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.     

The design of a body RF coil for low-field open MRI using pseudo electric dipole radiation and simulated annealing

The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B₁-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B₁-field below 0.49 dB (5.8%).     

Faraday shields within a solenoidal coil to reduce sample heating: Numerical comparison of designs and experimental verification

A comparison of methods to decrease RF power dissipation and related heating in conductive samples using passive conductors surrounding a sample in a solenoid coil is presented. Full-Maxwell finite difference time domain numerical calculations were performed to evaluate the effect of the passive conductors by calculating conservative and magnetically-induced electric field and magnetic field distributions. To validate the simulation method, experimental measurements of temperature increase were conducted using a solenoidal coil (diameter 3 mm), a saline sample (10 mM NaCl) and passive copper shielding wires (50 μm diameter). The temperature increase was 58% lower with the copper wires present for several different input powers to the coil. This was in good agreement with simulation for the same geometry, which indicated 57% lower power dissipated in the sample with conductors present. Simulations indicate that some designs should be capable of reducing temperature increase by more than 85%.     

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.     

Inverse design of an organ-oriented RF coil for open,vertical-field,MR-guided,focused ultrasound surgery

The advantages of open, vertical-field, magnetic resonance-guided, focused ultrasound surgery (MRgFUS) are attractive. The inverse technique using the bi-boundary conditions is proposed to design a uterine-oriented intraoperative RF coil with an ultrasound aperture for the MRgFUS system. In the current proposed scheme, the desired magnetic field of the RF coil was set to completely overlap the target organ. The current density distribution on the RF coil surface, accounting for the expected magnetic field, was solved using the inverse technique. The stream function was available through the ‘discretization’ of the current density distribution on the RF coil surface. The coil windings were obtained from the contour plot of the stream function. As a modification of previous designs, the bi-boundary conditions are proposed in the inverse technique for the existence of the ultrasound aperture. Based on the obtained coil windings, a prototype coil was constructed. MR imaging of the phantom and the human body was performed to show the efficacy of the prototype coil. The results of temperature measurement using the prototype coil in a 0.4-T MR system were satisfactory. The performance of the prototype coil improved compared with the previously reported design.     

Assessment of radio-frequency heating of a parallel transmit coil in a phantom using multi-echo proton resonance frequency shift thermometry

We propose a workflow for validating parallel transmission (pTx) radio-frequency (RF) magnetic field heating patterns using Proton-Resonance Frequency shift (PRF)-based MR thermometry. Electromagnetic (EM) and thermal simulations of a 7 T 8-channel dipole coil were done using commercially available software (Sim4Life) to assess RF heating. The fabrication method for a phantom with electrical properties matched to human tissue is also described, along with methods for its electrical and thermal characterisation. Energy was deposited to specific transmit channels, whilst acquiring 3D PRF data using a pair of interleaved RF shim transmit modes. A multi-echo readout and pre-scan stabilisation protocol were used for increased sensitivity and to correct for measurement-to-measurement instabilities. The electrical properties of the phantom were found to be within 10% of the intended values. Adoption of a 14-min stabilisation scan gave sufficient suppression of any evolving background spatial variation in the B₀ field to achieve <0.001 °C/mm thermometry drift over 10 min of subsequent scanning. Using two RF shim transmit modes enabled full phantom coverage and combining multiple echo times enabled a 13–54% improvement in the RMSE sensitivity to temperature changes. Combining multiple echoes reduced the peak RMSE by 45% and visually reduced measurement-to-measurement instabilities. A reference fibre optic probe showed temperature deviations from the PRF-estimated temperature to be smaller than 0.5 °C. Given the importance of RF safety in pTx applications, this workflow enables accurate validation of RF heating simulations with minimal additional hardware requirements.     

低场永磁体磁共振射频场映像

射频场映像是通过一定算法对磁共振射频线圈的发射场进行重建的方法.高场下的射频场经过生物组织时会发生明显变化,在其基础上可以反演生物组织体内电特性,进而对癌症等疾病进行早期诊断,是对生物组织的磁共振结构成像的有力补充.目前为止,射频场映像和电特性研究都以高场鸟笼线圈为主,对低场下的相控阵研究较少.本文主要研究了低场永磁体磁共振射频场的均匀度.有限元仿真和实验验证了在17.8 MHz激励下,射频场在空载和负载下均匀度均发生较大变化.射频场均匀度在负载下的改变在一定程度上可以反映负载生物组织的电特性,对磁共振电特性实用化研究提供了一定的参考价值.     

2-D analytic model for inductively coupled plasma sources. I.Electromagnetic model and kinetic analysis

Inductively coupled plasma sources (ICPS) have recently received increasing interest for semiconductor etching and deposition. A 2-D (r, z) electromagnetic (EM) model of ICPS is developed in terms of the mode analysis (MA) technique. Based upon the eigenmode expansion, a closed-form of the vector and scalar potentials of the EM fields is obtained, which provides exact and fast computing for the EM fields in the structure. By means of the Boltzmann-Vlasov equation, a 2-D model is established to describe the kinetic behaviors of plasmas. The closed-form solution for calculating induced space-currents is given, in which a collisionless coupled damping dominates the interaction mechanism between the induced RF wave and plasmas. In this connection, the 2-D coupled damping effect will be analyzed in detail in the following paper     

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.     

NMR strong off-resonance irradiation without sample overheating

A prototype NMR probe for long RF pulse has been constructed. Its main elements are two coils mounted in the concentric position. The first bigger coil is wound around a glass dewar tube and the second smaller coil is placed inside the dewar. These two coils are thermally isolated by the dewar. A long and strong RF pulse is applied to the bigger coil. The smaller detection coil inside the dewar contains a sample and to this coil a short RF pulse is applied. The two coils are independently tuned and electrically isolated. During the operation of the strong RF pulse the smaller coil has a high resistance to ground (very low Q factor) and does not absorb energy from the bigger coil. During the operation of the short on-resonance RF pulse the bigger coil is detuned to a higher frequency, but the resonance circuit with the small coil is in the electrical resonance. The NMR probe may be used in off-resonance experiments in which long and strong RF pulses are applied to the bigger coil and thereby the problem of the sample overheating is avoided.     

高温超导磁共振成像射频接收线圈的研究

为了提高低场磁共振成像系统的信噪比,提出了具有失谐电路的Bi2223带高温超导射频接收线圈.该线圈采用了电耦合方式传输超导谐振回路的磁共振信号,这种方式有利于进一步制成正交结构或相阵结构的超导接收线圈.为了防止趋肤效应降低超导接收线圈的性能,采用化学腐蚀的方法先将超导带的包套去掉,然后再制成超导主谐振电感.采用一种双探测线圈法对高温超导接收线圈和相同结构的常规铜线圈的Q值进行了测量,结果表明超导接收线圈比常规铜线圈的Q值约高一倍.     

High Q calcium titanate cylindrical dielectric resonators for magnetic resonance microimaging

At high magnetic fields radiation losses, wavelength effects, self-resonance, and the high resistance of typical components all contribute to increased losses in conventional RF coil designs. High permittivity ceramic dielectric resonators create strong uniform magnetic fields in a compact structure at high frequencies and can potentially solve some of the challenges of high field coil design. In this study an NMR probe was constructed for operation at 600 MHz (14.1 T) using an inductively fed CaTiO₃ (relative permittivity of 156) cylindrical hollow bore dielectric resonator. The design has an unmatched Q value greater than 2000, and the electric field is largely confined to the dielectric itself, with near zero values in the hollow bore which accommodates the sample. Experimental and simulation mapping of the RF field show good agreement, with the ceramic resonator giving a pulse width approximately 25% less than a loop gap resonator of similar inner dimensions. High resolution images, with voxel dimensions less than 50 μm³, have been acquired from fixed zebrafish samples, showing excellent delineation of several fine structures.     

一种用于开放式MRI的射频发射线圈设计

介绍了一种用于开放式MRI系统的射频发射线圈. 此发射线圈为上下2个相同的线圈,分别安装在磁体的2极,两线圈采用非对称的正交方式放置. 线圈为矩形螺线管结构,通过电磁场数值计算的方法对线圈的匝间距进行了优化,使线圈在300 mm的球形区域内达到偏差不超过3 dB的均匀性要求. 根据优化结果制作了一套用于0.23 T开放式MRI系统的发射线圈,并对线圈的均匀性及射频发射的效率进行了测试. 测试结果表明,线圈具有较高的发射效率和较好的均匀性,由此验证了设计方案的可行性.     

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.     

B1 transmit phase gradient coil for single-axis TRASE RF encoding

Purpose

TRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B₀ field gradients for k-space traversal and high-resolution MR image formation. Transmit coil performance is a key determinant of TRASE image quality. The purpose of this work is to design an optimized RF transmit phase gradient array for spatial encoding in a transverse direction (x- or y- axis) for a 0.2 T vertical B₀ field MRI system, using a single transmitter channel. This requires the generation of two transmit B₁ RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume.

Materials and Methods

A two-element array consisting of a double Maxwell-type coil and a Helmholtz-type coil was designed using 3D field simulations. The phase gradient polarity is set by the relative phase of the RF signals driving the simultaneously energized elements.

Results

Field mapping and 1D TRASE imaging experiments confirmed that the constructed coil produced the fields and operated as designed. A substantially larger imaging volume relative to that obtainable from a non-optimized Maxwell-Helmholtz design was achieved.

Conclusion

The Maxwell (sine)–Helmholtz (cosine) approach has proven successful for a horizontal phase gradient coil. A similar approach may be useful for other phase-gradient coil designs.     

低场核磁共振短死时间射频线圈与射频开关的设计

本文以核磁共振（NMR）射频线圈振铃信号产生原理为对象进行分析研究,提出了一种适用于低场环境下由环状间隙腔线圈与螺线管线圈构成的收发分离式短死时间射频线圈设计方案,采用优化调谐匹配网络提高发射效率;根据射频线圈方案需求设计了快速切换的射频开关及驱动．在此基础上依据仿真结果制作了短死时间射频线圈,并应用于自主研制的低场9.51 MHz便携式NMR谱仪系统,进行NMR实验,结果显示可将收发切换时间缩短至10 μs以内,验证了该设计方案的可行性．     

Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain

MRI is proving to be a very useful tool for sodium quantification in animal models of stroke, ischemia, and cancer. In this work, we present the practical design of a dual-frequency RF surface coil that provides (1)H and (23)Na images of the rat head at 4 T. The dual-frequency RF surface coil comprised of a large loop tuned to the (1)H frequency and a smaller co-planar loop tuned to the (23)Na frequency. The mutual coupling between the two loops was eliminated by the use of a trap circuit inserted in the smaller coil. This independent-loop design was versatile since it enabled a separate optimisation of the sensitivity and RF field distributions of the two coils. To allow for an easy extension of this simple double-tuned coil design to other frequencies (nuclei) and dimensions, we describe in detail the practical aspects of the workbench design and MRI testing using a phantom that mimics in vivo conditions. A comparison between our independent-loop, double-tuned coil and a single-tuned (23)Na coil of equal size obtained with a phantom matching in vivo conditions, showed a reduction of the (23)Na sensitivity (about 28 %) because of signal losses in the trap inductance. Typical congruent (1)H and (23)Na rat brain images showing good SNR ((23)Na: brain 7, ventricular cerebrospinal fluid 11) and spatial resolution ((23)Na: 1.25 x 1.25 x 5mm(3)) are also reported. The in vivo SNR values obtained with this coil were comparable to, if not better than, other contemporary designs in the literature.     

Physics‐based modeling and microwave characterization of graphene co‐planar waveguides

In this study, large area mono‐, bi‐, and multi‐layer graphenes (MLG) were synthesized using low pressure chemical vapor deposition (LPCVD) process and co‐planar waveguides (CPW) with graphenes being fabricated using conventional photolithography. The intrinsic room temperature radio frequency (RF) characteristics were measured via direct contact to the graphene surface with a probe–tip in the range of 0.1 GHz to 40 GHz without using metal electrodes. The device characteristics were also evaluated by physics‐based RLC equivalent circuit simulation. The proposed equivalent circuit model reproduced all of the measured characteristics within 3% of RMSE and we numerically extracted the set of component values which minimize the error between the measured data and the simulation results over the measured regime. Our findings demonstrate that MLG is an effective candidate in RF interconnect device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Design of an interventional magnetic resonance imaging coil for cerebral surgery

In clinical magnetic resonance imaging(MRI),the design of the radiofrequency(RF) coil is very important.For certain applications,the appropriate coil can produce an improved image quality.However,it is difficult to achieve a uniform B1 field and a high signal-to-noise ratio(SNR) simultaneously.In this article,we design an interventional transmitter-and-receiver RF coil for cerebral surgery.This coil adopts a disassembly structure that can be assembled and disassembled repeatedly on the cerebral surgery gantry to reduce the amount of interference from the MRI during surgery.The simulation results and the imaging experiments demonstrate that this coil can produce a uniform RF field,a high SNR,and a large imaging range to meet the requirements of the cerebral surgery.     

A Modified Alderman–Grant Coil makes possible an efficient cross-coil probe for high field solid-state NMR of lossy biological samples

The design, construction, and performance of a cross-coil double-resonance probe for solid-state NMR experiments on lossy biological samples at high magnetic fields are described. The outer coil is a Modified Alderman–Grant Coil (MAGC) tuned to the ¹H frequency. The inner coil consists of a multi-turn solenoid coil that produces a B₁ field orthogonal to that of the outer coil. This results in a compact nested cross-coil pair with the inner solenoid coil tuned to the low frequency detection channel. This design has several advantages over multiple-tuned solenoid coil probes, since RF heating from the ¹H channel is substantially reduced, it can be tuned for samples with a wide range of dielectric constants, and the simplified circuit design and high inductance inner coil provides excellent sensitivity. The utility of this probe is demonstrated on two electrically lossy samples of membrane proteins in phospholipid bilayers (bicelles) that are particularly difficult for conventional NMR probes. The 72-residue polypeptide embedding the transmembrane helices 3 and 4 of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) (residues 194–241) requires a high salt concentration in order to be successfully reconstituted in phospholipid bicelles. A second application is to paramagnetic relaxation enhancement applied to the membrane-bound form of Pf1 coat protein in phospholipid bicelles where the resistance to sample heating enables high duty cycle solid-state NMR experiments to be performed.