Magnetic Resonance Frequency Research Articles

Dynamic nuclear polarization via the cross effect and thermal mixing: B. Energy transport.

The fundamental process of dynamic nuclear polarization (DNP) via the cross effect (CE) and thermal mixing (TM) is a triple spin flip, in which two interacting electron spins and a nuclear spin interacting with one of these electron spins flip together. In the previous article (Wenckebach, 2018) these triple spin flips were treated by first determining the eigenstates of the two interacting electron spins exactly and next investigating transitions involving these exact eigenstates and the nuclear spin states. It was found that two previously developed approaches-the scrambled states approach and the fluctuating field approach-are just two distinct limiting cases of this more general approach. It was also shown that triple spin flips constitute a single process causing two flows of energy: a flow originating in the electron Zeeman energy and a flow originating in the mutual interactions between the electron spins. In order to render their definitions more precise, the former flow was denoted as the CE and the latter as TM. In this article the treatment is extended to a glass containing NI equivalent nuclear spins I=12 and NS randomly distributed and oriented electron spins S=12. Rate equations are derived for the two flows of energy to the nuclear spins. The flow originating in the electron Zeeman energy-i.e. the CE-is found to lead to the same stationary state as was previously predicted by the scrambled states approach, though the rate may be smaller due to limitations imposed by conservation of energy. The flow originating in the mutual interactions between the electron spins-i.e. TM-is found to involve the full spectrum of the mutual interactions between the electron spins, while the fluctuating field approach only accounts for the component of this spectrum at the nuclear magnetic resonance (NMR) frequency. Still, TM is found to induce equal spin temperature for different nuclear spin species during nuclear spin-lattice relaxation and, at least in some cases also during polarization. It is also confirmed that TM couples local nuclear spins near the electron spins so strongly to the mutual interactions between electron spins, that they may constitute a single energy reservoir (Cox et al., 1973). Hence such local nuclear spins may have to be included in treatments of the dynamics of the electron spins.

NMR Shifts in 3He in Aerogel Induced by Demagnetizing Fields

Magnetic materials generate demagnetizing field that depends on geometry of the sample and results in a shift of magnetic resonance frequency. This phenomenon should occur in porous nanostructures as well, e.g., in globally anisotropic aerogels. Here we report results of nuclear magnetic resonance (NMR) experiments with liquid $^3$He confined in anisotropic aerogels with different types of anisotropy (nematic and planar aerogels). Strands of aerogels in pure $^3$He are covered by a few atomic layers of paramagnetic solid $^3$He which magnetization follows the Curie-Weiss law. We have found that in our samples the NMR shift in solid $^3$He is clearly seen at ultralow temperatures and depends on value and orientation of the magnetic field. The obtained results are well described by a model of a system of non-interacting paramagnetic cylinders. The shift is proportional to the magnetization of solid $^3$He and may complicate NMR experiments with superfluid $^3$He in aerogel.

Dynamical coupling between a nuclear spin ensemble and electromechanical phonons

Dynamical coupling with high-quality factor resonators is essential in a wide variety of hybrid quantum systems such as circuit quantum electrodynamics and opto/electromechanical systems. Nuclear spins in solids have a long relaxation time and thus have the potential to be implemented into quantum memories and sensors. However, state manipulation of nuclear spins requires high-magnetic fields, which is incompatible with state-of-the-art quantum hybrid systems based on superconducting microwave resonators. Here we investigate an electromechanical resonator whose electrically tunable phonon state imparts a dynamically oscillating strain field to the nuclear spin ensemble located within it. As a consequence of the dynamical strain, we observe both nuclear magnetic resonance (NMR) frequency shifts and NMR sidebands generated by the electromechanical phonons. This prototype system potentially opens up quantum state engineering for nuclear spins, such as coherent coupling between sound and nuclei, and mechanical cooling of solid-state nuclei.

EVALUATION OF LOW LEVEL LASER THERAPY ON THE STABILITY OF IMPLANTS USING RESONANCE FREQUENCY ANALYSIS (CLINICAL STUDY)

INTRODUCTION: The implant supported overdentures are considered a very successful treatment option for edentulous patients. Low level laser therapy (LLLT) has gained greater awareness in the last decade for implant surgery, reduces postoperative pain after surgery, promotes the osseointegration of implants, particularly, improving stability and enhances new bone formation without causing any tissue destruction. OBJECTIVES: : The aim of this study was the effect of low-level laser therapy on the biostimulation of bone repair by enhancing or accelerating osseointegration through stimulating the expression of osteoblastic phenotype in cells cultured on Titanium specimens. MATERIALS AND METHODS: 10 patients with age ranged between 45-60 years old were involved in the study. Each patient received 2 implants in the mandibular edentulous ridge at the canine area. The right side acted as study group has received one implant with a Semiconductor diode LASER (type IV) application, while the left acted as control one of same patients having normal loading in left mandibular canine area without LLLT application. All implants osseointegration was assessed by Magnetic Resonance Frequency Analysis to evaluate the implant stability. RESULTS Resonance Frequency Analysis evaluation revealed great difference in the stability after three months when irradiated with LLLT, implant stability quotient changes were found to be statistically significant between the two studied groups. (P2= <0.001). CONCLUSIONS: Effect of LLLT on bone remodeling is evident and improves implant stability.

An investigation of BaLaxFe12−XO19 magneto dielectric material to develop miniaturized antenna for navigational applications

The paper presents the synthesis, characterization of Lanthanum substituted Barium ferrite material and its real time application in the development of miniaturised navigational antenna at 2.492 GHz. The Lanthanum substitution is observed to increase the ferro magnetic resonance frequency of BaFe12O19 parent material because of its octahedral site preference. Among the synthesized material, the BaLa0.2Fe11.8O19 material has resulted in higher effective magnetic permeability of around 2.5 and dielectric permittivity of around 5.5. The Quadrifilar helix antenna which is miniaturised by loading the BaLa0.2Fe11.8O19 material has the axial height reduction of 59% compared to the conventional quadrifilar helix antenna, while retaining the radiation characteristics like gain and band width.

Magnetic Resonance Frequency Shifts of Spin-Polarized Cesium Atoms in a Cs–Rb Mixture

The interaction of spin-polarized cesium atoms with cesium and rubidium atoms under conditions of optical orientation of atoms is considered. On the basis of data on spin-exchange complex cross sections in the Cs–Cs and Cs–Rb systems, a calculation of a magnetic resonance frequency shift of cesium atoms in a Cs–Rb mixture, as well as comparison of the calculated values of the frequency shift with experimental data, is performed.

Electrically and Optically Controlled Wideband Matching of Radio-Absorbing Composites to the Ambient Space

Microwave radio-absorbing metastructures are proposed that possess controlled broadband matching to the ambient space by virtue of an array of varactor-loaded chiral and annular elements in which magnetic resonance are excited. Experimentally in a rectangular waveguide and numerically in free space, it is shown that the tuning of the magnetic resonance frequency leads to a shift in the matching frequency region. The electric control of the magnetic resonance and, accordingly, the matching (25% relative tuning) is achieved by the variation in the reverse bias voltage at the varactor, and the optical control is achieved by directing a red laser pointer to a photodiode connected to the varactor in the photodiode mode.

NMR Relaxometry to Characterize the Drug Structural Phase in a Porous Construct.

Nuclear magnetic resonance (NMR) frequency spectra and T2 relaxation time measurements, using a high-power radio frequency probe, are shown to characterize the presence of an amorphous drug in a porous silica construct. The results indicate the ability of non-solid-state NMR methods to characterize crystalline and amorphous solid structural phases in drugs. Two-dimensional T1- T2 magnetic relaxation time correlation experiments are shown to monitor the impact of relative humidity on the drug in a porous silica tablet.

Effect of low-frequency AC magnetic susceptibility of Ru/Co60Fe20V20 and Ta/Co60Fe20V20 films

In this study, the maximum alternating current (AC) magnetic susceptibility (χac) and optimal resonance frequency (fres) of (a) glass/Ru (X nm)/Co60Fe20V20(5 nm) and (b) glass/Ta(Y nm)/Co60Fe20V20(5 nm) films at room temperature (RT) were investigated through low-frequency AC magnetic susceptibility (χac) measurement, where X and Y each ranged from 5 to 10 nm. The variation of the highest χac at the optimal resonance frequency (fres), at which the spin sensitivity is maximal, was examined using an experiment. The results indicated that χac decreased as the frequency was increased. The maximum χac of glass/Ru(9 nm)/Co60Fe20V20(5 nm) and glass/Ta(8 nm)/Co60Fe20V20(5 nm) was 20.6 × 10−3 and 15.5 × 10−3 at fres 100 Hz and 1000 Hz, respectively. The maximum χac of glass/Ru(9 nm)/Co60Fe20V20(5 nm) was 1.3 times larger than that of glass/Ta(8 nm)/Co60Fe20V20(5 nm). This indicated that the effect of the Ru seed layer is apparently suitable for low-frequency magnetic recording applications.

Analyzing the effect of geometric parameters of double split ring resonator on the effective permeability and designing a cloak of invisibility in microwave regime

A metamaterial unit cell based on double split ring resonator has been designed. The SRRs consist of a pair of concentric square copper rings. The metamaterial unit cell designed in millimeter scales. Effect of some geometric parameters of the split ring resonator such as the size of the gap, the width of the split ring resonator copper wires, the length of the inner ring and the length of the outer ring on the magnetic permeability has been investigated. The magnetic resonance frequency, the magnetic plasma frequency, the resonance bandwidth and the effective magnetic permeability at 9.5 GHz related to each metamaterial unit cell with specified geometric parameters has been calculated. Using these metamaterial unit cells, an invisible cloak operating in microwave regime is designed. The situation of the gaps in the double split ring resonator with two and four gaps is also changed and the effective permeability for each of them is obtained.

Imaging Agent for Redox‐Active Molecules

Redox‐active and cofactor molecules (such as glutathione, NAD+/NADH, and NADP+/NADPH) are involved in a wide array of important biological reactions. Their cellular concentrations are maintained within a range of values, and an imbalance is often linked to decreased cellular defense capability against oxidative stress. Currently, the most common techniques for quantifying these molecules require cell lysis. The goal of this project is to design sensor molecules that can be used in live cells. Our approach relies on the use of oligodeoxynucleotide strands (called aptamers) as the recognition moiety for the redox compound of interest. Aptamers, when selected for a specific target molecule, can display selectivity and affinity rivalling those of antibodies. Conjugation of the aptamer to a xenon‐containing organic molecule will yield an imaging agent that will report the concentration of the target as a measurable frequency value in the xenon nuclear magnetic resonance.Isolation of aptamers for a target molecule is achieved via an iterative in vitro selection technique called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Starting from a large pool of combinatorial library of aptamers (70‐base long strands containing 30 randomized bases), each iteration consists of: 1) incubation with the target molecule, 2) partitioning of the target‐binding and non‐binding aptamers, 3) amplification of the binding aptamers through polymerase chain reaction (PCR), and 4) separation of the amplicons into single‐stranded aptamers. This newly generated pool is then used as the starting library for the consecutive SELEX cycle. In the last round, the aptamer pool is cloned in E.coli and the sequence of each target binding strand is elucidated.The magnetic resonance frequency reporter is composed of 129Xe atom encapsulated in an organic ‘cage’ molecule. Starting from commercially available precursors, the synthesis of the Xe‐based component follows an established multi‐step organic synthesis procedure. In the final stage, conjugation of the aptamer to the organic ‘cage’ bearing the Xe atom is achieved via Huisgen 1,3‐dipolar cycloaddition or carboxylic acid‐amine condensation reaction.Toward this end, we have completed several rounds of SELEX and closely monitored the progress of each cycle. We started by incubating the aptamer pool with a high concentration of glutathione (10 mM), and observed aptamer binding to the target after several rounds. We have subsequently increased the selection pressure by gradually reducing the glutathione solution to 0.5 mM. At the same time, we have successfully completed several synthetic steps toward the Xe organic ‘cage’ molecule. The product of each step was fully characterized using standard organic chemistry methods.Upon completion of our project, we will have developed a magnetic resonance imaging (MRI) agent for glutathione which will be extended to other redox‐active and cofactor molecules.Support or Funding InformationFunding for the project came from the California State University East Bay (CSUEB) Faculty Support Grant. D.S. was a recipient of the CSUEB Center for Student Research Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Frequency-specific attentional modulation in human primary auditory cortex and midbrain

Paying selective attention to an audio frequency selectively enhances activity within primary auditory cortex (PAC) at the tonotopic site (frequency channel) representing that frequency. Animal PAC neurons achieve this ‘frequency-specific attentional spotlight’ by adapting their frequency tuning, yet comparable evidence in humans is scarce. Moreover, whether the spotlight operates in human midbrain is unknown.To address these issues, we studied the spectral tuning of frequency channels in human PAC and inferior colliculus (IC), using 7-T functional magnetic resonance imaging (FMRI) and frequency mapping, while participants focused on different frequency-specific sounds. We found that shifts in frequency-specific attention alter the response gain, but not tuning profile, of PAC frequency channels. The gain modulation was strongest in low-frequency channels and varied near-monotonically across the tonotopic axis, giving rise to the attentional spotlight. We observed less prominent, non-tonotopic spatial patterns of attentional modulation in IC.These results indicate that the frequency-specific attentional spotlight in human PAC as measured with FMRI arises primarily from tonotopic gain modulation, rather than adapted frequency tuning. Moreover, frequency-specific attentional modulation of afferent sound processing in human IC seems to be considerably weaker, suggesting that the spotlight diminishes toward this lower-order processing stage. Our study sheds light on how the human auditory pathway adapts to the different demands of selective hearing.

Verdazyl-ribose: A new radical for solid-state dynamic nuclear polarization at high magnetic field

Solid-state dynamic nuclear polarization (DNP) using the cross-effect relies on radical pairs whose electron spin resonance (ESR) frequencies differ by the nuclear magnetic resonance (NMR) frequency. We measure the DNP provided by a new water-soluble verdazyl radical, verdazyl-ribose, under both magic-angle spinning (MAS) and static sample conditions at 9.4 T, and compare it to a nitroxide radical, 4-hydroxy-TEMPO. We find that verdazyl-ribose is an effective radical for cross-effect DNP, with the best relative results for a non-spinning sample. Under non-spinning conditions, verdazyl-ribose provides roughly 2× larger 13C cross-polarized (CP) NMR signal than the nitroxide, with similar polarization buildup times, at both 29 K and 76 K. With MAS at 7 kHz and 1.5 W microwave power, the verdazyl-ribose does not provide as much DNP as the nitroxide, with the verdazyl providing less NMR signal and a longer polarization buildup time. When the microwave power is decreased to 30 mW with 5 kHz MAS, the two types of radical are comparable, with the verdazyl-doped sample having a larger NMR signal which compensates for its longer polarization buildup time. We also present electron spin relaxation measurements at Q-band (1.2 T) and ESR lineshapes at 1.2 and 9.4 T. Most notably, the verdazyl radical has a longer T1e than the nitroxide (9.9 ms and 1.3 ms, respectively, at 50 K and 1.2 T). The verdazyl electron spin lineshape is significantly affected by the hyperfine coupling to four 14N nuclei, even at 9.4 T. We also describe 3000-spin calculations to illustrate the DNP potential of possible radical pairs: verdazyl-verdazyl, verdazyl-nitroxide, or nitroxide-nitroxide pairs. These calculations suggest that the verdazyl radical at 9.4 T has a narrower linewidth than optimal for cross-effect DNP using verdazyl-verdazyl pairs. Because of the hyperfine coupling contribution to the electron spin linewidth, this implies that DNP using the verdazyl radical would improve at lower magnetic field. Another conclusion from the calculations is that a verdazyl-nitroxide bi-radical would be expected to be slightly better for cross-effect DNP than the nitroxide-nitroxide bi-radicals commonly used now, assuming the same spin-spin coupling constants.

Controlling Magnetic Dipole Resonance in Raspberry-like Metamolecules

Raspberry-like metamolecules (RMMs), clusters of closely packed noble metal nanobeads self-assembled on a dielectric core, exhibit emergent optical properties not available in simple nanoparticles. Examples include broad band far-field extinction and artificial optical magnetism. An important feature of these clusters is that their magnetic plasmon resonance and the breadth of their extinction spectra can be tuned via simple synthetic routes, such as by changing the bead size, core size, or the average interbead distance. However, the effect of each of these variables on the final magnetic resonance frequency and strength has not been studied in depth. Understanding how to tune the electric and magnetic resonance modes in these clusters can help improve the design of novel metamaterials for various applications. In this article, we combine theoretical analyses using numerical finite-different time-domain modeling and analytical dipole–dipole coupling theory to study the role of these variables in the glob...

The effect of the dielectric layer thickness on the negative permeability in metamaterials

In this work, we investigate the influences of the dielectric layer on the magnetic resonance of the cut-wire pair structure (CWP). The interaction between the cut-wires is modeled through a LC circuit, based on which, the magnetic resonant frequency is calculated. Furthermore, the dependence of the resonant bandwidth on the structure parameters is also determined. By tuning the dielectric layer thickness, we obtained a noticeable broadening of the negative permeability regime of 17%, which represents the enhancement of the magnetic resonance. A good agreement between the theory, simulation, and practical experiment has been demonstrated. We believe that our results should be consequential with regard to the determination of the mechanism behind the wave-matter interaction in the GHz frequency regime.

Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser.

The Earth's magnetic field has significant effects that protect us from cosmic radiation and provide navigation for biological migration. However, slow temporal variations originating in the liquid outer core invariably exist. To understand the working mechanism of the geomagnetic field and improve accuracy of navigation systems, a high-precision magnetometer is essential to measure the absolute magnetic field. A helium optically pumping magnetometer is an advanced approach, but its sensitivity and accuracy are directly limited by the low-frequency relative intensity noise and frequency stability characteristics of a light source. Here, we demonstrate a near quantum-noise limited and absolute frequency stabilized 1083nm single-frequency fiber laser. The relative intensity noise is only 5dB higher than the quantum-noise limit, and the root mean square of frequency fluctuation is ∼17 kHz after locked. This fiber laser could suppress the fluctuation of magnetic resonant frequency and improve the signal-to-noise ratio of the magnetic resonance signal detection.

Multi-resonance frequency spin dependent charge pumping and spin dependent recombination - applied to the 4H-SiC/SiO2 interface

We report on a new electrically detected magnetic resonance (EDMR) approach involving spin dependent charge pumping (SDCP) and spin dependent recombination (SDR) at high (K band, about 16 GHz) and ultra-low (360 and 85 MHz) magnetic resonance frequencies to investigate the dielectric/semiconductor interface in 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). A comparison of SDCP and SDR allows for a comparison of deep level defects and defects with energy levels throughout most of the bandgap. Additionally, a comparison of high frequency and ultra-low frequency measurements allows for (1) the partial separation of spin-orbit coupling and hyperfine effects on magnetic resonance spectra, (2) the observation of otherwise forbidden half-field effects, which make EDMR, at least, in principle, quantitative, and (3) the observation of Breit-Rabi shifts in superhyperfine measurements. (Observation of the Breit-Rabi shift helps in both the assignment and the measurement of superhyperfine parameters.) We find that, as earlier work also indicates, the SiC silicon vacancy is the dominating defect in n-MOSFETs with as-grown oxides and that post-oxidation NO anneals significantly reduce their population. In addition, we provide strong evidence that NO anneals result in the presence of nitrogen very close to a large fraction of the silicon vacancies. The results indicate that the presence of nearby nitrogen significantly shifts the silicon vacancy energy levels. Our results also show that the introduction of nitrogen introduces a disorder at the interface. This nitrogen induced disorder may provide at least a partial explanation for the relatively modest improvement in mobility after the NO anneals. Finally, we compare the charge pumping and SDCP response as a function of gate amplitude and charge pumping frequency.

B1-control receive array coil (B-RAC) for reducing B1+ inhomogeneity in abdominal imaging at 3T-MRI

B1+ inhomogeneity in the human body increases as the nuclear magnetic resonance (NMR) frequency increases. Various methods have thus been developed to reduce B1+ inhomogeneity, such as a dielectric pad, a coupling coil, parallel transmit, and radio-frequency (RF) shimming. However, B1+ inhomogeneity still remains in some cases of abdominal imaging.In this study, we developed a B1-control receive array coil (B-RAC). Unlike the conventional receive array coil, B-RAC reduces B1+ inhomogeneity by using additional PIN diodes to generate the inductive loop during the RF transmit period. The inductive loop can generate dense and sparse regions of the magnetic flux, which can be used to compensate for B1+ inhomogeneity. First, B-RAC is modeled in the numerical simulation, and the spatial distributions of B1+ in a phantom and a human model were analyzed. Next, we fabricated a 12-channel B-RAC and measured receive sensitivity and B1+ maps in a 3T-MRI experiment.It was demonstrated that B-RAC can reduce B1+ inhomogeneity in the phantom and human model without increasing the maximum local specific absorption rate (SAR) in the body. B-RAC was also found to have almost the same the receive sensitivity as the conventional receive coil. Using RF shimming combined with B-RAC was revealed to more effectively reduce B1+ inhomogeneity than using only RF shimming.Therefore, B-RAC can reduce B1+ inhomogeneity while maintaining the receive sensitivity.

Theoretical Study on the 1.185-THz Third Harmonic Gyrotron

We discuss how the existing University of Fukui (FIR UF) second harmonic double-beam gyrotron with the operating frequency 0.79 THz can be adopted for operation at the third harmonic. The new gyrotron will operate at the frequency 1.185 THz and will significantly increase the frequency of the dynamic nuclear polarization-nuclear magnetic resonance (DNP-NMR) spectrometer. This will allow one to study new bio-molecules.A special attention is payed to the mode competition between the operating $$ {TE}_{3,11}^{+} $$ mode at the third harmonic and the parasitic modes at the second and fundamental harmonics. The operating parameters of the modified gyrotron are U = 20 kV, α = 1.3, I = 0.35 A, and B = 14.60 T and the expected output power about $$ 100\mathrm{W} $$ .

A diffusive atmospheric pressure glow discharge in a coaxial pin-to-ring gap with a transverse magnetic field

Atmospheric pressure glow discharge (APGD) has been widely used in the industrial field. The industrial applications are based on achieving stable and diffusive APGD in a relatively large space. The existing sources only achieved stable and diffusive APGD between a short inter-electrode distance within 5 millimeters. In this paper, the effect of a transverse stationary magnetic field on the diffusion of filamentary APGD was studied in a pin-to-ring coaxial gap. The APGD was driven by a high-voltage resonant power supply, and the stationary magnetic field was supplied by a permanent magnet. The stable and diffusive APGD was achieved in the circular area, which diameter was 20 millimeters. The experimental results revealed that more collision ionization occurred and the plasma was distributed diffusively in the discharge gap by applying the external transverse magnetic field. Besides, it is likely to obtain more stable and diffusive APGD in the coaxial pin-to-ring discharge gap when adjusting the input voltage, transverse magnetic flux density and resonant frequency of the power supply.