Regard to drawing ROI & background.

Operating system:
Slicer version: 5.2.2
Expected behavior:
Actual behavior:

I have question regarding drawing 11 ROIs around the Catphan phantom, but im wondering:

  1. Is it necessary to draw a background around ROI? If so, could you please guide me on how to do it effectively?
  2. Is it required to draw the ROIs to cover the entire area 100%, or can there be some areas that are partially missing? I would like to know if this would have any impact on the resulting analysis.

Thank you in advance for your valuable support and guidance.