Magnetic Tensor Research Articles

Robust Estimation of Metal Target Shape Using Time-Domain Electromagnetic Induction Data

Discriminating metal parts of buried hazardous targets from ordinary metallic clutter is a very difficult and time-consuming task. For that purpose, electromagnetic induction (EMI) sensors composed of multiple transmitter/receiver coils in multiaxis arrangements are commonly used. In this way, data of high fidelity and spatial diversity are obtained so that the target can be characterized in terms of its geometric and electromagnetic properties. However, this often increases sensor size and reduces its portability, which is a problem for humanitarian demining applications, where compact, robust, and lightweight sensors are needed. If such sensors are to be used for target characterization, robust estimation algorithms are required, capable of coping with limited spatial information content and uncertainties related to sensor positioning and its coil geometry model. In this paper, we present a robust concept for estimating the general shape of magnetic metal targets using time-domain EMI sensors with single-axis coil geometries. We introduce the signature matrix, a parameter derived from time-dependent eigenvalues of the target’s magnetic polarizability tensor, and use it for shape estimation. The proposed method was evaluated both through simulations and experiments, using two different sensor platforms (laboratory-based experimental sensor platform and a commercial metal detector mounted on a mobile robot). The obtained results clearly indicate that the target shape can be estimated from sensor data of limited spatial diversity and under the uncertainties of sensor positioning and coil geometry.

Cloaking the magnons

We propose two approaches to cloak the spin waves (magnons) by investigating magnetization dynamics. One approach is based on a spatially inhomogeneous anisotropic magnetic moment tensor. The other mechanism is using a spatially inhomogeneous anisotropic gyromagnetic factor tensor and an inhomogeneous external magnetic field. For both approaches, the damping tensor is also inhomogeneous and anisotropic. The magnetic characteristic functions of the magnetic materials have been theoretically derived for both mechanisms. A non-magnetic core, which prevents magnons from entering and consequently distorts the spin wave propagation, can be cloaked by a structured magnetic shell to redirect the spin wave around the core using the above design mechanisms. We discuss the feasibility of the proposed mechanisms in an ensemble of quantum dot molecules and magnetic semiconductors. The proposed approaches shed light on transformation magnonics, and can be utilized for future spin-wave lenses, concentrators, low back-scattering waveguides, and ultimately quantum computing.

Eliminating magnetic disturbance of satellites by method of magnetic field gradient tensor measurement

Eliminating magnetic disturbance of satellites by method of magnetic field gradient tensor measurement

Cognitive insight is associated with cortical thickness in first-episode psychosis

Compared to non-clinical subjects, people with psychosis show poor cognitive insight as reflected in low Self-Reflectiveness and high Self-Certainty. Neuroimaging studies have reported that 1) low Self-Reflectiveness is associated with volumetric reductions in ventrolateral prefrontal cortex (VLPFC), 2) higher Self-Certainty is associated with volumetric reductions in hippocampus, and 3) higher Self-Certainty is associated with fractional anisotropy in the fornix, in people with psychosis. The aims of the current study were to expand on this research by 1) performing an exploratory whole-brain cortical thickness analysis of the neural correlates of cognitive insight, to reveal whether regions outside the VLPFC are important for cognitive insight, and 2) to evaluate associations between cognitive insight and subfields of the hippocampus, which are distinct, interacting, and have different functions. We also aimed to replicate previous research documenting associations between cognitive insight and 3) total hippocampal volumes and 4) fornix fractional anisotropy. Fifteen people with a first-episode psychosis completed the Beck Cognitive Insight Scale and provided magnetic resonance and diffusion tensor imaging scans. Cortical thickness and hippocampal volumes were analyzed in FreeSurfer, and fornixfractional anisotropy was analyzed in Diffusion Toolkit/TrackVis. Higher Self-Reflectiveness and lower Self-Certainty significantly associated with thickness and thinness in VLPFC, respectively, as well as thickness and thinness in widespread frontal, parietal and temporal cortices. No associations emerged between Self-Reflectiveness or Self-Certainty and hippocampal total or sub-field volumes, or fornix fractional anisotropy. Results suggest that the neural correlates of cognitive insight involve a network of frontal, temporal and parietal brain regions.

Mathematical and numerical analysis of low-frequency scattering from a PEC ring torus in a conductive medium

The electric and magnetic fields scattered off a non-penetrable ring torus, being characterized as perfect conductor, embedded in a homogeneous conductive medium and illuminated by a low-frequency magnetic dipole of arbitrary orientation and harmonic time-dependence are investigated herein. Upon definition of the complex wave number of the exterior medium k via its skin-depth, the 3-D scattering boundary value problem is handled via convenient low-frequency expansions in terms of powers of (ik)n, n ≥ 0 for the fields. A Maxwell-type problem is transformed into intertwined Laplace's or Poisson's potential-type boundary value problems with impenetrable boundary conditions. Using a toroidal coordinate system attached to the torus, they are solved as infinite series expansions for the fields in terms of toroidal eigenfunctions. In practice, what is accessible to the measurement is the scattered magnetic field. The static term (n=0) provides most of its real (or in-phase) part and the second-order term (n=2) consists of most of its imaginary part (quadrature), where in both cases a small contribution of the third-order term (n=3) is being calculated. For n=1, there exists no field, while the terms for n ≥ 4 and for such kind of applications have been proved to be of minor significance, hence they are neglected. The resulting infinite linear systems can be solved at any accuracy level through a cut-off process or via an analytical technique based on the method of finite continuous fraction solutions. Basics of the far-field approximation and the magnetic polarizability tensor are also included. At implementation stage, simulations are proposed in various situations, where a full-wave, finite-element approach is discussed.

Calculation of Magnetization and Permeability Tensor of a Partially Magnetized Cylindrical Ferrite Resonator

A accurate method of calculating a magnetization and permeability tensor of ferrites is proposed in this paper. We introduce a cylindrical ferrite resonator that is located between parallel metal plates. We theoretically derive exact eigenvalue equations for the mode-splitting phenomenon of the HE 111 mode. We also show that the mode-splitting behavior depends on a single magnetization value of the ferrite. The magnetization and permeability tensor components are computed from the measured resonant frequencies of the resonator. We present an analysis of cylindrical ferrite resonator and its experimental results.

Recovery of injured fornical crura following neurosurgical operation of a brain tumor: a case report.

Recovery of injured fornical crura following neurosurgical operation of a brain tumor: a case report.

Intrinsic conductivity of carbon nanotubes and graphene sheets having a realistic geometry.

The addition of carbon nanotubes (CNTs) and graphene sheets (GSs) into polymeric materials can greatly enhance the conductivity and alter the electromagnetic response of the resulting nanocomposite material. The extent of these property modifications strongly depends on the structural parameters describing the CNTs and GSs, such as their shape and size, as well as their degree of particle dispersion within the polymeric matrix. To model these property modifications in the dilute particle regime, we determine the leading transport virial coefficients describing the conductivity of CNT and GS composites using a combination of molecular dynamics, path-integral, and finite-element calculations. This approach allows for the treatment of the general situation in which the ratio between the conductivity of the nanoparticles and the polymer matrix is arbitrary so that insulating, semi-conductive, and conductive particles can be treated within a unified framework. We first generate ensembles of CNTs and GSs in the form of self-avoiding worm-like cylinders and perfectly flat and random sheet polymeric structures by using molecular dynamics simulation to model the geometrical shapes of these complex-shaped carbonaceous nanoparticles. We then use path-integral and finite element methods to calculate the electric and magnetic polarizability tensors (αE, αM) of the CNT and GS nanoparticles. These properties determine the conductivity virial coefficient σ in the conductive and insulating particle limits, which are required to estimate σ in the general case in which the conductivity contrast Δ between the nanoparticle and the polymer matrix is arbitrary. Finally, we propose approximate relationships for αE and αM that should be useful in materials design and characterization applications.

Conductivity Anomalies in Central Europe

This paper is a review of studies which, by applying the magnetotelluric, geomagnetic deep sounding, and magnetovariational sounding methods (the latter refers to usage of the horizontal magnetic tensor), investigate Central Europe for zones of enhanced electrical conductivity. The study areas comprise the region of the Trans-European Suture Zone (i.e. the south Baltic region and Poland), the North German Basin, the German and Czech Variscides, the Pannonian Basin (Hungary), and the Polish, Slovakian, Ukrainian, and Romanian Carpathians. This part of the world is well investigated in terms of data coverage and of the density of published studies, whereas the certainty that the results lead to comprehensive interpretations varies within the reviewed literature. A comparison of spatially coincident or adjacent studies reveals the important role that the data coverage of a distinct conductivity anomaly plays for the consistency of results. The encountered conductivity anomalies are understood as linked to basin sediments, asthenospheric upwelling, large differences in lithospheric age, and—this concerns most of them, which all concentrate in the middle crust—tectonic boundaries that developed during all mountain building phases that have taken place on the continent.

Recent Advances in Theoretical and Physical Aspects of NMR Chemical Shifts

In the first part of this review, theoretical aspects of nuclear magnetic shielding include (a) general theory, for example, newly developed approaches in relativistic theory of nuclear shielding, the relation between the spin-rotation tensor and shielding in relativistic theory, ab initio methods for treating open shell systems and a complete theory of chemical shifts in paramagnetic systems, the link between the definitions of the elusive concepts aromaticity and anti-aromaticity and the magnetic properties: the magnetizability tensor and the nuclear magnetic shielding tensor via delocalized electron currents and electron current maps, (b) ab initio and DFT calculations, both relativistic and non-relativistic, for various nuclei in various molecular systems using various levels of theoretical treatment. Physical aspects include (a) anisotropy of the shielding tensor, usually from solid state measurements, and calculations to support these, (b) shielding surfaces and rovibrational averaging, paying special attention to the sensitive relationship between shielding and bond angles or torsion angles that makes shielding such a powerful tool for structural/conformational determination in macromolecules, (c) chemical shifts that arise from isotopic substitution of NMR nucleus or neighboring nuclei, (d) intermolecular effects on nuclear shielding, and (e) absolute shielding scales.

Magnetic tensor gradiometry using Ramsey interferometry of spinor condensates

We have realized a magnetic tensor gradiometer by interferometrically measuring the relative phase between two spatially separated Bose-Einstein condensates (BECs). We perform simultaneous Ramsey interferometry of the proximate $^{87}$Rb spin-1 condensates in freefall and infer their relative Larmor phase -- and thus the differential magnetic field strength -- with a common-mode phase noise suppression exceeding $50\,\mathrm{dB}$. By appropriately biasing the magnetic field and separating the BECs along orthogonal directions, we measure the magnetic field gradient tensor of ambient and applied magnetic fields with a nominal precision of $30\,\mathrm{\mu G\,cm^{-1}}$ and a sensor volume of $2\times10^{-5}\,\mathrm{mm}^3$. We predict a spin-projection noise limited magnetic energy resolution of order $\hbar$ for typical Zeeman coherence times of trapped condensates with this scheme, even with the low measurement duty cycle inherent to BEC experiments.

Cylindrically symmetric relativistic fluids and the purely electric Weyl tensor

In General Relativity (GR), the analysis of electric and magnetic Weyl tensors has been studied by various authors. The present study deals with cylindrically symmetric relativistic fluids in GR characterized by the vanishing of magnetic Weyl tensor-purely electric (PE) fields. A very new assumption has been adapted to solve the Einstein's equations and the obtained solution is shearing at all. We signified the importance of PE fields in the context of expansion scalar, energy density, shear and acceleration.

Protein Dynamics in the Solid State from (2)H NMR Line Shape Analysis. II. MOMD Applied to C-D and C-CD3 Probes.

Deuterium line shape analysis from mobile C–D and C–CD3 groups has emerged as a particularly useful tool for studying dynamics in the solid state. The theoretical models devised so far consist typically of sets of independent dynamic modes. Each such mode is simple and usually case-specific. In this scenario, model improvement entails adding yet another mode (thereby changing the overall model), comparison of different cases is difficult, and ambiguity is unavoidable. We recently developed the microscopic order macroscopic disorder (MOMD) approach as a single-mode alternative. In MOMD, the local spatial restrictions are expressed by an anisotropic potential, the local motion by a diffusion tensor, and the local molecular geometry by relative (magnetic and model-related) tensor orientations, all of adjustable symmetry. This approach provides a consistent method of analysis, thus resolving the issues above. In this study, we apply MOMD to PS-adsorbed LKα14 peptide and dimethylammonium tetraphenylborate (C–CD3 and N–CD3 dynamics, respectively), as well as HhaI methyltransferase target DNA and phase III of benzene-6-hexanoate (C–D dynamics). The success with fitting these four disparate cases, as well as the two cases in the previous report, demonstrates the generality of this MOMD-based approach. In this study, C–D and C–CD3 are both found to execute axial diffusion (rates R⊥ and R∥) in the presence of a rhombic potential given by the L = 2 spherical harmonics (coefficients c02 and c22). R⊥ (R∥) is in the 102–103 (104–105) s–1 range, and c02 and c22 are on the order of 2–3 kBT. Specific parameter values are determined for each mobile site. The diffusion and quadrupolar tensors are tilted at either 120° (consistent with trans–gauche isomerization) or nearly 110.5° (consistent with methyl exchange). Future prospects include extension of the MOMD formalism to include MAS, and application to 15N and 13C nuclei.

Gut Microbiota Interacts With Brain Microstructure and Function.

Evidence from animals suggests that gut microbiota affects brain structure and function but evidence in humans is scarce. This study sought to evaluate potential interactions among gut microbiota composition, brain microstructure, and cognitive tests in obese and nonobese subjects. This was a cross-sectional study at a tertiary hospital including 20 consecutive obese and 19 nonobese subjects similar in age and sex. Gut microbiota (16S bacterial gene pyrosequencing), brain microstructure (diffusion tensor imaging of brain white and gray matter and R2* sequences in magnetic resonance imaging) and cognitive tests. Hierarchical clustering revealed a specific gut microbiota-brain map profile for obese individuals who could be discriminated from nonobese subjects (accuracy of 0.81). Strikingly, Shannon index was linked to R2* and fractional anisotropy of the hypothalamus, caudate nucleus, and hippocampus, suggesting sparing of these brain structures with increased bacterial biodiversity. Microbiota profile also clustered with cognitive function. The relative abundance of Actinobacteria phylum was linked not only to magnetic resonance imaging diffusion tensor imaging variables in the thalamus, hypothalamus, and amygdala but also to cognitive test scores related to speed, attention, and cognitive flexibility. In sum, obesity status affects microbiota-brain microstructure and function crosstalk.

Analysis and Simulation of Flight Effects on an Airborne Magnetic Gradient Tensor Measurement System

A magnetic tensor gradiometer is used to measure the spatial derivatives of three orthogonal magnetic field components. Being relatively insensitive to the geomagnetic field affords it the capability of being loaded onto flying platforms to conduct an airborne geophysical survey. It is especially suitable for exploring shallow buried mines and moving magnetic targets. The aerodynamic model was provided by the open source flight simulator, FlightGear. Estimations of the magnetic gradient tensor model of the magnetic dipole, the magnetic interference model of the aircraft, and the error model of the measuring device were all used to investigate the effects of flight on the airborne magnetic gradient tensor measurement system (AMGTMS). A thorough analysis of its causes, such as the vibration of the aircraft, the different rotating sequences of the coordinate system, and the reasonable sequence of data processing, was also done. The results can be applied to the guide design, integration, calibration, and data processing of the AMGTMS, thus reducing the research and development risk.

NEGATIVE MAGNETIC EDDY DIFFUSIVITIES FROM THE TEST-FIELD METHOD AND MULTISCALE STABILITY THEORY

The generation of large-scale magnetic field in the kinematic regime in the absence of an alpha-effect is investigated by following two different approaches, namely the test-field method and multiscale stability theory relying on the homogenisation technique. We show analytically that the former, applied for the evaluation of magnetic eddy diffusivities, yields results that fully agree with the latter. Our computations of the magnetic eddy diffusivity tensor for the specific instances of the parity-invariant flow-IV of G.O. Roberts and the modified Taylor-Green flow in a suitable range of parameter values confirm the findings of previous studies, and also explain some of their apparent contradictions. The two flows have large symmetry groups; this is used to considerably simplify the eddy diffusivity tensor. Finally, a new analytic result is presented: upon expressing the eddy diffusivity tensor in terms of solutions to auxiliary problems for the adjoint operator, we derive relations between magnetic eddy diffusivity tensors that arise for opposite small-scale flows v(x) and -v(x).

Towards metal detection and identification for humanitarian demining using magnetic polarizability tensor spectroscopy

This paper presents an inversion procedure to estimate the location and magnetic polarizability tensor of metal targets from broadband electromagnetic induction (EMI) data. The solution of this inversion produces a spectral target signature, which may be used in identifying metal targets in landmines from harmless clutter. In this process, the response of the metal target is modelled with dipole moment and fitted to planar EMI data by solving a minimization least squares problem. A computer simulation platform has been developed using a modelled EMI sensor to produce synthetic data for inversion. The reconstructed tensor is compared with an assumed true solution estimated using a modelled tri-axial Helmholtz coil array. Using some test examples including a sphere which has a known analytical solution, results show the inversion routine produces accurate tensors to within 12% error of the true tensor. A good convergence rate is also demonstrated even when the target location is mis-estimated by a few centimeters. Having verified the inversion routine using finite element modelling, a swept frequency EMI experimental setup is used to compute tensors for a set of test samples representing examples of metallic landmine components and clutter for a broadband range of frequencies (kHz to tens of kHz). Results show the reconstructed spectral target signatures are very distinctive and hence potentially offer an efficient physical approach for landmine identification. The accuracy of the evaluated spectra is similarly verified using a uniform field forming sensor.

Full magnetic gradient tensor from triaxial aeromagnetic gradient measurements: Calculation and application

The full magnetic gradient tensor (MGT) refers to the spatial change rate of the three field components of the geomagnetic field vector along three mutually orthogonal axes. The tensor is of use to geological mapping, resources exploration, magnetic navigation, and others. However, it is very difficult to measure the full magnetic tensor gradient using existing engineering technology. We present a method to use triaxial aeromagnetic gradient measurements for deriving the full MGT. The method uses the triaxial gradient data and makes full use of the variation of the magnetic anomaly modulus in three dimensions to obtain a self-consistent magnetic tensor gradient. Numerical simulations show that the full MGT data obtained with the proposed method are of high precision and satisfy the requirements of data processing. We selected triaxial aeromagnetic gradient data from the Hebei Province for calculating the full MGT. Data processing shows that using triaxial tensor gradient data allows to take advantage of the spatial rate of change of the total field in three dimensions and suppresses part of the independent noise in the aeromagnetic gradient. The calculated tensor components have improved resolution, and the transformed full tensor gradient satisfies the requirement of geological mapping and interpretation.

Improving reliability for classification of metallic objects using a WTMD portal

In this paper, a walk-through metal detection (WTMD) portal is used for classification of metallic objects. The classification is based on the inversion of the magnetic polarisability tensor (tensor) of the object. The nature of bias and noise components in the tensor are examined by using real walk-through data, and consequently, a novel classifier is introduced. Furthermore, a novel method for detecting poorly inverted tensors is presented, enabling self-diagnostics for the WTMD portal. Based on the results, the novel methods increase the accuracy of metal object classification and have the potential to improve the reliability of a WTMD system.

Microstructure of the superior longitudinal fasciculus predicts stimulation-induced interference with on-line motor control

A cortical visuomotor network, comprising the medial intraparietal sulcus (mIPS) and the dorsal premotor area (PMd), encodes the sensorimotor transformations required for the on-line control of reaching movements. How information is transmitted between these two regions and which pathways are involved, are less clear. Here, we use a multimodal approach combining repetitive transcranial magnetic stimulation (rTMS) and diffusion tensor imaging (DTI) to investigate whether structural connectivity in the ‘reaching’ circuit is associated to variations in the ability to control and update a movement. We induced a transient disruption of the neural processes underlying on-line motor adjustments by applying 1Hz rTMS over the mIPS. After the stimulation protocol, participants globally showed a reduction of the number of corrective trajectories during a reaching task that included unexpected visual perturbations. A voxel-based analysis revealed that participants exhibiting higher fractional anisotropy (FA) in the second branch of the superior longitudinal fasciculus (SLF II) suffered less rTMS-induced behavioral impact. These results indicate that the microstructural features of the white matter bundles within the parieto-frontal ‘reaching’ circuit play a prominent role when action reprogramming is interfered. Moreover, our study suggests that the structural alignment and cohesion of the white matter tracts might be used as a predictor to characterize the extent of motor impairments.