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The hierarchy of three levels

1.VOI, consisting of ROIs in different slices

2.ROI, consisting of contours within a slice

3.Contour, one closed polygon

is reflected in the VOI user interface. Each level has a corresponding tool bar for object manipulations. For instance, when selecting the VOI button and shifting, the contours in all slices will be shift. When selecting the ROI button and shifting, only the contours in the current slice will be shifted.