1. An image processing apparatus comprising: a data acquisition device for acquiring image data of a subject including a target bone; and a data processor for segmenting the image data into a plurality of segments, determining one of the plurality of segments as a target image based on image characteristics of the target bone, determining a longitudinal section of the target image, the longitudinal section passing through a center of the target image, and measuring a length of the target bone based on the longitudinal section.