System and method for electromagnet coil construction and operation
A method of manufacturing electromagnet coils for use in a magnetic resonance imaging (MRI) system is provided. The method comprises forming a coil representation of a coil surface for the electromagnet coils; setting a plurality of performance metric requirements for a plurality of performance metrics for the electromagnet coils, the plurality of performance metrics including a magnetic field-shape metric and an eddy-field metric; forming a performance functional, based on the coil representation and the plurality of performance metrics, for generating a current density pattern over the coil surface; optimizing the performance functional based on the plurality of performance metric requirements; generating a current density pattern over the coil surface based on the minimized performance functional; and obtaining coil windings from the current density pattern.
1. A method of manufacturing electromagnet coils for use in a magnetic resonance imaging (MM) system, the method comprising:
forming a coil representation of a coil surface for the electromagnet coils;
setting a plurality of performance metric requirements for a plurality of performance metrics for the electromagnet coils, the plurality of performance metrics including a magnetic field-shape metric and an eddy-field metric;
forming a performance functional, based on the coil representation and the plurality of performance metrics, for generating a current density pattern over the coil surface;
optimizing the performance functional based on the plurality of performance metric requirements;
generating a current density pattern over the coil surface based on the minimized performance functional;
obtaining coil windings from the current density pattern;
wherein the MM system further comprises correction coils;
wherein the eddy-field metric is a predetermined eddy field shape;
wherein optimizing the performance functional further comprises constraining an eddy current density representation over an eddy-field producing structure surface such that eddy fields predicted by the eddy-current density representation conform to said predetermined eddy field shape; and
obtaining further coil windings for said correction coils from said eddy-current density representation thereby enabling the correction coils to compensate for said eddy fields.
2. The method of claim 1 wherein said correction coils are second order correction coils and wherein the eddy current density representation is constrained by suppressing third and higher order modes.
3. The method of claim 2 further including weighting said higher orders modes for defining said predetermined eddy field shape.
4. The method of claim 1
wherein the eddy-field metric further comprises a target eddy field shape; and
wherein the optimizing of the performance functional further comprises minimizing the difference between the target eddy field shape and a predicted eddy field generated based on the performance functional.
5. The method of claim 1 wherein the optimizing of the performance functional further comprises minimizing a predicted eddy-current density pattern generated over an eddy-field producing structure surface based on the performance functional.
6. The method of claim 1 wherein the performance metrics can further comprise at least one of: a net force metric, a net torque metric, a dissipative power metric and an energy metric.
7. The method of claim 1 further comprising:
forming a plurality of performance metric functions based on the plurality of performance metrics, and
wherein the performance functional is formed from the plurality of performance metric functions.
8. The method of claim 1 wherein the coil representation is based on a boundary element method.
9. The method of claim 1 wherein the coil surface is cylindrical.
10. The method of claim 1 wherein the eddy-field producing structure is at least one of a bore of a main magnet and end flanges of a main magnet.
11. The method of claim 1 further comprising:
generating simulated eddy currents based on the optimized performance functional;
determining compensation currents based on the simulated eddy currents for correcting eddy fields producible by the simulated eddy currents.
12. The method of claim 11 wherein the electromagnet coils are gradient coils and the compensation currents are to be generated by gradient coils during the operation of the MRI.
13. The method of claim 1 wherein the determining further comprises:
minimizing the eddy fields producible by the simulated eddy currents.
14. The method of claim 1 wherein the electromagnet coils are gradient coils and wherein determining further comprises:
when the MM is constructed:
measuring the time and spatial variations of the eddy currents for a pulse sequence; and
determining the compensation currents based on the measured time and spatial variations of the eddy currents for the pulse sequence.
15. The method of claim 14 further comprising:
after the determining, storing the compensation currents in association with the pulse sequence.
16. The method of claim 1 wherein the electromagnet coils are one of a main magnet, gradient coils and said correction coils.
17. The method of claim 1 , wherein said eddy current density is based on spherical harmonics.
18. The method of claim 11 , wherein said compensation currents are generated by shim coils of at least 2 nd order.
19. The method of claim 11 , wherein compensation currents are generated by compensation coils configured to correct for said predicted eddy fields.
20. The method of claim 1 , wherein said compensation coils are designed based on one of either measured or simulated eddy currents.
21. An MM system manufactured in accordance with the method of claim 1 .