How to scan your body in 3d?

In S. N. Goyenka Vipassana meditation you have to scan your body from head to foot and foot to head. Now I take my attention or [*ध्यान* *dhyaan*] at top point of my head, so it’s a 2d point now I move from this point in 2d on surface of my body and I can either go from my front side or from back toward my foot. I'm not getting how to scan my body in 3d? And if I scan in 2d its take too much time to scan whole body if I go inch by inch. People working in this mediation may get my point what I’m trying to ask.