Parallel excitation holds strong promises to mitigate the impact of large transmit B1 (B1+) distortion at very high magnetic field. Accelerated RF pulses, however, inherently tend to require larger values in RF peak power which may result in substantial increase in Specific Absorption Rate (SAR) in tissues, which is a constant concern for patient safety at very high field. In this study, we demonstrate adapted rate RF pulse design allowing for SAR reduction while preserving excitation target accuracy. Compared with other proposed implementations of adapted rate RF pulses, our approach is compatible with any k-space trajectories, does not require an analytical expression of the gradient waveform and can be used for large flip angle excitation. We demonstrate our method with numerical simulations based on electromagnetic modeling and we include an experimental verification of transmit pattern accuracy on an 8 transmit channel 9.4 T system.
Bibliographical noteFunding Information:
The authors would like to acknowledge Lance DelaBarre for his help with setting up the 9.4 T multi-transmit system, Gregor Adriany and Carl Snyder for building the RF coils used in this study, Dinesh Deelchand and Pierre-Gilles Henry for their assistance with 3D AFI sequence development. We would also like to acknowledge the anonymous reviewers for their useful suggestions. This work was supported by the KECK Foundation and NIH funding (EB006835, EB007327, P41 RR008079 and P30 NS057091).
- 2D VERSE pulse
- High magnetic field MRI
- Parallel excitation
- RF pulse design
- Specific absorption rate