Optimization of a quadrature birdcage coil for functional imaging of squirrel monkey brain at 9.4T

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided.     

Design and Analysis of Microstrip-Based RF Birdcage Coil for 1.5 T Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) technique is widely used to capture the images of the liquid items inside the human body. The radio-frequency (RF) coil is one of the important modules present inside an MRI system, which plays a major role in image quality. In this work, a microstrip-based high-pass RF birdcage coil is proposed for 1.5 T MRI. The cylindrical-shaped birdcage coil consists of 12 microstrip radiating elements and tuning capacitors to achieve a resonance at 63.85 MHz. The coil is made up of 10 mm polytetrafluoroethylene substrate coated by a conducting transmission line of desired length and width. A finite difference time domain simulation is carried out to analyze the return loss (S₁₁), magnetic field homogeneity and Specific Absorption Rate (SAR) parameters of the RF coil. The SAR values of the proposed microstrip-based 1.5 T birdcage coil was compared with 3 T RF birdcage coil. The simulation results indicate the proposed birdcage coil structure gives optimal values of S₁₁, magnetic field homogeneity and SAR.     

A Degenerate Birdcage with Integrated Tx/Rx Switches and Butler Matrix for the Human Limbs at 7 T

The theoretically known degeneracy condition of the band-pass birdcage coil has rarely been exploited in transmit coil designs. We have created an eight-channel degenerate birdcage for the human limbs at 7 T, with dedicated Tx/Rx switches and a Butler matrix. The coil can be split into two half cylinders, as required for its application to patients with limited mobility. The design of the coil, the Butler matrix, and Tx/Rx switches relied on a combination of analytical, circuital, and numerical simulations. The birdcage theory was extended to the degenerate case. The theoretical and practical aspects of the design and construction of the coil are presented. The performance of the coil was demonstrated by simulations, workbench, and scanner measurements. The fully assembled prototype presents good performance in terms of efficiency, B₁ homogeneity, and signal-to-noise ratio, despite the asymmetry introduced by the splittable design. The first in vivo images of the knee are also shown. A novel RF coil design consisting of an eight-channel splittable degenerate birdcage has been developed, and it is now available for 7 T MRI applications of the human lower limbs, including high-resolution imaging of the knee cartilages and of the patellar trabecular structure.     

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.     

Comparison of 12 Quadrature Birdcage Coils with Different Leg Shapes at 9.4 T

The purpose of this study was to analyse the relationship between the radio frequency (RF) coil performance and conductor surface shape for ultra-high field (UHF) magnetic resonance imaging. Twelve different leg-shaped quadrature birdcage coils were modeled and built, e.g., 4 mm-width-leg conventional birdcage coil, 7 mm-width-leg conventional birdcage, 10 mm-width-leg conventional birdcage coil, 13 mm-width-leg conventional birdcage coil, inside arc-shape-leg birdcage coil, outward arc-shape-leg birdcage coil, inside right angle-shape-leg birdcage coil, outward right angle-shape-leg birdcage coil, vertical 4 mm-width-leg vertical birdcage, 6 mm-width-leg vertical birdcage, 8 mm-width-leg vertical birdcage and 10 mm-width-leg vertical birdcage. Studies were carried out in both electromagnetic simulations with finite element method as well as in vitro saline phantom experiments at 9.4 T. Both the results of simulation and experiment showed that conventional birdcage coil produces the highest signal-to-noise ratio (SNR) while the vertical birdcage coil produces the most homogeneous RF magnetic (B ₁) field at UHF. For conventional birdcage coils, as well as the vertical birdcage coils, only the proper width of legs results in the best performance (e.g., B ₁ homogeneous and SNR). For vertical birdcage coils, the wider the leg size, the higher RF magnetic (B ₁) field intensity distribution.     

4T split TEM volume head and knee coils for improved sensitivity and patient accessibility

Split RF coils offer improved patient access by eliminating the need for the coil to be slid over the region of interest. For unshielded birdcage coils, the presence of end ring currents necessitates a direct electrical connection between two halves of the coil. For high-field (>3T) shielded birdcage coils, both the shield and the coil must be split and reliably connected electrically. This problem can be circumvented by the use of split TEM volume coils. Since the elements of a TEM coil are coupled inductively, no direct electrical connection between the halves is necessary. In this work we demonstrate that the effects of splitting the shield for head and knee TEMs can be compensated for, and performance retained. For the knee, the improved access allowed the coil diameter to be reduced, enhancing the sensitivity by 15-20%.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

Multichannel transceiver dual-tuned RF coil for proton/sodium MR imaging of knee cartilage at 3 T

Sodium magnetic resonance (MR) imaging is a promising technique for detecting changes of proteoglycan (PG) content in cartilage associated with knee osteoarthritis. Despite its potential clinical benefit, sodium MR imaging in vivo is challenging because of intrinsically low sodium concentration and low MR signal sensitivity. Some of the challenges in sodium MR imaging may be eliminated by the use of a high-sensitivity radiofrequency (RF) coil, specifically, a dual-tuned (DT) proton/sodium RF coil which facilitates the co-registration of sodium and proton MR images and the evaluation of both physiochemical and structural properties of knee cartilage. Nevertheless, implementation of a DT proton/sodium RF coil is technically difficult because of the coupling effect between the coil elements (particularly at high field) and the required compact design with improved coil sensitivity. In this study, we applied a multitransceiver RF coil design to develop a DT proton/sodium coil for knee cartilage imaging at 3 T. With the new design, the size of the coil was minimized, and a high signal-to-noise ratio (SNR) was achieved. DT coil exhibited high levels of reflection S11 (～-21 dB) and transmission coefficient S12 (～-19 dB) for both the proton and sodium coils. High SNR (range 27-38) and contrast-to-noise ratio (CNR) (range 15-21) were achieved in sodium MR imaging of knee cartilage in vivo at 3-mm(3) isotropic resolution. This DT coil performance was comparable to that measured using a sodium-only birdcage coil (SNR of 28 and CNR of 20). Clinical evaluation of the DT coil on four normal subjects demonstrated a consistent acquisition of high-resolution proton images and measurement of relative sodium concentrations of knee cartilages without repositioning of the subjects during the same MR scanning session.     

Design and construction of an interchangeable RF coil system for rodent spinal cord MR imaging at 9.4 T

Rodent models of spinal cord injury (SCI) have been widely used in pre-clinical studies. Injuries may occur at different levels of the lumbar and thoracic cord, and the number of segments injured and their depths may vary along the spine. It is thereby challenging to build one universal RF coil that exhibits optimal performance for all spinal cord imaging applications, especially in an animal scanner with small in-bore space and limited hardware configurations. We developed an interchangeable RF coil system for a 9.4 T small animal MRI scanner, in which the users can select an optimal coil specialized for imaging specific parts of a rat spine. We also developed the associated animal management device for immobilization and positioning. The whole system allows ease of RF coil exchange, animal fixation, and positioning, and thus reduces the animal preparation time before the MRI scan significantly. Compared to a commercial general-purpose 2-cm-diameter coil that was used in our previous studies, the specialized coil optimized for Sprague-Dawley rat lumbar spinal cord imaging exhibits up to 2.4 times SNR improvement.     

Variations on the slotted-tube resonator: rectangular and elliptical coils.

Two designs (one rectangular, one elliptical) are proposed as efficient alternatives to noncylindrical birdcage RF coils. These designs are based on the slotted-tube resonator and their performance relies on the natural current distribution in the conductors due to the eddy current effects at high frequencies. A Finite element method program, solving the full set of Maxwell's equations, has been employed to accurately characterize and optimize the field homogeneity of the proposed noncylindrical coils. The optimum configuration of each design is presented, taking into account the effect of the RF shield. The proposed designs are compared to several configurations presented in the literature. Two coils (one rectangular, one elliptical) have been constructed and tested in a 0.6 T imaging system. A rectangular coil has been built to operate at 300 MHz. MR images substantiate the usefulness of these coils.     

高通鸟笼线圈激发螺旋波等离子体特性研究

研制了高通鸟笼线圈,使用氩气作为工质气体对射频天线的工作性能进行了初步评估。利用COMSOL5.4模拟出了鸟笼天线在13.56MHz的工作频率下,电场和磁场呈线性极化分布。对鸟笼线圈进行了电路结构解剖,推导出了其谐振频率计算公式。利用热耦合红外测温仪测试了正常工作状态下的鸟笼线圈外表温度明显低于传统射频天线,电容器最高温度仅65.8°。使用光谱仪对螺旋波等离子体放电光谱特性进行诊断。通过朗缪尔探针诊断了不同压强和磁场强度下螺旋波等离子体密度,在1.0Pa、600Gs、射频功率700W条件下等离子体密度达到1.62×10¹⁸m^-3。诊断了正向功率和反向功率对应的等离子体密度,其与磁滞现象变化趋势雷同。测试了螺旋波等离子体的径向密度分布,其在轴心处密度达到最高。探究了无磁场条件下等离子体特性,其密度值不会发生大幅度跃迁,纵向磁场是引发螺旋波等离子体放电的关键因素,低压条件下有利于得到更低的电子温度,最低达到2.67eV。表明鸟笼线圈低热耗、高馈入的特性使其在激发大体积的高密度螺旋波等离子体方面具有明显优势,可以投入到下一阶段氢气螺旋波等离子体的激...     

Radiation-damping effects in a birdcage resonator with hyperpolarised 3He gas NMR at 1.5 T

The presence and diagnosis of radiation damping could have major implications in NMR experiments with hyperpolarised gases, where accurate knowledge of the flip angle is imperative. In this work radiation damping was observed and investigated in a low-pass birdcage resonator (Q=250) with samples of hyperpolarised 3He at 1.5 T. With an initially highly polarised (P=38%) sample of 3He in a spherical cell, the observed FID had a distorted line shape with a spectral line width that was three times that of the same sample in a virtually depolarised state (1 Hz line width for P<1%). Moreover a linear relation between the sample's magnetisation (M0) and the line width of the spectrum was observed which is indicative of radiation damping. With highly polarised samples, significant radiation damping was observed and the effect was a lower than expected rate of depletion of M0 in RF flip angle calibration experiments, which led to significant underestimate of the RF flip angle. To our knowledge this is the first report of radiation damping in a birdcage resonator with samples hyperpolarised or otherwise. Experimental observation of radiation damping could be used as means of measuring coil efficiency as an alternative to the geometrical filling factor (eta) the definition of which is open to question for a birdcage resonator. Estimates of the birdcage filling factor from the measured damping time constants (eta(RD)=0.4%) are compared to those derived from electromagnetic energy ratios (eta(E)=1.6%) and metallic sphere frequency shift methods (eta(fs)=1.4%). These figures are much lower than the simple volume geometrical upper limit of eta(v)=3.7% derived from the ratio of cell volume to total coil volume (shield included). The physical explanation for this shortfall is that the bulk of the magnetic energy stored in the birdcage is spatially distributed predominantly between the rungs and the shield, and not in the coil centre where the sample is placed and where the B1+ field has its highest spatial homogeneity.     

Development of Birdcage Resonators for the 3 Tesla NMR Imaging System

An optimized birdcage resonator configuration with a low-pass filter was designed and built to facilitate the acquisition of high-resolution three-dimensional images of small animals on the 3 T nuclear magnetic resonance imaging system. The birdcage resonator had a 12-element structure in order to ensure B ₁ homogeneity over the image volume and for obtaining the maximum filling factor. The quality of the manufactured birdcage resonators was evaluated on the basis of the return loss following the matching and tuning process. The experimental magnetic resonance images of the phantoms were obtained by the various manufactured birdcage resonators to compare the signal-to-noise ratio (SNR) in accordance with the size of the objects. The ratio of the size of an object to the size of the coil was identified by the parameters that were estimated from the images of a phantom. This study demonstrated that the SNR could be different depending on the size of the object even if birdcage resonators have the same cylindrical forms. Authors' address: Bo-Young Choe, Department of Biomedical Engineering, Kangnam St. Mary's Hospital, College of Medicine, Catholic University of Korea, 505 Banpo-Dong, Seocho-Gu, Seoul 137-040, Korea     

A novel radio frequency coil for veterinary magnetic resonance imaging system

In this article,a novel designed radio frequency (RF) coil is designed and built for the imaging of puppies in a V-shape permanent magnetic resonance imaging (MRI) system.Two sets of Helmholtz coil pairs with a V-shape structure are used to improve the holding of an animal in the coil.The homogeneity and the sensitivity of the RF field in the coil are analysed by theoretical calculation.The size and the shape of the new coil are optimized and validated by simulation through using the finite element method (FEM).Good magnetic resonance (MR) images are achieved on a shepherd dog.     

Surface coil imaging of rat spine at 7.0 T

An inductively coupled surface coil for imaging the rat spine at 7 T is described. This planar circular probe was made from microwave substrate to limit the size of the coil and to minimize the magnetic susceptibility. The surface coil was used as a single transmit/receive coil and as a receive-only coil with a birdcage body coil for excitation. The signal-to-noise ratio (SNR) of the probe was compared to a 5-cm birdcage coil and exceeded the birdcage coil's SNR by three to six times at superficial structures. The main advantages of the probe are an improved SNR for superficial structures and a simple design and use. Images with 50 × 50 × 500 μm voxels were obtained of the rat spine with excellent anatomical detail.     

Design of small volume HX and triple-resonance probes for improved limits of detection in protein NMR experiments

Three- and four-frequency nuclear magnetic-resonance probes have been designed for the study of small amounts of protein. Both "HX" (1H, X, and 2H channels) and "triple-resonance" (1H, 15N, 13C, and 2H) probes were implemented using a single transmit/receive coil and multiple-frequency impedance matching circuits. The coil used was a six-turn solenoid with an observe volume of 15 microl. A variable pitch design was used to improve the B1 homogeneity of the coil. Two-dimensional HSQC spectra of approximately 1mM single labeled 15N- and double labeled 15N/13C-proteins were acquired in experimental times of approximately 2h. Triple-resonance capability of the small-volume triple-resonance probe was demonstrated by acquiring three-dimensional HNCO spectra from the same protein samples. In addition to enabling very small quantities of protein to be used, the extremely short pulse widths (1H = 4, 15N = 4, and 13C = 2 micros) of this particular design result in low power decoupling and wide-bandwidth coverage, an important factor for the ever-higher operating frequencies used for protein NMR studies.     

A Method for Accurate Calculation ofB1Fields in Three Dimensions. Effects of Shield Geometry on Field Strength and Homogeneity in the Birdcage Coil

A method for calculatingB₁field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB₁field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB₁field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B₁homogeneity is greater than in an unshielded coil.B₁field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement.     

Potential advantage of higher-order modes of birdcage coil for parallel imaging

The potential advantages of the higher-order resonant modes of a low-pass birdcage coil for parallel imaging are investigated using FDTD simulations in a spherical phantom. Better parallel imaging performance can be achieved in axial planes than in sagittal planes. If more modes are employed, the average g-factor (gmean) and maximum g-factor (gmax) will be improved for a specific acceleration factor (R). G-factor performance at 3 T (gmean=1.79, gmax=3.55) and 7 T (gmean=1.60, gmax=2.45) can be achieved even with an acceleration factor as high as four when all six order modes of 12-rung low-pass birdcage coil are incorporated for imaging on a mid-axial plane. For a specific number of channels, the optimum combination of corresponding modes can be obtained for different acceleration factors. Based on the g-factor and SNR performance, the total degenerate multi-mode birdcage coil with six order resonant modes has better homogeneous coverage and SENSE performance than the 8-element phased array coil, although requiring fewer channels. In addition, the dielectric effects at high field can improve the parallel imaging performance.     

A volume microstrip RF coil for MRI microscopy

Quantitative magnetic resonance imaging (MRI) studies of small samples such as a single cell or cell clusters require application of radiofrequency (RF) coils that provide homogenous B₁ field distribution and high signal-to-noise ratio (SNR).We present a novel design of an MRI RF volume microcoil based on a microstrip structure. The coil consists of two parallel microstrip elements conducting RF currents in the opposite directions, thus creating homogenous RF field within the space between the microstrips. The construction of the microcoil is simple, efficient and cost-effective.Theoretical calculations and finite element method simulations were used to optimize the coil geometry to achieve optimal B₁ and SNR distributions within the sample and predict parameters of the coil. The theoretical calculations were confirmed with MR images of a 1-mm-diameter capillary and a plant obtained with the double microstrip RF microcoil at 11.7 T. The in-plane resolution of MR images was 24 μm×24 μm.     

A novel RF coil configuration for in-vivo and ex-vivo imaging of arthritic rabbit knee joints

The purpose of this study was to design and build an optimized Radio Frequency (RF) coil configuration, that would facilitate the acquisition of high resolution 3-dimensional (3D) images of arthritic and normal rabbit knees. A surface coil transmit surface coil receive configuration was built, in order to ensure adequate B(1) homogeneity over the imaging volume and maximum filling factor, and hence to maximize the Signal to Noise ratio (SNR) and resolution of the 3-dimensional images. The two coils were passively decoupled using crossed diodes and lambda/4 lines, both during the transmit and receive phases of the imaging experiment. A specialized animal bed, to optimize the use of the coils and minimize the experiment setup time was designed and constructed. Three dimensional images of resolution 156 x 156 x 468 microm, were acquired in 20 min; the results, in terms both of the high resolution images and the ease with which the experimental setup could be reproduced, demonstrated that this configuration is ideal for imaging rabbit knee joints.