Navarrete-Leon, Carlos Felipe; (2025) Multi-contrast x-ray microtomography with two-dimensional amplitude modulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

X-ray microtomography is a powerful non-destructive imaging technique that provides three-dimensional insights into the internal structure of an object. It has broad applications across disciplines, spanning industrial inspection, biomedical research, and materials science. In addition to the conventional attenuation-based contrast, phase-contrast and dark-field imaging exploit the wave nature of x-rays to extend the capabilities of microtomography. Phase-contrast imaging detects the refraction of x-rays, enabling visualisation of low-attenuation details within samples composed, for example, by soft tissues and carbon composites. Dark-field imaging measures the scattering from unresolved microstructures, thus generating contrast from sub-resolution features within the volume of interest. Together, these modalities provide a multi-contrast approach to volumetric imaging, enabling complementary characterisation of complex samples. This thesis presents the development and experimental validation of an x-ray phase-contrast and dark-field tomography technique based on two-dimensional amplitude modulation of the x-ray beam, which we have called two-directional beam-tracking (2DBT). This method requires only a single optical element and is implemented in both a compact laboratory system and a synchrotron setup, leading to a simple and stable setup for this type of advanced X-ray imaging technique. The underlying principles and image formation models of 2DBT are presented, followed by its application in a laboratory system. The system is characterised in terms of spatial resolution and angular sensitivity for planar imaging and is demonstrated for phase-contrast and attenuation tomography using various biological tissues. When implemented with synchrotron radiation, this approach achieves nanoradian angular sensitivity, providing excellent contrast for visualising soft tissue morphology with phase-contrast tomography. The laboratory system is further extended to directional dark-field imaging, allowing the characterization of isotropic and anisotropic scatterers in planar imaging and tomography. This work demonstrates the potential of the 2DBT method to simplify and broaden the accessibility of phase-contrast and dark-field tomography, providing multi-contrast imaging capabilities to a wider user base.

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