JPH08126628A - Magnetic resonance tomographic imaging apparatus - Google Patents

Abstract

PURPOSE: To provide a magetic resonance tomographic imaging apparatus equipped with an inclined magnetic field coil or static magnetic field correcting shim coil reduced in resistance value as low as possible and easy to produce. CONSTITUTION: This magnetic resonance tomographic imaging apparatus is equipped with an inclined magnetic field coil and/or a static magnetic field correcting shim coil formed by forming short strip-shaped conductors 12 becoming the filaments of the coil by alternately providing deep notches to the opposed sides of an almost rectangular conductor flat plate at an interval determined so as to show a desired change in the current density of the coil and winding this notched conductor flat plate around a coil bobbin 20 while folding back the same at the connection parts 13 of the short strip-shaped conductors 12 on the deep side of the respective notches.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance tomography (MRI) apparatus for taking a tomographic image by utilizing a nuclear magnetic resonance phenomenon, and more particularly to a gradient magnetic field coil and a static magnetic field correction provided in the MRI apparatus. The structure of the shim coil is related.

[0002]

2. Description of the Related Art FIG. 6 shows a main structure of an MRI apparatus. As is well known, in the gantry 1 of the MRI apparatus, a main magnet 3 for forming a uniform static magnetic field space in an imaging area in a through hole 2 through which a subject is inserted, and three axes orthogonal to the imaging area. Gradient magnetic field coils 4 (4 _X , 4 _Y , for forming a gradient magnetic field whose magnetic field strength changes in the directions (X, Y, Z)
4 _Z ) and a static magnetic field correction shim coil 5 for correcting the non-uniformity of the static magnetic field due to the main magnet 3. The gradient magnetic field coil 4 and the static magnetic field correction shim coil 5 differ only in the order of the function of the generated magnetic field distribution,
Since the basic structure and the action of intentionally changing the distribution of the static magnetic field by the main magnet 3 are the same, the gradient magnetic field coil 4 will be described below as an example.

FIG. 7 (a) shows a schematic structure of a gradient magnetic field coil for generating a gradient magnetic field whose magnetic field strength changes in the axial direction (Z direction) of the through hole 2 of the gantry 1. The gradient magnetic field coil 4 _Z has Z
For example, the coils 4 _Z1 to 4 _Z6 , which are arranged so as to face each other and are divided into six, are electrically connected in series. The coil pairs 4 _Z1 and 4 _Z4 , the coil pairs 4 _Z2 and 4 _Z5 , and the coil pairs 4 _Z3 and 4 _Z6 have the same number of turns respectively, but the winding directions are set in reverse, so that it is shown in FIG. 7 (b). Thus, a gradient magnetic field in which the magnetic field strength changes linearly in the Z direction can be obtained. The linearity of the gradient magnetic field greatly contributes to the quality of the tomographic image (accuracy of the position of the data), and thus ensuring the linearity is important.

However, according to the above-mentioned gradient magnetic field coil, even if the linearity is secured, the current is concentrated in the region where the densely wound coils 4 _Z1 to 4 _Z6 are arranged (the current density is large). Therefore, the self-inductance of the coil increases. As a result, the rise of the gradient magnetic field is delayed, and there is a drawback that it is not possible to perform tomography at high speed.

Therefore, recently, in order to secure the linearity of the gradient magnetic field and reduce the inductance of the coil, a highly linear gradient magnetic field is obtained for a model (function system) in which the current density continuously changes. Therefore, a method of determining the positional relationship of the filaments (conductors) of the coil has been adopted by optimizing for this purpose and discretizing this functional system. Although some such optimization methods have been proposed, a method called a stream function method will be described below as an example with reference to FIGS. 3 to 5.

FIG. 3 is a diagram showing the relationship between the current density and the position in the Z direction used to explain the stream function, FIG. 4 is a diagram showing the positional relationship between the stream function and the filament of the gradient magnetic field coil, and FIG. 5 is the stream function method. It is a top view of the conventional gradient magnetic field coil obtained by.

First, the concept of the stream function will be described with reference to FIG. An explanation will be given taking a gradient magnetic field coil in the Z direction as an example. In order to reduce the self-inductance of this coil, a model in which the current density changes linearly in the Z direction is assumed as shown in FIG. However, since the Z direction is actually finite, the change in the current density is not completely linear because of the necessity of correction, but here, for convenience of description, it is described as being linear. In addition, the symbol O in the figure
Is the center of the imaging area. In this model, the total amount of current flowing through a certain area A _i (i is 1 to 10) divided into 10 in the Z direction is represented by the area S _i of the hatched area shown in FIG. Thus, the divided areas A ₁ to A area S ₁ to S ₁₀ corresponding to the ₁₀ divides the area A ₁ to A ₁₀ to be equal, the areas A ₁ to A ₁₀ single filaments f at the center of the _{If 1 to} f ₁₀ are set, the current density distribution equivalent to that shown in FIG. 3 can be obtained.

The stream function shown in FIG. 4 is an integral function of the current density shown in FIG. Therefore, the stream function shown in FIG. 4 is approximately a quadratic function. Therefore, when the number of turns T of the coil is determined according to the output of the gradient magnetic field coil, the vertical axis (axis of the stream function) of FIG. 4 is divided into Ts (for example, 10 divisions) at equal intervals by a number equal to the number of turns T. ) Do. Then, as shown in FIG. 4C, each divided area A ₁
Filament f ₁ each one of the center position of the to A ₁₀ ~f ₁₀
With the above, it is possible to obtain a linear current density distribution similar to that described in FIG. As shown in FIG. 5, according to the conventional gradient magnetic field coil obtained by the above-described stream function method, the filament (conductor wire) f wound around the coil bobbin 20 has a narrow winding interval at both ends of the bobbin and has a central portion of the bobbin. And the winding direction of the filament f is left-right reversed with the imaging center O interposed therebetween.

According to the above-described method, the function is optimized to ensure the high linearity of the gradient magnetic field, and the local concentration of the filament is avoided, so that the self-inductance of the coil is reduced and the gradient magnetic field is reduced. The rise time is shortened, and a gradient magnetic field coil suitable for high-speed imaging can be realized.

[0010]

However, the prior art having such a structure has the following problems. That is, since the cross-sectional area of the filament, which is a conductor wire having a circular cross-sectional shape, is limited by the minimum filament spacing at both ends of the gradient magnetic field coil, the cross-sectional area of the filament cannot be increased so much. As a result, there is a problem that the resistance value of the gradient magnetic field coil becomes large, a power source for driving the gradient magnetic field coil needs a large output, and the amount of heat generation of the gradient magnetic field coil becomes large.

There is also a problem that a manufacturing error is likely to occur at the filament position during the process of winding the filament.

The above-mentioned problems can be applied to other gradient magnetic field coils in the X and Y directions and a shim coil for correcting a static magnetic field, which have the same structure and function.

The present invention has been made in view of the above circumstances, and a magnetic resonance tomography having a gradient magnetic field coil or a static magnetic field correction shim coil whose resistance value is as small as possible and which is easy to manufacture. The purpose is to provide a device.

[0014]

The present invention has the following configuration to achieve the above object. That is, in the magnetic resonance tomography apparatus according to the present invention, deep cuts are alternately formed on opposite sides of a substantially rectangular conductor flat plate at intervals determined so that the current density of the coil exhibits a desired change. By forming a strip-shaped conductor to be a filament of the coil, and winding the conductor flat plate in which the notch is made around the coil bobbin while being folded back at the connecting portion of the strip conductor at the back side of each notch and / Alternatively, a shim coil for static magnetic field correction is provided.

[0015]

According to the present invention, the width of the strip-shaped conductor serving as the filament of the coil changes according to the intervals of the cuts determined so that the current density of the coil exhibits the desired change. In other words, when the current density is set high, the gap between the cuts becomes narrow, so the width of the strip conductor becomes narrow, but when the current density is set low, the gap between the cuts becomes wide, so the width of the strip conductor also becomes narrow. Get wider Therefore, since the cross-sectional area of each strip-shaped conductor takes the maximum value in relation to the required change in the current density of the coil, the resistance value of the coil becomes as small as possible. Further, since the positional relationship between the strip-shaped conductors (filaments) wound around the coil bobbin is determined by the cut intervals of the flat plate conductors, a manufacturing error in the positional relationship of the filaments when winding around the coil is unlikely to occur.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. Since the configuration of the gantry and the like of the magnetic resonance tomography apparatus according to the embodiment is similar to that of the conventional apparatus shown in FIG. 6,
The description is omitted here. The configuration of the gradient magnetic field coil, which is the main part of the embodiment, will be described below.

FIG. 1 is a plan view of a conductor flat plate for a gradient magnetic field coil, and FIG. 2 is a plan view of a gradient magnetic field coil formed by winding a conductor flat plate. As shown in FIG. 1, the conductor flat plate 10
Has a substantially rectangular shape, and is formed of a copper plate having a thickness of about 1 to 3 mm. The length L of the conductor flat plate 10 is equal to
0 is substantially equal to the length of the coil bobbin 20 (see FIG. 2) wound around. Further, the width W of the conductor flat plate 10 is a length obtained by subtracting the inter-conductor spacing α (see FIG. 2) from the circumferential length S of the coil bobbin 20 plus the length δ of the folded portion (S−α).
+ Δ) is set.

The opposite sides 10a of the conductor flat plate 10,
Alternate deep cuts 11 (11 ₁ , 11 ₂ , 1 in 10b)
1 _3, ...) by is entered, the strip-shaped conductor 12 made a filament coil (12 _1, 12 _2, 12 _3,
…) Is formed. The notch 11 is formed by an appropriate means such as machining or etching. The width of each notch 11 is about 1 to 3 mm due to processing restrictions. Since the winding direction of the strip-shaped conductor 12 around the coil bobbin 20 can be switched at the central portion of the bobbin, there is no folded portion at the central portion of one side 10a of the conductor flat plate 10, and there is no cut as shown in FIG. The notch 14 is formed.

The interval between the notches 11, in other words, the width of the strip-shaped conductor 12 is determined from the relationship between the stream function and the Z direction position shown in FIG. That is, when the number of turns T of the coil is determined according to the output of the gradient magnetic field coil, the vertical axis (axis of the stream function) of FIG. 4 is divided into Ts at equal intervals by a number equal to the number of turns T. The position in the Z direction corresponding to each dividing line is the position where the cut 11 is formed. As a result, as shown in FIG. 4B, the gap between the cuts 11 (that is, the width of the strip-shaped conductor 12) is wide in the central portion (coil central portion) of the conductor flat plate 10 corresponding to the imaging center O, and It becomes narrower toward both ends (both ends of the coil) of the flat plate 10.

The conductor flat plate 10 as described above is cut 11
The strip-shaped conductor 12 is wound around the coil bobbin 20 while being folded back at the connecting portion 13 of the strip-shaped conductor 12 which is the inner side of the strip-shaped conductor 12. Then, the strip-shaped conductor 12 is wound around the notch 14 in the central portion of the conductor flat plate 10 without being folded back, and thereafter, the strip-shaped conductor 12 is wound around by being folded back at the connecting portion 13. As a result, like the gradient magnetic field coil shown in FIG. 2, coils L _L and L _{R in} which the right half and the left half have opposite winding directions and are connected in series are obtained. After winding the strip-shaped conductor 12 as described above, the strip-shaped conductor 12 is fixed with resin for fixing the position.

Since the gradient magnetic field coil used in the magnetic resonance tomography apparatus is composed of three gradient magnetic field coils corresponding to the X, Y, and Z directions, each of the gradient magnetic field coils has an insulating sheet interposed between them. In the same manner as described above, the layers are formed on the same coil bobbin.

The present invention is not limited to the above-mentioned embodiment, but can be modified as follows.

(1) In the embodiment, the gradient magnetic field coil has been described as an example, but the present invention can also be applied to a static magnetic field correction shim coil for correcting the nonuniformity of the static magnetic field of the main magnet. .

(2) In the embodiment, the cut 11 formed in the conductor flat plate 10 is orthogonal to the side edges 10a and 10b, but the cut 11 may be formed in an oblique direction. In this case, the strip-shaped conductor 12 is spirally wound around the coil bobbin 20.

(3) In the embodiment, one conductor flat plate is used to form one coil having the number of turns T, but one coil is formed by stacking a plurality of conductor flat plates. May be. With such a laminated structure, the plate thickness of each conductor flat plate can be reduced, so that the processing for forming the notches and the winding operation on the bobbin are facilitated.

[0026]

As is apparent from the above description, according to the present invention, the cross-sectional area of each strip-shaped conductor of the conductor flat plate wound around the coil bobbin is different from the required change in the current density of the coil. Since the maximum value is adopted because of the relationship, the resistance value of the coil can be minimized. Further, the positional relationship of the strip-shaped conductors (filaments) is determined by the cut intervals of the flat plate conductors, so that a manufacturing error in the positional relationship of the filaments when wound around the coil is unlikely to occur.

[Brief description of drawings]

FIG. 1 is a plan view of a conductor flat plate used in Examples.

FIG. 2 is a plan view of a gradient magnetic field coil according to an embodiment.

FIG. 3 is a graph showing a relationship between a current density and a Z-direction position, which is used for explaining a flow function.

FIG. 4 is a graph showing a relationship between a stream function and a Z-direction position.

FIG. 5 is a plan view of a gradient magnetic field coil according to a conventional example.

FIG. 6 is a sectional view showing a schematic configuration of a gantry of the MRI apparatus.

FIG. 7 is a schematic diagram of another gradient magnetic field coil according to a conventional example.

[Explanation of symbols]

 10 ... Conductor flat plate 11 ... Notch 12 ... Strip-shaped conductor 13 ... Connection part 20 ... Coil bobbin

Claims (1)

[Claims] 1. A strip that becomes a filament of a coil by alternately making deep cuts on opposite sides of a substantially rectangular conductor flat plate at intervals determined so that the current density of the coil exhibits a desired change. A magnetic field coil and / or a shim coil for correcting static magnetic field, which is formed by winding a conductor flat plate in which a notch is formed and winding it around a coil bobbin while folding back the conductor flat plate at the back side of each notch A magnetic resonance tomography apparatus characterized by the above.