Dr. Ralf Hoffman - Phase contrast imaging techniques in near field, Karlsruhe Institute of Technology

Title: Contrast transfer in propagation based, single-distance X-ray phase-contrast imaging

Abstract:

We study how a 2D phase map, representing the absorption-free projection of an object along a hard, quasi-monochromatic and sufficiently spatially coherent  X-ray beam, is converted into a 2D map of intensity variations upon  free-space propagation over a distance z. 

In Fresnel theory, the according Fourier-space expansion in the strength of phase variations starts with a linear and local term which is modified by an infinite series of nonlocal corrections. We show simulationally and by an actual experiment  that two important properties of linear contrast transfer are conserved for a wide range of nonlinear phase variations and propagation distances and how this can be understood in terms of the breaking pattern of a phase-scaling symmetry which is exact in the limit of vanishing phase variations. An according phase-retrieval algorithm is deviced which, thanks to large values of z, exhibits high spatial resolution and contrast at low photon statistics. Finally, we compare our results of 3D reconstructed, time-lapsed electron density in living vertebrate model embryos with those obtained by a conventional phase-retrieval algorithm.