Showing papers on "Relaxation (NMR) published in 1984"

A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1-100 MHz: dependence on tissue type, NMR frequency, temperature, species, excision, and age.

Abstract: The longitudinal (T1) and transverse (T2) hydrogen (1H) nuclear magnetic resonance (NMR) relaxation times of normal human and animal tissue in the frequency range 1-100 MHz are compiled and reviewed as a function of tissue type, NMR frequency, temperature, species, in vivo versus in vitro status, time after excision, and age. The dominant observed factors affecting T1 are tissue type and NMR frequency (V). All tissue frequency dispersions can be fitted to the simple expression T1 = AVB in the range 1-100 MHz, with A and B tissue-dependent constants. This equation provides as good or better fit to the data as previous more complex formulas. T2 is found to be multicomponent, essentially independent of NMR frequency, and dependent mainly on tissue type. Mean and raw values of T1 and T2 for each tissue are tabulated and/or plotted versus frequency and the fitting parameters A, B and the standard deviations determined to establish the normal range of relaxation times applicable to NMR imaging. The mechanisms for tissue NMR relaxation are reviewed with reference to the fast exchange two state (FETS) model of water in biological systems, and an overview of the dynamic state of water and macromolecular hydrogen compatible with the frequency, temperature, and multicomponent data is postulated. This suggests that 1H tissue T1 is determined predominantly by intermolecular (possibly rotational) interactions between macromolecules and a single bound hydration layer, and the T2 is governed mainly by exchange diffusion of water between the bound layer and a free water phase. Deficiencies in measurement techniques are identified as major sources of data irreproducibility.

Measurement of radiation dose distributions by nuclear magnetic resonance (NMR) imaging

Abstract: A method is described for determining the spatial distribution of radiation dose in a tissue-equivalent phantom using nuclear magnetic resonance imaging. The conversion of ferrous ions to ferric by ionising radiation alters the magnetic moment and electron spin relaxation times of the metal ion. The spin relaxation times (T1 and T2) of the hydrogen nuclei in an aqueous solution of a ferrous salt are consequently reduced substantially. These changes in T1 and T2 can be measured using standard NMR techniques. The same conversion is used in conventional Fricke dosimetry, which can be used to calibrate the technique.

Theoretical study of relaxation mechanism in magnetic field effects on chemical reactions.

Abstract: External magnetic field and magnetic isotope effects on the dynamic behavior of radical pairs in solutions have been studied theoretically, where the relaxation of their electron spins was taken into account. The decay observed with some radical pairs in micellar solutions under high magnetic fields is successfully interpreted in terms of the relaxation mechanism, and the magnetic field dependence of the relaxation rates is calculated for a model system.

Pulsed Field Gradient Nuclear Magnetic Resonance as a Probe of Liquid State Molecular Organization

Abstract: The imposition of magnetic field gradients on a nuclear magnetic resonance experiment imparts a spatial tag to the molecules via the Larmor frequencies of the nuclei (commonly protons) within. A sensitive measure of changes in the frequency distribution is afforded by detecting the loss of phase coherence in a spin-echo experiment performed in the presence of gradient pulses. Here the molecular displacements may be measured over the duration given by the pulse separation. Pulsed field gradient nuclear magnetic resonance (PFGNMR) employs timesca1es of tens of milliseconds and has a displacement sensitivity of the order 100 nm.

Excited states at metal surfaces and their non-radiative relaxation

Abstract: ?We present a general overview of the spectroscopy and relaxation dynamics of vibrational and electronic excitations of molecules and atoms adsorbed on metal surfaces. We discuss briefly some experimental tools, in particular, electron energy loss spectroscopy, and present experimental illustrations of rotational, vibrational, and electronic excitations of adsorbates. For excitations at surfaces, new effective nonradiative decay paths are opened involving, for example, excitation of electron-hole pairs and phonons. For excited molecules not in direct contact with the metal substrate (or where this overlap can be neglected), the nonradiative quenching can only result from the interaction between the oscillating electric field of the excited molecule and the metal. We first discuss the "classical theory" of this coupling and point out its limitations. We then present an improved theory and compare its predictions with experimental data. For excited molecules in direct contact with the substrate, electron transfer between the molecule and the metal can often occur, and this leads in general to strongly nonadiabatic processes. We illustrate this type of decay path for both vibrational and electronic excitations. Finally, multiphonon and phase relaxation of vibrations at surfaces are briefly discussed.

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.

Vibrational energy relaxation in liquids

Abstract: The de-excitation of the vibrational population of small molecules in the liquid state is considered. Experimental techniques applicable to the measurement of relaxation times in dense phases are first described. Theoretical approaches are subsequently developed with special emphasis on the relationship between ab-initio quantum methods and binary interaction models. Finally, a selection of experimental results is analysed in the light of these theories. Special attention is given to the dependence of the relaxation time on experimental parameters such as density, temperature or the concentration of a mixture. The behaviour of the relaxation time across the liquid/solid phase transition is also treated.

The structure, thermodynamic properties and infrared spectra of liquid water and ice

Abstract: Monte Carlo simulations are used, together with models of the intramolecular and intermolecular potential surfaces, to model liquid water and several phases of ice. Intramolecular relaxation makes important contributions to both thermodynamic and structural properties. A quantum local mode analysis of the Monte Carlo configurations is used to predict the density of states and infrared absorption intensities for the intramolecular bending and stretching vibrations. The large shifts from the gas phase OH stretch frequencies observed experimentally in the liquid and solid phases are due to anharmonic terms in the intramolecular surface rather than to harmonic intermolecular coupling. A significant contribution to observed changes in IR intensity on condensation arises from the large molecular polarisability.

Magnetic field dependence of solvent proton relaxation induced by Gd3+ and Mn2+ complexes

Abstract: The effect of solute paramagnetic ions on the longitudinal magnetic relaxation rate 1/T1 of solvent water protons depends on magnetic field strength and on the chemical environment of the ions. The variation of 1/T1 with field has been measured for solutions of Gd3+ and Mn2+ ions in three grossly different environments near physiological pH: the hydrated aquoion; chelated by EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid); and bound to the protein concanavalin A. It is demonstrated that over the field range at which NMR imaging is currently being done, the chemical environment can alter 1/T1 of solvent protons by more than an order of magnitude. The relevance of these results to the potential utility of these ions as agents for enhancement of contrast in NMR images is discussed.

Femtosecond Orientational Relaxation of Photoexcited Carriers in GaAs

Abstract: Mesure de l'anisotropie de la saturation de l'absorption optique par des impulsions intenses de lumiere polarisee lineairement, dans GaAs a 77 K, pour des energies de photons legerement superieures a la bande interdite. Pour des densites de paires electron-trou d'environ 6×10 17 cm −3 , le temps de relaxation d'orientation de la quantite de mouvement est 190 fs

Theory of spin‐lattice relaxation in lipid bilayers and biological membranes. Dipolar relaxation

Abstract: In this work we have calculated spin‐lattice (T1) relaxation time expressions for the homo‐ and heteronuclear dipolar relaxation of lipid bilayers, in addition to the heteronuclear Overhauser enhancement (NOE), using correlation functions derived previously. The results can be applied to the analysis of the 1H and 13C T1 times of lipid bilayers and the 13C–1H NOE. Three different models for the segmental fluctuations of the membraneous lipid molecules have been considered: (i) a simple diffusion‐type model for the local segmental motions; (ii) a noncollective model in which relatively slow bilayer fluctuations are described by a single correlation time; and (iii) a collective model for the slow motions characterized by a continuous distribution of correlation times. For the diffusion model, the dependence on the bilayer orientation, order parameters 〈 P2〉 and 〈 P4〉, and the diffusion tensor anisotropy have been included in a general manner. Depending on the degree of segmental ordering and the anisotropy of the diffusion tensor, the maximum 13C–1H NOE can be either greater or less than the value of 2.988 obtained in the absence of an ordering potential. The various relaxation models were fit to 13C T1 data recently obtained for vesicles of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC) at seven different magnetic field strengths, i.e., resonance frequencies, in the liquid crystalline state. A simple diffusion‐type model (i) based on analogies to paraffinic liquids provides a very poor fit to the above 13C T1 data as a function of temperature and frequency, even for extreme values of the ordering and diffusion tensor anisotropy, and thus can be rejected at the present time. The 13C results can be fit satisfactorily over the range 15.0–126 MHz by models which include contributions from relatively slow bilayer fluctuations. A noncollective model (ii) with three or four adjustable parameters or a collective model (iii) with two parameters both describe the data to within experimental error. At present, the 13C T1 results suggest that the relaxation of the bilayer hydrocarbon region, in the liquid crystalline state, can best be accounted for by a collective model with a relaxation expression of the type T−11≂Aτ(2)f+BS2CH ω−1/2I, as concluded from a similar analysis of the 2H T1 data for DPPC multilamellar dispersions. In the above expression, τ(2)f is the correlation time for the local motions, SCH(=SCD) is the observed bond segmental order parameter, ωI is the resonance frequency, and A and B are constants which depend on the nucleus considered. Thus, the observed relaxation rate includes contributions from fast or local‐type motions, in addition to cooperative fluctuations of a more long‐range character. For the collective model, extrapolation of the 13C T−11 values obtained for the DPPC vesicles to infinite frequency yields estimates of τ(2)f which agree with those calculated from the frequency‐independent T−11 rates of n‐hexadecane at the same absolute temperature, suggesting that the segmental microviscosities of the two systems are similar, in agreement with 2H NMR studies.

Molecular dynamic simulation of quadrupole relaxation of atomic ions in aqueous solution

Abstract: Molecular dynamics (MD) simulations were performed in order to investigate the molecular origin of the nuclear quadrupole relaxation mechanism for Li+, Na+, and Cl− ions in dilute aqueous solution. Different boundary conditions were investigated in the simulations, but neither the boundary conditions nor the system size have any significant effect on the different time correlation functions (tcf:s) calculated. It is found that the field gradient tcf, determining the NMR relaxation rate for these ions, is almost completely due to the water molecules in the first shell. The calculated field gradient tcf:s show a complex time‐dependence not describable with a mono‐exponential decay. The very rapid decay shown by the field gradient tcf for all three ions can partly be attributed to a correlated water motion in the first hydration shell. The relaxation rates obtained from the simulations are in excellent agreement with experimental data for Na+ and Cl−, while the Li+ relaxation rate is less well produced.

Determinants of Proton Relaxation Rates in Tissue

Abstract: It is well established that the longitudinal magnetic relaxation rate of solvent water protons, 1/T1, increases markedly in homogeneous protein solutions as the magnetic field is reduced well below the traditional NMR range. For a 5% solution of protein of 10(5) Da, for example, 1/T1 increases from about 50% above the pure water rate of 20 MHz to five times the water rate at 0.01 MHz. For tissue, including blood, the behavior is similar. Data for blood show that extracellular water has ready access to the hemoglobin inside red blood cells, which causes the enhanced relaxation. The extent to which cell water can sample the spatial structure of soft tissue, and how this structure influences relaxation rates, is as yet unknown. Nonetheless, the relaxation data for tissue can be accommodated within the conceptual framework developed earlier for analyzing homogeneous solutions of diamagnetic proteins. The variation of 1/T1 with field differs among tissues, and its magnitude at a given field can vary by more than a factor of three, far more than does the water content of the tissues. Solute complexes of paramagnetic ions with macromolecules, which increase the relaxation rates of solvent protons, can be introduced intravenously in tissue. They are known to accumulate in specific organs, and therefore have potential utility as contrast-enhancing agents in NMR imaging. Mn2+ and Gd3+, for example, produce characteristic dependences of 1/T1 on magnetic field that vary with the chemical state of the agent. The possibility exists, therefore, for monitoring the in vivo biochemistry of these agents.

Studies of tissue NMR relaxation enhancement by manganese. Dose and time dependences

Abstract: Manganese is a powerful paramagnetic material and potential NMR contrast agent. It drastically affects the NMR properties of solutions and tissues and is less toxic than most other transition elements. It also possesses some unusual and advantageous features; it alters T1 and T2 to different degrees, and it can bind to macromolecules to become even more effective at reducing proton relaxation times. The dose dependence of tissue relaxation rate increases has been measured in mice, and proton relaxation enhancement ratios that describe binding effects have been evaluated. These ratios imply that a tenfold reduction in manganese dose is achievable when the ion binds to intracellular components, and it is demonstrated that such binding effects can be a major factor in the efficacy of contrast enhancement. The effect of manganese on the ratio T1/T2 is dose dependent so that lower doses may be more useful for some imaging techniques. The postmortem time course of relaxation times in organs containing manganese varies between organs and with manganese content, and demonstrates that the relationship between tissue relaxation enhancement and metal content is not a simple correlation with concentration since large variations in T1 and T2 can occur even when metal and water content are fixed.

Multi-exponential water proton spin-lattice relaxation in biological tissues and its implications for quantitative NMR imaging

Abstract: This in vitro study was undertaken to examine whether water proton spin-lattice relaxation in biological tissues is adequately described by a single time constant T1, to define under what circumstances a multi-exponential approach is indicated, and to study the implications of multi- exponentiality for quantitative NMR imaging. Water proton relaxation curves were measured with the 180-tau-90 method at 60 MHz. Uni- and bi-exponential curves were fitted to the empirical curves using chi 2 as a criterion for the goodness of fit. An F-test was applied to test the validity of each exponential term as it was added to the fitting function. Taking into account experimental accuracy, the uni-exponential model appeared to be an adequate description of the relaxation data for necrotic tissue. Eyelens and fat showed distinct bi- exponentiality , while liver, spleen, salivary gland, tumour, and muscle presented intermediate cases. The bi-exponential analysis generally yields a minor component with a fast relaxation time, T11 less than 20 ms, and a slow relaxation major component with T12 greater than 300 ms. A simplified bi-exponential model is proposed for implementation in quantitative NMR imaging. The results seem to be consistent with current views about water proton spin-lattice relaxation in biological tissues.

Principles of Nuclear Magnetic Resonance

Abstract: The basic principles of nuclear magnetic resonance (NMR) are discussed. The concepts presented include a qualitative quantum-mechanical approach to NMR spectroscopy and a classical-mechanical approach to time-dependent NMR phenomena (relaxation effects). The spectroscopic concepts discussed include absorption of radiation by matter, spin and energy quantization , chemical shift, and spin-spin splitting. The time-dependent phenomena include the concepts of T1 and T2, the spin-lattice and spin-spin relaxation time, and Fourier-transform NMR spectroscopy.

Studies of factors affecting the design of NMR contrast agents: manganese in blood as a model system.

Abstract: Some factors affecting the performance of paramagnetic ions as contrast agents for proton NMR imaging have been studied. It is demonstrated that the relaxation rate of an aqueous solution of the ion is not reliably predicted by its magnetic moment, but that significant relaxation enhancement may result when the ion is complexed with large molecules, which increases the dominant correlation time. This enhancement in turn can be altered by factors such as pH and competition for binding. Chelation of the paramagnetic ion, which may be implemented to lower its toxicity, can considerably reduce its efficacy by not only limiting its access to water but also by preventing the enhancement from associations and macromolecules. For manganese the ratio T1/T2 is a useful parameter which is sensitive to the degree of metal binding. These features of paramagnetic relaxation enhancement in tissue are demonstrated in a series of experiments on systems consisting of blood components and manganese.

Enthalpy relaxation behaviour of (Fe, Co, Ni)75Si10B15 amorphous alloys upon low temperature annealing

Abstract: The effect of composition on the anneal-induced enthalpy relaxation for (Fe, Co, Ni)75Si10B15 amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures well belowT g (T g

A picosecond holographic grating approach to molecular dynamics in oriented liquid crystal films

Abstract: The picosecond transient grating technique offers a new approach to the characterization of rotational dynamics and mechanical properties of thin liquid crystal films. Sample excitation by two crossed 100 ps pulses having parallel polarization results in two kinds of phase gratings: one due to the optical Kerr effect, and the other to a standing longitudinal acoustic wave. Rotational reorientation times are calculated from the relaxation of the Kerr grating, while the ultrasonic velocity and absorption are obtained by monitoring the acoustic response. If the excitation pulses are perpendicularly polarized, no longitudinal acoustic waves are generated, so that the signal is due exclusively to the Kerr effect. Whereas previous workers using ∼20 ns excitation pulses observed a single exponential Kerr relaxation in the isotropic phase, we are able to resolve the decay into a fast nonexponential component followed by a slow exponential component. While the slow component disappears below the isotropic→nematic ...

Potential problems with selective pulses in NMR imaging systems

Abstract: Most nuclear magnetic resonance(NMR)imaging systems require pulses whose frequency spectrum is shaped so as to selectively excite a given plane in the presence of a magnetic field gradient. We demonstrate by both computer simulation and experiment that linear Fourier transform theory is not a reliable guide to the uniformity of flip angle in the slice. We show by simulation that the nonuniformity can have serious implications for the measurement of relaxation timeT 1 if selective 180° pulses are used; the exact results depend also on the details of data analysis and criteria for adjusting the rephasing gradients. We describe an experiment and a phantom in which the axial nonuniformity can be demonstrated on clinical NMR imaging machines.

Distribution of relaxation times in ( KBr ) 0.5 ( KCN ) 0.5

Abstract: Measurements of the dielectric response of ${(\mathrm{KBr})}_{0.5}$${(\mathrm{KCN})}_{0.5}$ covering nine decades of frequency are reported. We have shown how the relaxation times proliferate as the temperature is lowered. The anomalously wide distribution of relaxation times can be generated from a Gaussian distribution of energy barriers. As temperature is decreased not only does the spread of relaxation times increase, but more importantly the width of the distribution of activation energies itself increases.

Unified picture for spin-lattice relaxation of lipid bilayers and biomembranes

Abstract: The present study compares and interprets the 1H, 2H, and 13C spin‐lattice (T1) relaxation times of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC), in the liquid crystalline phase, in terms of models for the molecular dynamics of lipid bilayers. The 1H T1 times of the DPPC bilayer hydrocarbon region at two frequencies and 13C T1 data at seven frequencies, for which the relaxation is dipolar in origin, as well as the 2H T1 data at three frequencies, due to the quadrupolar interaction, can be unified and interpreted in terms of a collective model for order fluctuations. In normalizing the 13C T1 data to the 1H and 2H T1 values, a vibrationally corrected 13C–1H distance parameter of r0CH=1.14 A has been assumed, rather than the equilibrium bond length of 1.09 A. The analysis suggests that the behavior of the individual acyl chain segments of lipid bilayers, in the liquid crystalline phase, is similar to that of molecules in nematic fluids.

Polarons and Excitons in Polar Semiconductors and Ionic Crystals

Abstract: Studies of the Free and Bound Magneto-Polaron and Associated Transport Experiments in n-InSb and Other Semiconductors- Experimental Study of Hot Electrons in Silver Halides at Crossed Electric and Magnetic Fields- Dynamical and Nonlinear Profiles of Polarons and Excitons in Pure and Ultrapure AgCl, AgBr, and AgClxBr1-x- Some Recent Developments in the Theory of Polarons- Two Applications of Polaron Theory: Cyclotron Resonance and Relaxation of Hot Charge Carriers- Coupled Plasmon-Polar Phonon Modes in Semiconductors: Spatial Dispersion and Other Properties- General Aspects of the Functional-Integral Approach to the Polaron and Related Systems- to the Theory of Excitons- Excitons and Exciton Relaxation in Silver Halides- Electron-Hole Liquid Condensation in Semiconductors- Transient Luminescence, Transport and Photoconductivity in Chalcogenide Glasses- Surface State Electrons above a Liquid Helium Film and the Surface Polaron Problem- Author Index- Material Index