X-ray phase-contrast and dark-field imaging using near-field speckle

Although conventional absorption X-ray imaging is still the routine method in medical clinics, it has been recognised that the phase-contrast signal, measuring the refraction of the X-rays in a specimen, is much more sensitive to small density differences. This effect is particularly beneficial for low absorbing samples such as biomedical soft-tissues, for which the contrast of the absorption image is quite limited especially at higher X-ray energies. The growing interest in X-ray phase-contrast imaging techniques in the last two decades has led to the development of a number of different methods to retrieve the phase-shifting properties of a sample. Most of them either measure the second or first derivative of the wavefront phase.

The youngest addition to the latter group of differential phase-contrast methods is the X-ray speckle-based technique. It relies on using an X-ray near-field speckle pattern, created by a random phase modulator such as a piece of sandpaper, as a wavefront marker. The sample-induced modulations of the reference speckle pattern are analysed to retrieve the transmission, differential phase and dark-field signals of the sample under study. The latter visualises the small-angle scattering in the specimen, which can provide valuable complementary information.

X-ray speckle-based phase-contrast and dark-field imaging has seen rapid development in the last years, including the demonstration of various operational modes, the extension to 3D tomographic imaging and the translation to polychromatic laboratory sources and higher X-ray energies. Recent efforts have focussed on optimising and simplifying acquisition and reconstruction approaches and making the technique more flexible and adaptable to specific demands on signal sensitivity and spatial resolution.

Speckle-based imaging benefits from a simple experimental setup, cost-effectiveness and relaxed requirements on the spatial and temporal coherence of the X-ray beam. The recent developments furthermore provide the method with a flexible character and adaptability to existing setups.

Thanks to these properties the speckle-based technique has received increasing attention in the X-ray imaging community and is highly promising for a wide range of applications in particular for medical imaging and materials science, but also at-wavelength metrology and optics characterisation.

I will present the basic principles and existing operational modes of X-ray speckle-based multimodal imaging, followed by an outline of the recent developments of the technique. I will then focus on one of the lately proposed operational modes, namely the unified modulated pattern analysis (UMPA), which allows flexibly adapting the signal sensitivity and spatial resolution and can be applied not only to random, but also periodic reference patterns. In the end I will show some examples of the main applications of the speckle-based technique and will conclude with a summary and anticipated future developments.