11 research outputs found
Reduzierung von Missing Wedge Artefakten mit DIRECTT
Wir stellen eine Prozedur vor, mit der typische Missing Wedge MW Artefakte in CT Rekonstruktionen effektiv unterdrĂŒckt werden können. Dazu wird der iterative DIRECTT Algorithmus Direkte Iterative Rekonstruktion Computertomographischer Trajektorien mit einigen Zyklen diskreter Rekonstruk tion eingesetzt. Die Kernfunktionen des Algorithmus , die wiederholte Auswahl und Gewichtung von Elementen einer Zwischenrekonstruktion, werden beibehalten. Abgesehen von SonderfĂ€llen der Röntgen und Neutronentomographie tritt die MW Restriktion neben einer Vielzahl anderer in Standardkonfigurationen der Elektronentomographie auf. Um den MW Effekt isoliert zu untersuchen und beurteilen zu können, werden hier vollstĂ€ndige experimentelle DatensĂ€tze einer bekannten Porenstruktur um einen Sektor beschnitten sowie MW Rekonstruktionen von Porenmodellen erzeugt. Die Ergebnisse werden mit zurzeit verbreiteten Algorithmen Gefilterte RĂŒckprojektion FBP und ART Varianten verglichen. Die Bewertung im Fourierraum zeigt, dass DIRECTT die fehlende Information im MW erfolgreich ergĂ€nzt. Zur quantitativen Bewertung der lokalen Rekonstruktions qualitĂ€t werden Methoden der rĂ€umlichen Statistik eingesetz
Three-Dimensional Imaging of Magnetic Domains with Neutron Grating Interferometry
This paper gives a brief overview on3D imaging of magnetic domains with shearing grating neutron tomography. We investigated the three-dimensional distribution of magnetic domain walls in the bulk of a wedge-shaped FeSi single crystal. The width of the magnetic domains wasanalyzed at different locations within the crystal. Magnetic domains close to the tip of the wedge are much smaller than in the bulk. Furthermore, the three-dimensional shape of individual domains wasinvestigated. We discuss prospects and limitations of the applied measurement technique
Investigation of materials for catalysis with electron tomography
Elektronentomographie mit dem Transmissionselektronenmikroskop (TEM) ermöglicht die Erstellung dreidimensionaler Darstellungen (Tomogramme) von Proben in der GröĂenordnung von einigen Nanometern bis hin zu einigen Mikrometern. Im Rahmen dieser Arbeit wurden verschiedene auf Ruthenium basierende Werkstoffe fĂŒr die Katalyse in Brennstoffzellen untersucht. Die Tomographie liefert, im Gegensatz zu gewöhnlichen TEM Bildern (Projektionen), Aufschluss ĂŒber die Verteilung und Erreichbarkeit der Katalysatorpartikel auf bzw. in dem TrĂ€germaterial. Es konnte gezeigt werden, dass neben qualitativen Vergleichen der Verteilung der Rutheniumpartikel auf/in dem KohletrĂ€germaterial verschieden hergestellter Proben auch detaillierte quantitative Analysen möglich sind. Da die Katalyse an heterogenen Katalysatoren an der OberflĂ€che des Katalysators stattfindet, spielen neben der GröĂe der OberflĂ€che auch die unterschiedlichen Koordinationszahlen verschieden orientierter Facetten der Katalysatorpartikel eine Rolle. Dazu wurde erstmalig ein Algorithmus entwickelt, der es erlaubt, viele verschiedene Partikel in dreidimensionalen DatensĂ€tzen automatisch hinsichtlich Facettierung zu analysieren. Durch die teilweise Einbettung der Katalysatorpartikel in das TrĂ€germaterial ist eine Unterscheidung der bedeckten und unbedeckten OberflĂ€che nötig, da nur der unbedeckte Teil der KatalysatoroberflĂ€che von den Reaktanten erreicht werden kann. Neben dieser unbedeckten OberflĂ€che ist durch die teilweise Einbettung auch die Ausrichtung der Katalysatorpartikel in Bezug zur lokalen OberflĂ€che des TrĂ€gers bedeutsam, da so statistische Untersuchungen der unbedeckten Facettentypen möglich werden. Zu den durchgefĂŒhrten Charakterisierungen wie: Partikelverteilung innerhalb des TrĂ€gers, GröĂenverteilung, OberflĂ€chen, Volumina, Formanalyse und der lokalen Ausrichtung, wurden Erkenntnisse gewonnen, die es erlauben, den untersuchten Katalysatortyp wĂ€hrend der Herstellung weiter zu optimieren. Es konnte zudem gezeigt werden, dass die entwickelten Bildanalysemethoden sich auch auf tomographische DatensĂ€tze anderer Messmethoden wie z.B. Neutronen- und Focused Ion Beam-Tomographie anwenden lassen.Electron tomography with a transmission electron microscope (TEM) enables creation of three-dimensional representations (tomograms) of samples in the range of a few nanometres to a few micrometres. In the frame of this thesis different ruthenium-based materials for catalysis in fuel cells were investigated. Tomography, in contrast to common TEM images (projections), yields information about the distribution and accessibility of the catalyst particles on or in the support material. It was shown that in addition to qualitative comparisons of the distribution of ruthenium particles on/in the carbon support material of differently manufactured samples, quantitative analyses are also possible. Since catalysis on heterogeneous catalysts takes place at the surface of the catalyst, the amount of surface area matters as do the coordination numbers of differently oriented facets of the catalyst particles. For this purpose a new algorithm was developed that allows to automatically analyse faceting of many different particles in a three-dimensional dataset. Due to the partial embedding of the catalyst particles into the support material only the uncovered fraction of the catalyst surface is accessible to the reactants and therefore a differentiation between the covered and uncovered catalyst surface is necessary. Apart from this uncovered surface, the orientation of the catalyst particles relative to the local support surface is also important since this allows statistical investigation of the uncovered facet types. In addition to the conducted characterizations such as: particle distribution within the support, size distribution, surface areas, volumes, shape analysis and the local orientation, new insights were gained which allow optimization of the examined catalyst during production. Furthermore, it could be shown that the developed image analysis methods can be applied to tomographic datasets from other measurement techniques such as neutron and focused ion beam tomography
3D exemplar-based image inpainting in electron microscopy
In electron microscopy (EM) a common problem is the non-availability of data, which causes artefacts in reconstructions. In this thesis the goal is to generate artificial data where missing in EM by using exemplar-based inpainting (EBI). We implement an accelerated 3D version tailored to applications in EM, which reduces reconstruction times from days to minutes. We develop intelligent sampling strategies to find optimal data as input for reconstruction methods. Further, we investigate approaches to reduce electron dose and acquisition time. Sparse sampling followed by inpainting is the most promising approach. As common evaluation measures may lead to misinterpretation of results in EM and falsify a subsequent analysis, we propose to use application driven metrics and demonstrate this in a segmentation task. A further application of our technique is the artificial generation of projections in tiltbased EM. EBI is used to generate missing projections, such that the full angular range is covered. Subsequent reconstructions are significantly enhanced in terms of resolution, which facilitates further analysis of samples. In conclusion, EBI proves promising when used as an additional data generation step to tackle the non-availability of data in EM, which is evaluated in selected applications. Enhancing adaptive sampling methods and refining EBI, especially considering the mutual influence, promotes higher throughput in EM using less electron dose while not lessening quality.Ein hĂ€ufig vorkommendes Problem in der Elektronenmikroskopie (EM) ist die NichtverfĂŒgbarkeit von Daten, was zu Artefakten in Rekonstruktionen fĂŒhrt. In dieser Arbeit ist es das Ziel fehlende Daten in der EM kĂŒnstlich zu erzeugen, was durch Exemplar-basiertes Inpainting (EBI) realisiert wird. Wir implementieren eine auf EM zugeschnittene beschleunigte 3D Version, welche es ermöglicht, Rekonstruktionszeiten von Tagen auf Minuten zu reduzieren. Wir entwickeln intelligente Abtaststrategien, um optimale Datenpunkte fĂŒr die Rekonstruktion zu erhalten. AnsĂ€tze zur Reduzierung von Elektronendosis und Aufnahmezeit werden untersucht. Unterabtastung gefolgt von Inpainting fĂŒhrt zu den besten Resultaten. EvaluationsmaĂe zur Beurteilung der RekonstruktionsqualitĂ€t helfen in der EM oft nicht und können zu falschen SchlĂŒssen fĂŒhren, weswegen anwendungsbasierte Metriken die bessere Wahl darstellen. Dies demonstrieren wir anhand eines Beispiels. Die kĂŒnstliche Erzeugung von Projektionen in der neigungsbasierten Elektronentomographie ist eine weitere Anwendung. EBI wird verwendet um fehlende Projektionen zu generieren. Daraus resultierende Rekonstruktionen weisen eine deutlich erhöhte Auflösung auf. EBI ist ein vielversprechender Ansatz, um nicht verfĂŒgbare Daten in der EM zu generieren. Dies wird auf Basis verschiedener Anwendungen gezeigt und evaluiert. Adaptive Aufnahmestrategien und EBI können also zu einem höheren Durchsatz in der EM fĂŒhren, ohne die BildqualitĂ€t merklich zu verschlechtern
Automatic Differentiation for Inverse Problems in X-ray Imaging and Microscopy
Computational techniques allow breaking the limits of traditional imaging methods, such as time restrictions, resolution, and optics flaws. While simple computational methods can be enough for highly controlled microscope setups or just for previews, an increased level of complexity is instead required for advanced setups, acquisition modalities or where uncertainty is high; the need for complex computational methods clashes with rapid design and execution. In all these cases, Automatic Differentiation, one of the subtopics of Artificial Intelligence, may offer a functional solution, but only if a GPU implementation is available. In this paper, we show how a framework built to solve just one optimisation problem can be employed for many different X-ray imaging inverse problems
Complementary 2D/3D Imaging of Functional Materials using X-ray & Electron Microscopy
Catalysts and other functional materials are generally hierarchically structured materials. Hence, the detailed
characterization at different length scales, and especially under reaction conditions, are necessary
to unravel their mechanisms and to improve their performance and catalytic activities. Besides, a combination
of several techniques is required to acquire complementary information owing to the lack of a
single technique able to cover all the length scales. With respect to length, the best way to investigate is
by microscopy either in 2D or more preferably in 3D. The work began with an exploration of three different
3D imaging techniques, i.e. ptychographic X-ray computed tomography, electron tomography, and
focused ion beam slice-and view. Using nanoporous gold as the model, this study aimed to exhibit the
versatility of 3D microscopy as a method beyond imaging as well as to confirm the necessity of complementary
nature between them, where electron offers better spatial resolution and X-ray provides larger
field of view. The study then continued by utilizing ptychographic X-ray computed tomography for quasi
in situ thermal treatment of the same materials under atmospheric pressure. This study highlighted its
ease of use of implementing in situ studies, complemented by electron tomography to prove its powerful
ability to resolve what ptychographic tomography cannot. The resulting 3D volumes were then used for
air permeability and CO2 diffusion simulations, along with materialâs electrical and thermal conductivity
simulations in order to further expose another excellent benefit from 3D microscopy. Ultimately, the work
proceeded into developing two cells in order to perform full in situ investigations under controlled temperatures
and atmospheres, where one cell was built for 2D only (X-ray) ptychography experiments with
simultaneous X-ray fluorescence mapping, and the other was constructed with an additional capability
for 3D limited-angle ptychographic tomography experiments. The feasibility tests were conducted using
several functional materials, i.e. nanoporous gold, zeolite, and cobalt-manganese-oxides hollow sphere,
as the models for thermal annealing process under specific atmospheres. This work eventually attests the
importance of in situ studies in precisely determining the onset annealing temperatures under particular
environments, to visualize the morphology online either in 2D or 3D, and to simultaneously map elemental
distributions live. Moreover, a complementary technique via transmission electron microscopy
was also demonstrated on the same sample, adding up another advantage in using the cells. Despite the
preliminary results from in situ limited-angle ptychographic tomography experiments for limitation in data
reconstruction, a new tomographic reconstruction technique was recently developed as a solution to acquire
3D images with shortened acquisition times. In conclusions, the work here converges into the ideal
case of performing all-around in situ 3D imaging of functional materials for quantitative analysis and simulation
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Atomic-scale and three-dimensional transmission electron microscopy of nanoparticle morphology
The burgeoning field of nanotechnology motivates comprehensive elucidation of nanoscale materials. This thesis addresses transmission electron microscope characterisation of nanoparticle morphology, concerning specifically the crystal- lographic status of novel intermetallic GaPd2 nanocatalysts and advancement of electron tomographic methods for high-fidelity three-dimensional analysis.
Going beyond preceding analyses, high-resolution annular dark-field imaging is used to verify successful nano-sizing of the intermetallic compound GaPd2. It also reveals catalytically significant and crystallographically intriguing deviations from the bulk crystal structure. So-called ânon-crystallographicâ five-fold twinned nanoparticles are observed, adding a new perspective in the long standing debate over how such morphologies may be achieved.
The morphological complexity of the GaPd2 nanocatalysts, and many cognate nanoparticle systems, demands fully three-dimensional analysis. It is illustrated how image processing techniques applied to electron tomography reconstructions can facilitate more facile and objective quantitative analysis (ânano-metrologyâ). However, the fidelity of the analysis is limited ultimately by artefacts in the tomographic reconstruction.
Compressed sensing, a new sampling theory, asserts that many signals can be recovered from far fewer measurements than traditional theories dictate are necessary. Compressed sensing is applied here to electron tomographic reconstruction, and is shown to yield far higher fidelity reconstructions than conventional algorithms. Reconstruction from extremely limited data, more robust quantitative analysis and novel three-dimensional imaging are demon- strated, including the first three-dimensional imaging of localised surface plasmon resonances. Many aspects of transmission electron microscopy characterisation may be enhanced using a compressed sensing approach
Reduzierung von Missing Wedge Artefakten der CT mit DIRECTT
Reduction of Missing Wedge Artifacts in Computerised Tomography by DIRECTT. Typical missing wedge MW artifacts in reconstructions of computerised tomography CT can be suppressed by the iterative algorithm DIRECTT Direct Iterative Reconstruction of Computed Tomography Trajectories . The MW restrictions due to missing projection angles are presently limited to specific applications of X ray and neutron tomography. However, in the field of nanometer resolving electron tomography they are an inevitable standard. The MW artifacts can be removed much better by an additional procedure of the DIRECTT algorithm applying partially discrete intermediate solutions. Reconstructions of complete and incomplete data sets from measurements and models are created in order to evaluate their quality, and to compare them to the results of other algorithms filtered back projection FBP and ART approaches . The Fourier transforms of the reconstructions demonstrate the contribution of supplementary information to the missing wedge sector. For further assessment of the reconstruction quality, adapted techniques of spatial statistics are applie