How to automatically get the region of interest and crop the creases part of the finger?

I have undergo the morphology process and now I need to automatically crop the the creases part of the finger only. But I dont know how to get the region of interest automatically. I have seen about the boundarybox, regionprops but I still got confuse. Then, after get the ROI I need to mapped back to the grayscale image. Can anyone help me for the code? Kindly refer to the sample image and the morphology image that I have done.