Eddy-Current-Compensated RF Pulse Design for Parallel Excitation

Introduction: High-performance RF coils and shielded Gradient coils [1] for high or ultra high field MRI often require the use of RF shields that are in close proximity to the imaged volume. These shields can sometimes generate Eddy-Currents that are not adequately compensated for using the pre-emphasis algorithm [2] of the scanner. K-space trajectory measurements [3] can be used to compensate for such Eddy-Currents but they require timeconsuming calibrations that are not suited for parallel transmit (PTX) RF pulse design. The Eddy-Current distortions induced by the RF coil’s shield are heavily dependent on the excitation trajectory and can sometimes severely compromise the practical implementation of highly desirable excitation patterns. In this work, we present a method to produce RF-coil-induced Eddycurrent–compensated RF pulse design in PTX. The approach is demonstrated with both simulated and experimental data at 7T. Methods: Theory: Various mathematical techniques have been proposed for calculating the characteristic time constants and amplitudes for multi-exponential decay from eddy current field measurements [6-7]. The proposed method relies on such multi-exponential fitting of the eddy current fields in order to produce a model of the expected distortion in the magnetization profile. The mathematical model used for the gradient waveform’s distortions due to the eddy currents is as follows,