1. A system for scanning the interior of a cargo container, comprising: a propagator device for producing a magnetic field at each of (I)positions adjacent a first side of a cargo container; a detector device for sensing magnetic field strength at each of (J) positions adjacent a second side of the cargo container; wherein the (J) positions are spaced apart in a direction substantially parallel to a Y axis of a Cartesian coordinate system, and wherein the propagator and detector devices are spaced apart in a direction substantially parallel to a Z axis of the Cartesian coordinate system a predetermined distance sufficient to permit passage of the cargo container between the propagator and detector devices in a direction substantially parallel to an X axis of the Cartesian coordinate system; and mapping means responsive to the detector device for processing signals indicative of the magnetic field strength at each of the (J) positions relative to the cargo container to generate a image representing contents of the cargo container caused by perturbations of the magnetic field strength caused by the contents of the cargo container.