Fig. 1. Schematic of neutron exterior CT scan

Fig. 2. Schematic of external CT scan phantom. (a) Phantom; (b) scanning process; (c) sinusoidal image

Fig. 3. Images reconstructed by different algorithms. (a) FBP algorithm; (b) SART algorithm

Fig. 4. Calculation method of local directional difference ^[23]. (a) Coordinate system of local directional difference; (b) calculation method of local directional difference

Fig. 5. Calculation method of local directional difference in improved method. (a) Coordinate system of local directional difference; (b) calculation method of local directional difference

Fig. 6. Reconstruction results of different algorithms. (a) SART algorithm; (b) SART-TV algorithm; (c) SART-WDTV algorithm

Fig. 7. Comparison of reconstruction results of different algorithms. (a) SART-WDTV algorithm; (b) improved SART-WDTV algorithm

Fig. 8. Complete projected sinogram and reconstructed image of gear sample. (a) Complete projected sinogram; (b) reconstructed image

Fig. 9. Reconstruction images of gear sample by different algorithms. (a) Sinogram after symmetrical complement data; (b) FBP algorithm; (c) sinogram obtained by exterior CT scan; (d) SART algorithm

Fig. 10. Gear sample reconstruction image with different algorithms. (a) SART-TV algorithm; (b) SART-WDTV algorithm; (c) improved SART-WDTV algorithm

Fig. 11. Local magnification results of reconstructed images with different algorithms. (a) Complete data reconstruction; (b) SART-TV algorithm; (c) SART-WDTV algorithm; (d) improved SART-WDTV algorithm

Fig. 12. RMSE curves obtained by different reconstruction algorithms