1. 3D Polyline must be selected.
  1. Use the Navigator panel to select the Z plane from which the feature starts.
  2. Click on the image to add as many points as you need.
  3. Navigate up and down through the planes to follow all features through the depth of the image.
  4. Press the green button to confirm the polyline, or the red button to delete it.

The region can be edited to modify its size, or to extend it along the Z planes or over the time points.
For this, use the Edit Region tool.
X and Y commands allow to modify the region bounds, while the Z (Planes) and the T (Time points) will extend the region in the 4D space.