B1+ Homogenization in 7T MRI Using Mode-shaping with High Permittivity Materials

TL;DR

This paper introduces a novel mode-shaping approach using high permittivity materials to improve B1+ homogeneity in 7T MRI, reducing inhomogeneity and SAR without complex parallel transmission systems.

Contribution

The study proposes a mode-shaping method with HPM structures to address B1+ inhomogeneity in 7T MRI, offering a simpler alternative to existing techniques.

Findings

B1+ inhomogeneity reduced by 54% in simulations

Peak SAR decreased by 42%

Effective inhomogeneity mitigation with simplified setup

Abstract

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.