666 research outputs found
Self-organized nanoscale pattern formation on vicinal Si(111) surfaces via a two-stage faceting transition
Journal ArticleWe demonstrate a self-organized pattern formation on vicinal Si(111) surfaces that are miscut toward the [211] direction. All the patterns, consisting of a periodic array of alternating (7 X 7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle, while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition stages: the first stage forms a stress-domain structure defining the universal periodicity; the second stage forms the low-energy facets controlling the facet width
Resonant Charge Transfer In Symmetric Alkali-ion Alkali-atom Collisions
Resonant charge transfer in alkali-ion alkali-atom collisions is investigated by using the molecular-orbital expansion method incorporating the use of electron translation factors. Molecular wave functions and eigen energies are obtained by the pseudopotential method. Molecular properties, Re, De, and e, obtained in the present calculation are in good accord with other recent theoretical results, as well as spectroscopic measurements. Three-state close-coupling calculations reproduce the positions of the maxima and minima in the oscillatory structure seen experimentally in the resonant-charge-transfer cross sections for the Li2 + and Cs2 + systems. The magnitude of the total cross sections and their velocity dependence are in agreement with experimental measurements. © 1986 The American Physical Society
Pattern formation via a two-step faceting transition on vicinal Si(111) surfaces
Journal ArticleWe demonstrate a self-organized pattern formation on vicinal Si(l 11) surfaces that are miscut toward the [211] direction. All the patterns, consisting of a periodic array of alternating (7x7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle; while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition steps: the first step forms a stress-domain structure defining the universal periodicity; the second step forms the low-energy facets controlling the facet width
GPU-based Iterative Cone Beam CT Reconstruction Using Tight Frame Regularization
X-ray imaging dose from serial cone-beam CT (CBCT) scans raises a clinical
concern in most image guided radiation therapy procedures. It is the goal of
this paper to develop a fast GPU-based algorithm to reconstruct high quality
CBCT images from undersampled and noisy projection data so as to lower the
imaging dose. For this purpose, we have developed an iterative tight frame (TF)
based CBCT reconstruction algorithm. A condition that a real CBCT image has a
sparse representation under a TF basis is imposed in the iteration process as
regularization to the solution. To speed up the computation, a multi-grid
method is employed. Our GPU implementation has achieved high computational
efficiency and a CBCT image of resolution 512\times512\times70 can be
reconstructed in ~5 min. We have tested our algorithm on a digital NCAT phantom
and a physical Catphan phantom. It is found that our TF-based algorithm is able
to reconstrct CBCT in the context of undersampling and low mAs levels. We have
also quantitatively analyzed the reconstructed CBCT image quality in terms of
modulation-transfer-function and contrast-to-noise ratio under various scanning
conditions. The results confirm the high CBCT image quality obtained from our
TF algorithm. Moreover, our algorithm has also been validated in a real
clinical context using a head-and-neck patient case. Comparisons of the
developed TF algorithm and the current state-of-the-art TV algorithm have also
been made in various cases studied in terms of reconstructed image quality and
computation efficiency.Comment: 24 pages, 8 figures, accepted by Phys. Med. Bio
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Deciphering the genetic basis for polyketide variation among mycobacteria producing mycolactones.
BACKGROUND: Mycolactones are immunosuppressive and cytotoxic polyketides, comprising five naturally occurring structural variants (named A/B, C, D, E and F), produced by different species of very closely related mycobacteria including the human pathogen, Mycobacterium ulcerans. In M. ulcerans strain Agy99, mycolactone A/B is produced by three highly homologous type I polyketide megasynthases (PKS), whose genes (mlsA1: 51 kb, mlsA2: 7.2 kb and mlsB: 42 kb) are found on a 174 kb plasmid, known as pMUM001. RESULTS: We report here comparative genomic analysis of pMUM001, the complete DNA sequence of a 190 kb megaplasmid (pMUM002) from Mycobacterium liflandii 128FXT and partial sequence of two additional pMUM replicons, combined with liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis. These data reveal how PKS module and domain differences affecting MlsB correlate with the production of mycolactones E and F. For mycolactone E these differences from MlsB in M. ulcerans Agy99 include replacement of the AT domain of the loading module (acetate to propionate) and the absence of an entire extension module. For mycolactone F there is also a reduction of one extension module but also a swap of ketoreductase domains that explains the characteristic stereochemistry of the two terminal side-chain hydroxyls, an arrangement unique to mycolactone F CONCLUSION: The mycolactone PKS locus on pMUM002 revealed the same large, three-gene structure and extraordinary pattern of near-identical PKS domain sequence repetition as observed in pMUM001 with greater than 98.5% nucleotide identity among domains of the same function. Intra- and inter-strain comparisons suggest that the extreme sequence homogeneity seen among the mls PKS genes is caused by frequent recombination-mediated domain replacement. This work has shed light on the evolution of mycolactone biosynthesis among an unusual group of mycobacteria and highlights the potential of the mls locus to become a toolbox for combinatorial PKS biochemistry.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are
GPU-based Low Dose CT Reconstruction via Edge-preserving Total Variation Regularization
High radiation dose in CT scans increases a lifetime risk of cancer and has
become a major clinical concern. Recently, iterative reconstruction algorithms
with Total Variation (TV) regularization have been developed to reconstruct CT
images from highly undersampled data acquired at low mAs levels in order to
reduce the imaging dose. Nonetheless, TV regularization may lead to
over-smoothed images and lost edge information. To solve this problem, in this
work we develop an iterative CT reconstruction algorithm with edge-preserving
TV regularization to reconstruct CT images from highly undersampled data
obtained at low mAs levels. The CT image is reconstructed by minimizing an
energy consisting of an edge-preserving TV norm and a data fidelity term posed
by the x-ray projections. The edge-preserving TV term is proposed to
preferentially perform smoothing only on non-edge part of the image in order to
avoid over-smoothing, which is realized by introducing a penalty weight to the
original total variation norm. Our iterative algorithm is implemented on GPU to
improve its speed. We test our reconstruction algorithm on a digital NCAT
phantom, a physical chest phantom, and a Catphan phantom. Reconstruction
results from a conventional FBP algorithm and a TV regularization method
without edge preserving penalty are also presented for comparison purpose. The
experimental results illustrate that both TV-based algorithm and our
edge-preserving TV algorithm outperform the conventional FBP algorithm in
suppressing the streaking artifacts and image noise under the low dose context.
Our edge-preserving algorithm is superior to the TV-based algorithm in that it
can preserve more information of fine structures and therefore maintain
acceptable spatial resolution.Comment: 21 pages, 6 figures, 2 table
GPU-based fast Monte Carlo simulation for radiotherapy dose calculation
Monte Carlo (MC) simulation is commonly considered to be the most accurate
dose calculation method in radiotherapy. However, its efficiency still requires
improvement for many routine clinical applications. In this paper, we present
our recent progress towards the development a GPU-based MC dose calculation
package, gDPM v2.0. It utilizes the parallel computation ability of a GPU to
achieve high efficiency, while maintaining the same particle transport physics
as in the original DPM code and hence the same level of simulation accuracy. In
GPU computing, divergence of execution paths between threads can considerably
reduce the efficiency. Since photons and electrons undergo different physics
and hence attain different execution paths, we use a simulation scheme where
photon transport and electron transport are separated to partially relieve the
thread divergence issue. High performance random number generator and hardware
linear interpolation are also utilized. We have also developed various
components to handle fluence map and linac geometry, so that gDPM can be used
to compute dose distributions for realistic IMRT or VMAT treatment plans. Our
gDPM package is tested for its accuracy and efficiency in both phantoms and
realistic patient cases. In all cases, the average relative uncertainties are
less than 1%. A statistical t-test is performed and the dose difference between
the CPU and the GPU results is found not statistically significant in over 96%
of the high dose region and over 97% of the entire region. Speed up factors of
69.1 ~ 87.2 have been observed using an NVIDIA Tesla C2050 GPU card against a
2.27GHz Intel Xeon CPU processor. For realistic IMRT and VMAT plans, MC dose
calculation can be completed with less than 1% standard deviation in 36.1~39.6
sec using gDPM.Comment: 18 pages, 5 figures, and 3 table
Exploratory genome-wide analyses of cortical inhibition, facilitation, and plasticity in late-life depression
Late-life depression (LLD) is a heterogenous mood disorder influenced by genetic factors. Cortical physiological processes such as cortical inhibition, facilitation, and plasticity may be markers of illness that are more strongly associated with genetic factors than the clinical phenotype. Thus, exploring the relationship between genetic factors and these physiological processes may help to characterize the biological mechanisms underlying LLD and improve diagnosis and treatment selection. Transcranial magnetic stimulation (TMS) combined with electromyography was used to measure short interval intracortical inhibition (SICI), cortical silent period (CSP), intracortical facilitation (ICF), and paired associative stimulation (PAS) in 79 participants with LLD. We used exploratory genome-wide association and gene-based analyses to assess for genetic correlations of these TMS measures. MARK4 (which encodes microtubule affinity-regulating kinase 4) and PPP1R37 (which encodes protein phosphatase 1 regulatory subunit 37) showed genome-wide significant association with SICI. EGFLAM (which encodes EGF-like fibronectin type III and laminin G domain) showed genome-wide significant association with CSP. No genes met genome-wide significant association with ICF or PAS. We observed genetic influences on cortical inhibition in older adults with LLD. Replication with larger sample sizes, exploration of clinical phenotype subgroups, and functional analysis of relevant genotypes is warranted to better characterize genetic influences on cortical physiology in LLD. This work is needed to determine whether cortical inhibition may serve as a biomarker to improve diagnostic precision and guide treatment selection in LLD
Orbital effects of a monochromatic plane gravitational wave with ultra-low frequency incident on a gravitationally bound two-body system
We analytically compute the long-term orbital variations of a test particle
orbiting a central body acted upon by an incident monochromatic plane
gravitational wave. We assume that the characteristic size of the perturbed
two-body system is much smaller than the wavelength of the wave. Moreover, we
also suppose that the wave's frequency is much smaller than the particle's
orbital one. We make neither a priori assumptions about the direction of the
wavevector nor on the orbital geometry of the planet. We find that, while the
semi-major axis is left unaffected, the eccentricity, the inclination, the
longitude of the ascending node, the longitude of pericenter and the mean
anomaly undergo non-vanishing long-term changes. They are not secular trends
because of the slow modulation introduced by the tidal matrix coefficients and
by the orbital elements themselves. They could be useful to indepenedently
constrain the ultra-low frequency waves which may have been indirectly detected
in the BICEP2 experiment. Our calculation holds, in general, for any
gravitationally bound two-body system whose characteristic frequency is much
larger than the frequency of the external wave. It is also valid for a generic
perturbation of tidal type with constant coefficients over timescales of the
order of the orbital period of the perturbed particle.Comment: LaTex2e, 24 pages, no figures, no tables. Changes suggested by the
referees include
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