Coil Magnetic Resonance Imaging Research Articles

Radio Frequency Modeling of Receive Coil Arrays for Magnetic Resonance Imaging

The numerical calculation of the signal-to-noise ratio (SNR) of magnetic resonance imaging (MRI) coil arrays is a powerful tool in the development of new coil arrays. The proposed method describes a complete model that allows the calculation of the absolute SNR values of arbitrary coil arrays, including receiver chain components. A new method for the SNR calculation of radio frequency receive coil arrays for MRI is presented, making use of their magnetic B 1 − transmit pattern and the S-parameters of the network. The S-parameters and B 1 − fields are extracted from an electromagnetic field solver and are post-processed using our developed model to provide absolute SNR values. The model includes a theory for describing the noise of all components in the receiver chain and the noise figure of a pre-amplifier by a simple passive two-port network. To validate the model, two- and four-element receive coil arrays are investigated. The SNR of the examined arrays is calculated and compared to measurement results using imaging of a saline water phantom in a 3 T scanner. The predicted values of the model are in good agreement with the measured values. The proposed method can be used to predict the absolute SNR for any receive coil array by calculating the transmit B 1 − pattern and the S-parameters of the network. Knowledge of the components of the receiver chain including pre-amplifiers leads to satisfactory results compared to measured values, which proves the method to be a useful tool in the development process of MRI receive coil arrays.

Equivalent magnetic dipole method used to design gradient coil for unilateral magnetic resonance imaging**Project supported by the National Natural Science Foundation of China (Grant Nos. 51677008, 51377182, 51707028, and 11647098), the Fundamental Research Funds of the Central Universities, China (Grant No. 106112017CDJQJ158834), and the State Key Development Program for Basic Research of China (Grant No. 2014CB541602).

The conventional magnetic resonance imaging (MRI) equipment cannot measure large volume samples nondestructively in the engineering site for its heavy weight and closed structure. In order to realize the mobile MRI, this study focuses on the design of gradient coil of unilateral magnet. The unilateral MRI system is used to image the local area above the magnet. The current density distribution of the gradient coil cannot be used as a series of superconducting nuclear magnetic resonance gradient coils, because the region of interest (ROI) and the wiring area of the unilateral magnet are both cylindrical side arc surfaces. Therefore, the equivalent magnetic dipole method is used to design the gradient coil, and the algorithm is improved for the special case of the wiring area and the ROI, so the X and Y gradient coils are designed. Finally, a flexible printed circuit board (PCB) is used to fabricate the gradient coil, and the magnetic field distribution of the ROI is measured by a Gauss meter, and the measured results match with the simulation results. The gradient linearities of x and y coils are 2.82% and 3.56%, respectively, less than 5% of the commercial gradient coil requirement.

The reproducibility of measuring trabecular bone parameters using a commercially available high-resolution magnetic resonance imaging approach: A pilot study

Bone imaging is currently the best non-invasive way to assess changes to bone associated with aging or chronic disease. However, common imaging techniques such as dual energy x-ray absorptiometry are associated with limitations. Magnetic resonance imaging (MRI) is a radiation-free technique that can measure bone microarchitecture. However, published MRI bone assessment protocols use specialized MRI coils and sequences and therefore have limited transferability across institutions. We developed a protocol on a Siemens 3 Tesla MRI machine, using a commercially available coil (Siemens 15 CH knee coil), and manufacturer supplied sequences to acquire images at the tibia. We tested the reproducibility of the FSE and the GE Axial sequences and hypothesized that both would generate reproducible trabecular bone parameters. Eight healthy adults (age 25.5 ± 5.4 years) completed three measurements of each MRI sequence at the tibia. Each of the images was processed for 8 different bone parameters (such as volumetric bone volume fraction). We computed the coefficient of variation (CV) and intraclass correlation coefficients (ICC) to assess reproducibility and reliability. Both sequences resulted in trabecular parameters that were reproducible (CV <5% for most) and reliable (ICC >80% for all). Our study is one of the first to report that a commercially available MRI protocol can result in reproducible data, and is significant as MRI may be an accessible method to measure bone microarchitecture in clinical or research environments. This technique requires further testing, including validation and evaluation in other populations.

Retraction notice to “The Globe’s Eccentric Rotational Axis: Why Medial Rectus Surgery is More Potent Than Lateral Rectus Surgery” (Ophthalmology. 2018;125:1234-1238)

Tablestypically recommend greater lateral rectus (LR) than medial rectus (MR) surgical doses for horizontal strabismus of any given magnitude, a difference unexplained by mechanical models that assume globe rotation about its center. We tested this assumption during horizontal ductions. Prospective observational study. Eighteen adult subjects with normal binocular vision. Surface coil magnetic resonance imaging at 390 or 430 μm resolution was obtained using 2-mm-thick contiguous axial planes while subjects fixated targets in central, right, and left gaze. Angular displacements of lines connecting the corneal apex through the minor lens axis to the retina were measured to approximate clinical ductions. Globe centers were calculated from their area centroids. Apparent lens and globe-optic nerve (ON) junction rotations around the globe center were then compared with clinical ductions. Apparent angular rotations of lenses and globe-ON junctions during horizontal ductions. Globe-ON junctions appeared to rotate significantly less around globe centers than did lenses for abduction (20.6°±4.7° vs. 27.4°±7.4°, ± standard deviation (SD), P < 0.001) and adduction (25.3°±6.7° vs. 31.9°±8.3°, P < 0.001). Both rotations differed significantly from clinical adduction (27.9°±8.3°, P < 0.007), but only in abduction was globe-ON junction rotation significantly less than clinical abduction (28.6°±9.4°, P < 0.001). The true geometric globe rotational center was 2.2±0.5 mm medial and 0.8±1.0 mm posterior to the geometric globe center and was displaced farther medially and posteriorly during adduction. This eccentricity imbues each millimeter of MR recession with approximately 30% more trigonometric rotational effect than equivalent LR recession. The medial and posterior eccentricities of the normal ocular rotational axis profoundly influence horizontal rectus action. The proximity of the globe's rotational axis to the MR shortens its lever arm relative to the LR, explaining why mechanical effects of smaller MR recessions are equivalent to larger LR recessions.

Cryogenic Preamplifiers for Magnetic Resonance Imaging.

Pursuing the ultimate limit of detection in magnetic resonance imaging (MRI) requires cryogenics to decrease the thermal noise of the electronic circuits. As cryogenic coils for MRI are slowly emerging cryogenic preamplifiers are required to fully exploit their potential. A cryogenic preamplifier operated at 77K is designed and implemented for C imaging at 3T (32.13MHz), using off-the-shelves components. The design is based on a high electron mobility transistor (ATF54143) in a common source configuration. Required auxiliary circuitry for optimal cryogenic preamplifier performance is also presented consisting of a voltage regulator (noise free supply voltage and optimal power consumption), switch, and trigger (for active detuning during transmission to protect the preamplifier). A gain of 18dB with a noise temperature of 13.7K is achieved. Performing imaging experiments in a 3T scanner showed an 8% increased signal-to-noise ratio from 365 to 399 when lowering the temperature of the preamplifier from 296 to 77K while keeping the coil at room temperature. This paper thus enables the merger of cryogenic coils and preamplifiers in the hopes of reaching the ultimate limit of detection for MRI.

An eight-channel sodium/proton coil for brain MRI at 3T.

The purpose of this work is to illustrate a new coil decoupling strategy and its application to a transmit/receive sodium/proton phased array for magnetic resonance imaging (MRI) of the human brain. We implemented an array of eight triangular coils that encircled the head. The ensemble of coils was arranged to form a modified degenerate mode birdcage whose eight shared rungs were offset from the z-axis at interleaved angles of ±30°. This key geometric modification resulted in triangular elements whose vertices were shared between next-nearest neighbors, which provided a convenient location for counter-wound decoupling inductors, whilst nearest-neighbor decoupling was addressed with shared capacitors along the rungs. This decoupling strategy alleviated the strong interaction that is characteristic of array coils at low frequency (32.6MHz in this case) and allowed the coil to operate efficiently in transceive mode. The sodium array provided a 1.6-fold signal-to-noise ratio advantage over a dual-nuclei birdcage coil in the center of the head and up to 2.3-fold gain in the periphery. The array enabled sodium MRI of the brain with 5-mm isotropic resolution in approximately 13min, thus helping to overcome low sodium MR sensitivity and improving quantification in neurological studies. An eight-channel proton array was integrated into the sodium array to enable anatomical imaging.

Radiative MRI Coil Design Using Parasitic Scatterers: MRI Yagi

Conventionally, radio frequency (RF) coils used for magnetic resonance imaging (MRI) are electrically small and designed for near-field operation. Therefore, existing antenna design techniques are mostly irrelevant for RF coils. However, the use of higher frequencies in ultrahigh field MRI allows for antenna design techniques to be adapted to RF coil designs. This communication proposes the use of parasitic scatterers to improve the performance of an existing 7T MRI coil called the single-sided adapted dipole (SSAD) antenna. The results reveal that scatterers arranged in a Yagi fashion can be applied to reduce local specific absorption rate maxima of a reference SSAD by 40% with only a 6% decrease in the propagated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B_{1}^{+}$ </tex-math></inline-formula> field at the tissue depth of 15 cm. The higher directivity of the proposed design also decreases the coupling with additional elements, making this antenna suitable for use in high-density arrays. These findings show the potential of parasitic scatterers as an effective method to improve the performance of existing radiative MRI coils.

Multiple Parallel Round Leg Design for Quadrature Birdcage Coil in Ultrahigh-Field MRI

Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B1) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B1 field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B1 field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI.

Control Device Thermal Modeling in MRI Receive Coil Q-Spoiling Circuits

Q-spoiling circuits for detuning receive coils during high-power magnetic resonance imaging (MRI) slice-select pulses protect both sensitive electronics as well as the patient from undesired high energy signals and associated heating. These circuits have traditionally used PIN diodes and, more recently, field effect transistors to switch in small values of inductance to shift the coil resonance away from the Larmor frequency. These semiconductor devices can see the full radio frequency (RF) power during operation, and will exhibit significant heating, potentially leading to device failure. Even if the device does not reach a destructive heating level, the resistance of the device can increase and effectively degrade the level of blocking in the RF coil. A useful tool for MRI circuit designers would be a thermal model that predicts this change in resistance, and hence degradation in MRI coil blocking, as device temperature rises. This paper presents a method for modeling thermal effects in MRI Q-spoiling control devices. A PIN diode resistance–temperature model is reviewed, and two approaches for field effect transistors that model the temperature rise and subsequent device resistance variation with temperature using SPICE-compatible equivalent circuit elements are covered. Experimental results are presented that validate the form of the models and show that increased device resistance due to temperature rise will degrade MRI blocking by several decibels. The ability to model the control device's thermal characteristics will lead to improved understanding of circuit operation, aid in the selection of the control device for the specific application, and will lead to more reliable MRI instrumentation.

Simulation, design, and test of an elliptical surface coil for magnetic resonance imaging and spectroscopy

AbstractThe simplest design of surface coils for magnetic resonance imaging (MRI) applications is circular and square loops, both producing a magnetic field perpendicular to the coil plane in the central region‐of‐interest (ROI), with an amplitude that decreases along the coil axis. However, a surface coil constituted by a loop with different geometry could be necessary when particular field‐of‐views (FOVs) are desired, especially for performing imaging in an elongated region. This can be achieved by using an elliptical loop, which can guarantee a wide longitudinal FOV and a good penetration in deep sample regions. This work proposes the application of a method for elliptical coil Signal‐to‐Noise Ratio (SNR) estimation previously developed for circular and square loop design, in which coil inductance and resistance are analytically calculated and the magnetic field pattern is estimated using the magnetostatic approach, while the sample‐induced resistance is calculated with the vector potential calculation method. In the second part of the paper, we propose the simulation and the design of a transmit/receive elliptical coil for MRI in mice with a 3T clinical scanner. We also evaluated the coil performance in a preliminary magnetic resonance spectroscopy (MRS) study in phantom.

Recording and post-processing speech signals from magnetic resonance imaging experiments

Speech recordings during an Magnetic Resonance Imaging (MRI) experiment yield valuable but noisy acoustic data for modelling purposes. Despite recent improvements in optical microphones for MRI, using an acoustic sound recording system in dipole configuration, based on shielded electret microphones and waveguides, has some inherent advantages: For example, the bandwidth can be made very wide, and the extremely linear behaviour of all components facilitates the numerical post-processing of signals. Here, the full measurement chain and the critical design decisions are reviewed. In particular, one must take into account the resonant behaviour of the necessarily non-optimally terminated waveguides as well as the environment acoustics within the MRI coils. Two competing approaches for the signal post-processing were developed: (i) Optimal subtraction of the noise and speech signals, and (ii) spectral peak removal by adaptively fitted comb filters. The main objective is the high-quality spectral identification, estimation, and classification of source features, e.g., vowel formants. Approach (i) preserves the phase behaviour of the original signal. Approach (ii) produces excellent speech sound quality when the noise consists of only few harmonic sources; a salient property of MRI acoustic noise. Both of the approaches must compensate the frequency dependent damping of the measurement chain.

Degree of RF MRI coil detuning for an anatomically realistic respiratory cycle modeled with the finite element method.

Respiratory motion is an important problem in Magnetic Resonance Imaging (MRI), contributing to image blurring during data acquisition and coil detuning. Using the concept of an ideal (perfectly matched and tuned at all available ports) RF transmit coil and the VHP-Female v4.0 dynamic CAD model, we numerically model and estimate the detuning of a full-body RF coil during the respiratory cycle.

CAD-Based Virtual Humans : Design and Safety Assessment of MRI Coils.

Magnetic resonance imaging (MRI) is a ubiquitous tool used in clinical settings around the world to provide detailed three-dimensional information on the internal anatomy and physiology of human patients without the use of ionizing radiation, which is the primary safety concern associated with computed tomography. This information is obtained noninvasively and can be used in the diagnosis of pathological conditions as well as the monitoring of treatments.

Experimental Setup to Compare Measurements and Numerical Simulations in Magnetic Resonance Imaging RF Dosimetry

Many of the parameters associated with the magnetic resonance imaging (MRI) coils are estimated by means of numerical simulations. Taking into account the unavoidable numerical approximations and imperfections of the models, an experimental validation of the theoretical results becomes essential. This paper describes two measuring setups which allow the comparison between measurements of electromagnetic fields and the same quantities computed numerically. The experimental activity highlighted some critical aspects of the numerical results that could bring to a wrong estimation of the parameters associated with the MRI coils. Results show the importance and feasibility of a dosimetry experimental setup suitable for MRI coils characterization.

Numerical Evaluation on Irregular Field Generated by Screening Current in High-Field REBCO Coil for Whole-Body MRI

In recent years, high-field magnets wound with REBCO tape and their applications to magnetic resonance imaging (MRI) have undergone considerable research and development. MRI coil systems require a highly accurate field in the imaging space. However, in REBCO coils, the irregular magnetic field generated by the screening current is induced by the radial component of the magnetic field in the REBCO tape, and may deteriorate the field quality both spatially and temporally. Therefore, a design process with consideration for the effect of the screening-current field is required for the reduction and correction of this field for a high-accuracy-field REBCO coil. In this study, the screening-current distribution in the REBCO tape and the irregular magnetic field were analyzed by using the simulation we developed for a REBCO model coil system for whole-body MRI. We additionally discuss the effect of the screening-current field in the REBCO coil based on the coil shape and arrangement to use it as feedback to the design stage.

Oval gradient coils for an open magnetic resonance imaging system with a vertical magnetic field

Existing open magnetic resonance imaging (MRI) systems use biplanar gradient coils for the spatial encoding of signals. We propose using novel oval gradient coils for an open vertical-field MRI. We designed oval gradients for a 0.3T open MRI system and showed that such a system could outperform a traditional biplanar gradient system while maintaining adequate gradient homogeneity and subject accessibility. Such oval gradient coils would exhibit high efficiency, low inductance and resistance, and high switching capability. Although the designed oval Y and Z coils showed more heat dissipation and less cooling capability than biplanar coils with the same gap, they showed an efficient heat-dissipation path to the surrounding air, which would alleviate the heat problem. The performance of the designed oval-coil system was demonstrated experimentally by imaging a human hand.

Improving $B_{1}$ Efficiency and Signal-to-Noise-Ratio of a Surface Coil by a High-Impedance-Surface RF Shield for 7-T Magnetic Resonance Imaging

In this paper, we present a fundamental investigation to improve the $B_{1}$ efficiency and the signal-to-noise ratio (SNR) of a radio frequency (RF) surface coil for ultrahigh-field magnetic resonance imaging (MRI) by utilizing a high-impedance surface (HIS) as the RF shield. An analytical investigation indicates that a circular loop backed by a perfect magnetic conductor (PMC), which is the ideal case of an HIS, suggests an improved magnetic field compared with the case of a perfect electric conductor (PEC) and the case without any shield. This improvement is verified by a full-wave simulation, where the surface coil is modeled by an ideal impressed current model with azimuthal component ( $J_{\text {surf}, \alpha }= 1$ A/m). The electromagnetic field is effectively shielded out behind the PEC and PMC shields compared with the case without any shield. Furthermore, the surface coil with uniform current distribution and the PMC shield is realized by a series resonant loop structure and a 2-D HIS structure, respectively. Since the normal component of the magnetic field is supported at the surface of an HIS, whereas suppressed by a conventional PEC, the ${\boldsymbol{B}}_{1}$ field in the vicinity of the HIS shield is enhanced compared with the case with a PEC shield. Hence, an improvement on SNR and ${\boldsymbol{B}}_{1}$ efficiency is achieved by utilizing an HIS shield, especially in the regions adjacent to the surface coil. It has been found that the improvement of $B_{1}$ efficiency is more prominent than the improvement of SNR due to different normalizations. The difference of peak SAR between considered shields, which is used for $B_{1}$ efficiency normalization, is considerably larger than the difference of the power loss within the phantom, which is used for the SNR normalization. The proposed approach is validated by full-wave finite-element method simulations and near-field measurements, which reveal good agreement with each other.

MRI画像計測へのマイクロファブリケーション応用

This paper reports on microfabrication technique for fabricating micro coils for MRI (Magnetic Resonance Imaging). The standing micro coils, the screw-shaped micro coils, the bicone-shaped micro coils, and the gradient micro coils were fabricated for high resolution MRI. We introduced the structures of micro coils and the fabrication process of these micro coils.

An Observational Study to Assess Brain MRI Change and Disease Progression in Multiple Sclerosis Clinical Practice-The MS-MRIUS Study.

ABSTRACTBACKGROUND & PURPOSETo describe methodology, interim baseline, and longitudinal magnetic resonance imaging (MRI) acquisition parameter characteristics of the multiple sclerosis clinical outcome and MRI in the United States (MS‐MRIUS).MATERIAL & METHODSThe MS‐MRIUS is an ongoing longitudinal and retrospective study of MS patients on fingolimod. Clinical and brain MRI image scan data were collected from 600 patients across 33 MS centers in the United States. MRI brain outcomes included change in whole‐brain volume, lateral ventricle volume, T2‐ and T1‐lesion volumes, and new/enlarging T2 and gadolinium‐enhancing lesions.RESULTSInterim baseline and longitudinal MRI acquisition parameters results are presented for 252 patients. Mean age was 44 years and 81% were female. Forty percent of scans had 3‐dimensional (3D) T1 sequence in the preindex period, increasing to 50% in the postindex period. Use of 2‐dimensional (2D) T1 sequence decreased over time from 85% in the preindex period to 65% in the postindex. About 95% of the scans with FLAIR and 2D T1‐WI were considered acceptable or good quality compared to 99–100% with 3D T1‐WI. There were notable changes in MRI hardware, software, and coil (39.5% in preindex to index and 50% in index to postindex). MRI sequence parameters (orientation, thickness, or protocol) differed for 36%, 29%, and 20% of index/postindex scans for FLAIR, 2D T1‐WI, and 3D T1‐WI, respectively.CONCLUSIONSThe MS‐MRIUS study linked the clinical and brain MRI outcomes into an integrated database to create a cohort of fingolimod patients in real‐world practice. Variability was observed in MRI acquisition protocols overtime.

The magnetic field homogeneity of coils by means of the space harmonics suppression of the current density distribution

Abstract. Electromagnetic coils are ubiquitously used in the modern world in motors, antennas, etc. In many applications (magnetic field coil calibration and nuclear magnetic resonance spectroscopy and imaging) there is a strong need for a homogeneous magnetic field. In this paper, we propose a simple modelling based on serial Fourier decomposition allowing the determination of the electrical conductor distribution to make the magnetic field homogeneous. The method is valid both for plane and axisymmetric geometries. The method allows the retrieval of the classical configurations of saddle coil for the plane geometry and Helmholtz coil for the axisymmetric one. The method is generalized for any number of electrical conductors and brings the perspective of new homogeneous magnetic resonance imaging (MRI) coil configuration.