1. A device for determining actual tissue layer boundaries of a body, from two or more ultrasound images acquired via a probe at adjacent positions of a surface of the body, the device comprising: a converter that separately converts said ultrasound images to depth signals, wherein a depth signal is obtained by summing intensities of a respective one of said ultrasound images along lines of substantially constant depth in the body; a detector that separately detects a set of candidate tissue layer boundaries for a respective ultrasound image by thresholding the depth signal obtained for said respective ultrasound image; a selector that separately selects from a set of candidate tissue layer boundaries for each respective ultrasound image a nearest candidate tissue layer boundary that is nearest to the surface of the body for the respective ultrasound image, wherein the selector further selects the nearest candidate tissue layer boundary only from among those candidate tissue layer boundaries that have a tissue boundary width exceeding a minimum tissue boundary width; and a processor that determines an actual tissue layer boundary from the nearest candidate tissue layer boundaries selected for each respective ultrasound image of the two or more ultrasound images.