TMS- coils and simulated magnetic field

Transcranial Magnetic Stimulation


2004-2007. Soluscience


Transcranial Magnetic Stimulation (TMS) is a technique for brain stimulation. As another techniques like ElectroConvulsive Therapy (ECT) or the implantation of electrodes into motor cortex, TMS excites neurones. But contrary to those techniques, TMS is painless and noninvasive. Indeed, neurones are excited by electrical current induced by a rapidly changing magnetic field. This field is created by the discharge of thyristor (during about 300 μs) into a coil i.e a copper winding surrounded by a water-cooled system. The current circulating through the coil is about 10000 A in order to allow magnetic field to cross the skull of the patient.

Using TMS is sometimes still empirical. The clinician who wants to stimulate a specific cortex area given an expected result (phosphene, modification of vision, motor effect) will perform several stimulations, scanning over the patient’s scalp to find the most important response.

We thus proposed in this project to develop a transcranial magnetic stimulation simulator : according to position and orientation of the coil and to the parameters of the stimulation, the simulator will give a modelling of the stimulation effects. Result will be mapped on MRI image of the patient.

The TMS simulator is composed of three main tools :

  • A numerical tool, to compute magnetic field generated by a given coil at any point of the empty-space. The aim is first to assess the magnetic field induced by the coil, then to provide a finite-element scheme boundaries conditions and finally to finite-element computation in free space.
  • A design tool, able to take into account the specificity of each winding copper shape. For a given contour, the user has only to define a 3D-parametrical curve and the expression of its derivative
  • An interface tool, providing some results such as the maps of magnetic and potential vector field, mapped on a 3D brain of the patient, and some interactions ( position/orientation of the coil w.r.t. the patient…)

TMS simulator