FIELD SYNTHESIS IN INHOMOGENEOUS MEDIA: JOINT CONTROL OF POLARIZATION, UNIFORMITY AND SAR IN MRI B1-FIELD

The homogeneity of the amplitude of one of the polarizations of the RF field B1 is a crucial issue in Magnetic Resonance Imaging (MRI), and several methods have been proposed for enhancing this uniformity (``Shimmingʹʹ). The existing approaches aim at controlling the homogeneity of B+1 and limiting the Specific Absorption Rate (SAR) of the RF field by independently controlling magnitude and phase of individual excitation currents in MRI scanners, either birdcage or TEM coil system. A novel approach is presented here which allows a joint control of B+1 uniformity, SAR, and purity of polarization of the total RF B1 field. We propose a convex optimization procedure with convex constraints, and special attention has been devoted to the issue of convexity of the proposed functional. The method is applied to MRI brain imaging; numerical tests have been performed on a realistic head model at low, medium, and high RF field in order to assess the effectiveness of the proposed method. We found that maintaining a specific polarization plays an important role also in maintaining the homogeneity of B+1 amplitude.