1,365,591 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
Holographic imaging with a hard x-ray nanoprobe : Ptychographic versus conventional phase retrieval
No full textWe have performed near-field x-ray imaging with simultaneous object and probe reconstruction. By an advanced ptychographic algorithm based on longitudinal and lateral translations, full-field images of nanoscale objects are reconstructed with quantitative contrast values, along with the extended wavefronts used to illuminate the objects. The imaging scheme makes idealizing assumptions on the probe obsolete, and efficiently disentangles phase shifts related to the object from the imperfections in the illumination. We validate this approach by comparison to the conventional reconstruction scheme without simultaneous probe retrieval, based on the contrast transfer function algorithm. To this end, a set of semiconductor nanowires with controlled chemical composition (InP core, insulating SiO2 layer, and indium tin oxide cover) is imaged using the quasi-point source illumination realized by the hard x-ray nanofocus (26 nm x 39 nm spot size) of the ID16A Nano-Imaging beamline at the European Synchrotron Radiation Facility
Validity of the empty-beam correction in near-field imaging
No full textExtended wavefronts are used for coherent full field imaging of objects based on solving the inverse Fresnel diffraction problem. To this end, the conventional data correction step is given by division of the recorded object image by the intensity pattern of the empty beam. This division of intensities in the detection plane is a rather crude approximation for the separation of the complex valued object and probing fields. Here we present a quantitative error estimate, along with its mathematical proof, and confirm the prediction with numerical simulations. Finally the problem is illustrated with experimental results
Phase retrieval for object and probe using a series of defocus near-field images
No full textFull field x-ray propagation imaging can be severely deteriorated by wave front aberrations. Here we present an extension of ptychographic phase retrieval with simultaneous probe and object reconstruction suitable for the near-field diffractive imaging setting. Update equations used to iteratively solve the phase problem from a set of near-field images in view of reconstruction both object and probe are derived. The algorithm is tested based on numerical simulations including photon shot noise. The results indicate that the approach provides an efficient way to overcome restrictive idealizations of the illumination wave in the near-field (propagation) imaging. (C) 2013 Optical Society of Americ
Phase retrieval for object and probe in the optical near-field
Full text linkLensless, holographic X-ray microscopy is a non-invasive imaging technique that provides resolution on the nanometer scale. Therefore, a divergent, coherent and especially clean wave front impinging on the sample is needed. Yet, focusing X-rays by even the most advanced X-ray mirrors causes so called figure errors of high spatial frequency content. The results are strongly deteriorated intensity profiles that are often even more pronounced than the holographic image of the sample itself. A common strategy to compensate these figure errors is to divide the hologram by the pure intensity profile of the beam (the so called flat field). However, this division is only valid in the limiting case of an illumination focused down to a point source. In reality, as a consequence of a fi nite spot size, one has to accept a loss in resolution when performing the flat field correction. An approach different from the described straightforward procedure is necessary. Here, the simultaneous reconstruction of object and probe is proposed using holograms which were not flat field corrected before phase retrieval. To this end, a method has been developed that allows simultaneously reconstructing object and probe in amplitude and phase from holographic intensity recordings. The experimental way of proceeding was mainly inspired by well-established holographic full-field X-ray imaging techniques that require holograms defocused to different degrees. Consequently, the conclusion seems reasonable that diversity in the optical near-field arises mainly from variation of the propagation distance of light. This so called longitudinal diversity is used to properly phase the transmission function of the sample of interest. The algorithmic strategy of simultaneous phase retrieval for object and probe draws on far-field ptychography where lateral translations of the sample create diverse diffraction patterns. In view of the need for longitudinal diversity realized by shifts of the sample along the optical axis, ptychography has been generalized and adapted for the optical near-field. Hence, translations of the sample in all three dimensions of space need to be exploited to collect enough information about object and probe such that both can be reconstructed simultaneously in amplitude and phase. Concepts have been put into practice by simulations as well as by experiments with coherent visible light and hard X-rays from synchrotron sources. The presented approach offers the opportunity to perform high resolution imaging, to be extended to tomography and to be adapted to super-resolution experiments
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
Probe reconstruction for holographic X-ray imaging
No full textIn X-ray holographic near-field imaging the resolution and image quality depend sensitively on the beam. Artifacts are often encountered due to the strong focusing required to reach high resolution. Here, two schemes for reconstructing the complex-valued and extended wavefront of X-ray nano-probes, primarily in the planes relevant for imaging (i.e. focus, sample and detection plane), are presented and compared. Firstly, near-field ptychography is used, based on scanning a test pattern laterally as well as longitudinally along the optical axis. Secondly, any test pattern is dispensed of and the wavefront reconstructed only from data recorded for different longitudinal translations of the detector. For this purpose, an optimized multi-plane projection algorithm is presented, which can cope with the numerically very challenging setting of a divergent wavefront emanating from a hard X-ray nanoprobe. The results of both schemes are in very good agreement. The probe retrieval can be used as a tool for optics alignment, in particular at X-ray nanoprobe beamlines. Combining probe retrieval and object reconstruction is also shown to improve the image quality of holographic near-field imaging
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Square Dancing with the Stars to Enhance Dynamic Hirschman Linkages?
No full textIn this Presidential Address, the author takes the reader on a reconnaissance of his life and time as a regional scientist. He points out scenery he found scintillating along the way, hoping that some may pick up the banner and chew on a few of the ideas for a while. He suggests a revisit to Albert O. Hirschman’s notion of key sectors and more empirical analysis related to Marcus Berliant’s and Masahisa Fujita’s notion of knowledge creation and transfer.Presidential Address, San Antonio, Texas, March 29, 2014 (53rd Meetings of the Southern Regional Science Association
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