1. An apparatus for assisting with examining a body emitting wave beams, comprising: means for receiving said body to examine; sensor means for collecting said wave beams along a plurality of respective axes termed "acquisition lines," each of said axes being designated by at least two axis parameters, and for transforming said collected wave beams into signals defining an image of projections of part of said body, each of said signals representing intensity of one of said wave beams; means for determining differences between selected ones of said signals relative to said axis parameters; means responsive to said differences for detecting jumps in intensity of said signals, each jump being a critical point corresponding to an acquisition line tangential to a boundary of said body and wherein said axis parameters of said critical points identify a position of at least one portion of said boundary.