Dragonfly provides you a tool to create a 3D ROI by drawing the ROI on two 2D slices in distance and then “interpolating” on all the between slices.

First of all, load your dataset and create a new ROI:

You can use any 2D ROI painter to draw on any slice of MPR view (XY, YZ, or XZ). Please note you have to select the ROI in the top-right list to work on it, and everything we do in this tutorial is on this same ROI object. 
The following example shows a ROI made by the Polygon painter on a slice on XY plane (slice #181):  

Then we scroll to another 2D slice and draw the ROI again. Following example shows the painting of ROI on slice #204 on XY plane. Now the ROI has  got some regions on two 2D XY slices (note that we keep working on the same ROI), which will be displayd as two straight lines  in YZ or XZ plane (green arrows):

Still with the ROI object selected in the top-right list, go to the Operations panel in Segment tab on left sidebar, choose Interpolate on Z axis (remember we painted on two XY slices) , and click “Apply”. This operation will change the ROI by auto painting all the slices between #181 and #204 (shown as below, XY slice #189 is now shown as painted, and on YZ plane the ROI between two straight lines is now “filled” ):  

Thus we have produced a 3D-shaped ROI from two 2D initial drawings using the Interpolate tool. 
Keep it in mind the two initial drawings of ROI (on two 2D slices in distance) have to overlap on the direction perpendicular to the slice. Otherwise the program won’t be able to interpolate. 
 Walnut data set courtesy of European Institute for Molecular Imaging (EIMI), Muenster, Germany  (http://www.uni-muenster.de/EIMI/).