Contribuição para um modelo de circulação do LCR na cabeça

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do Grau de Mestre em Engenharia BiomédicaA circulação do líquido cefaloraquidiano (LCR) é de extrema importância para a compreensão de patologias importantes como a Hidrocefalia de Pressão Normal(HPN), que é causada por um desequilíbrio entre a produção e a absorção do LCR. É portanto fundamental o desenvolvimento de um modelo que abranja todo o sistema do LCR de maneira a melhorar o conhecimento do sistema e permitir tratamentos mais eficazes. A quantificação de parâmetros de fluxo de LCR medidos no aqueduto de Sylvius tem sido utilizada para indicar se o paciente sofre de HPN. Há quem defenda que valores de fluxo total (FT) superiores a 18 ml/min indicam HPN, há quem defenda que valores de volume bidireccional médio (VBM) superiores a 42 μl justificam a terapia por derivação ventricular, levando a potenciais diagnósticos contraditórios. Neste estudo pretendemos contribuir para um modelo de circulação do LCR acrescentando dados obtidos na charneira da base do crânio, para estudarmos a dinâmica de circulação no interior da caixa craniana. Como se assume que o LCR seja produzido principalmente no plexo coroideu dos ventrículos laterais e absorvido nas granulações aracnoideias, podemos verificar a quantidade de LCR que flui dentro do cérebro entre o terceiro e quarto ventrículos, no aqueduto de Sylvius, e a quantidade de LCR que flui para o espaço subaracnoideu craniano e raquidiano na charneira da base do crânio, verificando o comportamento dinâmico dos parâmetros de circulação e da onda de propagação de fluxo, controlada pelo fluxo sanguíneo que entra no crânio. Estes exames quantitativos foram obtidos com a técnica de contraste de fase, PC-MRI por Ressonância Magnética que permite uma quantificação do fluxo durante um ciclo cardíaco in vivo e sem perturbação do sistema. Verificamos que tanto o VBM como o FT diminuem com o aumento do ritmo cardíaco, diminuindo mais drasticamente o VBM do que o FT e que o pico da sistole e da diástole se aproximam com esse aumento cardíaco. Foram feitas duas medições em regiões de interesse diferentes para a charneira da base do crânio devido à dificuldade de rodear o espaço subaracnoideu exteriormente e interiormente. O resultado para a medição em redor do espaço subaracnoideu mostrou-se mais concordante com os valores adquiridos no aqueduto de Sylvius. Verificamos também que os parâmetros de circulação estão correlacionados com a área do aqueduto de Sylvius, necessitando esta relação de ser mais aprofundada em estudos futuros

Polarization control of high order harmonics in the EUV photon energy range

International audienceWe report the generation of circularly polarized high order harmonics in the extreme ultraviolet range (18-27 nm) from a linearly polarized infrared laser (40 fs, 0.25 TW) focused into a neon filled gas cell. To circularly polarize the initially linearly polarized harmonics we have implemented a four-reflector phase-shifter. Fully circularly polarized radiation has been obtained with an efficiency of a few percents, thus being significantly more efficient than currently demonstrated direct generation of elliptically polarized harmonics. This demonstration opens up new experimental capabilities based on high order harmonics, for example, in biology and materials science. The inherent femtosecond time resolution of high order harmonic generating table top laser sources renders these an ideal tool for the investigation of ultrafast magnetization dynamics now that the magnetic circular dichroism at the absorption M-edges of transition metals can be exploite

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network

International audienceFemtosecond magnetization phenomena have been challenging our understanding for over a decade. Most experiments have relied on infrared femtosecond lasers, limiting the spatial resolution to a few micrometres. With the advent of femtosecond X-ray sources, nanometric resolution can now be reached, which matches key length scales in femtomagnetism such as the travelling length of excited 'hot' electrons on a femtosecond timescale. Here we study laser-induced ultrafast demagnetization in [Co/Pd]30 multilayer films, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses. This allows us to follow the femtosecond demagnetization process in a magnetic system consisting of alternating nanometric domains of opposite magnetization. No modification of the magnetic structure is observed, but, in comparison with uniformly magnetized systems of similar composition, we find a significantly faster demagnetization time. We argue that this may be caused by direct transfer of spin angular momentum between neighbouring domains

Imagerie cohérente de nano-objets avec impulsions ultra-rapides de XUV

The use of X-rays is fundamental to obtain a spatial resolution in the order of the dozen of nanometers. The duration of the flashes of radiation is placed between the dozen of femtoseconds (1 fs =10-15 s) to the hundreds of attoseconds (1as=10-18 s). During this time frame nano-objects are static in time, image wise it translates as a precise image. Exciting these nano-objects with flashes of X-ray beams it is possible to follow its temporal evolution and record a "movie" of the evolution due to excitation. This type of information is extremely important since it can allow the identification of intermediary structural states and therefore attaining a better understanding of their reactional power.This type of studies it is making its debut in the scientific community due to the recent development of ultra-fast and intense X-ray sources needed to perform this type of imaging. The referenced source is a free electron laser (FEL) and there are only tree of them in the world nowadays. One in Germany, one in the USA and one in Japan. The small amount of FELs is mainly due to its elevated costs. From some years the LOA has shown that lasers can also provide an X-ray beam in the femtosecond region and intense enough to produce images of nano-objects with equivalent temporal and spatial resolutions.This present thesis was built in tree phases: realization of an X-ray laser source, circularly polarized; realization of a new improved imaging system and testing of the nano-samples possessing nano-structures. These nano-structures have a velocity of evolution after excitation in the range of 100 fs. These studies have had place at LOA, LCLS, Laboratoire de Chimie-Physique, Matière et Rayonnement (LCPMR), the CEA de Saclay, BESSY-II in Germany. These cooperations have insured a specific training and expertise in the world of nanometric imaging based on the new technique developed during this work.L'utilisation des rayons X est indispensable pour obtenir la résolution spatiale de la dizaine de nanomètres. La durée de ces flashs va de la dizaine de femtosecondes (1 fs =10-15 s) à la centaine d'attosecondes (1as=10-18 s). Durant ce laps de temps, les nano-objets n'ont pas le temps d'évoluer, assurant l'obtention d'une image précise. En excitant les nano-objets entre deux flashs de rayons X, il est alors possible de suivre sont évolution temporelle et ainsi de réaliser un « film » de son évolution suite à une excitation. Cette information est extrêmement importante car elle permettra d'identifier les états structuraux intermédiaires des nano-objets qui sont connus comme étant les plus importants pour comprendre leur pouvoir réactionnel.Ce genre d'études vient uniquement de débuter dans le monde en raison de l'apparition très récente des sources de rayons X suffisamment brèves et intenses pour réaliser ce genre d'images. La source de référence est un laser dit à « électrons libres » (LEL) dont il existe trois exemplaires au monde, en Allemagne, aux USA et au Japon. Le faible nombre d'exemplaires provient du coût extrêmement élevé de cette machine. Depuis plusieurs années, nous avons montré au LOA que les lasers pouvaient produire un rayonnement X femtoseconde et suffisamment intense pour réaliser des images de nano-objets avec des résolutions spatiales et temporelles équivalentes à celles obtenues sur LEL.La présente thèse a etait construite autour de trois phases : réalisation d'une source de rayons X polarisés circulairement, réalisation d'un nouveau système plus performant d'imagerie, et test sur des échantillons possédants des nano-structures dont la vitesse d'évolution après excitation est prévue aux environs de 100 fs. Ces études ont eu lieu ao LOA, LCLS, Laboratoire de Chimie-Physique, Matière et Rayonnement (LCPMR) et le CEA de Saclay et BESSY-II en Alemagne. Ils ont permis de acquérir une forte expertise en imagerie nanométrique basée sur la nouvelle technique que nous avons développé

Table-top resonant magnetic scattering with extreme ultraviolet light from high-order harmonic generation

International audienceWe demonstrate for the first time the applicability of high-order harmonic generation for probing magnetization properties with nanometer spatial resolution. High harmonics were generated by focusing an infrared femtosecond laser into a neon-filled gas cell. Using a high throughput monochromator, EUV pulses with a photon energy resonant to the magnetically dichroic Co M2, 3 absorption resonance were obtained. These were focused onto a CoPd alloy film and the magnetic scattering pattern was recorded in a transmission geometry. The scattering pattern induced by the magnetic domain structure consists of two well-defined bright spots revealing the presence of stripe domains of about 63 nm in width. With the inherent femtosecond time resolution given by high harmonics, this demonstration paves the way to investigate ultrafast magnetization dynamics with femtosecond time and nanometer spatial resolutions, in jitter-free experiment based on table-top EUV light sources

Observation of spectral gain narrowing in a high-order harmonic seeded soft-x-ray amplifier

International audienceWe report an observation of spectral gain narrowing of a high-order harmonic amplified by a soft-x-ray optical-field-ionized plasma. The temporal coherence and spectral linewidth of both the seeded and unseeded soft-x-ray lasers were experimentally measured using a varying-path-difference interferometer. The results showed that the high-order harmonic is subject to a strong spectral narrowing during its propagation in the plasma amplifier without rebroadening at saturation. This is in good agreement with a radiative transfer calculation including gain narrowing and saturation rebroadening