1. An apparatus for extracting a boundary of an object in an image, comprising: a position determining unit, configured to determine a start point and an end point of the boundary of the object in the image and to determine a position of a reference point relevant to the start point and the end point; a first direction determining unit, configured to determine a first direction of the boundary; a second direction determining unit, configured to determine a second direction intersecting the first direction; a gradient map obtaining unit, configured to determine in the image a first region comprising the start point, the end point and the reference point and to obtain a gradient map of the first region; a gradient attenuating unit, configured to determine at least one second region on at least one of two sides of the reference point along the second direction and to attenuate in the gradient map the gradients of the second region; and an extracting unit, configured to extract a boundary between the start point and the end point based on the attenuated gradient map to obtain the boundary of the object.