Gradient coil for suppressing stray magnetic field

Abstract

The invention relates to a gradient coil for suppressing a stray magnetic field, in which the lengths of an X main coil and a Y main coil are consistent and are both smaller than the length of a Z main coil, and the lengths of an X shielding coil and a Y shielding coil are consistent and are both smaller than the length of a Z shielding coil. The Z gradient coil is formed by rolling four turns at the central area and eight turns at the end part by using a copper plate and then is welded together with a solenoid coil wound by a lead of the same specification, and the X gradient coil and the Y gradient coil are formed by cutting the copper plate. When the Z gradient coil is designed, magnetic field constraint is applied to the main shielding area and the expanded shielding area, the magnetic field constraint value of the Z gradient coil is smaller than that of the expanded shielding area, and when the X gradient coil and the Y gradient coil are designed, magnetic field constraint is only applied to the main shielding area. After the Z gradient coil is obtained through design, the widths of the four-turn wires in the center area and the eight-turn wires at the ends of the Z gradient coil are optimized and adjusted, the high shielding effect of the Z gradient coil is finally achieved, and the wide wires at the ends serve as passive shielding layers of the X gradient coil and the Y gradient coil.

Description

Technical Field

[0001] The invention belongs to the field of electrical engineering theory and new technology, and in particular relates to a gradient coil for suppressing stray magnetic fields. Background Art

[0002] The gradient coil current in the magnetic resonance imaging system needs to switch its size and direction continuously, thus generating eddy currents on the superconducting magnet. For imaging, the focus is generally on reducing the secondary magnetic field strength generated by eddy currents in the uniform area of ​​the gradient coil to avoid interfering with imaging encoding. However, as magnetic resonance imaging technology develops towards stronger magnetic fields, higher resolutions, and faster scanning speeds, the gradient magnetic field strength and switching rate are moving towards higher indicators. Although the use of active shielding technology can largely suppress the secondary magnetic field effect of eddy currents, this efficient magnetic field shielding effect is often limited to the middle area of ​​the gradient coil, while the shielding effect is greatly weakened in the end area of ​​the gradient coil, and the strong stray magnetic field is radiated to the superconducting magnet. Although the stray magnetic field at the end of the gradient coil does not cause obvious secondary magnetic field interference due to its distance from the uniform area of ​​the gradient coil, the violent eddy currents at the end have a significant thermal effect on the superconducting magnet, which may cause a sharp increase in the volatilization of liquid helium or cause the temperature of the magnet coil without liquid helium to rise, or even cause the superconducting magnet to quench. In the field of high-field magnetic resonance imaging, suppressing the radiation of stray magnetic fields at the end of the gradient coil is of great significance for improving the safety of magnetic resonance imaging systems. Summary of the invention

[0003] To solve the above technical problems, the present invention provides a gradient coil for suppressing stray magnetic fields. The present invention is applied in a superconducting magnetic resonance imaging system to solve the problem of eddy current induction heating caused by rapid switching of gradient magnetic fields on a superconducting magnet.

[0004] The technical scheme adopted by the present invention is as follows: a gradient coil for suppressing stray magnetic fields, wherein the gradient coils are arranged in the order of X main coil, Y main coil, Z main coil, X shielding coil, Y shielding coil and Z shielding coil from inner layer to outer layer, wherein the Z main coil and the Z shielding coil are all wound with wires of equal specifications except four turns at the center area and eight turns at the end, and the four turns at the center area and eight turns at the end are rolled with copper plates, and then welded together with the solenoid coils wound with wires of equal specifications; the X main coil, the Y main coil, the X shielding coil and the Y shielding coil are made by cutting copper plates.

[0005] The cylindrical shell made of copper plates with four turns in the center area of ​​the Z main coil and the Z shielding coil and eight turns at the end is not closed in the circumferential direction and has a cutout, which is used as a turn of conductor.

[0006] The four turns of copper plates in the central area of ​​the Z main coil and the Z shield coil are divided along the circumferential direction, and the circumferential angle of the divided and removed part is smaller than the entire circumference, so that the divided multiple thin copper strips are connected side by side; the end eight turns of the Z main coil and the Z shield coil are rolled with a whole piece of copper plate and are not divided circumferentially.

[0007] The lengths of the X main coil and the Y main coil are the same, and are both shorter than the length of the Z main coil; the lengths of the X shielding coil and the Y shielding coil are the same, and are both shorter than the length of the Z shielding coil.

[0008] The shielding area of ​​the gradient coil is divided into two parts, the middle is the main shielding area, and the two ends are the extended shielding areas. The length of the main shielding area and the extended shielding area is greater than the length of the shielding coil.

[0009] For the Z gradient coil, magnetic field constraints are applied in both the main shielding area and the extended shielding area, namely:

[0010] ,

[0011] Among them, z is the axial coordinate with the center position of the gradient coil axis as the origin, z1 is the boundary position of the main shielding area, -z1≤z≤z1 is the axial range of the main shielding area, z<-z1 and z>z1 are the axial ranges of the expansion area, B is the magnetic field within the corresponding axial range, S1 is the magnetic field constraint value of the main shielding area, S2 is the magnetic field constraint value of the expansion area, and the two constraint values ​​satisfy the condition S1 <S2。

[0012] For the X-gradient coil and the Y-gradient coil, magnetic field constraints are applied only in the main shielding area, that is:

[0013] ,

[0014] Among them, S3 is the magnetic field constraint value of the main shielding area, and the condition S3 is satisfied. <S2。

[0015] After the wire position of the Z gradient coil is obtained according to the conditions of claim 6, a rectangular cross section of equal specifications is given to each turn of the wire, and its cross-sectional size is determined according to the actual wire specifications. The magnetic field integration method of a hollow cylinder is used to calculate the magnetic field distribution in the uniform area and the shielding area. The calculation method is as follows:

[0016] ,

[0017] Among them, B z is the axial magnetic field, r is the radial coordinate of the magnetic field calculation sampling point, z is the axial coordinate of the magnetic field calculation sampling point, M is the number of turns of the wire, k is the corresponding subscript of the kth turn of the wire, N is the number of Gaussian integration points, i is the corresponding subscript of the i-th Gaussian integration point, μ0 is the vacuum permeability, J k is the current density of the kth turn of wire, π is the circumference of a circle, ωi is the weight of the Gaussian integration point, β i is the Gaussian integration point, L is the magnetic field integration operator, r 1k is the inner radius of the kth turn of wire, r 2k is the outer radius of the kth turn of wire, z 1k is the smaller axial coordinate of the kth turn of wire, z 2k is the larger axial coordinate of the kth turn of wire.

[0018] The width of the four turns of the conductor at the center area of ​​the Z main coil and the Z shielding coil and the eight turns of the conductor at the end is increased on the basis of the conductor of the same specification in claim 7, that is, the center position of the conductor is kept unchanged and the z 1k , increase z 2k The technical principle for increasing the width is that the wires do not overlap and an insulation gap is left.

[0019] The width optimization principle of the four-turn conductor at the center area of ​​the Z main coil and the Z shielding coil and the eight-turn conductor at the end is to minimize the magnetic field in the shielding area. The optimization judgment criterion for the width of the four-turn conductor at the center area of ​​the Z main coil and the Z shielding coil is to minimize the maximum magnetic field in the main shielding area. The optimization judgment criterion for the width of the eight-turn conductor at the end of the Z main coil and the Z shielding coil is to minimize the maximum magnetic field in the extended shielding area.

[0020] The present invention has the following beneficial technical effects:

[0021] The gradient coil structure involved in the present invention does not increase any engineering processing complexity. The gradient coil still maintains a layered structure. According to the specified radial arrangement of the coil, the regional magnetic field shielding optimization design method and the modified coil center and end structure are used to improve the active shielding effect, while effectively increasing the passive shielding function, thereby achieving a significant improvement in the shielding efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the radial arrangement of the gradient coils of the present invention;

[0023] Figure 2 It is the three-dimensional structure of Z main coil;

[0024] Figure 3 It is a three-dimensional structure of Z shielding coil;

[0025] Figure 4 It is the three-dimensional structure of the X main coil;

[0026] Figure 5 It is a three-dimensional structure of X-shielding coil;

[0027] Figure 6 It is the three-dimensional structure of the Y main coil;

[0028] Figure 7It is a three-dimensional structure of Y shielding coil.

[0029] Explanation of the accompanying drawings: 1 is the X main coil, 2 is the Y main coil, 3 is the Z main coil, 3a, 3b, 3c, 3d are four turns of wire in the central area of ​​the Z main coil, 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l are eight turns of wire in the end area of ​​the Z main coil, 4 is the X shielding coil, 5 is the Y shielding coil, 6 is the Z shielding coil, 6a, 6b, 6c, 6d are four turns of wire in the central area of ​​the Z shielding coil, 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l are eight turns of wire in the end area of ​​the Z shielding coil, 7 is the uniform area, 8 is the main shielding area, and 9 is the extended shielding area. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0031] like Figure 1 As shown, the present invention provides a gradient coil for suppressing stray magnetic fields, wherein the gradient coils are arranged in the order of X main coil 1, Y main coil 2, Z main coil 3, X shielding coil 4, Y shielding coil 5, and Z shielding coil 6 from the inner layer to the outer layer, wherein the Z main coil 3 and the Z shielding coil 6 are all wound with wires of the same specification except for four turns in the center area and eight turns at the end, and the four turns in the center area and eight turns at the end are rolled with copper plates, and then welded together with the solenoid coils wound with wires of the same specification; as shown Figure 4-7 As shown, the X main coil 1, the Y main coil 2, the X shielding coil 4, and the Y shielding coil 5 are made by cutting copper plates.

[0032] like Figure 2 and Figure 3 As shown, the cylindrical shell made of copper plates is not closed in the circumferential direction and has a cutout, and is used as a turn of conductor. The four turns 3a, 3b, 3c, 3d and 6a, 6b, 6c, 6d in the central area of ​​the Z main coil 3 and the Z shielding coil 6 and the eight turns 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l and 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l at the end.

[0033] like Figure 2 and Figure 3 As shown, the four turns 3a, 3b, 3c, 3d and 6a, 6b, 6c, 6d of copper plates in the central area of ​​the Z main coil 3 and the Z shielding coil 6 are divided along the circumferential direction, and the circumferential angle of the divided and cut parts is smaller than the entire circumference, so that the divided multiple thin copper strips are connected side by side; the eight turns 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l and 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l at the end parts of the Z main coil 3 and the Z shielding coil 6 are rolled from a whole piece of copper plate and are not divided circumferentially.

[0034] like Figure 1 As shown, the lengths of the X main coil 1 and the Y main coil 2 are the same, both shorter than the length of the Z main coil 3 ; the lengths of the X shielding coil 4 and the Y shielding coil 5 are the same, both shorter than the length of the Z shielding coil 6 .

[0035] like Figure 1 As shown, the shielding area of ​​the gradient coil is divided into two parts, the middle is a main shielding area 8, and the two ends are extended shielding areas 9. The length of the main shielding area 8 and the extended shielding area 9 is greater than the length of the shielding coil.

[0036] For the Z gradient coil, magnetic field constraints are applied in both the main shielding area 8 and the extended shielding area 9, namely:

[0037] ,

[0038] Among them, z is the axial coordinate with the center position of the gradient coil axis as the origin, z1 is the boundary position of the main shielding area, -z1≤z≤z1 is the axial range of the main shielding area, z<-z1 and z>z1 are the axial ranges of the expansion area, B is the magnetic field within the corresponding axial range, S1 is the magnetic field constraint value of the main shielding area, S2 is the magnetic field constraint value of the expansion area, and the two constraint values ​​satisfy the condition S1 <S2。

[0039] For the X-gradient coil and the Y-gradient coil, magnetic field constraints are applied only in the main shielding area 8, that is:

[0040] ,

[0041] Among them, S3 is the magnetic field constraint value of the main shielding area, and the condition S3 is satisfied. <S2。

[0042] After the conductor position of the Z gradient coil is designed according to the above conditions, a rectangular cross-section of equal specifications is given to each turn of the conductor, and its cross-sectional size is determined according to the actual conductor specifications. The magnetic field integration method of the hollow cylinder is used to calculate the magnetic field distribution in the uniform area and the shielding area. The calculation method is as follows:

[0043] ,

[0044] Among them, B z is the axial magnetic field, r is the radial coordinate of the magnetic field calculation sampling point, z is the axial coordinate of the magnetic field calculation sampling point, M is the number of turns of the wire, k is the corresponding subscript of the kth turn of the wire, N is the number of Gaussian integration points, i is the corresponding subscript of the i-th Gaussian integration point, μ0 is the vacuum permeability, J k is the current density of the kth turn of wire, π is the circumference of a circle, ω i is the weight of the Gaussian integration point, β i is the Gaussian integration point, L is the magnetic field integration operator, r 1k is the inner radius of the kth turn of wire, r2k is the outer radius of the kth turn of wire, z 1k is the smaller axial coordinate of the kth turn of wire, z 2k is the larger axial coordinate of the kth turn of wire.

[0045] The width of the wires of the four turns 3a, 3b, 3c, 3d and 6a, 6b, 6c, 6d at the center of the Z main coil 3 and the Z shielding coil 6 and the eight turns 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l and 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l at the end are increased on the basis of the wires of the same specifications, that is, the center position of the wire is kept unchanged and the width of the wire is reduced. 1k , increase z 2k The technical principle for increasing the width is that the wires do not overlap and an insulation gap is left.

[0046] The width optimization principle of the four turns 3a, 3b, 3c, 3d and 6a, 6b, 6c, 6d of the conductors at the center area of ​​the Z main coil 3 and the Z shielding coil 6 and the eight turns 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l and 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l of the conductors at the end is to minimize the magnetic field in the shielding area, wherein the width optimization judgment criterion of the four turns 3a, 3b, 3c, 3d and 6a, 6b, 6c, 6d of the conductors at the center area of ​​the Z main coil 3 and the Z shielding coil 6 is to minimize the maximum magnetic field in the main shielding area 8, and the width optimization judgment criterion of the eight turns 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l and 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l of the conductors at the end of the Z main coil 3 and the Z shielding coil 6 is to minimize the maximum magnetic field in the extended shielding area 9.

[0047] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A gradient coil for suppressing stray magnetic fields, characterized in that: The arrangement order of the gradient coil from the inner layer to the outer layer is X main coil, Y main coil, Z main coil, X shield coil, Y shield coil, Z shield coil, among which the Z main coil and Z shield coil are all wound with wires of the same specification except for four turns in the center area and eight turns at the end, while the four turns in the center area and eight turns at the end are rolled with copper plates, and then welded together with the solenoid coils wound with wires of the same specification; the X main coil, Y main coil, X shield coil, and Y shield coil are made by cutting copper plates.

2. A gradient coil for suppressing stray magnetic fields according to claim 1, characterized in that: The cylindrical shell made of copper plates with four turns in the center area of ​​the Z main coil and the Z shielding coil and eight turns at the end is not closed in the circumferential direction and has a cutout, which is used as a turn of conductor.

3. The gradient coil for suppressing stray magnetic field according to claim 1, characterized in that: The four turns of copper plates in the central area of ​​the Z main coil and the Z shield coil are divided along the circumferential direction, and the circumferential angle of the divided and removed part is smaller than the entire circumference, so that the divided multiple thin copper strips are connected side by side; the end eight turns of the Z main coil and the Z shield coil are rolled with a whole piece of copper plate and are not divided circumferentially.

4. The gradient coil for suppressing stray magnetic field according to claim 1, characterized in that: The lengths of the X main coil and the Y main coil are the same, and are both shorter than the length of the Z main coil; the lengths of the X shielding coil and the Y shielding coil are the same, and are both shorter than the length of the Z shielding coil.

5. The gradient coil for suppressing stray magnetic field according to claim 1, characterized in that: The shielding area of ​​the gradient coil is divided into two parts, the middle is the main shielding area, and the two ends are extended shielding areas. The length of the main shielding area and the extended shielding area is greater than the length of the shielding coil.

6. A gradient coil for suppressing stray magnetic fields according to claims 1-5, characterized in that: For the Z gradient coil, the method for calculating the wire position of the Z gradient coil specifically includes: applying magnetic field constraints in both the main shielding area and the extended shielding area, that is: ， Among them, z is the axial coordinate with the center position of the gradient coil axis as the origin, z1 is the boundary position of the main shielding area, -z1≤z≤z1 is the axial range of the main shielding area, z<-z1 and z>z1 are the axial ranges of the expansion area, B is the magnetic field within the corresponding axial range, S1 is the magnetic field constraint value of the main shielding area, S2 is the magnetic field constraint value of the expansion area, and the two constraint values ​​satisfy the condition S1 <S2; For the X-gradient coil and the Y-gradient coil, the method for calculating the conductor position of the X-gradient coil and the Y-gradient coil specifically includes: applying magnetic field constraints only in the main shielding area, that is: ， Among them, S3 is the magnetic field constraint value of the main shielding area, and the condition S3 is satisfied. <S2。 7. The gradient coil for suppressing stray magnetic field according to claim 6, characterized in that: After calculating the wire position of the Z gradient coil, each turn of the wire is given a rectangular cross-section of equal specifications, and its cross-sectional size is determined according to the actual wire specifications; the magnetic field integration method of the hollow cylinder is used to calculate the magnetic field distribution in the uniform area and the shielding area. The calculation method is as follows: ， Among them, B z is the axial magnetic field, r is the radial coordinate of the magnetic field calculation sampling point, z is the axial coordinate of the magnetic field calculation sampling point, M is the number of turns of the wire, k is the corresponding subscript of the kth turn of the wire, N is the number of Gaussian integration points, i is the corresponding subscript of the i-th Gaussian integration point, μ0 is the vacuum permeability, J k is the current density of the kth turn of wire, π is the circumference of a circle, ω i is the weight of the Gaussian integration point, β i is the Gaussian integration point, L is the magnetic field integration operator, r 1k is the inner radius of the kth turn of wire, r 2k is the outer radius of the kth turn of wire, z 1k is the smaller axial coordinate of the kth turn of wire, z 2k is the larger axial coordinate of the kth turn of wire.

8. The gradient coil for suppressing stray magnetic field according to claim 7, characterized in that: The width of the wire is increased based on the specifications of the wire, such as the four turns in the center area of ​​the Z main coil and the eight turns at the end, that is, the center position of the wire is kept unchanged and the z is reduced. 1k , increase z 2k The technical principle for increasing the width is that the wires do not overlap and an insulation gap is left.

9. The gradient coil for suppressing stray magnetic field according to claim 8, characterized in that: The width optimization principle of the four-turn conductor at the center area of ​​the Z main coil and the Z shielding coil and the eight-turn conductor at the end is to minimize the magnetic field in the shielding area. The optimization judgment criterion for the width of the four-turn conductor at the center area of ​​the Z main coil and the Z shielding coil is to minimize the maximum magnetic field in the main shielding area. The optimization judgment criterion for the width of the eight-turn conductor at the end of the Z main coil and the Z shielding coil is to minimize the maximum magnetic field in the extended shielding area.

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