Design and simulation of a multilayer Halbach magnet for NMR

Halbach magnet is a type of permanent magnet generating a relatively high and homogeneous magnetic field. It is suitable for Nuclear Magnetic Resonance (NMR) studies of small volume chemical or biological samples. In this article, the model of a Halbach magnet made from an odd number of cylindrical layers is proposed for the first time. Then after the optimization of interlayer distances for odd layers Halbach cylinders, the model is verified by the simulation with a magnet inner radius of 30 mm and an outer radius of 49 mm. Moreover, the disturbance of uniformity in 5 mm DSV (Diameter of Spherical Volume) is presented with errors in magnetic strength and angular variation. As a result, a minimum uniformity of 46 ppm inside a 5 mm DSV is achieved, while it increases practically in the presence of magnetic blocks errors. The good performance of the Halbach magnet with odd layers may find potential applications in NMR. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 134–141, 2015     

Peripheral nerve stimulation measures in a composite gradient system

Purpose: This work compared peripheral nerve stimulation (PNS) thresholds for the MRI scanner body gradients (BODY), a head/neck insert gradient (INSERT), and the combination of both gradient sets used simultaneously (COMPOSITE). Methods : For BODY, INSERT and COMPOSITE gradients, PNS thresholds were determined by exposing subject volunteers to stepped increases in gradient strength. For COMPOSITE mode, the INSERT was applied at equal (experiment 1) or double (experiment 2) the BODY gradient strength. Results: The locations and thresholds of peripheral nerve stimulation depended on the gradient system configuration, gradient axis, and gradient strength. Stimulation in the body occurred when using the BODY Y‐gradient axis either singly (110 T/m/s) or in COMPOSITE mode (315 T/m/s experiment 2) and adding the insert gradient had negligible effect on stimulation. Stimulation in the head/sinus area generally occurred when using the INSERT X‐gradient either singly (213 T/m/s) or in COMPOSITE mode (320 T/m/s) and adding the body gradient had negligible effect on stimulation. In the COMPOSITE mode, both the location of stimulation and the limiting gradient strength matched location and strength of the limiting component gradient. Conclusion: Stimulation, to a first‐order approximation, is independent for the two gradient systems. In COMPOSITE mode, PNS can be dominated by either of the individual gradient components, indicating that the contribution of each component can be increased until the threshold limit of each component gradient is reached. COMPOSITE gradients provide increased gradient performance with PNS thresholds higher than either component gradient system operating alone. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 66–74, 2015     

Analytical expressions for magnetic fields and field gradients of current‐carrying triangles

We present an analytical method for calculating magnetic field gradients generated by arbitrary triangulated surfaces. Our work builds upon the results published by Pissanetzky and Xiang, who presented formulas for calculating the magnetic field of current‐carrying faceted surfaces. We show that the analytical gradient expressions can be computed considerably faster than finite field value differences. We also find that the aforementioned published expressions for the magnetic field can be simplified and optimized substantially. Closer inspection of the algorithms, for both field and gradient, reveals a number pathological parameter constellations, which require special treatment. We present a detailed discussion on this. Our results can be directly applied in the optimization of complex magnetic field coils, such as magnetic resonance gradient coils. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 18–25, 2014     

Acoustic analysis for a split MRI system using FE method

In some MRI‐LINAC (Magnetic Resonance Imaging and Linear Accelerator) hybrid systems, the MRI scanner is split into two parts to form a central gap for the accommodation of the patient or a LINAC. Little is known about the acoustic characteristics of the split gradient coil structure needed for this system; however, it is believed to be very different from its typical configurations. It is important to develop dedicated numerical methods for the characterization of the unique acoustic properties, to provide engineering solutions for the noise attenuation for such a new system. In this article, we modeled the acoustic fields of a split MRI system and traditional gradient structures using the finite element method. The models were validated against acoustic experimental results obtained from a conventional MRI scanner. The acoustic field distribution analysis showed that the average sound pressure levels in the central gap were lower than those in the cylindrical tunnels of the split MRI system at most frequencies. This was also true when both the x coils or z coils were energized independently. Thus, if the patient bed is placed perpendicular to the axis of the main magnet of the split MRI system, the patient will be subjected to relatively lower acoustic intensities compared with that if the patient bed is placed parallel to the axis of the main magnet. Further work is planned to reduce the sound level in the central gap where the patient bed may be placed for this split system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 85–96, 2015     

Development and Characterization of An Unshielded PatLoc Gradient Coil for Human Head Imaging

A cylindrical head gradient insert for human imaging with non‐linear spatial encoding magnetic fields (SEMs) has been designed, optimized and successfully integrated with a modified 3T clinical MR system. This PatLoc (parallel acquisition technique using localized gradients) SEM coil uses SEMs that resemble second‐order magnetic shim fields, but with much higher amplitude as well as the possibility for rapid switching. This work describes the optimization of a coil design and measurement methods to characterize its SEMs, induced self‐eddy currents and concomitant fields. Magnetic field maps of the SEMs are measured and it is demonstrated that the induced self‐eddy current magnetic fields are small and can be compensated. A method to measure concomitant fields is presented and those fields are compared to simulated data. Finally, in vivo human images acquired using the PatLoc system are presented and discussed. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 111–125, 2013     

Estimation of occupational exposure to static magnetic fields due to usual movements in magnetic resonance units

The purpose of this work is to estimate the induced electric field E due to movements of magnetic resonance (MR) workers in static magnetic field. Different operator walking speeds and paths are taken into account, based on real‐world scenarios for a 3T MR scanner. The spectral components of magnetic field gradient are estimated to verify the compliance with the specific International Commission on Non‐Ionizing Radiation Protection safety guidelines for MR workers. The induced electric fields on the chosen paths are estimated using a previously described software tool based on an analytical model. Finally, a real situation is considered in which an operator performs the patient preparation for a brain MR examination. From the spectral components of the motion‐induced magnetic field gradient, it is possible to see that the highest values are concentrated on frequencies below 1 Hz. For the basic paths considered, the worst‐case scenario is a walking perpendicular to the magnet cylinder along the x‐axis. For the example of a real path, the critical zone where the worker exposure is highest is highlighted. The results of this study could be used for training MR workers to follow correct behavior in the scanner room to avoid high‐exposure scenarios. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 75–81, 2015     

An RF breast coil for 0.2T MRI

Breast cancer is the most frequently diagnosed cancer in women. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. Low field MRI offers typical MRI contrast at substantially lower cost, but has suffered from lower spatial resolution. Specificity of breast MRI can potentially be increased by acquiring MR imaging with higher spatial or temporal resolution, but the signal‐to‐noise ratio (SNR) achievable in a given imaging time becomes limiting. SNR for the particular pulse sequence and magnet field strength is strongly influenced by the characteristics of the radio‐frequency coil. An optimal breast coil should yield excellent SNR but also generate a homogeneous B₁ field, while allowing imaging of the both breasts simultaneously and maintaining patient comfort. RF receiver coil design is a key determinant of image quality, thus to address this we have designed and constructed a low field breast imaging coil. The coil was tested with a 4‐post 0.2T MRI providing high quality breast images. Designed and constructed saddle rf coil allows to obtain good quality image of the breast using low 0.2 T MRI system within 2 minutes. The coil provides patient comfort as breast compression is not required and minimizes artefacts caused by respiration or motion. A high contrast, low‐cost and pain‐free breast examination using optimized low field MRI system has the potential to serve a large patient population for whom current technologies have deficiencies. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 3–7, 2016     

Performance of a novel NMR apparatus with a solenoidal tape‐shaped antenna and a split‐type superconducting magnet

Since 2003, the authors have been developing a new configuration NMR that consists of a solenoid‐type antenna and a split‐type superconducting magnet to improve the signal‐to‐noise ratio (SNR). The SNR (standard 0.1% ethylbenzene) of the system reached 9,850 in 2009. Refinement of the radiofrequency components, which include an antenna coil, a low‐temperature preamplifier, and a signal switch, led to a reduction in the system noise. In this study, the line shape of the spectrum was improved by reducing the residual magnetization of the antenna coil using a low‐magnetic sheet laminated with a tungsten sheet and a copper sheet. The measured SNR showed a good agreement with the predicted value, and the result shows the validity of this approach to improve the SNR based on the theoretical prediction. In this article, the outline and the performance of the NMR system are reported. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 79‐89, 2013     

A circuit for synchronizing external stimuli and events to the pulse sequence of a clinical magnetic resonance scanner

Interfacing experiments to a clinical magnetic resonance imaging scanner which require synchronization with an imaging pulse sequence can be challenging unless access to a trigger‐out is available. We present a simple, alternative approach to synchronize an experiment to a scanner using an external trigger to drive the scanner through the peripheral optical pulse rate monitor and cardiac trigger available on most clinical imagers. A trigger circuit and pulse rate monitor interface are described for applying a stimulus in the form of a pulsed voltage to a sample at a specific time in a spin‐echo, echo planar imaging sequence. The apparatus and approach could be used for many other types and numbers of experimental stimuli or events. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 50–52, 2014     

Practical design of multi‐channel MOSFET RF transmission system for 7 T animal MR imaging

In this work, we developed and tested a multi‐channel radio frequency (RF) transmission system with compact metal‐oxide semiconductor field effect transistor (MOSFET) amplifiers for parallel excitation in 7 T animal MRI scanner. The system is composed of a multi‐channel RF controller and four independent RF power amplifiers. Each power amplifier contains two amplification stages. The design was validated by simulation and bench test. The power gain for the amplifier is 18.7 dB at 300 MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel excitation applications at 7 T. This compact RF power amplifier can be potentially used for on‐coil amplification in multichannel RF array system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 191–200, 2015     

CMOS based Q‐switch for low‐power pulsed14N quadruple resonance

In order to reduce the recovery time of the nuclear quadrupole resonance probe (which implies the insertion of a dead time between the end of the pulse and the beginning of signal acquisition), several strategies have been proposed in the case of high‐power radio frequency (rf) pulses. In fact, these strategies rely on a common principle, which is to lower, for an appropriate duration, the probe quality factor immediately after the transmit pulse. The quality factor is then restored for the receive period. In the case of a low‐power rf pulses, we propose a very simple circuit that can alleviate the ringing following the rf pulse just by controlling a rf complementary metal–oxide–semiconductor (CMOS) switch inside the probe circuit. Such a technology features a low insertion loss and a low consumption, the switch being driven by a single‐pin low voltage CMOS or transistor‐transistor logic (TTL) control input. These properties are quite suitable for portable applications. Moreover, we have found that, for simple one‐pulse experiments, this technology provides undistorted ¹⁴N quadrupole resonance spectra along with a significant improvement of the signal‐to‐noise ratio in the case of relatively broad lines involving a fine structure. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 12–17, 2014     

Multiturn surface coil: Theoretical considerations on unloaded to loaded Q ratio and SNR

Radiofrequency coils in magnetic resonance systems are used for exciting the nuclei in the object to be imaged and for picking up the signals emitted by the nuclei. The quality of obtained images strongly depends on the correct choice of the coils geometry and type. Although the coils' performance are influenced by the cross‐sectional shape of the coil conductors, for multiturn surface coils proximity effects between conductors can significantly influence coil behavior. This work describes how the use of a multiturn conductor affects a coil's performance in terms of unloaded to loaded quality factors ratio and signal‐to‐noise ratio, taking into account for the proximity effect between conductors of the coil. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 27–31, 2014     

Birdcage coils: Equivalent capacitance and equivalent inductance

Birdcage coil is extensively used in MR systems thanks to its possibility to provide high signal‐to‐ noise ratio and high radiofrequency magnetic field homogeneity that guarantee a large field of view. This work describes how to schematize the birdcage coil in terms of an equivalent inductance and an equivalent capacitance, whose knowledge can be useful for coil design and characterization. In particular, the knowledge of equivalent capacitance and equivalent inductance permits to estimate theoretically coil resonant frequency, quality factors and matching circuit capacitor values in a quick way, while workbench tests permit to estimate coil resistance and sample‐induced resistance. The presented theory is validated for both lowpass and highpass birdcage coils.by using literature data. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 32–38, 2014     

Image‐based estimation method for field inhomogeneity in brain echo‐planar images with geometric distortion using k‐space textures

Echo‐planar imaging (EPI) can suffer from geometrical distortion due to magnetic field inhomogeneity. To correct the geometric distortions in EPI, a magnetic field map is used. Our purpose was to develop a novel image‐based method for estimating the field inhomogeneity map from the distorted EPI image and T1‐weighted image of the brain using k‐space textures. Based on magnetic resonance imaging physics, our method synthesizes the distorted image to match the measured EPI image through the generating process of EPI image by updating the estimated field inhomogeneity map. The estimation process was performed to minimize the cost function, which was defined by the synthesized EPI image and the measured EPI image with geometric distortion, using an iterative conjugate gradient algorithm. The proposed method was applied to simulation and human data. To evaluate the performance of the proposed method quantitatively, we used the normalized root mean square error (NRMSE) between the ground truth and the results estimated by our proposed method. In simulation data, the values of the NRMSE between the ground truth and the estimated field inhomogeneity map were <0.08. In both simulation and human data, the estimated EPI images were very similar to input EPI images, and the NRMSE values between them were <0.09. The results of the simulated and human data demonstrated that our method produced a reasonable estimation of the field inhomogeneity map. The estimated map could be used for distortion correction in EPI images. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 142–152, 2015     

Retrospective 3D Modeling of RF Coils Using a 3D Tracker for EM Simulation

The 3D geometry of the RF coil in use is often unavailable when the RF coil is a commercial one or the RF coil has been developed through ad hoc modification of the coil shape at the laboratory. Without the coil geometry information, making a 3D model of the RF coil may be necessary to simulate the RF coil performance using a finite difference time domain (FDTD) solver. We used a stylus‐type 3D tracker to measure the 3D positions of the landmarks on the coil wires. From the measured landmark positions, we built 3D models of the coil wires using a 3D design tool. We also carried out FDTD simulation of the RF coil performances after transferring the 3D model data to the FDTD solver. For demonstration, we built 3D models of a shoulder coil and a 36‐channel helmet‐style array coil, and we computed B₁ field maps of the coils using the FDTD solver. We think the proposed method can be greatly used for FDTD simulation of the RF coils in use whose geometries are unknown. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 126–132, 2013     

Characterization of a dedicated double loop,endoluminal coil for anal sphincter MR imaging at 1.5 T and 3 T

MRI has proven its usefulness in the prediction of surgical anterior anal repair that cannot be done with the reference endosonographic exam. Conventional endorectal coils are often based on a single loop coil design and do not possess satisfactory radial uniformity which could impede the correct assessment of the anal sphincter. In this study, several double loop endorectal coils were designed, built, and assessed in simulations, on phantoms and in vivo. The optimum was found for a 50°–70° double loop endorectal coil which presents a better radial uniformity especially at close distance from the coil where the SNR is the highest. First in vivo experiments proved enhanced readability of the MR exam for the radiologist. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 39–49, 2014     

Approaching ultimate intrinsic SNR in a uniform spherical sample with finite arrays of loop coils

We investigated to what degree and at what rate the ultimate intrinsic (UI) signal‐to‐noise ratio (SNR) may be approached using finite radiofrequency detector arrays. We used full‐wave electromagnetic field simulations based on dyadic Green's functions to compare the SNR of arrays of loops surrounding a uniform sphere with the ultimate intrinsic SNR (UISNR), for increasing numbers of elements over a range of magnetic field strengths, voxel positions, sphere sizes, and acceleration factors. We evaluated the effect of coil conductor losses and the performance of a variety of distinct geometrical arrangements such as “helmet” and “open‐pole” configurations in multiple imaging planes. Our results indicate that UISNR at the center is rapidly approached with encircling arrays and performance is substantially lower near the surface, where a quadrature detection configuration tailored to voxel position is optimal. Coil noise is negligible at high field, where sample noise dominates. Central SNR for practical array configurations such as the helmet is similar to that of close‐packed arrangements. The observed trends can provide physical insights to improve coil design. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 53–65, 2015     

Universal radio‐frequency receive‐coil connector: Concept and validation at 3 T

In most clinical magnetic resonance imaging systems, only commercial receive coils with the appropriate connector and encoding can be plugged in. When willing to use a dedicated receive coil for a specific study which cannot be achieved with commercial coils, the researcher faces the connecting issue related to the specificity of the proprietary connector. In this work, a universal device is proposed which allows for the connection of any single channel dedicated coil on any magnetic resonance (MR) system, as long as it is provided with at least one commercial receive coil. Technical feasibility of the universal connecting device was demonstrated on a 3 T MR clinical imager. The device included an independent active decoupling circuit while signal transmission to the data cabinet was achieved by electromagnetic coupling with a commercial receive coil plugged to the MR device. The universal connecting device was notably characterized in terms of signal‐to‐noise ratio (SNR) and compared to the standard connection. Image SNR was comparable using both means of connection. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 125–133, 2015     

Dependence of NMR noise line shapes on tuning,matching, and transmission line properties

The tuning and matching conditions of rf circuits, as well as the properties of the transmission lines connecting these to the preamplifier, have direct consequences for NMR probe sensitivity and as for the optimum delivery of rf power to the sample. In addition, tuning/matching conditions influence radiation damping effects, which manifest themselves as fast signal flip‐back and line broadening effects, and can lead to concentration‐dependent frequency shifts. Previous studies have also shown that the appearance of spin‐noise and absorbed circuit noise signals heavily depended on tuning settings. Consequently, all these phenomena are linked together. The mutual connections and interdependences of these effects are highlighted and reviewed here. © 2014 The Authors Concepts in Magnetic Resonance, Part B: Magnetic Resonance Engineering Published by Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 1–11, 2014