Parallel-Excitation Techniques for Ultra-High-Field MRI

Abstract

Theoretical work on parallel RF transmission and recent experimental implementations on prototype systems indicate that parallel excitation has the potential to overcome critical obstacles to robust and routine human scanning at high field strength. As these developments are extended to 16- and 32-channel neuroimaging arrays, it seems likely that very-high-field human brain imaging will be possible with an essentially constant flip angle across the human head with RF pulse durations comparable to current slice-selective pulses. While most work has been concentrated on head-sized transmitters, the problem of B 1 homogeneity in the torso becomes significant at lower B 0 fields. Thus, the first clinical applications might be for body transmit coils at 3 T. Of course, intriguing research questions remain open in several areas, including optimal coil array designs that minimize element couplings and maximize spatial orthogonality of individual channels, the estimation and real-time monitoring of local SAR during simultaneous application of RF in excitation coil arrays, and the development of rapid and robust RF pulse designs that extend the current low-flip-angle domain to an arbitrary excitation angle, such as spin echoes, saturation, and inversions pulses. However, with continued active research in these areas, progress is likely to accelerate, and one can envision logical additions to the architecture of a current clinical scanner that readily accommodates the requirements of a general parallel RF excitation system. These include coil arrays optimized for parallel transmit and receive, modular blocks of RF synthesis and amplification with individual characterization of amplifier nonlinearities, subject-specific models of local SAR deposition for monitoring, and a rapid B 1-estimation pre-scan that feeds fast RF-pulse-design software capable of incorporating E 1 constraints.