Artifacts in X-ray dark-field measurements

Grating-based X-ray phase-contrast imaging with a Talbot-Lau interferometer is a promising method which might be able to increase soft tissue contrast and to gain additional information in comparison to attenuation-based imaging. The method provides an attenuation image, a differential phase image and a dark-field image. A conventional polychromatic X-ray tube can be used together with a Talbot-Lau interferometer consisting of a source grating, a phase grating and an absorption grating. The dark-field image shows information about the sub-pixel-size granularity of the measured object. This supplemental information is supposed to be suitable in applications, such as mammography or nondestructive testing. In this contribution we present results of measurements investigating the thickness-dependent behavior of dark-field imaging. The measurements are performed with a wedge-shaped, granular object with our X-ray phase-contrast imaging set-up and calculating the dark-field image. Measurements with this special phantom show a resurgence of visibility contrast with increasing thickness of the object after passing a minimum. The reason of this artifact is not completely clear up to now, but might be found in attenuation effects in the object in combination with the polychromatic X-ray spectrum or in residual amplitudes in our fitting algorithm for low visibilities and low intensities at large thicknesses. Understandig the thickness-dependent behavior of the X-ray dark-field advances the understanding of the formation of the dark-field image.