Inhomogeneous Field Conditions Research Articles

Clipped DeepControl: Deep neural network two-dimensional pulse design with an amplitude constraint layer

Advanced radio-frequency pulse design used in magnetic resonance imaging has recently been demonstrated with deep learning of (convolutional) neural networks and reinforcement learning. For two-dimensionally selective radio-frequency pulses, the (convolutional) neural network pulse prediction time (a few milliseconds) was in comparison more than three orders of magnitude faster than the conventional optimal control computation. The network pulses were from the supervised training capable of compensating scan-subject dependent inhomogeneities of B0 and B1+ fields. Unfortunately, the network presented with a small percentage of pulse amplitude overshoots in the test subset, despite the optimal control pulses used in training were fully constrained. Here, we have extended the convolutional neural network with a custom-made clipping layer that completely eliminates the risk of pulse amplitude overshoots, while preserving the ability to compensate for the inhomogeneous field conditions.

Advanced design of MRI inversion pulses for inhomogeneous field conditions by optimal control.

Non-selective inversion pulses find widespread use in MRI applications, where requirements on them are increasingly demanding. With the use of high and ultra-high field strength systems, robustness to and inhomogeneities, while tackling SAR and hardware limitations, has rapidly become important. In this work, we propose a time-optimal control framework for the optimization of - and -robust inversion pulses. Robustness is addressed by means of ensemble formulations, while allowing inclusion of hardware and energy limitations. The framework is flexible and performs excellently for various optimization goals. The optimization results are analyzed extensively in numerical experiments. Furthermore, they are validated, and compared with adiabatic RF pulses, in various phantom and in vivo measurements on a 3 T MRI system.

High-resolution diffusion-order NMR spectroscopy in inhomogeneous magnetic fields via intermolecular zero-quantum coherences

Diffusion-order NMR spectroscopy (DOSY) presents a powerful tool for studying solution mixtures by recording diffusion coefficients of individual components and separating their signals into respective 1D NMR spectra. Existing DOSY experiments, however, are generally unsuitable for measurements under adverse magnetic field conditions, because calculations for diffusion coefficients strictly rely on resolved resonances in the 1D NMR spectral domain. Herein, we propose a general DOSY method by introducing intermolecular zero-quantum coherence mechanism into molecular diffusion evolution to overcome the challenge of magnetic field inhomogeneity and to record high-resolution DOSY spectra free of magnetic field inhomogeneity. Our experimental results and theoretical interpretation suggest that the proposed method allows to diffusion analysis and component discrimination on solution mixtures under externally inhomogeneous magnetic field conditions and on intact biological tissues with field inhomogeneity internally caused by magnetic susceptibility variations. This study provides a previously unreported NMR protocol for high-resolution DOSY measurements in inhomogeneous magnetic fields, thus broadening the scope of DOSY applications and showing the promising prospect for studies on chemical and biological mixture samples.

A discrete-modules-based frequency domain hydroelasticity method for floating structures in inhomogeneous sea conditions

Based on the three-dimensional (3D) potential theory and finite element method (FEM), this paper proposes a new numerical method for hydroelastic predictions of floating structures in inhomogeneous seabed and wave field conditions. The continuous floating structure is first discretized into rigid modules connected by elastic beams. The motion equations of the entire floating structure are established according to the six degrees of freedom (6DOF) motions of each module by coupling the hydrodynamics of the modules with the structural stiffness matrix of the elastic beams in the frequency domain. By applying different wave excitation forces onto different modules, this discrete-modules-based method then uniquely realizes application of various wave excitation forces onto different modules of the structures in inhomogeneous waves. The hydroelastic responses of a plate and a Wigley hull under an even and uneven seabed using the proposed method are verified against the results from the published model tests and the conventional 3D hydroelastic method. Finally, the effects of inhomogeneous waves on the distributions of the bending moment, shear force and vertical displacements of the freely floating plate are investigated. The results show that the inhomogeneity of waves may induce about 2∼3 times increase of the force responses in a specific wave frequency.

Ultrahigh-Resolution NMR Spectroscopy for Rapid Chemical and Biological Applications in Inhomogeneous Magnetic Fields.

NMR spectroscopy is a commonly used analytical technique in practical applications, and its applicability is further promoted by pure chemical shift techniques based on spectral simplification for analyses. Unfortunately, magnetic field inhomogeneity caused by adverse experimental conditions remains an obstacle restricting NMR applications. In this study, we introduce a new NMR method for high-resolution pure shift proton (1H) NMR measurements in inhomogeneous magnetic fields. We demonstrate that the method allows one to perform chemical analyses on complex solutions in deshimmed magnetic fields, to obtain metabolite information on intact biological tissues with intrinsic field inhomogeneities and to achieve in situ electrochemical detection under externally adverse field conditions. This approach is readily implemented on common commercial NMR instruments without field shimming and locking procedures, specialized hardware requirements as well as complicated sample pretreatments. It provides an effective tool for NMR applications to high-resolution chemical and biological measurements under inhomogeneous magnetic field conditions.

The Concept of Stationary and Moving Boundaries Modelled as Accelerating or Decelerating Planes in the Understanding of Sweeping Processes Employed for Online Focusing in Capillary Zone Electrophoresis and Electrokinetic Chromatography

Sweeping is an important online focusing method in capillary zone electrophoresis and electrokinetic chromatography, which is employed in numerous methods for analyte enrichment and improvement of detection and quantitation limits. This review intends to summarize the present state of developments in the understanding of sweeping processes with special emphasis on the modelling of moving and stationary boundaries with assumed moving and/or stationary accelerating or decelerating planes. Starting from the description of sweeping for a neutral analyte under homogeneous field conditions, it is shown that the methodology of modelling zone focusing processes with moving and/or stationary accelerating or decelerating planes allows extending the description to charged analytes, inhomogeneous field conditions, retention factor gradient effect conditions, complex formation ligand as sweeping carrier and dynamic pH junction conditions. The present article introduces a generalized theoretical basis that allows a better understanding of the various factors influencing sweeping processes. The discussion is completed with a section on limitations of the presented approach.

Spectral fitting using basis set modified by measured B0 field distribution.

This study sought to demonstrate and evaluate a novel spectral fitting method to improve quantification accuracy in the presence of large magnetic field distortion, especially with high fields. MRS experiments were performed using a point-resolved spectroscopy (PRESS)-type sequence at 7 T. A double-echo gradient echo (GRE) sequence was used to acquire B0 maps following MRS experiments. The basis set was modified based on the measured B0 distribution within the MRS voxel. Quantification results were obtained after fitting the measured MRS data using the modified basis set. The proposed method was validated using numerical Monte Carlo simulations, phantom measurements, and comparison of occipital lobe MRS measurements under homogeneous and inhomogeneous magnetic field conditions. In vivo results acquired from voxels placed in thalamus and prefrontal cortex regions close to the frontal sinus agreed well with published values. Instead of noise-amplifying complex division, the proposed method treats field variations as part of the signal model, thereby avoiding inherent statistical bias associated with regularization. Simulations and experiments showed that the proposed approach reliably quantified results in the presence of relatively large magnetic field distortion. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Transient, polarity-dependent dielectric response in a twisted nematic liquid crystal under very low frequency excitation.

The electric Freedericksz transition is a second-order quadratic effect, which, in a planarly aligned nematic liquid crystal layer, manifests above a threshold field as a homogeneous symmetric distortion with maximum director-tilt in the midplane. We find that, upon excitation by a low frequency (<0.2Hz) square-wave field, the instability becomes spatially and temporally varying. This is demonstrated using calamitic liquid crystals, initially in the 90°-twisted planar configuration. The distortion occurs close to the negative electrode following each polarity switch and, for low-voltage amplitudes, decays completely in time. We use the elastically favorable geometry of Brochard-Leger walls to establish the location of maximum distortion. Thus, at successive polarity changes, the direction of extension of both annular and open walls switches between the alignment directions at the two substrates. For high voltages, this direction is largely along the midplane director, while remaining marginally oscillatory. These results are broadly understood by taking into account the time-varying and inhomogeneous field conditions that prevail soon after the polarity reverses. Polarity dependence of the instability is traced to the formation of intrinsic double layers that lead to an asymmetry in field distribution in the presence of an external bias. Momentary field elevation near the negative electrode following a voltage sign reversal leads to locally enhanced dielectric and gradient flexoelectric torques, which accounts for the surface-like phenomenon observed at low voltages. These spatiotemporal effects, also found earlier for other instabilities, are generic in nature.

Spatiotemporal character of the Bobylev-Pikin flexoelectric instability in a twisted nematic bent-core liquid crystal exposed to very low frequency fields.

The Bobylev-Pikin striped-pattern state induced by a homogeneous electric field is a volume flexoelectric instability, originating in the midregion of a planarly aligned nematic liquid crystal layer. We find that the instability acquires a spatiotemporal character upon excitation by a low frequency (0.5 Hz) square wave field. This is demonstrated using a bent-core liquid crystal, initially in the 90°-twisted planar configuration. The flexoelectric modulation appears close to the cathode at each polarity reversal and, at low voltage amplitudes, decays completely as the field becomes steady. Correspondingly, at successive polarity changes, the stripe direction switches between the alignment directions at the two substrates. For large voltages, the stripes formed nearly along the alignment direction at the cathode gradually reorient toward the midplane director. These observations are generally attributed to inhomogeneous and time-dependent field conditions that come to exist after each polarity reversal. Polarity dependence of the instability is attributed to the formation of intrinsic double layers that bring about an asymmetry in surface fields. Momentary field elevation near the cathode following a voltage sign reversal and concomitant gradient flexoelectric polarization are considered the key factors in accounting for the surfacelike modulation observed at low voltages.

Sweeping as a multistep enrichment process in micellar electrokinetic chromatography: The retention factor gradient effect

The application of a new method developed for the assessment of sweeping efficiency in MEKC under homogeneous and inhomogeneous electric field conditions is extended to the general case, in which the distribution coefficient and the electric conductivity of the analyte in the sample zone and in the separation compartment are varied. As test analytes p-hydroxybenzoates (parabens), benzamide and some aromatic amines are studied under MEKC conditions with SDS as anionic surfactant. We show that in the general case – in contrast to the classical description – the obtainable enrichment factor is not only dependent on the retention factor of the analyte in the sample zone but also dependent on the retention factor in the background electrolyte (BGE). It is shown that in the general case sweeping is inherently a multistep focusing process. We describe an additional focusing/defocusing step (the retention factor gradient effect, RFGE) quantitatively by extending the classical equation employed for the description of the sweeping process with an additional focusing/defocusing factor. The validity of this equation is demonstrated experimentally (and theoretically) under variation of the organic solvent content (in the sample and/or the BGE), the type of organic solvent (in the sample and/or the BGE), the electric conductivity (in the sample), the pH (in the sample), and the concentration of surfactant (in the BGE). It is shown that very high enrichment factors can be obtained, if the pH in the sample zone makes possible to convert the analyte into a charged species that has a high distribution coefficient with respect to an oppositely charged micellar phase, while the pH in the BGE enables separation of the neutral species under moderate retention factor conditions.

Processes involved in sweeping under inhomogeneous electric field conditions as sample enrichment procedure in micellar electrokinetic chromatography

Sweeping under inhomogeneous electric field conditions has been described as a process that includes stacking or destacking of the micelles when entering the sample zone, sweeping of analytes by the stacked or destacked micelles, and destacking or stacking of the swept analyte zone. However, there is ongoing debate that not only the retention factor of the analyte but also the electric conductivity of the sample solution or the concentration of an added salt can have an impact on the enrichment efficiency. Revisiting the equations describing sweeping, a factor θ (phase ratio shift factor) is defined to quantitatively describe the change of the retention factor between the sample and separation zones. The influence of the sample matrix composition on the experimentally obtained sweeping efficiency is studied with SDS as pseudostationary phase taking parabens, benzamide and anilines as model analytes. To this end, a robust and reliable method for the assessment of the sweeping efficiency is developed. The values obtained via this method are very precise and agree well with theoretically predicted ones. The results obtained for varied buffer concentration and varied concentration of NaCl in the sample solution show that under the conditions of our experimental study, the approximation of assuming θ to be equal to the reciprocal value of the field strength enhancement factor γ is valid. Accordingly, the sweeping efficiency for neutral analytes is independent of the electric conductivity of the sample matrix. It is also shown that under specific conditions unexpectedly high enrichment factors are obtained which are ascribed to the focusing of neutral analytes by micellar transient isotachophoresis (mtITP). The results obtained in this study can be used as a guide for better understanding of the sweeping process and the factors affecting the sweeping efficiency in micellar electrokinetic chromatography (MEKC).

Numerical Stress Investigation for Piezoelectric Elements with a Circular Cross Section and Interdigitated Electrodes

The scientific community has put significant effort into the development and optimization of sensors and actuators manufactured as piezoelectric composites with interdigitated electrodes (IDEs), well known as active fiber composite (AFC) and macro fiber composite (MFC). The advantages of these elements are their higher actuation performance and flexibility as compared to monolithic piezoceramic (PZT) elements. In general, their mechanical properties are calculated based on the classical lamination theory and the uniform field model (UFM). These two theories are well suited for predicting the stiffness and piezoelectric strain constants of the AFC or MFC. Although there are a variety of numerical investigations related to their electromechanical properties, there are no appropriate tools for accessing the stresses within these piezoelectric elements (including the inhomogeneous electric field conditions as well as the change in material properties). Explanations are given for this situation indicating the problems in investigating these types of piezoelectric elements with respect to stress states. In this work a finite element modeling approach is presented, which shows the influence of the IDE on the mechanical properties of PZT fibers. Experimental evidence is presented, which affirms the location of critical stress predicted in this model and explains the reported cracking in AFC in past research.

Do geomagnetic storms change the behaviour of the stingless bee guiruçu (Schwarziana quadripunctata)?

Six behavioural experiments were carried out to investigate the magnetic field effects on the nest-exiting flight directions of the honeybee Schwarziana quadripunctata (Meliponini). No significant differences resulted during six experiment days under varying geomagnetic field and the applied static inhomogeneous field (about ten times the geomagnetic field) conditions. A surprising statistically significant response was obtained on a unique magnetic storm day. The magnetic nanoparticles in these bees, revealed by ferromagnetic resonance, could be involved in the observed effect of the geomagnetic storm.

Mechanism of high field electroluminesence and determination of the space charge limited field in polymeric dielectrics

The mechanism of electroluminescence under inhomogeneous field conditions is investigated, as is the use of electroluminescence for determination of the space charge limited field. The latter investigation indicates that if the volume of polymer within the space charge limited field region is too small, the photon flux can be too low to permit accurate determination of the space charge limited field. Further, no means is available to assure that the photon flux is sufficient to assure accuracy.

Guarded needle for “charge injection” measurement

The ability to measure resistive current resulting from high-field carrier mobility in dielectrics provides insight into transport phenomena and high-voltage performance. The threshold electric field for the formation of a space-charge limited field region under highly inhomogeneous field conditions has fundamental significance in that above this field, rapid degradation takes place from hot electrons and UV photons generated by carrier recombination. A guarded needle electrode with a tip radius in the range of 5 μm and an electrical guard which extends to within about 35 μm of the tip provides the basis for measuring high-field mobility as the resistive current can be measured independent of the displacement current. The residual displacement current resulting from the fF capacitance from the needle tip to the opposite plane electrode is compensated out of the measurement. In this article, we provide construction details for such a guarded needle.

Systematic Errors in Radar Wind Estimation: Implications for Comparative Measurements

SA, IDI, and DBS horizontal wind estimators are examined and demonstrated to share common biases under inhomogeneous flow field conditions. The biases can reach magnitudes of several tens of meters per second at mesospheric heights, contaminate gravity wave flow field measurements, cause distortions in measured flow field spectra, and possibly account for large wind measurement errors evidenced in recent experimental studies including the AIDA'89 effort. However, being a linear response to gravity wave flow field fluctuations, the biases are expected to cancel out in long‐term wind and tidal statistics compiled with radar wind data.

Electrical breakdown in ZnS:Mn films grown by rf-magnetron sputtering

We have studied the electrical breakdown field strength Ebr of ZnS films doped with Mn(ZnS:Mn), produced by rf magnetron sputtering, as a function of film thickness d in the system Al-ZnS:Mn-Al. Electron-microscope studies have shown that breakdown is initiated under inhomogeneous field conditions imposed both by the roughness of the aluminum electrode and by the polycrystallinity of the ZnS:Mn film. An observed increase in Ebr as d is reduced below 0.7 µm, with no polarity effect in breakdown under dc field conditions, is explained in terms of an electron-thermal breakdown model.

New Efficient Treatment of Impact Ionization in Submicron Metal-Oxide-Semiconductor Field-Effect Transistors

Shockley's lucky electron model has been widely used for modeling impact ionization. Various nonlocal generalizations of this model have been proposed in order to model impact ionization more accurately for strongly inhomogeneous material and field conditions which exist, e.g., in n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). Recently it has been shown that the spatial distribution of impact ionization events resulting from the nonlocal variant is in close agreement with corresponding results derived from an advanced Monte Carlo model. However, the former lucky electron model fails to closely reflect the underlying microscopic processes and is based on the restrictive assumption that the electron trajectories are electrostatic field lines. In order to incorporate more microscopic features, an improved lucky electron modeling approach including ballistic and diffusive transport effects is proposed, which yields comparably favorable results if compared to Monte Carlo data.

Temperature and Space‐Charge Effects in Liquid Hydrocarbons

Preliminary results on the residual d‐c conduction and space‐charge polarization in n‐hexane and n‐heptane under highly inhomogeneous field conditions (thin wire in coaxial cylinder) are presented. A residual current is observed under high applied voltage for either polarity of the test cell. At 20deg;C the current is one order of magnitude larger when the wire is the positive electrode. As temperature is increased, the residual current for the wire positive decreases and for the wire negative increases, until at 50°C it is about the same for both polarities. By heating only the wire, observations supporting and extending these findings were made. The gross features of the current‐voltage characteristics can be explained by dissociation of polar impurities in the region of high field strength. The dependence on polarity can be explained by space‐charge distortion of the field. For negative wire, field emission appears to occur from the wire, surrounding it with a current‐limiting space charge.