Parallel excitation in the human brain at 9.4 T counteracting k-space errors with RF pulse design.

Multidimensional spatially selective radiofrequency (RF) pulses have been proposed as a method to mitigate transmit B1 inhomogeneity in MR experiments. These RF pulses, however, have been considered impractical for many years because they typically require very long RF pulse durations. The recent development of parallel excitation techniques makes it possible to design multidimensional RF pulses that are short enough for use in actual experiments. However, hardware and experimental imperfections can still severely alter the excitation patterns obtained with these accelerated pulses. In this note, we report at 9.4 T on a human eight-channel transmit system, substantial improvements in two-dimensional excitation pattern accuracy obtained when measuring k-space trajectories prior to parallel transmit RF pulse design (acceleration x4). Excitation patterns based on numerical simulations closely reproducing the experimental conditions were in good agreement with the experimental results.