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; utilizing a 2D image reading process of observing an axial cross-sectional image of said colon on an axial image plane for: marking in a current cross-sectional image of a colon in a current image plane, a current point of intersection of said centerline and said current image plane as a focus point; displaying said current cross-sectional image; extracting a colon surface portion in a connected colon component containing said focus point; marking voxels of said colon surface portion as “seen”, wherein said step of marking said voxels of said colon surface portion as “seen” comprises coloring in a distinctive color; and fading said distinctive color in accordance with the distance of said respective portions of said centerline from said current image, said distance being in terms of sequence progression along said centerline.