Apparatus and methods for screening inanimate objects for detecting concealed radioactive substances. The apparatus and method use a combination of at least one gamma-emission detector and the generation of a transmission image to non-invasively evaluate the contents of an inanimate object, such as a container, in order to determine the presence of absence of a radioactive substance. A variety of configurations of gamma-emission detectors and transmission detectors may be used to rapidly scan objects for the presence or absence of a radioactive substance.