  Ultra-high field MRI offers unprecedented detail for non-invasive
visualization of the human brain. However, brain imaging is challenging at 7T
due to the B$_1^+$ field inhomogeneity, which results in signal intensity drops
in temporal lobes and a bright region in the brain center. This study aims to
evaluate using a metasurface to improve brain imaging at 7T and simplify the
investigative workflow. Two flexible metasurfaces, each comprising a periodic
structure of copper strips and parallel-plate capacitive elements printed on an
ultra-thin substrate, were optimized for brain imaging and implemented via PCB.
We considered two setups: (1) two metasurfaces located near the temporal lobes;
and (2) one metasurface placed near the occipital lobe The effect of
metasurface placement on the transmit efficiency and specific absorption rate
was evaluated via electromagnetic simulation studies with voxelized models. In
addition, their impact on SNR and diagnostic image quality was evaluated in
vivo for male and female volunteers. Placement of metasurfaces near the regions
of interest led to an increase in homogeneity of the transmit field by 5$\%$
and 10.5$\%$ in the right temporal lobe and occipital lobe for a male subject,
respectively. SAR values changed insignificantly and were under recommended
limits. In vivo studies also confirmed the numerically predicted improvement in
field distribution and receive sensitivity in the desired ROI. Optimized
metasurfaces enable homogenizing transmit field distribution in the brain at
7T. The proposed lightweight and flexible structure has the potential to
provide MR examination with higher diagnostic value images.
