Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array.

Objective: To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array. Materials and methods: A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis. Results: Numerical results were consistent with experiments regarding component values, S-parameters and B pattern, though the B field was about 25 % weaker in measurements than simulations. Global shim settings were able to prevent B field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B standard deviations of below 10 % without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B efficiency and SAR limitations must be considered. Conclusion: Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B homogeneity against power-efficiency and local SAR. [ABSTRACT FROM AUTHOR]