@phdthesis{GraetzgebDittmann2022,
  author    = {Graetz [geb. Dittmann], Jonas},
  title     = {X-Ray Dark-Field Tensor Tomography : a Hitchhiker's Guide to Tomographic Reconstruction and Talbot Imaging},
  doi       = {10.25972/OPUS-28143},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-281437},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {X-ray dark-field imaging allows to resolve the conflict between the demand for centimeter scaled fields of view and the spatial resolution required for the characterization of fibrous materials structured on the micrometer scale. It draws on the ability of X-ray Talbot interferometers to provide full field images of a sample's ultra small angle scattering properties, bridging a gap of multiple orders of magnitude between the imaging resolution and the contrasted structure scale. The correspondence between shape anisotropy and oriented scattering thereby allows to infer orientations within a sample's microstructure below the imaging resolution. First demonstrations have shown the general feasibility of doing so in a tomographic fashion, based on various heuristic signal models and reconstruction approaches. Here, both a verified model of the signal anisotropy and a reconstruction technique practicable for general imaging geometries and large tensor valued volumes is developed based on in-depth reviews of dark-field imaging and tomographic reconstruction techniques. To this end, a wide interdisciplinary field of imaging and reconstruction methodologies is revisited. To begin with, a novel introduction to the mathematical description of perspective projections provides essential insights into the relations between the tangible real space properties of cone beam imaging geometries and their technically relevant description in terms of homogeneous coordinates and projection matrices. Based on these fundamentals, a novel auto-calibration approach is developed, facilitating the practical determination of perspective imaging geometries with minimal experimental constraints. A corresponding generalized formulation of the widely employed Feldkamp algorithm is given, allowing fast and flexible volume reconstructions from arbitrary tomographic imaging geometries. Iterative reconstruction techniques are likewise introduced for general projection geometries, with a particular focus on the efficient evaluation of the forward problem associated with tomographic imaging. A highly performant 3D generalization of Joseph's classic linearly interpolating ray casting algorithm is developed to this end and compared to typical alternatives. With regard to the anisotropic imaging modality required for tensor tomography, X-ray dark-field contrast is extensively reviewed. Previous literature is brought into a joint context and nomenclature and supplemented by original work completing a consistent picture of the theory of dark-field origination. Key results are explicitly validated by experimental data with a special focus on tomography as well as the properties of anisotropic fibrous scatterers. In order to address the pronounced susceptibility of interferometric images to subtle mechanical imprecisions, an efficient optimization based evaluation strategy for the raw data provided by Talbot interferometers is developed. Finally, the fitness of linear tensor models with respect to the derived anisotropy properties of dark-field contrast is evaluated, and an iterative scheme for the reconstruction of tensor valued volumes from projection images is proposed. The derived methods are efficiently implemented and applied to fiber reinforced plastic samples, imaged at the ID19 imaging beamline of the European Synchrotron Radiation Facility. The results represent unprecedented demonstrations of X-ray dark-field tensor tomography at a field of view of 3-4cm, revealing local fiber orientations of both complex shaped and low-contrast samples at a spatial resolution of 0.1mm in 3D. The results are confirmed by an independent micro CT based fiber analysis.},
  subject      = {Dreidimensionale Rekonstruktion},
  language  = {en}
}
@phdthesis{Wiest2022,
  author    = {Wiest, Wolfram},
  title     = {Entwicklung einer Apparatur zur In-situ-Erm{\"u}dungspr{\"u}fung von Zahnimplantaten mittels Synchrotron Micro-CT},
  doi       = {10.25972/OPUS-25770},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-257702},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Die vorliegende Arbeit besch{\"a}ftigt sich mit der volumenbildgebenden Untersuchung von mechanischen Erm{\"u}dungsprozessen in Titan-Zahnimplantaten. Im Vordergrund steht die Entwicklung einer neuen Messmethode der In-situ-Mikrotomografie am Synchrotron. Zahnimplantate werden beim Gebrauch mechanisch wiederholt belastet (Wechsellast). Nach vielen zyklischen Belastungen k{\"o}nnen aufgrund von mikroplastische Verformungen Erm{\"u}dungssch{\"a}den auftreten. Diese k{\"o}nnen im Extremfall zum Versagen und Verlust eines Implantats f{\"u}hren. Die Computertomographie ist eine sehr geeignete zerst{\"o}rungsfrei Pr{\"u}fmethode, um Zahnimplantate zu untersuchen. Diese Arbeit erweitert die bisherige CT-Methode insofern, dass In-situ-Beobachtungen bei mechanischer Belastung m{\"o}glich sind. Die in dieser Arbeit untersuchten Zahnimplantate weisen an der Implantat-Abutment-Grenzfl{\"a}che bei eintretender Erm{\"u}dung einen Mikrospalt auf. Dieser wird als Indikator f{\"u}r einsetzende Fatigue- Prozesse benutzt. Der in der Synchrotron CT verf{\"u}gbare Inlinephasenkontrast erm{\"o}glicht eine verbesserte Bestimmung der Mikrospaltgr{\"o}ße. Da die schnellen Bewegungen der Erm{\"u}dungspr{\"u}fung mittels Standard-CT-Verfahren schwer zu erfassen sind, war die stroboskopische Aufnahmemethode das zielf{\"u}hrende Messverfahren, um in-situ-Pr{\"u}fung zu erm{\"o}glichen. Die 4 kommerziellen Zahnimplantattypen werden neben der In-situ-Fatigue Pr{\"u}fung auch mittels klassischer Erm{\"u}dungspr{\"u}fung untersucht und mit der Neuen Messmethode verglichen. Die hier entwickelte In-situ-Fatigue-Pr{\"u}fstation kann Proben bis zu 345 N tomographisch untersuchen. Neben den experimentellen Untersuchungen wird eine statische FEM-Betrachtung durchgef{\"u}hrt und mit experimentellen Messdaten verglichen. Zuletzt wird mit der entwickelten Messtation Knochenrisse in der Implantat Umgebung untersucht.},
  subject      = {Mikrocomputertomographie},
  language  = {de}
}