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Relaxation Time Constant (relaxation + time_constant)
Selected AbstractsImpact of Left Ventricular Function on the Pulmonary Vein Doppler Spectrum:ECHOCARDIOGRAPHY, Issue 1 2003Nonsimultaneous Assessment with Load-Insensitive Indices Pulmonary vein Doppler spectrum is highly load-dependent and thus has been used to estimate left ventricular (LV) filling pressure. However, the impact of LV function on pulmonary vein Doppler spectrum remains obscure because only load-sensitive indices were studied previously. In the present study, measurements of the pulmonary vein Doppler spectrum were correlated with load-insensitive LV systolic (end-systolic elastance [Ees]) and diastolic (relaxation time constant [tau] and beta coefficient of the end-diastolic pressure volume relationship) function indices obtained from an invasive catheterization study nonsimultaneously. The peak velocity, velocity time integral, and duration of systolic forward spectrum were significantly correlated with Ees (r = 0.35, r = 0.36, andr = 0.41, respectively;P < 0.05). The pulmonary vein diastolic velocity time integral (PVDVTI) and duration of the diastolic forward spectrum were significantly correlated with Ees (r = 0.51andr = 0.57, respectively;P < 0.01). PVDVTI was correlated with tau and the end-diastolic pressure-volume relationship (EDPVR) (r = 0.42andr = 0.40respectively,P < 0.05). On the other hand, the systolic fraction of the forward spectrum was significantly correlated with ejection fraction (for peak velocity,r = 0.63, P < 0.01; for velocity time integral,r = 0.37, P < 0.05) but not with Ees, and the diastolic fraction of the forward spectrum was significantly correlated with minimum pressure derivative over time (for peak velocity,r = 0.48, P < 0.05; for velocity time integral,r = 0.44, P < 0.05, respectively) but not with tau or EDPVR. In summary, the systolic and diastolic components of the pulmonary vein Doppler spectrum are affected variably by LV systolic and diastolic function, independent of the loading condition. The systolic and diastolic fraction of pulmonary vein Doppler spectrum appears to depend more on the loading condition than the LV systolic or diastolic function. (ECHOCARDIOGRAPHY, Volume 20, January 2003) [source] Effect of gluten content on recrystallisation kinetics and water mobility in wheat starch gelsJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 4 2004Xin Wang Abstract The effect of gluten on starch retrogradation at 5 °C was studied using 1H NMR relaxometry. Gels were made from gluten and starch at 27.8 and 38.5% total solids and with gluten comprising either 10, 15 or 20% of the solids. Changes in the transverse relaxation time constant (T2) were related to water mobility. Mono-exponential analysis of relaxation curves showed that, in general, gluten retarded starch retrogradation. T2 values in gluten gels also decreased during storage, but to a much lesser extent. Distributed exponential analysis showed that two distinct regions of T2 were observed in all samples. During aging, the peak values of both regions shifted to lower values for all gels. Starch gel samples showed the most significant shift, and gluten gels showed the least. The three levels of gluten addition in starch/gluten gels produced similar shifts. For all samples the signal intensity of the less mobile region decreased more dramatically than that of the more mobile region during storage. It was suggested that gluten retards water loss in the granule remnants. Copyright © 2004 Society of Chemical Industry [source] Proton T2 relaxation study of water, N-acetylaspartate, and creatine in human brain using Hahn and Carr-Purcell spin echoes at 4T and 7TMAGNETIC RESONANCE IN MEDICINE, Issue 4 2002Shalom Michaeli Abstract Carr-Purcell and Hahn spin-echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T values of proton resonances (water, N-acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T decay can be substantially reduced using a Carr-Purcell-type SE sequence. Magn Reson Med 47:629,633, 2002. © 2002 Wiley-Liss, Inc. [source] Heart rate reduction by inhibition of If or by ,-blockade has different effects on postsystolic wall thickeningBRITISH JOURNAL OF PHARMACOLOGY, Issue 3 2007L Lucats Background and purpose: Postsystolic wall thickening (PSWT) is part of thickening that occurs after end-systole and represents wasted effort as it does not contribute to ejection. The effects of antianginal drugs on PSWT remain to be established. We compared the effects on PSWT of two agents that reduce heart rate, the ,-blocker atenolol and the selective inhibitor of If current, ivabradine. Experimental approach: Six dogs were prepared to measure wall thickening by sonomicrometry in the conscious state, at rest and during exercise, after administration of saline, atenolol (1mg.kg -1) or ivabradine (1mg.kg -1). Key results: Atenolol and ivabradine similarly reduced heart rate vs saline at rest (about 10-20%) and during exercise (about 30%). Atenolol but not ivabradine decreased dP/dtmax. Concomitantly, PSWT increased with atenolol vs saline at rest (0.35±0.07 vs 0.21±0.03mm, respectively) and during exercise (0.30±0.04 vs 0.15±0.04mm, respectively). In contrast, ivabradine did not alter PSWT. Importantly, atenolol but not ivabradine increased the ratio of postsystolic to systolic wall thickening by 80±23%. This enhanced thickening during diastole with atenolol was accompanied by impeded isovolumic relaxation of the left ventricle, as illustrated by the significant correlation between the isovolumic relaxation time constant , and the postsystolic to systolic wall thickening ratio. None of these effects of atenolol were abolished when heart rate was controlled with atrial pacing. Conclusion and implications: For a similar heart rate reduction at rest and during exercise, ivabradine, but not atenolol, did not alter PSWT and preserved the part of thickening contributing to ejection. British Journal of Pharmacology (2007) 150, 335,341. doi:10.1038/sj.bjp.0706996 [source] Singlet states open the way to longer time-scales in the measurement of diffusion by NMR spectroscopyCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2008Simone Cavadini Abstract Nuclear magnetic resonance is a powerful nonintrusive technique for measuring diffusion coefficients through the use of pulsed field gradients. The main limitation to the application range of this method is imposed by the relaxation time constants of the magnetization. The recently introduced singlet-state spectroscopy affords obtaining relaxation time constants for pairs of coupled spins which can be longer by more than an order of magnitude than the spin-lattice relaxation time constants. We review in this paper the advantages that are offered by these long relaxation time constants for diffusion measurements. Using experiments that combine singlet-state and diffusion spectroscopy, slower diffusion constants can be determined. The coupling of the two methods constitutes an alternative to the use of special probes equipped with strong gradients for the study of large molecules that diffuse slowly in solution. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 68,78, 2008. [source] Effect of non-exponential and multi-exponential decay behavior on the performance of the direct exponential curve resolution algorithm (DECRA) in NMR investigationsJOURNAL OF CHEMOMETRICS, Issue 11 2003Todd M. Alam Abstract The effect of non-exponential and multi-exponential decay or relaxation behavior on the performance of the direct exponential curve resolution algorithm (DECRA) is investigated through a series of numerical simulations. Three different combinations of decay or relaxation behavior were investigated through DECRA analysis of simulated pulse gradient spin echo (PGSE) NMR diffusion spectra that contained the combination of two individual components. The diffusion decay behavior of one component was described by a single-exponential decay, while the second component was described by either (1) a multi-exponential decay, (2) a decay behavior described by the empirical Kohlrausch,Williams,Watts (KWW) relation or (3) a multi-exponential decay behavior correlated with variations in the NMR spectral line shape. The magnitudes and types of errors produced during the DECRA analysis of spectral data with deviations from a pure single-exponential decay behavior are presented. It is demonstrated that the deviation from single-exponential decay impacts the resulting calculated line shapes, the calculated relative concentrations and the quantitative estimation of the decay or relaxation time constants of both components present in the NMR spectra. Copyright © 2004 John Wiley & Sons, Ltd. [source] Thermal characterization of light-emitting diodes in the frequency domainPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Pranci, kus Vitta Abstract We report on a method for measurement of thermal relaxation time constants within light-emitting diodes (LEDs) using harmonic modulation of the driving current. The method is based on the phase shift of the forward voltage waveform in respect to that of the modulated current. The phase shift is due to the sensitivity of the forward voltage to junction temperature, which responds to the modulation of the heat generation depending on the thermal relaxation rate. The frequency dependence of the phase shift was shown to exhibit characteristic dips at angular frequencies equal to inverse thermal time constants. Such an approach for thermal characterization was demonstrated for common GaP, AlGaAs, AlInGaP, and InGaN LEDs. In particular, low-power p-n and double-heterostructure LEDs as well as high-power truncated-inverted-pyramid and flip-chip LEDs were investigated. The measured thermal time constants (, 0.1,100 ms) were tentatively assigned to heat flows within the multilayer structure of the LEDs and collated with the numerical estimates based on thermal resistance and heat capacitance of the LED components. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||