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Rapid B 1 mapping using orthogonal, equal‐amplitude radio‐frequency pulses
Author(s) -
Chang Yulin V.
Publication year - 2012
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.23051
Subject(s) - flip angle , transmitter , amplitude , radio frequency , physics , phase (matter) , pulse sequence , excitation , dimension (graph theory) , field (mathematics) , noise (video) , pulse (music) , algorithm , computational physics , computer science , acoustics , nuclear magnetic resonance , mathematics , optics , channel (broadcasting) , telecommunications , combinatorics , medicine , quantum mechanics , magnetic resonance imaging , pure mathematics , radiology , artificial intelligence , detector , image (mathematics)
We present a new phase‐based method for mapping the amplitude of the radio‐frequency field ( B 1 ) of a transmitter coil in three‐dimension. This method exploits the noncommutation relation between rotations about orthogonal axes. Our implementation of this principle in the current work results in a simple relation between the phase of the final magnetization and the flip angle (FA). In this study, we focus on FAs less than 90°. Our method is rapid and easy to implement compared with the existing B 1 mapping schemes. The mapping sequence can be simply obtained by adding to a regular three‐dimensional gradient‐echo sequence a magnetization preparation radio‐frequency pulse of the same FA but orthogonal in phase to the excitation radio‐frequency pulse. This method is demonstrated capable of generating reliable maps of the B 1 field within 1 min using FAs no larger than 60°. We show that it is robust against T 1 , small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B 1 mapping in situations (e.g., long‐ T 1 and hyperpolarized‐gas imaging) where magnitude‐based methods are not readily applicable. A noise calculation of the FA map using this method is also presented. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.