Shaping the RF Transmit\nField in 7T MRI Using a Nonuniform\nMetasurface Constructed of Short Conducting Strips

The ability of metamaterial structures to offer unique\nproperties\nand new solutions has opened new avenues in a wide range of applications,\nincluding super-resolution in optics and efficient antennas in radiofrequency\n(RF) engineering. In magnetic resonance imaging (MRI), metamaterials\nhold the promise of increasing the RF magnetic field intensity while\nminimizing power deposition. Here, we propose a metasurface based\non a two-dimensional (2D) array of short conducting strips combined\nwith a high dielectric substrate, which was tuned to operate at ultrahigh\nfield 7T human MRI. While studied in optics and electromagnetics in\nthe GHz-to-THz range, this study is the first to design such a metasurface\nfor proton imaging at 7T MRI. We performed electromagnetic (EM) simulation\nof the brain MRI setup with the new metasurface placed in the proximity\nto the temporal lobe, which showed 2.2-fold local increase in the\nRF transmit efficiency, with superior performance than an array of\nelectric dipoles. In this study, we also investigate the effect of\nthe spatial distribution of the subunits to control the target RF\nfield’s distribution. While the common design is based on a\nuniform distribution of the subunits, nonuniform distribution, such\nas a denser center (convex) or more condensed edges (concave), provides\nan extra dimension to tailor both the magnetic and electric fields.\nThe concave distribution achieved 1.5–1.8-fold reduction in\nthe power deposition compared to the uniform distribution in the brain\nMRI setups examined.