Double-Structure Dielectrics for Efficient Traveling-Wave Magnetic Resonance Imaging

Ultra-high-field (7 T) MRI offers high SNR but suffers from severe B₁⁺ inhomogeneity and standing-wave effects. Although traveling-wave (TW) propagation in an ideal homogeneous medium maintains constant field amplitude and therefore suggests intrinsic advantages for UHF excitation, practical MRI environments introduce conductive and dielectric loading, radiation losses, and mode mismatches that degrade TW efficiency and uniformity. Conventional dielectric-based approaches rely on a single dielectric structure, which cannot simultaneously optimize transmit efficiency, field homogeneity, and wave-impedance matching. To address this limitation, this study introduces a double-structure dielectric design that integrates a dielectric waveguide with a separate dielectric pad to maximize power-transmission efficiency while also improving field uniformity. The combined structure was optimized with respect to geometric parameters and placement. Simulations show that, compared with one structure, double-structure predict a 33.3% increase inPtissue. Experimental validation confirms a 39.4% gain in excitation efficiency over the conventional TW MRI through using the proposed double-structure dielectrics.