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Slice‐selective RF pulses for in vivo B inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16‐element coil

2008; Wiley; Volume: 60; Issue: 6 Linguagem: Inglês

10.1002/mrm.21739

1522-2594

Kawin Setsompop, Vijayanand Alagappan, Borjan Gagoski, Thomas Witzel, Jon̈athan R. Polimeni, Andreas Potthast, F. Hebrank, Ulrich Fontius, Franz Schmitt, Lawrence L. Wald, Elfar Adalsteinsson,

Advanced NMR Techniques and Applications

Abstract Slice‐selective RF waveforms that mitigate severe B inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16‐element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least‐squares (MLS) criteria with an optimized k ‐space excitation trajectory to significantly improve profile uniformity compared to conventional least‐squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in‐plane flip angle (FA) with slice selection in the z ‐direction were demonstrated and compared with conventional sinc‐pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B on the excitation FA. The optimized parallel RF pulses for human B mitigation were only 67% longer than a conventional sinc‐based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in‐plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse‐duration penalty. Magn Reson Med 60:1422–1432, 2008. © 2008 Wiley‐Liss, Inc.