1,721,135 research outputs found
Iterative micro-tomography of biopsy samples from truncated projections with quantitative gray values
No full textAbstractThree-dimensional reconstruction from truncated two-dimensional projections cannot be solved analytically without prior knowledge of the sample or access to the non-truncated projections. To suppress reconstruction artifacts in region of interest (ROI) or local tomography, an iterative algorithm has been devised based on back-projection and re-projection, assuming an approximately cylindrical shape of the entire sample of general homogeneity, which is especially applicable for micro-tomography of biopsy samples from biological tissue. Tomographic reconstruction is iteratively refined by minimizing the mismatch between an empty ROI and the reconstruction from the difference between measured sinogram and forward projected ROI reconstruction. By numerical simulation and experimental demonstration, it is shown that the algorithm is not only able to reconstruct quantitative gray values, but also to reduce artifacts of peripheral glow, and may lead to increased image sharpness. The method is particularly suitable for examination of biopsy/autopsy-samples of soft tissue by micro/nano-computed tomography.Bundesministerium für Bildung und Forschunghttp://dx.doi.org/10.13039/50110000234
Actin bundles cross-linked with α -actinin studied by nanobeam X-ray diffraction
No full textWe have performed scanning nano-beam small-angle X-ray scattering (nano-SAXS) experiments on in vitro-formed actin filaments cross-linked with -actinin. The experimental method combines a high resolution in reciprocal space with a real space resolution as given by the spot-size of the nano-focused X-ray beam, and opens up new opportunities to study local super-molecular structures of actin filaments. In this first proof-of-concept, we show that the local orientation of actin bundles formed by the cross-linking can be visualized by the X-ray darkfield maps. The filament bundles give rise to highly anisotropic diffraction patterns showing distinct streaks perpendicular to the bundle axes. Interestingly, some diffraction patterns exhibit a fine structure in the form of intensity modulations allowing for a more detailed analysis of the order within the bundles. A first empirical quantification of these modulations is included in the present work
Viral ion channel proteins in model membranes: a comparative study by x-ray reflectivity
No full textWe have investigated the effect of the transmembrane domain of three viral ion channel proteins on the lipid bilayer structure by X-ray reflectivity and scattering from oriented planar bilayers. The proteins show a similar effect on the lipid bilayer structural parameters: an increase in the lipid bilayer hydrophobic core, a decrease in the amplitude of the vertical density profile and a systematic change in the ordering of the acyl chains as a function of protein-to-lipid ratio. These results are discussed in a comparative view
X-ray reflectivity of solid-supported, multilamellar membranes
No full textWe have investigated the structure of solid-supported,
multilamellar membranes by X-ray
reflectivity. The density profile is obtained by fitting the full -range to a model
using the bilayer Fourier coefficients as fitting parameters. The effect of hydration
and the substrate boundary condition are discussed in view of the well-known
Landau-Peierls effect and its implications for structure determination. The resulting
bilayer density profile agrees remarkably well with previously published data of a
molecular dynamics (MD) simulation for 1,2-oleoyl-palmitoyl-sn-glycero-3-phosphocholine
(OPPC)
Using sparsity information for iterative phase retrieval in x-ray propagation imaging
No full textFor iterative phase retrieval algorithms in near field x-ray propagation imaging experiments with a single distance measurement, it is indispensable to have a strong constraint based on a priori information about the specimen; for example, information about the specimen’s support. Recently, Loock and Plonka proposed to use the a priori information that the exit wave is sparsely represented in a certain directional representation system, a so-called shearlet system. In this work, we extend this approach to complex-valued signals by applying the new shearlet constraint to amplitude and phase separately. Further, we demonstrate its applicability to experimental data
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
How many photons are needed to reconstruct random objects in coherent X-ray diffractive imaging?
No full textPeerReviewe
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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