title: Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil.
abstract: 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.
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. kawin@mit.edu
authors.forename: Kawin
authors.initials: K
authors.lastname: Setsompop
authors.forename: Vijayanand
authors.initials: V
authors.lastname: Alagappan
authors.forename: Borjan
authors.initials: B
authors.lastname: Gagoski
authors.forename: Thomas
authors.initials: T
authors.lastname: Witzel
authors.forename: Jonathan
authors.initials: J
authors.lastname: Polimeni
authors.forename: Andreas
authors.initials: A
authors.lastname: Potthast
authors.forename: Franz
authors.initials: F
authors.lastname: Hebrank
authors.forename: Ulrich
authors.initials: U
authors.lastname: Fontius
authors.forename: Franz
authors.initials: F
authors.lastname: Schmitt
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
authors.forename: Elfar
authors.initials: E
authors.lastname: Adalsteinsson
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted