Transverse Radio Frequency Research Articles

Dual-Channel Transverse Fields Radiofrequency Coils for 1.5 T Magnetic Resonance Imaging.

This theoretical study presents the design and analytical/numerical optimization of novel dual-channel transverse fields radiofrequency (RF) surface coils for 1.5 T Magnetic Resonance Imaging (MRI). The research explores a planar setup with two channels on a row with aligned spatial orientation of the RF coils, aiming to solve a common design drawback of single-channel transverse field RF coils: the reduced Field Of View (FOV) along the direction of the RF field. A significant challenge in this design is the efficient decoupling of two sets of transverse field RF coils to prevent mutual interference. Our modeling approach integrates thin wire theoretical modeling, magnetostatic computation for strip conductor coils, and their full-wave electromagnetic simulation. Key findings at 64 MHz demonstrate that strategic geometric placement among the two-channel RF coil and the introduction of geometrical asymmetry in the design of the individual RF coils does minimize the mutual inductance, paving the way for effective dual-channel MRI applications. This decoupling approach allows to enhance the FOV, providing a theoretical framework for the development of optimized dual-channel transverse field RF coil configurations. The current design was validated with full-wave numerical study at 64 MHz (1H, 1.5 T), has the potential to be extended at lower or higher frequencies, and the presence of lossy samples needs to be considered in the latter case.

Magnetostatic Simulation and Design of Novel Radiofrequency Coils Based on Transverse Field Current Elements for Magnetic Resonance Applications.

Radiofrequency (RF) coils are key components in Magnetic Resonance (MR) systems and can be categorized into volume and surface coils according to their shapes. Volume RF coils can generate a uniform field in a large central sample's region, while surface RF coils, usually smaller than volume coils, typically have a higher Signal-to-Noise Ratio (SNR) in a reduced Region Of Interest (ROI) close to the coil plane but a relatively poorer field homogeneity. Circular and square loops are the simplest and most used design for developing axial field surface RF coils. However, for specific MR applications, the use of dedicated transverse field RF coils can be necessary or advantageous. Building on a previously developed and validated RF coil simulator, based on the magnetostatic approach, here we explore the potential applications of novel multiple axial field and transverse field surface RF coils in non-standard configurations. We demonstrate via numerical simulations that simple volume RF coils, matching a Helmholtz-like design, can be built with two identical transverse field RF coils separated by a given distance. Following well-known principles, the SNR of such novel configurations can be improved by a factor of up to √2 by combining two 90° rotated coils, producing, inside a central ROI, a circularly polarized B1 field.

Biot–Savart-Based Design and Workbench Validation at 100 MHz of Transverse Field Surface RF Coils

Radiofrequency (RF) surface coils are extensively used as receivers in magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) systems thanks to their high signal-to-noise ratio (SNR). For specific magnetic resonance applications, the design of dedicated RF surface coils with a transverse (to the coil’s plane) RF magnetic field pattern can be necessary. Such transverse-field RF coils are constituted by several central linear (parallel or crossing) conductor elements connected by return current paths. Typically, the outer shape of such RF coils is circular or squared, although other geometries can be used. This paper describes the implementation and validation of a transverse-field RF surface coil simulator based on magnetostatic analysis, which permits the design and optimization of square butterfly and figure-of-eight RF coils with adjustable size and mutual distance between the central linear current elements. The simulation results, compared with the ones provided by a standard square loop RF coil, were validated with 100 MHz workbench measurements performed on three home-built prototypes. Finally, two novel quadrature RF coil structures designed by overlapping two orthogonal square butterfly and figure-of-eight RF coils were simulated and theoretically characterized. The RF coils described here should be suitable for a wide range of MRI/MRS preclinical/clinical applications, mainly at fields below 3 T.

Self-generated Floquet modulation in a multiharmonic-dressed alignment-based magnetic-resonance system

Quantum systems with multiharmonic dressing have been extensively investigated and exhibited various novel phenomena and technologies by virtue of their robust electromagnetic response. Here we mainly investigate the self-generated longitudinal Floquet modulation phenomenon of an alignment-based magnetic-resonance system with pump-probe structure dressed by two different transverse radio-frequency (rf) fields in a warm atomic cesium ensemble, both theoretically and experimentally. The self-generated Floquet modulation includes first and second harmonics, which are contributed to by linear and nonlinear processes from the weak-dressed field, respectively. The self-generated Floquet modulation dresses the alignment absorption spectra into two continua. The asymmetry of each dressed continuum is due to the generalized parity symmetry of the system being broken by the second harmonic modulation. Moreover, the nonlinear modification effect of the weak rf field on the probe verifies the existence of the four-wave mixing process in the rf band. The analytical expression, establishment process, and asymmetry analysis of the continua are given concisely. Our results can be applied to the fields of precision magnetic-field measurement, quantum control, quantum simulation, and so on. In addition, because the physical mechanism is universal, it can be applied not only to an atomic ensemble, but also to solid-state devices.

A Linear Paul Trap without the use of the Transverse Quadrupole Field

A detailed experimental study has been conducted to demonstrate the efficient confinement of ions in the popular four-rod configuration of a linear Paul trap without exciting the transverse radio-frequency (rf) quadrupole field. The three-dimensional (3D) ion confinement is achieved with an identical rf voltage applied to the end electrodes. The optimum operating region is visualized in the stability tune diagram, which indicates that a large number of ions can be stored by adjusting a few fundamental parameters. The lifetime of an ion cloud in the present linear trap is over a second (corresponding to a million rf cycles), long enough for various practical applications. It is also shown through 3D numerical simulations that one can easily extract ions from the trap at a low loss rate below 10%.

Experimental and theoretical study of density, potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

AbstractPotential and density structures in the vicinity of an radio frequency (RF) electrode/antenna in a magnetized plasma are investigated using an RF‐compensated cylindrical Langmuir probe. These measurements were performed in the ALINE plasma device in which only electrons can be considered well magnetized. Very precise 2‐D maps of the plasma parameters are drawn thanks to a 3‐D automatic manipulator on which the probe is mounted. The effect of the tilted magnetic angle between the RF‐biased surface and the magnetic lines is also studied thanks to a tilting electrode. Comparison of several simplistic models with the experiments proved the reliability of simple Langmuir probe measurements in such an RF and magnetized environment (space potential vs. tilting angle of the antenna with respect to magnetic field lines and recovery of the floating potential structure using measured currents). A fluid model based on total current density and ion diffusion equations over the biased flux tube provides the same density structures in front of the electrode as the measurements. Those density structures display a “bunny ears” shape and can be explained using transverse RF and collisional current behaviour: In front of the antenna, the transverse ion currents deplete the magnetized flux tube, while at the edge of the biased flux tube, the same currents increase the density.

Cesium optical-radio-frequency resonance line reversals and doubling

We report an experimental study on the ground-state magnetic resonances of optically pumped cesium vapors at lower temperatures of about 30°C–40°C. A longitudinal static magnetic field is applied along the single laser beam to control the splitting of the Zeeman sublevels of the ground state. When the laser frequency is set to the cesium D1 spectral line (F=3→F′=4), by modifying the transverse radio frequency field, we find that the unusual weaker-absorption and zero-dip resonance phenomena occur in the optical-magnetic resonance (OMR) effects, which is called the cesium optical-RF resonance line reversals and doubling in this paper. We explore a theoretical description for these phenomena by using density matrix equations, and our calculated results are in approximate agreement with the experimental results.

A loop-gap resonator for chirality-sensitive nuclear magneto-electric resonance (NMER).

Direct detection of molecular chirality is practically impossible by methods of standard nuclear magnetic resonance (NMR) that is based on interactions involving magnetic-dipole and magnetic-field operators. However, theoretical studies provide a possible direct probe of chirality by exploiting an enantiomer selective additional coupling involving magnetic-dipole, magnetic-field, and electric field operators. This offers a way for direct experimental detection of chirality by nuclear magneto-electric resonance (NMER). This method uses both resonant magnetic and electric radiofrequency (RF) fields. The weakness of the chiral interaction though requires a large electric RF field and a small transverse RF magnetic field over the sample volume, which is a non-trivial constraint. In this study, we present a detailed study of the NMER concept and a possible experimental realization based on a loop-gap resonator. For this original device, the basic principle and numerical studies as well as fabrication and measurements of the frequency dependence of the scattering parameter are reported. By simulating the NMER spin dynamics for our device and taking the (19)F NMER signal of enantiomer-pure 1,1,1-trifluoropropan-2-ol, we predict a chirality induced NMER signal that accounts for 1%-5% of the standard achiral NMR signal.

Imaging tooth enamel using zero echo time (ZTE) magnetic resonance imaging.

In an event where many thousands of people may have been exposed to levels of radiation that are sufficient to cause the acute radiation syndrome, we need technology that can estimate the absorbed dose on an individual basis for triage and meaningful medical decision making. Such dose estimates may be achieved using in vivo electron paramagnetic resonance (EPR) tooth biodosimetry, which measures the number of persistent free radicals that are generated in tooth enamel following irradiation. However, the accuracy of dose estimates may be impacted by individual variations in teeth, especially the amount and distribution of enamel in the inhomogeneous sensitive volume of the resonator used to detect the radicals. In order to study the relationship between interpersonal variations in enamel and EPR-based dose estimates, it is desirable to estimate these parameters nondestructively and without adding radiation to the teeth. Magnetic Resonance Imaging (MRI) is capable of acquiring structural and biochemical information without imparting additional radiation, which may be beneficial for many EPR dosimetry studies. However, the extremely short T2 relaxation time in tooth structures precludes tooth imaging using conventional MRI methods. Therefore, we used zero echo time (ZTE) MRI to image teeth ex vivo to assess enamel volumes and spatial distributions. Using these data in combination with the data on the distribution of the transverse radio frequency magnetic field from electromagnetic simulations, we then can identify possible sources of variations in radiation-induced signals detectable by EPR. Unlike conventional MRI, ZTE applies spatial encoding gradients during the RF excitation pulse, thereby facilitating signal acquisition almost immediately after excitation, minimizing signal loss from short T2 relaxation times. ZTE successfully provided volumetric measures of tooth enamel that may be related to variations that impact EPR dosimetry and facilitate the development of analytical procedures for individual dose estimates.

Coherence resonance on a forbidden magnetic-dipole transition excited in optically oriented Cs atoms by two magnetic fields: an Inclined SHF field and a transverse RF field

It is shown that the coherence created by a resonance transverse superhigh-frequency (SHF) field induces, under the action of a longitudinal SHF and transverse radio-frequency (RF) nonresonance fields, the Zeeman coherences on two adjacent allowed Δm = 1 transitions. Under the action of an RF transverse field these coherences induce further a coherence on a forbidden Δm = 2 transition. The resonance behavior of such coherence was earlier observed experimentally when the RF-field frequency coincided with the Δm = 2 transition frequency. The theoretical conclusions are in quantitative agreement with the experiment.

Femtosecond x-ray pulse temporal characterization in free-electron lasers using a transverse deflector

We propose a novel method to characterize the temporal duration and shape of femtosecond x-ray pulses in a free-electron laser (FEL) by measuring the time-resolved electron-beam energy loss and energy spread induced by the FEL process, with a transverse radio-frequency deflector located after the undulator. Its merits are simplicity, high resolution, wide diagnostic range, and noninvasive to user operation. When the system is applied to the Linac Coherent Light Source, the world's most powerful x-ray FEL, it can provide single-shot measurements of the electron-beam and x-ray pulses with a resolution on the order of 1--2 femtoseconds rms.

MRI Coil Design Using Boundary-Element Method With Regularization Technique: A Numerical Calculation Study

A new boundary element method (BEM) based method is described for the design of coils for magnetic resonance imaging (MRI) systems. BEM is an effective approach for solving the electromagnetic forward problem and has been used in the design of MRI gradient coils. However, BEM-based gradient coil design faces an ill-posed mathematical problem, which is conventionally handled by means of a Lagrange multiplication method. This work attempts to improve the BEM method for MRI coil designs by applying the Tikhonov regularization scheme to solve the ill-posed matrix system formulated by the BEM forward model. The objective of the study is not to design some specific gradient or radio-frequency (RF) coils for MRI system, but to discuss the design scheme with practical regularization and constraints. The proposed approach was explained in the design of MRI coils including biplanar transverse gradient coils and RF phased array coils. With the consideration of the practical engineering requirements, physical constraints such as wire intervals are transformed into mathematical constraints and formulated into BEM equations. The examples demonstrate that the proposed method is efficient and flexible for the design of MRI coils with arbitrary geometries and engineering constraints.

Radio Frequency Fragment Separator at NSCL

A new device has been designed and built at NSCL which provides additional filtering of radioactive beams produced via projectile fragmentation. The Radio Frequency Fragment Separator (RFFS) uses the time micro structure of the beams accelerated by the cyclotrons to deflect particles according to their time-of-flight, in effect producing a phase filtering. The transverse RF (Radio Frequency) electric field of the RFFS has superior filtering performance compared to other electrostatic devices, such as Wien filters. Such filtering is critical for radioactive beams produced on the neutron-deficient side of the valley of stability, where strong contamination occurs at intermediate energies from 50 to 200MeV/u.

Dynamics of entanglement in one-dimensional Ising chains with two- and three-body interactions

The evolution of entanglement in a one-dimensional Ising chain with both two-body and three-body interactions, under two types of initial states, is numerically simulated. We analyse three problems concerning the dynamics of pairwise entanglement: (i) the possibility of generating large entanglement from an initial separable state by the use of a selective irradiation scheme; (ii) the effect of three-body interaction on the generation of entanglement from an initial separable state; (iii) the effect of three-body interaction on the decay of the entanglement from a state with only (m,n)-pair maximal entangled, and the rest in product form. It is shown that a large pairwise concurrence C mn can be obtained when the resonant, transverse radio-frequency fields are selectively switched on from the mth to nth spins. Three-body interaction will decrease the oscillation amplitude of the nearest neighbour concurrence, while the oscillation amplitude of remote pairwise concurrence will be greatly increased with the consideration of three-body interactions. For an initial (m,n)-pair maximal entangled state, a slow decay of the pairwise concurrence C mn is found with the introduction of three-body interactions.

Influence of frequency on O2(a 1Δ) and oxygen atom production in transverse radio-frequency discharges

Experimental investigations of radio-frequency discharges in O2/He/NO mixtures in the pressure range 1–100 Torr and power range 0.1–1.2 kW have indicated that O2(a 1Δ) production is a strong function of geometry, excitation frequency, pressure and diluent ratio. The goal of these investigations was maximization of both the yield and the flow rate (power flux) of O2(a 1Δ) in order to produce favourable conditions for application to an electric oxygen–iodine laser (EOIL). At lower pressures, improvements in yield are observed when excitation frequency is increased from 13.56 MHz. As pressure is increased in the baseline 50 mm gap configuration, increasing excitation frequency becomes detrimental. O2(a 1Δ) yield performance is improved at high pressure by reducing the discharge gap and increasing the diluent ratio. Numerous measurements of O2(a 1Δ), oxygen atoms and discharge excited states are made in order to describe the discharge performance dependent on various parameters.

An exciplex-halogen high-frequency radiation source on mixtures of krypton and xenon with chlorine molecules

The results of optimization of a broadband ultraviolet (UV) radiation source, which is excited with a transverse radio-frequency low-pressure discharge with a planar aperture, are presented. The UV source operates on Kr-Cl2 and Kr-Xe-Cl2 working mixtures. The UV source operates on the system of electron vibrational bands of XeCl* and KrCl* exciplex molecules and chlorine molecules in the spectral range 210–310 nm. The average radiation power reaches 30–50 W at an efficiency of 10–15%.

NMR frequencies of domain walls in a (110) plate of yttrium iron garnet upon the magnetization in the $$[\bar 110]$$ direction

Numerical methods have been used to study the shapes of NMR absorption lines of domain walls (DWs) in a (110) plate of yttrium iron garnet upon the magnetization along the \([\bar 1 10]\) direction. NMR spectra obtained from 57Fe nuclei located in tetrahedral and octahedral positions in Bloch and Neel walls in a longitudinal and transverse radiofrequency fields have been studied. The dependences of the frequencies of the NMR absorption maxima of DWs on the magnitude of the applied magnetic field permit one to unambiguously determine the type of rotation of magnetization in the transition layer. It is shown that by changing the conditions of excitation of oscillations of the radiofrequency field, signals from nuclei located in various layers of the DW can be obtained.

Optimization of multi-element transverse field radio frequency surface coils

In several MRI and MRS applications it is necessary to select a well-defined region of interest (ROI) with optimized signal-to-noise ratio and radio frequency (RF) field homogeneity. We have shown, theoretically and experimentally, that transverse field RF surface coils comprising a multitude of parallel linear elements are characterized by improved B1 spatial homogeneity and sensitivity in the central ROI, with respect to the standard square loop design. Finite-element modelling of RF coils tuned to 64.0 MHz was used to study the effect of the number of linear current elements and relative positioning with respect to B1 field homogeneity and sensitivity in the central ROI, both when empty and in the presence of a homogeneous tissue model. Workbench B1 field calibrations obtained with RF coil prototypes tuned to 64.0 MHz are in good agreement with our theoretical finite-element modelling. We have shown that the number of linear current elements and their relative positioning can be carefully selected to optimize either the B1 field homogeneity or sensitivity in the central ROI. We have shown that a number of multi-element RF coil geometries can be chosen to obtain an optimized RF field distribution and/or field amplitude. For example, we found that a six-element RF coil with 6 mm element separation is a good compromise for optimizing the B1 sensitivity (up to a factor 2) and the spatial homogeneity (about 30 mm with ΔB1/B1 ⩽ 20%) in the central ROI, with respect to the standard square loop or the two-element FO8 design. MRI images obtained at 2.35 T (proton frequency 100.34 MHz) with standard GE pulse sequences showed that the six-element FO8 prototype significantly increases the RF field homogeneity and RF field amplitude, with respect to the SL coil design.

Measurements of combustion efficiency in nonequilibrium RF plasma-ignited flows

The paper presents results of combustion efficiency measurements using Fourier transform infrared absorption spectroscopy in premixed hydrocarbon–air and CO–air flows excited by a low-temperature transverse radiofrequency (RF) discharge plasma. The results demonstrate that significant fractions of these fuels are burned in RF plasma-generated and -stabilized flames (up to 100% of ethylene, 70% of methane, and 40% of CO). The results also show that fuel oxidation in the RF plasma-excited flows is most efficient in lean mixtures. Measurements of reaction product concentrations in hydrocarbon–air and CO–air mixtures show that significant amounts of fuel react under conditions when there is no flame detected in the test section. Under these conditions, fuel species oxidation occurs in plasma chemical reactions, without producing ignition. Also, experiments in CO–air flows demonstrated ignition and combustion at the equivalence ratios well below the lean flammability limit for CO. Finally, experiments in higher power RF discharges (500 W) showed that nonequilibrium plasma ignition occurs at flow velocities up to at least u = 60 m / s and at flow residence times exceeding ∼ 1 s . The effect of significant fuel oxidation, which is observed for lean fuel–air mixtures outside the flammable range, before the ignition occurs, provides additional evidence for the nonthermal fuel oxidation triggered by plasma-generated radicals. The present results, combined with previous experiments in which RF plasma ignition was demonstrated to occur in a low-temperature plasma, suggest the following nonequilibrium plasma ignition mechanism: (i) active radical species generation by the nonequilibrium plasma, (ii) plasma chemical reactions of fuel species oxidation with participation of these radicals, (iii) flow heating due to net exothermal plasma chemical fuel oxidation process, and (iv) subsequent thermal ignition and combustion.

Mass separation of ion beams using transverse radio frequency electric field (abstract)

A condition to achieve a mass separation of an ion beam at a given energy by means of applying a transverse radio frequency (rf) electric field was derived. Effectiveness of the scheme was examined with a He ion beam of energy below 2 kV. An array of four-stage electrodes produced distributed 13.56 MHz rf field to realize a function of a velocity filter. When the extraction voltage of the beam was adjusted to yield a velocity predicted from the theory, the beam current showed transmission as large as 80% through the array of electrodes.