Transmit‐only/receive‐only radiofrequency coil configuration for hyperpolarized 129Xe MRI of rat lungs

We report a novel radiofrequency (RF) transmit‐only/receive‐only (TO/RO) coil configuration providing excellent transmit B₁₊ field uniformity as well as high sensitivity for hyperpolarized ¹²⁹Xe MR lung imaging of rats at 3T (35.34 MHz). The TO/RO coil configuration consisted of two separate components: (i) a high‐pass birdcage transmit coil which produces a homogeneous B₁₊ magnetic field, (ii) a saddle‐shaped single‐turn receive‐only surface coil that couples closely to the rat lung. On transmit, the receive‐only coil is decoupled from the transmit coil using a detuning circuit. On receive, the bird‐cage coil is deactivated through the use of PIN diodes. The sensitivity and uniformity of the saddle‐shaped receive coil were optimized solving the Biot‐Savart equation using 3D finite element modeling. The electrical performance of the new TO/RO configuration in transmit/receive (T/R) mode was compared with a commercial T/R birdcage coil of similar diameter, which was considering to be the gold standard for conventional T/R mode imaging. Experimental results in phantoms confirm that our novel TO/RO coil configuration provides a factor of three increase in SNR without compromising B₁ transmit uniformity compared with the commercial T/R birdcage coil configuration. The novel TO/RO coil was successfully tested for in vivo rat lung imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 115–124, 2015     

Design of a quadrature surface coil for hyperpolarized 13C MRS cardiac metabolism studies in pigs

This work describes the design of a quadrature surface coil constituted by a circular loop and a butterfly coil, employed in transmit/receive (TX/RX) mode for hyperpolarized ¹³C studies of pig heart with a clinical 3T scanner. The coil characterization is performed by developing an SNR model for coil performance evaluation in terms of coil resistance, sample‐induced resistance and magnetic field pattern. Experimental SNR‐vs.‐depth profiles, extracted from the [1–13C]acetate phantom chemical shift image (CSI), showed good agreement with the theoretical SNR‐vs.‐depth profiles. Moreover, the performance of the quadrature coil was compared with the single TX/RX circular and TX/RX butterfly coil, in order to verify the advantage of the proposed configuration over the single coils throughout the volume of interest for cardiac imaging in pig. Finally, the quadrature surface coil was tested by acquiring metabolic maps with hyperpolarized [1–13C]pyruvate injected i.v. in a pig. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 69–77, 2013     

Multislice localized parallel excitation for EPI applications in humans

In this work, the opportunities and challenges for the use of parallel transmission in combination with 2D RF pulses designed on EPI‐based excitation trajectories for diffusion‐weighted imaging (DWI) with reduced FOV are presented and analyzed in detail. The use of localized excitation allows for shortening of the EPI read‐out, which is especially important for EPI applications outside of the brain. DWI is chosen as a practically important and relevant example demonstrating the key aspects of 2D spatial selection. The properties of accelerated pulses are explored experimentally in phantoms for two different schemes, in which the thickness of the excited limited slices is encoded either along the frequency or phase encoding directions of the excitation trajectory. The feasibility of application of parallel transmission for MR imaging in humans is analyzed based on several pilot experiments. Although the parallel transmission acceleration is demonstrated to work in some examples in the spinal cord and abdomen, the results also uncover a number of challenges. Nonetheless, the reduction of FOV in the phase encoding direction of the read‐out train along with the associated substantial shortening of the minimum echo train length and reduction of geometric distortions motivates further search for an advantageous use of the parallel transmit technology in EPI applications. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 153–173, 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     

Maximally spaced projection sequencing in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) provides 3D images of absolute oxygen concentration (pO₂) in vivo with excellent spatial and pO₂ resolution. When investigating such physiologic parameters in living animals, the situation is inherently dynamic. Improvements in temporal resolution and experimental versatility are necessary to properly study such a system. Uniformly distributed projections result in efficient use of data for image reconstruction. This has dictated current methods such as equal‐solid‐angle (ESA) spacing of projections. However, acquisition sequencing must still be optimized to achieve uniformity throughout imaging. An object‐independent method for uniform acquisition of projections, using the ESA uniform distribution for the final set of projections, is presented. Each successive projection maximizes the distance in the gradient space between itself and prior projections. This maximally spaced projection sequencing (MSPS) method improves image quality for intermediate images reconstructed from incomplete projection sets, enabling useful real‐time reconstruction. This method also provides improved experimental versatility, reduced artifacts, and the ability to adjust temporal resolution post factum to best fit the data and its application. The MSPS method in EPRI provides the improvements necessary to more appropriately study a dynamic system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 33–45, 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     

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     

A vertical‐loop‐array transmit coil for two‐channel B1 shimming at 3T

We propose a whole‐body transmit coil structure for two‐channel B₁ shimming at 3T. The transmit coil consists of four identical vertical‐loop‐coils placed on a cylindrical former by equiangular basis. A vertical loop coil consists of an inner‐conductor plane and an outer‐conductor plane both of which constitute a vertical loop when the two planes are connected by tuning capacitors at both ends. To improve the B₁ homogeneity of the transmit coil, we split the inner‐ and outer‐conductor planes into two rungs. We made the transmit coil with the length of 500 mm on a frame that had the inner and outer diameters of 550mm and 600mm, respectively. In the B₁ shimming experiment with the transmit coil, we drove the coil with two RF power amplifiers of 18kWp each through two quadrature hybrids. Before the B₁ shimming, we performed B₁ mapping for each driving channel, and then, we determined the optimal magnitude and phase of the input signals to the RF power amplifiers. In the B₁ shimming experiment on a human‐body‐mimicking phantom, the optimized transmission improved the image uniformity by 24.7% as compared to the conventional quadrature transmission. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 59–68, 2013     

A novel switched frequency 3He/1H high‐pass birdcage coil for imaging at 1.5 tesla

The ability to produce hyperpolarized noble gases ³He and ¹²⁹Xe has opened up exciting possibilities for pulmonary magnetic resonance imaging (MRI). We have recently built a hyperpolarizer with the goal of using hyperpolarized ³He gas for MRI in neonatal lungs in a dedicated small foot‐print 1.5 T MR scanner developed at our institution and sited in our Neonatal Intensive Care Unit. Although hyperpolarized gas imaging can provide unique insights into lung ventilation, acinar microstructure, and gas‐exchange dynamics, there is an undiminished need for ¹H MRI of the lung to provide anatomic references, B₁ and B₀ maps, and ¹H images of lung parenchyma. To address this need, we designed, built and tested a novel radiofrequency body coil that provides a high‐pass birdcage coil that can be used for both ³He and ¹H frequencies (48.65 and 63.86 MHz, respectively, at 1.5 T). To switch between frequencies, the birdcage coil has a large mechanical actuator that simultaneously changes the capacitance between every rung of the birdcage. Advantages of this coil design include: 1) quadrature excitation and reception at the ³He and ¹H frequencies, 2) identical B₁ field maps for ³He and ¹H imaging, 3) excellent signal‐to‐noise ratio and B₁ homogeneity at both frequencies, and 4) rapid (10–20 s) switching times between ³He and ¹H operation. This report provides details of the coil's design and fabrication. Images of hyperpolarized ³He and ¹H in phantoms and ex vivo rabbit lungs demonstrate the image quality obtained with the coil. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 174–182, 2015     

Real‐time image reconstruction for pulse EPR oxygen imaging using a GPU and lookup table parameter fitting

The importance of tissue oxygenation has led to a great interest in methods for imaging pO₂ in vivo. Electron paramagnetic resonance imaging (EPRI) provides noninvasive, near absolute 1 mm‐resolved 3D images of pO₂ in the tissues and tumors of living animals. Current EPRI image reconstruction methods tend to be time consuming and preclude real‐time visualization of information. Methods are presented to significantly accelerate the reconstruction process in order to enable real‐time reconstruction of EPRI pO₂ images. These methods are image reconstruction using graphics processing unit (GPU)‐based 3D filtered back‐projection and lookup table parameter fitting. The combination of these methods leads to acceleration factors of over 650 compared to current methods and allows for real‐time reconstruction of EPRI images of pO₂ in vivo. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 46–57, 2015     

Circuit level simulation of MRI receive chain using excitation derived from images

We introduce a novel component‐level simulation method with which to characterize the performance of the electronic interfaces and circuitry of a magnetic resonance imaging (MRI) receiver. The Matlab‐based toolbox first reconstructs idealized coil free induction decay signals from real MR images, then uses these signals to drive a transistor‐level and extracted‐layout simulation of the desired receiver architecture. The simulated receiver's outputs are read back into the toolbox, which then reconstructs the MR images. By comparing the reconstructed images with the original ones, important design characteristics, such as the circuit's noise figure, linearity, phase noise, and inter‐channel coupling, can be investigated in terms of image quality. To validate the method, a new MRI receiver architecture is designed and simulated. The architecture is a 16‐channel multiplexer, designed for a commercially available 0.35 μm CMOS technology, and employs both frequency‐division and time‐division multiplexing to form a combined output signal. The SNR degradation and the circuit's linearity determined from the MR images compare well with the SNR and linearity point predictions from conventional circuit simulations. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 102–113, 2015     

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     

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     

Numerical methods and software tools for simulation,design, and resonant mode analysis of radio frequency birdcage coils used in MRI

Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015     

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 advanced,integrated large‐volume high‐pressure autoclave and 1h/13c double‐tuned resonator for chemistry and materials nuclear magnetic resonance spectroscopy and microscopy investigations

Nuclear magnetic resonance spectroscopy and imaging are well‐established tools in chemistry, physics, and life sciences. Nevertheless, most applications are performed at room temperature and atmospheric pressure. To study the processes in supercritical fluids, sample containers and coils have to be redesigned to especially allow for higher pressures up to several hundred times the atmospheric pressure. In this study, we present a setup for performing spectroscopic and imaging experiments on wood immersed in supercritical CO₂ at up to 20 MPa for drying. A magnetic resonance‐compatible autoclave as well as a double‐tuned ¹H/¹³C‐birdcage coil was designed and a setup for regulating pressure and storing gases was assembled. We were able to successfully perform measurements on the wood and water during the drying process and gaininsights into the displacement of water and its chemical reactions with the highly pressurized CO₂. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 49–58, 2013     

A novel acoustically quiet coil for neonatal MRI system

Magnetic resonance imaging (MRI) acoustic exposure has the potential to elicit physiological distress and impact development in preterm and term infants. To mitigate this risk, a novel acoustically quiet coil was developed to reduce the sound pressure level experienced by neonates during MR procedures. The new coil has a conventional high‐pass birdcage radio frequency design, but is built on a framework of sound abating material. We evaluated the acoustic and MR imaging performance of the quiet coil and a conventional body coil on two small footprint neonatal intensive care unit MRI systems. Sound pressure level and frequency response measurements were made for six standard clinical MR imaging protocols. The average sound pressure level, reported for all six imaging pulse sequences, was 82.2 dBA for the acoustically quiet coil, and 91.1 dBA for the conventional body coil. The sound pressure level values measured for the acoustically quiet coil were consistently lower, 9 dBA (range 6–10 dBA) quieter on average. The acoustic frequency response of the two coils showed a similar harmonic profile for all imaging sequences. However, the amplitude was lower for the quiet coil, by as much as 20 dBA. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 107–114, 2015     

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     

2D B0 mapping of micro solenoids with and without FC‐84 and SU‐8 at 9.4 T

This work addresses the effect of susceptibility matching improvement of micro‐solenoid coil materials on decreasing the B0 deviation in MR imaging of mass‐limited samples at high Tesla animal scanners. For this purpose, I investigated the effect of improving the solenoids of 1 and 0.5 mm diameters “susceptibility matching” by surrounding them in FC‐84 and SU‐8. Comparing 2D B0 maps of solenoids of 1 mm show that the mean value of B0 deviation has decreased by factors of 15.6 and 4.72 for the coils embedded with FC‐84 and SU‐8 respectively. Likewise, the mean of B0 deviation has decreased by factors of 13.15 and 5.27 for the solenoids of 0.5 mm diameter embedded in FC‐84 and SU‐8, respectively. MR images acquired by the solenoids 0.5 and 1 mm are clearly verifying the role of using susceptible materials in the coil structure in reducing the geometrical artifacts due to B0 deviation. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 69–77, 2015