How to get the CT value (HU) of ROI; and area of ROI using markup closed curve?

How to get the CT value (unit:Hu) of ROI for muscle and vetebra using 3D Slicer? In addition, i don’t ensure whether the value of area of ROI is correct using the method “markup-closed curve-Quantification-segment statistics”, and if click “segment editor-add-paint–segment statistics”, there is no value about the cross-area, but only the volume. Hope getting your help for addressing my confusion, thanks !