A description is given of a method of spectral analysis of a radio frequency ultrasonic signal reflected by a structure subjected to echographic examination, comprising the steps of: a) transmitting an ultrasonic excitation signal into a portion of said structure under examination; b) receiving a radio frequency response signal from said structure; c) applying a time-frequency transform to said radio frequency response signal, dividing the radio frequency response signal into a plurality of frequency bands; d) calculating a local spectral parameter from the values of the time-frequency transform.