  Ultra high field (UHF) brain MRI has proved its value by providing enhanced
SNR, contrast, and higher resolution derived from the higher magnetic field
(B0). Nonetheless, with the increased B0 of UHF MRI, the transmit RF magnetic
field (B1+) inhomogeneity also became one of the critical issues requiring
attention. As the effective wavelength of RF becomes comparable or smaller than
the dimension of the brain at B0 larger than 7 Tesla, the increased B1+
inhomogeneity of UHF MRI results in poor SNR and uneven contrast. While
parallel transmission techniques (PTx) and high permittivity material (HPM)
structures for the mitigation of B1+ inhomogeneity have been suggested, the
associated complexity in PTx and restricted volume of homogenization with HPM
approach still remain as challenges. In this work, we address the B1+
inhomogeneity in the notion of mode-shaping. Treating a brain phantom as a
dielectric potential-well resonator, we apply a phantom-conformal HPM
potential-well in combination with a low-index potential barrier (air), to
achieve the homogeneity of B1+ in the region of interest (ROI). Based on the
electromagnetic simulations using a realistic brain model at 7T, we show that
the proposed HPM structure reduces both the average deviation of B1+ in axial
slices by 54% and peak SAR by 42%, respectively.
