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Dielectric resonance effects and radio-frequency (RF) power deposition have become challenging issues for magnetic resonance imaging at ultrahigh-field (UHF) strengths. The use of transmit (Tx) coil arrays with independently-driven RF sources using a parallel transmission system is a promising method for alleviating the resulting RF inhomogeneities. In this study, the effect on homogeneity and RF-power when employing a higher number of transmit channels with multi-slice acquisition in vivo at high field strength (7T) is scrutinized. An 8-channel head coil array was driven to emulate circular polarized (CP) and 2-, 4-, and 8-channel independent transmit configurations at 7T. Static RF shimming was employed on human subjects in order to homogenize the B₁⁺ field in the excited volume. Slice-selective and global RF shimming methods were applied with CP and 2-, 4-, and 8-channel transmit channel configurations. RF shimming was performed from CP to 2-, 4-, and 8-channel Tx configurations globally and slice-selectively. Systematic improvement in B₁⁺ homogeneity and/or reduction in RF-power were observed. RF shimming in the human brain with 8-channel transmit and slice-selective shimming yields an increase in B₁⁺ homogeneity of 43% and/or reduces RF-power by 68% when compared with CP global RF shimming at 7T.

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Gain of Imaging Fidelity by Employing a Higher Number of Independent Transmit Channels Together with Slice-Selective Radio-Frequency (RF) Shimming at 7T