Dissertations / Theses on the topic 'Single-shot imaging'

L’imagerie sans lentille a élargi le champ d’applications de l’imagerie aux sources cohérentes de courte longueur d’onde dans le domaine XUV, pour lequel les systèmes optiques pour l’imagerie ne sont pas facilement disponibles. En outre, les sources pulsées ultra brèves XUV et X basées sur la génération d’harmoniques laser d’ordre élevé (HHG) et les lasers à électrons libres (FEL) offrent une très bonne résolution temporelle (femto 10-15s - atto 10-18s). Ce sont donc les outils indispensables pour suivre les dynamiques ultrarapides à l’échelle nanométrique. Il est donc nécessaire de disposer de techniques d’imagerie en un tir unique pour profiter pleinement des capacités de ces sources XUV. Les techniques d’imagerie sans lentille sont basées sur la mesure directe du champ électromagnétique diffracté lors de l’interaction de la source avec l’échantillon. La diffraction est liée à la transmittance de l’objet mais aussi à la cohérence spatiale de la source et à son front d’onde. La caractérisation en un tir unique de ces propriétés permet l’amélioration de la résolution de la reconstruction de l’objet.Les résultats de cette thèse sont présentés en deux parties dans ce manuscrit. La première partie est consacrée à la caractérisation des sources XUV et la deuxième au développement de nouvelles techniques d’imagerie multidimensionnelle. Nous présentons différentes applications de la mesure du front d’onde en un tir unique des sources XUV. Les résultats sont le produit de différentes campagnes expérimentales, sur des sources HHG et les FEL LCLS (Stanford) et FERMI (Trieste). Nous présentons également une nouvelle méthode pour la caractérisation en simple tir de la cohérence spatiale qui ne nécessite pas la connaissance de la distribution d’intensité du faisceau incident. De plus, nous présentons une nouvelle technique d’imagerie basée sur l’holographie par transformée de Fourier pour améliorer la résolution dans la reconstruction de l’objet dans le cas de l’utilisation d’une source partialement cohérente.La deuxième partie est consacrée à deux techniques d’imagerie multidimensionnelle développées pendant cette thèse. Une nouvelle technique d’imagerie 3D en simple tir, facile à implémenter et réduisant fortement la dose de rayonnement reçu par l’échantillon, est présentée. Différents schémas expérimentaux pour la génération de deux sources XUV synchronisées pour cette technique d’imagerie stéréographique 3D sont proposés. D’autre part, nous présentons une technique holographique compatible avec une source de large bande spectrale. Deux applications sont envisagées. La première est l’imagerie ultrarapide résolue spectralement, la deuxième est l’imagerie attoseconde. A la fin du manuscrit des conclusions générales du travail accompli pendant la thèse, ainsi que des perspectives sont présentées
Lensless imaging techniques have broadened imaging applications to coherent sources in the short wavelength XUV domain, where optical systems to create an image are still not readily available. Furthermore, high harmonic generation sources (HHG) and free electron lasers (FEL) have the advantage of providing short temporal resolutions (atto 10-18s - femto 10-15s), opening the way towards ultrafast time resolved nanoscale imaging. Single shot imaging techniques are then highly important to exploit the shortest temporal resolution that can be reached with XUV sources. Lensless imaging is based on the direct measurement of the electric field diffracted by the sample. The diffraction pattern depends on the object transmittance but also on the source spatial coherence and wavefront. Single shot characterization of those properties thus leads to an improvement of the resolution of the object reconstruction.The results presented in this thesis are divided in two parts; the first one is focused on the characterization of the sources and the second on the development of new multidimensional imaging techniques. We will present different applications of single shot wavefront sensing of XUV sources. The results presented are the product of different experimental campaigns performed during this thesis using HH sources and FEL facilities at LCLS (Stanford) and FERMI (Trieste). Furthermore, a new method for single shot characterization of the spatial coherence that does not require the simultaneous measurement of the intensity distribution is presented. Additionally, we present a new holographic technique to improve the resolution of the object reconstruction when a partially coherent source is used.The second part is dedicated to two new multidimensional imaging techniques developed during the thesis. A new tri-dimensional imaging technique that is single shot, easy to implement and that lowers drastically the X-ray dose received by the sample, is presented. Different experimental setups for the generation of two synchronized XUV sources suitable for this ultrafast single shot 3D stereo imaging technique are presented. In addition, we present a holographic technique to extend imaging using a broadband source towards spectrally resolved single shot imaging and attosecond applications. Finally, we present the general conclusions from the work done during the thesis, together with the perspectives drawn from this work

Echo planar imaging (EPI) is widely used for applications where speed and/or sensitivity are essential, such as functional MRI (fMRI) and perfusion MRI. Gradient echo EPI suffers from signal drop-out due to through-slice magnetic field (Bo) inhomogeneity, which prevents certain brain areas from being imaged, e.g. around the frontal sinus and auditory canals. Signal loss can be corrected using the z-shim method, which approximates local 80 variations as linear and recovers signal by applying an opposite refocusing gradient. However, the required gradient is spatially varying and unknown, and. each refocused image requires a separate acquisition. Multiple z-shims are required for full correction making the method inefficient. Signal loss increases with increasing magnetic field strength. The need for an effective correction strategy is driven by the current trend towards high field strengths of 3T and above. This work explores signal loss and recovery in the brain at 3T, and investigates signal loss correction methods culminating in a method which achieves successful signal recovery from two optimally chosen z-shims. The linear approximation made by z-shim methods was found to be valid for the majority of pixels in the brain ,at 3T. Parallel imaging in the through-slice direction to reconstruct points of the k-space slice profile not originally measured was investigated but found to be limited by current hardware. An efficient signal loss correction method was developed which requires only two optimally spaced zshims, positioned by a rapid calibration method for maximum signal recovery. Full signal restoration (to within 2% of the correct value) was achieved in 96% of all brain pixels for 3mm slices, and partial correction in pixels outside this range. This method was applied to a language fMRI study which suffers from signal loss, and recovered activation in regions of 80 inhomogeneity revealing language activation not detectable by conventional fMRI.

The quantum properties of matter waves, in particular quantum correlations and entanglement are an important frontier in atom optics with applications in quantum metrology and quantum information. In this thesis, we report the first observation of sub-Poissonian fluctuations in the magnetization of a spinor 87Rb condensate. The fluctuations in the magnetization are reduced up to 10 dB below the classical shot noise limit. This relative number squeezing is indicative of the predicted pair-correlations in a spinor condensate and lay the foundation for future experiments involving spin-squeezing and entanglement measurements. We have investigated the limits of the imaging techniques used in our lab, absorption and fluorescence imaging, and have developed the capability to measure atoms numbers with an uncertainly < 10 atoms. Condensates as small as ≈ 10 atoms were imaged and the measured fluctuations agree well with the theoretical predictions. Furthermore, we implement a reliable calibration method of our imaging system based on quantum projection noise measurements. We have resolved the individual lattice sites of a standing-wave potential created by a CO2 laser, which has a lattice spacing of 5.3 µm. Using microwaves, we site-selectively address and manipulate the condensate and therefore demonstrate the ability to perturb the lattice condensate of a local level. Interference between condensates in adjacent lattice sites and lattice sites separated by a lattice site are observed.

Imager des objets non-périodiques à une échelle nanométrique et à une échelle femto seconde est un vrai challenge à notre époque. Les techniques d’imagerie « sans lentille » sont des moyens puissants pour répondre à ce besoin. En utilisant des sources ultrarapide (~fs) et cohérente (ex. laser à électron libre ou harmoniques d’ordres élevés), ces techniques nous permettent de reconstruire des objets à partir de leur figure de diffraction, remplaçant les optiques conventionnelles du système d’imagerie par un algorithme informatique. Dans ce travail de thèse, je présent des expériences d’imageries en utilisant un rayonnement extrême-UV (15~40 nm) produit par la génération d’harmoniques d’ordre élevé d’un laser infrarouge puissant. Ce manuscrit est constitué d’une introduction, un chapitre de background théorique, trois chapitres de travail de thèse et une conclusion générale avec perspectives. La première partie du travail de thèse porte sur les développements et caractérisations de la ligne de lumière avec l’objectif de générer maximum de photons harmoniques cohérents avec un front d’onde plat. La deuxième partie est consacrée aux expériences et analyses de trois techniques d’imageries « sans lentille » : Imagerie par diffraction cohérente (CDI), Holographie par la transformée de Fourier (FTH) et Holographie avec références étendues (HERALDO). Ces derniers nous permettent de reconstruire des objets avec une résolution spatiale de 78 nm dans le cas de CDI et de 112 nm dans le cas de HERALDO, tous les deux avec une résolution temporaire de 20 fs. La troisième partie est une première application physique de l’imagerie sur la ligne harmonique. Il s’agit des études statiques et dynamiques de nano-domaines magnétique avec une résolution spatiale sub-100 nm à l’échelle femto seconde. Perspective des techniques d’imagerie 3D et développement potentiel de la ligne d’harmoniques sont présentés à la fin
Ultrafast imaging of isolated objects with nanometric spatial resolution is a great challenge in our time. The lensless imaging techniques have shown great potential to answer this challenge. In lensless imaging, one can reconstruct sample images from their diffraction patterns with computational algorithms, which replace the conventional lens systems. Using ultrafast and coherent light sources, such as free electron laser and high order harmonics, one can investigate dynamic phenomena at the femtosecond time scale. In this thesis work, I present the lenless imaging experiments using XUV radiation provided by a laser driven high order harmonic beamline. The manuscript is composed of an introduction, a chapter of theoretical background, three chapters of main research work and a general conclusion with perspectives. The first part of this work concerns the development of the harmonic beamline to optimize the illumination condition for lensless imaging. The second part concentrates on the imaging techniques: the Coherent Diffraction Imaging (CDI), the Fourier Transform Holography (FTH) and the Holography using extended references (HERALDO). The reconstructions have achieved 78 nm spatial resolution in case of CDI and 112 nm resolution in case of HERALDO, both in single-shot regime corresponding to a temporal resolution of 20 fs. The third part presents the first physical application on the harmonic beamline using the lensless imaging. Samples with magnetic nano-domains have been studied with sub-100 nm spatial resolution, which paves the way for ultrafast magnetic dynamic studies. At the end, single-shot 3D imaging and further beamline development have been discussed

碩士
國立臺灣大學
醫學工程學研究所
102
Magnetic resonance spectroscopy (MRS) is a non-invasive technique that has been used to investigate the metabolic changes in living tissues. Fast magnetic resonance spectroscopy imaging (MRSI) using the proton-echo-planar-spectroscopy-imaging (PEPSI) technique can provide spatial distribution of metabolites in one single radio-frequency (RF) excitation. This method significantly reduces the scan time of 2-dimension MRSI down to 1 minute. Inverse imaging (InI) uses a highly parallel RF coil array to achieve 100-millisecond temporal resolution with the whole brain coverage. Combining PEPSI sequence with InI can further accelerate the MRSI data acquisition. InI reconstruction utilizes coil sensitivities to reconstruct the omitted partition/phase encoded data by solving an under-determined inverse problem. This study shows that the acceleration rate of the combined PEPSI and INI method is not as fast as expected. Specifically, using a 32-channel head coil array, we can achieve up to 6-fold acceleration in 2D PEPSI. Such scan time reduction can still increase the potential of applying MRSI to clinical applications.

Rapid development of genetically encoded fluorescent indicators has provided a diverse chemical toolkit to probe complex biological systems, leading to the expansion of fluorescence microscopy for biological research and applications. However, the inherent constraints on resolution, speed and field of view have hindered the development of high speed, three dimensional fluorescence imaging over large spatial scales for biological microscopy. This thesis describes two strategies based on confocal microscopy to provide single-shot volumetric fluorescence imaging over large scales. In the first part, we describe a multiplane line-scan imaging strategy, which uses a series of axially distributed reflecting slits to probe different depths within a sample volume. Our technique, called line-scan multi-z confocal microscopy, enables the simultaneous imaging of an optically sectioned image stack with a single camera at frame rates of hundreds of hertz, without the need for axial scanning. We demonstrate the applicability of our system to monitor fast dynamics in biological samples by performing calcium imaging of neuronal activity in mouse brains and voltage imaging of cardiomyocytes in cardiac samples. In the second part, we describe a fiber bundle-based endomicroscopy technique, which provides pseudo-volumetric imaging over large field of views, without the need for axial scanning. Our technique uses a gradient refractive index lens to achieve an axially extended illumination and a series of reflecting pinholes of different diameters to simultaneously probe different number of fiber cores. The fluorophores are localized from the series of acquired images using a convolution neural network. We validate our system by localizing fluorescent beads distributed in a volume sample.

An intense laser or charged particle pulse propagating through matter excites light-speed refractive index structures in its wake via Kerr effect, ionization, or displacement of electrons from background ions. Examples include plasma wakes used to accelerate charged particles and self-guided filaments used for atmospheric analysis and micromachining. Such applications constrain the shape, size and evolution of the index structure, yet often these are known in detail only through intensive computer simulations based on estimated initial conditions. Here we develop and demonstrate three methods for visualizing evolving light-speed structures directly in the laboratory in a single shot : (1) frequency-domain streak camera, (2) frequency-domain tomography, and (3) multi-object-plane phase-contrast imaging. All three methods are based on analyzing phase perturbations that an evolving object imprints on one or more probe laser pulses that cross its path obliquely. The methods are tailored to different propagation lengths, material densities, and dimensionality of imaging. Using these techniques, evolving laser-driven filaments in glass and air and plasma wakes in helium gas driven by laser pulses up to petawatt peak power are visualized in one shot, revealing underlying nonlinear laser-plasma interaction physics that is compared in detail to computer simulations.
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碩士
高雄醫學大學
醫學影像暨放射科學系碩士班
102
The brain is the most important organ with complex system to our body. Creating a global architecture of the anatomical connectivity network could help us more understanding how brain organizes and how brain was altered by disorders. Previous studies showed that DTI can successfully trace neuronal fiber tracts in vivo; however, DTI with single shot echo-planar imaging (EPI) has geometric distortion problem which might cause false fiber tracts in tractography. One way to achieving less distortion in DTI images is to use a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) EPI technique. Therefore, the purpose of this study is to construct the anatomical connectivity network of human brain using DTI with PROPELLER EPI acquisition in order to reduce the distortion problem and improve the accuracy of connectivity analysis. In this study, we compare the connectivity network between DTI datasets with PROPELLER and single shot EPI, and we also attempt to observe sex differences in connectivity network between both techniques. The study enrolled 40 healthy subjects. All subjects underwent MRI scan using a 3T MR System. After acquiring 3D T1WI and distortion-free T2WI, PROPELLER DTI (pDTI) and single-shot DTI (ssDTI) data were acquired for comparison. After data acquisition, DTI tractography was performed to reconstruct all of white matter fiber tracts. Finally, the mean fiber tracts of every pair of regions were calculated as the connectivity indices of the network. The results showed that the PROPELLER DTI images exhibit substantially reduced geometric distortions as compared with single-shot DTI, and the anatomical connectivity network with PROPELLER DTI exhibited stronger connectivity between two brain regions. In addition, the results also found sex differences that the male subjects exhibited stronger connectivity mostly within hemispheres, but the female subjects had stronger connectivity mostly between hemispheres. Moreover, since DTI with PROPELLER EPI acquisition has less susceptibility distortions and more accurate fiber tracts. We concluded that the connectivity network with PROPELLER DTI would be more suitable for investigating the connectivity alterations of human brain in the future.