Described herein are systems and methods for efficient and accurate, automated detection of a region of interest interior to the ribcage from an in vivo mammalian image. It is found that efficient, automated identification of the region of interest interior to the ribcage can be achieved by the use of both a bone distance mask and a surface distance mask. The technique solves the problem of accurate and fast identification of the region of interest for a wide range of sizes and shapes of mammals, e.g., small mammals such as mice.