1. A method of determining the 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 the phase shift between two successive pseudo echographical signals and deducing therefrom the magnitude and the direction of said motion vector.