Capturing and visualizing transient X-ray wavefront topological features by single-grid phase imaging

The detection, localisation and characterisation of stationary and singular points in the phase of an X-ray wavefield is a challenge, particularly given a time-evolving field. In this paper, the associated difficulties are met by the single-grid, single-exposure X-ray phase contrast imaging technique, enabling direct measurement of phase maxima, minima, saddle points and vortices, in both slowly varying fields and as a means to visualise weakly-attenuating samples that introduce detailed phase variations to the X-ray wavefield. We examine how these high-resolution vector measurements can be visualised, using branch cuts in the phase gradient angle to characterise phase features. The phase gradient angle is proposed as a useful modality for the localisation and tracking of sample features and the magnitude of the phase gradient for improved visualization of samples in projection, capturing edges and bulk structure while avoiding a directional bias. In addition, we describe an advanced two-stage approach to single-grid phase retrieval.