The ability to control the electromagnetic near field with metasurfaces offers potential applications over the frequency range from radio frequency to optical domains. In this work, we show an... Show moreThe ability to control the electromagnetic near field with metasurfaces offers potential applications over the frequency range from radio frequency to optical domains. In this work, we show an essential feature of metasurfaces, subwavelength field confinement via excitation of a large number of eigenstates in a narrow frequency range, and demonstrate an innovative way of visualizing profiles of metasurface eigenmodes with the aid of a magnetic resonance imaging (MRI) system. We show that by tuning different eigenmodes of the metasurface to the Larmor frequency, we can passively tailor the near-field distribution to adjust the desired pattern of radio-frequency excitation in a MRI experiment. Our work demonstrates a practical nonperturbed rapid way of imaging metasurface eigenmodes. Show less
Dielectric resonators have previously been constructed for ultra-high frequency magnetic resonance imaging and microscopy. However, it is challenging to design these dielectric resonators at... Show moreDielectric resonators have previously been constructed for ultra-high frequency magnetic resonance imaging and microscopy. However, it is challenging to design these dielectric resonators at clinical field strengths due to their intrinsically large dimensions, especially when using materials with moderate permittivity. Here we propose and characterize a novel approach using artificial-dielectrics which reduces substantially the required outer diameter of the resonator. For a resonator designed to operate in a 3 Tesla scanner using water as the dielectric, a reduction in outer diameter of 37% was achieved. When used in an inductively-coupled wireless mode, the sensitivity of the artificial-dielectric resonator was measured to be slightly higher than that of a standard dielectric resonator operating in its degenerate circularly-polarized hybrid electromagnetic modes (HEM11). This study demonstrates the first application of an artificial-dielectric approach to MR volume coil design. (c) 2018 Elsevier Inc. All rights reserved. Show less
Shchelokova, A.V.; Slobozhanyuk, A.P.; Bruin, P. de; Zivkovic, I.; Kallos, E.; Belov, P.A.; Webb, A. 2018
Metasurfaces represent a new paradigm in artificial subwavelength structures due to their potential to overcome many challenges typically associated with bulk metamaterials. The ability to make... Show moreMetasurfaces represent a new paradigm in artificial subwavelength structures due to their potential to overcome many challenges typically associated with bulk metamaterials. The ability to make very thin structures and change their properties dynamically makes metasurfaces an exceptional meta-optics platform for engineering advanced electromagnetic and photonic metadevices. Here, we suggest and demonstrate experimentally a tunable metasurface capable of enhancing significantly the local image quality in magnetic resonance imaging. We present a design of the hybrid metasurface based on electromagnetically coupled dielectric and metallic elements. We demonstrate how to tailor the spectral characteristics of the metasurface eigenmodes by changing dynamically the effective permittivity of the structure. By maximizing a coupling between metasurface eigenmodes and transmitted and received fields in the magnetic resonance imaging (MRI) system, we enhance the device sensitivity that results in a substantial improvement of the image quality. Show less
Shchelokova, A.V.; Slobozhanyuk, A.P.; Bruin, P. de; Zivkovic, I.; Kallos, E.; Belov, P.A.; Webb, A. 2018
In this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high... Show moreIn this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high permittivity low loss medium. Placement of such a structure inside the scanner results in strong coupling of the radiofrequency field produced by the body coil with the lowest frequency electromagnetic eigenmode of the metasurface. This leads to spatial redistribution of the near fields with enhancement of the local magnetic field and an increase in the transmit efficiency per square root maximum specific absorption rate in the region-of-interest. We have investigated this structure in vivo and achieved a factor of 3.3 enhancement in the local radiofrequency transmit efficiency. (C) 2017 The Authors. Published by Elsevier Inc. Show less
