Computed Tomography imaging of large cargos

Abstract: A feasibility study of the megavoltage (MV) computed tomography (CT) for imaging large cargos has been performed. The MV cargo CT system has imaging field of view of 3.25m in diameter. The system design includes a compact x-ray source and detectors that are already used in existing MV cargo radiography systems. Although compact x-ray sources provide limited x-ray output, it is shown that this output is sufficient to perform CT imaging of large cluttered cargo contents for which MV radiography may be inconclusive. The scan time is 60sec. at the x-ray dose levels lower than the permitted limit of 5mGy. A steel tank phantom with 2m diameter including different materials was used. The MV CT images were generated at 3.5MV, 6MV, and 9MV beam energies. The materials were separated based on their CT numbers as well as using dual energy subtraction method. The materials with close densities and x-ray attenuations such as W and U were reliably separated and quantified based on their measured CT numbers. The boundary conditions for the system design were determined including scan time, radiation dose, maximum object thicknesses, and spatial resolution. Based on the findings of this feasibility study, it is concluded that the MV cargo CT system with large field of view can be developed based on existing technology already used in MV cargo radiography. Such a CT system can be extremely useful as adjunct to existing MV radiography on resolving the special cases that cannot be resolved conclusively with radiography alone.