Improved field localization in transcranial magnetic stimulation of the brain with the utilization of a conductive shield plate in the stimulator

In this paper, a carefully designed conductive shield plate is presented, which helps to improve localization of the electric field distribution induced by transcranial magnetic stimulation for neuron stimulation. The shield plate is introduced between a figure-of-eight coil and the head. In order to accurately predict the field distribution inside the brain and to examine the effects of the shield plate, a realistic head model is constructed from magnetic resonance image data with the help of image processing tools and the finite-element method in three dimensions is employed. Finally, to show the improvements obtained, the results are compared with two conventional coil designs. It is found that an incorporation of the shield plate into the coil, effectively improves the induced field localization by more than 50%, and prevents other parts of the brain from exposure to high pulsed magnetic fields.     

Magnetic stimulation of the motor cortex-theoretical considerations

The aim of this paper is to present a first approximation model for the computation of the electric fields produced in the brain tissues by magnetic stimulation. Results are given in terms of induced electric field and current density caused by coils of different radii and locations. Nontraditional coil locations and assemblies are also considered (multicoil arrangements). Model simulations show that a good control of the excitation spread can be achieved by proper positioning of the coil. It is also predicted that one of the major drawbacks of the technique, i.e., the poor ability to concentrate the current spread into a small brain area can be partially overcome by more effective coil positioning and/or assembly. Finally, some comparisons are made among the results obtained from electric and magnetic stimulation. This is thought to be of great help in the design of experiments aimed to understand the relative role of the different brain structures responsible for the motor response.     

Modeling of magnetic field stimulation of bent neurons

The authors consider a simple model of magnetic stimulation of a long bent neuron located in a semi-infinite volume conductor with a planar interface. It is shown that the stimulating coil characteristics (size, shape and location) and the neuron shape affect the location of the stimulation. The activating function, defined as the electric field derivative along the neuron, has two components. One component depends on the derivative of the electric field along the straight section of the neuron, and the other on the field magnitude. The maximal stimulation point is at the bent part of the nerve and its position depends on the nerve shape and coil parameters. The analysis also has shown a better performance (a stronger stimulus) for a double-circular (figure eight) coil than for a double-square coil     

Focal magnetic stimulation of an axon

The induced electric field produced by a circular coil during magnetic stimulation of an axon is derived from Maxwell's equations. The foci and virtual cathodal and anodal regions are predicted as a function of coil radius and orientation. Two virtual anode and cathode pairs are predicted, one lying outside the coil's perimeter and predominant in the far field, and one lying within the perimeter of the coil which may stimulate the axon when the coil and nerve are in close proximity. When the coil is positioned tangent to the nerve, an orientation commonly used in clinical magnetic stimulation, the foci of the predominant cathode and anode pair are extremely sensitive to changes in coil placement. In addition, the radius of curvature of the activating function, a measure of the size of the virtual cathode at threshold, is predicted to decrease with decreasing coil diameter and distance to the nerve. These predictions may help explain observed variability in measurements of conduction velocity and latency during magnetic stimulation of peripheral axons     

An inverse methodology for high-frequency RF coil design for MRI with de-emphasized B1 fields.

An inverse methodology for the design of biologically loaded radio-frequency (RF) coils for magnetic resonance imaging applications is described. Free space time-harmonic electromagnetic Green's functions and de-emphasized B1 target fields are used to calculate the current density on the coil cylinder. In theory, with the B1 field de-emphasized in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the central overemphasis effect caused by field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T (170 MHz) is calculated using an inverse methodology with de-emphasized B1 target fields. An in-house finite-difference time-domain routine is employed to evaluate B1 field and signal intensity inside a homogenous cylindrical phantom and then a complete human head model. A comparison with a conventional RF birdcage coil is carried out and demonstrates that this method can help in decreasing the normal bright region caused by field/tissue interactions in head images at 170 MHz and higher field strengths.     

Computation of electric and magnetic stimulation in human head using the 3-D impedance method

A comparative, computational study of the modeling of transcranial magnetic stimulation (TMS) and electroconvulsive therapy (ECT) is presented using a human head model. The magnetic fields from a typical TMS coil of figure-eight type is modeled using the Biot-Savart law. The TMS coil is placed in a position used clinically for treatment of depression. Induced current densities and electric field distributions are calculated in the model using the impedance method. The calculations are made using driving currents and wave forms typical in the clinical setting. The obtained results are compared and contrasted with the corresponding ECT results. In the ECT case, a uniform current density is injected on one side of the head and extracted from the equal area on the opposite side of the head. The area of the injected currents corresponds to the electrode placement used in the clinic. The currents and electric fields, thus, produced within the model are computed using the same three-dimensional impedance method as used for the TMS case. The ECT calculations are made using currents and wave forms typical in the clinic. The electrical tissue properties are obtained from a 4-Cole-Cole model. The numerical results obtained are shown on a two-dimenaional cross section of the model. In this study, we find that the current densities and electric fields in the ECT case are stronger and deeper penetrating than the corresponding TMS quantities but both methods show biologically interesting current levels deep inside the brain.     

An Inverse Methodology for High-Frequency RF Coil Design for MRI With De-emphasized$B _1$Fields

An inverse methodology for the design of biologically loaded radio-frequency (RF) coils for magnetic resonance imaging applications is described. Free space time-harmonic electromagnetic Green's functions and de-emphasized$B_1$target fields are used to calculate the current density on the coil cylinder. In theory, with the$B_1$field de-emphasized in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the central overemphasis effect caused by field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T (170 MHz) is calculated using an inverse methodology with de-emphasized$B_1$target fields. An in-house finite-difference time-domain routine is employed to evaluate$B_1$field and signal intensity inside a homogenous cylindrical phantom and then a complete human head model. A comparison with a conventional RF birdcage coil is carried out and demonstrates that this method can help in decreasing the normal bright region caused by field/tissue interactions in head images at 170 MHz and higher field strengths.     

透射电子显微镜原位磁场双倾样品杆的研制

本文简述了透射电镜磁场双倾样品杆的设计方案,展示了Philips/FEI透射电镜磁场双倾样品杆的研制成果.利用该样品杆可以产生100 Oe的连续磁场,也可以产生140 Oe以上的瞬间磁场.通过“U”型磁组件和样品杯的巧妙设计,尽可能的减小了电子束在横向磁场中的偏移量.     

A 3-D differential coil design for localized magnetic stimulation

A novel three-dimensional (3-D) differential coil has been designed for improving the localization of magnetic stimulation. This new coil design consists of a butterfly coil with two additional wing units and an extra bottom unit, both perpendicular to the plane of the butterfly coil. The wing units produce opposite fields to restrict the spread of induced fields while the bottom unit enhances the induced fields at the excitation site. The peak induced field generated by this new design is located at the center of the coil, providing an easy identification of the excitation site. The field localization of the new coil is comparable with that of much smaller coils but with an inductance compatible to current magnetic stimulators. Numerical computations based on the principles of electromagnetic induction and using a human nerve model were performed to analyze the induced fields and the stimulation thresholds of new coil designs. The localization of the coil design was assessed by a half power region (HPR), within which the magnitude of the normalized induced field is greater than 1/square root of 2. The HPR for a 3-D differential coil built is improved (decreased) by a factor of three compared with a standard butterfly coil. Induced fields by this new coil were measured and in agreement with theoretical calculations.     

Effect of surface boundary on neuronal magnetic stimulation

The effect of the surface boundary between free space and a conducting medium on the excitation properties of neurons by magnetic fields are analyzed. The electric field and the spatial derivative of the induced field generated by a coil mounted both parallel and perpendicular to the surface of a semi-infinite conducting medium were calculated using the method of images. An imaginary axon is located in the same relative position from the coil in both configurations and the excitation properties are compared. The calculations are expressed in terms of the activating function for the electrical stimulation of axons. The calculations indicate that the activating function for magnetic stimulation is biphasic as opposed to triphasic for electrical stimulation. The large spatial extent of the magnetically induced electric field compared to the electric field generated by point source electrode suggests a different mode of excitation for neuronal structures in the CNS. The field distribution have been verified experimentally and are important for the understanding of the mechanisms of magnetic stimulation of neural tissue.     

Effects of head shape on EEGs and MEGs

This paper presents results of computer modeling studies of the effects of head shape on electroencephalograms (EEG's) and magnetoencephalograms (MEG's) and on the localization of electrical sources in the brain using these measurements. The effects of general, nonspherical head shape on EEG's and MEG's are determined by comparisons of EEG and MEG maps from nonspherical head models with corresponding maps from a spherical head model. The effects on source localization accuracy are determined by calculating moving dipole inverse solutions in a spherical head model using EEG's and MEG's from the nonspherical models and comparing the solutions with the known sources. It was found that nonspherical head shape can produce significant changes in the maps produced by some sources in the cortical region of the brain. However, it was also found that such deviations of the head from sphericity produce localization errors of less than approximately 1 cm. No significant differences in the effects of such deviations on EEG's and MEG's were found. Finally, it was found that most such deviations do not cause a dipolar source which is perpendicular to the surface of the head model to produce a significant magnetic field; such a source produces zero magnetic field in a sphere.     

Toroidal coil models for transcutaneous magnetic stimulation of nerves

A novel design of coils for transcutaneous magnetic stimulation of nerves is presented. These coils consist of a toroidal winding around a high-permeability material (Supermendur) core embedded in a conducting medium. Theoretical numerical calculations are used to analyze the effect of the design parameters of these coils, such as coil width, toroidal radius, conducting layer thickness and core transversal shape on the induced electric fields in terms of the electric field strength and distribution. Results indicate that stimulation of nerves with these coils has some of the advantages of both electrical and magnetic stimulation. These coils can produce localized and efficient stimulation of nerves with induced electric fields parallel and perpendicular to the skin similar to surface electrical stimulation. However, they retain some of the advantages of magnetic stimulation such as no risk of tissue damage due to electrochemical reactions at the electrode interface and less uncomfortable sensations or pain. The driving current is reduced by over three orders of magnitude compared to traditional magnetic stimulation, eliminating the problem of coil heating and allowing for long duration and high-frequency magnetic stimulation with inexpensive stimulators     

Neuromagnetic localization using magnetic resonance images

Ionic flow associated with neural activation of the brain produces a magnetic field, called the neuromagnetic field, that can be measured outside the head using a highly sensitive superconducting quantum interference device (SQUID)-based neuromagnetometer. Under certain conditions, the sources producing the neuromagnetic field can be localized from a sampling of the neuromagnetic field. Neuromagnetic measurements alone, however, do not contain sufficient information to visualize brain structure. Thus, it is necessary to combine neuromagnetic localization with an anatomical imaging technique such as magnetic resonance imaging (MRI) to visualize both function and anatomy in vivo. Using experimentally measured human neuromagnetic fields and magnetic resonance images, the authors have developed a technique to register accurately these two modalities and have applied the registration procedure to portray the spatiotemporal distribution of neural activity evoked by auditory stimulation.     

头盔式动态可变多通道经颅磁刺激线圈的设计

为了实现真正意义上的动态可变多通道经颅磁刺激(TMS),首次提出了头盔式网状线圈的设计理念。基于该理念设计的TMS 线圈系统,可以在全头范围内实现多通道经颅磁刺激,而且在线圈定位、分辨率等方面得到极大改善,同时还可以实现刺激部位、面积、方式和强度的实时动态可变性。建立了人体头颅电磁模型来模拟TMS 线圈产生的诱导电流分布,同时还对线圈产生的诱导磁场分布进行了实际测量,模拟与实测结果均与理论值相符,证明了该设计的有效性与可行性。     

Frequency-related effects in the optimization of coils for the magnetic stimulation of the nervous system

Magnetic stimulation of the nervous system is a non-invasive technique with a large number of applications in neurological diagnosis, brain research, and, recently, therapy. New applications require engineering modifications in order to decrease power consumption and coil heating. This can be accomplished by optimized coils with minimized resistance. In this study the influence of some frequency-related effects (skin and proximity effect) on the coil resistance will be discussed, together with the role played by wire shape, wire section, and twisting effect. The results show that the coil resistance increases with frequency. As an example, for a 20-mm2 circular wire section, the skin effect in the typical frequency range of magnetic stimulator devices (2-4 kHz) increases the coil resistance up to about 45% with respect to its dc value. Moreover, the influence of the frequency is lower for flat wire sections and reasonably small helix twist angle of the coil.     

Influence of head tissue conductivity in forward and inverse magnetoencephalographic simulations using realistic head models

The influence of head tissue conductivity on magnetoencephalography (MEG) was investigated by comparing the normal component of the magnetic field calculated at 61 detectors and the localization accuracy of realistic head finite element method (FEM) models using dipolar sources and containing altered scalp, skull, cerebrospinal fluid, gray, and white matter conductivities to the results obtained using a FEM realistic head model with the same dipolar sources but containing published baseline conductivity values. In the models containing altered conductivity values, the tissue conductivity values were varied, one at a time, between 10% and 200% of their baseline values, and then varied simultaneously. Although changes in conductivity values for a single tissue layer often altered the calculated magnetic field and source localization accuracy only slightly, varying multiple conductivity layers simultaneously caused significant discrepancies in calculated results. The conductivity of scalp, and to a lesser extent that of white and gray matter, appears especially influential in determining the magnetic field. Comparing the results obtained from models containing the baseline conductivity values to the results obtained using other published conductivity values suggests that inaccuracies can occur depending upon which tissue conductivity values are employed. We show the importance of accurate head tissue conductivities for MEG source localization in human brain, especially for deep dipole sources or when an accuracy greater than 1.4 cm is needed.     

Magnetic coil design considerations for functional magnetic stimulation

Our studies have demonstrated effective stimulation of the bladder, bowel, and expiratory muscles in patients with spinal cord injury using functional magnetic stimulation. However, one limitation of the magnetic coils (MC) is related to their inability to specifically stimulate the target tissue without activation of surrounding tissue. The primary goal of this study was to determine the governing parameters in the MC design, such as coil configuration, diameter, and number of turns in one loop of the coil. By varying these parameters, our approach was to design, construct, and evaluate the induced electric field distributions of two sets of novel MC's. Based on the slinky coil design, the first set of coils was constructed to compare their abilities in generating induced electric fields for focal nerve excitation. The second set of coils was built to determine the effect that changes in two parameters, coil diameter and number of turns in one loop, had on field penetration. The results showed that the slinky coil design produced more focalized stimulation when compared to the planar round coils. The primary-to-secondary peak ratios of the induced electric field from slinky 1 to 5 were 1.00, 2.20, 2.85, 2.62, and 3.54. We also determined that coils with larger diameters had better penetration than those with smaller diameters. Coils with less number of turns in one loop had higher initial field strengths; when compared to coils that had more turns per loop, initial field strengths remained higher as distance from the coil increased. In our attempt to customize MC design according to each functional magnetic stimulation application and patients of different sizes, the parameters of MC explored in this study may facilitate designing an optimal MC for a certain clinical application.     

A transcranial magnetic stimulator inducing near-rectangular pulses with controllable pulse width (cTMS)

A novel transcranial magnetic stimulation (TMS) device with controllable pulse width (PW) and near-rectangular pulse shape (cTMS) is described. The cTMS device uses an insulated gate bipolar transistor (IGBT) with appropriate snubbers to switch coil currents up to 6 kA, enabling PW control from 5 micros to over 100 micros. The near-rectangular induced electric field pulses use 2%-34% less energy and generate 67%-72% less coil heating compared to matched conventional cosine pulses. CTMS is used to stimulate rhesus monkey motor cortex in vivo with PWs of 20 to 100 micros, demonstrating the expected decrease of threshold pulse amplitude with increasing PW. The technological solutions used in the cTMS prototype can expand functionality, and reduce power consumption and coil heating in TMS, enhancing its research and therapeutic applications.     

Precision stage using a non-contact planar actuator based on magnetic suspension technology

The precision stage using a novel contact-free planar actuator based on magnetic forces, magnetized force and Lorentz force, is suggested. In the promising magnetic structure, a mover is levitated by magnetized force between iron-core electromagnets attached under the upper-side of a stator and ferromagnetic plates belonging to the mover. And the mover is driven by Lorentz force that acts on permanent magnets with an identical polarity put under magnetic field by air-core coils. Namely, the mover is driven directly without any transmission mechanism, and does not need any auxiliary driver for its posture calibration. Then it is estimated that the proposed operating principle is very suitable for work requiring high accuracy and cleanness, or general-purpose nano-stage. In this paper, we discuss a driving principle of the planar system including the magnetic force generation mechanism, a framework for the force model, governing characteristics of the levitated plate, and a planar motion control of the constructed prototype. And experimental results are given to verify the derived theoretical model and the feasibility of the system.     

基于C8051单片机的金属探测器系统设计

为提升和完善金属探测器的性能,并且降低其成本,设计了一种基于单片机C8051F350单片机的金属探测的方案。通过采用平衡式线圈作为接收线圈来感应通电线圈周围磁场的变化,并将磁场的变化转化为电压的变化,经放大器AD620进行放大,在单片机内进行MD转换,并将测得的电压与预设的基准电压相比较来确定是否检测到了金属。本设计采用软硬件结合的方法消除干扰,提高探测器的性能,确保系统的精确性。