Flexible metasurface for improving brain imaging at 7T

Abstract: 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.