The basic principles of x-ray image formation in radiology have remained essentially unchanged since Röntgen first discovered x-rays over a hundred years ago. The conventional approach relies on x-ray attenuation as the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image formation. Phase-contrast or coherent scatter imaging techniques, which can be understood using wave optics rather than ray optics, offer ways to augment or complement the conventional approach by incorporating the wave-optical interaction of x-rays with the specimen. With a recently developed approach based on x-ray optical gratings, advanced phase-contrast and dark-field scatter imaging modalities are now in reach for routine medical imaging and non-destructive testing applications. To quantitatively assess the new potential of particularly the grating-based dark-field imaging modality, we here introduce a mathematical formalism together with a material-dependent parameter, the so-called linear diffusion coefficient and show that this description can yield quantitative dark-field computed tomography (QDFCT) images of experimental test phantoms.
Pore network geometries of intra‐aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non‐invasive three‐dimensional imaging techniques such as synchrotron‐radiation‐based x‐ray microtomography (SR‐μCT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three‐dimensional pore‐space geometry analyses of small scale (∼5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three‐dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 μm were analyzed for pore‐space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore‐size distributions (PSDs), throat‐area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore‐space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Conv.T.) aggregate (mode ≈ 7.9 × 10−7 mm3/≈ 63 μm2) than for the grassland aggregate (mode ≈ 5.0 × 10−6 mm3/≈ 400 μm2), respectively. Path lengths were shorter for the Conv.T. aggregate (maximum lengths < 200 μm) compared with the grassland aggregate (maximum lengths > 600 μm). In summary, the soil aggregate from the Conv.T site showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.
Phase-contrast imaging using conventional polychromatic x-ray sources and grating interferometers has been developed and demonstrated for x-ray energies up to 60 keV. Here, we conduct an analysis of possible grating configurations for this technique and present further geometrical arrangements not considered so far. An inverse interferometer geometry is investigated that offers significant advantages for grating fabrication and for the application of the method in computed tomography ͑CT͒ scanners. We derive and measure the interferometer's angular sensitivity for both the inverse and the conventional configuration as a function of the sample position. Thereby, we show that both arrangements are equally sensitive and that the highest sensitivity is obtained, when the investigated object is close to the interferometer's phase grating. We also discuss the question whether the sample should be placed in front of or behind the phase grating. For CT applications, we propose an inverse geometry with the sample position behind the phase grating.
We introduce a novel x-ray imaging approach that yields information about the local texture of structures smaller than the image pixel resolution inside an object. The approach is based on a recently developed x-ray dark-field imaging technique, using scattering from sub-micron structures in the sample. We show that the method can be used to determine the local angle and degree of orientation of bone, and fibers in a leaf. As the method is based on the use of a conventional x-ray tube we believe that it can have a great impact on medical diagnostics and non-destructive testing applications.
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