Pulsed Gradient Spin-echo Method Research Articles

Solvent self-diffusion dependence on the swelling degree of a hydrogel.

This article describes high-field nuclear magnetic resonance experiments using pulsed gradient spin echo methods to investigate water mobility inside homogeneously swollen polyacrylamide hydrogel beads. The NMR data permitted determining the solvent self-diffusion coefficient dependence on either the swelling time or the polymer volume fraction. The sensitivity to the structure of the hydrogel network could be tuned to a certain extent via controlling the diffusion probing time. Relaxation measurements have helped substantiate a theoretical description for these self-diffusion dependencies based upon a fast-exchange two-site model. A tortuosity-porosity dependence is extracted from the model and then compared to other known tortuosity regimes. The diffusion dependence on the degree of swelling obtained here corroborates the existence of a gel swelling-front observed during inhomogeneous solvent imbibition experiments. Finally, we stress that the NMR experimental results reported here are of great value on performing theoretical modeling of gel swelling via solvent imbibition.

Magnetic resonance imaging of strain in elastic gels

Magnetic resonance imaging (MRI) can also be used for studying different material conditions, including strain, which is the subject of this study. A pair of pulsed magnetic field gradients combined with the spin-echo imaging sequence [pulsed-gradient spin-echo (PGSE)] enables the detection of small sample displacements that are induced in the time between the gradient pulses. The displacements are registered in the image phase shift, which is then processed by the phase unwarping algorithm and phase background correction. The processed phase shift can then be used to calculate sample displacements by multiplying it with the corresponding calibration constant. Finally, the strain tensor can be calculated from the map of sample displacements. In the study, the PGSE imaging method was tested on a gelatin sample. While being MRI scanned, the sample was deformed synchronously with the imaging sequence using a special MRI-compatible apparatus for inducing sample deformations. The apparatus was based on the use of compressed air that was directed via a nozzle on the surface of the sample in pulses controlled by transistor-transistor logic pulses of the spectrometer. With the 20 ms pulses of compressed air at a pressure of 0.5 bar, sample displacements of up to 300 μm were detected at the impact point of compressed air, while the detection threshold was at 0.7 μm. Results of the PGSE method were also compared with the tagging method. The study was performed in 2D; however, the method can be implemented also in 3D, thus enabling the measurement of the full strain tensor.

Dependence of temporal diffusion spectra on microstructural properties of biological tissues

The apparent diffusion coefficient (ADC) measured using magnetic resonance imaging methods provides information on microstructural properties of biological tissues, and thus has found applications as a useful biomarker for assessing changes such as those that occur in ischemic stroke and cancer. Conventional pulsed gradient spin echo methods are in widespread use and provide information on, for example, variations in cell density. The oscillating gradient spin echo (OGSE) method has the additional ability to probe diffusion behaviors more readily at short diffusion times, and the temporal diffusion spectrum obtained by the OGSE method provides a unique tool for characterizing tissues over different length scales, including structural features of intracellular spaces. It has previously been reported that several tissue properties can affect ADC measurements significantly, and the precise biophysical mechanisms that account for ADC changes in different situations are still unclear. Those factors may vary in importance depending on the time and length scale over which measurements are made. In the present work, a comprehensive numerical simulation is used to investigate the dependence of the temporal diffusion spectra measured by OGSE methods on different microstructural properties of biological tissues, including cell size, cell membrane permeability, intracellular volume fraction, intranucleus and intracytoplasm diffusion coefficients, nuclear size and T2 relaxation times. Some unique characteristics of the OGSE method at relatively high frequencies are revealed. The results presented in the paper offer a framework for better understanding possible causes of diffusion changes and may be useful to assist the interpretation of diffusion data from OGSE measurements.

Influence of cell cycle phase on apparent diffusion coefficient in synchronized cells detected using temporal diffusion spectroscopy

The relationship between the apparent diffusion coefficient of tissue water measured by MR methods and the physiological status of cells is of particular relevance for better understanding and interpretation of diffusion-weighted MRI. In addition, there is considerable interest in developing diffusion-dependent imaging methods capable of providing novel information on tissue microstructure, including intracellular changes. To this end, both the conventional pulsed gradient spin-echo methods and the oscillating gradient spin-echo method, which probes diffusion over very short distance (<<cell size) and time scales, were used to measure apparent diffusion coefficient of synchronized packed HL-60 cells at 7 T. The results show that the pulsed gradient spin-echo method with relatively long diffusion times does not detect changes in apparent diffusion coefficient when structural variations arise during cell division. On the contrary, the oscillating gradient spin-echo method can detect and quantify major changes in intracellular organization that occur during mitosis by appropriate choice of gradient frequency. Cell structural parameters, including cell size, intracellular diffusion coefficient, and surface-to-volume ratio were also obtained by fitting the oscillating gradient spin-echo data to simple analytical models. These oscillating gradient spin-echo features may be used in diffusion-weighted MRI to create parametric maps that may be useful for detecting cancer or changes caused by treatment.

Numerical simulations of motion-insensitive diffusion imaging based on the distant dipolar field effects

Diffusion weighting in MRI is commonly achieved with the pulsed-gradient spin-echo (PGSE) method. When combined with spin-warping image formation, this method often results in ghosts due to the sample's macroscopic motion. It has been shown experimentally (Kennedy and Zhong, MRM 2004;52:1-6) that these motion artifacts can be effectively eliminated by the distant dipolar field (DDF) method, which relies on the refocusing of spatially modulated transverse magnetization by the DDF within the sample itself. In this report, diffusion-weighted images (DWIs) using both DDF and PGSE methods in the presence of macroscopic sample motion were simulated. Numerical simulation results quantify the dependence of signals in DWI on several key motion parameters and demonstrate that the DDF DWIs are much less sensitive to macroscopic sample motion than the traditional PGSE DWIs. The results also show that the dipolar correlation distance (d(c)) can alter contrast in DDF DWIs. The simulated results are in good agreement with the experimental results reported previously.

H&lt;SUB&gt;2&lt;/SUB&gt;O self-diffusion coefficient of water-rich MX-80 bentonite gels

MX-80 is a bentonite clay from the Clay Spur bed, Wyoming, USA, consisting of ~80 wt.% Na-rich montmorillonite and is a leading candidate material for use in engineered barriers within underground nuclear waste disposal sites in Switzerland and Sweden. The diffusive invasion of groundwater into the engineered barriers is likely to cause the dry high-density bentonite to swell and evolve into a less-dense, water-rich gel during re-submergence of the shafts and galleries. Data on H 2 O diffusivity within water-rich bentonite gels are therefore essential to the safe design of the engineered barriers. To address this need, the self-diffusion coefficient, D , of 1 H 2 O molecules in water-rich MX-80 bentonite gels was measured using the pulsed-gradient spin-echo method of 1 H nuclear magnetic resonance spectroscopy at 0.47 T; D was measured over a wide range of temperatures (20.6–70.1°C) and of bentonite volume fractions (0 to 17.2 vol.%). The results showed that D increased markedly to values of bulk water self-diffusivity as the bentonite volume fraction decreased towards 0 vol.%. The activation energy of the diffusion process for the gels was approximately equal to that for bulk water. As a result, the normalized H 2 O self-diffusivity, D / D 0 , was approximately independent of temperature, where D 0 is D in bulk water. The D / D 0 data for ⩽6.39 vol.% bentonite fraction were successfully fitted to a porous clay gel model that involves unbound water molecules that diffuse in the pore space by avoiding randomly placed discoidal obstacles (clay particles) of a high aspect ratio. The D / D 0 data for MX-80 were similar to data for SWy-2 (Wyoming bentonite from the Newcastle formation) and Kunigel-V1 (bentonite from Yamagata, Japan), suggesting that all three bentonites are equally suitable candidate materials, in terms of water diffusivity performance, for use within engineered barriers.

A new approach for simultaneous measurement of ADC and T2 from echoes generated via multiple coherence transfer pathways

The study of rotational and translational diffusion requires the measurement of both T 2 and apparent diffusion coefficient (ADC), quantities that are typically measured in separate experiments. The exploitation of echoes generated via multiple coherence transfer pathways offers an opportunity for measuring T 2 and ADC values simultaneously in a single experiment. A series of RF pulses can generate multiple echoes via different coherence pathways with each one being uniquely encoded. Here, we demonstrate one pulse sequence that uses an initial θ RF pulse to generate three coherence orders ( C = 0, −1, +1). In the particular version of the method discussed here only two are used ( C = 0, +1). Each order is encoded with a different b value from which the ADC is derived. The coherence order echo C = 0 is refocused to quantify T 2. The performance of the method—dubbed simultaneous measurement of ADC and relaxation time (SMART)—is demonstrated on a set of samples differing in T 2 and ADC achieved by varying the relative volume fractions in mixtures of gadolinium-doped H 2O and D 2O. The regional SMART derived T 2 and ADC agree well with those obtained with conventional double-spin–echo and pulsed gradient spin–echo methods.

Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite

Kunigel-V1 bentonite was analyzed by proton nuclear magnetic resonance (NMR) spectroscopy over a wide range of water content and temperature at 0.47 T. Kunigel-V1 is a bentonite clay consisting of ~50 wt% Na-rich montmorillonite from Yamagata, Japan, and represents a candidate for engineered barriers of underground nuclear waste disposal sites in Japan. The NMR-related properties of bentonite are also important with respect to application of the material as a mud in boreholes for NMR well logging in the geophysical exploration of disposal sites. The proton relaxation times, surface relaxivity of montmorillonite, and H2O self-diffusion coefficient, were determined in this study for water-rich gel samples of bentonite at 11.0 to 70.0°C and for bentonite weight fractions of 0 to 37.7 wt%. The proton relaxation times (T1 and T2) were measured by the inversion recovery method and Carr-Purcell-Meiboom-Gill method, respectively, and the self-diffusion coefficient of H2O molecules (D) was measured by the pulsed-gradient spin-echo method. The results showed that T1, T2, and D increased with increasing temperature, and decreased with increasing bentonite weight fraction (w). The T1 and T2 surface-relaxivities were on the order of 10–7 m/s and also decreased with temperature. The activation energies of the T1 and T2 relaxation for the bentonite gels were significantly lower than those for bulk water (i.e., about 50 to 70%), whereas the activation energy of the diffusion process for the gels was nearly equal to that for bulk water. As a result, the normalized H2O self-diffusivity, D/D 0, obeys a temperature-independent master curve described by In(D/D 0)=1.54[exp(—0.0377w)— 1], where D 0 is D in bulk water and w is in wt%.

Time-dependent velocities in porous media dispersive flow

Pulsed Gradient Spin Echo (PGSE) NMR methods may be used to measure the asymptotic dispersion coefficient as well as the velocity autocorrelation function (VACF) in porous media flow. The VACF can be measured in the frequency domain using repetitive gradient pulse trains, and in the time domain using double PGSE encoding. The one dimensional double PGSE method, and the two dimensional velocity exchange experiment (VEXSY) are briefly outlined and their application to flow in monodisperse 0.5 mm diameter beads packs described, both axial and transverse VACFs being examined. The measured correlation times are shown to agree well with calculated values. The asymptotic dispersion coefficients agree with literature values in the case of transverse flow while in axial flow it is shown that asymptotic conditions are not achieved, even for observation times longer than the correlation time for flow around a bead.

Comparison of multiple sclerosis clinical subgroups using navigated spin echo diffusion-weighted imaging

The apparent diffusion coefficient (ADC) of tissue provides an indication of the size, shape, and orientation of the water spaces in tissue. Thus, pathologic differences between lesions in multiple sclerosis (MS) patients with different clinical courses may be reflected by changes in ADC measurements in lesions and white matter. Twelve healthy subjects and 35 MS patients with a relapsing-remitting ( n = 10), benign ( n = 8), secondary progressive ( n = 8) and primary progressive ( n = 9) clinical course were studied. T 2-weighted and post-gadolinium T 1-weighted images were obtained using a 1.5 T Signa Echospeed magnetic resonance imaging (MRI) system. Diffusion-weighted imaging was implemented using a pulsed gradient spin echo (PGSE) sequence with diffusion gradients applied in turn along three orthogonal directions in order to obtain the average apparent diffusion coefficient (ADC av). Navigator echo correction and cardiac gating were used to reduce motion artifact. ADC maps were derived using a two point calculation based on the Stejskal-Tanner formula. Diffusion anisotropy was estimated using the van Gelderen formula to calculate an anisotropy index. MS lesions had a higher ADC and reduced anisotropy compared with normal appearing white matter. Highest ADC values were found in gadolinium enhancing lesions and non-enhancing hypointense lesions on T 1-weighted imaging. MS white matter had a slightly higher ADC and lower anisotropy than white matter of healthy subjects. Lesion and white matter ADC values did not differ between patients with different clinical courses of MS. There was no correlation between lesion ADC and disability. Diffusion-weighted imaging with measurement of ADC using the PGSE method provides quantitative information on acute edematous MS lesions and chronic lesions associated with demyelination and axonal loss but does not distinguish between clinical subtypes of MS.

First Direct Measurement of the Spin Diffusion Rate in a Homogenous Solid

The first direct measurement of the rate of spin diffusion through a homogeneous sample is reported. The measurement was performed as an incoherent scattering experiment via a combination of pulsed gradient spin echo methods with multiple pulse, pulsed gradient spatial encoding. The measurement records the destruction of a spin magnetization grating by the random translation of spin magnetization associated with the flip-flop term of the homonuclear dipole-dipole interaction. For single crystal ${\mathrm{CaF}}_{2}$, the measured parallel components of the spin diffusion rates are $7.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{2}/\mathrm{s}$ along the [001] direction and $5.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{2}/\mathrm{s}$ along the [111] direction, in good agreement with theoretical predictions.

Structure of Networks Formed in Concentrated Solutions of Nonionic Surfactant Studied by the Pulsed-Gradient Spin-Echo Method

Surfactant self-diffusion coefficients (D) have been measured on concentrated micellar solutions (>10 wt %) of nonionic surfactant (C 16 E 7 ) and liquid crystal (cubic and hexagonal) phases by using the pulsed-gradient spin-echo method. In the lower temperature range, the self-diffusion coefficient in the micellar phase is much smaller than that in the liquid crystal phases where the lateral diffusion is dominant. Also, the activation energy for the self-diffusion processes calculated from the temperature dependence ofD is much larger than those in the liquid crystal phases and depends on concentration and temperature only slightly. These results confirm the validity of our diffusion model previously reported which takes into account intermicellar migration of surfactant molecules at the entanglement point. As the temperature is raised above about 45 °C (the lower critical solution temperature is 51 °C at about 1 wt %), however, the activation energy decreases rapidly toward the value for the liquid crystal phases. Above about 60 °C, the self-diffusion coefficient depends on concentration only slightly and its absolute value coincides with that expected from the temperature dependence of D in the cubic phase. From these results and the structure of the cubic phase, it is inferred that cross-links of wormlike micelles exist above about 45 °C and that the extent of cross linking increases with increasing temperature. Relations with phase behaviors are also discussed.

Intermicellar Migration of Surfactant Molecules in Entangled Micellar Solutions

Surfactant self-diffusion coefficients (D) have been measured on semidilute solutions of nonionic surfactants (C 16 E 7 , C 14 E 7 , and C 14 E 6 ) at different temperatures by using the pulsed-gradient spin echo method. The self-diffusion coefficient of these surfactants first decreases with increasing concentration (c) and then increases. Above about 10% by weight, the log D-log c plot gives a straight line whose slope is in good agreement with the theoretical prediction (2/3) of the previously reported model which takes into account intermicellar migration of surfactant molecules. In order to simulate the observed diffusion coefficient in the whole concentration range, this model has been modified by taking into account the micellar diffusion

One- and Two-Dimensional Pulse Sequences for Diffusion Experiments in the Fringe Field of Superconducting Magnets

Abstract The fringe field of superconducting magnets provides extremely strong and stable field gradients which can be used for diffusion experiments. A number of corresponding supercon fringe-field variants of the pulsed-gradient spin-echo method are presented. These include procedures and pulse sequences for the record of relaxation-independent diffusion decays, multislice excitation, and one- or two-dimensional Fourier-transform evaluation from the k to the z domain. Several test experiments are described. It is demonstrated that reliable diffusion decays are obtained. The one-dimensional z -domain evaluation produces grid patterns analogous to the optical forced Rayleigh scattering method. The two-dimensional experiments are suitable for the detection of spatially varying probability densities of diffusion or incoherent flow in heterogeneous media. As the resolution is particularly high, fluids can be studied close to solid surfaces.

Nuclear magnetic resonance in chemical engineering: Principles and applications

In recent years chemical engineers have shown an increasing interest in non-invasive measurement techniques; Nuclear Magnetic Resonance (NMR) is perhaps the ultimate technique of this kind. Over the past 10 years notable developments have been made in both spectrometer hadrdware our ability to understand and manipulate nuclear spin interactions, and it is now possible to address research areas in catalysis, materials science, mass transfer and flow visualisation which are of real interest to chemical engineers. This review is divided into two sections. Part I identifies three broad categories of magnetic resonance measurements: spectroscopy, diffusion measurement and imaging, and outlines the basic principles underlying these experiments. A summary of the various nuclear spin interactions and the chemical information they yield is given. In progressing to the introduction of diffusion measurements, the application of magnetic gradients is discussed and the basic Pulsed Gradient Spin Echo (PGSE) and related measurement techniques are presented. The principles of NMR imaging are then described in the context of the two popular experimental schemes: projection—reconstruction and spin-warp imaging. The principles and application of parameter-selective imaging experiments are outlined and limitations on attainable resolution are noted. Extension of NMR imaging to the study of flow phenomena is also discussed. Part II of the review reports a number of examples of NMR methods applied to problems of direct relevance to chemical engineers. This literature survey starts with an overview of applications of NMR spectroscopy in the fields of catalysis, adsorption, measurement of phase equilibria and the consideration of NMR as a quality control technique. The use of PGSE methods to study diffusion phenomena is discussed, with particular emphasis being placed on how theoretical models in combination with NMR experiments are being used to gain insight into transport processes occurring within porous media. Recent developments in NMR imaging and their application to the study of ceramics processing, polymers, porous media, catalysis, food processing, filtration processes, and transport within reactors and packed columns are also presented. Finally, the state-of-the-art in NMR flow imaging studies is discussed and the ability of NMR to study two-phase flow phenomena is highlighted.

Self-diffusion processes in semidilute solutions of nonionic surfactant (C16E7) studied by light scattering and pulsed-gradient spin echo methods

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSelf-diffusion processes in semidilute solutions of nonionic surfactant (C16E7) studied by light scattering and pulsed-gradient spin echo methodsTadashi Kato, Toshiaki Terao, Michiko Tsukada, and Tsutomu SeimiyaCite this: J. Phys. Chem. 1993, 97, 15, 3910–3917Publication Date (Print):April 1, 1993Publication History Published online1 May 2002Published inissue 1 April 1993https://doi.org/10.1021/j100117a043RIGHTS & PERMISSIONSArticle Views122Altmetric-Citations40LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (880 KB) Get e-Alerts

Poly(phthalidylidenearylene)s. Gelation in the homopolycondensation of 3‐aryl‐3‐chlorophthalides

AbstractThe Friedel‐Crafts homopolycondensations of 3‐(4‐biphenyly)‐3‐chlorophthalide (1 a), 3‐(2‐fluorenyl)‐3‐chlorophthalide (1 b), 3‐(4‐phenoxyphenyl)‐3‐chlorophthalide (1 c), 3‐(4‐phenylthiophenyl)‐3‐chlorophthalide (1 d) were studied. The homopolycondensation of 1 a was found to give linear, fully soluble polymers. A gelation, depending on reaction parameters, can occur in the reactions of 1 b–1 d. The methylene group of 1 b was shown to be reactive in the course of synthesis, giving cross‐links. Using the NMR pulsed gradient spin echo method, it was found, that degradation of macromolecular chains in the homopolycondensation of 1 c is followed by gelation. A proposed mechanism of gelation is based on the transformation of intermediate acyloxonium complexes. Similar transformations of acyl‐sulfonium complexes are suggested to occur in the course of the homopolycondensation of 1 d.

Non-spherical micelles in the sodium dodecylsulfate–brine system. A fluorescence quenching and nuclear magnetic resonance study

The size of elongated micelles in a system containing 2 wt.% sodium dodecylsulfate (SDS), 3.5 wt.% NaCl and 94.5 wt.% water has been studied by means of two different nuclear magnetic resonance (NMR) methods and time-resolved fluorescence quenching with special emphasis on a comparison between the results from the three methods. The two NMR methods were the determination of self-diffusion coefficients by means of the pulsed-gradient spin–echo method and the determination of relaxation rates for specifically deuterium labelled SDS. In the analysis of the former data, the obstruction effect due to the finite micellar concentration was taken into account. In the analysis of the relaxation data, the combined effect of lateral surfactant diffusion over the micellar surface and micellar rotational tumbling was taken into account in an exact manner for the case of prolates and in an approximate way for the case of hemisphere-capped rods. The results obtained from the three methods were in reasonable agreement. If the micelles are modelled as prolates, the axial ratios obtained was ca. 6 while if they are described as hemisphere-capped rods, the axial ratio is ca. 5.

Water diffusion in wood pulp cellulose fibers studied by means of the pulsed gradient spin-echo method

The self-diffusion of water sorbed in wood pulp cellulose fibers was studied by using the pulsed gradient spin-echo (PGSE) method. The observed echo attenuation profiles deviate significantly from those of bulk liquids and can be decomposed into two components: one with a self-diffusion coefficient independent of the diffusion time, and one with an apparent diffusion coefficient that depends on the diffusion time. The relative amplitude of these two components for the different diffusion times used in the PGSE experiment is nearly constant. We attribute the two components to bulk water between cellulose fibers and to water in pores within the fibers, respectively. From the relative amplitude of the components and the known water content in the samples, the amount of water in pores inside the fibers was calculated to 1.4 g/g of cellulose fibers. The self-diffusion coefficient of the bulk water between the fibers is lower than that of neat water. This reduction is mainly caused by the obstruction effect of the cellulose fibers whereas the hydration effect is of minor importance. The apparent self-diffusion coefficient of water trapped in pores inside fibers is approximately one-third of that of the bulk water between fibers when the diffusion time is 12 ms and is reduced further by another factor of 3 when the diffusion time is increased to 80 ms. By using a sheet sample and applying the magnetic field gradient perpendicular or parallel to the sheet it was found that the diffusional motion of water in pores is anisotropic. These results indicate that the pores are elongated along the fiber axis having lengths ranging from a few micrometers up to 20 μm.

Perfusion and diffusion MR imaging.

Diffusion-weighted images of the rat brain were obtained using the pulsed-gradient spin-echo method. An attempt was made to extract perfusion-related parameters from signal intensity data taken from the caudate-putamen region of the images, by using a nonlinear least-squares calculation to fit the Le Bihan biexponential expression (Le Bihan et al., Radiology, 168, 497 (1988)) to the data. The perfusion-related parameters could not be obtained with sufficient accuracy to be useful, although the perfusion-weighted images appear to contain meaningful qualitative information. An analysis of the perfusion model is presented and shows why the Le Bihan pseudo-diffusion coefficient is particularly difficult to measure with reasonable accuracy.