Composite RF pulses for B 1 + -insensitive volume excitation at 7 Tesla

• Published in: Journal of magnetic resonance (1997) Vol. 205; no. 1; pp. 50 - 62
• Main Authors: Moore, Jay, Jankiewicz, Marcin, Zeng, Huairen, Anderson, Adam W., Gore, John C.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.07.2010
• Subjects: BIR-4; Composite radiofrequency pulses; RF field inhomogeneity; Ultra-high field human imaging; RF field inhomogeneity; Composite radiofrequency pulses; Ultra-high field human imaging; BIR-4
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2010.04.002

A new class of composite RF pulses that perform well in the presence of specific ranges of B 0 and B 1 + inhomogeneities has been designed for volume (non-selective) excitation in MRI. The pulses consist of numerous (∼100) short (∼10 μs) block-shaped sub-pulses each with different phases and amplitudes derived from numerical optimization. Optimized pulses are designed to be effective over a specific range of frequency offsets and transmit field variations and are thus implementable regardless of field strength, transmit coil configuration, or the subject-specific spatial distribution of the static and RF fields. In the context of 7 T human brain imaging, both simulations and phantom experiments indicate that optimized pulses result in similar on-resonance flip-angle uniformity as BIR-4 pulses but with the advantages of superior off-resonance stability and significantly reduced average power. The pulse design techniques presented here are thus well-suited for practical application in ultra-high field human MRI.