  Most material discrimination in security inspections is based on dual-energy
x-ray imaging, which enables the determination of a material's effective atomic
number (Zeff) as well as electron density and its consequent classification as
organic or inorganic. Recently phase-based "dark-field" x-ray imaging
approaches have emerged that are sensitive to complementary features of a
material, namely its unresolved microstructure. It can therefore be speculated
that their inclusion in the security-based imaging could enhance material
discrimination, for example of materials with similar electron densities and Z
eff but different microstructures. In this paper, we present a preliminary
evaluation of the advantages that such a combination could bear. Utilising an
energy-resolved detector for a phase-based dark-field technique provides
dual-energy attenuation and dark-field images simultaneously. In addition,
since we use a method based on attenuating x-ray masks to generate the
dark-field images, a fifth (attenuation) image at a much higher photon energy
is obtained by exploiting the x-rays transmitted through the highly absorbing
mask septa. In a first test, a threat material is imaged against a non-threat
one, and we show how their discrimination based on maximising their relative
contrast through linear combinations of two and five imaging channels leads to
an improvement in the latter case. We then present a second example to show how
the method can be extended to discrimination against more than one non-threat
material, obtaining similar results. Albeit admittedly preliminary, these
results indicate that significant margins of improvement in material
discrimination are available by including additional x-ray contrasts in the
scanning process.
