Optimal Transmission Performance of Quadrature Wireless Resonators at 3 T.

PURPOSE: To demonstrate a method for optimizing the efficiency of wireless resonators using tailored excitation from a dual-transmit birdcage coil (BC), and to present the design and experimental investigation of a quadrature resonator for 3 T breast MRI application. METHODS: A double-mode quadrature resonator was designed to enhance both orthogonal components of the B 1 field in a 3 T BC. Flip-angle maps measurements were performed on phantom to validate the predicted transmit efficiency gains. Numerical simulations were performed on a human voxel model to demonstrate the ability to optimize the B 1 field and SAR metrics. RESULTS: Experiments confirmed that each linear polarization was enhanced by the resonator, yielding a 4.5-fold improvement in the transmit efficiency compared with the reference BC under conventional circularly polarized (CP) excitation. The optimal excitation was derived and validated experimentally, leading to an additional 2% increase in transmit efficiency. Simulations revealed that the resonator provided a 5.9-fold (left breast) and 5.2-fold (right breast) improvement in SAR efficiency under conventional CP excitation. Tailored excitations further increased SAR efficiency by 15% (left breast) and 31% (right breast), approaching a 7-fold improvement compared to the reference BC. CONCLUSION: A double-mode quadrature resonator was demonstrated for 3 T MRI. Combined with dual-transmit BC, it enabled B 1 focusing and significantly improved transmit and SAR efficiency. Our analytical model allowed the derivation of tailored BC excitation to approach the theoretical performance limit of wireless resonators.