An apparatus for defining the lumen-intima interface and media-adventitia interface of a blood vessel, for example the carotid artery, comprises a means for acquisition of a two-dimensional representation, generated by an ultrasonographic machine, of a longitudinal or transverse cross section of the vessel. In the two-dimensional representation a region of interest (ROI) is defined comprising the interface lumen-intima and the interface media-adventitia of the vessel. In the ROI N search paths are then defined from the inside to the outside of the vessel. Along each search path i the values f(n, m) are taken of the two-dimensional representation, which are then subject to filtering obtaining filtered values. This allows retrieving discontinuity points of the two-dimensional representation and then localizing the lumen-intima and media-adventitia interfaces through further logical operations.