METHOD FOR DESIGNING GRADIENT COILS FOR MRI SYSTEMS, GRADIENT COILS FOR MRI SYSTEMS OBTAINED BY THE SAID METHOD AND MRI SYSTEM COMPRISING SUCH GRADIENT COILS

• US 20200132791A1
• Filed: 07/12/2019
• Published: 04/30/2020
• Est. Priority Date: 07/09/2018
• Status: Active Grant

• Current Assignee: Esaote SpA
• Original Assignee: Esaote SpA
• Inventors: PUNZO, Vincenzo
• Granted Patent: US 11,204,406 B2
• Time in Patent Office: Days
• CPC Class Codes: G01R 33/3642 Mutual coupling or decoupling of multiple coils, e.g. decoupling of a receive coil from a transmission coil, or intentional coupling of RF coils, e.g. for RF magnetic field amplification
• CPC Class Codes: G01R 33/3806 Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils
• CPC Class Codes: G01R 33/3873 using ferromagnetic bodies ; Passive shimming
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/56572 caused by a distortion of a gradient magnetic field, e.g. non-linearity of a gradient magnetic field G01R33/56509, G01R33/56518, G01R33/56536 take precedence

Abstract

A method for designing gradient coils includes the following steps: a) defining an imaging volume as an ellipsoid; b) defining an elliptic-cylindrical surface enclosing the said ellipsoid; c) defining the current density at each point of the surface by a series of basis functions and corresponding coefficients expressed in elliptic cylindrical coordinates; d) describing the magnetic field generated at a generic point by the above defined current density integrated all over the said entire elliptic-cylindrical surface; e) determining the values of the coefficients of the basis functions by solving the inverse function for describing the magnetic field; f) generating a discrete winding patter of a gradient coil by using a stream function method from the continuous current density and by using a series of scattered contours of the stream function as the design of the winding patters according to a set total number of windings.

Claims

1. Method for designing gradient coils,

the gradient coils providing for a current distribution on the lateral surface of an elliptical cylinder generating a field being perpendicular to the longitudinal axis of the elliptical cylinder,

the method comprising the following steps:

a) defining an imaging volume as an ellipsoid;

b) defining an elliptic-cylindrical surface enclosing the said ellipsoid;

c) defining the current density at each point of the surface by a series of basis functions and corresponding coefficients expressed in elliptic cylindrical coordinates;

d) describing the magnetic field generated at a generic point by the above defined current density integrated all over the said entire elliptic-cylindrical surface;

e) determining the values of the coefficients of the basis functions by solving the inverse function for describing the magnetic field;

f) generating a discrete winding patter of a gradient coil by using a stream function method from the continuous current density and by using a series of scattered contours of the stream function as the design of the winding patters according to a set total number of windings.

2. Method according to claim 1 applying the target field method for calculating the coefficients of the series of trigonometric basis functions and the stream-function method for determining the winding pattern of the gradient coils according to steps c) to f).

3. Method according to claim 1 further comprising the steps of optimization of the dissipated power and the stored magnetic energy inductance of the continuous current on the defined elliptic cylindrical surface.

4. The method according to claim 1 comprising the steps of delimiting the winding pattern extension on the elliptic cylindrical surface to opposed space away sectors of the said elliptic cylindrical surface by leaving open passages between the gradient coils.

5. Gradient coils in combination with the magnet of an MRI system, the gradient coils being driven by a control unit for generating gradient fields along three Cartesian axis, the said magnet fields permeating a region of interest, the said gradient coils comprising discrete winding patterns configured to generate a current distribution on the surface of an elliptical cylinder the magnet of the MRI system generating a static magnetic field oriented perpendicular to the longitudinal axis of the said elliptic cylindrical surface.

6. Gradient coils according to claim 5 in which said gradient coils are configured to generate the said gradient field in a region of interest (ROI) defined by an ellipsoid enclosed in the elliptic cylindrical surface.

7. Gradient coils according to claim 5, wherein each one of the said gradient coils generating respectively a gradient field in one of the three directions of the coordinate system, each of the said gradient coils comprising discrete winding patterns configured to generate a current distribution on the surface of an elliptical cylindrical surface, the windings of the three gradient coils extending on elliptic cylindrical surfaces layered one over the other, thus forming three layers of discrete winding patterns each one of one of the said gradient coils.

8. Gradient coils according to claim 7 in which a gradient coil generating a gradient field in z-direction of the Cartesian coordinate system and extending on an elliptical cylindrical surface comprises identical winding patterns on two half of the elliptic-cylindrical surface along a separation plane of the said two halves which is a plane parallel to the longitudinal axis and to the maximum radius of the elliptic-cylindrical surface, while on each half of the elliptical-cylindrical surface two winding patterns are provided which are symmetric with respect of a cross section plane at the center of the elliptical-cylindrical surface and which plane is perpendicular to the longitudinal axis of the said elliptic-cylindrical surface and parallel to the minimum and maximum radius of the elliptic cross section of the elliptic cylindrical surface.

9. Gradient coils according to claim 8, in which each winding pattern of the two symmetrical winding pattern on each half of the elliptic cylindrical surface is formed by a plurality of annular windings which are positioned one inside the other having progressively reduced dimensions.

10. Gradient coil according to claim 8 in which the said closed loop or annular windings are eccentric.

11. Gradient coils according to claim 8 in which the said closed loop or annular windings are non-circular.

12. Gradient coils according to claim 8 in which, the said closed loop or annular windings are designed such that the more external one is essentially rectangular having a major diameter parallel to the major radius of the elliptical-cylindrical surface and the most inner one has an elliptic form while the intermediate windings progressively changes their shape from a quite rectangular of the most external one to the elliptic shape of the most internal one.

13. Gradient coils according to claim 5, wherein the gradient coil generating a gradient field in the y-direction of the Cartesian coordinate system is formed by only one winding pattern for each one of two halves of the elliptic-cylindrical surface along a separation plane of the said two halves which is a plane parallel to the longitudinal axis and to the major radius of the elliptic-cylindrical surface, the two winding patterns being symmetric relatively to a plane of symmetry which is coincident or parallel to the said plane of separation.

14. Gradient coils according to claim 13, in which each winding pattern is formed by a plurality of annular or closed loop windings positioned one inside the other.

15. Gradient coil according to claim 13, in which the said winding patterns have a major diameter, which is parallel to the longitudinal axis of the elliptic cylindrical surface.

16. Gradient coils according to claim 13, wherein the said closed loop or annular windings are designed such that the more external one is essentially rectangular having a major diameter parallel to the longitudinal axis of the elliptical-cylindrical surface and the most inner one has an elliptic shape while the intermediate windings progressively changes their shape from a quite rectangular of the most external one to the elliptic shape of the most internal one.

17. Gradient coils according to claim 5, wherein the gradient coil generating a gradient field in the y-direction of the Cartesian coordinate system and extending on an elliptical cylindrical surface comprises identical winding patterns on two half of the elliptic-cylindrical surface along a separation plane of the said two halves which is a plane parallel to the longitudinal axis and to the maximum radius of the elliptic-cylindrical surface, while on each half of the elliptical-cylindrical surface two winding patterns are provided which are symmetric with respect of a section plane at the center of the elliptical-cylindrical surface and which plane is parallel to the longitudinal axis of the said elliptic-cylindrical surface and parallel to the minor axis of the elliptic cross section of the elliptic cylindrical surface.

18. Gradient coils according to claim 17 in which, each winding pattern of the two symmetrical winding pattern on each half of the elliptic cylindrical surface is formed by a plurality of annular windings which are positioned one inside the other having progressively reduced dimensions.

19. Gradient coil according to claim 17 in which the said closed loop or annular windings are eccentric.

20. Gradient coils according to claim 17 in which the said closed loop or annular windings are designed such that the more external one is essentially rectangular having a major diameter parallel to the longitudinal axis of the elliptical-cylindrical surface and the most inner one has an elongated shape with rounded ends while the intermediate windings progressively changes their shape from a quite rectangular of the most external one to the shape of the most internal one.

21. An MRI system comprising:

a gantry for accommodating a target body under examination or a part thereof;

a magnet for generating a static magnetic field in a volume of space;

gradient coils for generating gradient magnetic fields in addition to the static magnetic field according to three spatial directions;

a control unit configured to drive and control the gradient coils and the magnet;

a transmission antenna connected to an excitation signal sequence generator for transmitting spin echo excitations signals into the cavity housing the target body;

a receipt antenna for receiving the MRI signals caused by the excitation signals;

an MRI receipt signals processing unit and image generation unit for processing the signals and extracting image data information and for generating the images;

a display unit for displaying the reconstructed images;

a control unit managing the excitation signal sequence generator, the receipt signals processing unit, image generation unit and the display;

a user interface for input of MRI image acquisition settings to be specified by the user before starting with the image acquisition session.

22. An MRI system according to claim 21 in which the said gradient coils comprising discrete winding patterns extending over an elliptic-cylindrical surface which is oriented with its longitudinal axis perpendicular to the direction of the static magnetic field generated by the magnet.

23. An MRI system according to claim 21, in which, the gradient coils are configured to be driven by the control unit for generating gradient fields along three Cartesian axis, the said magnet fields permeating a region of interest, the said gradient coils comprising discrete winding patterns configured to generate a current distribution on the surface of an elliptical cylinder the magnet of the MRI system generating a static magnetic field oriented perpendicular to the longitudinal axis of the said elliptic cylindrical surface.