Three-dimensional current density distribution under surface stimulation electrodes.

Overflow to non-target tissue during FNS can be reduced by controlling current density distribution under surface stimulating electrodes. A method is introduced for the acquisition of 3-D current density distributions under complex surface stimulating FNS electrode geometries. The method makes use of a phantom model in which a conventional homogeneous model has been improved by adding a layer to simulate skin impedance properties, based on specific FNS parameters. Signal acquisition and processing circuits have been developed to simulate the process by which excitable tissue responds to external stimulation. In addition, a data analysis method has been introduced to allow for the characterisation of stimulation current intensity, electrode geometry and pulse waveform required to achieve target muscle activation, with minimal overflow and to avoid pain or burning. Measurements of integrated differential voltage corresponding to current density distribution acquired under electrodes of various geometries are presented in terms of 3-D attenuation coefficient maps as examples of the applicability of the method.