Head‐and‐neck multichannel <scp>B1</scp><sup>+</sup> mapping and <scp>RF</scp> shimming of the carotid arteries using a <scp>7T</scp> parallel‐transmit head coil
Matthijs H. S. de Buck, James L. Kent, Peter Jezzard, Aaron T. Hess
Abstract
Abstract Purpose Neurovascular MRI suffers from a rapid drop in B 1 + into the neck when using transmit head coils at 7 T. One solution to improving B 1 + magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel‐transmit head coils. However, calculating such shims requires robust multichannel B 1 + maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multichannel B 1 + mapping techniques, and B 0 sensitivity. We therefore sought a robust, large‐dynamic‐range, parallel‐transmit field mapping protocol and tested whether RF shimming can improve carotid artery B 1 + magnitude in practice. Methods A pipeline is presented that combines B 1 + 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 B 1 + for multiple random transmit shims, and by assessing the ability of RF shimming to increase B 1 + in the carotid arteries. Results The proposed method achieved good agreement between predicted and measured B 1 + in both the head and the neck. The B 1 + magnitude in the carotid arteries can be increased by 43% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared with CP mode. Conclusion B 1 + in the neck can be increased using RF shims calculated from multichannel B 1 + maps in both the head and the neck. This can be achieved using universal phase‐only RF shims, facilitating easy implementation in existing sequences.