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Fourier modeling of the BOLD response to a breath-hold task: Optimization and reproducibility
No full textCerebrovascular reactivity (CVR) reflects the capacity of blood vessels to adjust their caliber in order to maintain a steady supply of brain perfusion, and it may provide a sensitive disease biomarker. Measurement of the blood oxygen level dependent (BOLD) response to a hypercapnia-inducing breath-hold (BH) task has been frequently used to map CVR noninvasively using functional magnetic resonance imaging (fMRI). However, the best modeling approach for the accurate quantification of CVR maps remains an open issue. Here, we compare and optimize Fourier models of the BOLD response to a BH task with a preparatory inspiration, and assess the test-retest reproducibility of the associated CVR measurements, in a group of 10 healthy volunteers studied over two fMRI sessions. Linear combinations of sine-cosine pairs at the BH task frequency and its successive harmonics were added sequentially in a nested models approach, and were compared in terms of the adjusted coefficient of determination and corresponding variance explained (VE) of the BOLD signal, as well as the number of voxels exhibiting significant BOLD responses, the estimated CVR values, and their test-retest reproducibility. The brain average VE increased significantly with the Fourier model order, up to the 3rd order. However, the number of responsive voxels increased significantly only up to the 2nd order, and started to decrease from the 3rd order onwards. Moreover, no significant relative underestimation of CVR values was observed beyond the 2nd order. Hence, the 2nd order model was concluded to be the optimal choice for the studied paradigm. This model also yielded the best test-retest reproducibility results, with intra-subject coefficients of variation of 12 and 16% and an intra-class correlation coefficient of 0.74. In conclusion, our results indicate that a Fourier series set consisting of a sine-cosine pair at the BH task frequency and its two harmonics is a suitable model for BOLD fMRI CVR measurements based on a BH task with preparatory inspiration, yielding robust estimates of this important physiological parameter. (C) 2016 Elsevier Inc. All rights reserved.LIFME
On the mechanisms of protein interactions: predicting their affinity from unbound tertiary structures
No full textLPD
A Contribution to the Theoretical Prediction of Life-Time in Glass Structures
No full textIn order to assess safety levels in glass structures a scattered and inhomogeneous variety of mostly complicated resistance criteria is presently available, very often requiring specially developed softwares. For this reason engineers who wants to assess with reliability the actual safety level of glass structures of relevant economical importance are still obliged to undertake expensive experimental tests. In the attempt to overcome this problem, it was formulated a new semi-probabilistic failure prediction method called "Design Crack Method” (DCM), which is a compromise between the necessity to accurately model the complex mechanical behaviour of glass at breakage and the need to reduce the analytic complexity of the calculations. On the basis of Linear Elastic Fracture Mechanics, such aim has been analitically reached in the present work by defining a new quantity called Design Crack, characterized by a mathematical expression that depends on the probability of failure and on the surface damaging level. The proposed method, which is in accordance with the basic principles of the Structural Eurocodes, allows to predict glass lifetime taking into due account the influence of parameters like the surface extension and the loading time-history of the structural element. The results obtained by some applications on the D.C.M. have been numerically compared in this paper with those of the existing most frequently used theoretical methods.IICEditor’s Note: The first author of this paper is one of the four winners of the 2011 Hangai Prize, awarded for outstanding papers that are submitted for presentation and publication at the annual IASS Symposium by younger members of the Association (under 30 years old)
The Electronic-Structure Genome of Inorganic Crystals
No full textElectronic structure theory plays a central role in understanding properties of crystalline materials. Among all the ab initio methods, density functional theory (DFT) is the most popular choice due to its balance between accuracy and computational cost, and has been applied successfully to numerous realistic materials. However, the computations of many advanced properties require extremely dense samplings of the Brillouin zone, resulting in a serious challenge for direct DFT calculations. The theory of maximally localized Wannier functions (MLWFs) provides an elegant framework to tackle such issues. MLWFs are localized orbitals for crystals, resembling the atomic orbitals or bonding/anti-bonding orbitals from chemical intuition. Moreover, they are low-rank approximations of the original electronic structure, and enable accurate and efficient Wannier interpolation of quantum-mechanical operators. In this thesis, we first develop two automated algorithms to construct MLWFs, addressing the cases of metals and insulators, respectively. Then, we implement fully automated workflows that only require the crystal structure as input, and robustly generate MLWFs. On top of these, we build three MLWF databases for over 20,000 3D inorganic crystals, over 5000 3D insulators, and over 2000 exfoliable 2D crystals, respectively. These databases are the electronic-structure genome: they are efficient compressed encodings of the electronic structure of each material. Moreover, they are also accurate interpolators, since they hide the details of the underlying electronic-structure calculations, and subsequent property computations can be fully performed within the Wannier representation. To demonstrate the power of this notion and the benefits of our databases, we choose three applications for discovering novel materials: (a) high-performance thermoelectrics, (b) materials with large nonlinear Hall effect due to Berry curvature dipole, and (c) heterostructures hosting two-dimensional electron gases at the interface owing to polar discontinuity.THEO
Irradiation creep and precipitation in a ferritic ODS steel under helium implantation
No full textFerritic oxide dispersion strengthened (ODS) steel, PM2000, has been homogeneously implanted with helium under uniaxial tensile stresses from 20 to 250 MPa to maximum doses of about 0.75 dpa (3000 ppm He) with displacement damage rates of 5.5 × 10−6 dpa/s at temperatures of 573, 673 and 773 K. Straining of a miniaturized dog-bone specimen under helium implantation was monitored by linear variable displacement transformer (LVDT) and meanwhile by their resistance also measured by four-pole technique. Creep compliance was almost constant at 5.7 × 10−6 dpa−1 MPa−1 for temperatures below 673 K and increased to 18 × 10−6 dpa−1 MPa−1 at 773 K. The resistivity of PM2000 samples decreased with dose and showed a tendency to saturation. Subsequent transmission electron microscopy observations indicated the formation of ordered Fe3−xCrxAl precipitates during implantation. Correlations between the microstructure and resistivity are discussed.LNM_PS
