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Minimum-TR RF pulse design for rapid gradient echo sequences

Research output: Contribution to journalArticlepeer-review

Original languageEnglish
Pages (from-to)182-196
Number of pages15
JournalMagnetic Resonance in Medicine
Volume86
Issue number1
Early online date15 Feb 2021
DOIs
Accepted/In press11 Jan 2021
E-pub ahead of print15 Feb 2021
PublishedJul 2021

Bibliographical note

Funding Information: The Wellcome Trust/EPSRC Centre for Medical Engineering (WT 203148/Z/16/Z), the Medical Research Council (MR/K006355/1), the National Institute for Health Research Biomedical Research Centre, the EPSRC Centre for Doctoral Training in Medical Imaging (EP/L015226/1), and the Engineering and Physical Sciences Research Council (EPSRC) and Philips Healthcare (EP/L00531X/1) Funding Information: This work was supported by the Wellcome Trust/EPSRC Centre for Medical Engineering at King’s College London (WT 203148/Z/16/Z), the Medical Research Council (MR/K006355/1), and the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy's and St Thomas' NHS Foundation Trust and King's College London. This work was additionally supported by Imperial College London EPSRC Centre for Doctoral Training in Medical Imaging (EP/L015226/1), and the Engineering and Physical Sciences Research Council (EPSRC) and Philips Healthcare (EP/L00531X/1). We would like to thank both referees for their appreciation and valuable critique, which improved the theory section in this work. The views expressed are those of the authors and not necessarily those of the National Health Service, the NIHR, or the UK Department of Health. Publisher Copyright: © 2021 International Society for Magnetic Resonance in Medicine Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

King's Authors

Abstract

Purpose: A framework to design radiofrequency (RF) pulses specifically to minimize the TR of gradient echo sequences is presented, subject to hardware and physiological constraints. Methods: Single-band and multiband (MB) RF pulses can be reduced in duration using variable-rate selective excitation (VERSE) VERSE for a range of flip angles; however, minimum-duration pulses do not guarantee minimum TR because these can lead to a high specific absorption rate (SAR). The optimal RF pulse is found by meeting spatial encoding, peripheral nerve stimulation (PNS) and SAR constraints. A TR reduction for a range of designs is achieved and an application of this in an MB cardiac balanced steady-state free-precession (bSSFP) experiment is presented. Gradient imperfections and their imaging effects are also considered. Results: Sequence TR with low-time bandwidth product (TBP) pulses, as used in bSSFP, was reduced up to 14%, and the TR when using high TBP pulses, as used in slab-selective imaging, was reduced by up to 72%. A breath-hold cardiac exam was reduced by 46% using both MB and the TR-optimal framework. The importance of RF-based correction of gradient imperfections is demonstrated. PNS was not a practical limitation. Conclusion: The TR-optimal framework designs RF pulses for a range of pulse parameters, specifically to minimize sequence TR.

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