Review and experimental verification of X-ray darkfield signal interpretations with respect to quantitative isotropic and anisotropic darkfield computed tomography

TL;DR

This paper reviews models of X-ray darkfield signals, introduces Fresnel scaling for cone beam geometries, and experimentally verifies models of isotropic and anisotropic darkfield contrast in synchrotron imaging.

Contribution

It provides a comprehensive review of darkfield signal models, introduces Fresnel scaling for cone beam geometries, and experimentally validates models for isotropic and anisotropic darkfield contrast.

Findings

Darkfield contrast depends on sample-detector distance and can be compensated by symmetric trajectories.

The anisotropic darkfield signal model accurately describes fibrous materials.

Experimental verification confirms the relation between darkfield contrast and microstructure.

Abstract

Talbot(-Lau) interferometric X-ray darkfield imaging has, over the past decade, gained substantial interest for its ability to provide insights into a sample's microstructure below the imaging resolution by means of ultra small angle scattering effects. Quantitative interpretations of such images depend on models of the signal origination process that relate the observable image contrast to underlying physical processes. A review of such models is given here and their relation to the wave optical derivations by Yashiro et al. and Lynch et al. as well as to small angle X-ray scattering is discussed. Fresnel scaling is introduced to explain the characteristic distance dependence observed in cone beam geometries. Moreover, a model describing the anisotropic signals of fibrous objects is derived. The Yashiro-Lynch model is experimentally verified both in radiographic and tomographic imaging in a monochromatic synchrotron setting, considering both the effects of material and positional dependence of the resulting darkfield contrast. The effect of varying sample-detector distance on the darkfield signal is shown to be non-negligible for tomographic imaging, yet can be largely compensated for by symmetric acquisition trajectories. The derived orientation dependence of the darkfield contrast of fibrous materials both with respect to variations in autocorrelation width and scattering cross section is experimentally validated using carbon fiber reinforced rods.