In one embodiment, a method of examining contents of an object is disclosed comprising scanning an object at first and second radiation energies, detecting radiation at the first and second energies, and calculating a function of the radiation detected at the first and second energies, for corresponding pixels. A pixel is a projection of radiation through the object onto the detector. The first functions of a plurality of pixels are grouped and a second function of the group is analyzed to determine whether the object at least potentially contains material having an atomic number greater than the predetermined atomic number. The second function may be compared to a third function, which may be a threshold having a value based, at least in part, on material having a predetermined atomic number. Systems are also disclosed.