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Purpose: A 32-channel parallel transmit (pTx) add-on for 7 Tesla whole-body imaging is presented. First results are shown for phantom and in-vivo imaging. Methods: The add-on system consists of a large number of hardware components, including modulators, amplifiers, SAR supervision, peripheral devices, a control computer, and an integrated 32-channel transmit/receive body array. B1+ maps in a phantom as well as B1+ maps and structural images in large volunteers are acquired to demonstrate the functionality of the system. EM simulations are used to ensure safe operation. Results: Good agreement between simulation and experiment is shown. Phantom and in-vivo acquisitions show a field of view of up to 50 cm in z-direction. Selective excitation with 100 kHz sampling rate is possible. The add-on system does not affect the quality of the original single-channel system. Conclusion: The presented 32-channel parallel transmit system shows promising performance for ultra-high field whole-body imaging. [ABSTRACT FROM AUTHOR]

Abstract: In this article, we introduce coaxial cables to tune the resonant frequency of metamaterials, the high impedance surfaces (HISs). The proposed structure is used as tuned artificial reflector for radio frequency (RF) coils in magnetic resonance imaging (MRI) machines. In this technique, coaxial cables are connected to the pins (vias) of the HIS to extend their lengths. Circulating the current in long paths aims to increase the total inductance of the HIS to shift its resonance down in frequency. The proposed method for MRI solves the problems of using the available tuning techniques, such as the inaccurate mechanical adjustments which lead to inaccurate electromagnetic properties. Further, it solves the problem of using electronic tuning circuits such as diodes because of the bulk power used by RF coils in MRI which may expose them to damage. In this article, the transverse magnetic reflection coefficient phase at oblique incidence and the transmission coefficient (S2, 1) of the proposed tuned HIS have been investigated. The experimental results showed an obvious shift in the resonant frequency of the tuned HIS compared to the untuned surface. [ABSTRACT FROM AUTHOR]

In this contribution, we investigate the B_\text1 distribution and coupling characteristics of a multichannel radio frequency (RF) coil consisting of different dipole coil elements for 7 T MRI, and explore the feasibility to achieve a compromise between field distribution and decoupling by combining different coil elements. Two types of dipole elements are considered here: the meander dipole element with a chip-capacitor-based connection to the RF shield which achieves a sufficient decoupling between the neighboring elements; and the open-ended meander dipole element which exhibits a broader magnetic field distribution. By nesting the open-ended dipole elements in between the ones with end-capacitors, the B_\text1 distribution, in terms of field penetration depth and homogeneity, is improved in comparison to the dipole coil consisting only of the elements with end-capacitors, and at the same time, the adjacent elements are less coupled to each other in comparison to the dipole coil consisting only of the open-ended elements. The proposed approach is validated by both full-wave simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, we present a fundamental investigation to improve the B1 efficiency and the signal-to-noise ratio (SNR) of a radio frequency (RF) surface coil for ultrahigh-field magnetic resonance imaging (MRI) by utilizing a high-impedance surface (HIS) as the RF shield. An analytical investigation indicates that a circular loop backed by a perfect magnetic conductor (PMC), which is the ideal case of an HIS, suggests an improved magnetic field compared with the case of a perfect electric conductor (PEC) and the case without any shield. This improvement is verified by a full-wave simulation, where the surface coil is modeled by an ideal impressed current model with azimuthal component ( J\text {surf, \alpha }= 1 A/m). The electromagnetic field is effectively shielded out behind the PEC and PMC shields compared with the case without any shield. Furthermore, the surface coil with uniform current distribution and the PMC shield is realized by a series resonant loop structure and a 2-D HIS structure, respectively. Since the normal component of the magnetic field is supported at the surface of an HIS, whereas suppressed by a conventional PEC, the \boldsymbolB1 field in the vicinity of the HIS shield is enhanced compared with the case with a PEC shield. Hence, an improvement on SNR and \boldsymbolB1 efficiency is achieved by utilizing an HIS shield, especially in the regions adjacent to the surface coil. It has been found that the improvement of B1 efficiency is more prominent than the improvement of SNR due to different normalizations. The difference of peak SAR between considered shields, which is used for B1 efficiency normalization, is considerably larger than the difference of the power loss within the phantom, which is used for the SNR normalization. The proposed approach is validated by full-wave finite-element method simulations and near-field measurements, which reveal good agreement with each other. [ABSTRACT FROM PUBLISHER]

In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole coil element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the B1 field of a stripline coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single- and multi-coil elements. For the experimental validation, two shielding plates—a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively—are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed. [ABSTRACT FROM PUBLISHER]

This contribution discusses the utilization of RF power in an MRI system with RF mode shimming which enables the superposition of circularly polarized modes of a transmit RF coil array driven by a Butler matrix. Since the required power for the individual modes can vary widely, mode-shimming can result in a significant underutilization of the total available RF power. A variable power combiner (VPC) is proposed to improve the power utilization: it can be realized as a reconfiguration of the MRI transmit system by the inclusion of one additional matrix network which receives the power from all transmit amplifiers at its input ports and provides any desired (combined) power distribution at its output ports by controlling the phase and amplitude of the amplifiers’ input signals. The power distribution at the output ports of the VPC is then fed into the “mode” ports of the coil array Butler matrix in order to superimpose the spatial modes at the highest achievable power utilization. The VPC configuration is compared to the standard configuration of the transmit chain of our MRI system with 8 transmit channels and 16 coils. In realistic scenarios, improved power utilization was achieved from 17% to 60% and from 14% to 55% for an elliptical phantom and a region of interest in the abdomen, respectively, and an increase of the power utilization of 1 dB for a region of interest in the upper leg. In general, it is found that the VPC allows significant improvement in power utilization when the shimming solution demands only a few modes to be energized, while the technique can yield loss in power utilization in cases with many modes required at high power level. [ABSTRACT FROM PUBLISHER]

Chipless radio frequency identification (RFID) technology is expected to replace barcode technology due to its ability to read in non-line-of-sight (NLOS) situations, long reading range, and low cost. Currently, there is extensive research being conducted on frequency-coded (FC) co-polarized radar cross-section (RCS)-based tags, which are widely used. However, detecting co-polarized chipless RFID tags in cluttered environments is still a challenge, as confirmed by measuring two co-polarized tags in front of a perfect metal reflector ( 30.5 cm × 22.5 cm ). To address this challenge, a realistic mathematical model for a chipless RFID system has been developed that takes into account the characteristics of the reader and the tag, as well as reflections from cluttered objects. This extensive mathematical model developed for linear chipless RFID systems in clutter scenarios holds the potential to greatly assist researchers in their exploration of RCS-based tags. By relying solely on simulations, this model provides a tool to effectively analyze and understand RCS-based tags, ultimately simplifying the process of generating more authentic tag designs. This model has been simulated and verified with measurement results by placing a single flat metal reflector behind two co-polarized one-bit designs: a dipole array tag and a square patch tag. The results showed that the interfering signal completely overlaps the ID of the co-polarized tag, severely limiting its detectability. To solve this issue, the proposed solution involves reading the tag in cross-polarization mode by etching a diagonal slot in the square patch tag. This proposed tag provides high immunity to the environment and can be detected in front of both dielectric and metallic objects. [ABSTRACT FROM AUTHOR]

In a recently proposed indoor self-localization system, we excite dielectric resonators (DRs) in linear polarization (LP) as retrodirective passive tags, either as single DR, small array of DRs, or in combination with a spherical dielectric lens. DR tags mounted close to a reflecting wall suffer from large clutter return, which can supercede the DR tag signatures. To separate both into orthogonal polarizations, we design a polarization twist tag with two DRs tilted by ±45° and spaced by $\lambda $ /4. In the EM simulation and experiment, we show that this tag design allows efficient retrodirective scattering in cross polarization. The realized cross-polarization radar cross section (RCS) is equal to the scattering of a single DR in LP and 6 dB better than the RCS from a polarization splitting single DR with 45° tilt. This polarization signature is shown to allow separation of the DR signature from the high scattering level of a flat plate reflector placed behind the polarization twist DR tag. The potential of improving lens-DR combinations by using the polarization twist DR tag with circular polarization is also discussed. [ABSTRACT FROM AUTHOR]

It is shown, using three specific examples—a series fed patch (SFP) array, a phase reversal (PR) array and a composite right/left-handed (CRLH) antenna—that one-dimensional periodic leaky-wave antennas scanning through broadside build a class of leaky-wave antennas sharing qualitatively similar and quantitatively distinct dispersion and radiation characteristics. Based on an equivalent transmission line (TL) model using linearized series and shunt immittances to approximate the periodic (Bloch) antenna structure, asymptotic TL formulas for the characteristic propagation constant, impedance, energy, power and quality factor are derived for two fundamentally different near- and off-broadside radiation regimes. Based on these formulas, it is established that the total powers in the series and shunt elements are always equal at broadside, which constitutes one of the central results of this contribution. This equal power splitting implies a severe degradation of broadside radiation when only one of the two elements series or shunt efficiently contributes to radiation and the other is mainly dissipative. A condition for optimum broadside radiation is subsequently established and shown to be identical to the Heaviside condition for distortionless propagation in TL theory. Closed-form expressions are derived for the constitutive (LCRG) parameters of the TL model for the specific SFP, PR and CRLH antenna circuit models, and quantitative information on the validity range of the TL model is subsequently provided. Finally, full-wave simulation and measurement LCRG parameter extraction methods are proposed and validated. [ABSTRACT FROM AUTHOR]

Abstract: In this contribution the issue of fire detection by means of thermal radiation in the microwave region of the electromagnetic spectrum is addressed. The aim of this work is to show that microwave radiation is a useful quantity to detect fires, which is confirmed by measurements. Some of the results even indicate that it could be possible to detect smoldering fires with microwaves earlier than with smoke or with gas detectors. The first part is a short summary of the basic concept of our previous paper [Kaiser T, Kempka T. Is microwave radiation useful for fire detection? Proceedings of 12th international conference on automatic fire detection AUBE ’01, vol. 965, 26–28 March 2001. Gaithersburg: NIST Special Publication; 2001.] published at AUBE ‘01, along with a short revisit of the physical fundamentals of thermal radiation, of electromagnetic wave propagation, and of attenuation. In the second part, the microwave receiver will be presented, which is used to conduct experiments in Duisburg''s fire detection lab. The receiver is able to measure thermal radiation in the frequency region from 2 to 40GHz, and the superheterodyne technique used by the receiver enables a rather flexible measurement range among arbitrary frequency bands with individual widths of 100MHz. The receiver is controlled by a normal PC, which makes the measurement setup configurable, i.e. one can choose any combination of frequency bands for a single measurement. Some measurement results for the standardized European test fires TF1 and TF2 according to ISO 7240 (EN 54 Part 9) are shown in the last part of this contribution. The gathered data is discussed and compared to ionization chamber (MIC), to extinction light sensor (MIREX), and to gas-concentration measurements. [Copyright &y& Elsevier]

For a novel indoor‐localization system, chipless tags with high retro‐directive radar cross‐section (RCS) under wide‐angle incidence are required as fixed landmarks. Tags based on dielectric resonators (DRs) were proposed to provide identification by resonance frequency coding. To achieve a satisfactory read range for the localization system, the low RCS levels of these tags require a major boost. A solution was found by adopting the metallic corner reflector which is known for high RCS levels over a wide bandwidth and over a wide angle of incidence. The study presents a novel corner reflector design where notches in the RCS spectral signature are created by the attachment of arrays of dielectric resonators to the metallic surfaces of corner reflectors. It is shown that notches appear due to the increased scattering of the resonators at resonance and by the power loss due to grating lobes formed in addition to the specular reflection from the arrays and from the metallic surfaces. Results from electromagnetic simulations are verified by measurements of an example dihedral corner reflector of 100 × 100 mm2 plate size with two arrays of 3 × 3 DRs producing a notch signature at about 7 GHz. [ABSTRACT FROM AUTHOR]

This paper presents a realistic feasibility of radio frequency identification (RFID) based self-localization of objects in real environments using simple infrastructure composed of chipless tags. These tags are designed as arrays of dielectric resonators (DR) in order to overcome the range limitations of the traditional chipless tags, where the DR array structure of the tags boosts the radar cross section (RCS) and, consequentially, extends the range coverage. Furthermore, an efficient algorithm for tag detection is proposed considering the environmental and hardware impairments that alter the backscattered signal from the tag. In this system, the reader interrogates the reference nodes, i.e. chipless tags, by a frequency-sweep RF signal, and the backscattered signals from the tags are captured and analyzed at the reader for detection, ranging and location estimation. A testbed is implemented with fabricated chipless RFID tags in order to evaluate the validity and accuracy of the proposed self-localization system. Experimental measurements based simulations are conducted in several scenarios in order to evaluate the proposed system. Moreover, an object (the reader) is moved to several locations in an interrogation area of $2.5\times 1.2$ m, and, afterwards, its location is estimated. The averaged measured location error achieved based on the proposed system is approximately 5 cm with a variation of 0.9 cm as a minimum observed error and 7 cm as a maximum error. [ABSTRACT FROM AUTHOR]

Passive chipless tags based on dielectric resonators (DRs) have been proposed for a novel millimeter-wave indoor self-localization system to mark fixed reference nodes. The tags employed in the system provide a unique spectral signature (resonance peaks) with much-increased radar cross section (RCS) by placing several identical DRs at the focal line of a spherical dielectric lens. Resonators of optimized geometry allow a modification of this original DR-lens tag by combining the lens with different size DRs which exhibit resonant frequencies separated by large frequency gaps. For easier manufacturing and testing, a demonstrator tag was designed for a “scaled” frequency range of 5–6 GHz with this configuration and is shown to produce spectral signatures of the monostatic RCS which are uniquely related to the angle of arrival (AoA) of the incident wave from a reader. Simulated signatures of a lens of 120-mm diameter with seven spherical DRs are presented and results are supported by experiment. Correlation processing of the signatures can give a resolution of few degrees in AoA while the bistatic RCS half-power beamwidth is on the order of 20°, and RCS levels of resonant peaks are produced between 0 and −6 dBm2. [ABSTRACT FROM AUTHOR]

Abstract: This contribution discusses the dynamic behavior of the EN54 part 9 pyrolysis wood fire and its succeeding EN54 part 7 counterpart. The discussion is based on the measurement of the thermal radiation in the microwave region of the electromagnetic spectrum. The measurements show that the fire test according to EN54 part 7 takes much longer than the fire test according to EN54 part 9, although the only difference between the two fire tests is the amount and the geometry of the used wood sticks. [Copyright &y& Elsevier]

This paper demonstrates that unit cell asymmetry with respect to the transversal axis —or transversal asymmetry— is an essential design parameter in periodic leaky-wave antennas (P-LWAs). Specifically, it shows that transversal asymmetry can be leveraged to fully and systematically solve the well-known radiation degradation of P-LWAs at broadside, where it provides both open-stopband closure and efficiency equalization. The problem is addressed via a generic equivalent circuit model composed of a series resonator, a shunt resonator and ideal transformers for modeling asymmetry by a single and simple parameter, namely the transformation ratio. Once the series and shunt frequencies have been balanced (frequency-balancing), equalization is ensured by adjusting the degree of asymmetry in the unit cell so to match the at-broadside Bloch impedance to the off-broadside Bloch impedance. This equalization condition is referred to as quality factor balancing (Q-balancing) and it is related to the Heaviside condition (distortionless propagation) in homogeneous transmission lines. Based on this theory, optimization schemes for employing commercial fullwave eigenmode and drivenmode solvers are proposed to design unit cells with equalized efficiencies. Finally, two examples of P-LWAs are presented, a composite right/left-handed (CRLH) P-LWA and a series-fed coupled patch (SFCP) P-LWA, and verified to fully confirm the predictions of the theory obtained by circuit modeling. [ABSTRACT FROM PUBLISHER]

This paper includes two contributions. First, it proves that the series and shunt radiation components, corresponding to longitudinal and transversal electric fields, respectively, are always in phase quadrature in axially asymmetric periodic leaky-wave antennas (LWAs), so that these antennas are inherently elliptically polarized. This fact is theoretically proven and experimentally illustrated by two case-study examples, a composite right/left-handed (CRLH) LWA and a series-fed patch (SFP) LWA. Second, it shows (for the case of the SFP LWA) that the axial ratio is controlled and minimized by the degree of axial asymmetry. [ABSTRACT FROM PUBLISHER]

This paper deals with the development of a microwave noise-adding radiometer, which is purposely designed for the fire detection in forest environments. The sensor operates at 12.65 GHz and exploits commercial Satellite Television (SAT-Tv) components such as a parabolic dish and a low-noise block. First, a simple system model is presented to estimate the radiometric contrast due to the presence of fire (increase in the antenna noise temperature with respect to the background) at a certain distance from the receiving antenna. Then, the design of the sensor is addressed, underlining the key technologies that allow the required performance to be attained at low industrial costs. An experimental characterization of the developed radiometer is reported focusing on both accuracy and sensitivity issues. Due to a continuous gain calibration based on the noise-adding procedure, the antenna noise temperature can be retrieved with an absolute error of 4 K without any thermal stabilization of the instrument electronics. Last, real fire detection experiments have been carried out both in laboratory and open-space environments. A radiometric contrast of 8.8 K has been recorded for a wooden fire of 0.2 m² placed at a distance of about 30 m from the antenna. [ABSTRACT FROM AUTHOR]

Indoor localization based on unsynchronized, low-complexity, passive radio frequency identification (RFID) using the received signal strength indicator (RSSI) has a wide potential for a variety of internet of things (IoTs) applications due to their energy-harvesting capabilities and low complexity. However, conventional RSSI-based algorithms present inaccurate ranging, especially in indoor environments, mainly because of the multipath randomness effect. In this work, we propose RSSI-based localization with low-complexity, passive RFID infrastructure utilizing the potential benefits of large-scale MIMO technology operated in the millimeter-wave band, which offers channel hardening, in order to alleviate the effect of small-scale fading. Particularly, by investigating an indoor environment equipped with extremely simple dielectric resonator (DR) tags, we propose an efficient localization algorithm that enables a smart object equipped with large-scale MIMO exploiting the RSSI measurements obtained from the reference DR tags in order to improve the localization accuracy. In this context, we also derive Cramer–Rao lower bound of the proposed technique. Numerical results evidence the effectiveness of the proposed algorithms considering various arbitrary network topologies, and results are compared with an existing algorithm, where the proposed algorithms not only produce higher localization accuracy but also achieve a greater robustness against inaccuracies in channel modeling. [ABSTRACT FROM AUTHOR]