Head-and-neck multi-channel B1+ mapping and carotid arteries RF shimming using a parallel transmit head coil

Abstract

Purpose: Neurovascular MRI suffers from a rapid drop in B1+ into the neck when using transmit head coils at 7T. One solution to improving B1+ magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel transmit (pTx) head coils. However, calculating such shims requires robust multi-channel B1+ maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multi-channel B1+ mapping techniques, and B0 sensitivity. We therefore sought a robust large-dynamic-range pTx field mapping protocol, and tested whether RF shimming can improve carotid artery B1+ in practice. Methods: A pipeline is presented that combines B1+ mapping data acquired using circularly polarized (CP-) and CP2-mode RF shims at multiple voltages. The pipeline was evaluated by comparing the predicted and measured B1+ for multiple random transmit shims, and by assessing the ability of RF shimming to increase the B1+ in the carotid arteries. Results: The proposed method achieved good agreement between predicted and measured B1+ in both the head and the neck. The B1+ magnitude in the carotid arteries can be increased by 42% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared to CP mode. Conclusion: B1+ in the neck can be increased using RF shims calculated from multi-channel B1+ maps in both the head and the neck. This can be achieved using universal phase-only RF shims, facilitating easy implementation in existing sequences.