Grating-based phase-contrast computed tomography of thick samples

We analyze how the position dependence of the measured signal in phase-contrast imaging using a grating interferometer influences tomographic imaging of samples that are thick in relation to the setup dimensions. The investigations show that there are fundamental differences in image formation depending on the position of the sample relative to the gratings. We discuss tomographic imaging in parallel and fan-beam geometry, with a highlight on tomographic parallel-beam reconstructions for sample positions upstream and downstream of the central beam-splitter grating. Alterations to the standard reconstruction algorithms that result from the position dependence of the measured signal are analyzed analytically and discussed in exemplary computer simulations. We find that the reconstruction results are determined by the position of the axis of rotation relative to the gratings. We conclude that artifact-free reconstructions are feasible for all sample positions within the interferometer, although with different requirements on the angular range and the reconstruction algorithms, respectively.