Nonlinear calculation of three-dimensional static magnetic fields

We present here the principle and structure of a method to calculate the three-dimensional (3-D) static magnetic fields which have already permitted us to study hybrid magnets for magnetic resonance imaging and ion confinement. Field sources can be issued from resistive or superconducting coils, permanent magnets, and other magnetic bodies such as soft iron. It can be extended to very low-frequency fields calculation as long as eddy current effects do not intervene. We call this method CALMAG3D     

Exposure to electromagnetic fields and the risk of childhood leukaemia: a review

Extremely low-frequency magnetic fields have been classified as possibly carcinogenic to humans, mainly based on epidemiological studies consistently showing an association between long-term average exposures to magnetic fields above 0.3/0.4 microT and the risk of childhood leukaemia. No mechanism to explain this finding has been established and no support for a causal link emerged from experimental studies. Chance or bias cannot be ruled out with reasonable confidence as an explanation for the observed association. If the association is causal, it explains only a small fraction of childhood leukaemia cases. There were some reports of childhood leukaemia clusters in the vicinity of high-power radio and television broadcast transmitters in studies in Australia and Italy. However, recent large-scale systematic studies in Korea and Germany show no association between exposure to radio frequency electromagnetic fields emitted from broadcast towers and childhood leukaemia risk. Studies on mobile phone use and leukaemia risk in adolescents and young adults may be indicated.     

Nonphysical Reverse Currents in Transient Finite-Element Magnetics Simulation

We are concerned with the simulation of low-frequency transient magnetic fields and currents in conductors as modeled by the vector diffusion equation. In particular, the key topic of this paper is the diffusion of fields and currents into a conductor due to a rapidly varying magnetic field in the surrounding air. As an example, an explosive magnetic flux compression generator may be designed to produce an exponentially increasing magnetic field in the in the air gap between armature and stator. As a second example, in a standard railgun the motion of the armature causes a rapidly increasing magnetic field in the air ahead of the armature, inducing currents in the rails ahead of the armature. It has been observed that for these types of transient magnetics problems finite-element simulations can produce nonphysical “reverse currents” in conductors, currents that are flowing in the wrong direction. In this paper the root cause of these nonphysical reverse currents is explained, and several mitigation strategies are discussed.      

Do electromagnetic fields enhance the effects of environmental carcinogens?

Epidemiological studies have reported an increased risk of leukaemia in children who are exposed to extremely low-frequency (ELF) magnetic fields (MF), suggesting that ELF MFs may be carcinogenic to humans. No carcinogenic effects have been found in animal studies that have tested ELF MFs alone. Similarly, genotoxicity studies have generally not shown effects from MFs alone. However, ELF MFs have been reported to enhance the effects of known carcinogenic or mutagenic agents in a few animal studies and in several in vitro studies. This paper discusses the findings of studies on such combined effects. The majority of in vitro studies have reported positive findings, which supports the conclusion that MFs of 100 microT or higher interact with other chemical and physical agents. Further studies should address biophysical mechanisms and dose-response relationship below 100 microT. Animal studies designed according to the classical initiation-promotion concept may not be sufficient for studying the cocarcinogenic effects of MFs, and further studies using novel study designs would be useful. Epidemiological data on the interaction between MFs and other environmental agents are scant and inconclusive, and any further studies may be difficult because of the scarcity of subjects with suitable combined exposures.     

Spectral estimation for a magnetostrictive magnetic field sensor

Recently there has been increasing concern regarding the effects of low-frequency magnetic fields on humans and livestock. A portable magnetometer is essential for monitoring the field dose encountered. There are some devices currently available for this purpose which use the induced EMF technique. The field sensitivity for these devices depends on the number of turns of coil used. This limits the minimum size of devices with reasonable performance. The magnetometer considered in this paper overcomes this size limitation by using a highly magnetostrictive material, Terfenol-D, as the magnetic field sensor. The primary focus of this paper is to determine the magnetic field spectral characteristics using digital signal processing. A survey of spectral estimation techniques are presented, followed by their application to the Terfenol-D magnetic field signal. The estimators are compared using data obtained from a magnetometer hardware prototype. Using these methods it was possible to detect signal for a field strength of 20 milligauss     

50-60 Hz electric and magnetic field effects on cognitive function in humans: a review

This paper reviews the effect of 50-60 Hz weak electric, magnetic and combined electric and magnetic field exposure on cognitive functions such as memory, attention, information processing and time perception, as determined by electroencephalographic methods and performance measures. Overall, laboratory studies that have investigated the acute effects of power frequency fields on cognitive functioning in humans are heterogeneous, in terms of both electric and magnetic field (EMF) exposure and the experimental design and measures used. Results are inconsistent and difficult to interpret with regard to functional relevance for possible health risks. Statistically significant differences between field and control exposure, when they are found, are small, subtle, transitory, without any clear dose-response relationship and difficult to reproduce. The human performance or event related potentials (ERPs) measures that might specifically be affected by EMF exposure, as well as a possible cerebral structure or function that could be more sensitive to EMF, cannot be better determined.     

Assessment of ELF magnetic fields produced by independent power lines

In this paper, the problem of assessing the ELF (extremely low-frequency) magnetic fields produced, in a certain area characterised by the presence of more than one independent power line, is faced. The use of the incoherent summation of the single contributions, as an advantageous estimator of the total magnetic field, is proposed and justified by means of a heuristic procedure. This kind of approach can be seen as a useful and practical tool to be employed in environmental impact analysis and in assessing long-term human exposure to ELF magnetic fields.     

A 3D Magnetic Hyaluronic Acid Hydrogel for Magnetomechanical Neuromodulation of Primary Dorsal Root Ganglion Neurons

Neuromodulation tools are useful to decipher and modulate neural circuitries implicated in functions and diseases. Existing electrical and chemical tools cannot offer specific neural modulation while optogenetics has limitations for deep tissue interfaces, which might be overcome by miniaturized optoelectronic devices in the future. Here, a 3D magnetic hyaluronic hydrogel is described that offers noninvasive neuromodulation via magnetomechanical stimulation of primary dorsal root ganglion (DRG) neurons. The hydrogel shares similar biochemical and biophysical properties as the extracellular matrix of spinal cord, facilitating healthy growth of functional neurites and expression of excitatory and inhibitory ion channels. By testing with different neurotoxins, and micropillar substrate deflections with electrophysical recordings, it is found that acute magnetomechanical stimulation induces calcium influx in DRG neurons primarily via endogenous, mechanosensitive TRPV4 and PIEZO2 channels. Next, capitalizing on the receptor adaptation characteristic of DRG neurons, chronic magnetomechanical stimulation is performed and found that it reduces the expression of PIEZO2 channels, which can be useful for modulating pain where mechanosensitive channels are typically overexpressed. A general strategy is thus offered for neuroscientists and material scientists to fabricate 3D magnetic biomaterials tailored to different types of excitable cells for remote magnetomechanical modulation.     

Nano functional neural interfaces

Engineered functional neural interfaces (fNIs) serve as essential abiotic–biotic transducers between an engineered system and the nervous system. They convert external physical stimuli to cellular signals in stimulation mode or read out biological processes in recording mode. Information can be exchanged using electricity, light, magnetic fields, mechanical forces, heat, or chemical signals. fNIs have found applications for studying processes in neural circuits from cell cultures to organs to whole organisms. fNI-facilitated signal transduction schemes, coupled with easily manipulable and observable external physical signals, have attracted considerable attention in recent years. This enticing field is rapidly evolving toward miniaturization and biomimicry to achieve long-term interface stability with great signal transduction efficiency. Not only has a new generation of neuroelectrodes been invented, but the use of advanced fNIs that explore other physical modalities of neuromodulation and recording has begun to increase. This review covers these exciting developments and applications of fNIs that rely on nanoelectrodes, nanotransducers, or bionanotransducers to establish an interface with the nervous system. These nano fNIs are promising in offering a high spatial resolution, high target specificity, and high communication bandwidth by allowing for a high density and count of signal channels with minimum material volume and area to dramatically improve the chronic integration of the fNI to the target neural tissue. Such demanding advances in nano fNIs will greatly facilitate new opportunities not only for studying basic neuroscience but also for diagnosing and treating various neurological diseases.     

Post-processing enhancement of reverberation-noise suppression in dual-frequency SURF imaging

A post-processing adjustment technique to enhance dual-frequency second-order ultrasound field (SURF) reverberation-noise suppression imaging in medical ultrasound is analyzed. Two variant methods are investigated through numerical simulations. They both solely involve post-processing of the propagated high-frequency (HF) imaging wave fields, which in real-time imaging corresponds to post-processing of the beamformed receive radio-frequency signals. Hence, the transmit pulse complexes are the same as for the previously published SURF reverberation-suppression imaging method. The adjustment technique is tested on simulated data from propagation of SURF pulse complexes consisting of a 3.5-MHz HF imaging pulse added to a 0.5-MHz low-frequency soundspeed manipulation pulse. Imaging transmit beams are constructed with and without adjustment. The post-processing involves filtering, e.g., by a time-shift, to equalize the two SURF HF pulses at a chosen depth. This depth is typically chosen to coincide with the depth where the first scattering or reflection occurs for the reverberation noise one intends to suppress. The beams realized with post-processing show energy decrease at the chosen depth, especially for shallow depths where, in a medical imaging situation, a body-wall is often located. This indicates that the post-processing may further enhance the reverberation- suppression abilities of SURF imaging. Moreover, it is shown that the methods might be utilized to reduce the accumulated near-field energy of the SURF transmit-beam relative to its imaging region energy. The adjustments presented may therefore potentially be utilized to attain a slightly better general suppression of multiple scattering and multiple reflection noise compared with non-adjusted SURF reverberation-suppression imaging.     

The effects of functional magnetic nanotubes with incorporated nerve growth factor in neuronal differentiation of PC12 cells

In this in vitro study the efficiency of magnetic nanotubes to bind with nerve growth factor (NGF) and the ability of NGF-incorporated magnetic nanotubes to release the bound NGF are investigated using rat pheochromocytoma cells (PC12 cells). It is found that functional magnetic nanotubes with NGF incorporation enabled the differentiation of PC12 cells into neurons exhibiting growth cones and neurite outgrowth. Microscope observations show that filopodia extending from neuron growth cones were in close proximity to the NGF-incorporated magnetic nanotubes, at times appearing to extend towards or into them. These results show that magnetic nanotubes can be used as a delivery vehicle for NGF and thus may be exploited in attempts to treat neurodegenerative disorders such as Parkinson's disease with neurotrophins. Further neurite outgrowth can be controlled by manipulating magnetic nanotubes with external magnetic fields, thus helping in directed regeneration.     

Generation of remote homogeneous magnetic fields

This paper presents a magnetic efficiency model for comparing efficiencies of various magnets for magnetic resonance imaging. It demonstrates that monohedral magnets, magnets with sources on one side, can generate remote saddle points in the field profile relatively efficiently. These magnets may be modeled by a minimum of two magnetic dipoles. The paper examines the field profile and magnetic dipole efficiency for the two-dipole model in detail, and develops some fundamental properties of homogeneous magnetic fields     

A Six-Loop Magnetic-Field Exposure System for Extremely Low-Frequency Applications

This paper describes a versatile exposure system for the reproduction of static and extremely low-frequency (ELF) magnetic fields. The system can be used for in vitro and in vivo experiments. The exposure setup consists of six circular loops, each placed on the face of a cubical box and suitably fed. The paper shows, by means of a genetic-algorithm approach to optimize the degrees of freedom of the system, that such a configuration allows a given magnetic field to be reproduced, with great accuracy, in amplitude, phase, and polarization, on a specific surface of interest. The paper provides details of the design procedure and presents extensive numerical simulations that confirm the validity of the proposed design procedure.     

Increased apoptosis and DNA double-strand breaks in the embryonic mouse brain in response to very low-dose X-rays but not 50 Hz magnetic fields

The use of X-rays for medical diagnosis is enhancing exposure to low radiation doses. Exposure to extremely low-frequency electromagnetic or magnetic fields is also increasing. Epidemiological studies show consistent associations of childhood leukaemia with exposure to magnetic fields but any causal relationship is unclear. A limitation in assessing the consequence of such exposure is the availability of sensitive assays. The embryonic neuronal stem and progenitor cell compartments are radiosensitive tissues. Using sensitive assays, we report a statistically significant increase in DNA double-strand break (DSB) formation and apoptosis in the embryonic neuronal stem cell compartment following in utero exposure to 10–200 mGy X-rays. Both endpoints show a linear response. We also show that DSB repair is delayed following exposure to doses below 50 mGy compared with 100 mGy. Thus, we demonstrate in vivo consequences of low-dose radiation. In contrast to these impacts, we did not observe any significant induction of DSBs or apoptosis following exposure to 50 Hz magnetic fields (100 or 300 µT). We conclude that any DSB induction by treatment with magnetic fields is lower than following exposure to 10 mGy X-rays. For comparison, certain procedures involving computed tomography scanning are equivalent to 1–5 mGy X-rays.     

The micromagnetic propertis of high-coercivity metallic thin films and their effects on the limit of packing density in digital recording

The micromagnetic structures of the high-coercivity, isotropic, and high-squareness thin films of sputtered Co-Re have been investigated using transmission electron microscope (TEM) Lorentz imaging and electron deflection methods. From the behavior of the magnetic ripple structure under applied field and the configuration of the local surface fields observed in these experiments, the existence of magnetic clusters in these films was verified. Based on the interpretation of the field dependence of the ripple formation and the hysteretic properties of the film, it is concluded that the formation of the magnetic clusters is a spontaneous process resulting from intercrystalline interactions and local inhomogeneities in the anisotropy. The effects of such cluster formation on longitudinal magnetic recording were investigated. The results show that the reduction of dipole energy at the transition region between two oppositely magnetized regions can be achieved by a stepwise rotation of the magnetization vector of an individual cluster in the form of a vortex. This type of rotation creates a finite transition length which is limited by the size of the magnetic cluster of the film. Consequently, it is concluded that the maximum packing density for saturation recording in these types of films would be less than that predicted by the phenomenological equation, which was derived based solely on considerations of the demagnetization field and the coercivity of the film.     

Manipulation and tracking of superparamagnetic nanoparticles using MRI

The use of magnetic fields in magnetic resonance imaging (MRI) for the tracking and delivery of chemotherapeutics bound to superparamagnetic nanoparticles offers a promising method for the non-invasive treatment of inoperable tumours. Here we demonstrate that superparamagnetic magnetite nanoparticles fabricated by an easily scalable method can be driven and tracked in real time at high velocities in vitro using MRI hardware. Force balance calculations are consistent with the magnetic properties of individual 10 nm diameter particles that move collectively as micron sized agglomerates with hydrodynamic diameter similar to that inferred from zero-magnetic-field dynamic light scattering measurements.     

Hamiltonian magnetic optics

Starting from Hamilton's principle, the imaging properties of charged particle beams in arbitrary static magnetic fields are outlined. The magnetic scalar potential is expressed as a series of multipoles about an arbitrary space curve which need not necessarily coincide with a trajectory of the beam. Appropriate power series expansions are given for the components of the magnetic vector potential where the expansion coefficients are related to the complex curvature of the axis and the strengths of the multipole components of the magnetic scalar potential. The general laws which govern the propagation of the charged particles are discussed by means of Hamilton's characteristic functions. It is shown that the Poisson and the Lagrange brackets are equivalent representations of the symplectic condition. The Poincaré invariant is used for elucidating some characteristic imaging properties of magnetic fields.A perturbation eikonal is introduced which allows a systematic and simultaneous calculation of the geometrical and chromatic aberrations to any order. The method has the advantage that it reveals at the very beginning all interrelations between the aberration coefficients. The iteration procedure starts from the paraxial rays which are supposed to be known. The algorithm is especially suited for numerical calculation of the higher-order aberrations.     

Effective scattering coefficient of the cerebral spinal fluid in adult head models for diffuse optical imaging

An efficient computation of the time-dependent forward solution for photon transport in a head model is a key capability for performing accurate inversion for functional diffuse optical imaging of the brain. The diffusion approximation to photon transport is much faster to simulate than the physically correct radiative transport equation (RTE); however, it is commonly assumed that scattering lengths must be much smaller than all system dimensions and all absorption lengths for the approximation to be accurate. Neither of these conditions is satisfied in the cerebrospinal fluid (CSF). Since line-of-sight distances in the CSF are small, of the order of a few millimeters, we explore the idea that the CSF scattering coefficient may be modeled by any value from zero up to the order of the typical inverse line-of-sight distance, or approximately 0.3 mm(-1), without significantly altering the calculated detector signals or the partial path lengths relevant for functional measurements. We demonstrate this in detail by using a Monte Carlo simulation of the RTE in a three-dimensional head model based on clinical magnetic resonance imaging data, with realistic optode geometries. Our findings lead us to expect that the diffusion approximation will be valid even in the presence of the CSF, with consequences for faster solution of the inverse problem.     

Magnetoactive Acoustic Metamaterials

Acoustic metamaterials with negative constitutive parameters (modulus and/or mass density) have shown great potential in diverse applications ranging from sonic cloaking, abnormal refraction and superlensing, to noise canceling. In conventional acoustic metamaterials, the negative constitutive parameters are engineered via tailored structures with fixed geometries; therefore, the relationships between constitutive parameters and acoustic frequencies are typically fixed to form a 2D phase space once the structures are fabricated. Here, by means of a model system of magnetoactive lattice structures, stimuli‐responsive acoustic metamaterials are demonstrated to be able to extend the 2D phase space to 3D through rapidly and repeatedly switching signs of constitutive parameters with remote magnetic fields. It is shown for the first time that effective modulus can be reversibly switched between positive and negative within controlled frequency regimes through lattice buckling modulated by theoretically predicted magnetic fields. The magnetically triggered negative‐modulus and cavity‐induced negative density are integrated to achieve flexible switching between single‐negative and double‐negative. This strategy opens promising avenues for remote, rapid, and reversible modulation of acoustic transportation, refraction, imaging, and focusing in subwavelength regimes.