124 research outputs found
Non-scattering Metasurface-bound Cavities for Field Localization, Enhancement, and Suppression
We propose and analyse metasurface-bound invisible (non-scattering) partially
open cavities where the inside field distribution can be engineered. It is
demonstrated both theoretically and experimentally that the cavities exhibit
unidirectional invisibility at the operating frequency with enhanced or
suppressed field at different positions inside the cavity volume. Several
examples of applications of the designed cavities are proposed and analyzed, in
particular, cloaking sensors and obstacles, enhancement of emission, and
"invisible waveguides". The non-scattering mode excited in the proposed cavity
is driven by the incident wave and resembles an ideal bound state in the
continuum of electromagnetic frequency spectrum. In contrast to known bound
states in the continuum, the mode can stay localized in the cavity infinitely
long, provided that the incident wave illuminates the cavity
Improving B1 homogeneity in abdominal imaging at 3 T with light and compact metasurface
Radiofrequency field inhomogeneity is a significant issue in imaging large
fields of view in high- and ultrahigh-field MRI. Passive shimming with coupled
coils or dielectric pads is the most common approach at 3 T. We introduce and
test light and compact metasurface, providing the same homogeneity improvement
in clinical abdominal imaging at 3 T as a conventional dielectric pad. The
metasurface comprising a periodic structure of copper strips and parallel-plate
capacitive elements printed on a flexible polyimide substrate supports
propagation of slow electromagnetic waves similar to a high-permittivity slab.
We compare the metasurface operating inside a transmit body birdcage coil to
the state-of-the-art pad by numerical simulations and in vivo study on healthy
volunteers. Numerical simulations with different body models show that the
local minimum of B1+ causing a dark void in the abdominal domain is removed by
the metasurface with comparable resulting homogeneity as for the pad without
noticeable SAR change. In vivo results confirm similar homogeneity improvement
and demonstrate the stability to body mass index. The light, flexible, and
cheap metasurface can replace a relatively heavy and expensive pad based on the
aqueous suspension of barium titanate in abdominal imaging at 3 T.Comment: 18 pages, 6 figures, 4 supplementary figure
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