Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency‐gradient delay: Example of bipolar spokes pulses in parallel transmission

摘要

Purpose To eliminate a slice‐position–dependent excitation error commonly observed in bipolar‐gradient composite excitations such as spokes pulses in parallel transmission. Theory and Methods An undesired timing delay between subpulses in the composite pulse and their bipolar slice‐selective gradient is hypothesized to cause the error. A mathematical model is presented here to relate this mismatch to an induced slice‐position–dependent phase difference between the subpulses. A new navigator method is proposed to measure the timing mismatch and eliminate the error. This is demonstrated at 7 Tesla with flip‐angle maps measured by a presaturation turbo‐flash sequence and in vivo images acquired by a simultaneous multislice/echo‐planar imaging (SMS‐EPI) sequence. Results Error‐free flip‐angle maps were obtained in two ways: 1) by correcting the time delay directly and 2) by applying the corresponding slice‐position–dependent phase differences to the subpulses. This confirms the validity of the mathematical description. The radiofrequency (RF)‐gradient delay measured by the navigator method was of 6.3?μs, which agreed well with the estimate from flip‐angle maps at different delay times. By applying the timing correction, accurately excited EPI images were acquired with bipolar dual‐spokes SMS‐2 excitations. Conclusion An effective correction is proposed to mitigate slice‐position–dependent errors in bipolar composite excitations caused by undesired RF‐gradient timing delays. Magn Reson Med 78:1883–1890, 2017. ? 2016 International Society for Magnetic Resonance in Medicine.