Radiofrequency Excitation Research Articles

Study on the Design and Application of the Three-electrode Ionization Particle Sensor

Accurate detection is the foundation of haze governance. The existing particle concentration detection methods and instruments have many shortcomings, such as complex structure, high cost, easy blockage, short life, and significant impact of environmental changes. Therefore, this paper proposes a silicon micropillar three-electrode ionization particle sensor based on the gas discharge principle and studies the sensor's sensitivity to PM2.5 particles. Three-electrode comprises a cathode with micro silicon pillars grown on the surface, an extracting electrode, and a collecting electrode. It shows that the cathode current of the sensor increases linearly with extraction voltage and nonlinearly with temperature. As the cathode material of the sensor, the silicon micropillar is compared with the carbon nanotube to solve the burn problem of the nanotube rising from positive ions bombardment during gas discharge. The collecting current of the sensor shows a decreasing trend with the increase in particle concentration. The collecting current and sensitivity of the sensor are much higher in radio frequency excitation than in direct current excitation. The sensor developed in this paper displays the advantages of simple technology, high integration, low cost, and high sensitivity, and it exhibits great application potential.

Deep learning corrects artifacts in RASER MRI profiles

A newly developed magnetic resonance imaging (MRI) approach is based on “Radiowave amplification by the stimulated emission of radiation” (RASER). RASER MRI potentially allows for higher resolution, is inherently background-free, and does not require radio-frequency excitation. However, RASER MRI can be “nearly unusable” as heavy distortions from nonlinear effects can occur. In this work, we show that deep learning (DL) reduces such artifacts in RASER images. We trained a two-step DL pipeline on purely synthetic data, which was generated based on a previously published, theoretical model for RASER MRI. A convolutional neural network was trained on 630′000 1D RASER projections, and a U-net on 2D random images. The DL pipeline generalizes well when applied from synthetic to experimental RASER MRI data.

Ultra-low-field magnetic resonance angiography at 0.05 T: A preliminary study.

We aim to explore the feasibility of head and neck time-of-flight (TOF) magnetic resonance angiography (MRA) at ultra-low-field (ULF). TOF MRA was conducted on a highly simplified 0.05 T MRI scanner with no radiofrequency (RF) and magnetic shielding. A flow-compensated three-dimensional (3D)gradient echo (GRE) sequence with a tilt-optimized nonsaturated excitation RF pulse, and a flow-compensated multislice two-dimensional (2D) GRE sequence, were implemented for cerebral artery and vein imaging, respectively. For carotid artery and jugular vein imaging, flow-compensated 2D GRE sequences were utilized with venous and arterial blood presaturation, respectively. MRA was performed on young healthy subjects. Vessel-to-background contrast was experimentally observed with strong blood inflow effect and background tissue suppression. The large primary cerebral arteries and veins, carotid arteries, jugular veins, and artery bifurcations could be identified in both raw GRE images and maximum intensity projections. The primary brain and neck arteries were found to be reproducible among multiple examination sessions. These preliminary experimental results demonstrated the possibility of artery TOF MRA on low-cost 0.05 T scanners for the first time, despite the extremely low MR signal. We expect to improve the quality of ULF TOF MRA in the near future through sequence development and optimization, ongoing advances in ULF hardware and image formation, and the use of vascular T1 contrast agents.

Helmet Radio Frequency Phased Array Applicators Enhance Thermal Magnetic Resonance of Brain Tumors.

Thermal Magnetic Resonance (ThermalMR) integrates Magnetic Resonance Imaging (MRI) diagnostics and targeted radio-frequency (RF) heating in a single theranostic device. The requirements for MRI (magnetic field) and targeted RF heating (electric field) govern the design of ThermalMR applicators. We hypothesize that helmet RF applicators (HPA) improve the efficacy of ThermalMR of brain tumors versus an annular phased RF array (APA). An HPA was designed using eight broadband self-grounded bow-tie (SGBT) antennae plus two SGBTs placed on top of the head. An APA of 10 equally spaced SGBTs was used as a reference. Electromagnetic field (EMF) simulations were performed for a test object (phantom) and a human head model. For a clinical scenario, the head model was modified with a tumor volume obtained from a patient with glioblastoma multiforme. To assess performance, we introduced multi-target evaluation (MTE) to ensure whole-brain slice accessibility. We implemented time multiplexed vector field shaping to optimize RF excitation. Our EMF and temperature simulations demonstrate that the HPA improves performance criteria critical to MRI and enhances targeted RF and temperature focusing versus the APA. Our findings are a foundation for the experimental implementation and application of a HPA en route to ThermalMR of brain tumors.

Toward Ultra-High-Quality-Factor Wireless Masing Magnetic Resonance Sensing.

It has recently been shown that a bolus of hyperpolarized nuclear spins can yield stimulated emission signals similar in nature to maser signals, potentially enabling new ways of sensing hyperpolarized contrast media, including most notably [1-13C]pyruvate that is under evaluation in over 50 clinical trials for metabolic imaging of cancer. The stimulated NMR signal emissions lasting for minutes do not require radio-frequency excitation, offering unprecedented advantages compared to conventional MR sensing. However, creating nuclear spin maser emission is challenging in practice due to stringent fundamental requirements, making practical in vivo applications hardly possible using conventional passive MR detectors. Here, we demonstrate the utility of a wireless NMR maser detector, the quality factor of which was enhanced 22-fold (to 1,670) via parametric pumping. This active-feedback technique breaks the intrinsic fundamental limit of NMR detector circuit quality factor. We show the use of parametric pumping to reduce the threshold requirement for inducing nuclear spin masing at 300 MHz resonance frequency in a preclinical MRI scanner. Indeed, stimulated emission from hyperpolarized protons was obtained under highly unfavorable conditions of low magnetic field homogeneity (T2* of 3 ms). Greater gains of the quality factor of the MR detector (up to 1 million) were also demonstrated.

Dynamic metastable vortex states in interacting vortex lines

The electron transport in current-biased superconducting nano-bridges is determined by the motion of the quantum vortex confined in the internal disorder landscape. Here we consider theoretically a simple case of a single or two neighbouring linear defects crossing a nano-bridge. The strong anharmonicity of the vortex motion along the defect leads, upon radio frequency (RF) excitation, to fractional Shapiro steps. In the case of two defects, the vortex motion becomes correlated, characterised by metastable states that can be locked to the RF-drive. The lock-unlock process causes sudden voltage jumps and drops in the voltage-current characteristics that can be observed in experiments. We analyse the parameters that promote these metastable dynamic states and discuss their possible experimental realisations.

Measurements of nematic susceptibility with phase sensitive nuclear magnetic resonance in pulsed strain fields.

We present nuclear magnetic resonance data in BaFe2As2 in the presence of pulsed strain fields that are interleaved in time with the radio frequency excitation pulses. In this approach, the preceding nuclear magnetization acquires a phase shift that is proportional to the strain and pulse time. The sensitivity of this approach is limited by the homogeneous decoherence time, T2, rather than the inhomogeneous linewidth. We measure the nematic susceptibility as a function of temperature and demonstrate a three orders of magnitude improvement in sensitivity. This approach will enable studies of the strain response in a broad range of materials that previously were inaccessible due to inhomogeneous broadening.

From strength to precision: A systematic review exploring the clinical utility of 7-Tesla magnetic resonance imaging in abdominal imaging.

After approval for clinical use in 2017 early investigations of ultra-high-field abdominal magnetic resonance imaging (MRI) have demonstrated the feasibility as well as diagnostic capabilities of liver, kidney, and prostate MRI at 7-Tesla. However, the elevation of the field strength to 7-Tesla not only brought advantages to abdominal MRI but also presented considerable challenges and drawbacks, primarily stemming from heightened artifacts and limitations in Specific Absorption Rate, etc. Furthermore, evidence in the literature is relatively scarce concerning human studies in comparison to phantom/animal studies which necessitates an investigation into the evidence so far in humans and summarizing all relevant evidence. To offer a comprehensive overview of current literature on clinical abdominal 7T MRI that emphasizes current trends, details relevant challenges, and provides a concise set of potential solutions. This systematic review adheres to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. A PubMed search, utilizing Medical Subject Headings terms such as "7-Tesla" and organ-specific terms, was conducted for articles published between January 1, 1985, and July 25, 2023. Eligibility criteria included studies exploring 7T MRI for imaging human abdominal organs, encompassing various study types (in-vivo/ex-vivo, method development, reviews/meta-analyses). Exclusion criteria involved animal studies and those lacking extractable data. Study selection involved initial identification via title/abstract, followed by a full-text review by two researchers, with discrepancies resolved through discussion. Data extraction covered publication details, study design, population, sample size, 7T MRI protocol, image characteristics, endpoints, and conclusions. The systematic review included a total of 21 studies. The distribution of clinical 7T abdominal imaging studies revealed a predominant focus on the prostate (n = 8), followed by the kidney (n = 6) and the hepatobiliary system (n = 5). Studies on these organs, and in the pancreas, demonstrated clear advantages at 7T. However, small bowel studies showed no significant improvements compared to traditional MRI at 1.5T. The majority of studies evaluated originated from Germany (n = 10), followed by the Netherlands (n = 5), the United States (n = 5), Austria (n = 2), the United Kingdom (n = 1), and Italy (n = 1). Further increase of abdominal clinical MRI field strength to 7T demonstrated high imaging potential, yet also limitations mainly due to the inhomogeneous radiofrequency (RF) excitation field relative to lower field strengths. Hence, further optimization of dedicated RF coil elements and pulse sequences are expected to better optimize clinical imaging at high magnetic field strength.

Transition to a Magnon Bose–Einstein Condensate

Parameters of the transition from classical dynamics of spin waves to the formation of a coherent magnon Bose–Einstein condensate have been obtained experimentally for the first time. The studies are performed on an yttrium iron garnet film beyond the radio frequency excitation region; thus, the coherent state of magnons is an eigenstate rather than a state induced by an external radio frequency field. The critical magnon density at the formation of the Bose–Einstein condensate is in good agreement with a theoretically predicted value. The transition is obtained at room temperature, which is possible owing to a small mass of magnons and their high density.

A workflow for predicting radiofrequency-induced heating around bilateral deep brain stimulation electrodes in MRI.

Heating around deep brain stimulation (DBS) in magnetic resonance imaging (MRI) occurs when the time-varying electromagnetic (EM) fields induce currents in the electrodes which can generate heat and potentially cause tissue damage. Predicting the heating around the electrode contacts is important to ensure the safety of patients with DBS implants undergoing an MRI scan. We previously proposed a workflow to predict heating around DBS contacts and introduced a parameter, equivalent transimpedance, that is independent of electrode trajectories, termination, and radiofrequency (RF) excitations. The workflow performance was validated in a unilateral DBS system. To predict RF heating around the contacts of bilateral (DBS) electrodes during an MRI scan in an anthropomorphic head phantom. Bilateral electrodes were fixed in a skull phantom filled with hydroxyethyl cellulose (HEC) gel. The electrode shafts were suspended extracranially, in a head and torso phantom filled with the same gel material. The current induced on the electrode shaft was experimentally measured using an MR-based technique 3cm above the tip. A transimpedance value determined in a previous offline calibration was used to scale the shaft current and calculate the contact voltage. The voltage was assigned as a boundary condition on the electrical contacts of the electrode in a quasi-static (EM) simulation. The resulting specific absorption rate (SAR) distribution became the input for a transient thermal simulation and was used to predict the heating around the contacts. RF heating experiments were performed for eight different lead trajectories using circularly polarized (CP) excitation and two linear excitations for one trajectory. The measured temperatures for all experiments were compared with the simulated temperatures and the root-mean-squared errors (RMSE) were calculated. The RF heating around the contacts of both bilateral electrodes was predicted with ≤ 0.29°C of RMSE for 20 heating scenarios. The workflow successfully predicted the heating for different bilateral DBS trajectories and excitation patterns in an anthropomorphic head phantom.

Electric-field modulation of the charge-density-wave quantum condensate in h-BN/NbS3 quasi-2D/1D heterostructure devices

We report on the field-effect modulation of the charge-density-wave quantum condensate in the top-gated heterostructure devices implemented with quasi-one-dimensional NbS3 nanowire channels and quasi-two-dimensional h-BN gate dielectric layers. The charge-density-wave phases and collective current in quasi-1D NbS3 nanowires were verified via temperature dependence of the resistivity, non-linear current–voltage characteristics, and Shapiro steps that appeared in the device response under radio frequency excitation mixed with the DC bias. It was demonstrated that the electric field of the applied gate bias can reversibly modulate the collective current of the sliding charge-density-wave condensate. The collective current reduces with more positive bias, suggesting a surface effect on the condensate mobility. The single-particle current, at small source–drain biases, shows small-amplitude fluctuation behavior, attributed to the variations in the background potential due to the pinned or creeping charge-density-wave condensate. The knowledge of the electric-field effect on the charge density waves in quasi-1D NbS3 nanowires is useful for potential electronic applications of such quantum materials.

MOSFET-Based RF Switch With Active Biasing

A biasing method for a stacked field-effect transistor (FET)-based radio-frequency (RF) switch is proposed. The advantage of the method over conventional passive resistive biasing is the ability to sustain a desirable operating point under the peak RF excitation conditions achieved by compensating the excessive leakage current of FETs in the stack. Operating point stabilization results in improved RF voltage-handling capabilities of the OFF-state switch. A single-pole double-throw (SPDT) hardware demonstrator implemented in a dedicated 130-nm bulk-CMOS RF switch technology shows an increase in breakdown voltage by 10% for a 35-V voltage-handling class switch device. The improvement is achieved at the expense of increased peak current consumption and no penalty in the RF performance of the switch.

Adiabatic approach for heteronuclear SABRE hyperpolarization at high magnetic field

Signal Amplification By Reversible Exchange (SABRE) is a technique aimed at enhancing weak NMR signals of heteronuclei by utilizing the non-equilibrium spin order of parahydrogen. SABRE polarization transfer takes place by means of metalorganic complexes that interact with parahydrogen and the substrate to be polarized in a reversible manner. To achieve substrate hyperpolarization in the high magnetic field of an NMR magnet, radiofrequency (RF) excitation is required. There are two general options for the RF field amplitude: constant or modulated. To date, there has been limited optimization of the adiabatic SABRE conditions. In SABRE, the presence of chemical exchange significantly complicates the spin dynamics involved in polarization transfer and the optimization of adiabatic RF sweeps. We conducted a comprehensive analysis of high-field SABRE pulse sequences with RF sweeps on the heteronuclear channel, specifically 15N. We proposed a simple method for optimizing the amplitude modulation profile of the RF field, which is efficient for systems undergoing chemical exchange. Our approach involved utilizing the dependence of 15N polarization on the amplitude of the constant RF field on the 15N channel. By employing the "optimal" adiabatic RF profile, we achieved a 2.5-fold increase in 15N SABRE-derived polarization at high magnetic field compared to a linear sweep. We theoretically assessed the benefit of RF sweeps over constant RF fields for SABRE at high magnetic field. We demonstrated experimentally that at temperatures −5∘C - +10∘C RF sweeps are more efficient than constant RF field. Maximal increase in 15N polarization achieved was 1.7-fold for bound and 1.4-fold for free substrate. We attribute this increase in polarization to the adiabaticity of the polarization transfer process. This behavior was explained via numerical solution of SABRE master equation for different dissociation rate constants.

Approaching the Ultimate Limit in Measurement Precision with RASER NMR

Radio-frequency Amplification by Stimulated Emission of Radiation (RASER) is a promising tool to study nonlinear phenomena or measure NMR parameters with unprecedented precision. Magnetic fields, J-couplings, and chemical shifts can be recorded over long periods of time without the need for radiofrequency excitation and signal averaging. One key feature of RASER NMR spectroscopy is the improvement in precision, which grows with the measurement time T_{{text{m}}}^{3/2}, unlike conventional NMR spectroscopy, where the precision increases with T_{{text{m}}}^{1/2}. However, when detecting NMR signals over minutes to hours, using available NMR magnets (ppb homogeneity), the achieved frequency resolution will eventually be limited by magnetic field fluctuations. Here, we demonstrate that full compensation is possible even for open low-field electromagnets, where magnetic field fluctuations are intrinsically present (in the ppm regime). A prerequisite for compensation is that the spectrum contains at least one isolated RASER line to be used as a reference, and the sample experiences exclusively common magnetic field fluctuations, that is, ones that are equal over the entire sample volume. We discuss the current limits of precision for RASER NMR measurements for two different cases: The single-compartment RASER involving J-coupled modes, and the two-compartment RASER involving chemically shifted species. In the first case, the limit of measurable difference approaches the Cramér-Rao lower bound (CRLB), achieving a measurement precision {sigma }_{f}<{10}^{-4} Hz. In the second case, the measured chemical shift separation is plagued by independently fluctuating distant dipolar fields (DDF). The measured independent field fluctuation between the two chambers is in the order of tens of mHz. In both cases, new limits of precision are achieved, which paves the way for sub-mHz detection of NMR parameters, rotational rates, and non-linear phenomena such as chaos and synchrony.

Evaluation of diffusion-weighted magnetic resonance imaging of the rectal cancers: comparison between modified reduced field-of-view single-shot echo-planar imaging with tilted two-dimensional radiofrequency excitation pulses and conventional full field-of-view readout-segmented echo-planar imaging.

To evaluate the image quality qualitatively and quantitatively, as well as apparent diffusion coefficient (ADC) values of modified reduced field-of-view diffusion-weighted magnetic resonance imaging (MRI) using spatially tailored two-dimensional radiofrequency pulses with tilted excitation plane (tilted r-DWI) based on single-shot echo planar imaging (SS-EPI) compared with full-size field-of-view DWI (f-DWI) using readout segmented (RS)-EPI in patients with rectal cancer. Twenty-two patients who underwent an MRI for further evaluation of rectal cancer were included in this retrospective study. All MR images were analyzed to compare image quality, lesion conspicuity, and artifacts between f-DWI with RS-EPI and tilted r-DWI with SS-EPI. Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and ADC values were also compared. The Wilcoxon signed-rank test or paired t test was performed to compare the qualitative and quantitative assessments. All image quality scores, except aliasing artifacts, were significantly higher (p < 0.01 for all) in tilted r-DWI than f-DWI with RS-EPI. CNR in tilted r-DWI was significantly higher than in f-DWI with RS-EPI (p < 0.01), while SNR was not significantly different. Regarding the ADC values, no significant difference was observed between tilted r-DWI and f-DWI with RS-EPI (p = 0.27). Tilted r-DWI provides a better image quality with fewer artifacts and higher rectal lesion conspicuity than f-DWI with RS-EPI, indicating the feasibility of this MR sequence in evaluating rectal cancer in clinical practice.

Performance of Cryogen-Free Superconducting Magnet in Isochronous Cyclotron for Proton Therapy

A superconducting azimuthally varying field (AVF) cyclotron, SC230, was developed for proton therapy by Sumitomo Heavy Industries, Ltd. in 2021. This isochronous cyclotron is compact owing to the high magnetic field generated by NbTi superconducting coils. The average magnetic field is 3.9 T at the outer region, and the extraction radius is 0.6 m. Its yoke weight is 65 t. Currently, it is the most compact isochronous cyclotron for this purpose. The coils are cryogen-free and cooled by conduction cooling using four 4 K-GM cryocoolers. The system is highly safe and easy to maintain, which reduces the effects of unstable helium supply. Thus, cryogen-free magnets have various advantages; however, in isochronous cyclotrons, the heat inputs to the coils during beam operation are larger owing to the leakage radio frequency (RF) and beam loss. The magnet is designed to have a sufficient temperature margin for stable operation. The coil temperature during the RF excitation and its dependence on the beam loss were measured to verify the stability of operations. Subsequently, it was confirmed that the coil cools sufficiently at the critical temperature.

Dynamic nuclear polarization weighted spectroscopy of multispin electronic-nuclear clusters

Nuclear spins and paramagnetic centers in a solid randomly group to form clusters featuring nearly degenerate, hybrid states whose dynamics are central to processes involving nuclear spin-lattice relaxation and diffusion. Their characterization, however, has proven notoriously difficult mostly due to their relative isolation and comparatively low concentration. Here, we combine magnetic field cycling experiments, optical spin pumping, and variable radiofrequency (RF) excitation to probe transitions between hybrid multispin states formed by strongly coupled electronic and nuclear spins in diamond. Leveraging bulk nuclei as a collective time-integrating sensor, we probe the response of these spin clusters as we simultaneously vary the applied magnetic field and RF excitation to reconstruct multidimensional spectra. We uncover complex nuclear polarization patterns of alternating sign that we qualitatively capture through analytical and numerical modeling. Our results unambiguously expose the impact that strongly hyperfine-coupled nuclei can have on the spin dynamics of the crystal, and inform future routes to spin cluster control and detection.

P‐3: Conductive Indium‐Tin‐Zinc Oxide Formed Using an Oxygen Plasma Treatment Through a Silicon Oxide Cover Layer

Indium‐tin‐zinc oxide (ITZO) covered with a silicon oxide layer can be made conductive when subjected to an oxygen plasma treatment. The resulting resistivity is sensitive to the thickness of the cover oxide, the plasma excitation power, and the treatment time. With 280 nm of cover oxide and 10 mins of treatment, the lowest resistivity achieved is 1.2 mΩ∙cm in a plasma biased with a radio frequency excitation power of 100 W and an inductively coupled plasma excitation power of 2000 W. The treatment has been deployed to form the source/drain regions of a self‐aligned, top‐gate ITZO thin‐film transistor.

Optimal control flow encoding for time-efficient magnetic resonance velocimetry

Phase contrast velocimetry relies on bipolar gradients to establish a direct and linear relationship between the phase of the magnetic resonance signal, and the corresponding fluid motion. Despite its utility, several limitations and drawbacks have been reported, the most important being the extended echo time due to the encoding after the excitation. In this study, we elucidate a new approach based on optimal control theory that circumvents some of these disadvantages. An excitation pulse, termed FAUCET (flow analysis under controlled encoding transients), is designed to encode velocity into phase already during the radiofrequency excitation. As a result of concurrent excitation and flow encoding, and hence elimination of post-excitation flow encoding, FAUCET achieves a shorter echo time than the conventional method. This achievement is a matter of significance not only because it decreases the loss of signal due to spin–spin relaxation and B0 inhomogeneity, but also because a shorter echo time is always preferred in order to reduce the dimensionless dephasing parameter and the required residence time of the flowing sample in the detection coil. The method is able to establish a non-linear bijective relationship between phase and velocity, which can be employed to enhance the resolution over a specific range of velocities, for example along flow boundaries. A computational comparison between the phase contrast and optimal control methods reveals that the latter’s encoding is more robust against remnant higher-order-moment terms of the Taylor expansion for faster voxels, such as acceleration, jerk, and snap.

Preparation of ultracold ions in strong magnetic field and application to gas-phase NMR spectroscopy II

This paper presents a gas-phase nuclear magnetic resonance (NMR) apparatus, which may enable a tandem combination with an ion cyclotron resonance mass spectrometer. The NMR technique is widely used for analyzing the physical and chemical properties of materials; however, it is mostly limited to materials in condensed phases. Here, we construct a gas-phase NMR apparatus to extend this technique to gas-phase molecular ions. We outline the principle of NMR detection based on a Stern-Gerlach-type experiment and describe the experimental procedures and results for the preparation and manipulation of ultracold ions, which are fundamental for the proposed NMR detection. The development of the gas-phase NMR probe and radio-frequency excitation system and the features of the apparatus are presented. We also discuss the feasibility of combining this apparatus with an ion cyclotron resonance cell for realizing an NMR apparatus for mass-selected molecular ions briefly.