model to model distance --> one distance

Operating system: Windows 10 Pro
Slicer version: 4.10.2
Expected behavior: calculate distance between model in specific region
Actual behavior: I have a ultrasound volume with Volume S and Volume T segmented. with model to model distance, I can computed the distance and show it in a colormap. However, I want the closest distance between the two margins but only computed at one side. This, because Volume T is at the surface of Volume S, so the clostest margin will be 0. But I want the closest margin to the surface at the other side of Volume S.

Is there a way to select a region of the surface of Volume S, to compute to closest distance?

Thank you very much!