Volumetric B1+ field homogenization in 7 Tesla brain MRI using metasurface scattering

TL;DR

This paper introduces a metasurface-based method to improve B1+ field homogeneity in 7 Tesla brain MRI, significantly reducing inhomogeneity and local heating, surpassing the performance of 3 Tesla MRI.

Contribution

The authors develop a systematic metasurface design inspired by scattering theory for volumetric B1+ field homogenization in ultrahigh field MRI, with a pruning technique for practical implementation.

Findings

Over twofold improvement in field homogeneity

Suppressed local heating in brain models

Better performance than 3 Tesla MRI

Abstract

Ultrahigh field magnetic resonance imaging (UHF MRI) has become an indispensable tool for human brain imaging, offering excellent diagnostic accuracy while avoiding the risks associated with invasive modalities. When the radiofrequency magnetic field of the UHF MRI encounters the multifaceted complexity of the brain, characterized by wavelength-scale, dissipative, and random heterogeneous materials, detrimental mesoscopic challenges such as B1+ field inhomogeneity and local heating arise. Here we develop the metasurface design inspired by scattering theory to achieve the volumetric field homogeneity in the UHF MRI. The method focuses on finding the scattering ansatz systematically and incorporates a pruning technique to achieve the minimum number of participating modes, which guarantees stable practical implementation. Using full-wave analysis of realistic human brain models under a 7 Tesla MRI, we demonstrate more than a twofold improvement in field homogeneity and suppressed local heating, achieving better performance than even the commercial 3 Tesla MRI. The result shows a noninvasive generalization of constant intensity waves in optics, offering a universal methodology applicable to higher Tesla MRI.