  Cargo imaging with Megavoltage (MV) radiography has important applications
for detecting illicit materials. It enables decomposing and quantifying
materials with different atomic numbers by imaging cargo at two different x-ray
energies, or dual energy (DE) radiography. The performance of both single
energy and DE radiography depends on beam energy, beam filtration, radiation
dose, object size, and object content. The purpose of this work was to perform
comprehensive qualitative and quantitative investigations of the image quality
in MV radiography depending on the above parameters. A digital phantom was
designed including Fe background with thicknesses of 2cm, 6cm, and 18cm, and
materials samples of Polyethylene, Fe, Pb, and U. The single energy images were
generated at x-ray beam energies 3.5MV, 6MV, and 9MV. The DE material
decomposed images were generated using interlaced low and high energy beams
3.5/6MV and 6/9MV. The x-rays beams were filtered by low-Z (Polyethylene) and
high-Z (Pb) filters with variable thicknesses. The radiation output of the
accelerator was kept constant for all beam energies. The image quality metrics
was signal-to-noise ratio (SNR) of the particular sample over a particular
background. It was found that the SNR depends on the above parameters in a
complex way, but can be optimized by selecting a particular set of parameters.
For some imaging setups increased filter thicknesses, while strongly absorbed
the beams, increased the SNR of material decomposed images. Beam hardening due
to polyenergetic x-ray spectra resulted in material decomposition errors, but
this could be addressed using region of interest decomposition. It was shown
that it is not feasible to separate the materials with close atomic numbers
using the DE method. Particularly, Pb and U were difficult to decompose, at
least at the dose levels allowed by radiation source and safety requirements.
