1. A method for displaying unseen areas in guided two dimensional (2D) colon screening, comprising: calculating segmentation and colon surface for a colon; calculating a centerline for said colon; designating as a first current focus point a current point of intersection of a first current portion of said centerline and a first current cross-sectional image of a colon in a first current image plane; and extracting a first colon surface portion in a first connected colon component containing said first current focus point marking voxels of said first colon surface portion as a first “seen” area.