ίπ / 2) (r / RO)}. Is calculated, and the magnetic field at each point in the area where a linear gradient magnetic field is required is calculated. In the same way, if the same calculation is performed for the terms a 2 and a 3 one after another, file A can be created and O ₀

The above procedure is the procedure for creating file A, which is created in advance. Next, an example of the procedure for setting the pattern of the coil A11 using the file A created as described above will be described with reference to the flowchart of FIG.

step 1

Set the gradient magnetic field linearity error to a desired limit, for example, 2% or less. Step 2

Select n and m with reference to file A, and find the optimal solution by a linear programming method or a least-squares method for each of al to an and bl to bm so that the gradient magnetic field is linear. That is, the linear programming is performed so that the gradient magnetic field calculated based on the coefficients of al to an and bl to bm according to the Beos-Savart law has a linearity error of 2% or less at each point in the area where the gradient magnetic field is required. Perform the operation to find the optimal solution by the method of least squares.

Step three

Check whether the optimal solution was obtained. If not, go to step 4. If so, go to step 7.

Step 4

In order to obtain the optimal solution, consider whether to relax the tolerance of the linearity error set in step 1. If loose, increase the tolerance and return to step 2. If not, go to step 5.

Step 5

Consider whether to find the optimal solution by increasing the number of n and m in a n and b in. If not, return to step 2. If more, go to step 6.

Step

Increase and set the number of n and m, and return to step 2 to calculate for the increased a n and b m. Step 7

Current distribution: Find the curve by the optimal solution of i (r) and calculate the coil pattern as shown below.

According to the above procedure, in the region shown in FIG. 3, the value of each r on the horizontal axis, that is, the current distribution J (r) on the circumference of each radius is obtained, and the current distribution curve 21 of FIG. 5 is obtained. This curve is one example. Let S be the area between the current distribution curve 2 1 and the r-axis. Assuming that the number of turns of the coil is n, the area obtained by dividing the area S by n is ΔS. Rl, r2, ..., rn, R0 in the figure are points at which the area S according to the current distribution curve 21 is divided into equal areas ΔS, and the winding position is set to a position at which ΔS is divided into two equal parts. Choose.

The positions of the windings selected as described above are as shown in Fig. 2. A concentric pattern is composed of a copper wire of about ø2. The lead outlets are C I and C 2. The specific manufacturing method is to machine a groove corresponding to the above-mentioned coil pad with a NC machine or the like on a disk with a diameter of about 80 mm and a thickness of about 5 mni formed by FRP, and embed a copper wire in the groove. Make it.

The characteristics of the gradient magnetic field coil of the present embodiment obtained as described above are compared with those of a conventional Maxwell-type gradient magnetic field coil.

Figure 6 shows the characteristic curve of a Maxwell coil. In this curve diagram, the horizontal axis shows the z-axis coordinates with respect to the zero point on the z-axis shown in Fig. 1, and the vertical axis shows the linearity error of the gradient magnetic field. The parameter X of each curve is the radial distance of coil A11. As is clear from the figure, the linearity error at each position in the radial direction varies greatly as it approaches the coil surface.

FIG. 7 is a characteristic curve obtained by the gradient magnetic field coil of the pattern obtained according to the present embodiment. The vertical axis and the horizontal axis are the same as in FIG. As is clear from this figure, the linearity error at each radial position is extremely small and within ± 3%.

Next, the inductance and the resistance of the Maxwell type gradient magnetic field coil and the gradient magnetic field coil of the pattern of the present embodiment are compared.

The following Table 1 shows the inductance and resistance of the coil necessary to obtain a gradient magnetic field strength of 1 gaussZcm by applying a current of 7 OA. Inductance (mH) Resistance (Ω) Number of turns (turn) Max.

(R0 = 260mm) 0.5 3 0. 1 5 2 8 Coil of this embodiment

(2 Z0 = 450mm) As is clear from the table, the gradient magnetic field coil of this embodiment has a larger number of turns but a smaller inductance than the Maxwell type gradient magnetic field coil.

As described above, according to the present embodiment, since the linearity is good, the image distortion caused by the gradient magnetic field can be improved as compared with the related art. Conventionally, image distortion has been corrected in image processing, and there have been problems such as long processing times and the occurrence of artifacts.However, since linearity errors can be kept within an allowable range. The distortion correction at the image processing stage is no longer necessary.

—On the other hand, since the inductance is almost the same as before, or rather less, it is not necessary to increase the power supply capacity.

The present invention is not limited to the above embodiment. In this embodiment, an example in which a coil pattern is formed by a copper wire has been described, but the coil pattern may be formed by etching a copper plate. Industrial applicability

As described above in detail, according to the present invention, a gradient magnetic field coil having good gradient magnetic field linearity can be realized without increasing the inductance of the coil, and the practical effect is great.

Claims

The scope of the claims

(1) In an opposed magnet gradient magnetic field coil composed of two coils installed opposite to each other across a gradient magnetic field generation region by flowing substantially circular currents in opposite directions, each of the coils installed opposite each other has a plurality of coils. It has a circular and substantially concentric current path, and the current path has a current distribution including a region where the current density is low near the center of the coil and a region where the current density is high near the edge of the coil. This is a gradient magnetic field coil for facing magnets.

2. The opposed magnet according to claim 1, wherein a plurality of substantially circular and substantially concentric current paths in each coil are arranged at a pitch substantially inversely proportional to the current distribution. Gradient field coil.

3. The gradient magnetic field coil for opposed-type magnet according to claim 2, wherein the plurality of substantially circular and substantially concentric current paths in each coil are substantially spiral continuous current paths.