Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil

• Published in: Magnetic resonance in medicine Vol. 60; no. 6; pp. 1422 - 1432
• Main Authors: Setsompop, Kawin, Alagappan, Vijayanand, Gagoski, Borjan, Witzel, Thomas, Polimeni, Jonathan, Potthast, Andreas, Hebrank, Franz, Fontius, Ulrich, Schmitt, Franz, Wald, Lawrence L, Adalsteinsson, Elfar
• Format: Journal Article
• Language: English
• Published: United States 01.12.2008
• Subjects: Algorithms; Brain - anatomy & histology; Image Enhancement - methods; Image Interpretation, Computer-Assisted - methods; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; Phantoms, Imaging; Reproducibility of Results; Sensitivity and Specificity; Signal Processing, Computer-Assisted
• ISSN: 1522-2594; 0740-3194; 1522-2594
• DOI: 10.1002/mrm.21739

Slice-selective RF waveforms that mitigate severe B1+ 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 B1+ on the excitation FA. The optimized parallel RF pulses for human B1+ 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.