The method of the invention comprises the stages of injecting an alternating signal AC (carrier) into at least one electrode positioned on the skin of the patient in a region of interest, measuring the signal induced into at least one electrode, extracting at least an envelope of the signal measured and determining the difference between said envelope and a reference signal. Said envelope may be extracted by demodulating the signal measured, and this demodulation may be carried out by way of an associated rectifier diode, in a preferred embodiment, and an RC filter. The reference signal may be an envelope corresponding to the demodulated signal of another electrode, the injected signal, earth or a fixed voltage value.
