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Optimized Amplitude Modulated Multiband RF Pulse Design

Research output: Contribution to journalArticle

Original languageEnglish
Pages (from-to)2185-2193
JournalMagnetic Resonance in Medicine
Volume78
Issue number6
Early online date17 Jan 2017
DOIs
Publication statusPublished - Dec 2017

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King's Authors

Abstract

Purpose: Multiband pulses are characterized by highly temporally
modulated waveforms. Rapid phase or frequency modulation
can be extremely demanding on the performance of radiofrequency
(RF) pulse generation, which can lead to errors that can
be avoided if pulses are restricted to amplitude modulation
(AM) only. In this work, three existingmultiband pulse design techniques
are modified to produce AMwaveforms.
Theory and Methods: Multiband refocusing pulses were
designed using phase-optimization, time-shifting, and rootflipping.
Each technique was constrained to produce AM pulses
by exploiting conjugate symmetry in their respective frequency
domain representations. Pulses were designed using the AM and
unconstrained techniques for a range of multiband factors (ie,
number of simultaneously excited slices), time-bandwidth products,
and slice separations. Performance was compared by
examining the resulting effective pulse durations. Phantom and in
vivo experiments were conducted for validation.
Results: Acquired data confirmed that AM pulses can produce
precise results when unconstrained designs may produce artifacts.
The average duration of AM pulses is longer than the
unconstrained versions. Averaged across a range of parameters,
the duration cost for AM pulses was 26, 38, and 20% for phaseoptimizing,
time-shifting and root-flipping, respectively.
Conclusions: Amplitude modulation multiband pulses can be
produced for a relatively small increase in pulse duration.

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