Eddy Current Compensation for Gradient Array Coils With Explicit Eddy Loss Constraints on the Cryostat: An Electromagnetic Approach

We aim to mitigate the effects of secondary fields generated by induced eddy currents in the context of gradient array coils while simultaneously managing the time-averaged ohmic power losses within the cryostat. The proposed electromagnetic approach streamlines the tuning process and provides customized solutions for array coils of diverse configurations. It can be considered a dynamic and array-specific extension of the pre-emphasis technique employed in conventional gradient coils. We sample and record the net (impressed and secondary) electromagnetic fields within the imaging region and on the cryostat's surface just once. Utilizing the electromagnetic Poynting theorem and the extracted data, we optimize for a complex constellation of array currents across discrete frequencies, adhering to a set of explicit constraints. These constraints fulfill the performance parameters within the imaging volume while minimizing ohmic power losses within the cryostat assembly and across the copper wires. For numerical demonstrations, we deploy a 48-element whole-body z-gradient array coil embedded within a stainless-steel cryostat assembly and compute 48 gradient waveforms that compensate for intense eddy currents generated by a trapezoidal pulse of 300 T/m/s slew rate. Our computational approach enables precise control over eddy and copper losses, simplifies array coil tuning on-the-fly, and can be adapted to various gradient coils. It enhances the performance and controllability of array-based medical imaging equipment affected by eddy currents.