Theory of FID NMR signal dephasing induced by mesoscopic magnetic field inhomogeneities in biological systems.

A theory of the NMR signal dephasing due to the presence of tissue-specific magnetic field inhomogeneities is developed for a two-compartment model. Randomly distributed magnetized objects of finite size embedded in a given media are modeled by ellipsoids of revolution (prolate and oblate spheroids). The model can be applied for describing blood vessels in a tissue, red blood cells in the blood, marrow within trabecular bones, etc. The time dependence of the dephasing function connected with the spins inside of the objects, s(i), is shown to be expressed by Fresnel functions and creates a powder-type signal in the frequency domain. The short-time regime of the dephasing function for spins outside the objects, s(e), is always characterized by Gaussian time dependence, s(e) approximately exp[-zeta(k)(t/tc)2], with zeta being a volume fraction occupied by the objects, t(c) being a characteristic dephasing time, and the coefficient k depending on the ellipsoid's shape through the aspect ratio of its axes (a/c). The long-time asymptotic behavior of s(e) is always "quasispherical"-linear exponential in time, s(e) approximately exp(-zetaCt/tc), with the same "spherical" decay rate for any ellipsoidal shape. For long prolate spheroids (a/c)<1, there exists an intermediate characteristic regime with a linear exponential time behavior and an aspect-ratio-dependent decay rate smaller than (zetaC/tc).     

Structure-specific magnetic field inhomogeneities and its effect on the correlation time

We describe the relationship between the correlation time and microscopic spatial inhomogeneities in the static magnetic field. The theory takes into account diffusion of nuclear spins in the inhomogeneous field created by magnetized objects. A simple general expression for the correlation time is obtained. It is shown that the correlation time is dependent on a characteristic length, the diffusion coefficient of surrounding medium, the permeability of the surface and the volume fraction of the magnetized objects. For specific geometries (spheres and cylinders), exact analytical expressions for the correlation time are given. The theory can be applied to contrast agents (magnetically labeled cells), capillary network, BOLD effect and so forth.     

Discovering the Higgs through highly-displaced vertices

We suggest that the Higgs could be discovered at the Tevatron or the LHC (perhaps at the LHCb detector) through decays with one or more substantially displaced vertices from the decay of new neutral particles. This signal may occur with a small but measurable branching fraction in the recently-described “hidden valley” models, hep-ph/0604261; weakly-coupled models with multiple scalars, including those of hep-ph/0511250, can also provide such signals, potentially with a much larger branching fraction. This decay channel may extend the Higgs mass reach for the Tevatron. Unusual combinations of b jets, lepton pairs and/or missing energy may accompany this signal.     

Non-markovian dynamics of spectral narrowing for excitons in the layered semiconductor GaSe observed using optical four-wave mixing spectroscopy

We have investigated non-markovian dephasing by using time-resolved and spectrally resolved four-wave mixing measurements in a layered semiconductor GaSe. When the time interval between the first and second excitation pulses is increased, photon echo spectra exhibit narrowing only in a regime of a few picoseconds. In the initial dephasing of these signals, fast damping is observed. The narrowing of the spectrally resolved signal is consistent with the Fourier transformation of the time-resolved signals. Spectral narrowing is crucial evidence of the transition from non-markovian to markovian dynamics. By comparing experimental data with calculation results based on the non-markovian theory, we have found that the correlation time of the exciton-phonon interaction is 1.1 ps.     

Pinch instability of a cylindrical couette flow in a nonuniform axial magnetic field

The paper addresses the linear stability to axisymmetric perturbations of an incompressible nonideal fluid between two rotating coaxial infinitely long cylinders in a nonuniform axial magnetic field. For conducting cylinders, the results for uniform and nonuniform magnetic fields are qualitatively identical. This is also observed for nonconducting cylinders in a magnetic field with a constant direction. Instability appears for nonconducting cylinders in a magnetic field with a varying direction, whose magnitude exceeds a certain critical value. This new instability also exists in the absence of rotation and, hence, is independent of its parameters. In addition, the critical magnetic field is independent of the magnetic Prandtl number, which facilitates experimental observation of the new instability.     

Quantum diffusion in polaron model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers

We numerically investigate quantum diffusion of an electron in a model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers with fluctuation of the parameters due to the impact of colored noise. The randomness is introduced by fluctuations of distance between two consecutive bases along the stacked base pairs. We demonstrate that in the model the decay time of the correlation can control the spread of the electronic wavepacket. Furthermore it is shown that in a motional narrowing regime the averaging over fluctuation causes ballistic propagation of the wavepacket, and in the adiabatic regime the electronic states are affected by localization.     

Muon-spin rotation as a tool for investigating the diffusion of positive muons in type-II superconductors

The measurement of the spin precession frequencies of immobile positive muons in the mixed state of uniformly magnetized type-II superconductors gives the distribution function of the magnetic field. Increasing muon diffusivity leads first to the suppression of the van Hove singularity associated with the field-strength maximum in the flux-line centres, then to an enhancement of the damping of the main precession frequency, and finally to a shift and motional narrowing of the precession frequency distribution. These phenomena can be used to determine muon diffusivities in Nb exceeding about 10^?12m²s^?1. The required information on the magnetic-field distribution function in the mixed state is presented in detail.     

Segmental anisotropy in strained elastomers detected with a portable NMR scanner

Single-side NMR is particularly suitable for measurements of segmental anisotropy induced in elastomers by uniaxial forces or local strain. Proton transverse nuclear magnetic relaxation was investigated with the NMR-MOUSE by recording the Hahn-echo decay in cross-linked natural rubber bands. This provided information on the dependence of the Hahn-echo decay on the angle between the direction of the uniaxial stretching force and the axis Z defined direction perpendicular to the magnet pole faces of the NMR-scanner. The anisotropy effect on the Hahn-echo decay is correlated with the extension ratio, and it is more evident in the liquid-like regime of the decay. A weaker segmental anisotropy is detected by 1H solid- and Hahn-echo decays recorded by multi-pulse sequences. A qualitative understanding of the angular dependence is obtained by an analytical theory of the Hahn-echo decay adapted to the case of stretched elastomers and to strongly inhomogeneous magnetic fields. Using angular-dependent 1H residual second van Vleck moments and correlation times reported previously [P.T. Callaghan and E.T. Samulski, Macromolecules 30, 113 (1997)] from stretched natural rubber bands the segmental anisotropy measured in inhomogeneous magnetic fields by the Hahn-echo decay was numerically simulated. As an example of a macroscopic distribution of local segmental anisotropy, 1H Hahn-echo decays were measured by the NMR-MOUSE sensor in a stretched cross-linked natural rubber plate with a circular cut in the center.     

Frequency dependence of EPR signal intensity, 248 MHz to 1.4 GHz

The electron paramagnetic resonance pulsed free induction decay (FID) of a degassed solution of a triaryl methyl radical, methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2-d:4,5-d']bis(1,3)dithiol-4-yl) tripotassium salt, 0.2 mM in H2O, was measured at VHF (247.5 MHz) and L-band (1.40 GHz). The calculated and observed FID signal amplitudes (in millivolts) agreed within 1 and 6%, and the ratio of the normalized FID signals at the two frequencies agreed within 5%. The FID decay time constant was 2.7 micros at both frequencies.     

Muon-hydrogen correlation in nickel-titanium metallic glass

Measurements of muon spin rotation in a transverse magnetic field in the Ni₃₅Ti₆₅ metallic glass saturated with hydrogen are reported. The experiment was carried out on the μSR facility at ISIS/RAL. Chilton, UK. The decay of the muon precession has a gaussian form below 150 K. Above this temperature motional narrowing occurs, the linewidth falling to the background value at about 300 K. The linewidth is almost entirely determined by the dipolar interaction between the muon and proton spins and the line narrowing is due to the correlated motion of the muon-hydrogen system. The activation energies and jump rates deduced from these measurements and from nuclear magnetic dipolar relaxation are compared.     

Vector model of electron spin echo envelope modulation due to nuclear hyperfine and zeeman interactions

The transverse electron spin magnetization of a paramagnetic center with effective spinS=1/2 interacting with nonquadrupolar nuclei may be presented as a function of pairs of nuclei magnetization vectors which precess around the effective magnetic field directions. Each vector of the pair starts its precession perpendicular to both effective fields. The free induction decay (FID) signal is proportional to the scalar product of the vectors for nuclear spinI=1/2. The electron spin echo (ESE) signal can be described with two pairs of magnetization vectors. The ESE shape is not equal to two back-to-back FID signals except in the absence of ESE envelope modulation. A recursion relation is obtained which allows calculation of ESE signals for larger nuclear spins in the absence of nuclear quadrupole interaction. This relation can be used to calculate the time course of the ESE signal for arbitrary nuclear spins as a function of the nuclear magnetization vectors. While this formalism allows quantitative calculation of modulation from nuclei, it also provides a qualitative means of visualizing the modulation based on simple magnetization vectors.     

Motional narrowing and ergodic bands in excited superdeformed states of 194Hg

The E(gamma) - E(gamma) coincidence spectra from the electromagnetic decay of excited superdeformed states in (194)Hg reveal surprisingly narrow ridges, parallel to the diagonal. A total of 100-150 excited bands are found to contribute to these ridges, which account for nearly all the unresolved E2 decay strength. Comparison with theory suggests that these excited bands have many components in their wave functions, yet they display remarkable rotational coherence. This phenomenon can be explained in terms of the combination of shell effects and motional narrowing.     

Study of Water States in Different Hydrated and Consolidated Clay Using Nuclear Magnetic Resonance

ABSTRACT

The aim of the present work was to test low-resolution NMR to characterize and quantify the state of water in some clay materials such as illite, montmorillonite, and bentonite with water content and in a compacted state. Low-field ¹H-NMR spectroscopy (0.54 T) was used to determine the most mobile fraction of water in different types of clay materials by analyzing free induction decay (FID) and Carr-Purcell-Meiboom-Gill (CPMG) relaxation curves at 20°C, ?7°C, and ?25°C. Results have shown that a fast exchange occurs between the first bound monolayer of water and the following ones for illite, montmorillonite, and bentonite; a multiexponential decay of some CPMG relaxation curves was also observed. FID measurements at ?25°C showed that 83% of the water in illite (41% water content), 44% in montmorillonite (31% w.c.), and 52% in bentonite (27% w.c.) is frozen at this temperature. Treatment of signals obtained with bentonite compacted in capillaries (28% w.c.) was complicated by a signal coming from the capillaries themselves and an increase in internal magnetic field gradients, which led to a dual quantification whose most likely result is 39% of freezable water.     

基于过抽样Gabor变换的核磁共振FID信号增强算法

基于作者先前提出的过抽样实值离散Gabor变换,本文提出了一有效的算法用于核磁共振自由感应衰减（NMR FID）信号的减噪. 由于NMR FID信号在时域中是一短暂的振荡衰减信号,使得变换后的NMR FID信号能量在时频域中集中在少数变换系数上,而噪声则遍布在整个变换系数上,因此通过对变换系数幅度进行阈值限制方法可达到明显地增强NMR FID信号的目的. 文中在理论和模拟实验上分析表明,过抽样Gabor变换比临界抽样Gabor变换更适宜于NMR FID信号的减噪,因为在过抽样条件下比在临界抽样条件下的综合窗及其对应的分析窗,无论是在时域中还是在频域中都可具有更好的局域分布集中性,同时,Gabor变换在过抽样条件下也比在临界抽样条件下具有更高的时频精度.     

Relationship between soot volume fraction and LII signal in AC-LII: effect of primary soot particle diameter polydispersity

Theoretical analysis and numerical calculations were conducted to investigate the relationship between soot volume fraction and laser-induced incandescence (LII) signal within the context of the auto-compensating LII technique. The emphasis of this study lies in the effect of primary soot particle diameter polydispersity. The LII model was solved for a wide range of primary soot particle diameters from 2 to 80 nm. For a log-normally distributed soot particle ensemble encountered in a typical laminar diffusion flame at atmospheric pressure, the LII signals at 400 and 780 nm were calculated. To quantify the effects of sublimation and differential conduction cooling on the determined soot volume fraction in auto-compensating LII, two new quantities were introduced and demonstrated to be useful in LII study: an emission intensity distribution function and a scaled soot volume fraction. When the laser fluence is sufficiently low to avoid soot mass loss due to sublimation, accurate soot volume fraction can be obtained as long as the LII signals are detected within the first 200 ns after the onset of the laser pulse. When the laser fluence is in the high fluence regime to induce significant sublimation, however, the LII signals should be detected as early as possible even before the laser pulse reaches its peak when the laser fluence is sufficiently high. The analysis method is shown to be useful to provide guidance for soot volume fraction measurements using the auto-compensating LII technique.     

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

We describe a ³He magnetometer capable to measure high magnetic fields (B> 0.1 T) with a relative accuracy of better than 10^-12. Our approach is based on the measurement of the free induction decay of gaseous, nuclear spin polarized ³He following a resonant radio frequency pulse excitation. The measurement sensitivity can be attributed to the long coherent spin precession time T₂ ^? being of order minutes which is achieved for spherical sample cells in the regime of “motional narrowing” where the disturbing influence of field inhomogeneities is strongly suppressed. The ³He gas is spin polarized in situ using a new, non-standard variant of the metastability exchange optical pumping. We show that miniaturization helps to increase T₂ ^? further and that the measurement sensitivity is not significantly affected by temporal field fluctuations of order 10^-4.     

Temperature dependence of the stability of written bits in a magnetic hard-disk medium investigated by magnetic force microscopy

The thermal stability of written bits in a magnetic hard-disk medium has been investigated with a magnetic force microscope (MFM), which was equipped with an in situ heating system capable of heating the medium up to 300 °C. It is shown that both the annealing temperature and the duration have significant effect on the decay of the MFM signal. No signal decay is observed when annealing for 30 min up to temperatures of 200 °C. The MFM signal decays rapidly with increasing temperature, for temperatures over 200 °C. Repeated annealing at 280 °C with a duration below 10 min does not cause any signal decay.     

Oscillatory free induction decay in angular distribution of nuclear radiation

A theoretical consideration of the free induction decay (FID) signal formation in the angular distribution of radiation, emitted by oriented nuclei after excitation by a long radiofrequency resonance pulse t_p ? T¹₂, is carried out. The well-known theorem on the oscillatory FID signal is expanded for the case of second rank tensors.