A method of performing a security inspection of a container including a plurality of objects includes irradiating the container with polychromatic x-rays, reconstructing a 4-D voxelized representation defining a momentum transfer spectrum, and generating a 3-D image by determining a single value at each voxel as a function of the momentum transfer spectrum. The method further includes segmenting the voxels into segments of contiguous voxels that map onto at least one object. The method also includes computing an aggregated momentum transfer spectrum over at least a portion of the contiguous voxels through at least one of aggregating the spectra of the contiguous voxels for each segment and defining a supervoxel that includes voxels with the object exclusively and voxels in a neighborhood, regardless of association with the object. The method further includes classifying the object as one of a threat and a non-threat based on the aggregated momentum transfer spectrum.