Minimum TR radiofrequency-pulse design for rapid gradient echo sequences

Magn Reson Med. 2021 Jul;86(1):182-196. doi: 10.1002/mrm.28705. Epub 2021 Feb 15.

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.

Keywords: cardiac; radiofrequency pulse design; rapid imaging; simultaneous multislice (SMS).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms*
  • Brain
  • Heart
  • Heart Rate
  • Magnetic Resonance Imaging*
  • Phantoms, Imaging
  • Radio Waves