Peripheral nerve stimulation by induced electric currents: Exposure to time-varying magnetic fields

The review evaluates thresholds of peripheral nerve stimulation by complex current waveforms. A neuroelectric model employing Frankenhaeuser-Huxley membrane nonlinearities is used to derive excitation thresholds for monophasic and biphasic pulse sequences, as well as sinusoidal stimuli. The model, along with principles of magnetic field induction, is used to derive criteria of acceptability for exposure to time-varying magnetic fields. Applications to pulsed gradient fields from magnetic resonance imaging devices are discussed.     

Theoretical model for investigating the magnetic and electric fields produced during pulsed magnetic field therapy for nonunion of the tibia

Pulsed magnetic field therapy for tibial nonunion has become an established orthopaedic procedure in many centres. The field is generated by passing pulses of current through coils positioned one each side of the limb but the magnitudes of the magnetic and induced electric fields produced are not usually known. The paper describes a method of calculating the fields that gives good agreement between theory and measurement. An improved model of a bone in a limb has been developed and this model predicts that the peak induced electric field close to the fracture site is between 0·03 and 0·6 Vm⁻¹, depending on which of the many clinically tested coil systems is used. The effect of changing geometry and the contribution of the outer surface of the limb are examined, and the implications for future experimental work are discussed.     

Some technical aspects of magnetic stimulation coil design with the ferromagnetic effect

The performance of the stimulation coil in a magnetic nerve stimulator can be improved by attaching a ferromagnetic structure to the coil. This reduces heat generation at the coil and increases magnetic field strength for a given unit of current. Some technical aspects of the design of a stimulation coil with a ferromagnetic structure have been studied. Finite element method analysis results are presented for the effect of size, depth and magnetic saturation of the ferromagnetic structure on the stimulation coil performance. The experimental results show that the stimulation coil performance is improved by up to 40% by the attaching of a ferromagnetic structure on the coil.     

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.     

A theoretical comparison of electric and magnetic stimulation of the brain

We present a theoretical comparison of the electric field produced in the brain by three modalities of transcranial stimulation of the cortex: magnetic stimulation, bifocal electric stimulation, and unifocal electric stimulation. The primary focus of this comparison is the focality and direction of the electric fields produced. A three-sphere model is used to represent the scalp, skull, and brain. All electric fields are calculated numerically. For magnetic stimulation we consider only a figure-of-eight coil. We find that magnetic stimulation produces the most focal field, while unifocal electric produces the least. Fields produced during magnetic stimulation are parallel to the head surface, while fields produced during electric stimulation have components both parallel and perpendicular to the head surface. The electric field produced by magnetic stimulation is shown to be insensitive to the skull conductivity, while that produced by electric stimulation is very sensitive to it.     

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.     

Evidence for a dose-dependent effect of pulsed magnetic fields on pain processing

Functional magnetic resonance imaging (fMRI) was used to investigate the dose–response relationship (sham, 100, 200, 1000 μT) between a pulsed extremely low frequency magnetic field (ELFMF) and acute thermal pain on the dominant right hand. Forty-seven participants were recruited, and pulsed ELFMF was applied through the MRI gradient system using a novel technique. Regions of interest (ROIs) matching those of previous studies were examined for a potential dose response. Significant correlations between applied field strength and change in BOLD activity were found in the anterior cingulate and the ipsilateral insula, indicating that there might be either a dose response or a threshold effect of the ELFMF.     

Topographic mapping of human motor cortex with transcranial magnetic stimulation: Homunculus revisited

Summany The purpose of this study was to non-invasively evaluate the homuncular organization of the motor cortex in man. We used transcranial magnetic stimulation to induce motor evoked potentials (MEP's) in Abductor Pollicis Brevis (APB) and Biceps Brachii (BB) muscles of 10 healthy volunteers. The practicality and accuracy of magnetic stimulation to scalp sites one cm apart was increased by the application to the scalp of a flexible nylon grid with grid size of 1×1 cm. Responsive scalp sites collectively contributed to distinct but overlapping muscle representational areas for the two muscles. The topography of these motor maps along and slightly anterior to the central fissure corresponds to the homuncular configuration as described by Penfield and Boldrey in 1937.     

体表脉冲电刺激大动物的安全性评估

背景：目前电刺激技术已成为研究热点,但其对机体心电活动的影响尚不明确,且既往此方面研究主要针对小型动物。目的：在不影响心电活动的前提下,寻找刺激猪肝区所能使用最大的脉冲电压及频率。方法：取健康雄性荣昌猪3头,将脉冲电刺激仪输出电极固定于猪肝右叶体表投影前后对应部位,设置固定电刺激参数为波宽10 ms、方波,刺激时间为1 h,分别进行5-100 V的电压耐受实验和1-5 Hz的频率耐受实验,观察猪的动态心电图变化、其不适反应和生活行为的改变。结果与结论：电压耐受实验中脉冲电刺激电压高低与心率增量呈正相关;当电压> 35 V时,可发生窦性心律不齐、室性早搏等轻度心律失常,无恶性心律失常、死亡;当电压> 60 V时,可见电刺激诱发形成的qRs波。频率耐受实验中当频率> 2 Hz时可见心律失常发生,无恶性心律失常,脉冲电刺激可诱发形成qRs波。提示选择参数为电压35 V、频率2 Hz的脉冲电刺激猪肝区是安全的,电压大于35 V或频率大于2 Hz的脉冲电刺激猪肝区均可引发心律失常,多为良性心律失常。中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

Effect of time-varying magnetic fields on the action potential in lobster giant axon

The possible influence of time-varying magnetic fields on action potential in the lobster giant axon was studied. The axon membrane was excited by galvanic stimulation and the action potential was recorded intracellularly with microelectrodes. During the propagation of the action potential along the axon, alternating or pulsed magnetic fields were applied across the middle part of the axon to study whether or not magnetic fields have any effect on parameters such as conduction velocity and refractory period of the nerve fibre and amplitude, duration and shape of the action potentials. No effect on these parameters was observed under different flux densities and frequencies of the magnetic fields. When simulating the conductive properties of tissue surrounding the nerve with the aid of an external conducting loop with a load resistance, action potentials were generated which made it possible to study the threshold value of the induced eddy current for nerve excitation. Based on the results of the experiment, the influence of magnetic flux density, frequency, conductivity, induced EMF and induced eddy current density is discussed, and a method is proposed for estimating the threshold values of magnetic flux density for nerve excitation.     

Detailed 3D models of the induced electric field of transcranial magnetic stimulation coils

Previous models neglected contributions from current elements spanning the full geometric extent of wires in transcranial magnetic stimulation (TMS) coils. A detailed account of TMS coil wiring geometry is shown to provide significant improvements in the accuracy of electric field (E-field) models. Modeling E-field dependence based on the TMS coil's wire width, height, shape and number of turns clearly improved the fit of calculated-to-measured E-fields near the coil body. Detailed E-field models were accurate up to the surface of the coil body (within 0.5% of measured) where simple models were often inadequate (up to 32% different from measured).     

压电生物材料在组织再生领域的研究与应用进展

人体的内源性电场调节着相关生理过程。电刺激已被证明能够促进组织修复和再生。压电生物材料作为一种能将机械能转化为电刺激的智能材料,在组织再生领域有着良好的应用前景。该文综述了目前压电生物材料的分类和其促进组织再生的作用机理,以及压电生物材料在骨组织、神经组织、皮肤组织、肌肉组织和心血管等组织再生领域的最新研究与应用进展,最后总结分析了目前压电生物材料在实际应用中存在的不足和相应的改进措施。     

Mapping location of excitation during magnetic stimulation: Effects of coil position

We determined the location of excitation for different positions of a round and butterfly coil duringin vitro magnetic stimulation of cut peripheral nerves. We analyzed the conditions under which excitation occurs, either at the termination or at the peak of the field gradients (first spatial derivative of the electric field). These results were then compared to predictions about the location of excitation sites from a theoretical model of magnetic stimulation of finite neuronal structures. Excitation along a straight nerve occurred at terminations when 1) a coil was positioned close to the end of a nerve (at least one diameter length from the end), 2) a nerve ended in a finite terminating impedance much greater than the axial resistance of the nerve, 3) the induced electric field was of sufficient magnitude, pointing in a direction away from the axis of a nerve. Excitation occurred at the negative peak of the field gradients along a nerve when 1) a coil was positioned far away from the ends of a nerve, 2) there were no geometric or volume conductor inhomogeneities around a nerve, and 3) it was of sufficient magnitude. Threshold strengths for excitation at terminations were significantly lower than that for field gradient excitation and comparable to that due to geometric and volume conductor inhomogeneities.     

构建腰椎手术失败综合征大鼠模型及肾俞磁刺激治疗的可能机制

背景：腰椎手术失败综合征出现的重要原因是术后硬膜外瘢痕粘连,因此,探索预防腰椎术后硬膜外瘢痕粘连的方法一直是脊柱外科研究的热点。目的：观察肾俞穴磁刺激疗法对腰椎手术失败综合征模型大鼠硬膜外粘连的作用。方法：60只SD大鼠应用椎板切除法建立腰椎手术失败综合征模型,建模成功后均分为2组,肾俞磁疗组于造模后1周开始进行双侧肾俞穴磁刺激,1周治疗5 d,连续6周;空白对照组不干预。造模后13周取材,光镜下观察两组硬膜外瘢痕所占面积比,粘连范围大小,胶原纤维形成情况,并计数成纤维细胞数量;并比较两组瘢痕组织中羟脯氨酸含量及转化生长因子β1表达量。结果与结论：肾俞磁刺激治疗后,模型大鼠硬膜外瘢痕形成少,仅少数成纤维细胞浸润,瘢痕羟脯氨酸含量与转化生长因子β1表达量少,与空白对照组比较差异有非常显著性意义(P < 0.001)。提示肾俞磁刺激疗法能明显减少腰椎术后硬膜外瘢痕形成,是其治疗腰椎手术失败综合征的可能机制。中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

重复磁刺激右侧大脑中动脉闭塞模型大鼠脑梗死区微环境及神经功能的变化

背景：国内外大量研究表明重复经颅磁刺激可使皮质兴奋性产生较刺激时间更加持久的改变,为磁刺激应用于脑梗死后康复治疗提供了一个新的研究方向,但其远期临床疗效与安全性尚需进一步研究。 目的：观察重复经颅磁刺激脑梗死大鼠对神经再生微环境及功能恢复的影响。 方法：将大鼠随机分为模型组、假刺激组及重复经颅磁刺激组(80%运动阈值(MT)组、100%MT组和120%MT组),采用线栓法制备大鼠右侧大脑中动脉闭塞模型。制模24 h后各重复经颅磁刺激亚组给予20 Hz相应强度磁刺激,假刺激组则给予假磁刺激,模型组制模后未给予特殊处理。 结果与结论：造模后7 d,重复经颅磁刺激组的脑梗死体积显著小于模型组及假刺激组(P < 0.05)。RT-PCR、Western blot检测显示,造模后72 h,重复经颅磁刺激组水通道蛋白4/9基因和蛋白表达均较模型组显著增高(P < 0.05)。与造模后第1天比较,造模后第15天重复经颅磁刺激组(100%MT)神经功能缺损评分得到明显改善(P < 0.05)。免疫组织化学检测结果显示,各重复经颅磁刺激亚组缺血半暗带区胶质纤维酸性蛋白表达与模型组比较均显著减少(P < 0.05)。结果证实,重复经颅磁刺激可减轻脑梗死模型大鼠神经功能缺损程度,通过诱导脑缺血耐受、减少神经细胞凋亡和降低水通道蛋白4/9基因和蛋白的表达,改善神经再生微环境。中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

Comparison of the pedalling performance induced by magnetic and electrical stimulation cycle ergometry in able-bodied subjects

The purpose of the study was to compare the mechanical power and work generated by able-bodied subjects during functional magnetic stimulation (FMS) vs. functional electrical stimulation (FES) induced ergometer training conditions. Both stimulation methods were applied at a 30 Hz frequency to the quadriceps muscles of 22 healthy able-bodied subjects to induce cycling for 4× four minutes or until exhaustion. FMS was performed via large surface, cooled coils, while FES was applied with a typical stimulation setup used for cycling. Significantly more (p < 10⁻³) muscular power was generated by FMS (23.8 ± 9.1 W [mean ± SD]) than by FES (11.3 ± 11.3 W). Additionally, significantly more (p < 10⁻⁶) work was produced by FMS than by FES (4.413 ± 2.209 kJ vs. 0.974 ± 1.269 kJ). The increase in the work was paralleled by a significant prolongation of time to cycling failure (181.8 ± 33.4 s vs. 87.0 ± 54.0 s, respectively, p < 10⁻⁵). Compared to FES, FMS can produce more intense and longer cycling exercise in able-bodied subjects. The differing dynamic behaviour of FMS and FES in the presented measurement setup might be related to stimulation induced pain and fatigue mechanisms of the neuromuscular system.     

Measurement of electric fields induced in a human subject due to natural movements in static magnetic fields or exposure to alternating magnetic field gradients

A dual dipole electric field probe has been used to measure surface electric fields in vivo on a human subject over a frequency range of 0.1-800 Hz. The low-frequency electric fields were induced by natural body movements such as walking and turning in the fringe magnetic fields of a 3 T magnetic resonance whole-body scanner. The rate-of-change of magnetic field (dB/dt) was also recorded simultaneously by using three orthogonal search coils positioned near to the location of the electric field probe. Rates-of-change of magnetic field for natural body rotations were found to exceed 1 T s(-1) near the end of the magnet bore. Typical electric fields measured on the upper abdomen, head and across the tongue for 1 T s(-1) rate of change of magnetic field were 0.15+/-0.02, 0.077+/-0.003 and 0.015+/-0.002 V m(-1) respectively. Electric fields on the abdomen and chest were measured during an echo-planar sequence with the subject positioned within the scanner. With the scanner rate-of-change of gradient set to 10 T m(-1) s(-1) the measured rate-of-change of magnetic field was 2.2+/-0.1 T s(-1) and the peak electric field was 0.30+/-0.01 V m(-1) on the chest. The values of induced electric field can be related to dB/dt by a 'geometry factor' for a given subject and sensor position. Typical values of this factor for the abdomen or chest (for measured surface electric fields) lie in the range of 0.10-0.18 m. The measured values of electric field are consistent with currently available numerical modelling results for movement in static magnetic fields and exposure to switched magnetic field gradients.