Experimental optimization of X-ray propagation-based phase contrast imaging geometry.

Propagation-based phase contrast imaging (PB-PCI) with an X-ray lab source is a powerful technique to study low absorption samples, e.g. soft tissue or plastics, on the micrometer scale but is often limited by the low flux and coherence of the source. The setup geometry is essential for the performance since there is a trade-off where a short source distance yields a high contrast-to-noise ratio (CNR) but a low relative fringe contrast. While theoretical optimization strategies based on Fresnel propagation have been reported, there is a need for experimental testing of these models. Here, we systematically investigate this trade-off experimentally using two different setups with high-resolution detectors: a custom-built system with a Cu X-ray source and a commercial system (Zeiss Xradia) with a W source. The fringe contrast, CNR and fringe separation for a low-absorption test sample were measured for 130 different combinations of magnification and overall distances. We find that these figures-of-merit are sensitive to the magnification and that an optimum can be found that is independent of the overall source-detector distance. In general, we find that the theoretical models show excellent agreement with the measurements. However, this requires the complicated X-ray spectrum to be considered, in particular for the broadband W source.