To maintain and ensure safe railroad operations new technologies must be developed for wood crosstie inspection before and after installation. Installed crossties are inspected at 25mph using a 2D fan beam backscatter radiography method. The Railway Tie Association forecasts 23,700,000 new crossties will be purchased in 2017. Currently newly fabricated wood crossties are not radiographically inspected for internal flaws and adequate penetration of degradation prevention treatments. There is a market need for a 3D radiographic inspection of newly fabricated railroad crossties. Building on the 2D backscatter technology, information about how radiation scatters within an object is valuable for 3D computed tomography. Transmission radiography methods are based on only the line attenuation between two points. Scatter radiography methods are the product of illumination beam attenuation, total scatter signal from the intersection of the illumination beam and detector field of view, and the attenuation of scattered radiation. Unique transmission, forward scatter, side scatter, and backscatter radiography perspectives of a translating object will be collected from each stationary X-ray and/or gamma ray fan beam illumination source. The reconstruction method iteratively converges the density, not the attenuation coefficient(s), of each voxel in the object region by comparing the predicted and measured transmission and scatter signals. Ranges of densities are correlated with the absorption and scatter cross sections of known materials such as air, wood, water, ballast/rock, Aluminum, Iron, and Lead. Signal differences of the transmission and scatter perspectives are combined to successively estimate voxel composition and density to minimize signal differences.

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