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71.
Sandra M. Meyers Cornelia Laule Irene M. Vavasour Shannon H. Kolind Burkhard Mädler Roger Tam Anthony L. Traboulsee Jimmy Lee David K.B. Li Alex L. MacKay 《Magnetic resonance imaging》2009
This study compared region of interest (ROI) and voxel-based analysis (VBA) methods to determine the optimal method of myelin water fraction (MWF) analysis. Twenty healthy controls were scanned twice using a multi-echo T2 relaxation sequence and ROIs were drawn in white and grey matter. MWF was defined as the fractional signal from 15 to 40 ms in the T2 distribution. For ROI analysis, the mean intensity of voxels within an ROI was fit using non-negative least squares. For VBA, MWF was obtained for each voxel and the mean and median values within an ROI were calculated. There was a slightly higher correlation between Scan 1 and 2 for the VBA method (R2=0.98) relative to the ROI method (R2=0.95), and the VBA mean square difference between scans was 300% lower, indicating VBA was the most consistent between scans. For the VBA method, mean MWF was found to be more reproducible than median MWF. As the VBA method is more reproducible and gives more options for visualization and analysis of MWF, it is recommended over the ROI method of MWF analysis. 相似文献
72.
In the present work, the estimation of the parameters for asymmetric binary mixtures of carbon dioxide + n-alkanols has been developed. The binary interaction parameter k12 of the second virial coefficient and non-random two liquid model parameters τ12 and τ21 were obtained using Peng–Robinson equation of state coupled with the Wong–Sandler mixing rules. In all cases, Levenberg–Marquardt minimization algorithm was used for the parameters optimization employing an objective function based on the calculation of the distribution coefficients for each component. Vapor–liquid equilibrium for binary asymmetric mixtures (CO2 + n-alkanol, from methanol to 1-decanol) was calculated using the obtained values of the mentioned parameters. The agreement between calculated and experimental values was satisfactory. 相似文献
73.
The molecular and vibrational structure of the title compound (C4-HSL) was studied by experimental and theoretical methods. The infrared (IR) absorption spectrum was measured in the solid state and in CCl4 suspension. The observed absorption bands were compared with transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT) calculations. Two stable molecular conformations were predicted, corresponding to an endo- and an exo-conformer with similar energies. Intermolecular amide–amide hydrogen bonding in the crystal state was approximated by a simple cluster model, leading to excellent agreement with the observed solid state IR spectrum. Due to the low solubility of C4-HSL in common solvents for IR spectroscopy, such as CS2 and CCl4, a liquid solution spectrum of pure, monomeric C4-HSL was not obtained. However, absorbance peaks observed in oversaturated CCl4 solution could be assigned to distinct contributions from suspended micro-crystalline aggregates and dissolved monomeric species. The key vibrational bands of the monomeric form of C4-HSL are reported here for the first time: 3425 cm?1 [ν(N–H)], 1784 cm?1 [ν(CO), lactone], 1688 cm?1 [amide I], and 1494 cm?1 [amide II] (CCl4). 相似文献
74.
Brendan J. Kennedy Jimmy Ting Zhaoming Zhang Michihiro Miyake 《Journal of solid state chemistry》2009,182(4):954-90
Fifteen perovskite-type compounds Sr0.9−xCaxCe0.1MnO3, x=0-0.9 in steps as fine as 0.05, have been synthesised by solid state methods, and the room temperature structures characterised using X-ray synchrotron powder diffraction. At low Ca contents (x?0.45) the structures are tetragonal in space group I4/mcm and at high Ca contents (x?0.55) the compounds are orthorhombic in space group Pbnm. At room temperature these two phases co-exist in the compound with x=0.5. XANES measurements show the Ce to be present as Ce4+ in all the oxides. High temperature structures are reported for selected members. 相似文献
75.
76.
77.
Zhang L Ai Z Jia F Liu L Hu X Yu JC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(15):4185-4190
Various nanostructured films of copper and silver tellurides were hydrothermally grown on the corresponding metal substrates through reactions between metal foils and tellurium powder in different media. Interesting morphologies including nanowires, nanorods, nanobelts, nanosheets, and hierarchical dendrites were obtained. The nanostructured films were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). A growth mechanism was proposed based on the characterization results. This study provides a low-temperature, solution-phase approach to grow low-dimensional, nanostructured metal tellurides with controllable morphologies. 相似文献
78.
Welch CJ DaSilva JO Nti-Gyabaah J Antia F Goklen K Boyd R 《Journal of chromatography. A》2006,1101(1-2):204-213
Preparation and evaluation of a number of stationary phases for improved chromatographic purification of pneumocandin B0, a key intermediate in the synthesis of the antifungal agent, Cancidas, has led to the identification of several materials with potential for improved performance. 相似文献
79.
Kuethe JT Marcoux JF Wong A Wu J Hillier MC Dormer PG Davies IW Hughes DL 《The Journal of organic chemistry》2006,71(19):7378-7390
A highly efficient synthesis of the potent and selective NK-1 receptor antagonist 1 is described. The key transformation involved the etherification reaction between cyclopentanol 12 and chiral imidate 30 which was catalyzed by HBF4 to initially give ether 14 as a 17:1 mixture of diastereomers and in 75% combined yield. The diastereoselectivity was upgraded to 109:1 by crystallization of the triethylamine solvate 44 which was isolated in 54% yield from 12. Mechanistic studies confirmed that the etherification reaction proceeds through an unprecedented S(N)2 reaction pathway under typical S(N)1 reaction conditions. 相似文献
80.
Jimmy?LamboleyEmail author Arian?Novruzi Michel?Pierre 《Archive for Rational Mechanics and Analysis》2012,205(1):311-343
We focus here on the analysis of the regularity or singularity of solutions Ω 0 to shape optimization problems among convex planar sets, namely:where \({\mathcal{S}_{\rm ad}}\) is a set of 2-dimensional admissible shapes and \({J:\mathcal{S}_{\rm ad}\rightarrow\mathbb{R}}\) is a shape functional. Our main goal is to obtain qualitative properties of these optimal shapes by using first and second order optimality conditions, including the infinite dimensional Lagrange multiplier due to the convexity constraint. We prove two types of results:
相似文献
$J(\Omega_{0})={\rm min} \{J(\Omega), \Omega \quad {\rm convex},\Omega \in \mathcal{S}_{\rm ad}\},$
- i)under a suitable convexity property of the functional J, we prove that Ω 0 is a W 2,p -set, \({p\in[1, \infty]}\). This result applies, for instance, with p = ∞ when the shape functional can be written as J(Ω) = R(Ω) + P(Ω), where R(Ω) = F(|Ω|, E f (Ω), λ1(Ω)) involves the area |Ω|, the Dirichlet energy E f (Ω) or the first eigenvalue of the Laplace–Dirichlet operator λ1(Ω), and P(Ω) is the perimeter of Ω;
- ii)under a suitable concavity assumption on the functional J, we prove that Ω 0 is a polygon. This result applies, for instance, when the functional is now written as J(Ω) = R(Ω) ? P(Ω), with the same notations as above.