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B 1 ‐insensitive fast spin echo using adiabatic square wave enabling of the echo train (SWEET) excitation
Author(s) -
Madhuranthakam Ananth J.,
Busse Reed F.,
Brittain Jean H.,
Rofsky Neil M.,
Alsop David C.
Publication year - 2008
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.21630
Subject(s) - excitation , amplitude , physics , adiabatic process , robustness (evolution) , nuclear magnetic resonance , pulse wave , radio frequency , pulse sequence , signal (programming language) , optics , computational physics , acoustics , computer science , chemistry , telecommunications , quantum mechanics , laser , biochemistry , gene , programming language
Abdominal images at 3T acquired with fast spin echo (FSE) sequences often exhibit signal voids due to RF transmit field inhomogeneities. Theory suggests, however, that the repeated refocusing pulses of FSE are capable of maintaining signal even at reduced RF amplitudes if the magnetization is suitably prepared. Here we propose a modified excitation strategy for FSE that is more robust to transmit field inhomogeneities than conventional FSE. The new excitation approach replaces the standard 90° excitation pulse with a discretely sampled hyperbolic secant pulse that creates a square wave longitudinal magnetization as a function of gradient and off‐resonance induced phase shifts between the subsequent echoes of the FSE sequence. This pulse is followed by the conventional train of refocusing pulses except that the first few pulses increase from near zero to the desired refocusing amplitude. Simulations and in vivo results at 3T indicate preserved image quality and much greater robustness of this new sequence to nonuniform RF fields. This robustness comes at the cost of 20% reduction in signal when the RF field is uniform and increased motion sensitivity. This RF field‐insensitive sequence may overcome challenges of body imaging at high field and in patients with ascites. Magn Reson Med 59:1386–1393, 2008. © 2008 Wiley‐Liss, Inc.