Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Nuclear relaxation of human brain gray and white matter: Analysis of field dependence and implications for MRI

Abstract: The dependence of 1/T1 on the magnetic field strength (the relaxation dispersion) has been measured at 37 degrees C on autopsy samples of human brain gray and white matter at field strengths corresponding to proton Larmor frequencies between 10 kHz and 50 MHz (0.0002-1.2 T). Additional measurements of 1/T1 and 1/T2 have been performed at 200 MHz (4.7 T) and 20 MHz (0.47 T), respectively. Absolute signal amplitudes are found to be proportional to the sample water content, not to the "proton density," and it is concluded that the myelin lipids do not contribute to the signal. Transverse magnetization decay data can be fitted with a triple exponential function, giving characteristic results for each tissue type, and are insensitive to variations of the pulse spacing interval. The longitudinal relaxation dispersion curves show characteristic shapes for each tissue type. The most striking difference is a large dispersion for white matter at very high fields. As a consequence, the relative difference in 1/T1 between gray and white matter shows a marked maximum around 10 MHz. Possible implications for MRI are discussed. A weighted least-squares fit of the dispersions has been performed using a four-parameter function of the form 1/T1 = 1/T1,w + D + A/(1 + (f/fc)beta'). The quality of the fit is superior to that of other functions proposed previously. The results of these fits are used to predict image contrast between gray and white matter at different field strengths.

Low-energy electron diffraction from Cu(111): Subthreshold effect and energy-dependent inner potential; surface relaxation and metric distances between spectra

Abstract: In the present low-energy electron-diffraction (LEED) study of the copper (111) surface the inner potential and the surface relaxation are determined independently from the subthreshold effect and from Bragg-type diffraction. A "subthreshold effect" is a narrow LEED intensity structure occurring at a setting where new beams have an emergence threshold in the metal: a "subthreshold." We reconsider the absorption of electrons with regard to its spatial distribution in the crystal and design a phenomenological model comprising two parameters which are adjusted to the absorptive scattering cross section of the ion cores and that of the interstitial region. The two-parameter model for the interlayer attenuation indicates the existence of a transparent scattering channel "pseudoparallel" to the surface for beams emerging in the crystal. The channeling extends over all layers penetrated by the LEED electrons, giving the subthreshold effect a peak width of about 2 eV. Each observable subthreshold effect fixes a point on the energy-dependent inner potential; for the copper (111) surface we are able to measure the inner potential at 19.5-, 73.6-, and 109-eV incidence energy. A local excited-state potential of the Hedin-Lundqvist type produces for the copper (111) surface an inner-potential curve that agrees well with the measured points. From LEED spectra for the 00, 10, and 01 beams from the copper (111) surface in the energy range 16-190 eV we infer a top-layer spacing contracted (0.7 \ifmmode\pm\else\textpm\fi{} 0.5)% relative to the layer spacing in the bulk. The theoretical and experimental spectra are compared by means of metric distances, which are stable with respect to noise in the data and give a linear response to small variations of the structural parameters.

Dielectric Relaxation in Nematic Liquid Crystals

Abstract: In homogeneously ordered liquid crystals of the nematic type, two principal dielectric constants can be measured: ϵ1 parallel and ϵ2 perpendicular to the symmetry axis of the liquid crystal. The two dielectric constants may show a very different relaxation behavior. With alkoxy derivatives of azobenzene, only the normal Debye dispersion region at microwave frequencies has been observed. With alkoxy derivatives of azoxybenzene, ϵ2 shows again only the normal Debye dispersion region at microwave frequencies, but ϵ1 has an additional dispersion at very unusually low frequencies in the radio region. This dispersion is due to the permanent electric dipole momment of the azoxy group, more exactly to its component parallel to the long molecular axis. The reorientation of this component in direction 1 is hindered by a potential barrier due to intermolecular forces. The corresponding relaxation time is increased by a factor g. Starting from a modification of the Debye theory of dielectric relaxation, this...

Glass transition and layering effects in confined water: A computer simulation study

Abstract: Single particle dynamics of water confined in a nanopore is studied through computermolecular dynamics. The pore is modeled to represent the average properties of a pore of Vycor glass. Dynamics is analyzed at different hydration levels and upon supercooling. At all hydration levels and all temperatures investigated a layering effect is observed due to the strong hydrophilicity of the substrate. The time density correlators show, already at ambient temperature, strong deviations from the Debye and the stretched exponential behavior. Both on decreasing hydration level and upon supercooling we find features that can be related to the cage effect typical of a supercooled liquid undergoing a kinetic glass transition. Nonetheless the behavior predicted by mode coupling theory can be observed only by carrying out a proper shell analysis of the density correlators. Water molecules within the first two layers from the substrate are in a glassy state already at ambient temperature (bound water). The remaining subset of molecules (free water) undergoes a kinetic glass transition; the relaxation of the density correlators agree with the main predictions of the theory. From our data we can predict the temperature of structural arrest of free water.

Nuclear magnetic resonance patterns of intracellular water as a function of HeLa cell cycle.

Abstract: Nuclear magnetic resonance relaxation time (T1) of the intracellular water protons and water content were measured in synchronized HeLa cells. The T1 was maximum (1020 milliseconds) in mitotic and minimum (534 milliseconds) in S phase cells. The cyclic pattern of T1 values correlated well with the chromosome condensation cycle. By treating cells with spermine, it was possible to alter T1 without a significant change in the water content. The results of this study suggest that an additional variable, namely, the conformational state of macromolecules, should be incluced in any expression explaining the shortened relaxation times of water protons in biological systems.