Encoding and consolidation of memories are two processes within the hippocampus, and connected cortical networks, that recruit different circuit level dynamics to effectively process and pass information from brain region to brain region. In the hippocampal CA1 pyramidal layer local field potential (LFP), these processes take the form of theta and sharp wave ripples (SPW-Rs) for encoding and consolidation, respectively. As an animal runs through an environment, neurons become active at specific locations in the environment (place cells) increasing their firing rate, functionally representing these specific locations. These firing rate increases are organized within the local theta oscillations and sequential activation of many place cells creates a map of the environment. Once the animal stops moving and begins consummatory behaviors, such as eating, drinking, or grooming, theta activity diminishes, and large irregular activity (LIA) begins to dominate the LFP. Spontaneously, with the LIA, the place cells active during the experience are replayed during SPW-Rs in the same spatial order they were encountered in the environment. Both theta and SPW-R oscillations and their associated neuronal firing are necessary for effective place recognition as well as learning and memory. As such, interruption or termination of SPW-R events results in decreased learning performance over days. During exploration, the associated theta and sequential place cell activity is thought to encode the experience. During quiet restfulness or slow wave sleep (SWS), SPW-R events, that replay experience specific place sequences, are thought to be the signal by which systems consolidation progresses and the hippocampus guides cortical synaptic reorganization. The granular retrosplenial cortex (gRSC) is an associational area that exhibits high frequency oscillations (HFOs) during both hippocampal theta and SPW-Rs, and is potentially a period when the gRSC interprets incoming content from the hippocampus during encoding and systems consolidation. However, the precise laminar organization of synaptic currents supporting HFOs, whether the local gRSC circuitry can support HFOs without patterned input, and the precise coupling of hippocmapla oscillations to gRSC HFOs across brain states remains unknown. We aimed to answer these questions using in vivo, awake electrophysiological recordings in head-fixed mice that were trained to run for water rewards in a 1D virtual environment. We show that gRSC synaptic currents supporting HFOs, across all awake brain states, are exclusively localized to layer 2/3 (L2/3), even when events are detected within layer 5 (L5). Using focal optogenetics, both L2/3 and L5 can generate induced HFOs given a strong enough broad stimulation. Spontaneous gRSC HFOs occurring outside of SPW-Rs are highly comodulated with medial entorhinal cortex (MEC) generated high gamma in hippocampal stratum lacunosum moleculare. gRSC HFOs may serve a necessary role in communication between the hippocampus during SPW-Rs states and between the hippocampus, gRSC, and MEC during theta states to support memory consolidation and memory encoding, respectively. / Doctor of Philosophy / As an animal moves through an environment, individual neurons in the hippocampus, known as place cells, increase and decrease their firing rate as the animal enters and exits specific locations in the environment. Within an environment, multiple neurons become active in different locations, this cooperation of spiking in various locations creates a place map of the environment. Now let's say when the animal moved from one corner of the environment to another, place cells 'A', 'C', 'B', 'E', and 'D' became active in that order. This means, at any given point in the environment, the animal is standing in a venn-diagram-esque overlap of place fields, or locations individual place cells represent. A key question that entranced researchers for many years was how do these neurons know when to be active to not impinge on their neighbor's locations? The answer to this question rested with population electrical activity, known as the local field potential (LFP), that place cell activity is paced to. During active navigation through an environment, place cells activity is coupled to the phase of a slow ~8 hertz (Hz) theta oscillation. Within one theta cycle, or peak to peak, multiple place cells are active, representing the venn diagram of location the animal is in. Importantly, this theta activity and encoding of place cell activity is largely seen during active running or rapid eye movement (REM) sleep. During slow wave sleep (SWS), after an animal has experienced a specific environment and has created a place map, place cells are reactivated in the same order the animal experienced them in. From our previous example, the content of this reactivation would be the place cells 'A', 'C', 'B', 'E', and 'D' which all would be reactivated in that same order. These reactivations or replays occur during highly synchronous and fast LFP oscillations known as sharp wave-ripples (SPW-Rs). SPW-Rs are thought to be a key LFP event that drives memory consolidation and the eventual conversion of short-term memory into long-term memory. However, for consolidation to occur, connected cortical regions need to be able to receive and interpret the information within SPW-Rs. The granular retrosplenial cortex (gRSC) is one proposed region that serves this role. During SPW-Rs the superficial gRSC has been shown to exhibit high frequency oscillations (HFOs), which potentially serve the purpose for interpreting SPW-R content. However, HFOs have been reported during hippocampal theta, suggesting HFOs serve multiple purposes in interregional communication across different states. In this study, we found that naturally occurring gRSC HFOs occur exclusively in layer 2/3 across all awake brain states. Using focal optogenetic excitation we were able to evoke HFOs in both layer 2/3 and 5. Spontaneous gRSC HFOs occurring without SPW-Rs were highly comodulated with medial entorhinal cortex (MEC) generated high gamma in hippocampal stratum lacunosum moleculare. gRSC HFOs may serve a general role in supporting hippocampo-cortical dialogue during SPW-R and theta brain states to support memory consolidation and encoding, respectively.

Glioblastoma (GBM) is the most common and deadliest of the malignant primary brain tumors in humans, with a reported 5-year survival rate of only 6.8% despite years of extensive research. Failure to improve local tumor control rates and overall patient outcome is attributed to GBM's inherent therapeutic resistance. Marked heterogeneity, extensive local invasion within the brain parenchyma, and profound immunosuppression within the tumor microenvironment (TME) are some of the unique features that drive GBM therapeutic resistance. Furthermore, tumor cells are sequestered behind the blood-brain barrier (BBB), limiting delivery of effective therapeutics and immune cell infiltration into the local tumor. Electroporation-based therapies, such as irreversible electroporation (IRE) and second generation, high-frequency IRE (H-FIRE) represent attractive alternative approaches to standard GBM therapy given their ability to induce transient BBB disruption (BBBD), achieve non-thermal tumor cell ablation and stimulate local and systemic anti-tumor immune responses without significant morbidity. The following work explores the use of H-FIRE to overcome GBM-induced therapeutic resistance and improve treatment success. Chapter 1 opens with an overview of GBM and known barriers to treatment success. Here, we emphasize the utility of spontaneous canine gliomas as an ideal translational model for investigations into novel treatment approaches. Chapter 2 introduces novel ablation methods (i.e. IRE/H-FIRE) capable of targeting treatment-resistant cancer stem cells. The focus of Chapter 3 is to highlight IRE applications in a variety of spontaneous tumor types. In Chapter 4, we investigate the feasibility and local immunologic response of percutaneous H-FIRE for treatment of primary liver tumors using a spontaneous canine hepatocellular carcinoma (HCC) model. In chapter 5, we characterize the mechanisms of H-FIRE-mediated BBBD in an in vivo healthy rodent model. In Chapter 6, we characterize the local and systemic immune responses to intracranial H-FIRE in rodent and canine glioma models to enhance the translational value of our work. Collectively, our work demonstrates the potential for H-FIRE to overcome therapeutic resistance in GBM, thereby supporting its use as a novel, alternative treatment approach to standard therapy. / Doctor of Philosophy / Glioblastoma (GBM) is the most common and deadliest form of primary brain cancer in humans, with only 6.8% of people surviving 5-years after their diagnosis. GBM is characterized by a number of unique features that make it resistant to standard treatments, such as surgery, radiation and chemotherapy. Examples include: (1) extensive invasion of tumor cells into the brain, making complete removal via surgery very difficult; (2) tumor cells are protected by a structure called the blood-brain barrier (BBB), which restricts the entry of most drugs (i.e. chemotherapy) and many immune cells, into the brain, thereby preventing them from reaching tumor cells; (3) tumor cells produce substances that block the immune system from being able to detect the tumor itself, which allows it to continue to grow undetected. High-frequency irreversible electroporation (H-FIRE) represents a new approach for the treatment of GBM. H-FIRE uses electric pulses to temporarily or permanently injure cell membranes without the use of heat, which allows for very precise treatment. The following work explores the ways in which H-FIRE can interfere with specific GBM features that drive its resistance to treatment. Here, we demonstrate that H-FIRE is capable of temporarily disrupting the BBB and characterize the mechanisms by which this occurs. This allows for drugs and immune cells within the blood to enter the brain and access the tumor cells, particularly those extending beyond the visible tumor mass and invading the brain. We also illustrate the potential for H-FIRE treatment within the brain to stimulate local and systemic immune responses by causing the release of proteins from injured cells. Similar to a vaccine, these proteins are recognized by the immune system, which becomes primed to help fight off cancer cells within the body. The end result is an anti-tumor immune response. Collectively, this work supports the use of H-FIRE as an alternative treatment approach to standard therapy for GBM given its potential to overcome certain causes of treatment resistance.

Οι μετατροπείς ηλεκτρονικών ισχύος χρησιμοποιούνται σε ένα ευρύ φάσμα εφαρμογών μικρής και μεγάλης ισχύος. Στην πλειονότητά τους οι μετατροπείς αυτοί περιλαμβάνουν μαγνητικά στοιχεία (μετασχηματιστές – πηνία), οι απώλειες ισχύος των οποίων καθορίζουν σε μεγάλο βαθμό την απόδοση της εκάστοτε διάταξης. Είναι λοιπόν μεγάλης σημασίας η ανάπτυξη θεωρητικών μοντέλων, αλλά και πειραματικών μεθόδων, για τον ακριβή προσδιορισμό των απωλειών των μαγνητικών στοιχείων, ώστε να είναι εφικτός ο βέλτιστος σχεδιασμός τους, με τελικό πάντα στόχο την εξοικονόμηση ενέργειας. Οι στόχοι της διατριβής κινούνται σε δύο βασικούς άξονες: α) Να γίνει διερεύνηση των φαινομένων που επηρεάζουν τις απώλειες χαλκού σε τυλίγματα που αποτελούνται από στρώσεις αγωγών, αλλά και να αναπτυχθεί ένα μοντέλο για τον υπολογισμό των απωλειών χαλκού σε τυλίγματα αγωγών κυκλικής διατομής με τυχαία κατανομή αυτών στο διαθέσιμο χώρο. β) Να γίνει ανάπτυξη μιας ολοκληρωμένης μεθοδολογίας λήψης πειραματικών μετρήσεων σε μαγνητικά στοιχεία μέσα από τη διερεύνηση των διαφόρων παραγόντων πειραματικών σφαλμάτων και τη σχεδίαση μετατροπέα συντονισμού κατάλληλου για τη διέγερση μαγνητικών στοιχείων με ημιτονοειδή τάση υψηλής συχνότητας. Στο πρώτο μέρος της διατριβής (κεφάλαια Κεφ. 1, Κεφ. 2 και Κεφ. 3) γίνεται μια γενική περιγραφή των φυσικών φαινομένων που λαμβάνουν χώρα στα μαγνητικά στοιχεία όταν αυτά διαρρέονται από ρεύμα περιοδικά μεταβαλλόμενο στο χρόνο, τα φαινόμενα δηλαδή της μαγνητικής υστέρησης και της ανάπτυξης δινορρευμάτων στο μαγνητικό υλικό του πυρήνα και τα φαινόμενα επιδερμικό και γειτνίασης, που οφείλονται στην ανάπτυξη δινορρευμάτων στα τυλίγματα. Επίσης, γίνεται παράθεση και αξιολόγηση των σπουδαιότερων ως τώρα θεωρητικών εργασιών που πραγματεύονται τα παραπάνω φαινόμενα και χρησιμοποιούνται για τον υπολογισμό των απωλειών που σχετίζονται με αυτά. Στο δεύτερο μέρος της διατριβής, (κεφάλαια Κεφ. 4 και Κεφ. 5) πρώτα γίνεται η παράθεση των τριών κλασικών μοντέλων για τον υπολογισμό των απωλειών χαλκού σε τυλίγματα που απαρτίζονται από στρώσεις και με τη βοήθεια λογισμικού πεπερασμένων στοιχείων διερευνάται η ακρίβεια και το πεδίο εφαρμογής καθενός εξ’ αυτών. Προκύπτει πως το μοντέλο του Dowell δίνει σαφώς πιο ακριβή αποτελέσματα και πως οι αποκλίσεις των μοντέλων από τα πραγματικά (σύμφωνα με τις προσομοιώσεις) αποτελέσματα οφείλεται στην αδυναμίας τους να λάβουν σωστά υπόψη τη δισδιάστατη ανάπτυξη του μαγνητικού πεδίου και τις πυκνότητας ρεύματος όταν αυξάνεται η συχνότητα ή όταν μειώνεται ο παράγοντας πλήρωσης χαλκού. Διερευνάται το φαινόμενο παρυφής σε τυλίγματα στρώσεων που αποτελούνται είτε από αγωγούς κυκλικής διατομής είτε από φύλλα χαλκού και περιγράφεται ποσοτικά και ποιοτικά η επίδρασή του στην τιμή της ενεργού αντίστασης, η οποία προκύπτει πως εμφανίζεται αυξημένη μόνο σε συχνότητες περί τη βασική αρμονική του ρεύματος. Μελετάται επίσης η γεωμετρική ανάπτυξη του φαινομένου στο χώρο του τυλίγματος και διαπιστώνεται η γενικά επιφανειακή του επίδραση που ελαχιστοποιεί την πιθανή εμφάνιση τοπικής υπερθέρμανσης. Ακόμη, για τυλίγματα στρώσεων με αγωγούς κυκλικής διατομής, αναλύεται η διαφοροποίηση στην τιμή της ενεργού αντίστασης μεταξύ των περιπτώσεων της τετραγωνικής και της εξαγωνικής διάταξης των αγωγών και αναδεικνύεται η μείωσή της στη δεύτερη περίπτωση. Ακολούθως αναπτύσσεται ένα νέο μοντέλο για τον υπολογισμό των απωλειών χαλκού σε τυλίγματα αγωγών κυκλικής διατομής που έχουν τοποθετηθεί στο παράθυρο του μαγνητικού στοιχείου με τυχαίο τρόπο, πράγμα το οποίο αποτελεί μια κοινότυπη σχεδιαστική πρακτική. Για τη διατύπωση της νέας έκφρασης χρησιμοποιούνται τα αριθμητικά αποτελέσματα πληθώρας προσομοιώσεων που έγιναν με το λογισμικό πεπερασμένων στοιχείων και αναζητείται η κατάλληλη εξίσωση περιγραφής τους. Η αναζήτηση αυτή στηρίζονται σε μεθόδους ελαχιστοποίησης του σφάλματος, που εδώ εφαρμόζονται με τη βοήθεια κατάλληλων λογισμικών. Η εξίσωση που τελικά προκύπτει είναι απλή και περιέχει μόνο τρεις εύκολα προσδιορίσιμες παραμέτρους σχετιζόμενες άμεσα με γνωστές κατασκευαστικές παραμέτρους. Δείχνεται πως η νέα έκφραση μπορεί να εφαρμοστεί και στην περίπτωση πολύκλωνου αγωγού και διερευνάται ως προς την ευαισθησία της σε σφάλματα μέτρησης κατά τον προσδιορισμό του πάχους του τυλίγματος, ενώ επίσης προτείνεται μια απλούστερη προσεγγιστική έκφραση για τις χαμηλές συχνότητες. Τέλος, η ισχύς της νέας έκφρασης επαληθεύεται με τη βοήθεια πειραματικών μετρήσεων. Η πρώτη εργασία που παρουσιάζεται στο τρίτο μέρος της διατριβής (κεφάλαια Κεφ. 6 και Κεφ. 7) είναι η σχεδίαση και κατασκευή ενός μετατροπέα συντονισμού κατάλληλου για την τροφοδότηση μαγνητικών στοιχείων με καθαρά ημιτονοειδή τάση υψηλής συχνότητας (ως και 1MHz) και πλάτους αρκετών εκατοντάδων Volt, σε επίπεδα ισχύος αρκετών δεκάδων Watt, για την πραγματοποίηση μετρήσεων σε αυτά. Η θεωρητική και πειραματική διερεύνηση των παραγόντων που επηρεάζουν τη λειτουργία του μετατροπέα αναδεικνύει την αλληλένδετη σημασία των σχεδιαστικών επιλογών τόσο στο κύκλωμα ισχύος όσο και στο ηλεκτρονικό κύκλωμα ελέγχου και οδηγεί στην κατάλληλη διαστασιολόγηση των στοιχείων που χρησιμοποιούνται σε αυτά τα δύο κυκλώματα έτσι ώστε να γίνει εφικτή η διεύρυνση του φάσματος συχνοτήτων λειτουργίας του μετατροπέα, η ελαχιστοποίηση του αρμονικού περιεχομένου και η μεγιστοποίηση του πλάτους της τάσης εξόδου. Στη συνέχεια περιγράφονται μέθοδοι μετρήσεων των απωλειών μαγνητικών στοιχείων και λήψης του βρόχου υστέρησης του μαγνητικού υλικού των πυρήνων τους. Αναλύονται οι διάφοροι παράγοντες σφάλματος και γίνονται μετρήσεις τόσο για τον προσδιορισμό των ειδικών απωλειών φερρίτη όσο και για τη λήψη του βρόχου υστέρησης φερριτών σε διάφορες συχνότητες. Προτείνονται μεθοδολογίες διόρθωσης του αποτελέσματος για τις απώλειες υστέρησης, όπως αυτές προκύπτουν από το εμβαδόν του μετρούμενου βρόχου υστέρησης, όταν υπάρχει γνωστό σφάλμα φάσης κατά τη λήψη της κυματομορφών της μαγνητικής έντασης και της μαγνητικής επαγωγής. Τέλος, εξηγείται η μείωση στις υψηλές συχνότητες της ενεργού αντίστασης των τυλιγμάτων όταν αυξάνεται η θερμοκρασία και δίνονται κάποια ενδεικτικά γραφικά παραδείγματα για τη σχετική διόρθωση για τυπικές αυξήσεις της θερμοκρασίας σε μαγνητικά στοιχεία. / Power electronics converters are used in a wide range of both low and high power applications. Most of these converters include magnetic components (transformers – inductors), the power losses of which determine in a major degree their efficiency. It is therefore very important to the power electronics converter designers to have available the proper theoretical models and experimental methods for the accurate determination of the magnetic component losses in order to make optimum design choices and achieve an effective energy saving. The present work has a twofold goal: a) To investigate the phenomena affecting the copper losses in windings that consist of conductor layers and to develop a new model for the calculation of copper losses in round cross section conductor windings with random distribution of these conductors in the available core window area. b) To develop a complete methodology of making experimental measurements on magnetics components, through the investigation of the several measurement error factors and to design a resonant converter suitable for the excitation of magnetic components with high frequency sinusoidal voltage. In the first part of this thesis (chapters Ch. 1, Ch. 2 and Ch. 3) there is a general description of the physical phenomena that take place in magnetic components when a periodically time variable current flows through them, i.e. magnetic hysteresis and eddy currents at the magnetic core material and skin as well as proximity effect at the windings, which are due to the development of eddy currents in them. Moreover, there is an overview citation and critical review of the most important by now theoretical works on these issues which are also widely used for the calculation of the losses related to them. In the second part of this thesis (chapters Ch. 4 and Ch.5) there is at first a short review of the three classic models for the calculation of copper losses in windings made of layers and then a finite element software is utilized for the investigation of the accuracy and field of application of each of them. It is shown that Dowell’s model is much more accurate and that the declination of the models from the real (according to simulations) results are due to their inherent inability to properly take into account the two-dimensional distribution of the magnetic field and the current density when the frequency increases or when the filling factor value decreases. The edge effect in layered windings with either round cross section or foil conductors is investigated and a qualitative as well as quantitative description of its effect on the effective resistance is given, showing that there can be an increase in it only at frequencies close to the fundamental frequency of the current waveform. There is also a study about the geometrical extent of the edge effect in the winding volume and it is concluded that the winding is generally affected only on its outer parts, a fact that minimizes the possibility for a hot spot to appear. Moreover, for layered windings with round cross section conductors, a study is carried out about the difference in the effective resistance between the cases of square and hexagonal fit schemes and it is shown that in the second case there can be an appreciable power loss reduction. Following this work, is the development of a new model for the calculation of copper losses in round cross section conductor windings with the several turns placed with a random manner in the available core window area, which is a common design choice. For the extraction of the new expression a computer aided curve fitting process has been applied on a large amount of numerical data coming from finite element simulations. The final equation of the model is simple and incorporates only three easily determinable parameters, directly related to known constructive parameters. It is shown that the new expression can also be applied in the case of stranded wire windings. Its sensitivity to the winding height measurement errors is investigated and a low frequency approximation is proposed. At last, the new expression is validated with experimental measurements. The first work presented in the third part of this thesis (chapters Ch. 6 and Ch. 7) is the design and construction of a resonant converter suitable for the excitation of magnetic components with a clearly sinusoidal voltage of high frequency (up to 1MHz) and amplitude of several hundreds volts, at several tenths of Watts power level, for the implementation of experimental measurements. The theoretical and experimental investigation of the factors affecting the converter performance reveals the interrelated importance of the design choices in the resonant tank and the electronic control board and leads to the proper component selection in both these circuits so as to expand the operating frequency range, minimize the harmonic distortion and maximize the amplitude of the output voltage. Following that, there is a description of the methods available to measure the power losses of magnetic components and acquire the hysteresis loop of their magnetic cores. The several error factors are analyzed and measurements are taken in order to determine the power losses and monitor the hysteresis loop of ferrite materials at several frequencies. Some methods are proposed for the correction of the measured hysteresis losses, if these are determined from area of the hysteresis loop, in the case of a known phase error when recording magnetic intensity or magnetic induction. At last, the reduction with temperature of ohmic resistance at high frequencies is explained and some indicative graphical examples are given for the correction in its calculated value for some typical magnetic component temperature rise values.

Ces travaux de recherche portent sur la problématique CEM (Compatibilité Electromagnétique) en électronique de puissance. Cette étude s'intéresse particulièrement à la modélisation HF (haute fréquence) des convertisseurs d'énergie dans le but d'étudier la propagation des perturbations conduites vers le réseau avec ou sans l’utilisation du RSIL (Réseau Stabilisateur d'Impédance de Ligne). Une première partie présente les outils de modélisation des sources de perturbation dans les convertisseurs statiques.La seconde partie est consacrée à la modélisation HF d'un système d'entraînement à vitesse variable. L'onduleur de tension triphasé est représenté par trois générateurs équivalents afin de reconstituer le découpage des tensions de sortie. Un modèle HF du câble blindé de 4 conducteurs tenant compte de la dissymétrie est ensuite proposé. Les résultats de simulation ont été validés par des relevés expérimentaux. La comparaison montre qu'il est possible de modéliser correctement le comportement HF du dispositif jusqu'à 20MHz.Une troisième partie porte sur la propagation des perturbations conduites vers le réseau sans RSIL. Il a fallu pour cela déterminer et modéliser l'impédance du réseau. L'influence du pont redresseur à diodes sur la propagation des perturbations de mode commun a ensuite été étudiée. Les résultats de simulation montrent que l'impact de ces perturbations sur la tension réseau est prépondérant lorsque le pont redresseur est à l'état bloqué. Les résultats expérimentaux ont permis de valider cette étude. Ce travail se termine par une étude préliminaire sur l'association de deux convertisseurs connectés localement à une même source d'énergie / These research works focus on EMC (Electromagnetic Compatibility) in power electronics systems. Specifically, the study focuses on HF (high frequency) modeling of the power converters in order to study the conducted emissions toward the power grids with or without the use of an LISN (Line Impedance Stabilisation Network). The first part presents the modeling tools of the electromagnetic emissions sources in power converters.The second part describes the HF modeling method of an adjustable speed drive (ASD) that will be used to estimate the conducted emissions. In a first part, a behavioural model of PWM inverter is proposed as an EMI noise source. Then, a model of the shielded 4-wire energy cable that taking into account of the dissymmetry structure is proposed and validated in the frequency domain for two different lengths. The comparison of measurement and simulation results of conducted emissions (with LISN) shows the good compromise between the simulation duration and accuracy of the results.A third part is dedicated to the propagation of conducted emissions toward the power grid without using a LISN. The first stage consists in proposing a model impedance of the network impedance. The influence of the bridge rectifier diodes on the propagation paths of common mode disturbances has been studied in the second step. The simulation results show that the impact of these disturbances on the network voltage is more important when the bridge rectifier is normally off-state. The experimental results have validated this study. This work concludes with a preliminary study on the association of two converters connected locally to a single energy source

L'Elastographie par Résonance Magnétique (ERM) est une technique d'imagerie permettant de caractériser in vivo les propriétés biomécaniques des tissus de façon non invasive. Dans ce contexte, la première partie de cette thèse s'intéresse à comparer les propriétés viscoélastiques obtenues par ERM avec une technique de rhéologie haute-fréquence, pouvant atteindre des fréquences de sollicitation mécanique communes à l'ERM, contrairement à ce qui est classiquement fait dans la littérature. Pour effectuer les mesures ERM, le dispositif d'excitation mécanique et la séquence IRM ont été développés et un algorithme de reconstruction des propriétés viscoélastiques, fondé sur l'inversion de l'équation de Helmholtz, a été implémenté et évalué en simulation. La comparaison ERM/rhéologie a ensuite été effectuée, d'abord sur des fantômes de plastisol présentant différentes propriétés viscoélastiques, puis sur des échantillons de foies bovins. Dans les deux cas, les résultats ont montré un bon accord entre les valeurs obtenues en ERM et celles issues de la rhéologie. Le second volet de ces travaux s'attache à présenter une nouvelle stratégie d'acquisition en ERM. Les séquences d'ERM conventionnelle utilisent des gradients oscillants afin d'encoder la propagation de l'onde. Ces gradients peuvent cependant restreindre les applications, en raison des limitations de leur fréquence de commutation et de leur amplitude maximale. Nous proposons ici d'encoder directement la propagation de l'onde à l'aide d'impulsions RF générées par la théorie du Contrôle Optimal (CO), combinées à un gradient constant. Une première expérience simple de contrôle de la phase par impulsions RF est présentée, consistant à créer des motifs de distribution spatiale de cette phase, en l'absence de propagation d'onde. Puis, des impulsions RF adaptées à l'ERM ont été générées : les images de phase obtenues ont été comparées avec celles acquises par ERM conventionnelle. Les propriétés viscoélastiques reconstruites dans les deux cas sont similaires, validant ainsi cette nouvelle méthode d'acquisition. / Magnetic Resonance Elastography is a non-invasive imaging method enabling in vivo characterization of viscoelastic properties of biological tissues. The first part of this thesis deals with the comparison of viscoelastic properties obtained with MRE and with a high-frequency rheometer having a large excitation frequency range. This large frequency range enables common frequency range to MRE, unlike the comparisons usually performed in the literature. To perform MRE measurements, the excitation device and the MRE sequence were developed and an algorithm reconstructing the viscoelastic properties based on an inversion of Helmholtz equation was implemented and evaluated through simulation. The comparison between MRE/rheology was then performed, first on plastisol phantoms with different viscoelastic properties, and then on bovine liver samples. In both cases, results show a very good agreement between values obtained with MRE and those coming from rheology. The second part of this work presents a new acquisition method for MRE. Conventional MRE sequences use oscillating gradients to encode the wave propagation into the phase image. However, these gradients can restrict MRE applications, as their switching frequencies as well as maximal amplitude are limited. The new acquisition strategy we propose in this thesis encodes the wave propagation directly with RF pulses generated with Optimal Control Theory (OCT), in combination with a constant gradient. An initial experiment of phase control with RF pulses is presented, consisting in creating non-trivial spatial phase patterns in MRI phase images, in the absence of wave propagation. Then, RF pulses adapted to the MRE problem are generated with OCT and phase images obtained with these pulses are compared with conventional MRE acquisitions. Viscoelastic properties reconstructed from these two techniques are similar, validating thus this new acquisition method.

This study investigates undergraduate students’ vocabulary size, and their use of formulaic language. Using the Vocabulary Levels Test (Laufer and Nation 1995), it measures the vocabulary size of native and non-native speakers of English and explores relationships between this and course of study, gender, age and home language, and their academic performance. A corpus linguistic approach is then applied to compare student writers’ uses of three high-frequency verbs (have, make and take) relative to expert writers. Multiword units (MWUs) featuring these verbs are identified and analysed, focusing on delexical MWUs as one very specific aspect of depth of vocabulary knowledge. Student and expert use of these MWUs is compared. Grammatically and semantically deviant MWUs are also analysed. Finally, relationships between the size and depth of students’ vocabulary knowledge, and between the latter and academic performance, are explored. Findings reveal that Literature students had larger vocabularies than Law students, females knew more words than males, and older students knew more than younger ones. Importantly, results indicated a relationship between vocabulary size and academic performance. Literature students produced more correct MWUs and fewer errors than Law students. Correlations suggest that the smaller students’ vocabulary, the poorer the depth of their vocabulary is likely to be. Although no robust relationship between vocabulary depth and academic performance emerged, there was evidence of an indirect link between academic performance and correct use of MWUs. In bringing together traditional methods of measuring vocabulary size with an investigation of depth of vocabulary knowledge using corpus analysis methods, this study provides further evidence of the importance of vocabulary knowledge to academic performance. It contributes to debates on the value of a sound knowledge of high-frequency vocabulary and a developing knowledge of at least 5000 words to academic performance, and the analysis and quantification of errors in MWUs adds to our understanding of novice writers’ difficulties with these combinations. The study also explores new ways of investigating relationships between size and depth of vocabulary knowledge, and between depth of vocabulary knowledge and academic performance. / Linguistics and Modern Languages / D. Litt. et Phil. (Linguistics)

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: A well-known problem with FM-CW radar systems is the leakage of transmitter power into the receiver which leads to the making of close-in targets, and can severely limit the system dynamic range performance. This thesis considers two solutions to this radar system problem for a low frequency radar operating in the VHF band. The first method to suppress coupling is using separate transmit and receive antennas designed in such a way as to reduce coupling between them. The second is to design a negative feedback loop as part of the radar receiver where the feedback loop adaptively reduces the amount of transmitter leakage through to the receiver. This project details the realisation of these two solutions. A number of antenna designs are modelled in software and simulated to determine their characteristics of which the transmit-to-receive coupling is the key parameter. As no low coupling configuration could be found a simple configuration is chosen and practical measurements are taken. These antennas are then used in the radar system that is to be built. An FM-CW radar system is designed and simulated using software with a negative feedback loop being designed and implemented into the radar simulation. A practical radar system is then made inclusive of the feedback loop. Measurements are then taken to determine the efficacy of the feedback loop. / AFRIKAANSE OPSOMMING: ʼn Bekende probleem met FM-CW radar stelsels is die lekkasie van versender krag tot in die ontvanger wat lei tot die maak van nabye teikens en kan die stelsel se dinamiese sendbereik steng beperk. Hierdie tesis oorweeg twee oplossings tot hierdie probleem vir ʼn lae frekwensie radar wat in die VHF band werk. Die eerste metode wat na gekyk word om die koppeling te onderdruk is om die twee antennas van die radar stelsel so te ontwerp sodat die hoeveelheid koppeling tussen hulle verminder is. Die tweede is om ʼn negatiewe terugvoerlus as deel van die ontvanger te ontwerp. Hierdie terugvoerlus sal die versender lekkasie sein aanpassend in die ontvanger verminder. In hierdie projek word die realisering van bogenoemde oplossings uiteengeset. ʼn Paar verskillende antenna ontwerpe word gemodelleer in sagteware en word gesimuleer om hul karakteristieke te bepaal. Die belangrikste van hierdie faktore is die versender na ontvanger koppeling. Sienend dat geen ontwerp met ʼn lae genoeg koppeling gevind kon word nie, is ʼn eenvoudige ontwerp gekies en praktiese metings daarvan geneem. Hierdie antennas word dan gebruik in die radar stelsel wat gebou sal word. ʼn FM-CW radar stelsel word ontwerp en gesimuleer in sagteware. Die negatiewe terugvoerlus word ook ontwerp en geïmplementeer in die radar simulasie. ʼn Praktiese radar stelsel word dan gemaak insluitend die terugvoerlus. Metings word dan geneem om die effektiwiteit daarvan te bepaal.

This paper follows Ghysels, Gourieroux, and Jasiak (1998), examines the causal relation between price and volume in Taiwan Futures Market. I use high frequency intraday data of Taiwan Stock Exchange Capitalization Weighted Stock Index in Taiwan Futures Exchange; and analyze the causality between returns and volume series, which are transformed into Markov chain, with Granger’s causal tests. I analyze the data with two different time category, trading time and calendar time. In our research we find out that Taiwan futures market has a bi-directional causality between price and volume in trading time analysis, as to the calendar time analysis, only price to volume unidirectional causality exists. Unlike the unidirectional causal relation that Ghysels, Gourieroux, and Jasiak (1998) observed in French security market.

Operational transconductance amplifier and capacitor (OTA-C) filters have outstood among different types of filter due to high frequency and low power capabilities in the main stream digital CMOS technology. They have been widely used in computer hard disk drive (HDD) and wireless communication transceivers. OTA-C filters based on cascade and passive ladder simulation are well-known. However, multiple loop feedback (MLF) OTA-C filters which have certain advantages still have the scope for further research. So far there have been no explicit formulas for current-mode leapfrog (LF) filter design and performance evaluation of current-mode MLF OTA-C filters are still lacking. From application viewpoints, read channels for computer hard disk drives require very high frequency continuous-time filters. This automatically disqualifies active- RC/MOSFET-C filters and OTA-C filters become the only solution. In wireless communications, active-RC/MOSFET-C filters have been proved useful for mobile systems whose baseband frequency falls below a few MHz. However, for wireless LANs with the frequency of several tens of MHz, OTA-C filters are a strong candidate. Whilst in HDD read channels, cascaded OTA-C architectures have been most utilized and in wireless receivers, OTA-C structures based on ladder simulation have been popular, MLF OTA-C filters have not been practically used in either of the applications. This thesis describes some novel designs and applications of multiple loop feedback OTA-C filters with extensive CMOS simulations. Analogue filters for computer hard disk drive systems are first reviewed; the state of the art and design considerations are provided. Three VHF linear phase lowpass OTA-C filters are then designed, which include a seventh-order and a fifth-order current-mode filter based on the follow-the-leader-feedback (FLF) structure and a seventh-order voltage-mode filter using the inverse FLF (IFLF) configuration. These filters all have very low power consumption. The synthesis and design of general current-mode LF OTA-C filters are conducted next. Iterative design formulas for both all-pole and finite-zero functions are derived and explicit formulas for up to sixth-orders are given. These formulas are very easy to use for designing any type of characteristics. Subsequently, linear phase lowpass OTA-C filter design for HDD read channels using LF structures are investigated in details. A current-mode filter and a voltage-mode filter using the fifth-order LF structure are presented. The two filters can operate up to 800MHz and have very small passband phase ripple. Analogue filters for wireless communication baseband applications are also reviewed thoroughly in this thesis, where the design of a fourth-order current-mode FLF Butterworth lowpass OTA-C filter for multi-standard receivers is presented. Then two fifth-order current-mode elliptic lowpass OTA-C filters based on respective LF and FLF structures for wireless communication baseband are designed. Fifth-order voltage-mode IFLF and LF elliptic lowpass filters are also presented. All these MLF baseband filters designed can operate up to 40MHz to cover all important wireless and mobile standards. Simulations show that the LF structures have better dynamic range and stopband attenuation performances than the FLF and IFLF configurations.

The availability of ultra high-frequency (UHF) data on transactions has revolutionised data processing and statistical modelling techniques in finance. The unique characteristics of such data, e.g. discrete structure of price change, unequally spaced time intervals and multiple transactions have introduced new theoretical and computational challenges. In this study, we develop a Bayesian framework for modelling integer-valued variables to capture the fundamental properties of price change. We propose the application of the zero inflated Poisson difference (ZPD) distribution for modelling UHF data and assess the effect of covariates on the behaviour of price change. For this purpose, we present two modelling schemes; the first one is based on the analysis of the data after the market closes for the day and is referred to as off-line data processing. In this case, the Bayesian interpretation and analysis are undertaken using Markov chain Monte Carlo methods. The second modelling scheme introduces the dynamic ZPD model which is implemented through Sequential Monte Carlo methods (also known as particle filters). This procedure enables us to update our inference from data as new transactions take place and is known as online data processing. We apply our models to a set of FTSE100 index changes. Based on the probability integral transform, modified for the case of integer-valued random variables, we show that our models are capable of explaining well the observed distribution of price change. We then apply the deviance information criterion and introduce its sequential version for the purpose of model comparison for off-line and online modelling, respectively. Moreover, in order to add more flexibility to the tails of the ZPD distribution, we introduce the zero inflated generalised Poisson difference distribution and outline its possible application for modelling UHF data.