1. A method of determining boundary pixels of a feature in a digital picture provided to a computer, the digital picture being a medical image and comprising a two dimensional orthogonal array of lines of adjacent pixels, the method comprising the steps of: (b) scanning a first line across the picture in a first direction from an outermost pixel of the picture towards the feature and calculating grayscale gradient across adjacent pixels until a significant gradient is determined; (c) confirming that the variation in grayscale gradient across adjacent pixels indicates a first true boundary pixel of the feature by ascertaining that grayscale gradient across subsequent pixels in a series of several pixels along the line is still significant; (d) iteratively repeating the steps (v) to (vi) of: (v) scanning next adjacent line in vicinity of said boundary pixel, and (vi) determining the boundary pixel of the adjacent tine by assessing the significance of grayscale intensity gradient along the row until, a failure to find a subsequent pixel occurs in which case; (vii) attempt scanning in an orthogonal direction by repeating steps (v) and (vi), and, if attempt fails, extrapolating to edge of image or to first line, until having determined the boundary around the feature to one side of the first line; (e) repeating step (d) on other side of the line until the boundary around the feature on the other side of the line is determined.