1. Processor for determining actual tissue layer boundaries of a body, from two or more ultrasound images acquired at adjacent positions of a surface of the body comprising a converter for converting said ultrasound images separately to depth signals, wherein a depth signal is obtained by summing intensities of one of said ultrasound images along a line of substantially constant depth in the body, a detector for detecting a set of candidate tissue layer boundaries for an ultrasound image by thresholding the depth signal obtained for said ultrasound image, a selection means for selecting from a set of candidate tissue layer boundaries a nearest candidate tissue layer boundary that is nearest to the surface of the body, and a processing means for determining an actual tissue layer boundary from the nearest candidate tissue layer boundaries obtained for the two or more ultrasound images.