Roi objects are static definitions of a region of interest on a particular image. Its shape definition and the underlying pixel values are considered fixed upon construction, and hence its various properties and statistics are non-changing.
New instances of a Roi should be constructed everytime the definition of the region of interest or the underlying image pixel data has changed. The IGraphic.GetRoi method allows client code to quickly construct a new instance of a Roi based on the current definition of the graphic and the image it currently belongs to.