Home About us Contact
|
Home About us Contact | ||
|
|
||
Model Function (model + function)
Selected AbstractsSpatial and temporal analysis of fMRI data on word and sentence readingEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 7 2007Sven Haller Abstract Written language comprehension at the word and the sentence level was analysed by the combination of spatial and temporal analysis of functional magnetic resonance imaging (fMRI). Spatial analysis was performed via general linear modelling (GLM). Concerning the temporal analysis, local differences in neurovascular coupling may confound a direct comparison of blood oxygenation level-dependent (BOLD) response estimates between regions. To avoid this problem, we parametrically varied linguistic task demands and compared only task-induced within-region BOLD response differences across areas. We reasoned that, in a hierarchical processing system, increasing task demands at lower processing levels induce delayed onset of higher-level processes in corresponding areas. The flow of activation is thus reflected in the size of task-induced delay increases. We estimated BOLD response delay and duration for each voxel and each participant by fitting a model function to the event-related average BOLD response. The GLM showed increasing activations with increasing linguistic demands dominantly in the left inferior frontal gyrus (IFG) and the left superior temporal gyrus (STG). The combination of spatial and temporal analysis allowed a functional differentiation of IFG subregions involved in written language comprehension. Ventral IFG region (BA 47) and STG subserve earlier processing stages than two dorsal IFG regions (BA 44 and 45). This is in accordance with the assumed early lexical semantic and late syntactic processing of these regions and illustrates the complementary information provided by spatial and temporal fMRI data analysis of the same data set. [source] A novel model for the interpretation of small-angle scattering experiments of self-affine structuresJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2010Gerald J. Schneider A novel theory is presented which allows, for the first time, the analytical description of small-angle scattering experiments on anisotropic shaped clusters of nanoparticles. Experimentally, silica-filled rubber which is deformed is used as an example. The silica can be modelled by solid spheres which form clusters. The experiments demonstrate that the clusters become anisotropic as a result of the deformation whereas the spheres are not affected. A comparison of the newly derived model function and the experiments provides, for the first time, microscopic evidence of the inhomogeneous deformation of clusters in the rubbery matrix. [source] Deconvolution of femtosecond time-resolved spectroscopy data in multivariate curve resolution.JOURNAL OF CHEMOMETRICS, Issue 7-8 2010Application to the characterization of ultrafast photo-induced intramolecular proton transfer Abstract In femtosecond absorption spectroscopy, deconvolution of the measured kinetic traces is still an important issue as photochemical processes that may possess shorter characteristic times than the time resolution of the experiment are usually considered. In this work, we propose to perform deconvolution of the time-dependent concentration profiles extracted from multivariate curve resolution (MCR) applied to spectrokinetic data. The profiles are fitted with a model function including a description of the instrumental response function (IRF) of the experiment. The method combines the potential benefits of soft-modeling data analysis with the ones of hard-modeling for parameter estimation. The potential of the method is demonstrated first analyzing five synthetic data sets for which IRF of different widths are simulated. It is then successfully applied to resolve femtosecond UV-visible transient absorption spectroscopy data investigating the photodynamics of salicylidene aniline, a photochromic molecule of wide interest. Considering a time resolution of 150,fs for the IRF, a characteristic time of 45,fs is recovered for the first step of the photo-induced process which consists of an ultrafast intramolecular proton transfer. Our results also confirm the existence of an intermediate species with a characteristic time of 240,fs. Copyright © 2010 John Wiley & Sons, Ltd. [source] Assessment of tumor microcirculation with dynamic contrast-enhanced MRI in patients with esophageal cancer: Initial experienceJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2008Katja Oberholzer MD Abstract Purpose To investigate the feasibility and impact of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) on tumor characterization and response to radiochemotherapy (RCT) in patients with esophageal cancer. Materials and Methods A total of 48 patients underwent DCE-MRI to assess tumor microcirculation based on a two-compartment model function. Effects of RCT on kinetic parameters were studied in 12 patients with squamous cell carcinoma. Results Tumor microcirculation differs with respect to histological subtype: squamous cell carcinomas showed lower values of amplitude A (leakage space, P = 0.015) and higher contrast agent exchange rates (k21, P = 0.225) compared with adenocarcinomas. RCT led to a significant decrease of the contrast agent exchange rate (P = 0.005), while amplitude A increased moderately after therapy (P = 0.136). Conclusion DCE-MRI is feasible in patients with esophageal cancer, reveals therapeutic effects, and may thus be useful in therapy management and monitoring. J. Magn. Reson. Imaging 2008;27:1296,1301. © 2008 Wiley-Liss, Inc. [source] Cellulose Acetate- graft -Poly(hydroxyalkanoate)s: Synthesis and Dependence of the Thermal Properties on Copolymer CompositionMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 14 2004Yoshikuni Teramoto Abstract Summary: Several different series of cellulose acetate- graft -poly(hydroxyalkanoate)s (CA- g -PHAs) were synthesized over a wide range of compositions by the graft copolymerization of lactic acid, L -lactide, (R,S)- , -butyrolactone, , -valerolactone and , -caprolactone onto the residual hydroxyl positions of CA, by virtue of a suitable catalyst, solvent and procedure for each individual case. To achieve a diversity of molecular architectures of the respective graft copolymer series, the degree of acetyl substitution (acetyl DS) of the CA starting material was also varied, resulting in different levels of the intramolecular density of grafts. The CA- g -PHAs thus obtained were subjected to differential scanning calorimetric measurements and the relationship between their molecular structure and thermal transition behavior was estimated, in comparison with some semi-empirical equations available for polymer blends or comb-like polymers. In particular, the composition dependence of the Tgs of the graft copolymers was represented well in terms of a formula proposed by Reimschuessel for comb-like polymers, when CAs of acetyl DS ,2 were employed as a trunk polymer. The deviation of the glass transition data from the model function was discussed in connection with the manner of graft modification. [source] Towards metabolic mapping of the human retinaMICROSCOPY RESEARCH AND TECHNIQUE, Issue 5 2007D. Schweitzer Abstract Functional alterations are first signs of a starting pathological process. A device that measures parameter for the characterization of the metabolism at the human eye-ground would be a helpful tool for early diagnostics in stages when alterations are yet reversible. Measurements of blood flow and of oxygen saturation are necessary but not sufficient. The new technique of auto-fluorescence lifetime measurement (FLIM) opens in combination with selected excitation and emission ranges the possibility for metabolic mapping. FLIM not only adds an additional discrimination parameter to distinguish different fluorophores but also resolves different quenching states of the same fluorophore. Because of its high sensitivity and high temporal resolution, its capability to resolve multi-exponential decay functions, and its easy combination with laser scanner ophthalmoscopy, multi-dimensional time-correlated single photon counting was used for fundus imaging. An optimized set up for in vivo lifetime measurements at the human eye-ground will be explained. In this, the fundus fluorescence is excited at 446 or 468 nm and the time-resolved autofluorescence is detected in two spectral ranges between 510 and 560 nm as well as between 560 and 700 nm simultaneously. Exciting the fundus at 446 nm, several fluorescence maxima of lifetime t1 were detected between 100 and 220 ps in lifetime histograms of 40° fundus images. In contrast, excitation at 468 nm results in a single maximum of lifetime t1 = 190 ± 16 ps. Several fundus layers contribute to the fluorescence intensity in the short-wave emission range 510,560 nm. In contrast, the fluorescence intensity in the long-wave emission range between 560 and 700 nm is dominated by the fluorescence of lipofuscin in the retinal pigment epithelium. Comparing the lateral distribution of parameters of a tri-exponential model function in lifetime images of the fundus with the layered anatomical fundus structure, the shortest component (t1 = 190 ps) originates from the retinal pigment epithelium and the second lifetime (t2 = 1,000 ps) from the neural retina. The lifetime t3 , 5.5 ns might be influenced by the long decay of the fluorescence in the crystalline lens. In vitro analysis of the spectral properties of expected fluorophores under the condition of the living eye lightens the interpretation of in vivo measurements. Taking into account the transmission of the ocular media, the excitation of NADH is unlikely at the fundus. Microsc. Res. Tech., 2007. © 2007 Wiley-Liss, Inc. [source] Optical response of single-wall carbon nanotube sheets in the far-infrared spectral range from 1 THz to 40 THzPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007Tobias Kampfrath Abstract The optical properties of single-wall carbon nanotube sheets in the far-infrared have been investigated with THz time-domain spectroscopy. Over a wide frequency range from 1 THz to 40 THz, the complex dielectric function of the nanotube sample has been derived. Our data can be excellently reproduced by a Drude,Lorentz model function. The extracted fit parameters such as Lorentz resonance frequency and plasma frequency are consistent with values obtained by scanning tunneling techniques. We discuss the origin of both the Lorentz and Drude contribution in terms of direct and indirect optical transitions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Measurement of time-resolved autofluorescenceACTA OPHTHALMOLOGICA, Issue 2008D SCHWEITZER Purpose Functional alterations are first signs of reversible pathologic processes. Whereas microcirculation studies metabolism globally, autofluorescence of endogenous fluorophores has the potential for description of cellular basic processes. Therefore, a discrimination of fluorophores is required in the tissue. Methods Besides excitation and emission spectra, the fluorescence lifetime after short-time excitation is a promising substance-specific mark. Using the opto-mechanical system of a HRA II (Heidelberg Engineering), a fluorescence lifetime mapper was developed. Picosecond pulse-lasers (448nm, 468nm, 100ps FWHM, 80MHz) can be used for excitation and the emission will be detected in 2 spectral ranges (490-560nm, 560-700nm). The dynamic fluorescence will be detected in time-correlated single photon counting (SPC 150, Becker/Hickl, Berlin). An on line image registration is realised by simultaneously detected infrared images during measuring time. Approximating the fluorescence decay by 3-exponential model function, images (lifetime and amplitudes), histograms, and cluster diagrams can be calculated for interpretation. Results Examples are given for healthy subjects, AMD patients (non-exudative, exudative, geographic atrophy), diabetic retinopathy, and oedema. Measurements of excitation and emission spectra as well as lifetimes are performed of expected substances and of anatomical ocular structures for comparison. Conclusion Fluorescence lifetime measurement at the eye is a new method for evaluation of functional metabolic state. [source] | |||||||||||||