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 B1(+) 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 B1(+) pattern, though the B1(+) field was about 25% weaker in measurements than simulations. Global shim settings were able to prevent B1(+) field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B1(+) 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 B1(+) 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 B1(+) homogeneity against power-efficiency and local SAR.