The use of dielectric pads to redistribute the radiofrequency fields is currently a popular solution for 7T MRI practical applications, especially in brain imaging. In this work, we tackle several downsides of the previous generation of dielectric pads. This new silicon carbide recipe makes them MR invisible and greatly extends the performance lifespan.

We produce a set of two 10x10x1cm3 dielectric pads based on silicon carbide (SiC) powder dispersed in 4-Fluoro 1, 3-dioxalan-2-one (FEC) and polyethylene Glycol (PEG). The stability of the complex permittivity and the invisibility of the pads are characterized experimentally. Numerical simulations are carried out to evaluate global and local SAR over the head in presence of the pads. B0, B1+ and standard imaging sequences are performed on healthy volunteers.

SiC pads are compared to state-of-the-art perovskite based dielectric pads with similar dielectric properties (barium titanate). Numerical simulations confirm that head and local SAR are similar. MRI measurements confirm that the pads do not induce susceptibility artefacts and improve B1+ amplitude in the temporal lobe regions by 25% on average.

We demonstrate the long-term performance and invisibility of these new pads to increase the contrast in the brain temporal lobes in a commercial 7 T MRI head coil.

DOI: https://doi.org/10.1016/j.mri.2022.04.002