Gradient Strength Research Articles

Slip hydrodynamics of combined electroosmotic and pressure driven flows of power law fluids through narrow confinements

Electroosmotic flows (EOF) in microfluidic devices can be greatly enhanced over superhydrophobic surfaces because the high shear rates within the electrical double layer can drive large slip velocities at the interface. Using the power law fluid model, we derive a novel formulation for the Helmholtz–Smoluchowski (HS) velocity and use it to examine the effect of slip on the hydrodynamics of a coupled pressure driven and EOF. Semi analytical relations for the velocity gradient are obtained for cases of a favourable pressure gradient but exact solutions of the velocity can be found only for certain power law indices. Cases of adverse pressure gradient and fractional power law indices are investigated using numerically using the Galerkin Finite Element Method. The validity of the semi analytical relations verified by comparison with the numerical method. The presence of velocity slip at the wall leads to an enhancement of the HS velocity that is most pronounced in shear thinning fluids. Adverse pressure gradients are observed to generate an inflection in the velocity profile and even a two-way flow for certain flow parameters. The strength of the adverse pressure gradient needed to setup a reverse flow at the channel centre reduces as the slip length is increased. The location of the point of inflection is found to depend on the channel height, pressure gradient, electric field, slip length, Debye length and non-Newtonian behaviour.

Zeeman-resolved TDLAS using metastable levels of Ar in the weakly magnetized plasma of the linear plasma device PSI-2

Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.

New insights into thermomechanical fatigue behavior of AISI Type 316 LN SS weld joint

AbstractCyclic deformation and fracture behavior of a type 316 LN austenitic stainless steel (SS) weld joint (WJ) were investigated under thermomechanical fatigue (TMF) and isothermal low cycle fatigue (IF) cycling at the maximum temperature (Tmax) of TMF. A higher cyclic stress response (CSR) and reduced cyclic softening were observed under TMF compared with IF tests. In‐phase (IP) TMF resulted in lower lives compared with IF cycling at the Tmax and out‐of‐phase (OP) TMF, which was attributed to the more pronounced creep‐induced intergranular damage. Characterization of microstructural features and microhardness variations revealed that the accumulation of damage and associated failure depends on the strength and microstructural gradient, together with the deformation incompatibility. The crack propagation is found to depend on the individual and synergistic interactions of the microstructural transformations, creep, and oxidation, depending on the type of fatigue cycle (IF and IP/OP TMF), strain amplitude, and thermal cycling effects.

A novel cyclical wellbore-fluid circulation strategy for extracting geothermal energy

Both analytical and numerical modeling methods have helped explore power generation by producing steam with the binary organic Rankine cycle or ORC. A steady-state fluid circulation rate becomes the norm in a closed-loop system in most studies, assuming a stable geothermal gradient. In this study, we explored the suitability of those assumptions, given that the geothermal gradient is a time-variant entity at a well's proximity. Furthermore, the near-wellbore formation cooling necessitates a variable-circulation rate for the cold surface fluid to ensure a stable, wellhead-fluid temperature.This article presents a new transient analytical modeling approach for geothermal energy extraction by invoking the cyclical fluid circulation rate to ensure a stable surface fluid temperature. This analytical model entails the reverse circulation of water down the annulus and up the tubing to the surface. Specifically, we introduce a cycle (a stepwise increasing circulation rate sequence, followed by the stepwise decreasing rate sequence) to retain a stable, wellhead-fluid temperature. Of course, this cycle gets repeated to manage transient heat transfer from cold fluid circulation and the wellbore formation.Reproduction of the undisturbed formation temperature from the temperature line-source solution at each step of the cycle provided the necessary proof of the proposed approach. Model validation entailed comparing the suggested analytical model results to demonstrate that multiple transient-rate steps yielded the same initial temperature, thereby reaffirming the stepwise increasing and decreasing rate sequence strategy. We also explored the model's application in diverse settings in a probabilistic frame. As expected, the geothermal gradient, vertical-well depth, and insulation strength became the most influential on the dependent variable, wellhead fluid temperature, among the various independent variables studied.

Scan-rescan repeatability of axonal imaging metrics using high-gradient diffusion MRI and statistical implications for study design

Axon diameter mapping using diffusion MRI in the living human brain has attracted growing interests with the increasing availability of high gradient strength MRI systems. A systematic assessment of the consistency of axon diameter estimates within and between individuals is needed to gain a comprehensive understanding of how such methods extend to quantifying differences in axon diameter index between groups and facilitate the design of neurobiological studies using such measures. We examined the scan-rescan repeatability of axon diameter index estimation based on the spherical mean technique (SMT) approach using diffusion MRI data acquired with gradient strengths up to 300 mT/m on a 3T Connectom system in 7 healthy volunteers. We performed statistical power analyses using data acquired with the same protocol in a larger cohort consisting of 15 healthy adults to investigate the implications for study design. Results revealed a high degree of repeatability in voxel-wise restricted volume fraction estimates and tract-wise estimates of axon diameter index derived from high-gradient diffusion MRI data. On the region of interest (ROI) level, across white matter tracts in the whole brain, the Pearson's correlation coefficient of the axon diameter index estimated between scan and rescan experiments was r = 0.72 with an absolute deviation of 0.18 μm. For an anticipated 10% effect size in studies of axon diameter index, most white matter regions required a sample size of less than 15 people to observe a measurable difference between groups using an ROI-based approach. To facilitate the use of high-gradient strength diffusion MRI data for neuroscientific studies of axonal microstructure, the comprehensive multi-gradient strength, multi-diffusion time data used in this work will be made publicly available, in support of open science and increasing the accessibility of such data to the greater scientific community.

Spin polarization induced by the hydrodynamic gradients

We systematically analyze the effects of the derivatives of the hydrodynamic fields on axial Wigner function that describes the spin polarization vector in phase space. We have included all possible first-order derivative contributions that are allowed by symmetry and compute the associated transport functions at one-loop using the linear response theory. In addition to reproducing known effects due to the temperature gradient and vorticity, we have identified a number of potentially significant contributions that are overlooked previously. In particular, we find that the shear strength, the symmetric and traceless part of the flow gradient, will induce a quadrupole for spin polarization in the phase space. Our results, together with hydrodynamic gradients obtained from hydrodynamic simulations, can be employed as a basis for the interpretation of the Λ (anti-Λ) spin polarization measurement in heavy-ion collisions.

Geographic patterns of genetic diversity in North American trees: a continent-wide analysis

In the Northern Hemisphere, many species have been reported to have greater genetic diversity in southern populations than northern populations — ostensibly due to migration northward following the last glacial maximum. The generality of this pattern, while well-established for some taxa, remains unclear for North American trees. To address this issue, I collected published population genetics data for 73 North American tree species and tested whether genetic diversity was associated with latitude or longitude and whether geographic trends were associated with dispersal traits, range, or study characteristics. I found there were no general geographic patterns in genetic diversity and the strength of the geographic gradients was not associated with any species or study characteristics. The northern and western regions of North America tended to have more species with genetic diversity that declined with latitude, but most species had no significant trend. This work shows that North American trees have complex and individualistic patterns of genetic diversity that may negate explanation by any particular dispersal trait or range characteristic.

Pre-Clinical Study of Ibrutinib for the Treatment of T-Cell Lymphoma Using Magnetic Resonance Imaging (MRI) Assessment

Peripheral T cell lymphoma (PTCL) has a poor prognosis and there is a need for new treatments.The sanroque mouse bears a mutation in the E3 ubiquitin ligase, Roquin, which leads to T-cell activation and autoimmunity in homozygous animals. In heterozygous mice (Roquinsan/+) T-cell lymphomas with histological similarity to human angioimmunoblastic lymphoma (AITL), the commonest subtype of PTCL, develop in 40 to 50% of animals by 6 - 12 months of age.In order to test the feasibility of using sanroque mice for pre-clinical work we carried out MRI scans of the axillary and inguinal regions before and after receiving 200mg/kg cyclophosphamide as an intra-peritoneal injection. After 28-days the MRI scans were repeated and lymph nodes, spleens and livers collected.MRI scanning was performed on a 9.4T Agilent scanner (Agilent Technologies, Santa Claire, CA, USA) with a 310mm bore diameter and 6cm inner-diameter gradient (1000mT/m maximum gradient strength). A 4cm millipede RF coil was used for RF transmission and reception. T2-weighted images were acquired using a respiratory-gated fast spin echo (FSE) sequence with TR/TE=3000/40ms, 40x40mm field of view (256x256 matrix), 32x0.8mm slices and two signal averages.Enlarged lymph nodes were identified on the pre-treatment MRI scan and compared to the equivalent lymph node following treatment (Figure 1A). There was a mean 94% reduction in lymph node size following cyclophosphamide treatment. This demonstrated that the enlarged lymph nodes were chemosensitive and that MRI scanning could be used to monitor the lymph node size in response to treatment.Interleukin-2-inducible kinase (ITK) is expressed and has essential functions in regulating normal T-cell proliferation and differentiation. ITK is also expressed in AITL and might be a therapeutic target but there is no pre-clinical data. The small molecule BTK/ITK inhibitor, ibrutinib, is effective and well tolerated in clinical practice in some B-cell lymphomas and ibrutinib is active against mouse ITK.In order to assess the effect of ibrutinib on (Roquinsan/+) T-cell lymphoma we identified twelve mice with palpable lymph nodes. Following an MRI scan, oral treatment with ibrutinib (25mg/kg in 1% (2-Hydroxypropyl)-β-cyclodextrin (2HPBD)) or equivalent volume of vehicle (1% 2HPBD) was given daily by gavage. Five mice were treated with ibrutinib for 28 days and three for reduced time periods (7 days, 14 days and 21 days). Four mice were treated with vehicle for 28 days. Following cessation of therapy a further MRI scan was performed prior to harvesting of tissues, including liver, spleen and lymph nodes. MRI images were analysed to determine the change in volume of lymph nodes during the treatment period.Responses to ibrutinib were mixed (Figure 1B & 1C): of those that received 28 days treatment 5/8 animals showed reduction in tumour size of between 10 and 86% whilst those that received shorter durations of treatment showed reductions of 8 and 11%. 3/8 ibrutinib treated animals showed increases in tumour size of 9 and 85% (mice treated for 28 days) whilst one animal treated for 7 days showed an increase of 117%. 3/4 vehicle treated animals showed stable or increasing lymphadenopathy whilst one mouse showed spontaneous tumour regression.In order to analyze the effects of ibrutinib at the cellular level we established flow cytometry protocols to determine cellular composition. Previous reports show that the majority of cells within sanroque enlarged lymph nodes are B-cells with a minority (1 to 2%) being malignant T-cells. This is similar to the pattern observed in human Tfh lymphoma. Lymph nodes were harvested following the post-treatment MRI scan and cell suspensions produced. The proportions of CD4+, CD8+ and B220+ lymphocytes were assessed. To determine the Tfh cell population CD4 positive lymphocytes were gated and the proportion of CXCR5hiPD1hi cells assessed. We did not observe any significant differences between responding and non-responding mice (Figure 1D) or treated and untreated mice.We have developed an MRI protocol to assess the response of a T-cell lymphoma model to novel agents to facilitate pre-clinical studies. We interpret our results as showing that ibrutinib is effective as a single agent in a proportion of the spontaneous T-cell lymphomas generated in the Roquinsan/+ model of Tfh lymphoma but further work is required to understand the differences between responder and non-responder tumours. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Design equation for stability of a circular tunnel in anisotropic and heterogeneous clay

The safety assessment of tunnel stability is critical to tunnel construction and requires accurate analysis to obtain a reliable prediction. Strength anisotropy is an important aspect of clay behavior, but it is mostly neglected in practical stability analyses. In this study, the effects of undrained strength anisotropy and strength nonhomogeneity on the stability of unlined circular tunnels in clays are investigated. The static approach of lower-bound (LB) analysis using finite-element and second-order cone programming is employed to examine the aforementioned effects. The anisotropic shear strength of clay is modeled by employing an elliptical yield function under plane strain conditions. A complete set of dimensionless parameters covering the cover depth ratios of tunnels, normalized overburden pressure ratios, normalized strength gradient ratios of clays, and anisotropic strength ratios, are systematically investigated. The new LB solutions indicate that the stability load factor of the problem has a nonlinear relationship with the cover depth ratio and the anisotropic strength ratio, and there exists a linear relationship with the normalized overburden pressure and the normalized strength gradient. Their influence on the predicted failure mechanism is parametrically evaluated. A statistically approximate stability equation of unlined circular tunnels in anisotropic and non-homogeneous clay is proposed for the first time, which contains four new stability factors, namely, constant undrained strength, linearly increasing strength gradient, undrained strength anisotropy, and soil unit weight, and it can serve as a fast and accurate tool for predicting the undrained stability of this problem in practice.

ENSO phase-locking biases from the CMIP5 to CMIP6 models and a possible explanation

The El Niño-Southern Oscillation (ENSO) system is the most significant equatorial interannual climate variability pattern and has enormous global climate impacts. The ability to forecast ENSO accurately is crucially important to human livelihoods worldwide. ENSO is characterized by strong winter-peaking sea surface temperature (SST) anomalies in the central-eastern tropical Pacific during the mature phase. However, the bias in simulating ENSO phase-locking behavior persists in climate models. In this study, the ENSO seasonal phase-locking behaviors simulated in 42 Coupled Model Intercomparison Project Phase 6 (CMIP6) models are evaluated by comparing 43 CMIP5 models and observations. Only a few models (12 CMIP5 and 15 CMIP6) simulated ENSO with a majority proportion of winter-peaking events, which indicates that reasonable ENSO seasonal phase-locking is still a challenge for state-of-the-art climate models. Furthermore, the seasonal cycle of the zonal SST gradient along the equator can explain approximately 24% and 27% of the variance in the ENSO phase-locking for CMIP5 and CMIP6, respectively. In particular, the strengths of the zonal SST gradient in the central-eastern Pacific during boreal spring and autumn are crucial. The biases in simulating the seasonal changes in the zonal SST gradient influence the zonal advective feedback's strengths that respond to the anomalous SST of ENSO, therefore disturbing the simulation of ENSO phase-locking. Improvement of the simulated ENSO phase-locking should be realized by focusing on the zonal SST gradient's seasonal cycle.

Multiparametric magnetic resonance imaging of the prostate: Lights and shadows.

Prostate cancer is the second most commonly diagnosed cancer in man. Since the first MRI was performed, enormous progress has been made in diagnosis, treatment, and follow up of PCa, mainly due to multiparametric prostatic MRI (mpMRI). Although mpMRI has become the best imaging tool for identifying PCa, some limitations still exist. Prostate imaging with mpMRI is, to date, the best way to locate suspicious lesions to trigger prostate biopsy, plan active surveillance, or definitive treatment. In case of relapse, mpMRI can help detect local disease and provide specific management. It is well known that there is a subset of patients in whom mpMRI fails to depict csPCa. These missed significant cancers demand great attention. Prostate mpMRI quality depends on several factors related to equipment (including equipment vendor, magnet field and gradient strength, coil set used, software and hardware levels, sequence parameter choices), patient (medications, body habitus, motion, metal implants, rectal gas), and most importantly the radiologic interpretation of images (learning curve effects, subjectivity of observations, interobserver variations, and reporting styles). Inter-reader variability represents a huge current limitation of this method. Therefore, mpMRI remains the best imaging tool available to detect PCa, guiding diagnosis, treatment, and follow up while inter-reader variability represents the best limitation. Radiomics can help identifying imaging biomarkers to help radiologist in detecting significant PCa, reducing examination times, and costs.

Stability of active trapdoors in axisymmetry

Trapdoor stability has been widely studied by many researchers in the field of tunneling engineering. A general question being frequently asked is that why most sinkholes have a near-perfect circular shape on the ground surface. This could be possibly explained by the current numerical study using finite element limit analysis under axisymmetric condition, where upper and lower bound solutions of active circular trapdoors are determined. The failure study of sinkholes and the associated failure mechanisms in this paper are for non-homogeneous clay with a linear increase of strength with depth under various cover depth ratios and dimensionless strength gradients. A design equation for predicting the stability solutions is also developed based on the novel three dimensional solutions using axisymmetry.

SPHERIOUSLY? The challenges of estimating sphere radius non-invasively in the human brain from diffusion MRI

The Soma and Neurite Density Imaging (SANDI) three-compartment model was recently proposed to disentangle cylindrical and spherical geometries, attributed to neurite and soma compartments, respectively, in brain tissue. There are some recent advances in diffusion-weighted MRI signal encoding and analysis (including the use of multiple so-called ’b-tensor’ encodings and analysing the signal in the frequency-domain) that have not yet been applied in the context of SANDI. In this work, using: (i) ultra-strong gradients; (ii) a combination of linear, planar, and spherical b-tensor encodings; and (iii) analysing the signal in the frequency domain, three main challenges to robust estimation of sphere size were identified: First, the Rician noise floor in magnitude-reconstructed data biases estimates of sphere properties in a non-uniform fashion. It may cause overestimation or underestimation of the spherical compartment size and density. This can be partly ameliorated by accounting for the noise floor in the estimation routine. Second, even when using the strongest diffusion-encoding gradient strengths available for human MRI, there is an empirical lower bound on the spherical signal fraction and radius that can be detected and estimated robustly. For the experimental setup used here, the lower bound on the sphere signal fraction was approximately 10%. We employed two different ways of establishing the lower bound for spherical radius estimates in white matter. The first, examining power-law relationships between the DW-signal and diffusion weighting in empirical data, yielded a lower bound of 7μm, while the second, pure Monte Carlo simulations, yielded a lower limit of 3μm and in this low radii domain, there is little differentiation in signal attenuation. Third, if there is sensitivity to the transverse intra-cellular diffusivity in cylindrical structures, e.g., axons and cellular projections, then trying to disentangle two diffusion-time-dependencies using one experimental parameter (i.e., change in frequency-content of the encoding waveform) makes spherical radii estimates particularly challenging. We conclude that due to the aforementioned challenges spherical radii estimates may be biased when the corresponding sphere signal fraction is low, which must be considered.

Nanoscale control of temperature operation ranges for magnetocaloric applications

We devised a proof-of-concept materials design that addresses the necessary requirements for magnetocaloric materials to have a constant magnetocaloric effect (MCE) over a large temperature range. For this purpose, we have fabricated epitaxial Co1−x(z)Ru x (z) films engineered to have a triangular gradient in exchange strength J along the thickness. Different from homogeneous Co1−x Ru x layers, where the maximum value of magnetic entropy change ΔS m falls rapidly with temperature away from the ferromagnetic (FM)–paramagnetic (PM) phase transition, the Co1−x(z)Ru x (z) graded structures exhibit high MCE over a large temperature range, leading to an improved cooling capacity. Theoretical modeling results confirm the enhanced temperature range and highlight a core aspect of our exchange graded materials approach, namely the ability to control and manipulate magnetism at nanoscale dimensions. As we demonstrate, this control is reliant on the fact that the temperature driven PM–FM phase transition does not occur in the entirety of the material system but only in well-defined nanoscopic regions of our samples at any given temperature, enabling us to significantly extend the useful temperature range for magneto-caloric utilization.

Metabolic engineering of Escherichia coli for efficient ectoine production

Ectoine is a high-value stabilizer and protective agent with various applications in enzyme industry, cosmetics, and biomedicine. In this study, rational engineering strategies have been implemented in Escherichia coli to efficiently produce ectoine. First, the synthetic pathway of ectoine was constructed in E. coli MG1655 by introducing an artificial thermal switch system harboring the ectABC cluster from Halomonas elongate, and the resulting strain produced 1.95 g/L ectoine. Second, crr encoding the glucose-specific enzyme II domain A of phosphotransferase system and iclR encoding the glyoxylate shunt transcriptional repressor were deleted in E. coli for enhancing the oxaloacetate supply, leading to the increasement of the ectoine titer to 9.09 g/L. Third, thrA encoding the bifunctional aspartokinase/homoserine dehydrogenase was removed from the genome to weaken the competitive pathway; simultaneously, an endogenous feedback-resistant lysC was overexpressed to complement the enzymatic activity deficiency of the aspartate kinase, leading to 30.36% increase of ectoine titer. Next, the expression of phosphoenolpyruvate carboxylase was modulated with varying gradient strength promoters to accelerate the biosynthesis efficiency of ectoine. Finally, aspDH encoding aspartate dehydrogenase from Pseudomonas aeruginosa PAO1 was overexpressed to further improve the biosynthesis of ectoine. The final strain MWZ003/pFT28-ectABC-EclysC*-aspDH-ppc3 produced 30.37 g/L ectoine after 36-h fed-batch fermentation with a yield of 0.132 g/g glucose and a productivity of 0.844 g/(L h).

The Guadiaro-Baños contourite drifts (SW Mediterranean). A geotechnical approach to stability analysis

Two Quaternary plastered contourite drifts, with terraced and low-mounded morphologies, make up the continental slope and base-of-slope in the northwestern Alboran Sea, respectively, between the Guadiaro and Baños turbidite systems, close to the Strait of Gibraltar.Considering their significant lateral extent, the link between the contourite drift deposits and landslides may be particularly important for hazard assessment. The physical properties, composition and geometry of contourite drifts have been proposed as key factors in slope stability, although this relationship still needs to be better constrained. In this work, new in-situ geotechnical data (cone penetration tests; CPTu) has been combined with morphostratigraphic, sedimentological, and (laboratory) geotechnical properties to determine the stability of the Guadiaro-Baños drifts.For the depositional domains of both drifts, the resulting sedimentary and geotechnical model describes low-plasticity granular and silty sands on the erosive terraced domain that evolve seawards to silty and silty-clay deposits with a higher plasticity and uniform geomechanical properties. For the shallower coarse-grained contourite sediments, the cohesion (c') and internal friction angle (ϕ') values are 0–9 kPa and 46–30°, respectively, whereas for the distal fine contourites the undrained shear strength gradient (∇Su) is 2 kPa/m. These properties allow us to establish high factors of safety for all the scenarios considered, including seismic loading. Slope failure may be triggered in the unlikely event that there is seismic acceleration of PGA > 0.19, although no potential glide planes have been observed within the first 20 m below the seafloor.This suggests that the contourite drifts studied tend to resist failure better than others with similar sedimentary characteristics. The interplay of several processes is proposed to explain the enhanced undrained shear strength: 1) the geometry of the drifts, defined by an upper contouritic terrace and lower low-mounded shapes; 2) recurrent low-intensity earthquakes with insufficient energy to trigger landslides, favouring increased strength due to dynamic compaction; and 3) cyclic loading induced by solitons/internal waves acting on the sediment.

Three-dimensional gradient and spin-echo readout for time-encoded pseudo-continuous arterial spin labeling: Influence of segmentation factor and flow compensation.

PurposeTo monitor the complete passage of the labeled blood through the vascular tree into tissue and improve the quantification of ASL maps, we evaluated the effect of 3D gradient and spin‐echo (GRASE) readout segments on temporal SNR (tSNR) and image blurriness for time‐encoded pseudo‐continuous arterial spin labeling and the effect of flow‐compensation gradients on the presence of intravascular signal.MethodsFifteen volunteers were scanned using time‐encoded pCASL with 2D EPI and single‐segment, two‐segments, and three‐segments 3D‐GRASE readouts with first‐order flow compensation (FC) gradients. Two‐segments 3D‐GRASE scans were acquired with 25%, 50%, 75%, and 100% of full first‐order FC. Temporal SNR was assessed, and cerebral blood flow and arterial blood volume were quantified for all readout strategies.ResultsFor single‐segment 3D GRASE, tSNR was comparable to 2D EPI for perfusion signal but worse for the arterial signal. Two‐segments and three‐segments 3D GRASE resulted in higher tSNR than 2D EPI for perfusion and arterial signal. The arterial signal was not well visualized for 3D‐GRASE data without FC. Visualization of the intravascular signal at postlabeling delays of 660 ms and 1060 ms was restored with FC. Adequate visualization of the intravascular signal was achieved from 75% of FC gradient strength at a postlabeling delay of 660 ms. For a postlabeling delay of 1060 ms, full‐FC gradients were the best option to depict intravascular signal.ConclusionSegmented GRASE provided higher effective tSNR compared with 2D‐EPI and single‐segment GRASE. Flow compensation with GRASE readout should be carefully controlled when applying for time‐encoded pCASL to visualize intravascular signal.

On the dependence of quantitative diffusion-weighted imaging on scanner system characteristics and acquisition parameters: A large multicenter and multiparametric phantom study with unsupervised clustering analysis.

The purpose of this multicenter phantom study was to exploit an innovative approach, based on an extensive acquisition protocol and unsupervised clustering analysis, in order to assess any potential bias in apparent diffusion coefficient (ADC) estimation due to different scanner characteristics. Moreover, we aimed at assessing, for the first time, any effect of acquisition plan/phase encoding direction on ADC estimation. Water phantom acquisitions were carried out on 39 scanners. DWI acquisitions (b-value=0-200-400-600-800-1000s/mm2) with different acquisition plans (axial, coronal, sagittal) and phase encoding directions (anterior/posterior and right/left, for the axial acquisition plan), for 3 orthogonal diffusion weighting gradient directions, were performed. For each acquisition setup, ADC values were measured in-center and off-center (6 different positions), resulting in an entire dataset of 84×39=3276 ADC values. Spatial uniformity of ADC maps was assessed by means of the percentage difference between off-center and in-center ADC values (Δ). No significant dependence of in-center ADC values on acquisition plan/phase encoding direction was found. Ward unsupervised clustering analysis showed 3 distinct clusters of scanners and an association between Δ-values and manufacturer/model, whereas no association between Δ-values and maximum gradient strength, slew rate or static magnetic field strength was revealed. Several acquisition setups showed significant differences among groups, indicating the introduction of different biases in ADC estimation. Unsupervised clustering analysis of DWI data, obtained from several scanners using an extensive acquisition protocol, allows to reveal an association between measured ADC values and manufacturer/model of scanner, as well as to identify suboptimal DWI acquisition setups for accurate ADC estimation.

Distribution of Temperature and Strength in the Central Andean Lithosphere and Its Relationship to Seismicity and Active Deformation

AbstractWe present three‐dimensional (3D) models of the present‐day steady‐state conductive thermal field and strength distribution in the lithosphere beneath the Central Andes. Our primary objective was to investigate the influence that the structure of the Central Andean lithosphere has on its thermal and rheological state, and the relationship between the latter and the active deformation in the region. We used our previous data‐driven and gravity‐constrained 3D density model as starting point for the calculations. We first assigned lithology‐derived thermal and rheological properties to the different divisions of the density model and defined temperature boundary conditions. We then calculated the 3D steady‐state conductive thermal field and the maximum differential stresses for both brittle and ductile behaviors. We find that the thickness and composition of the crust are the main factors affecting the modeled thermal field, and consequently also the strength distribution. The orogen is characterized by a thick felsic crust with elevated temperatures and a low integrated strength, whereas the foreland and forearc are underlain by a more mafic and thinner crust with lower temperatures and a higher integrated strength. We find that most of the intraplate deformation coincides spatially with the steepest strength gradients and suggest that the high potential energy of the orogen together with the presence of rheological lateral heterogeneities produce high compressional stresses and strong strain localization along the margins of the orogen. We interpret earthquakes within the modeled ductile field to be related to the weakening effect of long‐lived faults and/or the presence of seismic asperities.

Three-dimensional trench stability in non-uniform undrained clay with discretization-based kinematic analysis

This paper presents a three-dimensional trench stability analysis in non-uniform undrained clay adopting the kinematic approach of limit analysis. Rather than assuming a classic curved cylinder (“torus”) shape, the failure surface of the proposed rotational mechanism is formulated by a spatial discretization technique in order to accommodate the non-uniformity profile of soil’s undrained shear strength in a multi-layered ground as well as better adaptation to typical trench geometries in practice. The present study agrees well with the traditional torus rotational mechanism in terms of the normalized trench stability number in uniform clay when trench length to depth ratio is greater than 0.6, while significant improvement to the previous solution is achieved for shorter trench panels. A linear variation pattern between trench stability number and soil shear strength gradient is found for non-uniform soil profiles, in particular for shorter trenches. A simplified method is thereafter introduced for preliminary trench stability evaluation in non-uniform clay profiles. For trenches in two-layered clay, a more notable change in trench safety factor with layer thickness ratio is observed when the top layer is weaker than the bottom layer.