An experimental approach to optimising refraction sensitivity for lab-based edge illumination phase contrast set-ups

Refraction sensitivity can be optimised for differential x-ray phase contrast (XPC) imaging methods by modifying the set-up. Often, modifications involve changing source/detector parameters, propagation distances, or the design of optical components, i.e. parameters that are not readily changed without non-trivial time investment, replacing components, or performing high-precision recalibrations. The edge illumination (EI) XPC method provides a method of optimising the refraction sensitivity, by exploiting micrometric translations of its periodic masks, that bypasses the constraints listed above. These translations can be performed on-the-fly and allow optimising the refraction signal for specific applications without making significant changes to the set-up. The method can prove advantageous for lab-based systems that make use of larger sources but with limited available set-up space. In this paper, we study how refraction sensitivity varies as a function of illuminated pixel fraction (IPF) under two commonly encountered experimental conditions: (1) at approximately constant detected counts, and (2) at equal exposure time. We compare the standard deviation in the background of reconstructed refraction images at different IPFs and find that refraction sensitivity is optimal at 25% IPF under both conditions. Finally, we demonstrate that refraction sensitivity affects the visibility of weakly refracting features on an insect leg. The results suggest that IPFs lower than 50% can actually be preferable, especially in the case where the statistics is kept constant, and provide experimental validation that phase sensitivity in EI is not fixed once the system parameters are defined. (paper)