Thesis (doctoral)--Friedrich-Wilhelms-Universität zu Berlin, 1870. / Vita.

The motion of a gyroscope in a satellite orbiting the earth is considered. The axis of rotation of the gyroscope is assumed to be parallely propogated along its world line. Taking the satellite's path to be an ellipse, and using the true gravitational potential of the earth, including higher harmonics, one calculates the precession of the axis of rotation during one orbit of the earth with respect to the coordinate frames. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis (M.S.E.C.E.)--University of Delaware, 2008. / Principal faculty advisor: Ian Appelbaum, Dept. of Electrical & Computer Engineering. Includes bibliographical references.

Thesis (M.S.E.C.E.)--University of Delaware, 2009. / Principal faculty advisors: Ian Appelbaum and James Kolodzey, Dept. of Electrical & Computer Engineering. Includes bibliographical references.

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L’Imagerie par Résonance Magnétique (IRM) est une belle application de la physique et constitue sans aucun doute l’une des techniques les plus performantes d’imagerie médicale. Basée sur le phénomène de la Résonance Magnétique Nucléaire (RMN) du proton contenu dans les molécules d’eau, l’IRM permet d’investiguer en coupes les tissus mous du corps, sur base de contrastes différents. La méthode est non-invasive et n’utilise pas de radiations ionisantes. En plus des données morphologiques, l’IRM permet également d’obtenir des informations fonctionnelles et physiologiques. De nos jours, plus de 10 000 unités IRM existent dans le monde et des millions d’examens sont réalisés chaque année. La technique est en constant développement et le domaine de recherches est multidisciplinaire. Il concerne aussi bien les développements méthodologiques (imagerie rapide, imagerie de diffusion, etc.) que technologiques (imagerie à haut champ, systèmes de gradients à commutation rapide, etc.), le point central des recherches étant l’amélioration de la qualité des images et la diminution du temps d’acquisition. Ceci implique l'optimisation des différentes séquences IRM (séries d'impulsions radiofréquence et de gradients de champ magnétique) tenant compte des contraintes imposées par le matériel, ainsi que le développement et l'optimisation du matériel lui-même. Cette thèse est consacrée au design avancé des séquences d’impulsions et contribue donc à l'optimisation des schémas d’acquisition en IRM. En particulier, le présent travail est focalisé sur la compréhension et l’amélioration d’un certain type de séquences rapides, employant des échos de gradients : les séquences Steady-State Free Precession (SSFP) et plus précisément les séquences dites balanced-SSFP. Dans ce genre de schéma d’acquisition, le système est excité rapidement et périodiquement, conduisant à l’établissement d’un état stationnaire de l’aimantation. La première partie de la thèse est consacrée à une analyse approfondie des propriétés du signal dans une séquence balanced-SSFP, à la fois à l’état stationnaire et à l’état transitoire. Ensuite, de nouveaux schémas d’acquisition sont développés sur base de calculs analytiques et de simulations numériques et sont ensuite testés expérimentalement. D’une part, une manipulation de l’état stationnaire est présentée en vue de supprimer le signal de la graisse sur les images (qui peut être gênant pour le diagnostic de certaines lésions ou maladies). D’autre part, l’application d’une phase de préparation en vue d’obtenir un contraste basé sur le degré de diffusion des molécules d’eau dans les tissus est analysée en détails, afin d’améliorer la qualité d’image produite par des séquences de diffusion existantes. La présente thèse constitue donc un travail de recherches théoriques et expérimentales, allant de la conception de nouveaux schémas d’acquisition à leur expérimentation sur volontaires, en passant par leur implémentation sur un imageur IRM. Ce travail a été réalisé au sein de l’Unité d’IRM – Radiologie de l’Hôpital Erasme, sous la direction de Thierry Metens, Docteur en Sciences et Physicien IRM.

A aplicação de uma sequência de pulsos com tempo de repetição muito menor que os tempos de relaxação Tp << T2; T1, faz com que a magnetização atinja um estado estacionário descrito por H.Y. Carr como Estado Estacionário em Precessão Livre, Steady State Free Precession (SSFP). Nessa condição, o sinal é composto pela complexa sobreposição das componentes FID e eco. Sequências tipo SSFP são utilizadas na aquisição rápida de sinais, resultando em uma boa razão sinal ruído (s/r) em curto intervalo de tempo, porém introduzem fortes anomalias de fase e amplitude devido a complexa interação das componentes que formam o estado estacionário. Neste trabalho, desenvolvemos sequências de pulsos tipo SSFP para RMN em alta e baixa resolução com alternância e incremento de fase. Em alta resolução desenvolvemos as sequências SSFPdx e SSFPdxdt com incremento de fase linear e quadrático respectivamente. Os resultados mostram que espectros de núcleos com baixa sensibilidade podem ser obtidos com mesma razão s/r em menor tempo experimental e as sequências desenvolvidas removem as anomalias espectrais. Em baixa resolução, os resultados mostram que a introdução de alternâncias de fase na Continuous Wave Free Precession (CWFP) possibilita a remoção da dependência da sequência com o offset de frequência e com o tempo entre pulsos. Além disso, mostramos que a sequência CP-CWFPx-x com ângulo de refocalização pequeno (5&deg; a 10&deg;) possibilita a estimativa rápida do tempos de relaxação longitudinal. Apresentamos também resultados dos estudos e desenvolvidos no estágio de pesquisa no exterior, onde as sequências de pulsos no estado estacionário &ndash; DECPMG e Split 180&deg; &ndash; foram estudas numericamente e implementadas nos sistemas magnéticos compactos: mini-Halbach e MOUSE-NMR. Por fim, são apresentados resultados com os métodos de processamento de dados Krylov Basis Diagonalization Method (KBDM) e a Transformada Inversa de Laplace aplicados na análise de sinais SSFP. Resultados mostram que KBDM é uma ferramenta útil no processamento de dados em alta e baixa resolução, tanto na obtenção de espectros como na determinação da distribuição dos tempos de relaxação. / The application of a pulse sequence with repetition time much smaller than the relaxation times, Tp << T2; T1, causes the magnetization to reach a steady state, described by H. Y. Carr as a Steady State Free Precession (SSFP). In this condition, the signal is composed of the complex overlapping of the FID and eco components. SSFP type sequences are used in fast acquisition of NMR signals, resulting in a good signal to noise ratio (s/r) in a short time interval, however, they introduce phase and amplitude anomalies due to the complex interaction between the components of the steady state. In this work, we develop SSFP type pulse sequences for NMR in high and low resolution, with alternation and increment of phase. In high resolution, we develop SSFPdx and SSFPdxdt sequences, with linear and quadratic phase increment respectively. Results show that the low sensitivity nuclei spectra can be obtained with the same s/r ratio in smaller experimental time, about an order of magnitude, and the developed sequences can remove the spectral anomalies. In low resolution, the results show that the introduction of a phase alternation in the Continuous Wave Free Precession (CWFP) allows the elimination of the dependence of the sequence with the offset frequency and the time between pulses. Besides, we show that the CP-CWFPx-x sequence with a small refocalization angle (5° to 10°) allows the fast estimative of the longitudinal relaxation time in a single experiment. The results of the studies conducted during an international research internship are also presented. Steady state pulse sequences &ndash; DECPMG and Split 180° &ndash; were studied and implemented in compact magnetic systems: mini-Halbach and MOUSE-NMR. Finally, the results of the application of the Krylov Basis Diagonalization Method (KBDM) and the Inverse Laplace Transform for the analysis of SSFP signals are presented. The results show that KBDM is a useful tool in data processing for low and high resolution, both for obtaining spectra and determining the relaxation times distribution.

The hippocampus is a brain structure involved in episodic memory and spatial cognition. Neuronal activity within the hippocampus exhibits intricate temporal patterning, including oscillatory and sequential dynamics, which are believed to underlie these cognitive processes. In individual cells, a temporal activity pattern called phase precession occurs which leads to the organisation of neuronal populations into sequences. These sequences are hypothesised to form a substrate for episodic memory and the representation of spatial trajectories during navigation. In this thesis, I present a novel theory of the phenomenological properties of these neuronal activity sequences. In particular, I propose that the sequential organisation of population activity is governed by the independent phase precession of each cell. By comparison of models in which cells are independent and models in which cells exhibit coordinated activity against experimental data, I provide empirical evidence to support this hypothesis. Further, I show how independent coding affords a vast capacity for the generation of sequential activity patterns across distinct environments, allowing the representation of episodes and spatial experiences across a large number of contexts. This theory is then extended to account for grid cells, whose activity patterns form a hexagonal lattice over external space. By analysing simple forms of phase coding in populations of grid cells, I show how previously undocumented constraints on phase coding in two dimensional environments are imposed by the symmetries of grid cell firing fields. To overcome these constraints, I propose a more complex phenomenological model which can account for phase precession in both place cells and grid cells in two dimensional environments. Using insights from this theory, I then propose a biophysical circuit mechanism for hippocampal sequences. I show that this biophysical circuit model can account for the proposed phenomenological coding properties and provide experimentally testable predictions which can distinguish this model from existing models of phase precession. Finally, I outline a scheme by which this biophysical mechanism can implement supervised learning using spike time dependent plasticity in order to learn associations between events occurring on behavioural timescales. The models presented in this thesis challenge previous theories of hippocampal circuit function and suggest a much higher degree of flexibility and capacity for the generation of sequences than previously believed. This flexibility may underlie our ability to represent spatial experiences and store episodic memories across a seemingly unlimited number of distinct contexts.

Research into the fluidic precessing jet, used in industrial burners, has been carried out within the School of Mechanical Engineering at the University of Adelaide for over a decade. The flow field generated by the fluidic precessing jet (FPJ) is extremely complex, and there are many questions yet to be answered about the mechanisms by which precession influences the mixing of the jet and ambient fluid, and hence combustion. Some may be answered by studying a non-reacting precessing jet. The mechanical precessing jet (MPJ) nozzle generates a precessing jet for which the exit conditions are well known, unlike the fluidic precessing jet. The non-reacting flow from this 'mechanical analogue' of the FPJ forms the basis of the current study. The MPJ provides a means of controlling and changing the structure of turbulence in a precessing jet by varying its precessional frequency. The characteristics of the MPJ flow are primarily determined by a Strouhal number of precession, and may be categorised as belonging to either a 'low Strouhal number' or 'high Strouhal number' regime of behaviour. The fundamental aim of studying the mechanical precessing jet flow is to determine the influence of the structure of turbulent motions, and in particular the large scale motions, on jet mixing. The analyses presented in this thesis lead to a better understanding of the underlying mechanisms of precession-enhanced turbulent mixing and combustion. Simultaneously collected phase-averaged velocity and concentration fields of the MPJ flow are presented, and correlations between the fields analysed, for one low and one high Strouhal number. Additionally, because the turbulent flow produced by the MPJ nozzle is unsteady in nature and instantaneous realisations of the flow may differ significantly from the mean flow patterns, planar velocity and concentration measurements which show instantaneous flow structure over the entire field are presented. The phase-averaged velocity and concentration field data have enabled new analytical models of the MPJ trajectory to be developed, and the behaviour of the major flow features, including the stability of the counter-rotating vortex pair, to be studied. The strong entrainment and mixing characteristics of the MPJ flow are also illustrated. The data and analysis strongly suggest that the initial trajectory of the jet is essentially radial, during which the jet experiences axial compression. At larger radius the jet experiences axial stretching. A counter- rotating vortex pair is seen to form approximately two potential core lengths from the jet exit, where the jet appears to bend sharply towards the axis of rotation. These vortices dominate the jet motion in the near field and eventually merge in the transition region of the flow. The inner vortex of the counter-rotating vortex pair mixes at approximately half the rate of the outer vortex, thus delivering a rich fuel mixture to the transition region when the MPJ is used as a burner. This may explain in part earlier observations of highly radiant, fuel-rich flames in the transition region. This study also outlines the development of an experimental technique for the simultaneous measurement of velocity and concentration in a plane. The medium is air, and the technique combines Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) of acetone vapour in a unique manner. / Thesis (Ph.D.)--School of Mechanical Engineering, 2004.

This study investigates modes of atmospheric variability in response to changes in Earth's orbit and changes in 20th century sea surface temperatures (SST). The orbital forcing is manifested by a change in obliquity and precession, and changes the distribution of the top-of-atmosphere insolation. A smaller obliquity reduces the the annual insolation that the poles receive and increases the annual insolation in the tropics. As the meridional insolation gradient increases, the zonal mean atmospheric-ocean circulation increases. The resulting climate also has a reduced global mean temperature due to the effect of climate feedbacks. This cooling can be attributed to a reduced lapse rate, increased cloud fraction. reduced water vapor in the atmosphere, and an increase in the surface albedo. A change in the precession, as the perihelion shifts from the winter to the summer solstice, causes a strengthening as well as an expansion of the N. Pacific summer subtropical anticyclone. This anticyclonic anomaly can be attributed to the weakening of the baroclinic activity, but also represents the circulation response to remote and local diabatic heating. The remote diabatic heating is associated with monsoonal activity in the SE Asia and North Africa. Regarding the 20th century SST forcing, it is represented by a multidecadal variability in the inter-hemispheric SST difference. This change in the SST causes a latitudinal shift in the ascending branch of the Hadley cell and precipitation in the tropics, as well as an increase in the atmospheric meridional heat transport from the warmer to the colder hemisphere.