Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

Abstract

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple “dielectric pads” for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…As we continue uphill, there is also a place for new opportunities and disruptive approaches which go beyond traditional RF coil design. One intriguing trajectory includes structures based on metamaterials to better manage RF wave propagation [49]. What can this approach offer to the high field climbers?…”

Advanced Radio Frequency Applicators for Thermal Magnetic Resonance Theranostics of Brain Tumors

“…However, this is not necessarily required since the single-ring RF applicator can be moved along the cranial-caudal axis so that the tumor position is located in the center of the ring, providing a viable and cost-effective approach adapted to the needs of the patient. Incorporating novel metamaterial-surfaces or high dielectric pads offers another alternative approach and research direction to improve MRI performance and heating efficacy [ 97 , 98 , 99 , 100 , 101 ]. Metamaterial planar surfaces (metasurface) are conceptually appealing for pursuing the development of RF applicators for ThermalMR-based theranostics of brain tumors due to the extra degrees of freedom for shaping electromagnetic fields they can provide [ 97 , 98 , 99 , 100 , 101 ].…”

“…Incorporating novel metamaterial-surfaces or high dielectric pads offers another alternative approach and research direction to improve MRI performance and heating efficacy [ 97 , 98 , 99 , 100 , 101 ]. Metamaterial planar surfaces (metasurface) are conceptually appealing for pursuing the development of RF applicators for ThermalMR-based theranostics of brain tumors due to the extra degrees of freedom for shaping electromagnetic fields they can provide [ 97 , 98 , 99 , 100 , 101 ]. This benefit could be exploited for further SAR 10g reduction and B 1 + uniformity improvements for MRI and targeted RF power deposition in deep-seated brain tumors [ 101 , 102 ].…”

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