Radio Frequency Magnetic Field Research Articles

High-Field Pulsed ENDOR with Intra-cavity Radiofrequency Coil

This study compares the performance of two coil configurations for W-band pulsed ENDOR using a setup with both a radiofrequency ‘hairpin’ coil internal to a microwave non-radiative resonator and Helmholtz-like coils external to the resonator. Evaluation of the different coil performances is achieved via the ENDOR study of two model systems. The efficiencies of the coil configurations are first investigated numerically, showing that a higher radiofrequency current-to-magnetic field conversion factor can be achieved with the intra-cavity coil, with a similar radiofrequency magnetic field uniformity. This result is then confirmed by the broadband ENDOR spectra acquired with the two coil arrangements. A gain in the signal-to-noise ratio enabled by the internal coil of about a factor 10 was observed. In some cases, the high conversion factor of the intra-cavity coil led to a saturation of the ENDOR transitions. The possibility to implement a similar intra-cavity radiofrequency coil configuration in higher field spectrometers is finally discussed.

Magnetic induction imaging with a cold-atom radio frequency magnetometer

The sensitive detection of either static or radio frequency (RF) magnetic fields is essential to many fundamental studies and applications. Here, we demonstrate the operation of a cold-atom-based RF magnetometer in performing 1D and 2D imaging of small metallic objects. It is based on a cold 85Rb atomic sample and operates in an unshielded environment with no active field stabilization. It shows a sensitivity up to 200 pT/Hz in the 5−35 kHz bandwidth range and can resolve a 0.4 mm wide cut in a 0.8 mm thick metallic foil. The characteristics of our system make it a good candidate for applications in civil and industrial surveillance.

A radiofrequency voltage-controlled current source for quantum spin manipulation.

We present a wide-bandwidth, voltage-controlled current source that is easily integrated with radiofrequency magnetic field coils. Our design uses current feedback to compensate for the frequency-dependent impedance of a radiofrequency antenna. We are able to deliver peak currents greater than 100 mA over a 300 kHz to 54 MHz frequency span. The radiofrequency current source fits onto a printed circuit board smaller than 4 cm2 and consumes less than 1.3 W of power. It is suitable for use in deployable quantum sensors and nuclear magnetic resonance systems.

An artificial dielectric slab for ultra high-field MRI: Proof of concept

High-permittivity dielectric pads, i.e., thin, flexible slabs, usually consisting of mixed ceramic powders and liquids, have been previously shown to increase the magnetic field at high and ultra high-fields in regions of low efficiency of transmit coils, thus improving the homogeneity of images. However, their material parameters can change with time, and some materials they contain are bio incompatible. This article presents an alternative approach replacing ceramic mixtures with a low-cost and stable artificial dielectric slab. The latter comprises a stack of capacitive grids realized using multiple printed-circuit boards. Results in this article show that the proposed artificial dielectric structure can obtain the same increase in the local transmit radiofrequency magnetic field distribution in a head phantom at 7 T as the conventional dielectric pad.

Power absorption, plasma parameters and wave structure in inductive RF plasma source with low value external magnetic field

The efficiency of radio-frequency (RF) power absorption, RF magnetic field structure and plasma parameters were measured in cylindrical inductive RF plasma sources 20 cm in diameter and 22, 32, 53 cm in length with a low value external magnetic field. The experiments were carried out in argon at pressures of 13–140 mPa. The RF power supply changed from 200 W to 800 W. The spiral antenna was used for sustaining the discharge. It was shown that efficiency of RF power absorption depended nonlinearly on the external magnetic field values. At maximal values of the RF power absorption efficiency, the axial distributions of longitudinal Bz and azimuthal Bφ components of RF magnetic field manifested the formation of the partially standing wave with a half wavelength close to 8 cm. At the same conditions, the axial dependence of the radial RF magnetic field component Br differed drastically. It was concluded that the Bz and Bφ amplitudes were largely determined by the RF field of Trivelpiece-Gould wave, while Br amplitude represented the radial RF field of the helicon wave.

Coupled very-high permittivity dielectric resonators for clinical MRI

Dielectric resonators made of high permittivity materials with low losses have been extensively studied for application in magnetic resonance imaging (MRI) and magnetic resonance spectroscopy. They can focus, redistribute, and enhance the radio frequency magnetic field in a controlled way. In this Letter, we investigate coupled very-high permittivity dielectric resonators for clinical bilateral breast MRI. The resonators are tuned to support a TE mode within the frequency of 3 T MRI and implemented as a pair of coaxial hollow cylinders consisting of ceramic rings with extremely high permittivity (ε ∼ 870) and low loss. We study the electromagnetic coupling between two resonators placed in the near field and analyze the impact of symmetric and antisymmetric mode excitation on the sensitivity of the MRI machine. We experimentally verify that the coupling of the mode with symmetric field distribution to the radio frequency body birdcage coil allows us to enhance the receive sensitivity substantially and to decrease the excitation power needed to provide the optimal transmit efficiency, thus making the MRI procedure safer. Our work offers a practical method to realize a simple yet very effective system based on dielectric resonators for bilateral breast imaging.

Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI With Metasurfaces

Phased antenna arrays of dipoles are widely used in ultrahigh field (UHF) magnetic resonance imaging for creating the controllable radio frequency (RF) magnetic field distributions in a human body. Due to safety and imaging quality reasons each individual channel of the array should be decoupled-electromagnetically isolated from the others. The required number of channels is large and in some techniques the dipole antennas should be located in the close proximity of the human body. Their ultimately dense arrangement leads to a strong mutual coupling and makes the conventional decoupling structures inefficient. This coupling needs to be suppressed without a significant distortion of RF fields in the imaged area. In this work, we propose and study a novel decoupling technique for two UHF transceiver on-body dipole antennas. The decoupling is performed by a periodic structure of five parallel resonant wires referred to as a metasurface (MS). In contrast to conventional decoupling with a single resonant wire, the MS decoupled by means of excitation of a higher order coupled mode of the wires, which field is highly confined. The main advantage is a low distortion of the RF-field in the central region of the body.

Effects of pulse-modulated radiofrequency magnetic field (RF-EMF) exposure on apoptosis, autophagy, oxidative stress and electron chain transport function in human neuroblastoma and murine microglial cells

The use of body-worn wireless devices with different communication protocols and rapidly changing exposure scenarios is still multiplying and the need to identify possible health effects of radiofrequency electromagnetic field (RF-EMF) exposure with extremely low-frequency (ELF) modulation envelops. In this study, effects of ELF-modulated 935 MHz RF-EMF on apoptosis, autophagy, oxidative stress and electron exchange in N9 microglial and SH-SY5Y neuroblastoma cells were investigated.Cells were exposed at 4 W/kg or sham-exposed for 2 and 24 h. RF-EMF exposure of both cell types did not alter apoptosis, the number of living cells nor the apoptosis-inducing factor (AIF), irrespective of the exposure duration. RF-EMF exposure for 24, but not for 2 h, increased protein levels of the autophagy marker ATG5, whereas LC3B-I and II and pERK were not altered in both cell types and exposure times investigated. A transient increase in glutathione (GSH), but not hydrogen peroxide and cytochrome c oxidase was found only in SH-SY5Y cells, indicating that short-time RF-EMF at SAR levels accepted by today's safety guidelines might cause autophagy and oxidative stress with the effect being dependent on cell type and exposure duration.Further studies are needed to evaluate possible underlying mechanisms involved in pulse-modulated RF-EMF exposure.

Optimized Portable Unilateral Magnetic Resonance Sensor for Assessing the Aging Status of Silicon Rubber Insulators

Silicone rubber insulators (SRIs) play an important role in high-voltage power transmission lines. Given that SRIs critically threaten the safe operation of the power grid, quantitatively assessing the aging status of SRIs in situ is crucial. In this article, a portable unilateral magnetic resonance (UMR) sensor with a U-shaped magnet structure and its measuring circuit were designed for the nondestructive detection of the aging status of SRIs in situ. First, the optimization efficiency of the magnet structure was improved through a uniform design method. Then, the planar radio frequency (RF) coil was designed by the finite-element method. Furthermore, the demagnetization effect of eddy currents induced in metal materials on the RF magnetic field was analyzed. We measured the SRI sheds at different service times and obtained the corresponding 1H transverse relaxation decay curves by importing the Carr- Purcell-Meiboom-Gill (CPMG) sequence. Finally, the transverse relaxation decay curves were postprocessed using inverse Laplace transformation, and the 1-D spectrum distributions of transverse relaxation time (T2) were obtained. The results reveal that T2 of the surface of the SRI shed decreases as the service time of the SRI increases. In addition, the difference in T2 between the surface and interior of the SRI shed increases. The comparison experiments demonstrate that the U-shaped sensor designed in this article is superior to the curved sensor previously designed by our team. The effect of temperature variation on SRIs aging assessment was considered at last.

Pilot study on the therapeutic potential of radiofrequency magnetic fields: growth inhibition of implanted tumours in mice

ABSTRACTThe present study investigated possible therapeutic effects of radiofrequency or hypomagnetic fields on the growth rate of two types of implanted tumours. To this end, mice with implanted fibrosarcoma and pancreatic tumours were exposed continuously to a 2 µT, 10 MHz radiofrequency magnetic field (MF) perpendicular to a 45 µT static MF or to a hypomagnetic (~0.4–1 µT) field. The reasoning for a 10 MHz treatment was based on a current theoretical explanation for MF effects, which predicts a resonance phenomenon in this frequency range. Radiofrequency MFs reduced consistently the growth rate of two implanted tumour types (by ~30% in both cases). Also, hypomagnetic field hindered tumour growth in both tumour types, but the observation was not statistically significant with fibrosarcoma tumours. In conclusion, although experiments included a limited number of animals, the results indicate that MFs may offer a novel therapeutic strategy in the treatment of cancer.

Arabidopsis cryptochrome is responsive to Radiofrequency (RF) electromagnetic fields

How living systems respond to weak electromagnetic fields represents one of the major unsolved challenges in sensory biology. Recent evidence has implicated cryptochrome, an evolutionarily conserved flavoprotein receptor, in magnetic field responses of organisms ranging from plants to migratory birds. However, whether cryptochromes fulfill the criteria to function as biological magnetosensors remains to be established. Currently, theoretical predictions on the underlying mechanism of chemical magnetoreception have been supported by experimental observations that exposure to radiofrequency (RF) in the MHz range disrupt bird orientation and mammalian cellular respiration. Here we show that, in keeping with certain quantum physical hypotheses, a weak 7 MHz radiofrequency magnetic field significantly reduces the biological responsivity to blue light of the cryptochrome receptor cry1 in Arabidopsis seedlings. Using an in vivo phosphorylation assay that specifically detects activated cryptochrome, we demonstrate that RF exposure reduces conformational changes associated with biological activity. RF exposure furthermore alters cryptochrome-dependent plant growth responses and gene expression to a degree consistent with theoretical predictions. To our knowledge this represents the first demonstration of a biological receptor responding to RF exposure, providing important new implications for magnetosensing as well as possible future applications in biotechnology and medicine.

Measurement of induced currents in radio frequency magnetic fields based on near field antenna perturbations

A measurement system is developed utilizing electromagnetic compatibility test equipment for the study of induced current in conductive materials subjected to radio frequency (RF) magnetic field strengths similar to the 1.5 T magnetic resonance imaging (MRI) B1 magnetic field at ∼65 MHz. The intent of developing such a system was to produce μT range RF magnetic fields in the laboratory to facilitate characterization of induction in conductive materials with modified surface electromagnetic properties to address unintended eddy current issues like Joule heating caused by implanted devices during MRI. A Helmholtz coil (HHC) is used as the RF magnetic field source, and the radiated field is monitored using a receiving loop antenna positioned coaxially outside the HHC. The measurement system operates in continuous wave and pulsed wave modes. Analytical models of the system were derived, which calculate the spatial distribution of RF magnetic flux and the induced current within a coaxially located sample in the transmission path between the HHC and receiving (R/C) loop from output voltage measurements at a single coaxial position. Induced currents were evaluated at multiple flux densities and at different frequencies, showing direct proportionality over the flux densities tested. Induced current results recorded in samples of different sizes and electrical conductivities (ranging from 0.1 to 5.8 × 107 (Ω m)−1 produced changes, matching trends predicted by conductive, closed-loop antenna theory. Induced currents were also used with simultaneous temperature rise measurements to characterize the effective surface conductivity for wire with non-uniform properties at 65 MHz.

Saturation effect of atomic magnetic resonance

In this paper, we experimentally study the saturation effect of rubidium atomic magnetic resonance. A pump beam is applied to create atomic spin polarization, a radio frequency (RF) magnetic field is used to drive the spin precession about a static magnetic field, and an off-resonant linearly polarized probe beam is applied to detect the spin dynamics based on the Faraday-rotation effect. When the RF magnetic field intensity becomes strong, a saturation effect is observed. We measure the resonant peak under strong RF magnetic field driving and obtain a saturation dip as an indication of the saturation effect. Furthermore, we show that the line-width of the saturation dip is much smaller than that of magnetic resonance. Accordingly, the sensitivity of the precision measurement based on the saturation dip can be significantly improved. The relations between the saturation dip in the resonant peak and the driving RF magnetic field intensity, as well as pumping intensity, are also studied in our experiment.

Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities

We use scanning tunneling microscopy (STM) and spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS) to investigate the effect of nitrogen doping on the surface electronic and chemical structures of cutouts from superconducting $\mathrm{Nb}$ radio-frequency cavities. The goal of this work is to get insights into the fundamental physics and materials mechanisms behind the striking decrease of the surface resistance with the radio-frequency magnetic field, which has been observed on $N$-doped $\mathrm{Nb}$ cavities. Our XPS measurements reveal significantly more oxidized $\mathrm{Nb}$ $3d$ states and a thinner metallic suboxide layer on the $N$-doped $\mathrm{Nb}$ surfaces, which is also confirmed by tunneling spectroscopy measurements. In turn, tunneling measurements performed on native surfaces as well as on $\mathrm{Ar}$-ion sputtered surfaces allow us to separate the effect of $N$ doping on the surface-oxide layer from that on the density of states in the bulk. Analysis of our tunneling spectra in the framework of a model of a proximity-coupled normal layer at the surface [A. Gurevich and T. Kubo, Phys. Rev. B 96, 184515 (2017)] is consistent with the hypothesis that $N$-doping ameliorates lateral inhomogeneities of superconducting properties on the surface and shrinks the metallic suboxide layer. For the $\mathrm{Ar}$ sputtered surfaces, we also find evidence that $N$ doping changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value corresponding to a minimum surface resistance. The totality of our experimental data suggests that the $N$ doping provides an effective tuning of the density of states in such a way that it can result in a decrease of the surface resistance with the radio-frequency field, as predicted by calculations of the nonlinear low-frequency electromagnetic response of dirty superconductors. Furthermore, STM imaging of vortex cores shows a slightly reduced average superconducting gap and a shorter coherence length in the $N$-doped $\mathrm{Nb}$ samples as compared to typically prepared $\mathrm{Nb}$ samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

Graphene Synthesis by Inductively Heated Copper Foils: Reactor Design and Operation

We report on the design of a reactor to grow graphene via inductively heating of copper foils by radio frequency (RF) magnetic fields. A nearly uniform magnetic field induced by Helmholtz-like coils penetrates the copper foil generating eddy currents. While the frequency of the current is being rapidly varied, the substrate temperature increases from room temperature to ~1050 °C in 60 s. This temperature is maintained under Ar/H2 flow to reduce the copper, and under Ar/H2/CH4 to nucleate and grow the graphene over the entire copper foil. After the power cut-off, the temperature decreases rapidly to room temperature, stopping graphene secondary nucleation. Good quality graphene was obtained and transferred onto silicon, and coated with a 300 nm layer of SiO2 by chemical etching of the copper foil. After synthesis, samples were characterized by Raman spectroscopy. The design of the coils and the total power requirements for the graphene induction heating system were first estimated. Then, the effect of the process parameters on the temperature distribution in the copper foil was performed by solving the transient and steady-state coupled electromagnetic and thermal problem in the 2D domain. The quantitative effects of these process parameters were investigated, and the optimization analysis results are reported providing a root toward a scalable process for large-sized graphene.

The Relationship between MRI Radiofrequency Energy and Function of Nonconditional Implanted Cardiac Devices: A Prospective Evaluation.

Background The risks associated with MRI in individuals who have implanted cardiac devices are thought to arise from the interaction between the implanted device and static, gradient, and radiofrequency magnetic fields. Purpose To determine the relationship between the peak whole-body averaged specific absorption rate (SAR) and change in magnetic field per unit time (dB/dt), maximum specific energy dose, imaging region, and implanted cardiac device characteristics and their function in patients undergoing MRI. Materials and Methods This prospective observational cohort study was conducted from October 16, 2003, to January 22, 2015 (https://ClinicalTrials.gov, NCT01130896). Any individual with an implanted cardiac device who was referred for MRI was included. Clinical MRI protocols without SAR restriction were used. Exclusion criteria were newly implanted leads, abandoned or epicardial leads, and dependence on a pacemaker with an implantable cardioverter defibrillator without asynchronous pacing capability. For each MRI pulse sequence, the calculated whole-body values for SAR, dB/dt, and scan duration were collected. Atrial and ventricular sensing, lead impedance, and capture threshold were evaluated before and immediately after (within 10 minutes) completion of each MRI examination. Generalized estimating equations with Gaussian family, identity link, and an exchangeable working correlation matrix were used for statistical analysis. Results A total of 2028 MRI examinations were performed in 1464 study participants with 2755 device leads (mean age, 67 years ± 15 [standard deviation]; 930 men [64%]). There was no evidence of an association between radiofrequency energy deposition, dB/dt, or scan duration and changes in device parameters. Thoracic MRI was associated with decreased battery voltage immediately after MRI (β = -0.008 V, P < .001). Additionally, right ventricular (RV) lead length was associated with decreased RV sensing (β = -0.012 mV, P = .05) and reduced RV capture threshold (β = -0.002 V, P < .01) immediately after MRI. Conclusion There was no evidence of an association between MRI parameters that characterize patient exposure to radiofrequency energy and changes in device and lead parameters immediately after MRI. Nevertheless, device interrogation before and after MRI remains mandatory due to the potential for device reset and changes in lead or generator parameters. © RSNA, 2020 See also the editorial by Shellock in this issue.

Control of the RF magnetic field pattern by tunable metasurface in 1.5 T MRI

A metasurface with a unit cell in the form of a half-wave dipole can be used as a base for the creation of wireless coils for magnetic resonance imaging (MRI). In a real structure based on a metasurface concept, the number of elements is always limited. Therefore, some elements at the edges become weaker coupled and radiofrequency (RF) magnetic field pattern becomes distorted. In this paper, we investigate metasurface-based structure with added non-uniform electrical capacitance at the ends of the resonant unit cells. We show that such a structure can be easily adjusted to work within the frequency range of the clinical 1.5 T MRI machine. Also, we demonstrate that by adding of non-uniform edge capacitances at the resonant elements, it is possible to control the RF magnetic field pattern above the structure.

RF-resonator for clinical MR imaging in urology and andrology

New diagnostic visualisation and quantification methods of nerve tracks are needed in andrology and urology applications. In order for magnetic resonance imaging (MRI) to properly image the nerves of interest a dramatic signal-to-noise ratio (SNR) increase is required. For this purpose, radiofrequency (RF) resonator based on the birdcage operating principles has been designed, fabricated, measured and compared to simulation model obtained by time domain method in CST microwave studio. The resonator concentrate RF magnetic field in the centre of the resonator, while electric field remains on the edge thus preventing tissue heating. Design and parameters of resonator was optimised for human MRI on 1.5T scanners. Using the resonator allowed to improve RF-safety for 2 times, transmit efficiency for 10 times and SNR for 14 times compared to body coil only system. Measured and computed resonance frequencies and transmit efficiency are in good agreement.

Spin noise at electron paramagnetic resonance

We develop a microscopic theory of spin noise in solid-state systems at electron paramagnetic resonance, when the spin dynamics is driven by static and radio-frequency (rf) magnetic fields and the stochastic effective magnetic field stemming from the interaction with environment. The rf field splits the peaks in the power spectrum of spin noise into the Mollow-type triplets and also gives rise to additional spin-spin correlations which oscillate in the absolute time at the rf frequency and the double frequency. Even in systems with strong inhomogeneous broadening, the spin-noise spectrum contains narrow lines insensitive to the dispersion of the effective $g$ factors. Thus, the measurements of spin noise at electron paramagnetic resonance provide an access to the intrinsic spin lifetime of electrons.

Scaling mitigation in radio-frequency induction heated membrane distillation

Calcium sulfate (CaSO4) scaling is a limiting factor in membrane distillation as a result of concentration polarization and elevated temperatures. We assessed the influence of radio-frequency magnetic fields, used as a method of heating the membrane's surface, as part of a membrane distillation (RF-MD) process. The influence of RF heating on CaSO4 scaling is addressed in terms of distillate flux and crystal formation, and the results are compared to conventional MD systems. The RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface, and scaling was assessed at different feed flow velocities using feed solutions containing elevated concentrations of CaSO4 and NaCl and at ionic composition simulating RO brine. Results show only sporadic small CaSO4 crystals on the membrane surface following RF-MD, while high concentrations of small crystals were detected at the concentrate stream exiting the filtration module. Furthermore, the RF-MD system was able to produce enhanced distillate flux. The scaling mitigation mechanism is discussed in detail and hypothesized to be a result of the high-frequency movement and collision of the ions in the solution.