1. A method for examining an inspection volume, the method comprising: generating, based on data representing attenuation of higher-energy radiation having a peak energy of at least 1 MeV passing through a portion of an inspection volume, a first image including a projection of the portion; generating, based on data representing attenuation of lower-energy radiation passing through the portion of the inspection volume, a second image including a projection of the portion; creating a dual-pixel image from the first image and the second image, wherein a pixel of the dual-pixel image, a pixel of the first image, and a pixel of the second image represent substantially the same portion of the inspection volume; selecting, from the dual-pixel image, a region of interest including at least a portion of an item of interest and an associated background region; determining a first basis material of the associated background region; selecting a first basis function that is derived from an attenuation characteristic associated with the region of interest based on the determined first basis material; and representing the region of interest in terms of an amplitude associated with the first basis function and an amplitude associated with a second basis function.