Thorsten Liebig

MIM CRLH Series Mode Zeroth Order Resonant Antenna (ZORA) implemented in LTCC Technology

A metal-insulator-metal (MIM) composite right/left- handed (CRLH) series mode zeroth order resonant antenna (ZORA) implemented in LTCC technology is presented and demonstrated to exhibit excellent efficiency (71%) and co-to-cross polarization performances, in addition to a high gain (10 dB) provided by its large electrical size of 2 lambda0. Due to its versatile characteristics, high performances and capability to attain at low cost high directivities, conventionally available only with standard arrays, this antenna may find various applications in future.

Double-Lorentz Transmission Line Metamaterial and its Application to Tri-Band Devices

A double-Lorentz (DL) transmission line (TL) metamaterial is proposed for the first time. Both effective material parameters mueff and epsiveff of the corresponding line exhibit a Lorentz-type dispersion, hence the proposed terminology double-Lorentz. This type of TL presents the interest of being intrinsically tri-band, while the previously reported composite right/left-handed TL metamaterial is only dual-band. The DL TL theory is fully derived and demonstrated experimentally by a microstrip implementation. The tri-band property of the DL TL is illustrated by the design of a tri-band lambda/4 impedance transformer and a systematic design procedure is described.

Highly Directive Resonator Antennas based on Composite Right/Left-Handed (CRLH) Transmission Lines

A composite right/left-handed (CRLH) series mode zeroth order resonator antenna (ZORA) and a CRLH half-wavelength antenna (HWA) with tunable directivity are presented and demonstrated to exhibit substantially higher directivity than a conventional patch antenna in the case of a 2.4 GHz (WLAN) design. Due to the special dispersion relation of a CRLH transmission line (TL), the length of CRLH TL resonators can be enlarged while keeping their resonance frequency constant, which yields enhanced directivity. The ZORA has been shown to have a higher efficiency (etaZORA 70%) than the CRLH HWA (etaHWAap40%), comparable to that of a patch antenna, as a result of the perfectly uniform current distribution in the zeroth order mode. In particular, the ZORA due to its versatile characteristics and high performance, is expected to find wide applications in the future.

Tri-band and dual-polarized antenna based on composite right/left-handed transmission line

Relaxing the composite right/left-handed (CRLH) balance condition enables arbitrary tri-band CRLH resonant devices. This paper presents a tri-band and dual-polarized antenna based on this principle. The proposed CRLH antenna operates in the transversally-polarized zeroth order (ZO) shunt mode (n = 0) and in the two longitudinally-polarized half-wavelength (HW) modes (n = plusmn1). Explicit synthesis formulas are given for the CRLH tri-band operation. A specific microstrip design using MIM (metal-insulator-metal) series capacitors and shunt stub inductors is demonstrated. In this design, the three resonant modes n = -1, n = 0 and n = +1 exhibited similar input impedances (allowing a simple feeding structure), radiation patterns and efficiencies/gains.

Highly adaptive RF excitation scheme based on conformal resonant CRLH metamaterial ring antennas for 7-Tesla traveling-wave magnetic resonance imaging

We propose an adaptive RF antenna system for the excitation (and manipulation) of the fundamental circular waveguide mode (TE11) in the context of high-field (7T) traveling-wave magnetic resonance imaging (MRI). The system consists of «flat» composite right-/left-handed (CRLH) meta-material ring antennas that fully conforms to the inner surface of the MRI bore. The specific use of CRLH metamaterials is motivated by its inherent dispersion engineering capabilities, which is needed when designing resonant ring structures for virtually any predefined diameter operating at the given Larmor frequency (i.e. 298MHz). Each functional group of the RF antenna system consists of a pair of subsequently spaced and correspondingly fed CRLH ring antennas, allowing for the unidirectional excitation of propagating, circularly polarized B1 mode fields. The same functional group is also capable to simultaneously mold an incoming, counter-propagating mode. Given these functionalities we are proposing now a compound scheme (i.e. periodically arranged multiple antenna pairs) — termed as “MetaBore” — that is apt to provide a tailored RF power distribution as well as full wave reflection compensation virtually at any desired location along the bore.

Dual-band composite right/left-handed ring antenna with linear/circular polarization capability

A dual-band metal-insulator-metal (MIM) composite right/left-handed (CRLH) ¿-resonance ring antenna with multi-polarization (linear/circular) radiation capability is demonstrated by way of full-wave simulations. The theory, design guidelines, parameter extraction procedure and feeding structure of this antenna are presented and discussed.

Tailored RF magnetic field distribution along the bore of a 7-Tesla traveling-wave magnetic resonance imaging system

This paper provides a highly efficient method for tailoring the RF magnetic field (B1) distribution along the cylindrical bore of a high-field (7T) magnetic resonance imaging (MRI) scanner operating in the advanced traveling-wave scheme. Here the B1 wave field propagates as a circularly polarized TE11 waveguide mode and is excited, molded and dumped by a lengthwise equidistant array of thin quadrature-fed (metamaterial) ring antennas where each of them perfectly conforms to the inner surface of the cylindrical MRI bore. All individual antenna excitations associated to the desired longitudinal field profile are retrieved from an inverse problem that is efficiently solved in a (weighted) least-squares sense. The electromagnetic modeling is carried out with our equivalent-circuit (EC) FDTD simulation platform openEMS, and a convincing showcase involving a narrow illumination window for larynx diagnostics is presented.

Metamaterial-based transmit and receive system for whole-body magnetic resonance imaging at ultra-high magnetic fields

Magnetic resonance imaging (MRI) at ultra-high fields (UHF), such as 7 T, provides an enhanced signal-to-noise ratio and has led to unprecedented high-resolution anatomic images and brain activation maps. Although a variety of radio frequency (RF) coil architectures have been developed for imaging at UHF conditions, they usually are specialized for small volumes of interests (VoI). So far, whole-body coil resonators are not available for commercial UHF human whole-body MRI systems. The goal of the present study was the development and validation of a transmit and receive system for large VoIs that operates at a 7 T human whole-body MRI system. A Metamaterial Ring Antenna System (MRAS) consisting of several ring antennas was developed, since it allows for the imaging of extended VoIs. Furthermore, the MRAS not only requires lower intensities of the irradiated RF energy, but also provides a more confined and focused injection of excitation energy on selected body parts. The MRAS consisted of several antennas with 50 cm inner diameter, 10 cm width and 0.5 cm depth. The position of the rings was freely adjustable. Conformal resonant right-/left-handed metamaterial was used for each ring antenna with two quadrature feeding ports for RF power. The system was successfully implemented and demonstrated with both a silicone oil and a water-NaCl-isopropanol phantom as well as in vivo by acquiring whole-body images of a crab-eating macaque. The potential for future neuroimaging applications was demonstrated by the acquired high-resolution anatomic images of the macaque’s head. Phantom and in vivo measurements of crab-eating macaques provided high-resolution images with large VoIs up to 40 cm in xy-direction and 45 cm in z-direction. The results of this work demonstrate the feasibility of the MRAS system for UHF MRI as proof of principle. The MRAS shows a substantial potential for MR imaging of larger volumes at 7 T UHF. This new technique may provide new diagnostic potential in spatially extended pathologies such as searching for spread-out tumor metastases or monitoring systemic inflammatory processes.

Multi-functional RF coils for ultra-high field MRI based on 1D/2D electromagnetic metamaterials

Electromagnetic metamaterials have already proven very valuable for the enhancement and molding of RF magnetic fields within ultra-high field MRI scanners at 7T. We report on our development of coil elements based on composite right-/left-handed (CRLH) 1D electromagnetic (EM) metamaterial transmission lines (metalines) operating in the zeroth order resonance (ZOR) to foster uniform RF magnetic field distributions along the scanner axis. Tailored EM metalines supporting full-wave or quarter-wave resonances are used either as metamaterial ring antenna or as dual-band coil elements for simultaneous 1H/23Na imaging. The EM metalines are key to the MetaBore, which is a fully adaptive RF field control scheme based on a periodic axial arrangement of conformal metamaterial ring antennas in the framework of high-field traveling-wave MRI. With the 2D EM metamaterials (metasurfaces) we realized high-impedance surfaces (HIS) in order to enhance the uniformity and directivity of the RF magnetic field from e.g. overlaid (elongated) dipole elements towards the probe volume. The designs include simulation studies of the overall multichannel coil systems, which are carried out with our home-made, open source electromagnetic 3D EC-FDTD solver openEMS supporting conformal cylindrical inhomogeneous meshing. Experimental verifications of our coils have been carried out within 7T MRI scanners (Siemens Magnetom).