Microwave Imaging of Brain Stroke

Brain stroke is an age-related illness which has become a major issue in our ageing societies. Early diagnosis and treatment are of high importance for the full recovery of the patient, as reminded in Anglo-Saxon countries by the abbreviation FAST (Face, Arm, Speech, Time) referring to both the four major visible signs and the necessity to act fast. In this respect, Computed Tomography (CT) and Nuclear Magnetic Resonance (NMR) imaging are key diagnostic tools in clinical practice. Unfortunately, not only these modalities can neither be transported nor rapidly usable, which would allow early treatment (especially in rural environments), but also cannot be brought to the bedside of the patient to monitor the evolution of the disease. Microwave Imaging (MWI) is a potential candidate to provide fast and accurate diagnostic insights for brain stroke pathological states. The head of the patient is illuminated with low-power microwave waveforms (non-ionizing radiations), whose backscattered signals are used to generate either images of its internal structures, distributions, patterns and shapes (qualitative imaging) or directly its physical parameters such as the dielectric contrast and the permittivity values (quantitative imaging). The technology relies on the high sensitivity of microwaves on the water content of tissues to allow for the discrimination between pathological and healthy regions.

This thesis focuses on both the forward modeling of the electromagnetic phenomena arising in biological tissues and the inverse scattering problem for imaging in the differential MWI (dMWI) scenario for brain stroke monitoring. It is intrinsically interdisciplinary as it requires knowledge in Biology, Medicine, Physics, Chemistry, and Engineering. In order to investigate the challenges arising in brain MWI, it is crucial to have accurate and efficient solvers to model electromagnetic (EM) fields at UHF/SHF-bands. The head is a distributed, heterogeneous, and lossy scatterer for which existing solvers are known to struggle at higher frequencies. Volume Integral Equation (VIE) formulations and MultiGrid (MG) approaches are investigated to find the actual solution of the field distributions for large scale problems. The EM modeling also permits to analyze the feasibility of brain MWI, which depends on the power transmission from the antennas towards the human brain. In order to estimate this transmission, simplified but still representative models, including intermediate layers -skin, fat, bone, and CerebroSpinal Fluid (CSF) - of the head, are proposed in the framework of simulations (analytical tools) and experimental validations (3D printed head phantom). For the imaging task, the physics of the EM scattering, leads to complex non-linear inverse scattering problems (consisting in retrieving from a set of field measurements the physical parameters which produced them) for which reliable assumptions and approximations must be found. For brain MWI, estimating and quantifying the degree of non-linearity allows for determining the scope of application of existing algorithms, for which different regularizers are applied. Modeling and inverse problem resolution for brain MWI investigated in the present work are ultimately meant to contribute to the development of a technology dedicated to brain stroke detection, differentiation, and monitoring.

Power line communications

Emmanuel Marthe

With the deregulation of the electricity and telecommunications markets, a great deal of new operators providing some innovative services have appeared. In the telecommunications' field, the promise of broadband Internet access with easy installation procedure at low cost may help to win new customers. The Power Line Communications (PLC) technology is a way to fulfil this function. PLC uses the low voltage (LV) and the medium voltage (MV) networks as communication support. It consists in an additional signal carrying data information and superimposed to the 50 Hz power wave. As PLC is easy to install, it can be used to extend the Internet coverage to areas that are still badly covered by the other broadband technologies, or to provide high speed Internet access to every power socket of a building. This thesis, done as a part of research project called Digital Power Line Access, is devoted to the study of electromagnetic compatibility (EMC) aspects involved with the PLC. There are still a certain number of problems due to the fact that PLC, which is a quite recent technology, has not been regulated yet. Among these problems, all the aspects associated with the radiation created by the transmission of PLC signals over unshielded conductors and their potential disturbances are very significant. The aim of this thesis is the analysis of PLC data transmission over the low voltage network and the related EMC problems, mainly the radiation due to the transmission of PLC signals. The measurement and the analysis of these radiations lead us to develop a method to reduce these radiations. The mitigation of radiation created by the use of PLC within buildings is still an important EMC challenge, because there are no possibilities for network conditioning. The first two chapters of this thesis present a state of the art of PLC, its regulation and the related EMC problems. The second chapter describes the working of PLC and emphasizes the hostile nature of its environment. The problems (filtering, attenuation, generation of noise,...) caused by the use of a signal in the [1 – 30 MHz] band over a channel designed for a 50 Hz power wave are presented. On the other hand, the various technologies used to allow the PLC to work (suitable modulation scheme, use of repeater,...) are illustrated. An analysis of the different PLC concepts and of the successful and unfruitful commercial deployment ends this chapter. In the third chapter, we present the technical criterion (use of the PLC as a multi-purpose port: as a main port and as a telecommunication port) that we have to consider and the evolution of the conducted and radiated limits that we have to respect. The pollution of the radio amateur band, which could be the result of a massive deployment of PLC, is the subject of tremendous discussions and proves that if according to some persons the current limits are too permissive, for the PLC manufacturer they are already too restrictive. The coexistence of the PLC with other broadband technologies (xDSL, CA-TV, Wireless,...) is also addressed and we show that only the coexistence with an alternative of the DSL – VDSL – could be problematic. The main original contributions of this thesis are presented in chapters 4 to 6. We concentrated on the problems related to the electromagnetic radiations associated with PLC signals, especially in indoor environment. The reason for this is that the emission of electromagnetic noise, which can interfere with services such as public and amateur radio, remains an obstacle to the development of PLC. Furthermore, to the best of our knowledge, the mitigation of indoor radiation has not been addressed in the literature. In chapter 4, we present a complete characterization of the problems resulting from the use of PLC in an indoor environment. Two approaches are presented for the measurement of electromagnetic fields. The first one consists in the injection of a reference signal in the network and the measurement of the resulting radiations. The second one deals with the measurements of interferences due to the work of real PLC equipments. These two approaches enlight several aspects. The main aspect is the strong dispersion of the measurements, resulting from the influence of the position of the observation point and of the power network load. Measurements on real PLC networks show excessive radiations, which confirm that the limits are, at this time, too restrictive. We also present and discuss PLC radiated emissions in typical indoor environments. In particular, we analyze the field levels resulting from PLC signals with magnitudes given by EN55022/CISPR22 limits and compare them with various proposals for PLC radiated field limits. Thus, we show that the resulting fields exceed, for a wide range of frequencies, some of the current emission limits such as the German NB30, which is considered as a very strict limit. In this same chapter, we present a technique for the analysis of power line communication (PLC) signals, based on the formalism of the scattering parameters. This technique allows an efficient and fast characterization of any transmission medium of a power network. The analysis of the variations of the magnitude and phase of the scattering parameters gives useful insights on the increasing of the losses with frequency and confirm the need of a technique to reduce impedance mismatch at both injection and reception ports. The chapter 5 presents the modelling of the PLC environment and the computation of its radiations using a commercial code, the Numerical Electromagnetics Code (NEC), which solves the electric field integral equation in the frequency domain using the method of moments. We present some comparisons between the NEC simulations and the experimental data for the total magnetic field presented in the previous chapter. The simulations of simple models give results in good agreement with the experimental data and therefore allow the validation of certain principles. Unfortunately, the simulations on a model of increasing complexity give less satisfactory results. Because of the difficulty to respect the topology of the network and to take into account all of the components of the network coupled with the constraints of NEC, it is extremely difficult to obtain accurate results on the complete PLC band. The strong electromagnetic radiations appeared as the major problem of this technology. Therefore, the development of a method of mitigation was the logical continuation of this work, especially because there is still no solution to this problem. We developed a technique, which is presented in chapter 6, aiming at reducing the electromagnetic field radiated by indoor PLC signals. This technique is the major original contribution of this thesis and led us to file a patent. The central idea is to take advantage of the additional ground conductor and inject a signal similar to the PLC signal into the ground-neutral line, but with a reversed phase. As a result, the overall antenna mode current is significantly reduced, as well as the radiated electromagnetic fields. The method is specifically developed for indoor applications and applies to uni-directional and bi-directional data transmission systems with an implementation using switches. Experimental data obtained on a test network have shown that the use of the proposed technique implies a significant reduction (up to 20 dB) of the PLC electromagnetic radiation in the considered frequency range. It is also shown that the performance of the proposed technique increases when the ground-neutral line is terminated by an impedance equal to the one of the PLC modem. Other tests performed on 230 V network confirm this trend but show that further work is needed in this direction. This work could consist in the development of a system allowing the synchronisation of the real PLC signal and the auxiliary signal according to the behavior of the PLC channel, in order to optimise the performances for any frequency used by the PLC.

EPFL2005

Electromagnetic Simulation of CERN Accelerator Components and Experimental Applications

Carlo Zannini

Wakes and impedances of single accelerator elements can be obtained by means of theoretical calculation, electromagnetic (EM) simulations or bench measurements. Since theoretical calculations apply only to simple structures and bench measurements have some intrinsic limitations, EM simulations can be used as a reliable tool to determine wakes and impedances. This thesis will focus on the use of time domain 3D CST Particle Studio EM simulations to calculate wakes and/or impedances. First, the results of the EM simulations are compared with the known analytical solutions and other codes. In this exercise, the driving and the detuning terms of the wakes/impedances, in the transverse plane, are disentangled for both symmetric and asymmetric geometries. The sensitivity of the simulations results to the numerical parameters is discussed, as well as the limits of validity of the wake formalism and its extension to the nonlinear regime. Using the CST Wakefield Solver, the SPS kicker impedance contribution is then estimated. The simulation model was improved step by step, and successfully benchmarked with existing and new theoretical models, giving confidence in the numerical results and allowing a better understanding of the EM problems. In the case of the resistive wall impedance of simple chamber geometries a handy theoretical model has been proposed. In order to calculate the resistive wall impedance of a round chamber, a theoretical approach based on the transmission line (TL) theory, is demonstrated to be valid and practical to use. By means of appropriate form factors the method is then extended to rectangular or elliptical chambers. Moreover the method was successfully benchmarked with the most recent codes based on the field matching technique developed at CERN and was used to construct the SPS wall impedance model. For more complicated geometries (asymmetries, small inserts, holes etc.), a theoretical estimation without involving EM simulation becomes unworkable. An example of interest is the LHC beam-screen, for which CST 3D simulations were used to estimate the impedance. In order to allow the simulator to cover the whole frequency range of interest (few KHz to several tens of MHz) a novel scaling technique was developed and applied. Where possible, the EM models developed throughout this thesis were also successfully benchmarked with bench measurements (wire methods) and observations with beam. On the specific subject of bench measurements, a numerical investigation of coaxial wire measurements has been also presented. Finally, in order to verify the adopted EM models of the materials both in theoretical calculations and 3D simulations, an experimental setup for measuring EM properties (permittivity and permeability) of materials has been presented. The method is based on fitting the measured reflection transmission coefficients through a coaxial line filled with the material to be probed, with the outcome of EM simulations or theoretical models. It was successfully applied for measuring NiZn ferrites and dielectric material (e.g. SiC).

EPFL2013

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

Sound absorption at low frequencies still remains a challenge in both scientific research and engineering practice. Natural porous materials are ineffective in this frequency range, as well as acoustic resonators which present too narrow bandwidth of absorption, thus requiring alternative solutions based on active absorption techniques. In the present work, we propose an active control framework applied on a closed-box loudspeaker to enable the adjustment of the acoustic impedance at the loudspeaker diaphragm. More specifically, based on the proportionality between the pressure inside the enclosure and the axial displacement of the loudspeaker diaphragm at low frequencies, we demonstrate both analytically and experimentally that a PID-like feedback control approach allows tuning independently the compliance, the resistance and the moving mass of the closed-box loudspeaker to implement a prescribed impedance of a single-degree-of-freedom resonator. By considering different control combinations to tailor the resonator characteristics, a perfect absorption (with absorption coefficient equal to 1) is achievable at the target resonance frequency, while enlarging the effective absorption bandwidth. Moreover, the proposed feedback control strategy shows an excellent control accuracy, especially compared to the feedforward-based control formerly reported in the literature. The mismatches between the performance of experimental prototype and the model, likely to result from the control time delay and the inaccuracy in estimating the loudspeaker parameters, can be compensated directly by tuning the control parameters in the control platform. The active resonators implemented through the reported control scheme can be used to build more complex acoustic devices/structures to enable high-efficiency broadband sound absorption or other types of acoustic phenomena such as wavefront shaping.

2022