Gradient System Research Articles

Reducing PNS with minimal performance penalties via simple pulse sequence modifications on a high-performance compact 3T scanner

One of the major concerns associated with high-performance gradients is peripheral nerve stimulation (PNS) of the subject during MRI exams. Since the installation, more than 680 volunteer subjects (patients and controls) have been scanned on a compact 3 T MRI system with high-performance gradients, capable of 80 mT m−1 gradient amplitude and 700 T m−1 s−1 slew rate simultaneously. Despite PNS concerns associated with the high-performance gradients, due to the smaller physical dimensions of the gradient coils, minimal or no PNS sensation was reported with most pulse sequences. The exception was PNS reported by only five of 252 subjects (about 2%) scanned with a specific 3D fast spin echo pulse sequence (3DFLAIR). Rather than derating the entire system performance across all pulse sequences and all gradient lobes, we addressed reported PNS effect with a simple and specific modification to the targeted lobes of the problematic pulse sequence. in addition, the PNS convolutional model was adapted to predict sequence-specific PNS threshold level and its reduction after derating. The effectiveness of the targeted pulse sequence modification was demonstrated by successfully re-scanning four of the subjects who previously reported PNS sensations without further reported PNS. The pulse sequence modification did not result in noticeable degradation of image quality or substantial increase in scan time. The results demonstrated that PNS was rarely reported on the compact 3 T, and when it was, utilizing a specific modification of the gradient waveform causing PNS was an effective strategy, rather than derating the performance of the entire gradient system.

Response of cracking processes in variations of geophysical fields

This paper discusses a range of issues related to the interpretation of time variations of geophysical fields as a response of the geophysical environment to lunar-solar tidal influences and seismic events under conditions of submeridional compression. A model linking the mechanism of crack formation and generation of geophysical fields has been proposed. The concept of the energy characteristics of a seismic field of deep underground (endogenous) origin along the seismic wave field observed on the Earth's surface for three mutually perpendicular components has been introduced. The results of field studies of simultaneous recording of a seismic wave field by a three-component gradient system and a magnetotelluric field at monitoring sites of the Bishkek Geodynamic Proving Ground (BGPG in Northern Tien Shan) are presented. The test area is situated in an active intracontinental mountain Central Asian Orogenic Belt with the frequent seismicity. For the experiment purposes, we select the Ukok regime monitoring point in the Kochkor basin of Kyrgyz Tien Shan. Because the location is characterized by low-level of seismic noise and the technical-and-industrial influence. The applicability of such a joint approach in geophysics is discussed.According to the results of the experiments performed, we establish a possible causal relationship between lunar-solar tides and activation of the deep underground (endogenous) source. Based on the correlation analysis of gravitational tidal effects, and the results of magnetotelluric monitoring, the azimuthal dependence of the electrical conductivity of the medium was revealed. We detect characteristic changes in the studied electromagnetic parameters, which we identify as a result of the redistribution of fluids between the fracture systems, were. The joint analysis of data from regime magnetotelluric observations and seismicity distribution confirms the high sensitivity of the method to changes in the stress-strain state of the geological environment. The confinement of seismic events to zones with high gradients of variations of apparent electrical resistivity is shown.The work may be of interest to specialists in the field of Earth Sciences, dealing with the interpretation of experimental data from geophysical test sites.

The Effect of the Queen's Presence on Thermal Behavior and Locomotor Activity of Small Groups of Worker Honey Bees.

We examined effects of the queen’s presence on diurnal rhythms of temperature preference (TP) and locomotor activity (LA) in worker honeybees’ groups. TP and LA of six queenless and six queenright (with the queen) groups of bees, consisting of 7–8 worker bees, were recorded in a thermal gradient system for four days, under light to darkness (LD) 12:12 photoperiod. The same experiments were conducted on five virgin queens (of the same age as those in the queenright groups), which were placed individually in the gradient chambers. The single virgin queens showed signs of distress and no rhythms of TP and LA. In contrast, there were diurnal rhythms of TP and LA in both group variants with daytime activity and nighttime rest. However, the queen’s presence exerted a strong calming effect, reducing LA of bees both at day- and nighttime. The nighttime minimum LA of queenright groups was five times lower than that in queenless groups. Moreover, there was a reversal of the diurnal pattern of TP in queenright groups. The results are discussed in terms of the bee colony organization as a superorganism.

The study of peak purity of Chinese medicinal compositions by HPLC/DAD

Peak purity was studied by diode-array detector in high performance liquid chromatography (DAD-HPLC) to assay seven Chinese medicinal standards which were prepared of various different concentrations. Three injections were executed for each concentration sample, then the SI values were obtained by comparing the absorption spectra of three injections of each concentration's value. The results showed that SI values increased when the concentrations decreased. The absorption spectra of peaks of reference standards and samples are usually not matched while running a gradient system, because the background of the solvent is continuously changed. However, a good match was obtained by subtracting absorption spectrum of baseline before comparing absorption spectra of reference standards and analytical peaks. The effect of insufficient running time for HPLC analysis was also studied. For example, Ching-Shy-Huah-Tarn - Tang normally had a running time of 60 min. If the running time was changed from 60 min to 20 min, the peak area was significantly interfered by the previous run during a continuous eight injections trial.

Gradient systems of harmonic polynomials

We characterize all local phase-portraits of the finite and infinite singular points of the gradient systems defined by the real harmonic polynomials in two variables.We classify all the non-equivalent topological phase portraits of the gradient systems in the Poincaré disc defined by harmonic polynomials of degree less than five.

Free-breathing self-gated 4D lung MRI using wave-CAIPI.

The aim of this study was to compare the wave-CAIPI (controlled aliasing in parallel imaging) trajectory to the Cartesian sampling for accelerated free-breathing 4D lung MRI. The wave-CAIPI k-space trajectory was implemented in a respiratory self-gated 3D spoiled gradient echo pulse sequence. Trajectory correction applying the gradient system transfer function was used, and images were reconstructed using an iterative conjugate gradient SENSE (CG SENSE) algorithm. Five healthy volunteers and one patient with squamous cell carcinoma in the lung were examined on a clinical 3T scanner, using both sampling schemes. For quantitative comparison of wave-CAIPI and standard Cartesian imaging, the normalized mutual information and the RMS error between retrospectively accelerated acquisitions and their respective references were calculated. The SNR ratios were investigated in a phantom study. The obtained normalized mutual information values indicate a lower information loss due to acceleration for the wave-CAIPI approach. Average normalized mutual information values of the wave-CAIPI acquisitions were 10% higher, compared with Cartesian sampling. Furthermore, the RMS error of the wave-CAIPI technique was lower by 19% and the SNR was higher by 14%. Especially for short acquisition times (down to 1 minute), the undersampled Cartesian images showed an increased artifact level, compared with wave-CAIPI. The application of the wave-CAIPI technique to 4D lung MRI reduces undersampling artifacts, in comparison to a Cartesian acquisition of the same scan time. The benefit of wave-CAIPI sampling can therefore be traded for shorter examinations, or enhancing image quality of undersampled 4D lung acquisitions, keeping the scan time constant.

Development and validation of bioanalytical methods for LNA-i-miR-221 quantification in human plasma and urine by LC-MS/MS

LNA-i-miR-221, a 13-mer oligonucleotide, has proved favorable efficacy and safety profiles in the preclinical studies, leading to being approved for use in clinical trials by regulatory authorities. The objective of this study was to develop and validate LC–MS/MS methods to quantify LNA-i-miR-221 in human plasma and urine. Chromatographic separation was performed with a gradient system on HALO C18 column using hexafluoro-2-propanol/triethylamine buffer and methanol as mobile phase. LNA-i-miR-221 was detected on tandem mass spectrometer with electrospray ionization source in negative ion mode. The methods showed good linearity within the calibration range of 50−25000 ng/mL and 50−50000 ng/mL for human plasma and urine, respectively. The methods proved to be accurate, precise and selective in both human matrices. These validated methods are reliable and are currently in use to support a first-in-human clinical trial of LNA-i-miR-221 in patients affected by refractory multiple myeloma and advanced solid tumors.

Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system.

Diffusion weighted imaging (DWI) is commonly limited by low signal-to-noise ratio (SNR) as well as motion artifacts. To address this limitation, a method that allows to maximize the achievable signal yield and increase the resolution in motion robust single-shot DWI is presented. DWI was performed on a 3T scanner equipped with a recently developed gradient insert (gradient strength: 200 mT/m, slew rate: 600 T/m/s). To further shorten the echo time, Stejskal-Tanner diffusion encoding with a single-shot spiral readout was implemented. To allow non-Cartesian image reconstruction using such strong and fast gradients, the characterization of eddy current and concomitant field effects was performed based on field-camera measurements. An echo time of only 19 ms was achieved for a b-factor of 1000 s/mm2 . An in-plane resolution of 0.68 mm was encoded by a single-shot spiral readout of 40.5 ms using 3-fold undersampling. The resulting images did not suffer from off-resonance artifacts and blurring that are common to single-shot images acquired with regular gradient systems. Spiral diffusion imaging using a head gradient system, together with an accurate characterization of the encoding process allows for a substantial reduction of the echo time, and improves the achievable resolution in motion-insensitive single-shot DWI.

Unconditionally Stable Algorithm for Copolymer and Copolymer-Solvent Systems

In the time evolution simulation of a copolymer system towards its equilibrium configuration, it is common to use the Otha-Kawasaki approach for free energy and time evolution by means of a Cahn-Hilliard diffusion equation. The conventional numerical resolution is to use the cell dynamics simulation method (CDS). Although this method gives an adequate response, it is limited since it needs very small time steps to present both appropriate resolution and stability. Unconditionally stable methods have recently been used in gradient systems that provide adequate resolution and stability with a greater time step in solving Cahn-Hilliard equations. In this paper we develop and implement unconditionally stable algorithms for copolymer-solvent systems and for resolution of the time evolution of block copolymer systems under the Otha-Kawasaki functional.

Field camera versus phantom-based measurement of the gradient system transfer function (GSTF) with dwell time compensation

PurposeThe gradient system transfer function (GSTF) can be used to describe the dynamic gradient system and applied for trajectory correction in non-Cartesian MRI. This study compares the field camera and the phantom-based methods to measure the GSTF and implements a compensation for the difference in measurement dwell time. MethodsThe self-term GSTFs of a MR system were determined with two approaches: 1) using a dynamic field camera and 2) using a spherical phantom-based measurement with standard MR hardware. The phantom-based GSTF was convolved with a box function to compensate for the dwell time dependence of the measurement. The field camera and phantom-based GSTFs were used for trajectory prediction during retrospective image reconstruction of 3D wave-CAIPI phantom images. ResultsDifferences in the GSTF magnitude response were observed between the two measurement methods. For the wave-CAIPI sequence, this led to deviations in the GSTF predicted trajectories of 4% compared to measured trajectories, and residual distortions in the reconstructed phantom images generated with the phantom-based GSTF. Following dwell-time compensation, deviations in the GSTF magnitudes, GSTF-predicted trajectories, and resulting image artifacts were eliminated (< 0.5% deviation in trajectories). ConclusionWith dwell time compensation, both the field camera and the phantom-based GSTF self-terms show negligible deviations and lead to strong artifact reduction when they are used for trajectory correction in image reconstruction.

Tracing the Spatial Distribution of Whole-Lake Exchange of Groundwater and Lake Water in Low-Hydraulic Gradient Systems Using δ18O and Electrical Conductivity and Uncertain End-Member Mixing Analysis

δ18O and electrical conductivity (EC) were used successfully to trace the spatial distribution of whole-lake groundwater-lake exchange for a small (four ha) groundwater-fed lake situated in a low relief and low hydraulic gradient area. The method relies on quick sampling of shallow groundwater, direct analysis of EC in the field, and relatively in-expensive analysis of δ18O in the laboratory. Ternary uncertain end-member mixing analysis (precipitation, groundwater, and lake water) quantified the composition of water discharging to and recharging from the lake. The tracer distribution and mixing analysis were in agreement with the interpreted groundwater flow near the lake. The use of only one tracer (either δ18O or EC) gave the same results for the recharge segments, but the discharge segments changed the origin of the water from being groundwater to precipitation controlled. The two tracers complemented each other, especially with different signals in precipitation and groundwater. The uncertain end-members were assessed based on local (groundwater and lake water) and off-site (precipitation) data. The off-site data were found to be useful if it contained representative information on local-site seasonality (uncertainty, variance). Final end-member concentrations could explain the transience of the hydrology at the site (i.e., flooding of the area adjacent to the lake during periods with high precipitation, and variability of the δ18O signal in precipitation). This methodology potentially represents a new option to study groundwater-lake systems. The tracer information collected over only two days is useful by itself for developing the next steps like the quantification of fluxes based on other standard methods (Darcy approach, seepage meters, or temperature). The tracer information can provide quantitative estimation of inputs and outputs by using the mixing analysis.

Retrospective Correction of ADC for Gradient Nonlinearity Errors in Multicenter Breast DWI Trials: ACRIN6698 Multiplatform Feasibility Study.

The presented analysis of multisite, multiplatform clinical oncology trial data sought to enhance quantitative utility of the apparent diffusion coefficient (ADC) metric, derived from diffusion-weighted magnetic resonance imaging, by reducing technical interplatform variability owing to systematic gradient nonlinearity (GNL). This study tested the feasibility and effectiveness of a retrospective GNL correction (GNC) implementation for quantitative quality control phantom data, as well as in a representative subset of 60 subjects from the ACRIN 6698 breast cancer therapy response trial who were scanned on 6 different gradient systems. The GNL ADC correction based on a previously developed formalism was applied to trace-DWI using system-specific gradient-channel fields derived from vendor-provided spherical harmonic tables. For quantitative DWI phantom images acquired in typical breast imaging positions, the GNC improved interplatform accuracy from a median of 6% down to 0.5% and reproducibility of 11% down to 2.5%. Across studied trial subjects, GNC increased low ADC (<1 µm2/ms) tumor volume by 16% and histogram percentiles by 5%–8%, uniformly shifting percentile-dependent ADC thresholds by ∼0.06 µm2/ms. This feasibility study lays the grounds for retrospective GNC implementation in multiplatform clinical imaging trials to improve accuracy and reproducibility of ADC metrics used for breast cancer treatment response prediction.

Effective diffusion in thin structures via generalized gradient systems and EDP-convergence

The notion of Energy-Dissipation-Principle convergence (EDP-convergence) is used to derive effective evolution equations for gradient systems describing diffusion in a structure consisting of several thin layers in the limit of vanishing layer thickness. The thicknesses of the sublayers tend to zero with different rates and the diffusion coefficients scale suitably. The Fokker–Planck equation can be formulated as gradient-flow equation with respect to the logarithmic relative entropy of the system and a quadratic Wasserstein-type gradient structure. The EDP-convergence of the gradient system is shown by proving suitable asymptotic lower limits of the entropy and the total dissipation functional. The crucial point is that the limiting evolution is again described by a gradient system, however, now the dissipation potential is not longer quadratic but is given in terms of the hyperbolic cosine. The latter describes jump processes across the thin layers and is related to the Marcelin-de Donder kinetics.

Simultaneous Determination of Active Ingredients in Multicomponent Common Over the Counter Tablets in the Present of Parabens and 4-Aminophenol by HPLC

A new selective reverse-phase high-performance liquid chromatography method with an attitude of practical use in pharmaceutical laboratories for simultaneous determination of Ascorbic acid, Acetaminophen, phenylephrine, Caffeine, Pamabrom, Guaifenesin, Dextromethorphan, Pyrilamine, Chlorpheniramine maleate, Ibuprofen in the presence of Methyl, Ethyl, Butyl Parabens as preservatives has been developed and validated. These active pharmaceutical ingredients widely used in a variety of cough and cold medicines. The separation was carried out on an Atlantis dC18 column including 4.6 mm inner diameter, 150 mm length and particle size of 3 μm, using a mobile phase which consisted of an ion-pairing agent (sodium salt of heptane sulphonic acid) as a buffer solution mixed with acetonitrile as an organic modifier at a flow rate of 1 mL min−1 on a gradient system. The investigation was carried out on wavelength program detection to quantify the very low concentration of Dextromethorphan and Chlorpheniramine and high concentration of Acetaminophen, Guaifenesin, Ibuprofen, Ascorbic Acid at the wavelengths of 290 and 215 nm. The temperature was maintained at 40 °C. The method was shown to be linear (R2 > 0.996), accurate (recoveries 97.77–103.94%), precise (RSD ≤ 1.0%), sensitive, specific, simple, fast and robust. The proposed reverse phase-liquid chromatography method was successfully applied to the routine analysis of the Over-The-Counter tablets containing these active pharmaceutical ingredients without any interference with excipients and parabens and 4-aminophenol.

Convergence Rate of Inertial Proximal Algorithms with General Extrapolation and Proximal Coefficients

In a Hilbert space setting ${\mathcal{H}}$, in order to minimize by fast methods a general convex lower semicontinuous and proper function ${\Phi }: {\mathcal{H}} \rightarrow \mathbb {R} \cup \{+\infty \}$, we analyze the convergence rate of the inertial proximal algorithms. These algorithms involve both extrapolation coefficients (including Nesterov acceleration method) and proximal coefficients in a general form. They can be interpreted as the discrete time version of inertial continuous gradient systems with general damping and time scale coefficients. Based on the proper setting of these parameters, we show the fast convergence of values and the convergence of iterates. In doing so, we provide an overview of this class of algorithms. Our study complements the previous Attouch–Cabot paper (SIOPT, 2018) by introducing into the algorithm time scaling aspects, and sheds new light on the Guler seminal papers on the convergence rate of the accelerated proximal methods for convex optimization.

Preferencia termal de postlarvas del camarón de río Cryphiops caementarius previamente aclimatados a diferentes temperaturas

Reyes Avalos, W., Campoverde Vigo, L., Ferrer Chujutalli, K., Sernaque Jacinto, J. 2020. Thermal preference of postlarvae of the river shrimp Cryphiops caementarius previously acclimated to different temperatures. Ecosistemas 29(1):1802. https://doi.org/10.7818/ECOS.1802 Temperature is one of the main environmental parameters that affects growth, reproduction and migration of ectothermic animals. The aim of this research was to determine the thermal preference of the postlarvae of the river shrimp Cryphiops caementarius, previously acclimatized to different temperatures. The postlarvae were collected from the Mala River and acclimatized for 60 days in the laboratory at 19.4 ± 0.6 oC (control), 22, 24, 26, 28 and 30 oC. The experimental thermal gradient system (10 oC to 33 oC) consisted of a PVC gutter (3 m long and 10 cm deep), with 18 compartments. Thermoregulators (100 W) or frozen hydrogel bags were placed on opposite ends of the gutter. The thermal preference was determined with the acute method and the tests were carried out between 08:00 and 14:00 h. The postlarvae acclimatized to 19 oC and 22 oC were moved to compartments with higher temperatures than their acclimatization temperature. The postlarvae acclimatized to 28 oC and 30 oC were moved to compartments with lower temperatures than their acclimatization temperature. The postlarvae acclimatized to 24 oC and 26 oC were moved to compartments with similar acclimatization temperature. The acclimatization temperature had no significant effect (P>0.05) on the thermal preference (23.9 oC to 24.7 oC) of the postlarvae and the final thermal preference was 24.5 oC. We propose, that this thermal preference is taken into account for the culturing this species.

Complementary approach for analysis of phospholipids by liquid chromatography hyphenated to elemental and molecular mass spectrometry.

Phospholipids are one of the most important lipid categories with multiple functions in biological systems. Their analysis can contribute to a better understanding of metabolomic and kinetic processes in living cells. Comprehensive methods based on liquid chromatography coupled to mass spectrometry are available for phospholipid identification and quantification. However, quantification of phospholipids using electrospray ionization-mass spectrometry with internal standards is still challenging due to several reasons. In particular, the detector response of phospholipid species differs with variation of the head group as well as the fatty acid chain length and double bond number. Inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) provides an alternative approach for their absolute quantification with universal detector response for phosphorus independent of its chemical form and proportional to its quantity. Therefore, a quantification method based on compound-independent calibration using hydrophilic interaction liquid chromatography (HILIC) coupled to ICP-MS/MS was developed. An inverse gradient system was implemented for constant mobile phase composition after HILIC separation, which provides steady plasma ionization conditions. Isobaric phosphorus interferences were decreased by using the oxygen reaction mode of the triple quadrupole based ICP-MS/MS instrument. Complementary molecular information was obtained by ESI-high-resolution MS and MS/MS. The applicability of this approach was demonstrated in a proof of concept by complementary analysis of a total lipid extract of baker's yeast.

Stochastic homogenization of \Lambda -convex gradient flows

<p style='text-indent:20px;'>In this paper we present a stochastic homogenization result for a class of Hilbert space evolutionary gradient systems driven by a quadratic dissipation potential and a <inline-formula><tex-math id="M2">\begin{document}$ \Lambda $\end{document}</tex-math></inline-formula>-convex energy functional featuring random and rapidly oscillating coefficients. Specific examples included in the result are Allen-Cahn type equations and evolutionary equations driven by the <inline-formula><tex-math id="M3">\begin{document}$ p $\end{document}</tex-math></inline-formula>-Laplace operator with <inline-formula><tex-math id="M4">\begin{document}$ p\in (1, \infty) $\end{document}</tex-math></inline-formula>. The homogenization procedure we apply is based on a stochastic two-scale convergence approach. In particular, we define a stochastic unfolding operator which can be considered as a random counterpart of the well-established notion of periodic unfolding. The stochastic unfolding procedure grants a very convenient method for homogenization problems defined in terms of (<inline-formula><tex-math id="M5">\begin{document}$ \Lambda $\end{document}</tex-math></inline-formula>-)convex functionals.

A new iterative refinement for ill-conditioned linear systems based on discrete gradient

In this paper, a new iterative refinement for ill-conditioned linear systems is derived based on discrete gradient methods for gradient systems. It is proved that the new method is convergent for any initial values irrespective of the choice of the stepsize h. Some properties of the new iterative refinement are presented. It is shown that the condition number of the coefficient matrix in the linear system to be solved in every step can be improved significantly compared with Wilkinson’s iterative refinement. The numerical experiments illustrate that the new method is more effective and efficient than Wilkinson’s iterative refinement when dealing with ill-conditioned linear systems.

Spatiotemporal Variation of Plant Diversity Under a Unique Estuarine Wetland Gradient System in the Yellow River Delta, China

Multiple natural and human factors in estuarine wetlands result in complicated land surface characteristics with distinct spatial and temporal heterogeneities, thereby contributing to the difficulty in identifying spatiotemporal variations and influencing factors of plant diversity. A unique estuarine wetland gradient system (UEWGS) consisting of soil, vegetation, heat, distance, landscape, and anthropogenic gradients was established based on the ecological features of estuarine wetland through remote sensing and field investigation methods. It resolved the complicated land surface characteristics, covered all aspects of factors influencing plant diversity, and possessed distinct spatiotemporal heterogeneities. The Yellow River Delta, the largest estuarine wetland in the northern China, was selected as the study area to demonstrate UEWGS in four seasons in 2017. A total of 123 species were recorded with considerable seasonal difference. Phragmites australis, Suaeda salsa, and Tamarix chinensis were the dominant species, and crop species also played important roles. In single effect, all aspects of gradients exerted significant influences, yet only vegetation gradient possessed significant influences in all seasons. In comprehensive effect, soil, vegetation, heat, and distance gradients showed significant gross influences. Moisture content in soil gradient and net primary productivity in vegetation gradient possessed significant net influences in all seasons and can be considered as the main driving factor and indicator, respectively, of plant diversity. The results validated the significance of UEWGS in revealing the plant diversity spatiotemporal characteristics and influencing factors, and UEWGS possessed universal applicability in the spatiotemporal analysis of plant diversity in estuarine areas.