1. A system ( 100 ) for extracting an object (Ob) from a source image, said object being delineated by a contour (C), the system ( 100 ) comprising: a gradient unit ( 110 ) for computing the source image gradient field, based on the source image, a smoothing unit ( 120 ) for smoothing the source image gradient field, and an integration unit ( 130 ) for calculating an object image by integrating the smoothed source image gradient field, thereby extracting the object (Ob) from the source image, characterized in that at each point of the source image, the smoothing is defined by a 2-dimensional convolution kernel which is a product of a first 1-dimensional convolution kernel in the first direction substantially parallel to the contour (C), and of a second 1-dimensional convolution kernel in the second direction substantially normal to the contour (C).