X-ray computed tomography (CT)systems can produce high resolution images, in which small (sub-millimeter) features can be detected. This requires the X-rays to sufficiently penetrate the object and interact strongly enough to produce measurable attenuation. Low atomic number (low Z), low density objects shielded by high atomic number (high Z) materials result in X-ray reconstructions that lack sufficient contrast to differentiate interior features from noise and reconstruction artifacts. Fast neutron CT offers complementary information to X-rays with superior penetration through high Z shielding and with less severe beam hardening artifacts. However, spatial resolution in X-ray imaging systems is generally superior to that of fast neutron imagers. Here, we quantitatively compare these two complementary modalities to demonstrate the ability to observe small feature locations within two multi-material objects. Quantitative measures include calculation of image gradient at material edges, contrast-to-noise ratio, and F1 score.
Material identification is challenging for X-ray Computed Tomography (CT) when objects of interest composed of low atomic number (Z) elements are shielded by dense materials. Fast neutron CT (FNCT) can compensate for this shortcoming by providing both penetration through high-Z materials and good contrast in low-Z materials. Here we investigate improvements in X-ray CT feature identification using information from fast neutron imaging. To demonstrate the complementarity of X-ray and FNCT, simulated CT data sets were generated for two heterogenous, nested, cylindrical phantoms using the Monte Carlo N-particle (MCNP) transport code for both imaging modalities. Xray radiographs were simulated for polychromatic 300 keV and 9 MeV e-Bremsstrahlung X-ray sources, while a generic Gaussian D-T source spectrum [En(pk) = 14.10 MeV, w/ FWHM ~ 0.75 MeV] was used for the neutron radiographs. A total of 360 projections taken at 1° intervals were simulated for both modalities and phantoms. All projection data were reconstructed with filtered-back projection (FBP) using the Livermore Tomography Tools (LTT) code. Our results indicate improved material discrimination and resolution of certain features with combined X-ray and neutron CT data sets.
Fast neutrons accessible from 14-MeV D-T neutron generators have higher transmission through high-Z materials compared to radiography X-rays due to a more uniform attenuation as a function of material Z. These neutrons can therefore image low-Z materials even when shielded by high-Z materials. The constraints in portable fast-neutron digital imaging systems include limited neutron output, limited light produced by neutron imaging scintillators, and lower resolution images due to large source spot size and required scintillator thickness. In addition, digital panel dark-noise counts can be 100x higher than the image signal, and variations across the panel can also be comparable to this signal. We will discuss recent efforts to develop a portable neutron-radiography system, including improved neutron scintillator, mitigation of digital panel noise, and new portable D-T neutron generators. We will also present MCNP efforts to model the system, including neutron scattering effects.
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