Straight-Line-Trajectory-Based X-Ray Tomographic Imaging for Security Inspections: System Design, Image Reconstruction and Preliminary Results

With the source and detector kept stationary while an inspected object moves between them, straight-line-trajectory-based tomographic imaging (SLTTI) has the capability to provide both digital radiologic images and tomographic images simultaneously, at high speed and with low-cost equipment. It has a promising future for fast security inspection for air cargo examinations. In order to fully investigate the feasibility of this approach, an experimental SLTTI platform has been developed. In this work, the system design is first presented. Then, a hybrid image reconstruction algorithm is proposed to obtain high quality tomographic images from the system to solve the limited-angle problem it represents. In the proposed algorithm, an initial reconstruction using either a direct filtered-backprojection (FBP) -type algorithm or the linogram technique is followed by a Gerchberg-Papoulis (GP) extrapolation to compensate for missing data. The GP extrapolation is implemented by the linogram technique, and constrained by a total variation (TV) minimization. By taking advantage of the FBP, the linogram and the TV techniques, we establish a fast and stable iterative reconstruction for the SLTTI. Finally, numerical simulations and realistic experiments are given, through which our proposals are validated to be feasible and effective. In the numerical examples, our algorithm can reduce the reconstruction inaccuracies for both high contrast and low contrast objects by factors of more than 15 and can generate nearly artifact free images as if complete projection data were available. A realistic experiment on a CatPhan©600 phantom suggests that high quality tomographic images can be achieved from 120-deg of source fan-beam angle with undersampled data. For a group of explosive simulacra using 90 projection views over 120-deg of source fan-beam angle, the reconstruction inaccuracy can be reduced to lower than 4%.