磁刺激中深度感应电场分布的计算

磁刺激是利用变化磁场产生的感应电场作用于可兴奋人体组织的过程.根据磁刺激感应电场理论,计算8字形和四叶形线圈刺激深度感应电场的分布.结果表明通过线圈的电流方向直接影响感应电场的聚焦性.8字形线圈电流方向相反时用于刺激大脑皮层神经效果较好,而四叶形线圈电流方向左右相反,上下相同时,刺激外周神经纤维效果较好.     

磁刺激八字线圈变形对空间磁场与感应电场分布的影响

在经颅磁刺激中,尽管传统八字线圈的聚焦性要优于圆形线圈的聚焦性,但是八字线圈的两侧边缘产生的磁场与感应电场的峰值仍然较高。设计一种基于折叠变形的八字线圈,通过数值计算分析变形八字线圈空间磁场与感应电场的分布,并与传统八字线圈进行对比。结果表明,当变形八字线圈的折叠角度在45°～60°范围内时,其中心位置产生的空间磁场和感应电场占传统八字线圈在中心位置的90%左右,而变形线圈两侧产生的空间磁场与感应电场占传统八字线圈两侧的30%左右。因此,在空间磁场和感应电场的聚焦度方面,折叠变形八字线圈均优于传统八字线圈。所提出的折叠变形八字线圈为设计新型经颅磁刺激线圈提供了新的思路。     

磁刺激人体可兴奋组织的模及其感应电场的三维分析

磁刺激是利用时变电流流入线圈,产生时变磁场,从而在组织内感应出电流使某些可兴奋性组织产生兴奋的一种无创的诊断和治疗技术。本文建立了磁刺激可兴奋性组织的一般模型。确定了一些无量纲参数,对感应电场的分布函数进行无量纲化,并给出强度三维分布的计算机仿真。文中分析和讨论了决定刺激聚焦和刺激深度的因素,确定了设计合理的磁刺激线圈和可以使刺激效果和磁刺激装置都得到优化的依据。     

磁刺激人体可兴奋组织的建模及其感应电场的三维分析

磁刺激是利用时变电流流入线圈,产生时变磁场,从而在组织内感应出电流使某些可兴奋性组织产生兴奋的一种无创的诊断和治疗技术.本文建立了磁刺激可兴奋性组织的一般模型.确定了一些无量纲参数,对感应电场的分布函数进行无量纲化,并给出了电场强度三维分布的计算机仿真.文中分析和讨论了决定刺激聚焦和刺激深度的因素,确定了设计合理的磁刺激线圈和可以使刺激效果和磁刺激装置都得到优化的依据.     

磁共振六通道线圈阵列的数字化仿真与设计

近年来,多通道线圈阵列被广泛应用于磁共振成像,以提高图像的质量。针对局部感兴趣区域内的射频场优化,提出一种由不同尺寸单元构成的六通道线圈阵列,可优化盆腔组织中局部感兴趣区域内的射频场。使用宽度为10和20 cm的两种不同尺寸的线圈单元来构建六通道线圈阵列模型,并对其采用几何重叠法和电容网络法进行去耦,以及运用时域有限差分(FDTD)方法进行仿真和计算,分析和评估其在感兴趣区域内产生的射频场。仿真结果表明,在加载盆腔组织椭圆柱电磁模型情况下,提出的线圈阵列的去耦效果S₁₂和S₁₃分别为-27.19和-33.46 dB,在感兴趣区域内产生的射频场B⁺₁强度平均值,比由宽度为15 cm的相同单元构成的常规线圈阵列高出约5.21%。由不同尺寸单元构成的六通道线圈阵列能够优化感兴趣区域内的射频场,为磁共振线圈设计提供新的思路和方法。     

磁刺激8字形线圈的结构对感应电场分布影响

磁刺激是利用变化磁场产生的感应电场作用于可兴奋人体组织的过程.8字形线圈由于结构简单,常用于磁刺激中.故根据磁刺激感应电场理论,对8字形线圈改进了两种空间结构,计算其刺激深度感应电场分布,并与改进前作比较,结果表明扇形空间结构聚焦性好,更利于磁刺激兴奋点定位.     

提高脑神经磁刺激感应电场聚焦性能的线圈参数优化方法的研究

本研究提出一个衡量磁刺激线圈聚焦性的比较系数,比较不同形状线圈的感应电场聚焦性,研究线圈参数的优化方法.我们建立了磁刺激感应电场分布的数学模型,以Matlab作为仿真工具,以能够反映线圈刺激效果的比较系数ξ为指标,分析比较了几种不同线圈的聚焦性,以及成不同角度的线圈聚焦性.结果表明:四叶玫瑰形为刺激聚焦性最好的线圈形状,该线圈半径越小,聚焦性越好,成102°角的四叶玫瑰线形线圈具有最佳聚焦性,而且112°角四叶玫瑰形线圈与89°角线圈组合成的线圈阵列聚焦性最好.由此说明本研究中的比较系数可用于比较线圈聚焦性,对线圈参数的优化设计具有一定的指导意义.     

经颅磁刺激线圈的磁聚焦优化设计

经颅磁刺激是利用变化磁场产生的感应电场作用于可兴奋人体脑组织的过程,磁聚焦性能是经颅磁刺激线圈设计的一项重要指标。根据磁刺激线圈感应电场理论,我们设计了半圆螺线管用于经颅磁刺激,计算了其载流线圈随刺激深度的感应电场分布,并与传统的经颅磁刺激8字形线圈作比较。结果表明,半圆螺旋管线圈既继承了8字形线圈感应电场的主瓣聚焦性强的优良特性,又摒弃了其相对较大的旁瓣对浅表非靶组织的兴奋刺激的不良影响,完全达到了磁聚焦优化设计的目的,也更利于磁刺激兴奋点的定位。     

经颅磁刺激中刺激线圈的仿真研究

目的设计用于经颅磁刺激的线圈,要求能够对大脑皮质进行多点刺激,且具有聚焦性好、制作简单、使用方便等特点。方法利用电磁仿真方法,以圆形线圈和8字形线圈为基础,计算线圈在均匀人体模型中感应电场的分布情况,比较尺寸、绕法对经颅磁刺激线圈的聚焦性和刺激深度的影响。在此基础上设计了一种多圆相切线圈,并计算该线圈在均匀人体和真实头部模型中的电场分布。结果感应电场强度随刺激深度的增加呈指数式衰减。减小圆形线圈的尺寸,会提高聚焦性,同时可减弱感应电场强度。8字形线圈比圆形线圈具有更好的聚焦性,多层绕法综合效果较好。多圆相切线圈具有8字形线圈的优点,且可以进行多点刺激。结论尺寸、绕法等因素对线圈的聚焦性和刺激深度具有重要影响,多圆相切线圈在经颅磁刺激中具有很好的应用前景。真实头部模型仿真,对于线圈的设计和靶区定位具有重要意义。     

反演法设计磁共振射频阵列线圈的研究

磁共振射频阵列线圈技术的出现进一步提高了磁共振成像的质量,但现有的阵列线圈无法凸显局部感兴趣区域(ROI)。为了满足术中磁共振发展的需求,研究反演法在磁共振射频阵列发射线圈设计与优化中的应用。首先根据临床需求确定ROI,设计目标函数,应用反演法计算阵列线圈的电流密度,采用正则化技术优化计算中出现的高病态问题,采用流函数技术求解线圈绕组。根据临床不同需求设计了3种ROI位置的阵列发射线圈,ROI内的磁场强度达到 0.957 4 A/m以上,与目标磁场强度的误差在5%以内,10 cm ROI内磁场均匀度达到5×10^-8以内,满足理论设计要求。实验结果表明,基于反演法设计的射频阵列发射线圈在ROI内的磁场分布符合理论要求,证明反演法在射频阵列发射线圈设计中的适用性。     

Focusing and targeting of magnetic brain stimulation using multiple coils

Neurones can be excited by an externally applied time-varying electromagnetic field. Focused magnetic brain stimulation is attained using multiple small coils instead of one large coil, the resultant induced electric field being a superposition of the fields from each coil. In multichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusing of the stimulating field, and independent coil channels allow targeting of the stimuli on a given spot without moving the coils. The problem of shaping the stimulating field in multichannel stimulation is analysed, and a method is derived that yields the driving currents required to induce a field with a user-defined shape. The formulation makes use of lead fields and minimumnorm estimation from magneto-encephalography. Using these methods, some properties of multichannel coil arrays are examined. Computer-assisted multichannel stimulation of the cortex will enable several new studies, including quick determination of the cortical regions, the stimulation of which disrupts cortical processing required by a task.     

High-field head radiofrequency volume coils using transverse electromagnetic (TEM) and phased array technologies

This article describes technological advances in quadrature transverse electromagnetic (TEM) volume coils and phased arrays reported recently from our laboratory developed for MRI and MRS imaging of the human brain. The first part of this work presents a new method for tuning TEM volume coils based on measurements of the radiofrequency current distribution in the coil elements. This technique facilitates bench adjustment of the coils' homogeneity and is particularly important for tuning double-tuned TEM volume coils. We have also used this method to optimize other TEM configurations such as a quadrature TEM half-volume coil and a split TEM coil. TEM half-volume coils provide greater sensitivity over localized regions than conventional full-volume coils, and the split TEM coil provides greater patient access and ease of use. The second part of this work describes the development of single-tuned and double-tuned transmit TEM volume coils in combination with phased arrays. A variety of different techniques for active detuning of single-tuned and double-tuned TEM volume coils are presented along with the development of phased arrays and transmission line preamplifier decoupling. The final section describes the use of counter rotating current (CRC) surface coils in phased arrays. Because of the intrinsic isolation of CRC coils from transmit volume coils, CRC arrays can be used simultaneously with volume coils for both reception and transmission. Near the center of the human head where both the phased array and the volume coil produce similar sensitivities, simultaneous reception enhances the signal-to-noise ratio. Conversely, simultaneous transmission can be used to boost the transmit field in peripheral brain regions from the volume coil to provide a more homogeneous transmit field.     

Numerical optimization of a three-channel radiofrequency coil for open, vertical-field, MR-guided, focused ultrasound surgery using the hybrid method of moment/finite difference time domain method

The numerical optimization of a three-channel radiofrequency (RF) coil with a physical aperture for the open, vertical-field, MR-guided, focused ultrasound surgery (MRgFUS) system using the hybrid method of moment (MoM)/finite difference time domain (FDTD) method is reported. The numerical simulation of the current density distribution on an RF coil with a complicated irregular structure was performed using MoM. The electromagnetic field simulation containing the full coil-tissue interactions within the region of interest was accomplished using the FDTD method. Huygens' equivalent box with six surfaces smoothly connected the MoM and FDTD method. An electromagnetic model of the human pelvic region was reconstructed and loaded in the FDTD zone to optimize the three-channel RF coil and compensate for the lower sensitivity at the vertical field. In addition, the numerical MoM was used to model the resonance, decoupling and impedance matching of the RF coil in compliance with engineering practices. A prototype RF coil was constructed to verify the simulation results. The results demonstrate that the signal-to-noise ratio and the homogeneity of the B(1) field were both greatly improved compared with previously published results.     

Electromagnetic fields in biological studies

For biological or cellular experiments using electromagnetic fields, it is essential that the parameters defining the field be carefully specified if the results are to be meaningful and are to be compared with the same experiment conducted in a different laboratory. The interaction of living systems with electric and magnetic fields can come only through forces exerted on the charges on the system. If the charges are stationary, the only origin of the force is the electric field. This electric field may be established by charge distributions, as in “capacitive plate” experiments, or by time-varying magnetic fields. A geometry commonly used to produce time-varying magnetic fields consists of a pair of coaxial coils, each of equal radius and separated by a distance often equal to the radius. The electric field induced by a varying current in such a pair of coils varies both in space and in time. The electric field strength is zero on the axis of symmetry, and increases to a maximum near the radius of the coils. The strength is proportional to the time rate of change of the current in the coil, which depends not only on the amplitude and shape of the voltage pulse applied to the coil but also on the resistance and inductance of the coil. The purpose of this article is to describe how these important physical parameters may be determined for both geometries.     

Electromagnetic fields inside a lossy, multilayered spherical head phantom excited by MRI coils: models and methods

The precise evaluation of electromagnetic field (EMF) distributions inside biological samples is becoming an increasingly important design requirement for high field MRI systems. In evaluating the induced fields caused by magnetic field gradients and RF transmitter coils, a multilayered dielectric spherical head model is proposed to provide a better understanding of electromagnetic interactions when compared to a traditional homogeneous head phantom. This paper presents Debye potential (DP) and Dyadic Green's function (DGF)-based solutions of the EMFs inside a head-sized, stratified sphere with similar radial conductivity and permittivity profiles as a human head. The DP approach is formulated for the symmetric case in which the source is a circular loop carrying a harmonic-formed current over a wide frequency range. The DGF method is developed for generic cases in which the source may be any kind of RF coil whose current distribution can be evaluated using the method of moments. The calculated EMFs can then be used to deduce MRI imaging parameters. The proposed methods, while not representing the full complexity of a head model, offer advantages in rapid prototyping as the computation times are much lower than a full finite difference time domain calculation using a complex head model. Test examples demonstrate the capability of the proposed models/methods. It is anticipated that this model will be of particular value for high field MRI applications, especially the rapid evaluation of RF resonator (surface and volume coils) and high performance gradient set designs.     

Electric field induced in a spherical volume conductor from arbitrary coils: application to magnetic stimulation and MEG

A mathematical method is presented that allows fast and simple computation of the electric field and current density induced inside a homogeneous spherical volume conductor by current flowing in a coil. The total electric field inside the sphere is computed entirely from a set of line integrals performed along the coil current path. Coils of any closed shape are easily accommodated by the method. The technique can be applied to magnetic brain stimulation and to magnetoencephalography. For magnetic brain stimulation, the total electric field anywhere inside the head can be easily computed for any coil shape and placement. The reciprocity theorem may be applied so that the electric field represents the lead field of a magnetometer. The finite coil area and gradiometer loop spacing can be precisely accounted for without any surface integration by using this method. The theory shows that the steady-state, radially oriented induced electric field is zero everywhere inside the sphere for ramping coil current and highly attenuated for sinusoidal coil current. This allows the model to be extended to concentric spheres which have different electrical properties.     

Feasibility study of a unilateral RF array coil for MR-scintimammography

Despite its high sensitivity, the variable specificity of magnetic resonance imaging (MRI) in breast cancer diagnosis can lead to unnecessary biopsies and over-treatment. Scintimammography (SMM) could potentially supplement MRI to improve the diagnostic specificity. The synergistic combination of MRI and SMM (MRSMM) could result in both high sensitivity from MRI and high specificity from SMM. Development of such a dual-modality system requires the integration of a radio frequency (RF) coil and radiation detector in a strong magnetic field without significant mutual interference. In this study, we developed and tested a unilateral breast array coil specialized for MRSMM imaging. The electromagnetic field, specific absorption ratio and RF coil parameters with cadmium-zinc-telluride detectors encapsulated in specialized RF and gamma-ray shielding mounted within the RF coil were investigated through simulation and experimental measurements. Simultaneous MR and SMM images of a breast phantom were also acquired using the integrated MRSMM system. This work, we feel, represents an important step toward the fabrication of a working MRSMM system.