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, 2016. © 2016 International Society for Magnetic Resonance in Medicine.
- ultra-high-field MR
- parallel transmission
- excitation errors
- bipolar gradient spokes pulses
- SIMULTANEOUS MULTISLICE
- RF EXCITATION
- HUMAN BRAIN
- PEAK POWER