Enhanced Whole Brain Excitation Performance of Parallel Transmission with a Z-Encoding RF Coil Array at 7T

Xiaoping Wu¹, Sebastian Schmitter¹, Gregor Adriany¹, Edward J. Auerbach¹, Kamil Ugurbil¹, Pierre-Francois Van de Moortele¹

Multi-channel transmit B1 (B1+) manipulation techniques, such as static B1 shimming and parallel transmission (pTX), can be used to reduce the severe B1+ inhomogeneities that arise at 7T and above. One of the fundamental limits of multi-channel B1 manipulation is determined by the spatial encoding capabilities of the utilized multi-element RF coil, although different B1 methods, e.g. static B1 shim vs. spoke trajectories, will not be affected similarly by a given RF coil geometry. In a previous study it has been shown that distributing individual elements of a 7T head array coil in two rings along Z direction provided better RF efficiency than a standard single ring array when applying static B1 shimming over a large brain area. The goal of the current study was to determine, by comparing two transceiver arrays, whether, and to what extent, the use of kT points, a promising approach recently proposed to achieve homogeneous excitation over the whole brain at 7T, would also benefit from RF coil elements distributed along the Z axis.