1. A method for using natural cosmic muons as a radiation source to obtain muon tomographic images of an object, a cargo or vehicle under inspection, comprising: obtaining one or more cosmic muon images of an object, cargo or vehicle under inspection; subtracting a reference cosmic muon image of a reference object, cargo or vehicle that represents a background of the object, cargo or vehicle under inspection from an obtained cosmic muon image of the object, cargo or vehicle under inspection to obtain a subtracted image; and processing the subtracted image to determine whether the object, cargo or vehicle under inspection contains a target object that is absent from the reference cosmic muon image.