Using Dedicated Fieldprobes for Trajectory Measurements in Parallel Excitation Experiments

Introduction: Parallel spatially selective EXcitation (PEX) relies on an exact matching of RF pulses and a simultaneously traversed k-space trajectory. However, e.g. eddy currents or gradient amplifier bandwidth limitations may lead to deviations of the traversed trajectory resulting in reduced excitation accuracy due to the susceptibility of PEX pulses to trajectory errors [1]. Therefore, different methods for measuring actually traversed k-space trajectories and for calculating PEX pulses based on such calibration data have been presented and have proven to be advantageous [1,2]. However, most of the applied trajectory measurement techniques are based on phase evolutions in situ within the object and suffer therefore, especially at high field strengths and for long trajectories, from object-dependent limitations like fast T2 decays, off-resonances, intra-volume effects or motion artifacts. To overcome these limitations, this work exploits the field monitoring approach [3,4] using newly developed field probes on D2O basis in order to achieve object-independent trajectory measurements. This allows a robust acquisition of calibration data for the calculation of PEX pulses and results in parallel excitation with high accuracy.