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Grating Interferometry

Grating Interferometry

Venera Weinhardt

Figure 1: Grating interferometer setup mounted at the TOPO-TOMO beamline.
The beam arrives from the left, interacts with the sample, passes the grating and impinges on the detector.

Talbot Grating Interferometry as a new phase contrast imaging method simultaneously provides four different contrast modes, that is, absorption, differential phase, phase and dark-field contrast. Meanwhile, it relaxes beam requirements on monochromaticity and coherence, as a white beam with spatial coherence of a few micrometers can be used. First experimental results at the TOPO-TOMO beamline at ANKA show high phase sensitivity with a spatial resolution limit of 10 μm and 1 h exposure time for a complete tomographic data set (phase-stepping technique, 8 steps). The grating interferometer setup realized to carry out the experiment is illustrated in Figure 1. As an example, Figure 2 shows reconstructed images of a 40-50 million years old weevil urculionoidea in amber. Compared to the phase image (D), the differential phase-contrast image (C) clearly provides higher contrast of the beetle morphology and highlights the internal structure even through the surrounding amber, while in the phase image a higher contrast between different tissues is visible. Gratings are fabricated at the Institute for Micro Technologies (IMT, KIT) by the LIGA process. Within the collaboration between ANKA and IMT, phase contrast by means of interferometry will be established at the IMAGE beamline for fast tomography, laminography, and microscopy methods. The focus in propagation-based methods is on the processing of a single-distance intensity measurement with contrast induced solely by free-space propagation away from the exit plane of a pure-phase object. This is relevant for real-time 3D tomographic applications, prominently in developmental biology. The physics of the nonlinear algorithms at large relative phase shifts, as being developed in our group, is (i) a perturbative expansion in object-detector distance using a lowest-order a priori estimate and (ii) a Landau quasiparticle approach for high-resolution phase retrieval. The algorithms are computationally inexpensive which, in addition to the single-distance property, makes the method fit for real-time applications. A collaboration with developmental biologists at KIT is under way to exploit the full potential of our theoretical results in this important field of investigation in the life sciences.