93,795 research outputs found
Critical Parameter Values and Reconstruction Properties of Discrete Tomography: Application to Experimental Fluid Dynamics
We analyze representative ill-posed scenarios of tomographic PIV with a focus
on conditions for unique volume reconstruction. Based on sparse random seedings
of a region of interest with small particles, the corresponding systems of
linear projection equations are probabilistically analyzed in order to
determine (i) the ability of unique reconstruction in terms of the imaging
geometry and the critical sparsity parameter, and (ii) sharpness of the
transition to non-unique reconstruction with ghost particles when choosing the
sparsity parameter improperly. The sparsity parameter directly relates to the
seeding density used for PIV in experimental fluids dynamics that is chosen
empirically to date. Our results provide a basic mathematical characterization
of the PIV volume reconstruction problem that is an essential prerequisite for
any algorithm used to actually compute the reconstruction. Moreover, we connect
the sparse volume function reconstruction problem from few tomographic
projections to major developments in compressed sensing.Comment: 22 pages, submitted to Fundamenta Informaticae. arXiv admin note:
text overlap with arXiv:1208.589
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DNA-Packing Portal and Capsid-Associated Tegument Complexes in the Tumor Herpesvirus KSHV.
Assembly of Kaposi's sarcoma-associated herpesvirus (KSHV) begins at a bacteriophage-like portal complex that nucleates formation of an icosahedral capsid with capsid-associated tegument complexes (CATCs) and facilitates translocation of an ∼150-kb dsDNA genome, followed by acquisition of a pleomorphic tegument and envelope. Because of deviation from icosahedral symmetry, KSHV portal and tegument structures have largely been obscured in previous studies. Using symmetry-relaxed cryo-EM, we determined the in situ structure of the KSHV portal and its interactions with surrounding capsid proteins, CATCs, and the terminal end of KSHV's dsDNA genome. Our atomic models of the portal and capsid/CATC, together with visualization of CATCs' variable occupancy and alternate orientation of CATC-interacting vertex triplexes, suggest a mechanism whereby the portal orchestrates procapsid formation and asymmetric long-range determination of CATC attachment during DNA packaging prior to pleomorphic tegumentation/envelopment. Structure-based mutageneses confirm that a triplex deep binding groove for CATCs is a hotspot that holds promise for antiviral development
A hybrid 3d reconstruction/registration algorithm for correction of head motion in emission tomography
Even with head restraint, small head movements can occur during data acquisition for emission tomography, sufficiently large to result in detectable artifacts in the final reconstruction. Direct measurement of motion can be cumbersome and difficult to implement, whereas previous attempts to correct for motion based on measured projections have been limited to simple translation orthogonal to the projection. A fully 3D algorithm is proposed that estimates the patient orientation at any time based on the projection of motion-corrupted data, with incorporation of the measured motion within subsequent OSEM sub-iterations. Preliminary studies have been performed using a digital version of the Hoffman brain phantom. Movement was simulated by constructing a mixed set of projections in two discrete positions of the phantom. The algorithm determined the phantom orientation that best aligned each constructed projection with its corresponding, measured projection. In the case of simulated movement of 24 of 64 projections, all mis-positioned projections were correctly identified. The algorithm resulted in a reduction of mean square difference (MSD) between motion corrected and motion-free reconstructions compared to the MSD between uncorrected and motion-free reconstructions by a factor of 2.7
Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces
Polar discontinuities occurring at interfaces between two different materials
constitute both a challenge and an opportunity in the study and application of
a variety of devices. In order to cure the large electric field occurring in
such structures, a reconfiguration of the charge landscape sets in at the
interface via chemical modifications, adsorbates or charge transfer. In the
latter case, one may expect a local electronic doping of one material: one
sparkling example is the two-dimensional electron liquid (2DEL) appearing in
SrTiO once covered by a polar LaAlO layer. Here we show that tuning the
formal polarisation of a (La,Al)(Sr,Ti)O (LASTO:) overlayer
through chemical composition modifies the quantum confinement of the 2DEL in
SrTiO and its electronic band structure. The analysis of the behaviour in
magnetic field of superconducting field-effect devices reveals, in agreement
with calculations and self-consistent Poisson-Schr\"odinger
modelling, that quantum confinement and energy splitting between electronic
bands of different symmetries strongly depend on interface charge densities.
These results not only strongly support the polar discontinuity mechanisms with
a full charge transfer to explain the origin of the 2DEL at the celebrated
LaAlO/SrTiO interface, but also demonstrate an effective tool for
tailoring the electronic structure at oxide interfaces.Comment: 18 pages, 4 figures, 1 ancillary file (Supporting Information
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