TMS线圈与EEG电极线相对位置对TMS-EEG信号的影响

通过分析经颅磁刺激线圈放电时的电场分布,探索磁刺激输出脉冲对脑电采集帽产生的伪迹信号来源。我们通过建立八字线圈感应电场分布的半无界空间数学模型,获得八字线圈感应电场的分布特性,分析磁刺激影响脑电采集回路的两种影响因素,我们用matlab软件仿真刺激线圈与脑电极线成不同角度、不同距离时产生的伪迹信号的变化趋势,通过模型试验和人体实验验证这种趋势的准确性;结果显示,脑电导线重合于线圈长轴,产生脑电伪迹最小,当线圈转动其他角度或当线圈下移时伪迹会逐渐增大,最大伪迹幅值是最小伪迹幅值的约10倍。在TMS-EEG试验中,伪迹信号的幅值、持续时间与线圈摆放位置、角度有关。通过实验前合理排布脑电极线可降低伪迹信号幅值、持续时间等参数,提高磁刺激下脑电信号特征信号提取的准确性。     

基于真实头模型的H型线圈颅内感应电场分布的仿真研究

研究了H线圈作用下人体颅内感应电场分布情况,并且评估H型线圈的深部特性;建立包含边缘系统的真实头部电导率模型,沿头部模型轮廓建立H型线圈模型,分析其在头皮表面和边缘系统产生的感应电场的分布规律;与8字线圈颅内电场分布进行了比较,采用头皮电场峰值和边缘系统电场峰值之比来评估H型线圈和8字线圈脑深部电场分布特性;结果显示,H型线圈产生的感应电场在头皮不聚焦,但能集中刺激到边缘系统的前扣带回;与8字线圈相比,H型线圈能够减弱头皮电场强度,增强边缘系统电场强度,具有良好的深部特性;本文基于真实头模型的H型线圈研究,初步揭示了H型线圈感应电场的颅内分布特性,有助于脑深部磁刺激线圈的设计优化。     

射频美容电极在人脑中电磁场和比吸收率分布研究

研究在频率40.68 MHz下,使用多极射频电极头对人进行射频美容治疗时在人头部所产生磁场强度、电场强度和比吸收率(Specific Absorption Ratio,SAR)的分布,并将得出的结果与国际非电离辐射防护委员会(ICNIRP)所指定的安全限值进行对比。结果表明,脑组织中磁场强度最大值为0.06 A/m,约为ICNIRP限值的82.19%;电场强度最大值为5.5 V/m,约为INCIRP安全限值的19.64%;SAR最大约为0.03 W/kg,远小于ICNIRP标准的2 W/kg。所有结果均处于ICNIRP安全限值以内,说明射频美容电极所产生的电磁暴露不会对人体造成威胁。     

外磁驱动轴流式血泵磁场分布及红细胞电磁特性分析

外磁驱动轴流式血泵较强的磁场强度会对血液及周围组织细胞产生影响,因此对血泵及其周围红细胞进行电磁场理论计算和仿真分析。利用ANSYS Electronics Desktop中3D瞬态磁场模块对血泵进行瞬态磁场仿真,用理论方法建立细胞膜磁场分布模型,综合利用3D瞬态电场和磁场模块对红细胞膜及其内外电磁场进行研究。给出了血泵稳定状态时的3D和2D磁感应强度分布云图,得到了细胞膜受到的最大磁感应强度值;通过最大磁感应强度值和血泵工况特点得到红细胞膜电场时域上的分布规律和幅值;综合细胞膜静息电位得到细胞膜电场耦合分布规律;基于以上条件求得细胞膜上感应磁场分布及细胞膜所受最大磁场力。尽管钕铁硼材料剩余磁感应强度很大,但血液和红细胞所受最大磁感应强度值仅为812 mT。由此得到的各项红细胞电磁特性参数值可为红细胞受驱动磁场影响下受到的电磁损伤和血泵的临床应用以及优化设计提供理论基础。     

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。     

经颅神经电磁调控人体头部物理模型研究

经颅磁刺激（TMS）作为一种广泛应用的神经调控技术,对于治疗神经、精神疾病的效果已经得到证实。TMS引起的颅内电场与治疗效果密切相关,准确测量TMS产生的颅内电场具有重要意义,但直接对人体进行颅内测量将面临技术、安全、伦理等多种问题限制。因此,本文旨在构建一个能够模拟真实大脑电导率和解剖结构的人体头部物理模型,以便代替真实大脑实现颅内电场测量。本文根据真实大脑各层组织的电导率选取和制备了合适的模拟材料,并基于核磁共振图像对各层组织进行了图像分割、三维重建及三维打印等过程,完成了对人体头部物理模型各层组织的制作,最后将各层组织组合起来构成完整的人体头部物理模型。对TMS线圈施加于人体头部物理模型所产生的感应电场进行测量,进一步验证了该人体头部物理模型的导电性。本文的研究为TMS颅内电场分布研究提供了有效的实验工具。     

八字线圈激励下五层球头模型感应电场能量的分布

研究经颅磁刺激(TMS)下人体头部组织内感应电场能量的分布状况,即组织内焦耳热能损耗情况.提出通过感应电能分配率及感应电能集中性两方面因素,研究蚊香型八字线圈激励下头部模型感应电场能量分布情况.利用ANSYS有限元方法建立蚊香型八字线圈,及包含了头皮、头骨、脑脊液、脑灰质和脑白质的五层球头模型,用瞬态分析方法研究感应电场能量分布.仿真结果表明,对线圈施以8 000 A交流电流时,感应电场能量的63.37%消耗于头皮层,其次是脑脊液层上的30.82%,而脑灰质层仅分配了2.6%,而该层能量集中度为15.72%运用所建立的方法,可为建立逼近于真实条件的头部模型,提高脑神经磁刺激技术聚焦性能和定位性能,提供更加精确的仿真研究结果.     

陡脉冲电场分布仿真与在体兔肝组织的实验研究

本研究建立了陡脉冲在均匀介质中的有限元模型，采用ANSYS电磁场软件仿真双电极和七电极阵列的二维电场强度分布，并改变脉冲参数（电压值、电极间距和电极直径），分析电场强度的分布规律。通过陡脉冲处理在体大白兔肝脏的实验，定量观察兔肝组织病理改变的几何区域、组织损伤的实际形状和电极的有效杀伤半径。结合理论仿真结果，探讨导致兔肝细胞不可逆性电击穿所需达到的电场强度阈值，为今后的临床应用寻找电极阵列布置和脉冲电参数选择的依据。     

多电极联合电刺激神经调控模型建立与分析

目的:分析电刺激信号在人体组织中的扩散情况，为多电极联合刺激提供理论依据。方法:采用以电磁仿真标准单层人体头部模型为研究对象的几何结构模型，假定模型中填充物为肌肉组织，建立具有肌肉特性的有限元模型。向模型注入20 mA的直流电信号，通过多物理场仿真软件COMSOL Multiphysics 5.5计算分析信号在简化头部模型中的信号传播机制。结果:信号主要集中在电极周围；多电极联合刺激时，在模型的几何中心处存在信号叠加，信号在该处存在增强的趋势；同时，通过xy截面分析发现，在均匀的肌肉组织中，信号衰减强度约为8 dB/cm。结论:在简化头部模型中，多电极联合刺激能够在头部截面几何中心形成信号增强的趋势。     

磁共振成像检查中Bl场作用下人体头部比吸收率分布的研究

应用2D—阻抗法和由人体头部横断面切片获得的头部二维模型，计算人体头部在磁共振成像(magnetic resonance imaging,MRI)检查中由RF线圈产生的B1场作用下比吸收率(specific absorption rates,SAR)的分布，并对在MRI检查中人体头部的SAR只分布及相关问题进行了探讨。     

Instrumentation for the measurement of electric brain responses to transcranial magnetic stimulation

There is described a 60-channel EEG acquisition system designed for the recording of scalp-potential distributions starting just 2.5ms after individual transcranial magnetic stimulation (TMS) pulses. The amplifier comprises gain-control and sample-and-hold circuits to prevent large artefacts from magnetically induced voltages in the leads. The maximum amplitude of the stimulus artefact during the 2.5ms gating period is 1.7 μV, and 5 ms after the TMS pulse it is only 0.9 μV. It is also shown that mechanical forces to the electrodes under the stimulator coil are a potential source of artefacts, even though, with chlorided silver wire and Ag/AgCl-pellet electrodes, the artefact is smaller than 1 μV. The TMS-compatible multichannel EEG system makes it possible to locate TMS-evoked electric activity in the brain.     

Instrumentation for the measurement of electric brain responses to transcranial magnetic stimulation

There is described a 60-channel EEG acquisition system designed for the recording of scalp-potential distributions starting just 2.5ms after individual transcranial magnetic stimulation (TMS) pulses. The amplifier comprises gain-control and sample-and-hold circuits to prevent large artefacts from magnetically induced voltages in the leads. The maximum amplitude of the stimulus artefact during the 2.5ms gating period is 1.7 μV, and 5 ms after the TMS pulse it is only 0.9 μV. It is also shown that mechanical forces to the electrodes under the stimulator coil are a potential source of artefacts, even though, with chlorided silver wire and Ag/AgCl-pellet electrodes, the artefact is smaller than 1 μV. The TMS-compatible multichannel EEG system makes it possible to locate TMS-evoked electric activity in the brain.     

The effect of head and coil modeling for the calculation of induced electric field during transcranial magnetic stimulation

In the present work we studied some of the features related to transcranial magnetic stimulation (TMS) computational modeling. Particularly we investigated the impact of head model resolution on the estimated distribution of the induced electric field, as well as the role of the stimulating magnetic coil model in TMS. Using the impedance method we calculated the induced electric field inside a realistic numerical phantom of the human head from a commercially available eight-shaped coil, which was modeled in two ways. The results showed that finer resolution of the model has better performance at tissue interfaces eliminating numerical artifacts of local peaks. Furthermore, the geometrical details of a TMS coil must be taken into account since the predicted amount of volume of brain tissue involved can have great variation. Finally, the secondary magnetic field that is generated by the induced eddy currents in the tissues can be neglected.     

Intracranial measurement of current densities induced by transcranial magnetic stimulation in the human brain

Transcranial magnetic stimulation (TMS) is a non-invasive technique that uses the principle of electromagnetic induction to generate currents in the brain via pulsed magnetic fields. The magnitude of such induced currents is unknown. In this study we measured the TMS induced current densities in a patient with implanted depth electrodes for epilepsy monitoring. A maximum current density of 12 microA/cm2 was recorded at a depth of 1 cm from scalp surface with the optimum stimulation orientation used in the experiment and an intensity of 7% of the maximal stimulator output. During TMS we recorded relative current variations under different stimulating coil orientations and at different points in the subject's brain. The results were in accordance with current theoretical models. The induced currents decayed with distance form the coil and varied with alterations in coil orientations. These results provide novel insight into the physical and neurophysiological processes of TMS.     

Where does transcranial magnetic stimulation (TMS) stimulate? Modelling of induced field maps for some common cortical and cerebellar targets

Computational models have been be used to estimate the electric and magnetic fields induced by transcranial magnetic stimulation (TMS) and can provide valuable insights into the location and spatial distribution of TMS stimulation. However, there has been little translation of these findings into practical TMS research. This study uses the International 10-20 EEG electrode placement system to position a standard figure-of-eight TMS coil over 13 commonly adopted targets. Using a finite element method and an anatomically detailed and realistic head model, this study provides the first pictorial and numerical atlas of TMS-induced electric fields for a range of coil positions. The results highlight the importance of subject-specific gyral folding patterns and of local thickness of subarachnoid cerebrospinal fluid (CSF). Our modelling shows that high electric fields occur primarily on the peaks of those gyri which have only a thin layer of CSF above them. These findings have important implications for inter-individual generalizability of the TMS-induced electric field. We propose that, in order to determine with accuracy the site of stimulation for an individual subject, it is necessary to solve the electric field distribution using subject-specific anatomy obtained from a high-resolution imaging modality such as MRI.     

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。     

经颅磁刺激电场分析研究进展

经颅磁刺激是一种利用通电线圈在脑部的诱发电场来调节皮质兴奋性的技术,广泛应用于神经病学、康复学等领域。经颅磁刺激诱发电场分析与安全性、刺激效果密切相关,在优化刺激方案、线圈设计方面具有重要意义,成为相关领域的研究重点。首先介绍经颅磁刺激3种常见的临床副作用,然后阐述经颅磁刺激现有研究中的常规电场分析方法,包括解析法和数值分析法及其应用场景,并讨论与电场分析密切相关的生物模型建模方法。此外,由于磁刺激线圈与组织中电场分布的密切相关性,介绍常规的刺激线圈结构类型,并结合磁刺激线圈的7种典型设计,分析基于有限元分析的球模型下的电场分布特征。最后,展望经颅磁刺激电场分析研究未来的发展趋势。     

Methodology for Combined TMS and EEG

The combination of transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) provides us the possibility to non-invasively probe the brain’s excitability, time-resolved connectivity and instantaneous state. Early attempts to combine TMS and EEG suffered from the huge electromagnetic artifacts seen in EEG as a result of the electric field induced by the stimulus pulses. To deal with this problem, TMS-compatible EEG systems have been developed. However, even with amplifiers that are either immune to or recover quickly from the pulse, great challenges remain. Artifacts may arise from the movement of electrodes, from muscles activated by the pulse, from eye movements, from electrode polarization, or from brain responses evoked by the coil click. With careful precautions, many of these problems can be avoided. The remaining artifacts can be usually reduced by filtering, but control experiments are often needed to make sure that the measured signals actually originate in the brain. Several studies have shown the power of TMS–EEG by giving us valuable information about the excitability or connectivity of the brain.     

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.     

一种抑制反向感应电场的磁刺激线圈设计方法探讨

在无创性脑神经磁刺激技术中,多采用8字形磁刺激线圈,在线圈周围一定距离的空间中,对应8字线圈中心出现感应电场最大值,对应两个边缘处出现反向感应电场峰值,后者容易在刺激目标处产生副刺激,分析了8字形磁刺激线圈感应电场的分布,针对其反应方向感应电场幅值较大,容易引起副刺激的问题提出了新的磁刺激线圈设计方法,以抑制感应电场副峰,并进行了计算机模拟和验证。