Mihaela Cretu

Magnetic Stimulation of the Spinal Cord: Experimental Results and Simulations

This paper aims in interpreting the leg muscles responses recorded by electromyography during magnetic stimulation of the spinal cord by computing the electric field induced in the spinal cord and the nearby areas during this procedure. A simplified model of the spine was created and a Finite Difference Method algorithm was implemented in Matlab.

Magnetic coil design for evaluating the response of the spinal cord during magnetic stimulation

The purpose of this paper is to evaluate the response of the spinal cord and nearby area during lumbar magnetic stimulation. One of the most important issues about magnetic stimulation of nervous tissue is the lack of precision in the stimulated region. Based on the experimental measurements performed in cooperation with the Medical University of Vienna, the figure-of-eight coil configuration was proved to be not so efficient in spinal cord stimulation (its inability to specifically stimulate the target tissue, without activating the surrounding areas). In order to obtain the best results in stimulation, the electric circuit of the magnetic stimulator must be optimized (LRC series circuit). The most important is to establish an adequate design of the inductive coil in accordance with the medical application that is used. So, starting from these considerations, the present work aims to find a suitable design of magnetic coil capable to trigger stimulation of the spinal cord.

Assessment of the electric field induced in the human tissue during magnetic stimulation of the spinal cord

Purpose – The purpose of this paper is to estimate the electrical field induced in the spinal cord and nearby area during lumbar magnetic stimulation.Design/methodology/approach – The spinal cord is modelled as a continuous cylinder, while the vertebral column is also represented by a concentric interrupted cylinder. The coil used for magnetic stimulation is a figure of eight, whose centre is placed above T12‐L1 vertebras. The electrical field is induced and its derivative is computed using the finite difference method.Findings – Preliminary results suggest that magnetic stimulation may be able to induce a sufficiently intense electric field inside the spinal cord, leading to the direct activation of spinal nerve roots.Practical implications – If the spinal cord can be stimulated directly by magnetic stimulation, this technique can facilitate functional motor activities, including standing and stepping in paralyzed people, in a non‐invasive way.Originality/value – The authors revealed the fact that functi...

Evaluating the efficiency of stimulators used in magnetic stimulation of the spinal cord

This paper aims in evaluating the efficiency of magnetic stimulators capable of stimulating neural tracts of the spinal cord in healthy subjects. Our previous preliminary tests had shown that the commercial clinical magnetic stimulator Magstim Rapid2 was unable to activate nerve structures located in the spinal cord, with its maximum deliverable energy. In this study we evaluate the characteristics of magnetic stimulators designed and tested (by modelling) for this specific application and we show that the circular commercial coil is the most efficient applicable shape, and no improvements could be gained by changing the distribution of the windings of the magnetic coils. The main conclusion of the study was that a much higher initial voltage should be loaded by the stimulators capacitor in order to achieve activation of neural tracts within the spinal cord.

Modeling the activation of a non-homogenous nerve fiber by magnetic stimulation

In this paper we present a model that combines circuit analysis with Maxwells equations of electromagnetic theory and non-linear cable theory, to explain the action of the induced electric field upon a nerve fiber. The current source and stimulating coil are modeled as a series RLC circuit. The induced electric field distribution within a homogeneous cylindrical volume conductor modeling the arm is calculated for different time courses of the current. The effect of the induced electric field upon the nerve is determined with a cable model which contains active Hodgkin-Huxley elements. The possible non-homogeneities of the nerves electrical properties are also considered, and we assess their influence on the activation of the nerve.

Improved coil design for repetitive magnetic stimulation of the spinal cord

Purpose – The purpose of this paper is to evaluate the response of the spinal cord, the transmembrane potential, during lumbar magnetic stimulation, using a figure of eight coil. Design/methodology/approach – In order to obtain a precise stimulation of the spinal cord and not the nearby nervous fibres, the coil from the electric circuit of the magnetic stimulator is optimized. The new proposed design is based on the turns’ placement inside the coil, the number of turns required to produce activation. Once the coil configuration is established, the paper addresses other issues that need to be solved: reducing power consumption (the low efficiency of power transfer from the coil to the tissue is a major drawback) and reducing coil heating. Findings – The traditional commercial coils, used for magnetic stimulation in some preliminary experiments, had proved their inability to specifically stimulate the target tissue, without activating the surrounding areas and the low efficiency of power transfer from the c...

ORCAD implementation of a frequency response function using equivalent circuits

The paper describes the implementation in ORCAD of equivalent circuits, used to model an artificially created frequency response function. The function is implemented and graphically represented in Matlab. Next, we compute the input admittance/impedance of the two proposed equivalent circuits (that include controlled sources), and compare it to the initial function. Finally, we conclude that this method of synthesis of electrical models for frequency responses, approximated by rational functions, is correct.

Power factor compensation using OrCAD simulation. A new approach in teaching electrical engineering

Teaching electrical engineering with the help of simulation packages is very essential nowadays to explain/examine the circuit topologies and operation. The paper focuses on the power factor compensation in the three-phase electric circuits. The three-phase electric network, working on a known nominal power, will be simulated using OrCAD, a programme for electric circuit modeling and simulation.

Numerical modelling of a wind farm located in the south area of Romania through equivalent electrical circuits

Unexpected faults occurring in certain conditions on renewable energy sources connected to the power grid may result in severe damages, if capacitive load exists and consequently inductive power factor correction is applied with shunt coils. The paper presents some investigations on a case study.

Analysis of the activation of spinal nerves during magnetic stimulation of the lumbar area

This paper aims in estimating which nerve fibers can be activated during magnetic stimulation of the lumbar area of the thorax. For this purpose, we compute the electrical field and the activation function induced in the tissue. We used a simplified model, where the spinal cord is modeled as a continuous cylinder, while the vertebral column is also represented by a concentric interrupted cylinder. The general thorax is also modeled as a parallelipipedic domain that includes all these structures. The coil used for magnetic stimulation is a figure of eight, whose center is placed above L1-L2 vertebras.