RF Coil Research Articles

Development of an Add-on 23Na-MRI Radiofrequency Platform for a 1H-MRI System Using a Crossband Repeater: Proof-of-concept.

23Na-MRI provides information on Na+ content, and its application in the medical field has been highly anticipated. However, for existing clinical 1H-MRI systems, its implementation requires an additional broadband RF transmitter, dedicated transceivers, and RF coils for Na+ imaging. However, a standard medical MRI system cannot often be modified to perform 23Na imaging. We have developed an add-on crossband RF repeater system that enables 23Na-MRI simply by inserting it into the magnet bore of an existing 1H MRI. The three axis gradient fields controlled by the 1H-MRI system were directly used for 23Na imaging without any deformation. A crossband repeater is a common technique used for amateur radio. This concept was proven by a saline solution phantom and in vivo mouse experiments. This add-on RF platform is applicable to medical 1H MRI systems and can enhance the application of 23Na-MRI in clinical usage.

Complementing mTMS with neuroimaging: the TMSMR 28 Ch RF coil

Complementing mTMS with neuroimaging: the TMSMR 28 Ch RF coil

A single-coil-based method for electromagnetic interference reduction in point-of-care low field MRI systems

One of the main challenges for point-of-care (POC) MRI systems is electromagnetic interference (EMI), since such systems are intended for use outside conventional Faraday-shielded rooms. Many methods have been proposed based on EMI detection via sensors external to the MRI system, followed by different types of signal processing to reduce artifacts in the image. Although these methods can be very effective, they do increase the complexity of the overall system, and introduce more potential failure points for systems designed for challenging environments. In this work we introduce a new method that does not require external sensors, but rather uses the “MR-silent” mode of an RF coil to detect the EMI, followed by simple subtraction from the signal from the “MR-active” mode. This method can be performed post-acquisition if there are two receive channels available, or as demonstrated here can operate with a single-channel receive detection system with the addition of a simple passive 180° power splitter/combiner into the receive chain. Proof-of-concept in vivo results show that a reduction in the standard deviation of the EMI up to ∼ 97 % is possible, with average values ∼ 90 %.

Physical demonstration of vitrification of liter scale CPA systems and characterization of 120KW RF coil for nanowarming of liter scale systems

Physical demonstration of vitrification of liter scale CPA systems and characterization of 120KW RF coil for nanowarming of liter scale systems

T1 relaxation time of ISMRM/NIST T1 phantom spheres at 7T.

T1 relaxation times of the 14 T1 phantom spheres that make up the standard International Society for Magnetic Resonance in Medicine (ISMRM)/National Institute of Standards and Technology (NIST) system phantom are reported at 7 T. T1 values of six of the 14 T1 spheres at 7 T (with T1 > 270 ms) have been reported previously, but, to the best of our knowledge, not all of the T1s of the 14 T1 spheres at 7 T have been reported before. Given the increasing number of 7-T MRI systems in clinical settings and the increasing need for T1 phantoms that cover a wide range of T1 relaxation times to evaluate rapid T1 mapping techniques at 7 T, it is of high interest to obtain accurate T1 values for all the ISMRM/NIST T1 spheres at 7 T. In this work, T1 relaxation time was measured on a 7-T MRI scanner using an inversion-recovery spin-echo pulse sequence and derived by curve fitting to a signal equation that exhibits insensitivity to inhomogeneity. Day-to-day reproducibility was within 0.4% and differences between two different RF coils within 1.5%. T1s of a subset of the 14 spheres were also measured by NMR at 7 T for comparison, and the T1 results were consistent between the MRI and NMR measurements. T1 measurements performed at 3 T on the same 14 spheres using the same sequence and fitting method yielded good agreement (mean percentage difference of -0.4%) with the reference T1 values available from the NIST, reflecting the accuracy of the reported technique despite being without the standard phantom housing. We found that the T1 values of all 14 NiCl2 spheres are consistently lower at 7T than at 3T. Although our results were well reproduced, this study represents initial work to quantify the 7-T T1 values of all 14 NIST T1 spheres outside of the standard housing and does not warrant reproducibility of the ISMRM/NIST system phantom as a whole. A future study to assess the T1 values of a version of the ISMRM/NIST system phantom that fits inside typical commercial coils at 7T will be very helpful. Nonetheless, the details on our acquisition and curve-fitting methods reported here allow the T1 measurements to be reproduced elsewhere. The T1 values of all 14 spheres reported here will be valuable for the development of quantitative MR fingerprinting and rapid T1 mapping for a large variety of research projects, not only in neuroimaging but also in body MRI, musculoskeletal MRI, and gadolinium contrast-enhanced MRI, each of which is concerned with much shortened T1.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B0 and B1 magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B1 inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-13C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

Active EMI Suppression System for a 50 mT Unshielded Portable MRI Scanner.

Passive shielding is usually applied to block electro-magnetic interference (EMI) for portable very-low field MRI scanners, but it goes against the mobility of the scanners. Here, the reference channel-based active EMI suppression (AES) system was proposed to discard them. Different from the existing studies, this work started with analyzing the interference transmission paths and discovered that the human body coupling was the main path. Then, the "ring-" shaped EMI receiving coil designed here together with the electrocardiograph (ECG) electrode patch were used to suppress the human-body-coupled interferences. For the first time, the periphery data of the k-spaces of the RF coil and the EMI detectors were utilized to calculate the transfer factors in multiple frequency bands by using the least square method. The reference EMI signals were transferred to the interferences in the MR signal by multiplying the transfer factors in each frequency band, and the denoised MR signal could be obtained by subtracting the transferred reference EMI signals. The prototype of the AES system was applied to a 50 mT unshielded portable MRI scanner. The in-vivo experiments indicated that the interference suppression rate of the AES system equipped with the "ring-" shaped EMI receiving coil could reach 96.8%. Meanwhile, the SNR of the images after interference suppression by the AES system equipped with both types of detectors was 97.2% of that of the images scanned inside the shielded room. The proposed AES system ensures that the portable MRI scanner works normally in unshielded environment. Our study provides a solution to make portable MRI scanners truly movable.

Hairpin RF resonators for MR imaging transceiver arrays with high inter-channel isolation and B1 efficiency at ultrahigh field 7 T

Electromagnetic decoupling among a close-fitting or high-density transceiver RF array elements is required to maintain the integrity of the magnetic flux density from individual channel for enhanced performance in detection sensitivity and parallel imaging. High-impedance RF coils have demonstrated to be a prominent design method to circumvent these coupling issues. Yet, inherent characteristics of these coils have ramification on the B1 field efficiency and SNR. In this work, we propose a hairpin high impedance RF resonator design for highly decoupled multichannel transceiver arrays at ultrahigh magnetic fields. Due to the high impedance property of the hairpin resonators, the proposed transceiver array can provide high decoupling performance without using any dedicated decoupling circuit among the resonant elements. Because of elimination of lumped inductors in the resonator circuit, higher B1 field efficiency in imaging subjects can be expected. In order to validate the feasibility of the proposed hairpin RF coils, systematical studies on decoupling performance, field distribution, and SNR are performed, and the results are compared with those obtained from existing high-impedance RF coil, e.g., “self-decoupled RF coil”. To further investigate its performance, an 8-channel head coil array using the proposed hairpin resonators loaded with a cylindrical phantom is designed, demonstrating a 19 % increase of the B1+ field intensity compared to the self-decoupled coils at 7 T. Furthermore, the characteristics of the hairpin RF coils are evaluated using a more realistic human head voxel model numerically. The proposed hairpin RF coil provides excellent decoupling performance and superior RF magnetic field efficiency compared to the “self-decoupled” high impedance coils. Bench test of a pair of fabricated hairpin coils prove to be in good accordance with numerical results.

A radiofrequency coil to facilitate task-based fMRI of awake marmosets

BackgroundThe small common marmoset (Callithrix jacchus) is an ideal nonhuman primate for awake fMRI in ultra-high field small animal MRI scanners. However, it can often be challenging in task-based fMRI experiments to provide a robust stimulus within the MRI environment while using hardware (an RF coil and restraint system) that is compatible with awake imaging. New methodHere we present an RF coil and restraint system that permits unimpeded access to an awake marmoset’s head subsequent to immobilization, thereby permitting the setup of peripheral devices and stimuli proximal to the head. ResultsAs an example application, an fMRI experiment probing whole-brain activation in response to marmoset vocalizations was conducted—this paradigm showed significant bilateral activation in the inferior colliculus, medial lateral geniculate nucleus, and auditory cortex. Comparison with existing method(s)The coil performance was evaluated and compared to a previously published restraint system with integrated RF coil. The image and temporal SNR were improved by up to 58 % and 27 %, respectively, in the peripheral cortex and by 30 % and 3 % in the centre of the brain. The restraint-system topology limited head motion to less than 100 µm of translation and 0.30° of rotation when measured over a 15-minute acquisition. ConclusionsThe proposed hardware solution provides a versatile approach to awake-marmoset imaging and, as demonstrated, can facilitate task-based fMRI.

Accuracy of fat fraction estimation using Dixon: experimental phantom study

Objective. Quantitative assessment of Dixon two-point and three-point technologies operation using phantom modeling in the range from 0 to 70%.Materials and methods. To simulate substances with different concentrations of the fat phase we chose direct oil-in-water emulsions. Tubes with ready-made emulsions were placed in a phantom. Emulsions based on vegetable oils were presented in the range from 0–70%. The phantom was scanned on an Optima MR450w MRI tomograph (GE, USA) in two Dixon modes: the accelerated two-point method “Lava-Flex” and the three-point method “IDEAL IQ”. A scan was performed on a GEM Flex LG Full RF coil. We calculated fat fraction (FF) using two formulas.Results. There is a linear relationship of the determined values when calculating the fat concentration in “IDEAL IQ” mode and using the formula based on Water and Fat. The accuracy of body fat percentage measurement in “IDEAL IQ” mode is higher than in “Lava-Flex” mode. According to the MR-sequence “Lava-Flex” draws attention to the overestimation of the measured values of the concentration of fat in relation to the specified values by an average of 57.6% over the entire range, with an average absolute difference of 17.2%.Conclusion. Using the “IDEAL IQ” sequence, the results of the quantitative determination of fractions by formulas were demonstrated, which are more consistent with the specified values in the phantom. In order to correctly quantify the fat fraction, it is preferable to calculate from the Water and Fat images using Equation 2. Calculations from the In-phase and Out-phase images provide ambiguous results. Phantom modeling with direct emulsions allowed us to detect the shift of the measured fat fraction.

Scalable and modular 8-channel transmit and 8-channel flexible receive coil array for 19 F MRI of large animals.

To introduce an RF coil system consisting of an 8-channel transmit (Tx) and 8-channel receive (Rx) coil arrays for 19 F MRI of large animals. The Tx efficiency and homogeneity of the 8-element loop coil array (loop size: 6× 15 cm2 ) were simulated for two different pig models rendered from MR images. An 8-channel Rx coil array consisting of a flexible 6-channel posterior and a 2-channel planar anterior array was designed to fit on the abdomen of an average-sized pig in supine position. Measurements were performed in a grid phantom and ex vivo on a pig model with perfluoroctylbromide (PFOB)-filled tubes inserted in the thorax. Measured and simulated Tx efficiency and homogeneity for the 8-channel and 5-channel arrays were in good agreement: 1.87 ± 0.22μT/√kW versus 1.96 ± 0.29μT/√kW, and 2.29 ± 0.39μT/√kW versus 2.41 ± 0.37μT/√kW. An isolation of 38 ± 8dB is achieved between the 19 F Tx and Rx elements, and over 30dB between the 1 H and 19 F elements. The PFOB-filled vials could be clearly identified within the cadaver abdomen with an SNR of 275 ± 51 for a 3D gradient-echo sequence with 2-mm isotropic resolution and 12 averages, acquired in 9:52 min:s. Performance of the Tx array was robust against phase and amplitude mismatches at the input ports. A modular and scalable Tx array offers improved Tx efficiency in 19 F MRI of large animals with various sizes. Although conventional birdcage coils have superior Tx efficiency within the target region of interest, scalability of the Tx array to animal size is a major benefit. The described 19 F coil provides homogeneous excitation and high sensitivity detection in large pig models.

Generating accurate tip angles for NMR outside the rotating-wave approximation

The generation of accurate tip angles is critical for many applications of nuclear magnetic resonance. In low static field, with a linear rather than circular polarized rf field, the rotating-wave approximation may no longer hold and significant deviations from expected trajectories on the Bloch sphere can occur. For rectangular rf pulses, the effects depend strongly on the phase of the rf field and can be further compounded by transients at the start and end of the pulse. The desired terminus can be still be achieved, however, through the application of a phase-dependent Bloch–Siegert shift and appropriate consideration of pulse timings. For suitably shaped rf pulses, the Bloch–Siegert shift is largely phase independent, but its magnitude can vary significantly depending on details of the pulse shape as well as the characteristics of the rf coil circuit. We present numerical simulations and low-field NMR experiments with 1H and 3He that demonstrate several main consequences and accompanying strategies that one should consider when wanting to generate accurate tip angles outside the validity of the rotating-wave approximation and in low static field.

An End-to-End Recurrent Neural Network for Radial MR Image Reconstruction.

Recent advances in deep learning have contributed greatly to the field of parallel MR imaging, where a reduced amount of k-space data are acquired to accelerate imaging time. In our previous work, we have proposed a deep learning method to reconstruct MR images directly from k-space data acquired with Cartesian trajectories. However, MRI utilizes various non-Cartesian trajectories, such as radial trajectories, with various numbers of multi-channel RF coils according to the purpose of an MRI scan. Thus, it is important for a reconstruction network to efficiently unfold aliasing artifacts due to undersampling and to combine multi-channel k-space data into single-channel data. In this work, a neural network named ‘ETER-net’ is utilized to reconstruct an MR image directly from k-space data acquired with Cartesian and non-Cartesian trajectories and multi-channel RF coils. In the proposed image reconstruction network, the domain transform network converts k-space data into a rough image, which is then refined in the following network to reconstruct a final image. We also analyze loss functions including adversarial and perceptual losses to improve the network performance. For experiments, we acquired k-space data at a 3T MRI scanner with Cartesian and radial trajectories to show the learning mechanism of the direct mapping relationship between the k-space and the corresponding image by the proposed network and to demonstrate the practical applications. According to our experiments, the proposed method showed satisfactory performance in reconstructing images from undersampled single- or multi-channel k-space data with reduced image artifacts. In conclusion, the proposed method is a deep-learning-based MR reconstruction network, which can be used as a unified solution for parallel MRI, where k-space data are acquired with various scanning trajectories.

The design and evaluation of single-channel loopole coils at 7T MRI

Objective. Improving the local uniformity of field for awake monkey brain magnetic resonance imaging (MRI) at ultra-high fields while facilitating convenient placement and fixation of MRI-compatible multimodal devices for neuroscience study, can eventually advance our understanding of the primate’s brain organization. Approach. A group of single-channel RF coils including conventional loop coils and loopole coils sharing the same size and shape were designed for comparison; their performance as the transmit coil was quantitatively evaluated through a series of numerical electromagnetic (EM) simulations, and further verified by using 7T MRI over a saline phantom and a monkey in vivo. Main results. Compared to conventional loop coils, the optimized loopole coil brought up to 23.5% uniformity improvement for monkey brain imaging in EM simulations, and this performance was further verified over monkey brain imaging at 7T in vivo. Importantly, we have systematically explored the underlying mechanism regarding the relationship between loopole coils’ current density distribution and uniformity, observing that it can be approximated as a sinusoidal curve. Significance. The proposed loopole coil design can improve the imaging quality in awake and behaving monkeys, thus benefiting advanced brain research at UHF.

Practical method for RF pulse distortion compensation using multiple square pulses for low-field MRI.

Since recovery time of the RF coil is long at low field MRI, the rising and the ring-down times of the square pulse are also long, which means the applied sinc pulse can easily be distorted from the changing amplitude. However, both the rising time and ring-down time can be calculated using Q-factor. Using this information, an RF square pulse were compensated by appending two square pulses before and after the RF pulse. The durations of these RF square pulses were calculated using the Q-factor. Since the amplitude of the sinc pulse changes continuously, a series of square pulses were applied to apply sinc pulse to the coil. The minimum number of square pulses and the amplitude of the square pulses were calculated. It was successfully demonstrated that the sinc pulse can be compensated using a series of square pulses. The more number of square pulses were used, the smoother sinc pulse was applied to the RF coil. The Q-factor was experimentally calculated from the ring-down time of a signal induced in a sniffer loop which was connected to an oscilloscope. The resulting Q-factor was then used to calculate both the duration and amplitude of the square pulses for compensation. Echo trains were also acquired in an inhomogeneous B0 field using the compensated RF pulses. In order to enhance the SNR of the echo trains, a pre-polarization pulse was added to the CPMG spin echo sequence. The SNRs of the echo signal acquired using compensated pulses were compared with those of signal obtained with uncompensated pulses and showed significant improvements of 61.1% and 51.5% for the square and sinc shaped pulses respectively.

A 5-channel local B0 shimming coil combined with a 3-channel RF receiver coil for rat brain imaging at 3T.

We aimed to improve B0 magnetic field homogeneity and minimize the interference between RF coils and local B0 shimming coils with few channel numbers. To design and construct the prototype for B0 shimming of the rat brain, we first evaluated the interferences of single shimming loops on RF receiver loops. Then, B0 shimming of the whole rat brain was implemented using an optimization procedure. The positions and currents of the local shimming coils with channel numbers from 3 to 6 were optimized to improve shimming performance. Based on the simulation results, a 5-channel local shimming coil, combined with a 3-channel RF receiver coil, was constructed and evaluated by animal experiments. There was marginal SNR loss within 5% after integrating the local shimming coil into the RF receiver coil. With respect to the Siemens standard shims up to second order, the B0 inhomogeneity in one whole rat brain was reduced from 39.6 Hz to 24.7Hz by using the local shimming coil. A large portion of the EPI distortions was recovered after using the 5-channel local shimming coil. The temporal SNR using the local shimming coil was higher than that using the Siemens standard shims up to second order, with an improvement of more than 24%. The local shimming coil can improve B0 magnetic field homogeneity despite minor effects on the RF coil and can benefit a variety of applications that are sensitive to B0 inhomogeneity. Nevertheless, EPI for rat brain is still very challenging.

Low-cost inductively coupled stacked wireless RF coil for MRI at 3 T.

Inductively coupled RF coils are an inexpensive and simple method to realize wireless RF coils in MRI. They are low cost and can greatly ease the MR scan setup and improve patient comfort, since they do not require bulky components such as cables, baluns, preamplifiers, and connectors. Previous works have typically used single-layer loops as wireless coils. In this work, we present a novel wireless coil, where two loops are stacked and decoupled with a shared capacitor. We found that such a stacked structure could increase the coil efficiency and SNR. Compared with the single-layer wireless coil, both electromagnetic simulation and MR experiment results demonstrate that the stacked wireless coil has a considerable SNR improvement of approximately 35%.

RF shielding designs for birdcage coils for preclinical MRI at 9.4 T

Birdcage coils are widely used in preclinical MRI as they perform well, allow for quadrature drive, and can provide a homogeneous transmit field. Unlike in larger bore scanners, an RF shield is essential to avoid strong cross-talk with gradient coils that are in close proximity. However, gradient switching induces eddy currents that heat the shield and coil and impair the temporal signal-to-noise ratio (tSNR). The motivation of this study is to investigate the performance of different designs of RF shields on a birdcage coil used for high resolution functional MRI of small primates at 9.4 T. We found the choice of materials for RF shields significantly affected ghosting and tSNR in fMRI scans. Both ultrathin foils and a slotted pattern reduce eddy currents and improve imaging quality. Our results also demonstrate that a 9-um-thick copper foil is sufficiently thin to reduce the eddy current effects for high-resolution fMRI scans and there is no need for high-cost 4-um-thick foil. For slotted shields, our results demonstrate that the number of slots should be carefully considered, and an excessive number of slots can lead to a lower SNR and tSNR. We believe the results from this study can be used as a reference to design future RF coil shields selection for preclinical scanners.

Assessing muscle-specific potassium concentrations in human lower leg using potassium magnetic resonance imaging.

Noninvasively assessing tissue potassium concentrations (TPCs) using potassium magnetic resonance imaging (39 K MRI) could give valuable information on physiological processes connected to various pathologies. However, because of inherently low 39 K MR image resolution and strong signal blurring, a reliable measurement of the TPC is challenging. The aim of this work was to investigate the feasibility of a muscle-specific TPC determination with a focus on the influence of a varying residual quadrupolar interaction in human lower leg muscles. The quantification accuracy of a muscle-specific TPC determination was first assessed using simulated 39 K MRI data. In vivo 39 K and corresponding sodium (23 Na) MRI data of healthy lower leg muscles (n = 14, seven females) were acquired on a 7-T MR system using a double-resonant 23 Na/39 K birdcage Tx/Rx RF coil. Additional 1 H MR images were acquired on a 3-T MR system and used for tissue segmentation. Quantification of TPC was performed after a region-based partial volume correction (PVC) using five external reference phantoms. Simulations not only underlined the importance of PVC for correctly assessing muscle-specific TPC values, but also revealed the strong impact of a varying residual quadrupolar interaction between different muscle regions on the measured TPC. Using 39 K T2 * decay curves, we found significantly higher residual quadrupolar interaction in tibialis anterior muscle (TA; ωq = 194 ± 28 Hz) compared with gastrocnemius muscle (medial/lateral head, GM/GL; ωq = 151 ± 25 Hz) and soleus muscle (SOL; ωq = 102 ± 32 Hz). If considered in the PVC, TPC in individual muscles was similar (TPC = 98 ± 11/96 ± 14/99 ± 8/100 ± 12 mM in GM/GL/SOL/TA). Comparison with tissue sodium concentrations suggested that residual quadrupolar interactions might also influence the 23 Na MRI signal of lower leg muscles. A TPC determination of individual lower leg muscles is feasible and can therefore be applied in future studies. Considering a varying residual quadrupolar interaction for PVC of 39 K MRI data is essential to reliably assess potassium concentrations in individual muscles.

Deuterium metabolic imaging of the human brain in vivo at 7T.

To explore the potential of deuterium metabolic imaging (DMI) in the human brain in vivo at 7T, using a multi-element deuterium (2 H) RF coil for 3D volume coverage. 1 H-MR images and localized 2 H MR spectra were acquired in vivo in the human brain of 3 healthy subjects to generate DMI maps of 2 H-labeled water, glucose, and glutamate/glutamine (Glx). In addition, non-localized 2 H-MR spectra were acquired both in vivo and in vitro to determine T1 and T2 relaxation times of deuterated metabolites at 7T. The performance of the 2 H coil was assessed through numeric simulations and experimentally acquired B1 + maps. 3D DMI maps covering the entire human brain in vivo were obtained from well-resolved deuterated (2 H) metabolite resonances of water, glucose, and Glx. The T1 and T2 relaxation times were consistent with those reported at adjacent field strengths. Experimental B1 + maps were in good agreement with simulations, indicating efficient and homogeneous B1 + transmission and low RF power deposition for 2 H, consistent with a similar array coil design reported at 9.4T. Here, we have demonstrated the successful implementation of 3D DMI in the human brain in vivo at 7T. The spatial and temporal nominal resolutions achieved at 7T (i.e., 2.7 mL in 28 min, respectively) were close to those achieved at 9.4T and greatly outperformed DMI at lower magnetic fields. DMI at 7T and beyond has clear potential in applications dealing with small brain lesions.