Evaluation of new MR invisible silicon carbide based dielectric pads for 7 T MRI.

PURPOSE: The use of dielectric pads to redistribute the radiofrequency fields is currently a popular solution for 7 T 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. METHOD: 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 done 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. RESULTS: 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. CONCLUSION: We demonstrate the long-term performance and invisibility of these new pads in order to increase the contrast in the brain temporal lobes in a commercial 7 T MRI head coil.

Compressed perovskite aqueous mixtures near their phase transitions show very high permittivities: New prospects for high-field MRI dielectric shimming.

PURPOSE: Perovskites are greatly used nowadays in many technological applications because of their high permittivity, more specifically in the form of aqueous solutions, for MRI dielectric shimming. In this study, full dielectric characterizations of highly concentrated CaTiO3 /BaTiO3 water mixtures were carried out and new permittivity maxima was reached. METHODS: Permittivity measurements were done on aqueous solutions from 0%v/v to dry powder. The permittivity dependence with pressure was investigated. Scanning electron microscopy images were performed on a few representative solutions. BaTiO3 pressed pads of different thicknesses, permittivities, and distances to the head were compared in a 7T MRI scanner. RESULTS: Perovskite aqueous mixtures undergo a pressure-dependent phase transition in terms of permittivity, with increasing water content. A new relative permittivity maximum of 475 was achieved. Microscopic images revealed structural differences between phases. A B1+ improvement in the temporal lobe was obtained with thin, high permittivity BaTiO3 head. CONCLUSIONS: This new preparation method allows improved pad geometry and placement, as a result of the high relative permittivity values achieved. This method has great significance for medical applications of MRI dielectric shimming, being easy to replicate and implement on a large scale. Magn Reson Med 79:1753-1765, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.