How to cumpute 3D binary region convex hull for image segmentation

hi dear all i have a 3D image of a ribcage

i need to segment the organs inside the abdomen (inside the ribcage), thus i want to calculate a binary mask like the bwconvhull but for a 3D image of the ribcage shown.... i have no idea... can anybody help? thanks. ( i dont want to use bwconvhull as it works on 2D only and i dont want to convert to 2D) i have matlab 2016a.