Contactless impedance measurement by magnetic induction--a possible method for investigation of brain impedance.

Using a primary coil that induces eddy currents in a volume conductor, and a pair of secondary coils configured to form a differential transformer, it is possible to measure the conductivity of a volume conductor by the magnetic fields of the induced eddy currents. This method is especially favourable for measuring impedance of the brain, as the low conductance of the surrounding skull interferes only very slightly with the measuring process. In a simulation experiment a life-size skull model was filled with NaCl solutions of different concentrations in each half, so as to model the impedances of a normal and an oedematous hemisphere. The impedance differences could be clearly detected by a coil system of 25 mm diameter.