Single Radio Frequency Channel Research Articles

Localized Shims Enable Low-Field Simultaneous Multinuclear NMR Spectroscopy.

This Article introduces a smart localized shimset, integrated with a radiofrequency (RF) microsolenoid and optimized as a single unit to locally achieve high-resolution NMR spectroscopy, as a significant advancement toward achieving parallel NMR spectroscopy. The shimset, consisting exclusively of linear shims, demonstrates the capability to improve NMR line width from 84 to 4 Hz in a 1.05 T preclinical MRI scanner. This enhancement enables the resolution of j-couplings in the sample while requiring a total power of 545 mW. Furthermore, a novel method is presented for concurrently obtaining multinuclear NMR spectra using a single RF channel. This approach allows for the utilization of high-sensitivity NMR signals as a reference to correct field drift, facilitating signal averaging in a permanent magnet NMR scanner that is prone to significant temperature-dependent drift.

Frequency multiplexing enables parallel multi-sample EPR

Electron paramagnetic resonance (EPR) spectroscopy stands out as a powerful analytical technique with extensive applications in the fields of biology, chemistry, physics, and material sciences. It proves invaluable for investigating the molecular structure and reaction mechanisms of substances containing unpaired electrons, such as metal complexes, organic and inorganic radicals, and intermediate states in chemical reactions. However, despite their remarkable capabilities, EPR systems face significant limitations in terms of sample throughput, as current commercial systems only target the analysis of one sample at a time. Here we introduce a novel scheme for conducting ultra-high frequency continuous-wave EPR (CW EPR) targeting the EPR spectroscopy of multiple microliter volume samples in parallel. Our proof-of-principle prototype involves two decoupled detection cells equipped with high qualty factor Q=104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Q=104$$\\end{document} solenoidal coils tuned to 488 and 589 MHz, ensuring a significant frequency gap for effective radio frequency (RF) decoupling between the channels. To further enhance electromagnetic decoupling, an orthogonal alignment of the coils was adopted. The paper further presents an innovative radiofrequency circuit concept that utilizes a single physical RF channel to simultaneously conduct parallel EPR on up to eight cells. Parallel EPR experiments on two BDPA samples, each with a sample volume of 18.3 μL, registered signal-to-noise ratios of 255 and 252 for the two EPR measurement cells, with no observable coupling. The showcased prototype, built using cost-effective commercially available fabrication technology, is readily scalable and represents an initial step with promising potential for advancing sample screening with high-throughput parallel EPR.

Measurement of Complex-Ratio Between Two RF Signals With Spectrum Analysis

A novel method to measure the amplitude-phase differences between two radio frequency (RF) signals is presented, based on spectrum analysis after periodic modulation. Two input signals are alternatively selected to enter the single-channel RF channel by a single-pole double-throw (SPDT) switch, which generates harmonics related to the channel unbalance. Amplitude-phase differences between two input signals can be obtained by analyzing the spectrum of received signal using discrete Fourier transform (DFT) in digital domain. Compared to conventional multichannel RF receivers, the proposed method has a concise structure and low-computational complexity. Simulation results are provided to verify its effectiveness over dynamic ranges of 60 dB and 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^\circ$</tex-math> </inline-formula> . A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$</tex-math> </inline-formula> -band RF circuit is fabricated to measure the amplitude-phase differences over 35 dB and 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^\circ$</tex-math> </inline-formula> dynamic ranges. The measured results show good agreement to those by the vector network analyzer (VNA), certifying the effectiveness of proposed method.

An optimized quadrature RF receive coil for very-low-field (50.4 mT) magnetic resonance brain imaging

The radiofrequency (RF) receive coil is a direct probe for magnetic resonance imaging (MRI), and its performance determines the quality of MRI results. The RF coil employed for low-field MRI has a low working frequency, which makes its characteristic different from the RF coil exploited for conventional clinic MRI and may result in a different optimum RF coil configuration. To design and optimize a head RF receive coil for a very-low-field (50.4 mT) MRI system, we investigated the relationship between the structure and performance of the RF coil. Specifically, we focused on a quadrature RF coil consisting of a saddle coil and a modified Helmholtz coil wound around the surface of an elliptical cylinder. First, we evaluated the efficiency and RF magnetic field inhomogeneity of one-loop RF coil and determined the optimum dimension for saddle coil and modified Helmholtz RF coil. Then, we further studied the performance of the optimum-dimension RF coil from the perspective of the number of RF coil loops and revealed that the number of loops of RF coil for very-low-field MRI was a remarkable feature influencing the alternative current (AC) resistance of the RF coil and therefore make the SNR increase first and then decrease with the number of RF coil loops. We finally obtained the optimum number of loops for the saddle coil, modified Helmholtz coil, and fabricated a quadrature RF coil. The performance of the quadrature coil was evaluated through CuSO4 phantom imaging and in vivo human brain imaging. In phantom imaging, the SNR of quadrature RF coil increased by about 40% compared with that of single-channel RF coil.

Improving local SNR of a single-channel 54.6 mT MRI system using additional LC-resonator

Very-low field magnetic resonance imaging (VLF-MRI, B0 < 0.1T) has an essential application in medical imaging diagnosis because of its light weight and low cost. For single-channel RF coil VLF-MRI system, a planar spiral LC-resonator placed on the surface of samples was designed to improve the local SNR. First, an equivalent circuit model was established to evaluate the boosting effects on radiofrequency (RF) magnetic field and SNR. Second, the relationship between the resonant capacitance and the transmission coefficient was deduced according to the circuit model, and the appropriate resonant capacitance was obtained. Then, the influence of the diameter and the number of turns of the LC-resonator on the SNR is considered, and the structure of the LC-resonator was optimized to maximize the SNR. Finally, a phantom MRI experiment was carried out with our home-built 54.6 mT MRI system to compare the SNR of the experiment with the calculation, the SNR enhancement trend of the two was consistent. Additional experiments were conducted using orange and chicken leg to demonstrate the SNR enhancement abilities of the LC-resonator. The enhancement of SNR reached up to 1.8-fold and 2.2-fold depending on the distance between the sample and LC-resonator. For comparison, we conducted imaging experiments on surface receiving coil with the same parameters, and the results show that the SNR of the LC resonator is comparable to that of the surface coil. The reported LC-resonator provide a low-cost local enhancement method for VLF-MRI.

Selective RF excitation designs enabled by time-varying spatially non-linear ΔB0 fields with applications in fetal MRI.

To demonstrate, through numerical simulations, novel designs of spatially selective radiofrequency (RF) excitations of the fetal brain by both a restricted 2D slice and 3D inner-volume selection. These designs exploit a single-channel RF pulse, conventional gradient fields, and the spatially non-linear ΔB0 fields of a multi-coil shim array, using an auto-differentiation optimization algorithm. The design algorithm jointly optimizes the RF pulse and the time-varying ΔB0 fields, which is produced by a 64-channel multi-coil ΔB0 body array to augment the RF and the linear gradient fields, using an auto-differentiation approach. Two design targets were specified, one a 4-mm thick slice with a limited in-slice extent in one dimension ("restricted slice"), and the other a 3D inner-volume selection encompassing the fetal brain ("inner volume"). The RF duration was limited to 2ms for the restricted slice excitation and 6ms for the inner-volume excitation. Excitation profiles were achieved for both the restricted slice excitation task (one-minus-minimum magnitude, 8%) within the region of interest (ROI) and (maximum-minus-zero magnitude, 8%) in the suppressed regions and the fetal brain volume excitation task (13% and 9%, respectively). The proposed joint design of RF and time-varying, spatially non-linear ΔB0 fields achieves the target excitation profiles with short RF pulse durations and demonstrates the potential to enhance fetal MRI with multi-channel body shim arrays.

Single-Channel Spatial Spectrum Estimation Direction Finding by the Time-Modulated Linear Array

In this letter, a novel direction-finding method with spatial spectrum estimation is proposed and verified, based on the time-modulated linear array (TMLA) and a single radio-frequency channel. Its principle is as follows. The TMLA receives and periodically modulates the incident signal. After the power combiner, the modulated signal is down-converted to the intermediate frequency and sampled in the single-channel receiver. In the digital domain, the array manifold vector can be calculated from the Fourier coefficients and the harmonic characteristic matrix related to modulation sequences, and thus, the autocorrelation matrix can be estimated. Afterward, the multiple signal classification algorithm is applied to estimate the incident angle, and the spatial smoothing method is exploited to deal with the direction finding of coherent sources. The proposed method has a simple structure and low computational complexity. Numeric simulations and corresponding experiments have been conducted to verify its effectiveness.

Image Artifact Management for Clinical Magnetic Resonance Imaging on a 7 T Scanner Using Single-Channel Radiofrequency Transmit Mode.

The aim of this work was to devise mitigation strategies for addressing a range of image artifacts on a clinical 7 T magnetic resonance imaging scanner using the regulatory-approved single-channel radiofrequency transmit mode and vendor-supplied radiofrequency coils to facilitate clinical scanning within reasonable scan times. Optimized imaging sequence protocols were developed for routine musculoskeletal knee and neurological imaging. Sources of severe image nonuniformities were identified, and mitigation strategies were devised. A range of custom-made high permittivity dielectric pads were used to compensate for B1 and B1 inhomogeneities, and also for magnetic susceptibility-induced signal dropouts particularly in the basal regions of the temporal lobes and in the cerebellum. Significant improvements in image uniformity were obtained using dielectric pads in the knee and brain. A combination of small voxels, reduced field of view B0 shimming, and high in-plane parallel imaging factors helped to minimize signal loss in areas of high susceptibility-induced field distortions. The high inherent signal-to-noise ratio at 7 T allowed for high receiver bandwidths and thin slices to minimize chemical shift artifacts. Intermittent artifacts due to radiofrequency inversion pulse limitations (power, bandwidth) were minimized with dielectric pads. A patient with 2 implanted metallic cranial fixation devices located within the radiofrequency transmit field was successfully imaged, with minimal image geometric distortions. Challenges relating to severe image artifacts at 7 T using single-channel radiofrequency transmit functionality in the knee and brain were overcome using the approaches described in this article. The resultant high diagnostic image quality paves the way for incorporation of this technology into the routine clinical workflow. Further developmental efforts are required to expand the range of applications to other anatomical areas, and to expand the evidence- and knowledge-base relating to the safety of scanning patients with implanted metallic devices.

Field Comparison Tests of LDM and TDM in ATSC 3.0

In this paper, we present laboratory and field test results of layered division multiplexing (LDM) and time division multiplexing (TDM) technologies based on Advanced Television Systems Committee 3.0. The presented field test results include analysis in several scenarios such as rooftop, indoor, and mobile receptions. In order to provide performance comparison of the LDM and TDM technologies, reception power (field strength) and successful reception rate are measured for all the considered scenarios. For in-depth analysis in real field environment, further field measurements such as threshold of visibility, marginal (minimal) reception power, and erroneous second ratio are used for rooftop, indoor, and mobile receptions, respectively. The laboratory and field test results show that the LDM technology not only shows superior performance compared to the TDM technology in all the considered scenarios (rooftop, indoor, and mobile receptions), but also enables broadcaster to efficiently provide a variety of services (e.g., mobile, pedestrian, indoor, and stationary services) with different robustness within a single radio frequency channel.

Efficient Transmission of Multiple Broadcasting Services Using LDM and SHVC

In this paper, we present an efficient transmission method of multiple broadcasting services using layered division multiplexing (LDM) and scalable high efficiency video coding (SHVC) based on the next generation terrestrial digital broadcasting standard, advanced television systems committee 3.0. In a two-layer LDM system, each layer forms a physical layer pipe that carries base layer and enhancement layer video streams of SHVC, respectively. This combination of physical and presentation layers technologies can maximize channel utilization when multiple broadcasting services are delivered in a single radio frequency channel with different robustness and reception conditions. An intensive performance analysis is provided when the proposed combination of LDM and SHVC is compared with other approaches, such as time division multiplexing and/or high efficiency video coding simulcast broadcasting. Several performance measurements are also introduced, such as constant data rate measurement and constant quality measurement, which are used to evaluate the performance of video codecs in conjunction with different physical layer system parameters. A prototype hardware system with LDM and SHVC capable of selecting different parameter combinations is tested in the laboratory and in real field environments to verify the performance and feasibility of the proposed LDM and SHVC combination. Results show that the proposed LDM and SHVC combination provides significant gains on video service quality as well as reception robustness.

Single RF Channel Digital Beamforming Array Antenna Based on Compressed Sensing for Large-Scale Antenna Applications

A single radio frequency (RF) channel digital beamforming (DBF) array antenna based on compressed sensing (CS) was proposed to reduce the hardware costs, power consumption, and the design complexity of large-scale DBF systems. For DBF antenna systems, the general way to obtain the signals received by each sensor is to connect each sensor to an independent RF receiver channel. When the array contains a large number of sensors, the multichannel signal sampling scheme makes the system power hungry and expensive. A time sequence phase weighting (TSPW) technology provides a solution to this problem. The TSPW antenna array can obtain the signals received by each sensor with only one RF receiver channel by sequentially sampling the specific single channel signals, which are produced by the TSPW array. However, the number of single channel samples scales linearly with the number of sensors. The sampling time will be so long as to be unacceptable when the array contains a large number of sensors. To overcome this problem, we introduced CS theory to the TSPW array. With the help of CS, the sampling frequency can be simultaneously reduced in both time and spatial domain, which correspond to the reduction of the number of both the samples and sensors. Theoretical analyses have been proposed to show the conditions that should be met for successful reconstruction. The simulation results from an X-band array with an aperture size of $80\lambda $ under the signal-to-noise ratio of 15 dB and the scenario of six targets showed that the proposed array could save above 63.1% of sensor numbers and above 84.4% in sampling time when compared with those of conventional TSPW arrays.

3D Double-Quantum/Double-Quantum Exchange Spectroscopy of Protons under 100 kHz Magic Angle Spinning.

Solid-state 1H NMR spectroscopy has attracted much attention in the recent years due to the remarkable spectral resolution improvement by ultrafast magic-angle-spinning (MAS) as well as due to the sensitivity enhancement rendered by proton detection. Although these developments have enabled the investigation of a variety of challenging chemical and biological solids, the proton spectral resolution is still poor for many rigid solid systems owing to the presence of conformational heterogeneity and the unsuppressed residual proton-proton dipolar couplings even with the use of the highest currently feasible sample spinning speed of ∼130 kHz. Although a further increase in the spinning speed of the sample could be beneficial to some extent, there is a need for alternate approaches to enhance the spectral resolution. Herein, by fully utilizing the benefits of double-quantum (DQ) coherences, we propose a single radio frequency channel proton-based 3D pulse sequence that correlates double-quantum (DQ), DQ, and single-quantum (SQ) chemical shifts of protons. In addition to the two-spin homonuclear proximity information, the proposed 3D DQ/DQ/SQ experiment also enables the extraction of three-spin and four-spin proximities, which could be beneficial for revealing the dipolar coupled proton network in the solid state. Besides, the 2D DQ/DQ spectrum sliced at different isotropic SQ chemical shift values of the 3D DQ/DQ/SQ spectrum will also facilitate the identification of DQ correlation peaks and improve the spectral resolution, as it only provides the local homonuclear correlation information associated with the specific protons selected by the SQ chemical shift frequency. The 3D pulse sequence and its efficiency are demonstrated experimentally on small molecular compounds in the solid state. We expect that this approach would create avenues for further developments by suitably combining the benefits of partial deuteration of samples, selective excitation/decoupling pulses, heteronuclear spins for spectral editing, and nonuniform sampling.

Low-Complexity Decoding Algorithms for the LDM Core Layer at Fixed Receivers in ATSC 3.0

Gallager’s decoding algorithms ${B}$ (GDB) and ${E}$ (GDE) for low-density parity-check codes have much lower computational complexity and much less required memory size than the sum-product algorithm (SPA). This is because GDB and GDE only use binary or integer operations, while the SPA requires real operations and a look-up table. However, they are hardly used in commercial communication systems since they have a worse performance than the SPA. Layered-division multiplexing (LDM) is considered in ATSC 3.0 in order to deliver multiple broadcasting streams with distinct robustness in a single radio frequency channel. Due to the unique characteristic of the LDM, we propose to use GDB or GDE rather than the SPA for decoding the core layer signal at fixed receivers. Numerical results show that the computational complexity and the required memory size can be reduced without any performance loss by about 50 percent and 80 percent, respectively, when GDB and GDE are employed.

Multi-port-driven birdcage coil for multiple-mouse MR imaging at 7 T.

In ultra-high field (UHF) imaging environments, it has been demonstrated that multiple-mouse magnetic resonance imaging (MM-MRI) is dependent on key factors such as the radiofrequency (RF) coil hardware, imaging protocol, and experimental setup for obtaining high-resolution MR images. A key aspect is the RF coil, and a number of MM-MRI studies have investigated the application of single-channel RF transmit (Tx)/receive (Rx) coils or multi-channel phased array (PA) coil configurations under a single gradient coil set. However, despite applying a variety of RF coils, Tx (|B1+ |)-field inhomogeneity still remains a major problem due to the relative shortening of the effective RF wavelength in the UHF environment. To address this issue, we propose a relatively smaller size of individual Tx-only coils in a multiple birdcage (MBC) coil for MM-MRI to image up to three mice. We use electromagnetic (EM) simulations in the finite-difference time-domain (FDTD) environment to obtain the |B1 |-field distribution. Our results clearly show that the single birdcage (SBC) high-pass filter (HPF) configuration, which is referred to as the SBCHPF , under the absence of an RF shield exhibits a high |B1 |-field intensity in comparison with other coil configurations such as the low-pass filter (LPF) and band-pass filter (BPF) configurations. In a 7-T MRI experiment, the signal-to-noise ratio (SNR) map of the SBCHPF configuration shows the highest coil performance compared to other coil configurations. The MBCHPF coil, which is comprised of a triple-SBCHPF configuration combined with additional decoupling techniques, is developed for simultaneous image acquisition of three mice. SCANNING 38:747-756, 2016. © 2016 Wiley Periodicals, Inc.

MIMO for ATSC 3.0

This paper provides an overview of the optional multiple-input multiple-output (MIMO) antenna scheme adopted in ATSC 3.0 to improve robustness or increase capacity via additional spatial diversity and multiplexing by sending two data streams in a single radio frequency channel. Although it is not directly specified, it is expected in practice to use cross-polarized $2 {\times }2$ MIMO (i.e., horizontal and vertical polarization) to retain multiplexing capabilities in line-of-sight conditions. MIMO allows overcoming the channel capacity limit of single antenna wireless communications in a given channel bandwidth without any increase in the total transmission power. But in the U.S. MIMO can actually provide a larger comparative gain because it would be allowed to increase the total transmit power, by transmitting the nominal transmit power in each polarization. Hence, in addition to the MIMO gains (array, diversity, and spatial multiplexing), MIMO could exploit an additional 3 dB power gain. The MIMO scheme adopted in ATSC 3.0 re-uses the single-input single-output antenna baseline constellations, and hence it introduces the use of MIMO with non-uniform constellations.

Mitigation ofB1+inhomogeneity on single-channel transmit systems with TIAMO

In magnetic resonance imaging, there has been a constant drive to higher static magnetic field strengths (B0) to achieve a higher signal-to-noise ratio and new or enhanced contrasts. In today's high-field systems, severe problems regarding the homogeneity of the transmission field are encountered. Recently, an acquisition scheme called Time-Interleaved Acquisition of Modes has been proposed to tackle the inhomogeneity problems in high-field magnetic resonance imaging. The basic premise is to excite two (or more) different B1(+) modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In its usual implementation, a multichannel transmit system is required. In this work, the goal is to present a simple and inexpensive hardware setup which makes it possible to use time-interleaved acquisition of modes on any single-channel transmit system while making use of the vendor-provided single-channel radiofrequency safety system. To demonstrate the efficacy of this setup, spin echo images from the pelvis are acquired at 7 T exhibiting no complete signal dropouts.

A specific absorption rate prediction concept for parallel transmission MR

The specific absorption rate (SAR) is a limiting factor in high-field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single-channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so-called Q-matrices and further computational optimizations allow for a real-time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight-channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high-field MR.

Synthetic signal injection using a single radiofrequency channel

To demonstrate that, when injecting an artificial reference signal for quantitation purposes, the real and artificial signals can be acquired separately, using a single radiofrequency (RF) channel, with no loss of fidelity. Conversion of MR signals to units of concentration can be simplified by injection of a precalibrated, artificial reference signal, or pseudo-signal. In previous implementations, the pseudo-signal was acquired simultaneously with the real signals arising from the sample and this requires a second, integrated RF channel. We used in vivo spectroscopy and in vitro imaging measurements to test the validity of the separate acquisition method. There was very strong correlation (r = 0.94; P = 0.02) between the in vivo concentrations determined with separate and simultaneous acquisition methods. The in vitro measurements validated that the separate acquisition method compensates for differences in coil loading conditions as well as the simultaneous acquisition method. Separate acquisition eliminates the need for a second RF channel, which allows easier implementation at sites that have only one channel available, and relaxes the constraints on the number and amplitude of pseudo-signals. This flexibility can be exploited to increase the signal to noise ratio of the pseudo-signal and reduce variability when making the conversion to units of concentration.

A comparison of single-channel and multi-channel RF transmit coil for SSFP cine imaging at 3 Tesla

On high field MRI scanners uniform radio frequency (RF) excitation over the entire field-of-view (FOV) is often challenging with single-channel RF transmit coils. This may cause a reduction of image quality (e.g. shadowing artifacts). This problem is most pronounced in sequences that heavily rely on a homogenous magnetic field, such as steady state free precession (SSFP) sequences. Multi-channel RF transmission (Tx) has been shown to significantly improve the RF (B1 field) uniformity in high-field MRI. More accurate knowledge of the local B1 field also allows to better predicting the local specific absorption rate (SAR), thereby allowing for shorter repetition times (TR). Overall, this may allow for an improvement in image quality as a consequence of reduced artefact

Whole heart coronary MRA at 3.0T: a comparison between multi and single- RF transmission

On high field MRI scanners uniform radio frequency (RF) excitation over the entire field-of-view is often challenging with single-channel RF transmit coils. This may cause a reduction of image quality (e.g. shadowing artifacts). This problem is most pronounced in sequences which heavily rely on a homogenous magnetic field, such as steady state free precession (SSFP) sequences, despite having been shown to yield best results for MR coronary angiography at 1.5T. Multi-channel RF transmission (Tx) has been shown to significantly improve the RF (B1 field) uniformity in high-field MRI. More accurate knowledge of the local B1 field also allows to better predicting the local specific absorption rate (SAR), thereby allowing for shorter repetition times (TR). Altogether, this may allow for the use of SSFP sequences at 3T and thus to combine the high CNR benefits of 3D SSFP with the high SNR advantages of 3T for whole heart coronary MR angiography (WHCMRA).