共查询到20条相似文献,搜索用时 15 毫秒
1.
Magnetic field mapping in NMR imaging 总被引:1,自引:0,他引:1
The Hahn spin preparation sequence provides a practical means for rapid and sensitive mapping of magnetic field inhomogeneity in NMR imaging applications. Choice of the rf pulse delay times tau 1 and tau 2 as well as conditions and limitations on the proposed use of this sequence for chemical shift imaging are discussed. 相似文献
3.
The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimides, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows to probe the proton motion on a time scale of the microsecond. The NMR longitudinal relaxation rate R(1) measured over three decades of Larmor angular frequencies omega is particularly sensitive to the host-water interactions and thus well suited to study fluid dynamics in restricted geometries. In the polyimide membranes, we have observed a strong dispersion of R(1)(omega) following closely a 1/sqrt[omega] law in a low-frequency range (correlation times from 0.1 to 10 micros). This is indicative of a strong interaction of water with "interfacial" hydrophilic groups of the polymeric matrix (wetting situation). On the contrary, in the Nafion, we observed weak variations of R(1)(omega) at low frequency. This is typical of a nonwetting behavior. At early hydration stages, the proton-proton inter-dipolar contribution to R(1)(omega) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to 3D diffusion is observed. 相似文献
4.
P. C. Riedi 《Hyperfine Interactions》1989,49(1-4):335-355
A review is given of the information about a ferromagnetic system obtainable from swept frequency NMR. The distinction between
signals from nuclei in domains and domain walls is stressed and recent advances in instrumentation described. 相似文献
5.
Bryant HC Adolphi NL Huber DL Fegan DL Monson TC Tessier TE Flynn ER 《Journal of magnetism and magnetic materials》2011,323(6):767-774
We use dynamic susceptometry measurements to extract semiempirical temperature-dependent, 255-400 K, magnetic parameters that determine the behavior of single-core nanoparticles useful for SQUID relaxometry in biomedical applications. Volume susceptibility measurements were made in 5 K degree steps at nine frequencies in the 0.1-1000 Hz range, with a 0.2 mT amplitude probe field. The saturation magnetization (Ms) and anisotropy energy density (K) derived from the fitting of theoretical susceptibility to the measurements both increase with decreasing temperature; good agreement between the parameter values derived separately from the real and imaginary components is obtained. Characterization of the Néel relaxation time indicates that the conventional prefactor, 0.1 ns, is an upper limit, strongly correlated with the anisotropy energy density. This prefactor decreases substantially for lower temperatures as K increases. We find, using the values of the parameters determined from the real part of the susceptibility measurements at 300 K, that SQUID relaxometry measurements of relaxation and excitation curves on the same sample are well described. 相似文献
6.
A field-cycling NMR pulse sequence is described for studying cross-relaxation between unlike nuclear spins in the solid state. The technique has been applied to study proton tunneling in the hydrogen bonds of a carboxylic acid containing 19F and 1H spins. By studying the B-field dependence of the off-diagonal element of the relaxation matrix that characterizes the longitudinal polarizations, an accurate measure of the proton transfer rate is obtained. 相似文献
7.
8.
Kazimierczuk K Koźmiński W 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,174(2):287-291
A simple stabilization scheme of B(0) magnetic field fluctuations is described. The method is based on external measurements of time dependent magnetic field fluctuations by fluxgate sensors and generation of a compensating correction current in a coil mounted directly on an NMR magnet. It is shown that such an approach efficiently eliminates relatively slow magnetic field variations with frequency up to approximately 100 Hz. In combination with a standard (2)H field-frequency lock system, the method enables acquisition of reproducible lineshapes and dramatically improves overall performance of a high resolution NMR spectrometer. The presented solution might substitute for the internal lock system in these case where deuterium lock is not available. 相似文献
9.
10.
M. Singh P.R. Wallace S.D. Jog E. Arushanov 《Journal of Physics and Chemistry of Solids》1984,45(4):409-418
Starting from the band structure calculations of Yamada, we have undertaken to calculate in analytic form the Landau levels, de Haas-Shubnikov oscillation periods and effective masses in the valence band of CdSb. Because of the anomalous character of the E — k relation in the x-direction, it is necessaryto use different approximation methods for low and high hole energies; between the two is a region for which calculation is very difficult. Analytic forms have been obtained in both regimes, which should facilitate the calculation of magneto-transport properties and thus the determination of band parameters. 相似文献
11.
R. Kausik C. Mattea R. Kimmich N. Fatkullin 《The European physical journal. Special topics》2007,141(1):235-241
Polymer chain dynamics were studied with the aid of
field-cycling NMR relaxometry (time scale: 10-9s...
10-4s) supplemented by field gradient NMR diffusometry
(time scale: 10-4s...100s). Three sorts of samples
of mesoscopically confined polymer melts were examined. In the first
sample series, linear poly(ethylene oxide) was incorporated in
strands embedded in a quasi-solid and impenetrable methacrylate
matrix. The strand diameters ranged from 10 to 60 nm. It was shown
that chain dynamics becomes dramatically different from bulk
behavior. This so-called “corset effect” occurs both above
and below the critical molecular mass and reveals dynamic features
predicted for reptation. On the time scale of spin-lattice
relaxation, the frequency and molecular weight, signature of
reptation, T1 ∼M0 ν3/4, that is limit II of
the Doi/Edwards formalism corresponding to the mean squared segment
displacement law 〈r2 〉∼M0 t1/4, showed
up. A “tube” diameter of only 0.6 nm was concluded to be
effective on this time scale even when the strand diameter was
larger than the radius of gyration of the PEO random coils.
The corset effect is traced back to the lack of the local
fluctuation capacity of the free volume under nanoscopic
confinements. The confinement dimension at which the cross-over from
confined to bulk chain dynamics is expected was estimated to be
micrometers. Using the so-called roll-coating technique, micrometer
thick polymer melt layers between Kapton foils were prepared.
Perceptible differences from the bulk materials were found. The
polymer species studied in this case was perfluoropolyether with
Flory radii in the order of 7 nm. Remarkably, the confinement
effect was shown to reach polymer-wall distances of the order 100
Flory radii. As a third confinement system, melts of
perfluoropolyether were filled into a porous silica glass (Vycor;
4 nm nominal pore size). In this case, a crossover from Rouse
dynamics in the bulk to reptation in the Doi/Edwards limit III
(T1∼M-1/2 ν1/2 corresponding to 〈r2
〉∼M-1/2 t1/2) was observed. 相似文献
12.
N. A. Marigheto G. K. Moates M. E. Furfaro K. W. Waldron Brian P. Hills 《Applied magnetic resonance》2009,36(1):35-47
A combination of two-dimensional nuclear magnetic resonance (NMR) relaxometry and optical microscopy is used to investigate
the effects of ripening and high-pressure treatment on tomato pericarp tissue. It is shown that the relaxation times of the
vacuolar water increase during the early stages of ripening even when there is no visible change and that this could be the
basis of a useful on-line sensor for sorting tomatoes. The membrane rupturing and biopolymer denaturation associated with
high-pressure treatments are also reflected in the NMR relaxation spectra. 相似文献
13.
W E Maas A Bielecki M Ziliox F H Laukien D G Cory 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,141(1):29-33
Magnetic field gradients have proven useful in NMR for coherence pathway selection, diffusion studies, and imaging. Recently they have been combined with magic angle spinning to permit high-resolution measurements of semi-solids, where magic angle spinning averages any residual dipolar couplings and local variations in the bulk magnetic susceptibility. Here we show the first examples of coherence pathway selection by gradients in dipolar coupled solids. When the gradient evolution competes with dipolar evolution the experiment design must take into account both the strength of the dipolar couplings and the means to refocus it. Examples of both homonuclear and heteronuclear experiments are shown in which gradients have been used to eliminate the need for phase cycling. 相似文献
14.
Detection of anisotropic hyperfine transitions in zero magnetic field using field-cycling techniques
Sturm G Kilian D Lötz A Voitländer J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,142(1):139-144
For the first time, we describe the detection of hyperfine transitions in zero magnetic field using field-cycling techniques and pulsed EPR spectroscopy. The sample investigated was coal, which shows an anisotropic electron spin-(13)C hyperfine interaction. 相似文献
15.
It is shown that hydrodynamic flow has an effect on spin-lattice relaxation in water filled into a porous monolithic silica material. This is a rotational analogue of translational hydrodynamic (or Taylor-Aris) dispersion arising from incoherent Brownian motion in combination with coherent flow. The effect is demonstrated with the aid of field-cycling NMR relaxometry and confirmed by theoretical considerations. The results directly verify bulk mediated surface diffusion and reveal interfacial slip at fluid-solid interfaces. 相似文献
16.
The proton spin-lattice relaxation time T1, in the nematic liquid crystal 4-pentyl-4′-cyanobiphenyl confined in a glassy porous matrix has been measured in a wide Larmor frequency range of 1 · 102?2 · 107 Hz employing the fast field-cycling NMR technique. A strong influence of the restricted geometry on the character of the T1 dispersion was found. Our investigation clearly demonstrates the importance of the translationally induced molecular reorientations in inhomogeneous director field for the relaxation in the samples with 200 and 80 nm mean pore size. The experimental results are in a good agreement with the theoretical predictions. In the sample with 7 nm pore size the main contribution to the relaxation is ascribed to the slowing down of the molecular motion in the near-surface layer. Zero-field 1H NMR spectra of a microconfined liquid crystal are reported for the first time. 相似文献
17.
Concentrated oil-in-water emulsions have been studied using nuclear magnetic resonance relaxometry (NMRR). To characterize
the size distribution of water droplets, correlations between NMRR data and integral droplets diameters have been established
and used for data interpretation. The dependences of parameters of emulsion components have been obtained from proton relaxation
data after rotating magnetic field application and sedimentation. 相似文献
18.
R. Meier D. Kruk A. Bourdick E. Schneider E. A. Rössler 《Applied magnetic resonance》2013,44(1-2):153-168
Field cycling 1H nuclear magnetic resonance (NMR) relaxometry is applied to study rotational as well as translational dynamics in molecular liquids. The measured relaxation rates, $ T_{1}^{ - 1} \left( \omega \right) \equiv R_{1} \left( \omega \right) $ , contain intra- and intermolecular contributions, $ R_{{1,{\text{intra}}}}^{{}} \left( \omega \right) $ and $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ . The intramolecular part is mediated by rotational dynamics, the intermolecular part by translation as well as rotation. The rotational impact on the intermolecular relaxation (eccentricity effect) is due to the spins not located in the molecule’s center. The overall relaxation rate is decomposed into $ R_{{1,{\text{intra}}}}^{{}} \left( \omega \right) $ and $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ by isotope dilution experiments. It is shown that the eccentricity model (Ayant et al. in J. Phys. (Paris) 38:325, 1977) reproduces fairly well the bimodal shape of $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ for o-terphenyl and glycerol. As the relaxation contribution associated with translational dynamics dominates at lower frequencies, the overall relaxation rate shows a universal linear behavior when plotted versus square root of frequency. This allows determining the self-diffusion coefficient, D, in a model-independent way. It is demonstrated that the shape of NMR master curves comprising relaxation data for different temperatures, linked by frequency–temperature superposition, reflects the relative strength of translational and rotational contributions. 相似文献
19.
The magnetic hyperfine splitting frequencies of71AsNi and72AsNi in a 0.11 Tesla external magnetic field have been determined by NMR/ON method as 66.00(6) MHz and 106.17(13) MHz respectively.
Using the known magnetic moments of μ(71As)=1.6735(18) and μ(72As)=−2.1566(3), the hyperfine fields were deduced asB
hf(71AsNi)=12.824(19) Tesla andB
hf(72AsNi)=12.807(16) Tesla. 相似文献
20.
《Comptes Rendus Physique》2010,11(2):160-171
A salient characteristic of nuclear magnetic resonance (NMR) techniques is the possibility to scan nuclear spin evolutions within a broad Larmor frequency range. Special instrumentation was developed to extend nuclear spin relaxation studies up to proton Larmor frequencies in the sub-kilohertz regime, a technique known as field-cycling NMR relaxometry. This article refers to an experimental version where the sample under study is selectively subjected to ultrasonic irradiation. The fact that ultrasound couples selectively to the collective dynamics of liquid crystals, offers new insights for the study of the molecular dynamics in these materials using NMR relaxation. 相似文献