An autosegmentation/autocontouring method that may be used for quickly and accurately contouring the regions and boundaries around regions for the development of cross-sections that may be linearly disposed for the three-dimensional reconstruction of an image is described and claimed. The autosegmentation/autocontouring method includes at least four steps. The first step is to digitize a CT, MRI, or other suitable image and display it on a display screen. The two-dimensional image on the display screen will include gray-scale representations of the internal organs and tissue masses of the anatomical site through which the cross-section was made. The second step is to select the interior of a ROI and draw a polygon within the boundaries of the cross-sectional view of this ROI. This polygon could also be drawn in the interior of a cancerous mass or other diseased tissue. The third step of the method of the present invention is to expand the polygon in a novel manner by iteratively testing pixels of the image on the display outside of, but adjacent to, the pixels that the polygon currently subtends. Pixels will be added to the polygon if the value of a decision rule function has a predetermined value. The expansion of the polygon is continued until none of the pixels at the perimeter of the polygon can satisfy the decision rule. Once it is found that none of the perimeter pixels satisfy the decision rule, the perimeter of the polygon is considered the boundary of the ROI. And the fourth step is that the boundary of the ROI is computed and a contour is developed based on this boundary. This same process is repeated for other ROIs that the user may select.