Mapping a part of a 2D image to its correspondent in 3D

Hello Everyone,
I am very new to open3D and I am trying to map a part of a 2D image of the body (e.g. the chest in .jpg image of full body) to its correspondent in the .ply file of the full-body 3d point cloud. Do you guys have any idea that how I can detect the chest area in both 2D and 3D images? Let’s say I successfully cropped these areas in both 2D and 3D images. How can I map the 2D into the 3D file in python?

Thanks a lot in advance,