Robert Feidenhans'l

Quantitative x-ray dark-field computed tomography

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.

Soft-tissue phase-contrast tomography with an x-ray tube source.

We report the first experimental soft-tissue phase-contrast tomography results using a conventional x-ray tube source, with a millimeter-sized focal spot. The setup is based on a Talbot-Lau grating interferometer operated at a mean energy of 28 keV. We present three-dimensional ex vivo images of a chicken heart sample, fixated in formalin. The results clearly demonstrate the advantageous contrast attainable through phase-contrast imaging over conventional attenuation-based approaches.

Hard X-ray phase-contrast imaging with the Compact Light Source based on inverse Compton X-rays

The first imaging results obtained from a small-size synchrotron are reported. The newly developed Compact Light Source produces inverse Compton X-rays at the intersection point of the counter propagating laser and electron beam. The small size of the intersection point gives a highly coherent cone beam with a few milliradian angular divergence and a few percent energy spread. These specifications make the Compact Light Source ideal for a recently developed grating-based differential phase-contrast imaging method.

Multimodal x-ray scatter imaging

We describe a small-angle x-ray scattering-based imaging technique that reveals the distribution and orientation of nano-scale structures over extended areas. By combining two measurement and analysis schemes, complementary structural information is available which renders the technique suitable for a broad range of applications, e.g. in materials science and bio-imaging. Through a combination of current techniques and on-line analysis schemes, measurements with a so far unprecedented combination of speed, dynamic range and point density became feasible. This is illustrated by data recorded for a section of a mouse soleus muscle visualizing fine muscle and Achilles tendon structures down to the 10?nm range over a 10?mm2 sample area.

Directional x-ray dark-field imaging.

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.

McXtrace: a Monte Carlo software package for simulating X-ray optics, beamlines and experiments

This article presents the Monte Carlo simulation package McXtrace, intended for optimizing X-ray beam instrumentation and performing virtual X-ray experiments for data analysis. The system shares a structure and code base with the popular neutron simulation code McStas and is a good complement to the standard X-ray simulation software SHADOW. McXtrace is open source, licensed under the General Public License, and does not require the user to have access to any proprietary software for its operation. The structure of the software is described in detail, and various examples are given to showcase the versatility of the McXtrace procedure and outline a possible route to using Monte Carlo simulations in data analysis to gain new scientific insights. The studies performed span a range of X-ray experimental techniques: absorption tomography, powder diffraction, single-crystal diffraction and pump-and-probe experiments. Simulation studies are compared with experimental data and theoretical calculations. Furthermore, the simulation capabilities for computing coherent X-ray beam properties and a comparison with basic diffraction theory are presented.

Monitoring Shifts in the Conformation Equilibrium of the Membrane Protein Cytochrome P450 Reductase (POR) in Nanodiscs

Background: Investigating the mechanism of NADPH-dependent conformational changes of POR in nanodiscs. Results: The conformational equilibrium of compact and extended POR, shifts toward the compact form (from 30 to 60%) upon reduction by NADPH. Conclusion: The NADPH-dependent conformational changes follow the “swinging model.” Significance: This is the first time that the action of a membrane protein located in a lipid bilayer environment is probed by neutron reflectivity. Nanodiscs are self-assembled ∼50-nm2 patches of lipid bilayers stabilized by amphipathic belt proteins. We demonstrate that a well ordered dense film of nanodiscs serves for non-destructive, label-free studies of isolated membrane proteins in a native like environment using neutron reflectometry (NR). This method exceeds studies of membrane proteins in vesicle or supported lipid bilayer because membrane proteins can be selectively adsorbed with controlled orientation. As a proof of concept, the mechanism of action of the membrane-anchored cytochrome P450 reductase (POR) is studied here. This enzyme is responsible for catalyzing the transfer of electrons from NADPH to cytochrome P450s and thus is a key enzyme in the biosynthesis of numerous primary and secondary metabolites in plants. Neutron reflectometry shows a coexistence of two different POR conformations, a compact and an extended form with a thickness of 44 and 79 Å, respectively. Upon complete reduction by NADPH, the conformational equilibrium shifts toward the compact form protecting the reduced FMN cofactor from engaging in unspecific electron transfer reaction.

Sulfur induced Cu4 tetramers on Cu(111)

Abstract The structures of the Cu (111) | 4 1 −1 4 |- S and the Cu(111)(√7 × √7)R19.1°-S reconstructions have been studied by surface X-ray diffraction and two new structural models are proposed. Although the unit cells for the two structures are quite different, a high degree of similarity has been revealed. It consists of a Cu4S unit with sulfur adsorbed in the fourfold hollow site on top of a Cu tetramer. The | 4 1 −1 4 | structure contains two Cu4S units, the (√7 × √7)R19.1° structure only one. In both structures additional sulfur atoms are present. The sulfur coverages for the structures are 0.35 and 0.43 ML, respectively. The building block for the structures is similar to the Ni4S unit which we recently found in the Ni(111)(5√3 × 2)-S structure. We propose that the driving force for the reconstruction is the chemical stability of the Cu4S units given by the tendency of sulfur to be fourfold coordinated.

Strong light scattering and broadband (UV to IR) photoabsorption in stretchable 3D hybrid architectures based on Aerographite decorated by ZnO nanocrystallites

In present work, the nano- and microscale tetrapods from zinc oxide were integrated on the surface of Aerographite material (as backbone) in carbon-metal oxide hybrid hierarchical network via a simple and single step magnetron sputtering process. The fabricated hybrid networks are characterized for morphology, microstructural and optical properties. The cathodoluminescence investigations revealed interesting luminescence features related to carbon impurities and inherent host defects in zinc oxide. Because of the wide bandgap of zinc oxide and its intrinsic defects, the hybrid network absorbs light in the UV and visible regions, however, this broadband photoabsorption behavior extends to the infrared (IR) region due to the dependence of the optical properties of ZnO architectures upon size and shape of constituent nanostructures and their doping by carbon impurities. Such a phenomenon of broadband photoabsorption ranging from UV to IR for zinc oxide based hybrid materials is novel. Additionally, the fabricated network exhibits strong visible light scattering behavior. The developed Aerographite/nanocrystalline ZnO hybrid network materials, equipped with broadband photoabsorption and strong light scattering, are very promising candidates for optoelectronic technologies.

Molecular X-ray computed tomography of myelin in a rat brain

In this work we demonstrate the feasibility of applying small-angle X-ray scattering computed tomography (SAXS-CT) for non-invasive molecular imaging of myelin sheaths in a rat brain. Our results show that the approach yields information on several quantities, including the relative myelin concentration, its periodicity, the total thickness of the myelin sheaths, and the relative concentration of cytoskeletal neurofilaments. For example the periodicity of the myelin sheaths varied in the range from 17.0 to 18.2 nm around an average of 17.6 (±0.3) nm. We believe that imaging, i.e., spatially resolved measuring these quantities could provide general means for understanding the relation to a number of neurodegenerative diseases.