Mean Relaxation Time Research Articles

Simultaneous multislice cardiac magnetic resonance fingerprinting using low rank reconstruction.

This study introduces a technique for simultaneous multislice (SMS) cardiac magnetic resonance fingerprinting (cMRF), which improves the slice coverage when quantifying myocardial T1, T2 , and M0 . The single-slice cMRF pulse sequence was modified to use multiband (MB) RF pulses for SMS imaging. Different RF phase schedules were used to excite each slice, similar to POMP or CAIPIRINHA, which imparts tissues with a distinguishable and slice-specific magnetization evolution over time. Because of the high net acceleration factor (R=48 in plane combined with the slice acceleration), images were first reconstructed with a low rank technique before matching data to a dictionary of signal timecourses generated by a Bloch equation simulation. The proposed method was tested in simulations with a numerical relaxation phantom. Phantom and in vivo cardiac scans of 10 healthy volunteers were also performed at 3T. With single-slice acquisitions, the mean relaxation times obtained using the low rank cMRF reconstruction agree with reference values. The low rank method improves the precision in T1 and T2 for both single-slice and SMS cMRF, and it enables the acquisition of maps with fewer artifacts when using SMS cMRF at higher MB factors. With this technique, in vivo cardiac maps were acquired from three slices simultaneously during a breathhold lasting 16 heartbeats. SMS cMRF improves the efficiency and slice coverage of myocardial T1 and T2 mapping compared with both single-slice cMRF and conventional cardiac mapping sequences. Thus, this technique is a first step toward whole-heart simultaneous T1 and T2 quantification with cMRF.

Initial evidence for hypothalamic gliosis in children with obesity by quantitative T2 MRI and implications for blood oxygen‐level dependent response to glucose ingestion

In adults, hypothalamic gliosis has been documented using quantitative T2 neuroimaging, whereas functional magnetic resonance imaging (fMRI) has shown a defective hypothalamic response to nutrients. No studies have yet evaluated these hypothalamic abnormalities in children with obesity. Children with obesity and lean controls underwent quantitative MRI measuring T2 relaxation time, along with continuous hypothalamic fMRI acquisition to evaluate early response to glucose ingestion. Children with obesity (N=11) had longer T2 relaxation times, consistent with gliosis, in the mediobasal hypothalamus (MBH) compared to controls (N=9; P=0.004). Moreover, there was a highly significant group*region interaction (P=0.002), demonstrating that signs of gliosis were specific to MBH and not to reference regions. Longer T2 relaxation times correlated with measures of higher adiposity, including visceral fat percentage (P=0.01). Mean glucose-induced hypothalamic blood oxygen-level dependent signal change did not differ between groups (P=0.11). However, mean left MBH T2 relaxation time negatively correlated with glucose-induced hypothalamic signal change (P<0.05). Imaging signs of hypothalamic gliosis were present in children with obesity and positively associated with more severe adiposity. Children with the strongest evidence for gliosis showed the least activation after glucose ingestion. These initial findings suggest that the hypothalamus is both structurally and functionally affected in childhood obesity.

Comparison of native myocardial T1 and T2mapping at 1.5T and 3T in healthy volunteers : Reference values and clinical implications.

SummaryBackgroundMyocardial native T1 and T2 mapping are promising techniques for quantitative assessment of diffuse myocardial pathologies; however, due to conflicting data regarding normal values, routine clinical implementation of this method is still challenging.MethodsTo evaluate this situation during daily clinical practice the characteristics of normal values obtained in 60 healthy volunteers who underwent magnetic resonance imaging (MRI) scans on 1.5T and 3T scanners were studied. The T1 modified look-locker inversion recovery (MOLLI; 5(3)3; modified for higher heart rates) and T2 navigator gated black-blood prepared gradient-spin-echo (GraSE) sequences were used.ResultsWhile age and body mass index did not affect relaxation times, a gender and heart rate dependency was found showing higher T1 and T2 values in females, whereas at higher heart rates a prolongation of T1 and a shortening of T2 relaxation times was found. Particularly prone to artifacts were T2 measurements at 3T and the inferolateral wall. In the individual setting mean relaxation times for T1 were 995.8 ± 30.9 ms at 1.5T and 1183.8 ± 37.5 ms at 3T and 55.8 ± 2.8 ms at 1.5T and 51.6 ± 3 ms at 3T for T2 indicating a high dependency of reference values on MRI protocol when compared to the literature. Furthermore, as presumed mean T1 and T2 values correlated in the same individual.ConclusionsThe T1 and T2 relaxation times depend on physiological factors and especially on MRI protocols. Therefore, reference values should be validated individually in every radiological institution before implementing mapping protocols in daily clinical practice. Correlation of mean T1 and T2 values in the same proband at both field strengths indicates intraindividual reproducibility.

Nonlinear Mechano-Optical Behavior and Strain-Induced Structural Changes of l-Valine-Based Poly(ester urea)s

The uniaxial mechano-optical behavior of a series of amorphous l-valine-based poly(ester urea) (VAL-PEU) with varying diol lengths was studied to elucidate the molecular mechanism associated with their thermal shape memory properties. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during the temporary shape programming at stretching temperatures above the glass transition temperature (Tg). The mechano-optical response of VAL-PEUs exhibits an initial photoelastic behavior related to enhanced segmental correlation at low temperatures above the Tg. A characteristic temperature, defined as the liquid–liquid (Tll) transition (rubbery–viscous transition), was found at about 1.05 Tg (K) (at Tg + 15 °C) at strain rate of 0.017 s–1, above which the segmental contacts largely “melt” and the initial slope of the stress-optical curves becomes temperature independent. This temperature corresponds to the temperature where mean relaxation time for the polymer i...

Inter- and Intraobserver reproducibility of T2 relaxation times of the discus interpubicus: A feasibility study at 3 Tesla

ObjectiveTo quantify standard values of the discus interpubicus in healthy subjects and to determine reliability and repeatability using T2 relaxation time measurements at 3T.Methods20 asymptomatic participants (10 male, 10 female; mean age: 27.3 years ±4.1, BMI: 22.2 ±1.8) underwent a 3T Magnetic Resonance Imaging (MRI) of the pelvic region in a supine position. We included sagittal and para-axial T2w sequences centred over the pubic symphysis in order to identify the complete discus interpubicus. For quantitative analysis, a multi-echo Turbo Spin Echo (TSE) sequence (including 12 echo times between 6.4 and 76.8 ms) was acquired and analysed by using an in-house developed quantification plugin tool (qMapIt) extending ImageJ. Two readers in consensus defined three central slices of the pubic symphysis with the greatest length. For each slice, both readers separately placed three regions-of-interest (ROI) covering the whole discus interpubicus. Both readers repeated the ROI placements in identical fashion after a four-week interval on the original MRI images. Statistical analysis included intraclass correlation coefficient (ICC), nonparametric Wilcoxon test, Fisher exact test and mean relaxation time in ms and 95% confidence intervals.ResultsT2 relaxation time analysis was performed for all 20 participants. In total, a mean relaxation time of all analysed segments for both observers was 48.6 (±6.3 ms), with a mean relaxation time for observer 1 of 48.7 (±6.0 ms) and for observer 2 of 48.5 ms (±6.6ms). The calculated ICC comparing inter- and intrarater reproducibility was excellent in all segments (≥0.75).ConclusionT2 mapping of the discus interpubicus demonstrates good inter- and intrarater repeatability as well as reliability. Mean relaxation times were calculated with 48.6ms in healthy volunteers.

Microstructure and opto-electronic properties of Sn-rich Au-Sn diffusive solders

Microstructural and opto-electronic properties of Au⧹Sn and Sn⧹Au bilayers, obtained by sequential evaporating of metals on the Si substrate, were investigated by means of atomic force microscopy, X-ray diffraction and spectroscopic ellipsometry. Thicknesses of individual films were established to obtain the atomic ratio of Au:Sn atoms 1:1, 1:2 and 1:4, which were favor the formation of AuSn, AuSn2 and AuSn4, respectively. However, the produced intermatallic compounds were detected as AuSn and AuSn2. Additionally, the unbounded Sn was found. The sequence of deposition of Au and Sn films as well as their thickness strongly affect on the composition, microstructure, optical and electrical properties of the produced layers. The Au⧹Sn (Sn on the top) layers were more smooth than Sn⧹Au (Au on the top) films. Generally, the Au⧹Sn layers exhibit a better electrical and optical properties than Sn⧹Au films. The optical parameters: plasma energy, free-carrier damping, mean relaxation time of conduction electrons and optical resistivity were determined from the effective complex dielectric function of the formed Au, Sn and Au-Sn films. The optical resistivity values are in the range from 17.8 μΩ cm to 85.1 μΩ cm and from 29.6 μΩ cm to 113.3 μΩ cm for Au⧹Sn and Sn⧹Au layers, respectively.

Impact of fundamental thermodynamic fluctuations on light propagating in photonic waveguides made of amorphous materials

To improve the detection limit of optical sensors, it is of paramount importance to understand light–matter interaction processes at a fundamental level. At room temperature, the ultimate detection limit is governed amongst others by fundamental thermodynamic fluctuations. Their effect on the properties of light that propagates in amorphous materials is not well understood. Here, we unveil and model for the first time a dominating high-frequency (terahertz-range) noise contribution in the phase of laser light having propagated in optical waveguides, leading to Raman-like sidebands in the optical output spectrum. A salient feature of our approach is to consider a mean relaxation time of the spontaneous random heat flux in the medium, which leads to a spatial correlation of the thermo-refractive noise. The resulting phase noise can be several orders of magnitude larger than what was predicted by earlier models. Our model allows us to explain the origin and specificities of the background that is observed in the Raman optical spectra of silicon nitride waveguides and silica optical fibers. Not only do these findings add a previously unknown dimension to the fundamental knowledge about noise in light–matter interaction, they also need to be taken into account in any optical system in which dynamic fluctuations at the picosecond (ps) or sub-ps level play a role.

A slow atomic diffusion process in high-entropy glass-forming metallic melts

Quasi-elastic neutron scattering has been used to study atomic relaxation processes in high-entropy glass-forming metallic melts with different glass-forming ability (GFA). The momentum transfer dependence of mean relaxation time shows a highly collective atomic transport process in the alloy melts with the highest and lowest GFA. However, a jump diffusion process is the long-range atomic transport process in the intermediate GFA alloy melt. Nevertheless, atomic mobility close to the melting temperature of these alloy melts is quite similar, and the temperature dependence of the diffusion coefficient exhibits a non-Arrhenius behavior. The atomic mobility in these high-entropy melts is much slower than that of the best glass-forming melts at their respective melting temperatures.

Crystallization-Induced Confinement Enhances Glassy Dynamics in Star-Shaped Polyhedral Oligomeric Polysilesquioxane–Isotactic Polystyrene (POSS–iPS) Hybrid Material

In semicrystalline polymers, the segments around the crystallites typically relax significantly slower than in the purely amorphous phase. This results in an, on average, slower dynamics. Here we present a contrary effect in a star-shaped polymer based on a polyhedral oligomeric silesquioxane (POSS) molecule as center and isotactic polystyrene arms. Measurements by means of broadband dielectric spectroscopy reveal a reduction of the mean relaxation time by up to 1 decade. Analyzing the relaxation time distribution unravels three moieties of different dynamics beyond the crystalline fraction. These are assumed to form respective domains: a rigid amorphous fraction around crystallites, a mobile amorphous fraction, and a confined amorphous fraction of enhanced dynamics presumably located around the POSS centers. Probably, the crystallites in combination with the starlike architecture stabilize the average volume which balances the higher density of the growing crystallites by an increase in free volume in the amorphous domains.

Water desorption and absorption isotherms of sodium montmorillonite: A QENS study

Understanding the adsorption, movement and release of water in bentonites under unsaturated conditions remains a critical factor in improving design of hydraulic barrier technologies to minimise transport of contaminants. In situ neutron scattering measurements during controlled exposure to water vapour partial pressures provide an opportunity to probe dynamics of adsorbed water over a large range of unsaturated conditions. The hydration state of a sodium montmorillonite (Na+-Mt) sample was monitored in situ by both the position of the (001) reflection and the elastic scattering signal amplitude to ensure that equilibrium was achieved at the chosen vapour pressures (p/po=0.85, 0.75, 0.55, 0.35 and 0.01 during dehydration and rehydration). Quantification of total adsorbed water, confirmed by separate gravimetric analyses on the same sample, was achieved by integrating the total scattering contribution between the limits ±2.0meV. A further integration between ±0.8meV allowed separating total adsorbed water into mobile and immobile fractions. Hysteresis was observed for the p/po=0.55 and 0.35 vapour pressures, but little or no hysteresis was observed for the p/po=0.85 and 0.75 states and these were representative of hydration intermediate between pseudo-monolayer and pseudo-bilayer hydrate states. Fitting of the quasielastic neutron scattering spectra to the water model resulted in rational changes in translational and rotational diffusion coefficients (Dt and Dr, respectively), mean residence time, τ0 and the mean relaxation time, τr, of the water with respect to experimental p/po. The behaviour of Dt and τ0 suggested that on rehydration at low p/po, water initially occupied the external surfaces before entering the interlayer spaces.

Petrographic and Petrophysical Characteristics of the Upper Devonian Three Forks Formation, Southern Nesson Anticline, North Dakota

Petrographic and Petrophysical Characteristics of the Upper Devonian Three Forks Formation, Southern Nesson Anticline, North Dakota

Kinetic theory-based numerical modeling and analysis of bi-disperse segregated mixture fluidized bed

We discuss a series of continuum Euler-Euler simulations of an initially mixed bi-disperse fluidized bed which segregates under certain operating conditions. The simulations use the multi-phase kinetic theory-based description of the momentum and energy exchanges between the phases by Simonin's Group [see e.g. Gourdel, Simonin and Brunier (1999). Proceedings of 6th International Conference on Circulating Fluidized Beds, Germany, pp. 205–210]. The discussion and analysis of the results focus on the fluid-particle momentum exchange (i.e. drag). Simulations using mono- and poly-disperse fluid-particle drag correlations are analyzed for the Geldart D-type size bi-disperse gas-solid experiments performed by Goldschmidt et al. [Powder Tech., pp. 135–159 (2003)]. The poly-disperse gas-particle drag correlations account for the local particle size distribution by using an effective mixture diameter when calculating the Reynolds number and then correcting the resulting force coefficient. Simulation results show very good predictions of the segregation index for bi-disperse beds with the mono-disperse drag correlations contrary to the poly-disperse drag correlations for which the segregation rate is systematically under-predicted. The statistical analysis of the results shows a clear separation in the distribution of the gas-particle mean relaxation times of the small and large particles with simulations using the mono-disperse drag. In contrast, the poly-disperse drag simulations have a significant overlap and also a smaller difference in the mean particle relaxation times. This results in the small and large particles in the bed to respond to the gas similarly without enough relative time lag. The results suggest that the difference in the particle response time induce flow dynamics favorable to a force imbalance which results in the segregation.

Interfacial and space charge dielectric effects in Polypyrrole/ZnO composites

Polypyrrole/zinc oxide composites were studied by Broadband Dielectric Spectroscopy in the frequency range 10 mHz–1 MHz for temperatures ranging from 15 K to room temperature. Lowering temperature, the dc conductivity was suppressed revealing underlying dielectric relaxation mechanisms. For 20, 30 and 40 wt.% ZnO composites, a relaxation was detected in the vicinity of range 104–10­5 Hz. Its typical mean relaxation time and its concentration dependence are compatible with Sillar's model for interfacial polarization in heterogeneous matter. At high ZnO concentrtion, a low-frequency relaxation also appeared around 0.01–10 Hz, stemming from space charge polarization. The shift of the relaxation upon temperature provides an insight to the dynamics of relaxing charge entities. Composites consisting of 10 wt.% ZnO are most suitable optoelectronic material, as they combine good electrical conduction, low capacitance effects and ohmic sample-electrode contacts.

Statistical characterization of microstructure of packings of polydisperse hard cubes.

Polydisperse packings of cubic particles arise in several important problems. Examples include zeolite microcubes that represent catalytic materials, fluidization of such microcubes in catalytic reactors, fabrication of new classes of porous materials with precise control of their morphology, and several others. We present the results of detailed and extensive simulation and microstructural characterization of packings of nonoverlapping polydisperse cubic particles. The packings are generated via a modified random sequential-addition algorithm. Two probability density functions (PDFs) for the particle-size distribution, the Schulz and log-normal PDFs, are used. The packings are analyzed, and their random close-packing density is computed as a function of the parameters of the two PDFs. The maximum packing fraction for the highest degree of polydispersivity is estimated to be about 0.81, much higher than 0.57 for the monodisperse packings. In addition, a variety of microstructural descriptors have been calculated and analyzed. In particular, we show that (i) an approximate analytical expression for the structure factor of Percus-Yevick fluids of polydisperse hard spheres with the Schulz PDF also predicts all the qualitative features of the structure factor of the packings that we study; (ii) as the packings become more polydisperse, their behavior resembles increasingly that of an ideal system-"ideal gas"-with little or no correlations; and (iii) the mean survival time and mean relaxation time of a diffusing species in the packings increase with increasing degrees of polydispersivity.

Dielectric relaxation of near-percolated carbon nanofiber polypropylene composites

In this work, the morphological, structural and dielectric analysis of near-percolated polypropylene (PP) composites containing carbon nanofibers (CNF) processing by melt-mixing are investigated. Whereas the morphological analysis shows that CNF exhibit some tendency to agglomerate within the PP matrix, the structural analysis showed first a general decrease in the intensity of the IR bands as a consequence of the interaction between carbon nanofibers and PP matrix and second an increase of the crystallinity degree of the PP/CNF composites when compared to the pure PP. The dielectric analysis demonstrates enhanced dielectric constants (from 2.97 for neat polymer to 9.7 for 1.9vol% loaded composites at 200Hz) and low dielectric losses. Furthermore, the dielectric relaxation for composites with concentrations in the vicinity of percolation is evidenced and well described by the generalized polydispersive Cole-Cole model from which the values of static dielectric constant (εs), high frequency dielectric constant (ε∞), distribution of relaxation time (α) and mean relaxation time (τo), are determined, suggesting that this latter analysis constitutes a strong tool for understanding the relationships between microstructure and dielectric properties in this type of polymer composites.

Interfacial and space charge dielectric effects in Polypyrrole/Zinc Oxide composites

Polypyrrole/zinc oxide composites were studied by broadband dielectric spectroscopy in the frequency range 10mHz to 1MHz for temperature ranging from 15K to room temperature. Lowering temperature, the dc conductivity was suppressed, revealing underlying dielectric relaxation mechanisms. For pristine polypyrrole and 10wt% ZnO composites, no dielectric relaxation is detectable. For 20, 30 and 40wt% ZnO composites, a relaxation was detected in the vicinity of range 104–105Hz. Its typical mean relaxation time and its concentration dependence are compatible with Sillar's model for interfacial polarization in heterogeneous matter consisting of inclusions dispersed within a dielectric matrix. At high ZnO concentration, a low-frequency relaxation also appeared around 0.01–10Hz, stemming from space charge polarization. The shift of the relaxation upon temperature provides an insight to the dynamics of relaxing charge entities.

Impedance change and capacity fade of lithium nickel manganese cobalt oxide-based batteries during calendar aging

Abstract The calendar aging of commercial 18650 lithium-ion batteries with lithium nickel manganese cobalt oxide cathode and graphite anode is studied by regular electrochemical characterization of batteries stored at defined conditions. The cell capacity is found to decrease linearly with time and shows a faster decrease at higher storage temperatures. From current pulse tests, it is determined that both higher temperature and higher state of charge (SOC) cause accelerated resistance increase with storage time. Changes in different battery parameters during storage are also quantified by analyzing electrochemical impedance spectroscopy (EIS) spectra. The cell degradation causes a gradual increase of the ohmic and the total polarization resistance with storage duration, where the latter one is found to be the main contributor to the increased cell impedance. An increase in the mean relaxation time constant and changes in the porous structure for the electrode processes are observed from EIS analysis. Resistance for this cell chemistry is found to be current independent by comparing the cell resistance calculated from the current pulse method after 1s and from the EIS analysis at 1 Hz. Furthermore, it is seen that the additional charge throughput due to the periodic electrochemical characterization induces significant cell degradation effects.

Δself-energy at finite temperature and density and theπNcross section

The self energy of $\Delta$-baryon is evaluated at finite temperature and density using the real time formalism of thermal field theory. The Dyson-Schwinger equation is used to get the exact thermal propagator followed by the spectral function of $\Delta$. The $\pi N$ scattering cross section obtained using explicit $\Delta$ exchange is normalized to the experimental data in vacuum and its medium modification is implemented by means of the exact thermal propagator. A significant suppression of the peak of the cross-section is observed at higher temperature and baryon density. Effects on the mean relaxation time of nucleons and the temperature dependence of the shear viscosity of a pion nucleon gas are demonstrated.

Nonlocality of relaxation rates in disordered landscapes.

We investigate both analytically and by numerical simulation the relaxation of an overdamped Brownian particle in a 1D multiwell potential. We show that the mean relaxation time from an injection point inside the well down to its bottom is dominated by statistically rare trajectories that sample the potential profile outside the well. As a consequence, also the hopping time between two degenerate wells can depend on the detailed multiwell structure of the entire potential. The nonlocal nature of the transitions between two states of a disordered landscape is important for the correct interpretation of the relaxation rates in complex chemical-physical systems, measured either through numerical simulations or experimental techniques.

Multi-frequency electrical impedance tomography as a non-invasive tool to characterize and monitor crop root systems

Abstract. A better understanding of root–soil interactions and associated processes is essential in achieving progress in crop breeding and management, prompting the need for high-resolution and non-destructive characterization methods. To date, such methods are still lacking or restricted by technical constraints, in particular the charactization and monitoring of root growth and function in the field. A promising technique in this respect is electrical impedance tomography (EIT), which utilizes low-frequency (&lt; 1 kHz)- electrical conduction- and polarization properties in an imaging framework. It is well established that cells and cell clusters exhibit an electrical polarization response in alternating electric-current fields due to electrical double layers which form at cell membranes. This double layer is directly related to the electrical surface properties of the membrane, which in turn are influenced by nutrient dynamics (fluxes and concentrations on both sides of the membranes). Therefore, it can be assumed that the electrical polarization properties of roots are inherently related to ion uptake and translocation processes in the root systems. We hereby propose broadband (mHz to hundreds of Hz) multi-frequency EIT as a non-invasive methodological approach for the monitoring and physiological, i.e., functional, characterization of crop root systems. The approach combines the spatial-resolution capability of an imaging method with the diagnostic potential of electrical-impedance spectroscopy. The capability of multi-frequency EIT to characterize and monitor crop root systems was investigated in a rhizotron laboratory experiment, in which the root system of oilseed plants was monitored in a water–filled rhizotron, that is, in a nutrient-deprived environment. We found a low-frequency polarization response of the root system, which enabled the successful delineation of its spatial extension. The magnitude of the overall polarization response decreased along with the physiological decay of the root system due to the stress situation. Spectral polarization parameters, as derived from a pixel-based Debye decomposition analysis of the multi-frequency imaging results, reveal systematic changes in the spatial and spectral electrical response of the root system. In particular, quantified mean relaxation times (of the order of 10 ms) indicate changes in the length scales on which the polarization processes took place in the root system, as a response to the prolonged induced stress situation. Our results demonstrate that broadband EIT is a capable, non-invasive method to image root system extension as well as to monitor changes associated with the root physiological processes. Given its applicability on both laboratory and field scales, our results suggest an enormous potential of the method for the structural and functional imaging of root systems for various applications. This particularly holds for the field scale, where corresponding methods are highly desired but to date are lacking.