JP3299308B2 - Gradient magnetic field coil unit, gradient magnetic field coil and MRI apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradient magnetic field coil unit and a gradient magnetic field coil used in an MRI (magnetic resonance imaging apparatus), and an MR having such a gradient magnetic field coil.
I device.

[0002]

2. Description of the Related Art An MRI is an apparatus for obtaining a tomographic image of a subject by focusing on a specific nucleus using a magnetic resonance phenomenon.

The nuclei such as H, F, Na, C, and P in the subject have individual magnetic moments μ. When these are placed in a static magnetic field H ₀ in which the direction of the magnetic field is the z-axis direction. These nuclei precess. The angular frequency ω ₀ of this precession is given by the following equation.

Ω ₀ = γH ₀ γ is called a nuclear magnetic rotation ratio and is a constant peculiar to an atom.
The magnetic moment μ is oriented in various directions. If the average of μ is M ′, M ′ is oriented in the z-axis direction. When an electromagnetic field having the same angular frequency as ω ₀ is applied from the x-axis direction in this state, M ′ starts falling in the y-axis direction. At this time, when the receiving coil is arranged in the y-axis direction, a high-frequency current proportional to M 'is induced in the coil. A tomographic image of the subject can be obtained based on the high-frequency current.

[0005] In MRI, a gradient magnetic field is used to obtain a tomographic image in a subject, to position a slice of the tomographic image and to obtain positional information in a cross section. As a technique 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.

[0006]

In a magnet device for forming a static magnetic field, a facing magnet using a permanent magnet as shown in FIG. 12 is sometimes used. The figure shows a 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, which is provided facing up and down in the figure. Reference numeral 2 denotes a yoke that magnetically connects the vertically opposed permanent magnets 1 to form a magnetic circuit, and reference numeral 3 denotes a magnetic shunt plate for equalizing a magnetic field. The subject is arranged between the upper and lower permanent magnets in a direction perpendicular to the paper surface.

FIG. 13 is a schematic structural view of an Anderson coil used for a gradient magnetic field coil in the x-axis direction or the y-axis direction in a magnet device using this opposed permanent magnet. As shown in the drawing, the Anderson type coil has two rectangular coils placed on the same plane,
It is a coil installed in a lower part on the axis in parallel with the coil.

In the drawing, 4A and 4B are upper coils placed on the same plane, 4A is a left upper coil, and 4B is an upper right coil. 5A and 5B are upper left coils 4 respectively.
A, the lower coil being located at a distance of 2Z ₀ directly below the right upper coil 4B, 5A the lower left portion coil, 5B is a right lower coil.

When the long side of the rectangle of each coil is L, the short side is W, and the interval between the left and right coils is S, W = 1.55
A coil is formed while maintaining the relationship of Z ₀ , S = 0.83Z ₀ . In such an Anderson coil, the linearity of the gradient magnetic field is not good.

It is an object of the present invention to provide a gradient magnetic field coil unit and a gradient magnetic field coil for obtaining a gradient magnetic field with good linearity, and to provide an MRI apparatus having such a gradient magnetic field coil.

[0011]

According to the present invention for solving the above-mentioned problems, a first invention is a current distribution J in a two-dimensional space.
r and Jφ are the following formulas

[0012]

(Equation 2)

Wherein the coefficients S _n and C _m in the equation are obtained by performing an optimization calculation so as to obtain the required linearity of the gradient magnetic field in the required space area, and the obtained coefficients S _n, the current distribution Jr gradient field coil obtained on the basis of C _m, and is characterized in that it comprises the step of determining a coil pattern on a two-dimensional plane using j.phi.

The second invention is characterized in that the coefficients S _n and C _m are calculated by using the least squares method so as to obtain the linearity of the gradient magnetic field. The third invention is characterized in that optimization calculations for obtaining the coefficients S _n and C _m so as to obtain the linearity of the gradient magnetic field are performed by using a linear programming method.

According to a fourth aspect of the present invention, there are provided two electric paths formed mirror-symmetrically on two symmetrical semicircular planes.
Consists of multiple arcuate spiral patterns connected in series,
The plurality of arcuate spiral patterns are characterized by having substantially concentric arc portions that gradually increase from the center of the circle toward the periphery.

According to a fifth aspect of the present invention, there is provided a pair of coil units according to the fourth aspect of the present invention, which face each other at a predetermined distance in a static magnetic field space.

[0017]

[Action] In the formula of the current distribution, the coefficient in the equation S _n, C _m
Is optimized by a linear programming algorithm or a least-squares method so that the desired performance can be obtained with the linearity of the gradient magnetic field generated by the two-dimensional current distributions Jr and Jφ. Determine.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a pattern diagram of a gradient magnetic field coil according to one embodiment of the present invention, and FIG. 2 is a layout diagram of the gradient magnetic field coil.
As shown in FIG. 1, the coil pattern is two electric paths formed mirror-symmetrically on two symmetrical semicircular planes, and each electric path is composed of a plurality of arcuate spiral patterns connected in series. Has a substantially concentric arc portion that gradually increases from the center of the circle toward the periphery. 2, 11 is a coil arranged at a distance of 2Z ₀ below the coil A, 12 is a coil A11 with patterns of Figure 1, made in a plane perpendicular to the z axis A
In the coil B having the same current pattern as that of the coil 11, the outermost pattern of the coil A11 and the coil B12 has a radius of R
It is ₀ .

FIG. 1 is a diagram showing current patterns of the coils A11 and B12. The configuration of this current pattern is obtained as follows. To determine a pattern that minimizes the error in the magnetic field potential at each point on the coil plane, the magnetic field potential u is determined by the following equation. The magnetic field potential of the coil A11 has a distribution of u = f (r / R ₀ ) · cos φ in polar coordinates. Here, it is assumed that the function of f (r) is expanded by a trigonometric function, and the details are shown in equation (1). Here, the current distribution is assumed to be limited to Z = ± Z _m.

[0020]

(Equation 3)

From the equation (1), the equations for the current distributions Jr and Jφ in the equations (2) and (3) are obtained. Here, Jr is an r component, and Jφ is a φ direction component. Jr and Jφ are shown below.

[0022]

(Equation 4)

The procedure for calculating the coil pattern from equations (2) and (3) will be described below. First, the current distribution at φ = 0 is determined. φ = 0 means to find a point on the x-axis. Therefore, Jr = 0 in the equation (2).

[0024] (3) In the equation, a magnetic field produced when a current of unit current in the coil having a current distribution of each term alone contained in the formula to calculate the Biot-Savart law, for example, S ₁ except term Assuming that a unit current is applied to a coil having a current distribution of Jφ = −S ₁ · π · cos (πr / R ₀ ) where coefficients S _{2 to} S ₄ and C _{1 to} C ₃ are zero, the necessary area Calculate the magnetic field at each point in. Hereinafter, similarly, the term of S ₂ , S
_The file A can be created by calculating the _three terms one after another.

The above procedure is a procedure for creating the file A, and the file A is created in advance. Next, the coil A1 is performed using the file A created as described above.
The procedure of pattern setting 1 will be described with reference to the flowchart of FIG.

Step 1 The file A created earlier is opened. Step 2 Set the linearity error of the gradient magnetic field to 2% or less.

[0027] Select a S _n, C _m refer to Step 3 files A, Sn gradient magnetic field satisfies a predetermined Riniaritei by linear programming method or least square method to determine the optimal solution of Cm. That is,
A current distribution Jφ is obtained from various values of S _n and C _m by equation (3), a gradient magnetic field is calculated from this current distribution according to Biot-Savart's law, and an optimum solution that has a linearity of 2% or less is calculated. I do.

Step 4 It is checked whether an optimal solution has been obtained. If not, go to step 5. Step 9 if obtained
Proceed to.

Step 5 In order to obtain the optimal solution, it is examined whether the allowable value of the previously set linearity error is loosened. If loosened, proceed to step 6. If not loosened, the process proceeds to step 7.

Step 6 The reference value of the linearity error is set to a relaxed value, and the process returns to step 3. Step 7 It is examined whether or not the number of n and m of S _n and C _m is increased to obtain an optimal solution. If not, return to step 3. When increasing, go to step 8.

Step 8 The number of n and m is increased and set, and the process returns to step 3 to calculate the increased S _n and C _m .

Step 9 A curve based on the optimal solution of the current distribution Jφ is obtained, and the coil pattern is set by the following method.

According to the above procedure, the value of each x / R ₀ on the horizontal axis, that is, the current distribution Jφ at a point on the horizontal axis of the coil is obtained, and for example, the current distribution curve 21 of FIG. 4 is obtained. The intersection of this current distribution curve 21 and the line of current distribution 0.00 is r ₁ ,
Let r ₂ , r ₃ , r ₄ . In addition, area 0~r _1, r ₁ ~r
_{2, r 2 ~r 3, r} 3 ~r 4, r 4 ~1 0.00 lines and current distribution curve 21 and the sandwiching area of current distribution in the S
_1, and _{S 2, S 3, S 4} , S 5. Current distribution curve 21
The total area of the areas S ₁ , S ₃ and S ₅ on the positive side of
And the number of turns of the coil to be obtained is n, and the area S
Is divided by n, and the area obtained is ΔS. Here, a method of determining the position of the winding will be described with reference to FIG. The current distribution curve in FIG. 5 is the same as the curve in FIG. In FIG. 5, r ₂₁
, R ₂₂ , r _{41 to} r ₄₆ divide the area S of the positive portion of the current distribution curve 21 into equal areas ΔS, and set the winding position at the center of each division point.

Next, in the equations (2) and (3), φ, r
The coil position shown in FIG. 6 is obtained by determining the winding position while changing. FIG. 6 shows a detailed pattern of the coil of FIG. 1 and shows only the first image limit. The other image limit patterns are patterns that are line-symmetric with respect to the x-axis or y-axis of the adjacent portion. In the figure, r ₁ , r ₂ , r ₃ , r ₄ are the same as the points of the same reference numerals in FIGS. Position of the winding in each region of 0~r _1, r ₂ ~r ₃ and r ₄ to 1 is the position of the winding determined in FIG.

The pattern thus obtained is constituted by a printed board or a copper wire having an appropriate thickness. When a copper wire is used, a groove corresponding to the coil pattern is machined on a disk of FRP material having a thickness of about 5 mm and a diameter of about 800 mm by an NC machine or the like, and the copper wire is embedded in the groove.

Using the coils A11 and B12 produced as described above, an x, y gradient magnetic field coil is produced as follows. The facing magnetic permanent magnet 3 in FIG. 12 has a shape as shown in FIG. That is, the magnetic shunt plate 3 has a concave shape having a circular outer shape, and the coils A11 and B12 formed as shown in FIG. . The currents applied to the coils A11 and B12 flow in the same direction.

The characteristics of the gradient magnetic field coil of the present embodiment obtained as described above are compared with those of a conventional Anderson coil. 8 and 9 are curve diagrams of the linearity error of the Anderson coil, and FIGS. 10 and 11 are curve diagrams of the linearity error of the coil of this embodiment. 8 and 10, the abscissa indicates the radial x-axis coordinates of the coils A11 and B12 shown in FIG. 2, and the ordinate indicates the linearity error of the gradient magnetic field. The parameter Z of each curve is a position on the z-axis from a point of z = 0, which is a middle point between the coils A11 and B12. As is clear from the figure, the error in the x-axis direction is extremely small in the case of the present embodiment, and is within approximately 2% except for z = 12 cm. Although the linearity error at z = 12 cm is large, it is greatly improved as compared with the Anderson type coil.

FIGS. 9 and 11 are the same as FIGS. 8 and 10 in that the abscissa indicates the coordinates of the x-axis in the radial direction of the coils A11 and B12 and the ordinate indicates the linearity error. However, the parameter y of each curve is a coordinate on the z = 0 plane in the y-axis direction orthogonal to the x-axis and the z-axis. As is clear from this curve diagram, it can be said that the linearity error is within 3% except for y = 12 cm, and within y = 12 cm, it is within approximately 3% except for the end on the x-axis.
On the other hand, Anderson type coil has 10% error.
Is over.

The overall characteristics are as follows: 2Z ₀ = 46 cm,
The linearity error of the Anderson coil of L = 80 cm 2W + S = 90.4 cm is z = 200 mm, x = 175
It is 14% in mm.

The linearity error of the coil of this embodiment with 2Z ₀ = 46 cm and 2R ₀ = 80 cm is z = 200 mm, x =
It is 3% at the point of 175 mm, and the linearity error is remarkably improved.

As described above, according to this embodiment, a gradient coil having good linearity can be obtained. Since this coil has a circular shape, it can be easily attached to a permanent magnet having a circular magnetic shunt plate or a magnet made of a circular superconducting or normal conducting coil by a facing vertical magnetic field method.

[0042]

According to the present invention, it is possible to provide a gradient coil and an MRI apparatus having good linearity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a pattern of a gradient magnetic field coil according to an embodiment of the present invention.

FIG. 2 is a layout view of a gradient coil according to an embodiment of the present invention.

FIG. 3 is a flowchart of a procedure for setting a coil pattern.

FIG. 4 is a diagram of a current distribution curve obtained from a current distribution equation.

FIG. 5 is an explanatory diagram of a method of obtaining a winding position of a gradient magnetic field coil from a current distribution equation.

FIG. 6 is a diagram of a first quadrant of a gradient coil according to an embodiment of the present invention.

FIG. 7 is a diagram of a magnetic shunt plate on which a coil of the present invention is mounted.

FIG. 8 is a curve diagram of a linearity error of a conventional Anderson coil with z as a parameter.

FIG. 9 is a curve diagram of a linearity error of a conventional Anderson coil with y as a parameter.

FIG. 10 is a curve diagram of a linearity error using z of the coil according to the embodiment of the present invention as a parameter.

FIG. 11 is a curve diagram of a linearity error using y as a parameter of the coil according to the embodiment of the present invention.

FIG. 12 is a sectional view showing a schematic structure of a facing magnet of MRI.

FIG. 13 is a schematic configuration diagram of an Anderson coil.

[Explanation of symbols]

 3 Magnetic shunt plate 10, 11 coil

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (3)

(57) [Claims] 1. A gradient magnetic field coil unit having a pattern for a gradient magnetic field coil formed in a circular shape on one plane, wherein the pattern comprises two halves forming the circular circle on the one plane. Each of the circles comprises two electric paths formed mirror-symmetrically to each other, and each of the two electric paths has a plurality of arcs having a plurality of centers on a straight line extending outward from the center of the circle. A coil unit for a gradient magnetic field, comprising a spiral pattern, wherein an arc portion in the arc shape in the spiral pattern is oriented along a circumference of the circle. 2. A gradient magnetic field coil unit according to claim 1, wherein each gradient magnetic field coil unit in said pair faces each other at a predetermined distance within a static magnetic field space. Gradient field coil. 3. It has the gradient magnetic field coil of claim 2,
An MRI apparatus, wherein a gradient magnetic field is generated using the gradient magnetic field coil.