1. A method of determining a motion vector of tissues in a biological medium, said method comprising the steps of: acquiring a sequence of echographical images at a given repetition rate, digitalising said echographical images and storing the so-digitalised images, defining a digital pseudo echographical pulse, effecting in a plurality of directions a convolution of said digital pseudo echographical pulse with said digitalised images and deducing therefrom at each point of said medium a plurality of pseudo echographical signals associated with said plurality of directions, determining an estimate over said plurality of directions of a phase shift between two successive pseudo echographical signals and deducing therefrom the magnitude and the direction of said motion vector.