Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

一种永磁磁共振中磁体涡流场的测量方法

磁共振成像（Magntic Resonance Imaging,MRI）技术是一种先进的医疗影像技术.在MRI系统中,通过梯度线圈电流快速切换方向,对待测区域施加梯度磁场,产生的梯度磁场会在其周围的金属体内激发出变化的涡旋电场,进而导致金属体内闭合的回路中产生对原来的梯度电流起抑制作用的感生电流,也就是我们所说的涡流.本文介绍了一种测量磁体涡流场的方法,结合电磁感应定律,设计了一种磁体涡流场测量装置,通过硬件采集以及软件处理的方法,将理想梯度场与实际磁场进行相减并将波形实时呈现,实验结果表明该方法可实现对磁体涡流场的测量.     

Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet.It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications.An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications.This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards.Simulation and experimental results verify that the method is valid and efficient.     

MWECR CVD等离子体系统梯度磁场对沉积a-Si:H薄膜特性研究

分别研究了磁场线圈电流为115.2和137.7A以及137.7A并在加热台下加放SmCo永磁体的方法，来改变单磁场线圈分散场MWECR CVD系统等离子体室及沉积室磁场形貌.用洛伦兹拟合定量地得到了三种磁场形貌的磁场梯度.研究了磁场梯度对沉积a-Si:H薄膜性能的影响.研究表明：在衬底附近，高的磁场梯度可以获得高的沉积速率；在温度不很高时，高的磁场梯度可得到光敏性较好的a-Si:H薄膜. 关键词： 梯度磁场 洛伦兹拟合 a-Si:H薄膜 MWECR CVD系统     

一种实用的使用铁磁材料匀场的开放式超导MRI磁体的优化方法(英文)

磁共振成像(MRI)系统是一种重要的医学诊断设备。作者提出了一种针对使用铁磁材料匀场的开放式超导MRI主磁场匀场的优化方法。在这种方法中,为了使这种包含非线性铁磁轭结构的MRI磁体产生高均匀度的磁场,集成了改进的多输入多输出负反馈控制理论和有限元方法,来计算匀场区非线性铁磁材料的形状。特别要指出的是,这种方法对初始值的要求不高,一定可以收敛,如果配置得当,可以快速收敛。另外,由于收敛速度快,这种方法也可以应用于大型的非轴对称三维模型优化分析。在文中,作者在一个二维轴对称磁体模型上测试了该方法,表现良好。     

斜位电流注入磁共振电阻抗成像（MREIT）与硬件设计

现有的磁共振电阻抗成像(MREIT)方法只适用于主磁场平行水平面的磁共振系统,本文中提出了一种新的MREIT方法即斜位电流注入磁共振电阻抗成像,同时适用于主磁场垂直于水平面的磁共振系统. 硬件系统以磁共振主控计算机为控制中心,将MRI与EIT有机结合于一体：(1)能够测量注入电流在成像体内部感应磁场的磁感应强度B(x,y,z);(2)向成像体注入电流;(3)满足时序匹配要求;(4)测量边界电压. 文章还展示了利用此装置所得的初步实验结果.     

涡流损耗对爆炸去磁脉冲发生器输出性能的影响

 为了提高爆炸去磁脉冲发生器的输出能量,设计了绝缘层截断涡流回路磁体结构,以减小冲击去磁过程中磁体内产生涡流造成的能量损耗。把圆柱形钕铁硼磁体切分成4块,在块与块之间增加聚脂绝缘层,再组合成一个圆柱形整体,形成截断涡流回路的磁体结构。采用Maxwell 3D电磁场有限元分析软件,对未切分和切分后的钕铁硼磁体进行了静磁场计算,分析了两种结构下的磁感应强度分布。对这两种磁体结构的脉冲发生器进行了爆炸实验,测量了脉冲发生器输出的感生电动势,分析了涡流损耗对发生器输出电流的影响。结果表明：磁体中截断涡流回路的脉冲功率发生器涡流损耗较小,能够输出更大的电能。     

Vibration analysis and measurement of a gradient coil insert in a 4 T MRI

High speed switching of current in gradient coils within high magnetic field strength Magnetic Resonance Imaging (MRI) scanners may result in high acoustic sound pressure levels in and around these machines. Many studies have already been conducted to characterize the sound field in and around MRIs and various methods have been investigated to attenuate the noise generated. To characterize the vibration properties of the gradient coil, a modified Finite Element (FE) model was developed according to the dimensional design of an available gradient coil insert and the concentration of the copper windings in the coil. The finite element analysis results were verified through experimental modal testing of the same gradient coil in a free-free state (no boundary constraints). Comparisons show that the FE model predicts the vibration properties extremely accurately. Based on the verified FE model, boundary conditions (supports) were added to the model to simulate the operating condition when the gradient coil insert is in place in an MRI machine. Vibration analysis results from the FE model were again verified through experimental vibration testing with the gradient coil insert installed in a 4 T MRI and excited using swept sinusoidal time waveforms. Through a comparison of the vibration signals generated it was found that the vibration resonances, both from the FE model and the experimental vibration testing, shift to higher frequencies after the boundary constraints were applied, as was expected. The predicted vibration response was very close to that measured from the gradient coil insert in operation. The FE modeling procedure that has been developed could easily be used to accurately predict the vibration properties of other gradient coil designs. Furthermore, the vibration analysis results from the FE model could be used in acoustic noise analysis to predict the sound pressure level produced by different types of input current pulse sequences.     

Optimal design method for magnetization directions of a permanent magnet array

In many magnetic systems, the permanent magnet (PM) pattern has a great influence on their performance. This study proposes a systematic optimization method for designing discrete magnetization directions. While previous works have been mostly dependent on researchers’ intuition, the developed method is systematic and can be applied to a two-dimensional PM-type eddy current brake model. The effectiveness of the method is confirmed, where the design’s aim is to maximize the braking force on a moving conductor. The sensitivity analysis is accomplished by the adjoint variable method and the sequential linear programming is used as an optimizer. Several optimization results for various conditions through the proposed design method are compared to each other and the optimal magnet configuration for an eddy current brake is suggested.     

Equivalent magnetic dipole method used to design gradient coil for unilateral magnetic resonance imaging

The conventional magnetic resonance imaging(MRI) equipment cannot measure large volume samples nondestructively in the engineering site for its heavy weight and closed structure. In order to realize the mobile MRI, this study focuses on the design of gradient coil of unilateral magnet. The unilateral MRI system is used to image the local area above the magnet. The current density distribution of the gradient coil cannot be used as a series of superconducting nuclear magnetic resonance gradient coils, because the region of interest(ROI) and the wiring area of the unilateral magnet are both cylindrical side arc surfaces. Therefore, the equivalent magnetic dipole method is used to design the gradient coil, and the algorithm is improved for the special case of the wiring area and the ROI, so the X and Y gradient coils are designed.Finally, a flexible printed circuit board(PCB) is used to fabricate the gradient coil, and the magnetic field distribution of the ROI is measured by a Gauss meter, and the measured results match with the simulation results. The gradient linearities of x and y coils are 2.82% and 3.56%, respectively, less than 5% of the commercial gradient coil requirement.     

A distributed equivalent magnetic current based FDTD method for the calculation of E-fields induced by gradient coils

This paper evaluates a new, low-frequency finite-difference time-domain method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretization of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modelling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multi-layered spherical phantom model and a complete body model.     

Identification of magnet and superconductor contributions to the ac loss in a magnet-superconductor levitation system

By measuring the quality factor of the mechanical oscillator with an attached magnet we investigated the amplitude dependence of the energy loss in a permanent magnet-YBCO superconductor system. When a non-conducting ferrite magnet is used the energy loss is proportional to the amplitude to the 2.79 power. This value is close to the value of 3 expected for the hysteretic motion of flux lines from the Bean model. When a conducting magnet is used the exponent decreased to the value of 2.5. This reflects the contribution of eddy current dissipation in a conducting magnet to the overall loss. We identified a novel mechanism of ac loss in the magnet-superconductor system. According to this mechanism the eddy current in a magnet is induced by the ac field generated by the ac supercurrent in response to the motion of a magnet. This mechanism would take place in a rotary bearing if circumferential inhomogeneity of the rotor magnetization occurred.     

使用改进的形状优化方法设计高均匀度的超导MRI磁体

针对设计带铁磁的超导磁共振成像磁体,提出了一种确定类的优化算法来优化极靴的形状以达到匀场的目的。这种方法集合了灵敏度分析、形状优化的理论和有限元方法计算非线性铁磁材料极靴的配置,来改善两个极靴之间大体积成像区的磁场均匀度。需要指出的是,本优化方法不受极靴初始形状的限制,均可以快速收敛。我们通过一个紧凑型的1.2T双平面磁共振磁体,展示了这种方法。     

非金属低温杜瓦装置的研制

报道了一种非金属杜瓦装置的研制,以满足高温超导磁悬浮车系统对液氮低温容器的要求。这种长方体低温容器采用无磁非金属材料制备,从而可以避免由涡流导致的阻力和热损耗,以及由磁性导致的吸引力等不利于高温超导磁悬浮车系统的影响。容器的液氮损耗率为0.55L/h。综合性能测试表明,该非金属杜瓦可以作为高温超导磁浮车低温容器的理想选择之一。     

Mapping eddy current induced fields for the correction of diffusion-weighted echo planar images

Diffusion-weighted echo-planar magnetic resonance imaging is potentially of great importance as a diagnostic imaging tool; however, the technique currently suffers a number of limitations, including the image distortion caused by the eddy current induced fields when the diffusion-weighting magnetic field gradient pulses are applied. The distortions cause mis-registration between images with different diffusion-weighting, that then results in artifacts in quantitative diffusion images. A method is presented to measure the magnetic fields generated from the eddy currents for each of three orthogonal gradient pulse vectors, and then to use these to ascertain the image distortion that occurs in subsequent diffusion-weighted images with arbitrary gradient pulse vector amplitude and direction, and image plane orientation. The image distortion can then be reversed. Both temporal and spatial dependence of the residual eddy current induced fields are included in the analysis. Image distortion was substantially reduced by the correction scheme, for arbitrary slice position and angulation. This method of correction is unaffected by the changes in image contrast that occur due to diffusion weighting, and does not need any additional scanning time during the patient scan. It is particularly suitable for use with single-shot echo planar imaging.     

Field-cycled PEDRI imaging of free radicals with detection at 450 mT

This paper describes the design, construction and use of a field-cycled proton-electron double-resonance imaging (FC-PEDRI) system for the detection and imaging of free radicals. The unique feature of this imager is its use of a 450-mT detection magnetic field in order to achieve good image quality and sensitivity. The detection magnetic field is provided by a superconducting magnet, giving high stability and homogeneity. Field cycling is implemented by switching on and off the current in an internal, coaxial, resistive secondary magnet that partially cancels the superconducting magnet's field at the sample; the secondary magnet is actively shielded to avoid eddy currents. EPR irradiation takes place at approximately 5 mT, following which the field is switched to 450 mT in 40 ms for NMR signal detection. Full details of the imager's subsystems are given, and experiments to image the distribution of stable free radical contrast agents in phantoms and in anesthetized rats are described.     

MRI梯度预加重模块的分时复用设计

为获得更优的成像质量和更快的成像速度,磁共振成像（MRI）系统的梯度预加重模块需要具有更多的补偿通道和调节参数,常规预加重模块的设计方案使现场可编程门阵列（FPGA）面临巨大的资源消耗.为解决高性能梯度预加重模块的资源消耗大的问题,本文提出了一种基于分时复用技术的梯度预加重实现方案,以常规方案1/44的资源实现了11通道×4组参数的梯度预加重模块.将该模块用于0.35 T MRI系统,测试了补偿前后的涡流曲线和磁共振图像,结果表明该模块有效降低了系统的涡流,减小了磁共振图像中的涡流伪影.     

一种减小梯度线圈产生的涡流的方法

从原理上分析了减小梯度线圈的半径可以减小其在带抗涡流板磁体中引起的涡流.然后采用目标场方法设计了一组半径缩减的梯度线圈，并用Biot-Savart定理计算了这个梯度线圈的梯度线性区.最后通过磁共振成像实验证明了原理中分析得出的结论. 关键词： 梯度线圈 涡流 目标场 流函数     

Theoretical study on spin-polarized injection of electrical currents into polymer semiconductors

Different from electrons and holes in traditional inorganic semiconductors, the charge carriers in polymer semiconductors are spin polarons and spinless bipolarons. In this paper, a theoretical model is presented to describe the spin-polarized injection of electrical currents from a ferromagnetic contact into a nonmagnetic polymer semiconductor. In this model, a new relation of conductivity to concentration polarization for polymer semiconductors is introduced based on a three-channel model to describe the spin-polarized injection of electrical currents under large electrical current densities. The calculated results of the model reveal the effects of the polaron ratio, the carrier concentration polarization, the interfacial conductance, the bulk conductivity of materials, and the electrical current density, etc. on the spin polarization of electrical currents. As conclusions, the large and matched bulk conductivity of materials, the small spin-dependent interfacial conductance, the thin polymer thickness and the large enough electrical current are critical factors for upgrading the spin polarization of electrical currents in polymer semiconductors. Particularly, when the polaron ratio in polymer semiconductors approaches the concentration polarization of the ferromagnetic contact, a modest concentration polarization is sufficient for achieving a nearly complete spin-polarized injection of electrical currents.     

Multi-objective optimization of gradient coil for benchtop magnetic resonance imaging system with high-resolution

Significant high magnetic gradient field strength is essential to obtaining high-resolution images in a benchtop mag- netic resonance imaging （BT-MRI） system with permanent magnet. Extending minimum wire spacing and maximum wire width of gradient coils is one of the key solutions to minimize the maximum current density so as to reduce the local heating and generate higher magnetic field gradient strength. However, maximum current density is hard to optimize together with field linearity, stored magnetic energy, and power dissipation by the traditional target field method. In this paper, a new multi-objective method is proposed to optimize the maximum current density, field linearity, stored magnetic energy, and power dissipation in MRI gradient coils. The simulation and experimental results show that the minimum wire spacings are improved by 159% and 62% for the transverse and longitudinal gradient coil respectively. The maximum wire width increases from 0.5 mm to 1.5 mm. Maximum gradient field strengths of 157 mT/m and 405 mT/m for transverse and lon- gitudinal coil are achieved, respectively. The experimental results in BT-MRI instrument demonstrate that the MRI images with in-plane resolution of 50 ~tm can be obtained by using the designed coils.