title: On the origin and nature of the grating interferometric dark-field contrast obtained with low-brilliance x-ray sources.
abstract: The x-ray dark-field contrast accessible via grating interferometry is sensitive to features at length scales well below what is resolvable by a detector system. It is commonly explained as arising from small-angle x-ray scattering (SAXS), and can be implemented both at synchrotron beamlines and with low-brilliance sources such as x-ray tubes. Here, we demonstrate that for tube based setups the underlying process of image formation can be fundamentally different. For focal spots or detector pixels that comprise multiple grating periods, we show that dark-field images contain a strong artificial and system-specific component not arising from SAXS. Based on experiments carried out with a nanofocus x-ray tube and the example of an excised rat lung, we demonstrate that the dark-field contrast observed for porous media transforms into a differential phase contrast for large geometric magnifications. Using a photon counting detector with an adjustable point spread function, we confirm that a dark-field image can indeed be formed by an intra-pixel differential phase contrast that cannot be resolved as such due to a dephasing between the periodicities of the absorption grating and the Talbot carpet. Our findings are further corroborated by a link between the strength of this pseudo-dark-field contrast and our x-ray tube's focal spot size in a three-grating setup. These results must not be ignored when measurements are intended to be reproducible across systems.
authors.affiliation: Institute for Photon Science and Synchrotron Radiation (IPS) / Laboratory for Application of Synchrotron Radiation (LAS) & ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Koenig
authors.forename: Marcus
authors.initials: M
authors.lastname: Zuber
authors.forename: Barbara
authors.initials: B
authors.lastname: Trimborn
authors.forename: Tomas
authors.initials: T
authors.lastname: Farago
authors.forename: Pascal
authors.initials: P
authors.lastname: Meyer
authors.forename: Danays
authors.initials: D
authors.lastname: Kunka
authors.forename: Frederic
authors.initials: F
authors.lastname: Albrecht
authors.forename: Sascha
authors.initials: S
authors.lastname: Kreuer
authors.forename: Thomas
authors.initials: T
authors.lastname: Volk
authors.forename: Michael
authors.initials: M
authors.lastname: Fiederle
authors.forename: Tilo
authors.initials: T
authors.lastname: Baumbach
mesh_terms: D000818:Animals; D006321:Heart / Q000000981:diagnostic imaging*; D007368:Interferometry / Q000379:methods*; D008168:Lung / Q000000981:diagnostic imaging*; D008858:Microscopy, Phase-Contrast / Q000379:methods*; D017785:Photons; D051381:Rats; D017207:Rats, Sprague-Dawley; D053838:Scattering, Small Angle; D014132:Trachea / Q000000981:diagnostic imaging*; D014961:X-Ray Diffraction / Q000379:methods*