Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array
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- Hoffmann, J., Shajan, G., Scheffler, K. et al. Magn Reson Mater Phy (2014) 27: 373. doi:10.1007/s10334-013-0419-y
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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.
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.
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.