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

Tse, Desmond H.Y., Wiggins, Christopher J. and Poser, Benedikt A. (2016) Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency-gradient delay: Example of bipolar spokes pulses in parallel transmission. Magnetic Resonance in Medicine, . doi:10.1002/mrm.26586


Author Tse, Desmond H.Y.
Wiggins, Christopher J.
Poser, Benedikt A.
Title Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency-gradient delay: Example of bipolar spokes pulses in parallel transmission
Journal name Magnetic Resonance in Medicine   Check publisher's open access policy
ISSN 0740-3194
1522-2594
Publication date 2016-12-26
Year available 2016
Sub-type Article (original research)
DOI 10.1002/mrm.26586
Open Access Status Not yet assessed
Total pages 8
Place of publication Hoboken, NJ, United States
Publisher John Wiley & Sons
Collection year 2017
Language eng
Formatted abstract
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. 
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: HERDC Pre-Audit
Centre for Advanced Imaging Publications
 
Versions
Version Filter Type
Citation counts: Google Scholar Search Google Scholar
Created: Mon, 20 Mar 2017, 14:53:27 EST by Ms Kate Rowe on behalf of Learning and Research Services (UQ Library)