Safety monitoring for functional magnetic stimulation

Abstract

In the recent years the use of Functional Magnetic Stimulation (FMS) has gained growing importance as a complementary method to Functional Electrical Stimulation (FES). Unlike electrical stimulation, FMS relies on induction of a magnetic field with rapid intensity changes which bypass the skin¿s impedance and induce eddy currents in deeper tissues. Thereby they evoke action potentials on targeted nerve fibers whilst avoiding unpleasant perception in less conductive skin and subcutaneous areas. Their magnetic fields are applied via coils that are optimized to concentrate the field to specific anatomical structures. While the main design parameter of the coil is the shape of their magnetic field it is also important to consider the electrical characteristics of the target tissue and in particular the possible presence of metal implants which have significant influence on magnetic impedance and field distribution. Induction of eddy currents in metal implants (e.g. joint prostheses like artificial hip or osteosynthesis components) can lead to strong heat development which can lead to serious tissue damage and a severe health risk. In this investigation the electrical characteristics of three different FMS stimulation coils - one with a laminated magnetic iron core and two air coils - are examined and compared by recording the frequency dependency of their impedance. Further the coils were inserted into a parallel resonance circuit using an appropriate parallel capacitance. The deviation of the operating points were analysed when various standard orthopaedic implant components (e.g. Cobalt-Chrome hip implant sphere or Titanium cup of a hip implant) were placed along the coil¿s central axis with varying distances. Resonance frequencies for different set-ups range from 3kHz to 100kHz. Depending on the coil type results show a characteristic frequency range where the resonance frequency and voltage drop of the LC circuit was most affected by an implant insertion. Based on these results a metal detector was designed to identify potentially hazardous conductive materials within the magnetic field of an FMS stimulation device during application. The results also indicate a 3-fold increase of the resistance when the resonance frequency is increased from 3kHz to 10kHz, and a further increase of frequency leads to an exponential growth of the stimulation coil¿s resistance. These results have to be taken into consideration when designing an FMS stimulation system as the resistance leads to power losses and excessive heating of the stimulation coil.