Pulsed Gradient Spin Echo Research Articles

Synthesis and characterization of bis(amine)palladium(II) carboxylate complexes as precursors of palladium nanoparticles.

The synthesis and characterization of the adducts of n-alkyl amine and palladium n-alkyl carboxylate, [Pd(R2NH2)2(R1COO)2] (R1 = 1, 7, and 11; R2 = 8, 12, and 16), as precursors for the synthesis of palladium nanoparticles (PdNPs) was carried out via differential scanning calorimetry, FT-IR, Raman and UV-Vis spectroscopy, NMR spectroscopy (1H, 13C pulsed field gradient spin-echo (PGSE), and 13C CP-MAS), and powder X-ray diffraction. Pd n-alkyl carboxylates were obtained by a ligand exchange reaction from palladium acetate and the appropriate aliphatic carboxylic acid. It is proposed that carboxyl moieties in the presence of amine ligands are bound to palladium ions via monodentate bonding as opposed to bridging bidentate coordination of pure palladium carboxylate which exists in the form of polymer aggregates. All the studied palladium carboxylate/amine complexes form bilayer lamellar structures and exhibit first-order melting transitions. The evidence presented in this study shows that the phase behavior of bivalent metal carboxylates is mainly controlled by the type of coordination of carboxylate head groups. For n-alkyl carboxylates, linear chain type aggregates replace the trimeric units of Pd acetate. In solution, in the presence of amine, palladium salt aggregates disintegrate and the Pd complex is isolated and stabilized by amine molecules. Using bis(amine) palladium carboxylate adducts as precursors, palladium nanoparticles were fabricated. During high temperature thermolysis, the bis(amine) Pd carboxylate complex decomposes to form small sized Pd nanoparticles. Combining NMR techniques with FTIR spectroscopy, it was possible to follow an original stabilization mechanism. PdNPs are stabilized by weakly interacting long chain aliphatic amide and carboxylic acid derived from the palladium precursor.

The unseen evidence of Reduced Ionicity: The elephant in (the) room temperature ionic liquids

The unambiguous quantification of the proton transfer in Protic Ionic Liquids (PILs) and its differentiation from the concept of ionicity are still unsolved questions. Albeit researchers awfully quickly treat them as synonyms, the two concepts are intrinsically different and imply a dramatic modification in the expected chemical and physical properties of a PIL. Some attempts have been made to shed light on this discrimination, but single-technique-based approaches fail in giving a clear answer. Aiming at definitively figuring out the differentiation between proton transfer and ionicity, we performed a multi-technique analysis (NMR, Raman, IR, thermal and electrochemical analyses, among others). Indeed, thermal and spectroscopic analyses are employed to determine the acid strength's role in ions' complete formation. To overcome the ambiguity between ionicity and formation degree, we introduce a new paradigm where Reduced Ionicity accounts for both the quantities mentioned above. The reduced ionicity directly affects the thermal stability, the phase behavior, and the spectroscopic observations, resulting in particular features in NMR and vibrational spectra. The combination of physical-chemical analyses and Pulsed-Gradient Spin-Echo (PGSE) NMR allows determining the reduced ionicity (and not the ionicity, as reported so far) of the investigated systems. In this context, being the proton transfer not quantitatively accessible directly, the reduced ionicity of a reference series of triethylamine-based PILs is investigated through transport properties as a function of temperature. Our findings point towards a substantial dependence of the reduced ionicity by the acid strength and the anion's coordination power. Furthermore, some interesting insights about the proton transfer are obtained, combining all the findings collected.

Dual-Cation Electrolyte System Using Quaternary Ammonium Salts and Ionic Liquid for High Power and High Energy Li4Ti5O12//AC Hybrid Capacitor System

[Introduction] The Use of thick electrodes (> 100 µm) is a simple approach to increase the energy density of electrochemical energy storage devices. However, the high ionic resistance in thick electrodes limits the high-power applications of such devices. “Dual-cation” electrolytes for thick-electrode hybrid capacitor systems, typically composed of Li4Ti5O12 (LTO, 200 µm) and activated carbon (AC, 400 µm) electrodes, have been developed[1]. A dual-cation electrolyte is composed of lithium tetrafluoroborate (LiBF4) and spiro-(1,1')-bipyrrolidinium tetrafluoroborate (SBPBF4) dissolved in propylene carbonate (PC). An SBP-based dual-cation electrolyte, comprising 1 M LiBF4 + 2 M SBPBF4 / PC, exhibits a higher total ionic conductivity (7.67 S cm-1) than the single-cation electrolyte (3.40 S cm-1), consisting of 1 M LiBF4 / PC, while Li+ conductivity decreases with an increase of SBPBF4 concentration. Our interesting finding was that the presence of SBP+, despite the low Li+ transport in the electrolyte bulk, reduces overvoltage associated with migration limitation in the thick LTO electrode, and thus enhances power density in LTO//AC hybrid capacitor systems.In this study, we propose another dual-cation electrolyte using a 1-ethyl-3-methylimidazolium (EMI) cation in place of the SBP cation, which further enhances the power performance of LTO//AC hybrid capacitor systems. It has been shown that an EMI-based electrolyte exhibits a higher total ionic conductivity (10.9 S cm-1) than the SBP-based electrolyte. Herein we provide a detailed account of factors that improve the discharge properties of the EMI-based dual-cation electrolyte by using electrochemical and spectroscopic methods.[Experimental] The dual-cation electrolytes were prepared by dissolving 1 M LiBF4 and x M SBPBF4 (0 < x < 3) or x' M EMIBF4 (0 < x' < 4), in PC. The ionic conductivity of the prepared electrolytes was measured using a conductivity meter. The LTO(200 μm)//AC(400 μm) hybrid capacitor system assumed a coin type cell assembly; charge-discharge measurements were conducted at 1 mA cm-2 (at the charge) and 1 to 200 mA cm-2 (at the discharge). Pulsed Gradient Spin Echo (PGSE) NMR analysis was conducted to determine the self-diffusion coefficients of the Li(7Li), SBP (1H), and EMI (1H) cations and the BF4 (19F) anion in the electrolytes.. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the resistance of the LTO electrode using LTO//LTO symmetric cells designed at both the delithiated (SOC of LTO = 0%) and lithiated (SOC of LTO = 25%) states.[Results and discussion]The results of the ionic conductivity for all electrolytes are shown in Figure 1. The EMI-based dual-cation electrolyte exhibited higher ionic conductivity than the SBP-based electrolyte. The results of the charge-discharge test (Figure 2) showed that the EMI-based dual-cation electrolyte exhibits optimal discharge properties among the electrolytes. Moreover, the PGSE NMR analysis results indicated that the self-diffusion coefficient for all ions including Li+ decreased with an increase in EMIBF4 concentration. In contrast, the ionic resistance (R ion) of the LTO negative electrode calculated from the EIS results at the delithiated state (SOC of LTO = 0%) yielded lower values for the EMI-based dual-cation electrolyte (11.1 Ω cm2) than that for the SBP-based (14.0 Ω cm2) and single-cation (27.1 Ω cm2) electrolytes. We also evaluated the charge-transfer resistance (R ct) from the EIS results using LTO//LTO symmetric cells at the lithiated state (SOC of LTO = 25%). The R ct was found to be lower in the EMI-based dual-cation electrolyte (1.71 Ω cm2) than that in the SBP-based (2.28 Ω cm2) and single-cation (3.57 Ω cm2) electrolytes. These results showed that charge-transfer reactions at the LTO electrodes occur more readily in the dual-cation electrolyte.The results indicated that a reduction in R ct occurs because ion potential suppression occurs owing to the accelerated ion transport in the electrode, as well as in our previous study[1]. The suppression of the ionic potential increased the Li+ concentration and reaction current at the collector side, which decreased the R ct. This is because the advantage of an increase in total ionic conductivity is effective even when the transport ofLi+ is reduced in a thick-electrode system. This study confirms that the use of the EMI-based electrolyte, which has a higher total ionic conductivity than the SBP-based electrolyte, exhibits a remarkable suppression of R ion and R ct. The use of thick electrodes, therefore, facilitates an increase in total ionic conductivity, which has a more desirable effect than that of high Li+ conductivity, especially in the EMI-based dual-cation electrolyte, wherein the power density of the thick-electrode LTO//AC hybrid capacitor systems was significantly improved.[1] Y. Chikaoka et al., J. Phys. Chem. C, in press(doi.org/10.1021/acs.jpcc.0c01916). Figure 1

How Does the Solvent Composition Influence the Transport Properties of Electrolyte Solutions?

The electrolyte solutions used in lithium ion batteries have been optimized based on the empirical knowledge and concept. Yet the ion transport phenomena in the electrolyte solution, which composed of mixed solvent with salt concentration of about 1 mol kg−1, are complicated and significantly different from those in dilute solutions. In the present study, the characteristics of the electrolyte solutions are reanalyzed by using physical and spectroscopic data for the electrolyte solutions, which were obtained from our experiment, and by referring to recent theoretical studies 1,2) and the experimentally determined diffusion coefficient in previous studies 3,4).We experimentally measured the viscosity η and ionic conductivity σ of the electrolyte solutions of 1 mol kg−1 of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSA) dissolved in the binary mixture solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in the temperature range of 288 ≤ T/K ≤ 328 with varying the EC content from 0 to 60 vol.%, which translates into the molar fraction of EC in 0 ≤ x EC ≤ 0.6 and the solute to solvent ratio constant (total ca. 11 solvent molecules are present per solute in the system). The diffusion coefficients D of each species, Li+, PF6, FSA−, EC and DMC, were determined by pulse gradient spin echo NMR. The state of molecules around Li+ was examined by the Raman spectra of the solvents and anions.With the increase in x EC, DMC solvating Li+ is replaced by EC, resulting in the increase in the total number of solvents around Li+. Reanalysis of Raman spectra based on Borodin's calculations 2) reassessed the solvation numbers and confirmed that the preferential coordination of EC to Li+, but the extent of the preference was not as strong as previously conceived. The addition of EC, more viscous fluid than DMC, into the system leads to the increase in η, which should be responsible for the monotonous decrease in D of all the species with x EC. On the other hand, σ initially increases until marking the maximum at x EC = ~0.3. The opposite trend between D and σ, i.e., dD/dx EC < 0 while dσ/dx EC > 0, at low x EC suggests that the Nernst-Einstein relation does not hold as it ignores the inter-ionic correlations. The analysis of the Stokes radii r St based on η and D obtained in the present study implies that the increase in x EC results in the decrease in r St for all the species, among which the anions are most influenced. Although only an equivocal picture on the cation-anion correlation, such as the ion pair formation, was obtained from the Raman spectroscopy, the present study is consistent with the view that the highly Li+-solvating EC, with its higher dielectric shielding effect than DMC, liberates the anions from the attraction from Li+, enhancing the cation-anion anti-correlation, which positively contributes to the ionic conductivity until the viscosity prevails at high x EC. The Einstein-Stokes formula and van der Waals radius are certainly "classical" concepts; nonetheless, if properly formulated, they can be consistent with the solvation number obtained from Raman spectra not only qualitatively but also semi-quantitatively. References Borodin, G. V. Zhuang, P. N. Ross and K. Xu, Phys. Chem. C, 117, 7433 – 7444 (2013).Borodin, M. Olguin, P. Ganesh, P. R. C. Kent, J. L. Allen and W. A. Henderson, Chem. Chem. Phys., 18, 164 – 175 (2016).Hayamizu, Y. Aihara, S. Arai, C. Garcia-Martinez, J. Phys. Chem. B, 103, 519 – 524 (1999).Hayamizu, J. Chem. Eng. Data, 57, 2012 – 2017 (2012). Figure 1

New options for increasing the sensitivity, specificity and scope of synergistic contrast magnetic resonance imaging (scMRI) using Multiplied, Added, Subtracted and/or FiTted (MASTIR) pulse sequences.

This paper reviews magnetic resonance (MR) pulse sequences in which the same or different tissue properties (TPs) such as T1 and T2 are used to contribute synergistically to lesion contrast. It also shows how synergistic contrast can be created with Multiplied, Added, Subtracted and/or fiTted Inversion Recovery (MASTIR) sequences, and be used to improve the sensitivity, specificity and scope of clinical magnetic resonance imaging (MRI) protocols. Synergistic contrast can be created from: (i) the same TP, e.g., T1 used twice or more in a pulse sequence; (ii) different TPs such as ρm, T1, T2, and D* used once or more within a sequence, and (iii) additional suppression or reduction of signals from tissues and/or fluids such as fat, long T2 tissues and cerebrospinal fluid (CSF). The short inversion time (TI) inversion recovery (IR) (STIR) and double IR (DIR) sequences usually show synergistic positive contrast for lesions which have increases in both T1 and T2. The diffusion weighted pulsed gradient spin echo (PGSE) sequence shows synergistic contrast for lesions which have an increase in T2 and a decrease in D*; the sequence is both positively weighted for T2 and negatively weighted for D*. In the brain, when an IR sequence nulling white matter has subtracted from it an IR sequence nulling gray matter to form the subtracted IR (SIR) sequence, increases in the single TP T1 between the two nulling points of the original two sequences generate high synergistic positive contrast. In addition, the subtraction to produce the SIR sequence reduces fat and CSF signals. To provide high sensitivity to changes in TPs in disease the SIR sequence can be used (i) alone to provide synergistic T1 contrast as above; (ii) with T2-weighting to provide synergistic T1 and T2 contrast, and (iii) with T2- and D*-weighting to provide synergistic T1, T2, and D* contrast. The SIR sequence can also be used in reversed form (longer TI form minus shorter TI form) to produce very high positive synergistic T1 contrast for reductions in T1, and so increase the positive contrast enhancement produced by clinical gadolinium-based contrast agents (GBCAs) when they reduce T1. The specificity of MRI examinations can be improved by using the reversed SIR sequence with a long echo time (TE) gradient echo as well as echo subtraction to show synergistic high contrast from T1 and T2* shortening produced by organic iron. Other added and subtracted forms of the MASTIR sequence can be used synergistically to selectively show myelin, myelin water and fluids including blood and CSF. Protocols using MASTIR sequences to provide synergistic contrast in MRI of the brain, prostate and articular cartilage are included as illustrative examples, and the features of synergistic contrast MRI (scMRI) are compared to those of multiparametric MRI (mpMRI) and functional MRI (fMRI).

Unravelling the role of a green rejuvenator agent in contrasting the aging effect on bitumen: A dynamics rheology, nuclear magnetic relaxometry and self-diffusion study

This paper evaluated the potentialities of a green and biocompatible rejuvenator agent (HR) in conferring an appreciable resistance against the effects caused by artificial aging on a given bitumen. Both neat and aged bitumens were analyzed and compared to analogous samples modified with HR. Control samples containing a vegetable oil as softening agent were also tested for comparison. The tested samples were subjected to a second aging cycle. Structural differences between the samples were carried out through an inverse Laplace transform of the NMR spin-echo decay (T2) and self-diffusion measurements by pulsed gradient spin echo nuclear magnetic resonance (PGSE-NMR) spectroscopy. In addition, dynamic rheological analyses were conducted to determine the dependence of the gel-sol transition temperature on both the type of additive and ageing process. The present study clearly highlighted the fact that artificial ageing, realized here by the rolling thin film oven test (RTFOT) and the pressure ageing vessel (PAV) test, induced important structural modifications. The analysis of relaxation times and self-diffusion coefficients indicated that ageing promoted the formation of molecular populations characterized by a shift of the distribution toward higher molecular weights compared to unaged bitumen. Diffusion data showed also an Arrhenius-like temperature dependence. A correlation between all the data was attempted to understand the role of the investigated additives. The eco-friendly biocompatible rejuvenator helped not only to restore the structure of the aged bitumen, but even slowed down the processes of a second aging (aiming at the first aged sample).

Limits to flow detection in phase contrast MRI

Pulsed gradient spin echo (PGSE) complex signal behavior becomes dominated by attenuation rather than oscillation when displacements due to flow are similar or less than diffusive displacements. In this “slow-flow” regime, the optimal displacement encoding parameter q for phase contrast velocimetry depends on the diffusive length scale qslow=1/lD=1/2DΔ rather than the velocity encoding parameter venc=π/(qΔ). The minimum detectable mean velocity using the difference between the phase at +qslow and −qslow is 〈vmin〉=1/SNRD/Δ. These theories are then validated and applied to MRI by performing PGSE echo planar imaging experiments on water flowing through a column with a bulk region and a beadpack region at controlled flow rates. Velocities as slow as 6 μm/s are detected with velocimetry. Theories, MRI experimental protocols, and validation on a controlled phantom help to bridge the gap between porous media NMR and pre-clinical phase contrast and diffusion MRI.

Structural and Li-ion diffusion properties of lithium tantalum phosphate LiTa2PO8

We synthesized a polycrystalline LiTa2PO8 sample at 1323 K by a solid-state reaction. The crystal structure of LiTa2PO8 was examined by selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) techniques. Electrochemical impedance measurements showed that the total conductivity at room temperature was 4.85 × 10−4 S cm−1 and the activation energy was 0.52 eV. The high Li-ion conductive properties were confirmed by 7Li pulse-field gradient (PFG) or pulse-gradient spin-echo (PGSE) NMR spectroscopy. The activation energy of the Li diffusion (0.51 eV) measured by PFG NMR was well consistent with the result of impedance measurement. We believe that the LiTa2PO8 would be as one of the important candidates for the solid electrolyte used in all solid-state lithium batteries.

New and rapid pulse sequences for two-dimensional D-T1 correlation measurements

Longitudinal relaxation time (T1), transverse relaxation time (T2) and diffusion coefficient (D) values have been widely used for the characterizations of materials using low field Time Domain Nuclear Magnetic Resonance (TD-NMR). Each parameter can be determined using one-dimensional techniques or their values and correlations by multi-dimensional experiments such as T1-T2, D-T2, and T1-D-T2. In this work, we studied four D-T1 sequences for TD-NMR combining Stejskal-Tanner Pulse Gradient Spin Echo (PGSE) diffusion measurement with Inversion-Recovery (IR), Saturation-Recovery (SR), Small-Angle Continuous Wave Free Precession (CWFP-T1) and Small-Angle Flip-Flop (SAFF) for T1 measurement. The results show that rapid D-T1 measurements can be obtained with single shot CWFP-T1 and SAFF sequences. The two sequences were two and eight time fast than sequences based on SR and IR, respectively. Although the two fast sequences yield low signal-to-noise ratio signal, they can be as fast as the traditional D-T2 experiment, or even faster, because it is not necessary to wait a recycle delay of 5 T1. Another advantage of the CWFP-T1 and SAFF methods, when compared to the one based on SR or CPMG (for D-T2) are the low specific absorption rate (SAR) of these sequences due the low flip angles in the sequences, that reduces the sample heating problem. These sequences were initially studied using phantom samples. They also were used to study plant tissues to observe the anisotropic diffusion in asparagus. Therefore, they can be useful methods for practical application in TD-NMR.

Pulse sequences as tissue property filters (TP-filters): a way of understanding the signal, contrast and weighting of magnetic resonance images.

This paper describes a quantitative approach to understanding the signal, contrast and weighting of magnetic resonance (MR) images. It uses the concept of pulse sequences as tissue property (TP) filters and models the signal, contrast and weighting of sequences using either a single TP-filter (univariate model) or several TP-filters (the multivariate model). For the spin echo (SE) sequence using the Bloch equations, voxel signal intensity is plotted against the logarithm of the value of the TPs contributing to the sequence signal to produce three TP-filters, an exponential ρm-filter, a low pass T1-filter and a high pass T2-filter. Using the univariate model which considers signal changes in only one of ρm, T1, or T2 at a time, the first partial derivative of signal with respect to the natural logarithm of ρm, T1 or T2 is the sequence weighting for each filter (for small changes in each TP). Absolute contrast is then the sequence weighting multiplied by the fractional change in TP for each filter. For large changes in TPs, the same approach is followed, but using the mean slope of the filter as the sequence weighting. These approaches can also be used for fractional contrast. The univariate TP-filter model provides a mathematical framework for converting conventional qualitative univariate weighting as used in everyday clinical practice into quantitative univariate weighting. Using the multivariate model which considers several TP-filters together, the relative contributions of each TP to overall sequence and image weighting are expressed as sequence and imaging weighting ratios respectively. This is not possible with conventional qualitative weighting which is univariate. The same approaches are used for inversion recovery (IR), pulsed gradient SE, spoiled gradient echo (SGE), balanced steady state free precession, ultrashort echo time and other pulse sequences. Other TPs such as susceptibility, chemical shift and flow can be included with phase along the Y axis of the TP-filter. Contrast agent effects are also included. In the text TP-filters are distinguished from k-space filters, signal filters (S-filters) which are used in imaging processing as well as to describe windowing the signal width and level of images, and spatial filters. The TP-filters approach resolves many of the ambiguities and inconsistencies associated with conventional qualitative weighting and provides a variety of new insights into the signal, contrast and weighting of MR images which are not apparent using qualitative weighting. The TP-filter approach relates the preparation component of pulse sequences to voxel signal, and contrast between two voxels. This is complementary to k-space which relates the acquisition component of pulse sequences to the spatial properties of MR images and their global contrast.

Study of ion dynamics of LiI·6H2O in the supercooled liquid state using NMR spectroscopy and ionic conductivity measurements

Ion dynamics in the liquid and supercooled liquid state of LiI·6H2O were studied by nuclear magnetic resonance (NMR) spectroscopy and ionic conductivity measurements. The self-diffusion coefficients of the water molecules and lithium ions were measured by 1H and 7Li pulsed gradient spin echo NMR (PGSE-NMR). The temperature dependences of the DC ionic conductivity and diffusion coefficients of lithium ions and water molecules were well-fitted by the Vogel-Tamman-Fulcher (VTF) law, similar to those of the LiCl·RH2O system. The conductivity and lithium diffusion coefficients of the LiI·6H2O system were more than twice those of LiCl·7H2O in the supercooled liquid state below 200 K, which could be attributed to the larger size of the iodide anions than that of chloride anions. The temperature dependence of the correlation times calculated from NMR T1 relaxation deviated from the VTF law at low temperatures because of the local orientational fluctuation of the water molecules.

Comparison of PGSE and STEAM DTI acquisitions with varying diffusion times for probing anisotropic structures in human kidneys.

To evaluate the sensitivity of stimulated-echo acquisition mode (STEAM) and pulsed-gradient spin-echo (PGSE) diffusion tensor imaging (DTI) acquisitions with different diffusion times for measuring renal tissue anisotropy. Twelve healthy volunteers underwent an MRI examination at a 3T scanner including STEAM and PGSE DTI with variable diffusion times Δ (20.3, 37 and 125ms). Three volunteers were scanned twice to test the reproducibility for repeated examinations. Diffusion parameters fractional anisotropy (FA) and apparent diffusion coefficient (ADC) in the automatically segmented cortical and medullary regions of interests in both kidneys were calculated and averaged over all subjects for further analysis. Moreover, 5-grade qualitative evaluation of the FA and ADC maps from each sequence was conducted by two experienced radiologists in a consensus. The cortex-medulla difference in the STEAM sequence was significantly higher than that in PGSE with short ∆=20.3ms (P<0.001) and in PGSE with intermediate ∆=37ms (P<0.05) diffusion times. Reproducibility of the FA/ADC measurements was very good and comparable for all acquisition modes investigated. For the FA maps, the PGSE sequence with intermediate diffusion time scored highest in the subjective visual assessment of radiologists. The delineation of anisotropy in renal tissue is depending on the used diffusion time of the DTI sequence. A PGSE acquisition at a diffusion time of about 37ms provides reproducible results with optimal corticomedullary contrast in FA and ADC maps and good image quality.

Diffusion-time dependence of diffusional kurtosis in the mouse brain.

To investigate diffusion-time dependency of diffusional kurtosis in the mouse brain using pulsed-gradient spin-echo (PGSE) and oscillating-gradient spin-echo (OGSE) sequences. 3D PGSE and OGSE kurtosis tensor data were acquired from ex vivo brains of adult, cuprizone-treated, and age-matched control mice with diffusion-time (tD ) ~ 20 ms and frequency (f) = 70 Hz, respectively. Further, 2D acquisitions were performed at multiple times/frequencies ranging from f = 140 Hz to tD = 30 ms with b-values up to 4000 s/mm2 . Monte Carlo simulations were used to investigate the coupled effects of varying restriction size and permeability on time/frequency-dependence of kurtosis with both diffusion-encoding schemes. Simulations and experiments were further performed to investigate the effect of varying number of cycles in OGSE waveforms. Kurtosis and diffusivity maps exhibited significant region-specific changes with diffusion time/frequency across both gray and white matter areas. PGSE- and OGSE-based kurtosis maps showed reversed contrast between gray matter regions in the cerebellar and cerebral cortex. Localized time/frequency-dependent changes in kurtosis tensor metrics were found in the splenium of the corpus callosum in cuprizone-treated mouse brains, corresponding to regional demyelination seen with histological assessment. Monte Carlo simulations showed that kurtosis estimates with pulsed- and oscillating-gradient waveforms differ in their sensitivity to exchange. Both simulations and experiments showed dependence of kurtosis on number of cycles in OGSE waveforms for non-zero permeability. The results show significant time/frequency-dependency of diffusional kurtosis in the mouse brain, which can provide sensitivity to probe intrinsic cellular heterogeneity and pathological alterations in gray and white matter.

Oscillating diffusion-encoding with a high gradient-amplitude and high slew-rate head-only gradient for human brain imaging.

We investigate the importance of high gradient-amplitude and high slew-rate on oscillating gradient spin echo (OGSE) diffusion imaging for human brain imaging and evaluate human brain imaging with OGSE on the MAGNUS head-gradient insert (200 mT/m amplitude and 500 T/m/s slew rate). Simulations with cosine-modulated and trapezoidal-cosine OGSE at various gradient amplitudes and slew rates were performed. Six healthy subjects were imaged with the MAGNUS gradient at 3T with OGSE at frequencies up to 100 Hz and b = 450 s/mm2 . Comparisons were made against standard pulsed gradient spin echo (PGSE) diffusion in vivo and in an isotropic diffusion phantom. Simulations show that to achieve high frequency and b-value simultaneously for OGSE, high gradient amplitude, high slew rates, and high peripheral nerve stimulation limits are required. A strong linear trend for increased diffusivity (mean: 8-19%, radial: 9-27%, parallel: 8-15%) was observed in normal white matter with OGSE (20 Hz to 100 Hz) as compared to PGSE. Linear fitting to frequency provided excellent correlation, and using a short-range disorder model provided radial long-term diffusivities of D∞,MD = 911 ± 72 µm2 /s, D∞,PD = 1519 ± 164 µm2 /s, and D∞,RD = 640 ± 111 µm2 /s and correlation lengths of lc,MD = 0.802 ± 0.156 µm, lc,PD = 0.837 ± 0.172 µm, and lc,RD = 0.780 ± 0.174 µm. Diffusivity changes with OGSE frequency were negligible in the phantom, as expected. The high gradient amplitude, high slew rate, and high peripheral nerve stimulation thresholds of the MAGNUS head-gradient enables OGSE acquisition for in vivo human brain imaging.

Diffusion time dependency along the human corpus callosum and exploration of age and sex differences as assessed by oscillating gradient spin-echo diffusion tensor imaging

Conventional diffusion imaging uses pulsed gradient spin echo (PGSE) waveforms with diffusion times of tens of milliseconds (ms) to infer differences of white matter microstructure. The combined use of these long diffusion times with short diffusion times (<10 ​ms) enabled by oscillating gradient spin echo (OGSE) waveforms can enable more sensitivity to changes of restrictive boundaries on the scale of white matter microstructure (e.g. membranes reflecting the axon diameters). Here, PGSE and OGSE images were acquired at 4.7 ​T from 20 healthy volunteers aged 20–73 years (10 males). Mean, radial, and axial diffusivity, as well as fractional anisotropy were calculated in the genu, body and splenium of the corpus callosum (CC). Monte Carlo simulations were also conducted to examine the relationship of intra- and extra-axonal radial diffusivity with diffusion time over a range of axon diameters and distributions.The results showed elevated diffusivities with OGSE relative to PGSE in the genu and splenium (but not the body) in both males and females, but the OGSE-PGSE difference was greater in the genu for males. Females showed positive correlations of OGSE-PGSE diffusivity difference with age across the CC, whereas there were no such age correlations in males. Simulations of radial diffusion demonstrated that for axon sizes in human brain both OGSE and PGSE diffusivities were dominated by extra-axonal water, but the OGSE-PGSE difference nonetheless increased with area-weighted outer-axon diameter. Therefore, the lack of OGSE-PGSE difference in the body is not entirely consistent with literature that suggests it is composed predominantly of axons with large diameter. The greater OGSE-PGSE difference in the genu of males could reflect larger axon diameters than females. The OGSE-PGSE difference correlation with age in females could reflect loss of smaller axons at older ages. The use of OGSE with short diffusion times to sample the microstructural scale of restriction implies regional differences of axon diameters along the corpus callosum with preliminary results suggesting a dependence on age and sex.

Quantitative analysis of the interaction of ammonia with 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ionic liquid. Understanding the volumetric and transport properties of their mixtures

We used near infrared spectroscopy (NIR) and nuclear magnetic resonance, 1H NMR and 19F NMR to analyze the structural changes of 1-(2-hydroxyethyl)-3-methylimidazolium tetra fluoroborate ionic liquid (IL) when it interacted with ammonia. The NIR spectra of the ionic liquid recorded after it had interacted with ammonia gas at different pressures (between 1 and 6bar) are resolved using multivariate curve resolution (MCR-ALS) to quantitatively evaluate the interaction. The analysis was done at 20°C, 40°C and 60°C.The results show that the interaction of the ammonia with the ionic liquid is governed by the temperature. The interaction preferentially occurs through the hydroxyl group in the cation of the ionic liquid. The interaction through the anion is favored only at lower temperatures. The MCR-ALS results have shown that at 20°C, when the ammonia mole fraction in the mixture is higher than 0.564, ammonia molecules not associated with the ions are also present.The diffusion coefficients of the ions in NH3+IL mixtures prepared at 2.5bar and 6.0bar and in neat IL have been calculated from pulsed gradient spin echo (PGSE)-NMR experiments. The differences between them are discussed from the perspective of the quantitative analysis of the interaction between the NH3 and the IL. The results discussed throughout the study shed light on the trends of the excess molar volume and viscosity data reported in the literature for the mixture of ammonia and the ionic liquid studied.

Influence of preferred orientation of clay particles on the diffusion of water in kaolinite porous media at constant porosity

Solute transport in natural or artificial compacted clay porous media is receiving particular attention in the contexts of waste storage and the design of materials with tuneable physical properties. In these contexts, the porosity is commonly considered as a primary parameter controlling the diffusional properties of water and solutes in these systems. However, little attention has been given to the role played by anisotropy in the particle orientation. In this study, the influence of the preferred orientation of clay particles on the water diffusion anisotropy in two kaolinite porous media obtained by compaction and centrifugation methods (for a constant porosity value of ~0.5) was investigated by coupling experiments and simulations. An increase in the preferred orientation of kaolinite particles, as quantified by X-ray scattering analysis, was found to be logically associated with an enhanced anisotropy in water diffusion obtained from pulsed gradient spin echo attenuation measurements by nuclear magnetic resonance of protons. Brownian dynamics simulations performed on three-dimensional virtual porous media, mimicking the shape and orientation of the particles in the samples, led to calculated water diffusion coefficients in agreement with experimental data. Once validated, this computational work was extended to a wide range of degrees in the preferred orientation of particles. The results showed that this parameter leads to an increase and a decrease in pore water diffusion coefficients along and across the mean orientation plane, respectively, up to a factor ~2. The directional diffusion anisotropy was also found to range between 1 and ~5 for the most isotropic and anisotropic organisations, respectively. This study hence provides quantitative insights into the impact of the preferred orientation for the prediction of water diffusion in compacted clay media.

Analysis of Mechanical Performance of Bitumen Modified with Waste Plastic and Rubber Additives by Rheology and Self Diffusion NMR Experiments

In this study, the mechanical and physico-chemical properties of a new kind of modified bitumen are presented. The bituminous binders have been modified in order to understand the effect on the structural properties of several compounds such as a Polymer elastomer as Styrene Butadiene Rubber (SBR), Polymer thermoplastic polypropylene (PP) and a waste plastic (Waste PP). Laboratory tests have been focused on the characterization of bitumen modified with single product and their binary combinations compared with pristine binder as a reference. Characterization has been conducted by using conventional as well as advanced methods on bitumens. Fundamental rheological tests, based on dynamic shear rheometer in the temperature range from -30 °C to +160 °C have been performed and the structure of a bitumens and modified bitumens has been analysed by the mobility of the oily maltene by self-diffusion Pulsed field gradient spin-echo (PGSE) FT-NMR experiments.

SpinDoctor: A MATLAB toolbox for diffusion MRI simulation

The complex transverse water proton magnetization subject to diffusion-encoding magnetic field gradient pulses in a heterogeneous medium can be modeled by the multiple compartment Bloch-Torrey partial differential equation. Under the assumption of negligible water exchange between compartments, the time-dependent apparent diffusion coefficient can be directly computed from the solution of a diffusion equation subject to a time-dependent Neumann boundary condition.This paper describes a publicly available MATLAB toolbox called SpinDoctor that can be used 1) to solve the Bloch-Torrey partial differential equation in order to simulate the diffusion magnetic resonance imaging signal; 2) to solve a diffusion partial differential equation to obtain directly the apparent diffusion coefficient; 3) to compare the simulated apparent diffusion coefficient with a short-time approximation formula.The partial differential equations are solved by P1 finite elements combined with built-in MATLAB routines for solving ordinary differential equations. The finite element mesh generation is performed using an external package called Tetgen.SpinDoctor provides built-in options of including 1) spherical cells with a nucleus; 2) cylindrical cells with a myelin layer; 3) an extra-cellular space enclosed either a) in a box or b) in a tight wrapping around the cells; 4) deformation of canonical cells by bending and twisting; 5) permeable membranes; Built-in diffusion-encoding pulse sequences include the Pulsed Gradient Spin Echo and the Oscillating Gradient Spin Echo.We describe in detail how to use the SpinDoctor toolbox. We validate SpinDoctor simulations using reference signals computed by the Matrix Formalism method. We compare the accuracy and computational time of SpinDoctor simulations with Monte-Carlo simulations and show significant speed-up of SpinDoctor over Monte-Carlo simulations in complex geometries. We also illustrate several extensions of SpinDoctor functionalities, including the incorporation of T2 relaxation, the simulation of non-standard diffusion-encoding sequences, as well as the use of externally generated geometrical meshes.

Evidence of the diffusion time dependence of intravoxel incoherent motion in the brain.

To investigate the diffusion time (TD ) dependence of intravoxel incoherent motion (IVIM) signals in the brain. A 3-compartment IVIM model was proposed to characterize 2 types of microcirculatory flows in addition to tissue water in the brain: flows that cross multiple vascular segments (pseudo-diffusive) and flows that stay in 1 segment (ballistic) within TD . The model was first evaluated using simulated flow signals. Experimentally, flow-compensated (FC) pulsed-gradient spin-echo (PGSE) and oscillating-gradient spin-echo (OGSE) sequences were tested using a flow phantom and then used to examine IVIM signals in the mouse brain with TD ranging from ~2.5 ms to 40 ms on an 11.7T scanner. By fitting the model to simulated flow signals, we demonstrated the TD dependency of the estimated fraction of pseudo-diffusive flow and the pseudo-diffusion coefficient (D*), which were dictated by the characteristic timescale of microcirculatory flow (τ). Flow phantom experiments validated that the OGSE and FC-PGSE sequences were not susceptible to the change in flow velocity. In vivo mouse brain data showed that both the estimated fraction of pseudo-diffusive flow and D* increased significantly as TD increased. We demonstrated that IVIM signals measured in the brain are TD -dependent, potentially because more microcirculatory flows approach the pseudo-diffusive limit as TD increases with respect to τ. Measuring the TD dependency of IVIM signals may provide additional information on microvascular flows in the brain.