Abstract: An impedance tomography apparatus is disclosed for the measuring of the electrical activity of excitable tissue. The apparatus comprises a plurality of signal transfer devices adapted for transferring an input signal to a body and receiving an output signal from a body. An input signal generator is adapted to generate an electrical input signal and apply it to at least one of the signal transfer devices. A measurement device is arranged to measure the output signal from the body at least one or more of the remaining signal transfer devices, wherein the input signal generator is adapted to generate an input signal of frequency greater than 100 Hz and less that 5 kHz. This allows the depolarisation of nervous or muscle tissue to be imaged.