A method of detecting the extent of a lung nodule in a scan image comprises fine segmenting (14) a region around the nodule into foreground and background, filling holes within foreground areas (16), region growing the foreground (18) to identify an initial region of interest, determining a mask (20) by enlarging the initial region to contain background and to exclude other foreground regions, determining a spatial map (26) of connectivity within the mask to a point within the initial region, region growing (28) within the mask and determining the maximum edge contrast (30) during region growing. The region of maximum edge contrast identifies the extent of the nodule (32). The position of the nodule may a seed point identified by the user (12). Most preferably, an optimum seed point is determined by iterative determination of seed points (22, 24) so that the determined extent of the nodule is substantially independent of the precise location of the seed point identified by the user.