Magnetic Resonance in Medicine
Magnetic Resonance in Medicine
3 years ago

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

Christopher J. Wiggins, Benedikt A. Poser, Desmond H.Y. Tse
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.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/mrm.26586

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