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61.
Molality as a unit of measure for expressing 1H MRS brain metabolite concentrations in vivo 总被引:1,自引:0,他引:1
Knight-Scott J Haley AP Rossmiller SR Farace E Mai VM Christopher JM Manning CA Simnad VI Siragy HM 《Magnetic resonance imaging》2003,21(7):787-797
Absolute concentrations of cerebral metabolite in in vivo 1H magnetic resonance spectroscopy studies (1H-MRS) are widely reported in molar units as moles per liter of tissue, or in molal units as moles per kilogram of tissue. Such measurements require external referencing or assumptions as to local water content. To reduce the scan time, avoid assumptions that may be invalid under specific pathologies, and provide a universally accessible referencing procedure, we suggest that metabolite concentrations from 1H-MRS measurements in vivo be reported in molal units as moles per kilogram of tissue water. Using internal water referencing, a two-compartment water model, a simulated brain spectrum for peak identification, and a spectroscopic bi-exponential spin-spin relaxation segmentation technique, we measured the absolute concentrations for the four common 1H brain metabolites: choline (Cho), myo-inositol (mIno), phosphocreatine + creatine (Cr), and N-acetyl-aspartate (NAA), in the hippocampal region (n = 26) and along the Sylvian fissure (n = 61) of 35 healthy adults. A stimulated echo localization method (20 ms echo time, 10 ms mixing time, 4 s repetition time) yielded metabolite concentrations, uncorrected for metabolite relaxation or contributions from macromolecule resonances, that were expectantly higher than with molar literature values. Along the Sylvian fissure the average concentrations (coefficient of variation (CV)) in mmoles/kg of tissue water were 17.6 (12%) for NAA, 14.2 (9%) for Cr, 3.6 (13%) for Cho, and 13.2 (15%) for mIno. Respective values for the hippocampal region were 15.7 (20%), 14.7 (16%), 4.6 (19%), and 17.7 (26%). The concentrations of the two regions were significantly different (p = 0.001) for NAA, mIno, and Cho, a trend in agreement with previous studies. All gray matter Sylvian fissure CV values, except for NAA, were also in agreement with previous 1H-MRS gray matter studies. The reduced precision of the NAA concentration was attributed to overlapping signal contributions from glutamate and glutamine (Glx), suggesting that a detailed Glx model is critical for accurate quantitation of the NAA 2.02 ppm resonance. The reduced precision of the measurements in the hippocampal region was attributed to poor spectral resolution. 相似文献
62.
We show here that field emission (FE) can be used to directly observe the vibration resonances nu(R) of carbon nanotubes (CNTs) and that the tension created by the applied field allows the tuning of these resonances by up to a factor of 10. The resonances are observable by the changes they create in the FE pattern or the emitted FE current. The tuning is shown to be linear in voltage and to follow from the basic physics of stretched strings. The method allows one to study the mechanical properties of individual multiwall carbon nanotubes within an ensemble and follow their evolution as the CNTs are modified. The tuning and detection should be useful for nanometric resonant devices. 相似文献
63.
This paper gives lower estimates for the frequency modules of almost periodic solutions to equations of the form
, where A generates a strongly continuous semigroup in a Banach space
, F(t,x) is 2π-periodic in t and continuous in (t,x), and f is almost periodic. We show that the frequency module ℳ(u) of any almost periodic mild solution u of (*) and the frequency module ℳ(f) of f satisfy the estimate e
2π
iℳ(f)⊂e
2π
iℳ(u). If F is independent of t, then the estimate can be improved: ℳ(f)⊂ℳ(u). Applications to the nonexistence of quasi-periodic solutions are also given. 相似文献
64.
This paper gives sufficient conditions for the upper and lower semicontinuities of the solution mapping of a parametric mixed generalized Ky Fan inequality problem. We use a new scalarizing approach quite different from traditional linear scalarization approaches which, in the framework of the stability analysis of solution mappings of equilibrium problems, were useful only for weak vector equilibrium problems and only under some convexity and strict monotonicity assumptions. The main tools of our approach are provided by two generalized versions of the nonlinear scalarization function of Gerstewitz. Our stability results are new and are obtained by a unified technique. An example is given to show that our results can be applied, while some corresponding earlier results cannot. 相似文献
65.
66.
67.
Local shrinkage and stress induced by thermo-oxidation in composite materials at high temperatures 总被引:1,自引:0,他引:1
Marco Gigliotti Loic Olivier Dinh Quy Vu Jean-Claude Grandidier Marie Christine Lafarie-Frenot 《Journal of the mechanics and physics of solids》2011,59(3):696-712
The paper is concerned with the modelling, simulation and experimental characterisation of local shrinkage strains and stresses induced by thermo-oxidation phenomena in the IM7/977-2 carbon/epoxy composite material at elevated temperatures. The oxygen concentration and mechanical fields were established through a coupled model constructed from a unified multiphysical approach and the thermodynamics of irreversible processes. The model was implemented in the ABAQUS® finite element commercial code. Simulations of thermo-oxidation-induced matrix shrinkage were run at a local scale, i.e., the scale of the elementary constituents of the composite, the fibre and the matrix. The experimental assessment was done at the same scale, and the local matrix shrinkage profiles were measured by confocal interferometric microscopy.A good agreement was found between the simulated and measured profiles, validating the unified model. The thermo-oxidation induced stress field was analysed to understand the influence of the environment on the onset of damage in composite materials at elevated temperature. 相似文献
68.
Julien Iehl Thi Le Anh Nguyen Stéphane Frein Uwe Hahn Joaquín Barberá Jean-François Nierengarten 《Liquid crystals》2017,44(12-13):1852-1860
ABSTRACTLiquid-crystalline [60]fullerodendrimers were constructed via click chemistry based on the reaction between hexa-adducts of [60]fullerene (C60) bearing 12 azide groups and alkyne-terminated cyanobiphenyl dendrons of first- and second-generation. The structure of all the new compounds was confirmed by IR, UV, 1H and 13C NMR spectroscopies and mass spectrometry. The mesomorphic properties were studied by polarised optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. The hexa-adduct of C60 functionalised with the first-generation dendrons gave rise to the formation of a smectic A phase and a rectangular columnar phase (c2mm symmetry) while the hexa-adduct of C60 decorated with the second-generation dendrons displayed only a rectangular columnar phase (c2mm symmetry). Our results show that the hexa-adduct of C60 is a unique synthetic platform for the design of fullerodendrimers and dendronised materials. 相似文献
69.
Matilda Vojnović Ana J. Šetrajčić-Tomić Siniša M. Vučenović Jovan P. Šetrajčić 《Optical and Quantum Electronics》2018,50(4):198
Recent research in nano-optical engineering and in nanomedicine as well, seeks for methods of construction of various types of nano-markers, nano-carriers, and ways to deliver drugs to the exactly determined regions of body. In this process it is important to find methods of recognition of certain types of molecules. It is obvious that optical recognition would be the easiest and the most effective way to do it. Our research presents a model of a molecular ultrathin crystalline film and generated exciton system inside it and corresponding methodology of analysis of their optical characteristics. Properties of these spatially very restricted structures are very sensitive to their surrounding surfaces. Using the two-time Green’s functions adapted for crystalline structures with symmetry breaking, and graphical-numerical software, we have calculated the energy spectra and possible exciton states. We have shown that the appearance and the presence of localized states on the surfaces and in the boundary layers of the film depend on the thickness of the film and the film surroundings, presented through the perturbation of parameters on surfaces. Optical properties in these structures demonstrate discrete and very selective resonant absorption spectra, depending on the perturbation on their surfaces. 相似文献
70.
Tuan Anh Pham 《International journal of quantum chemistry》2019,119(1):e25795
Understanding physicochemical properties of liquid electrolytes is essential for predicting and optimizing device performance for a wide variety of emerging energy technologies, including photoelectrochemical water splitting, supercapacitors, and batteries. In this work, we review recent progress and open challenges in predicting structural, dynamical, and electronic properties of the liquids using first-principles approaches. We briefly summarize the basic concepts of first-principles molecular dynamics (FPMD), and we discuss how FPMD methods have enriched our understanding of a number of liquids, including aqueous solutions, organic electrolytes and ionic liquids. We also discuss technical challenges in extending FPMD simulations to the study of liquid electrolytes in more complex environments, including the interface between electrolytes and electrodes, which is a key component in many energy storage and conversion systems. 相似文献