## 3 Results and Discussion
### 3D-CT Primary Images
The results for the horizontal layers from L15 to L20 are shown in Figure 5. Two lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb) were inserted into the two holes, while the third hole remained unfilled as shown in Figure 5a. Figure 5b shows the reconstructed images from L\({}_{off}\)15 to L\({}_{off}\)20. The two high-attenuation areas corresponding to the \({}^{206}\)Pb and \({}^{208}\)Pb rods can be clearly observed but they cannot be distinguished. The low-intensity area at the third hole corresponds to the vacant space. Figure 4: (**a**) Cross-sectional slice images of the CT sample in the layers from L9 to L14. (**b**) Reconstructed CT images from L\({}_{off}\)9 to L\({}_{off}\)14 of the off-resonance attenuation (atomic effect) with a resolution of 1 mm/pixel. Figure 5: (**a**) Cross-sectional slice images of the CT sample in the layers from L15 to L20. (**b**) Reconstructed CT images from L\({}_{off}\)15 to L\({}_{off}\)20 of the off-resonance attenuation (atomic effect) at a resolution of 1 mm/pixel. Since Layers L21 and L22 were located above the inserted rods within the CT sample, the LCS gamma-ray beam did not pass through these rods. Therefore, there was no characterization for the rods in the two reconstructed images, L\({}_{off}\)21 and L\({}_{off}\)22. The 3D-CT image was, in general, primarily made up of the 2D-CT images taken at various positions. Thus, the 22 measured 2D gamma-CT images were visualized together to create one 3D gamma-CT image. We used the MicroAVS data visualization tool [22] to create the 3D gamma-CT image with a resolution of 1 mm/pixel. Movie S1 shows the visualized 3D gamma-CT image. Figure 6 shows a shot of the 3D gamma-CT image, which was captured from the visualized 3D movie (see Movie S1 in the Supplementary Materials). We adjust a value of approximately 60% of the maximum intensity of the 2D gamma-CT images as a threshold limit for visualizing the 3D surface. Since the Fe or Al rods induced less atomic attenuation than the lead rods, this threshold limit ensured that only the lead rods appeared on the 3D surface. Intensity values greater (less) than the limit appeared (disappeared) on the 3D image. The five high-attenuation areas caused by the atomic process, which correspond to the positions of the five enriched lead isotope (\({}^{208}\)Pb and \({}^{206}\)Pb) rods within the CT sample, are clearly visible. The gamma-CT imaging technique is capable of clearly identifying the presence of the materials to be examined. In addition, we obtained the 3D gamma-CT image with a higher resolution and shorter acquisition time than the NRF-CT imaging technique. However, it lacked the isotope-selective capability needed to distinguish between different isotopes of the same element. Therefore, we utilized the 3D gamma-CT image as an additional information source to improve the quality of the 3D isotope-selective CT imaging based on the NRF transmission method. For the primary 3D NRF-CT image, we chose the layers at the vertical distances of \(z=3\), 11, and 17 mm, measured from the holder's bottom, to reconstruct the 2D NRF-CT images of L\({}_{ NRF}\)4, L\({}_{ NRF}\)12, and L\({}_{ NRF}\)18, respectively. One 2D NRF-CT (\(7\times 7\)) image was obtained for each horizontal row of rods (see Figure 1a). In our previous study [14], we reported the visualization of the reconstructed 3D images with a resolution of 4 or 8 mm/pixel for the horizontal or vertical plane, respectively, which were based on the atomic attenuation and the pure NRF attenuation. The measurement procedure, numerical equations, and 2D NRF-CT images were reported in [14]. If a 3D NRF-CT image were measured under the  
Figure 6: A shot of the three-dimensional visualization of the atomic attenuation (3D gamma-CT image). See coresponding Movie S1 in the Supplementary Materials.