Relaxation (NMR)

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

Quantitation of the susceptibility difference between trabecular bone and bone marrow: experimental studies.

Abstract: In this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient-echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T2, while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T2* as demonstrated in gradient-echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro. We have measured the MR relaxation times T1, T2, and T2* of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro, at 1.5 T, the relaxation times T1 and T2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/T2*, of saline in the presence of trabecular bone increases at a rate of 0.2 s-1/mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density-related T2* variations were observed for fat protons within the trabeculae where the chemical-shift-induced modulations of signal intensity in an oil-water emulsion have been separated from the susceptibility-induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T2* are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations.

Dynamics of hydrogen‐bonded liquids confined to mesopores: A dielectric and neutron spectroscopy study

Abstract: In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen‐bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low‐ and high‐frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≊+47 K; and (ii) relatively ‘‘free’’ molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg...

Collective effects in a random-site electric dipole system: KTaO3: Li

Abstract: Li, substituting for K in KTa${\mathrm{O}}_{3}$, creates a local electric dipole, due to its off-center position with respect to the cubic site. We have studied such crystals with different amounts of Li (and in a few cases also doped with Nb, substituting for Ta) by nuclear magnetic resonance, dielectric relaxation, pyroelectricity, ultrasound, and birefringence methods. Birefringence and dielectric susceptibility results show that collective effects between the Li dipoles occur below a fairly well-defined concentration-dependent temperature of the order of 50 K, but nuclear magnetic and dielectric relaxation indicate the absence of criticality at the onset of these effects. These collective effects are related to those arising in spin-glasses. We discuss the data in the light of theoretical models and computer simulations of systems of randomly interacting moments, which predict an apparent condensation into a system of metastable clusters without long-range order.

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

Abstract: A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH2) and chlorination (14–22 Cl/1000 CH2). Both linear and branched polymers were studied. All of the relaxations between the melt and −196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (Tmax at constant frequency or log fmax at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log fmax and Tmax on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing emax with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (∼10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.