Analyse des Magnetfeldes für Experimente mit magnetischer Rückenmarksstimulation

Magnetic stimulation of the spinal cord is a new, painless and non-invasive method which is expected to allow simple movements in paralyzed people. To achieve that goal, neuronal spinal networks should be activated which enable standing or stepping-like movements. Stimulating the spinal cord by electrical pulses with implanted electrodes is currently the objective of several international research groups. Electric stimulation for paralyzed rats and the activation of stepping movements have already been demonstrated succesfully. The stimulation by induction, which means using an external applyied time-varying magnetic field, also produced stepping like movements in experiments by the Pavlov Institute in St.Petersburg. However, the site of excitation in the spinal area was not fully clear. Objectives of this thesis were the simulation of the electric field in the vicinity of spinal nerves by magnetic stimulation and the experimental evaluation. Stimulations were done by commercialised magnetic stimulating devices, as used in neurology for magnetic brain stimulation. The round application coil was used to stimulate the spinal cord of cats. Via a mathematical formulation of the field problem, induction measurement and considerations for the functionality of the stimulator, the resulting field could be identified. With the help of MRT-images of a cat a three dimensional model of the spinal cord area from the segments T13 to L6 was constructed. Different tissue compartments were included. By using finite-elements-software the induced field components were simulated. The resulting field distribution was plotted in the spinal matter and in the tissue around the bony spinal cord. Focal intensity distributions with different coil positions were discussed and interpreted in the context of previous experimental results. Additionally, an experiment with different coil positions at the cat's spinal cord was carried out. The muscle answers of the cat were recorded. The experiments gave insight about the required field intensity inside the tissue for succesful stimulation. Also the influence of different coil positions on the muscle excitation got clearer. Unfortunately the exact site of stimulation could not be found, since the direction of the axons in the electric field plays an important role in excitation process, and it was not possible to modell single nerv cords. However the calculation gives an insight which field distribution arises inside the body, by the application of an external magnetic field. Based on this knwoledge further descriptions or calculations can be found to identify the most effective stimulation points.