Internal Boundary delineation

I want to delineate the lowest and internal boundary of the attached image like the sample image I upload it (green line). Can anyone help me how I can do it?

If the top screenshot is a binary image, it is not possible to derive the bottom image from it. If top is not a binary image, adjust its window and level so detail can be seen. Then we might have suggestions.

The top screenshot is a binary image. What if I change the pixel type of the image ? Do you have any suggestion ?