Nuts & Bolts of Advanced Imaging: Theory and Demonstration for Coils, RF Shimming and SAR

Highlights:  At high frequency MRI operation and electrically large (≥ 7T head, ≥ 3T body), asymmetrical, inhomogeneous, and/or irregularly-shaped loading, integer multiples of phase-shifts and uniform amplitudes are not necessarily the ideal characteristics to impose on the voltages driving the transmit array/coil in order to obtain a homogeneous transmit (with safe electric field) and/or receive fields.  If none of the coils on the Tx Array is producing a good excitation in the ROI, PTX and/RF shimming will not work.  Image voids due lack of RF excitation are caused by inhomogeneities and/or unsuitable polarization of the electromagnetic fields.  At 7T, there is typically good intensity of RF magnetic field across the volume of human head. At many instances, it possess unsuitable polarization.  While the physics (Maxwell’s) is well established, it is really difficult to make up definitive rules regarding SARs (especially local SARs)