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Description
This study explores the design of high-permittivity materials (HPMs) for magnetic resonance imaging (MRI) at three distinct Larmor frequencies. The objective of the work was to assess the influence of heterogeneous head structures, commonly used in numerical simulations, on the design of HPMs when utilizing analytical tools. To achieve this, a brain MRI experiment was simulated, incorporating an HPM helmet surrounding the head and a current-carrying coil for radiofrequency illumination. The investigation employed both an analytical scattering model and numerical simulations using the xFDTD software (Remcom).
The novel theoretical tool introduced in this study is based on Mie scattering formulation but utilizes Hankel functions of the first and second kind to describe the radial dependence of the electromagnetic fields. This innovative approach extends the framework of transmission line theory, enabling a comprehensive analysis of scattering phenomena in terms of impedance and reflection coefficients.
The findings, in terms of magnetic induction fields as a function of helmet permittivity, reveal that the impact of brain tissue heterogeneity on HPM design becomes more pronounced as the RF radiation frequency increases. This effect is attributed to the shorter wavelength at higher frequencies, which interacts more significantly with the varying tissue properties. Despite this, the analytical model proves effective in predicting optimal permittivity values or material thicknesses for specific applications. As such, this tool can be proposed as a valuable aid to support numerical simulations in the design of these materials.
