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     

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     

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     

Evaluation of a new 1H/31P dual‐tuned birdcage coil for 31P spectroscopy

We introduce a new dual‐tuned hydrogen/phosphorus (¹H/³¹P) birdcage coil, referred to as split birdcage coil, and evaluate its performance using both simulations and magnetic resonance (MR) experiments on a 3 T MR scanner. The proposed coil simplifies the practical matters of tuning and matching, which makes the coil easily reproducible. Simulations were run with the finite difference in time domain method to evaluate the sensitivity and homogeneity of the magnetic field generated by the proposed ¹H coils. Following simulations, MR experiments were conducted using both a phantom and human thigh to compare the proposed design with a currently available commercial dual‐tuned flexible surface coil, referred to as flex surface coil, for signal to noise ratio (SNR) as well as homogeneity for the ³¹P coil. At regions deep within the human thigh, the split birdcage coil was able to acquire spectroscopic signal with a higher average SNR than the flex surface coil. For all regions except those close to the flex surface coil, the split birdcage coil matched or exceeded the performance of the flex surface coil. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 90‐99, 2013     

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     

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     

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     

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 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     

Theoretical analysis of magnetic wall decoupling method for radiative antenna arrays in ultrahigh magnetic field MRI

Radiative antenna techniques, e.g., dipole and monopole, have been proposed for radiofrequency (RF) coil array designs in ultrahigh field MRI to obtain stronger B₁ field and higher signal‐to‐noise ratio (SNR) gain in the areas deep inside human head or body. It is known that element decoupling performance is crucial to SNR and parallel imaging ability of array coil and has been a challenging issue in radiative antenna array designs for MR imaging. Magnetic wall or induced current elimination (ICE) technique has proven to be a simple and effective way of achieving sufficient decoupling for radiative array coils experimentally. In this study, this decoupling technique for radiative coil array was analyzed theoretically and verified by a simulation study. The decoupling conditions were derived and obtained from the theory. By applying the predicated decoupling conditions, the isolation of two radiative elements could be improved from about ? 8 dB to better than ? 35 dB. The decoupling performance has also been validated by current distribution along the radiative elements and magnetic field profiles in a water phantom. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 183–190, 2015     

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     

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     

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     

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     

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     

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     

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     

磁共振成像射频线圈的设计及仿真

目的 探讨0.3T永磁MRI系统中笼式线圈的计算机模拟和优化.方法 运用毕奥-萨伐尔定律,通过计算分析,设计了针对3H,129Xe共振频率的8列高通笼式线圈,利用IDL软件进行相关计算和仿真.结果 仿真结果证明了在低场条件下所设计的8列笼式高通线圈产生的射频磁场的均匀性和对称性.当列线数目减少为6列及4列时,笼式线圈产生的磁场均匀性和对称性大大降低.结论 仿真精确度较好,误差较低.线圈列数的降低对B1场均匀性造成破坏.     

猕猴脑部多通道接收线圈的仿真与设计

目的介绍一种用于消除磁共振线圈通道间耦合的仿真方法,指导猕猴脑部线圈的设计。材料与方法通过电磁场及电路仿真软件,计算线圈周围的空间电磁场分布及S参数,以观测通道间的耦合影响。结果在猕猴脑部接收线圈的设计过程中,通过仿真通道间的耦合可知:当相邻一组线圈的几何中心相距42.2 mm时,其耦合影响可以忽略不计,达到了线圈的设计要求。结论利用本文介绍的仿真方法,实现了猕猴脑部多通道线圈的去耦设计。在线圈的制作过程中,该方法可以明确优化方向,降低线圈的研发成本。     

Quadrature birdcage coil with distributed capacitors for 7.0 T magnetic resonance data acquisition of small animals

High static magnetic field magnetic resonance imaging (MRI) is commonly used for preclinical studies in rodents. In this context, minimization of coil losses is mandatory to scan samples that are small compared to the radiofrequency wavelength in the medium. In this study we construct a radiofrequency (RF) birdcage probe with distributed capacitors, operating in quadrature, tailored for 7.0T ¹H MRI of small animals. The design eliminates the need for extra electrical components on the probe structure and affords a high SNR, a uniform field (homogeneity of 93% in the axial plain of the phantom) and a coil sensitivity of 9.8 . Feasibility experiments of mouse imaging are conducted and the competitive capability of a 7.0 T human system equipped with the proposed coil is demonstrated in both body and brain preclinical imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 83–88, 2015