## 3 Results and Discussion
### Post-Multiply FV Method
Figure 8: _Cont._  
(IV) 2D-CT Image Overlapping  
We overlapped the primary 2D-CT images to obtain a 2D FV NRF-CT image as follows: the 2D NRF\({}^{\sim}-\) CT images of Layers L\({}_{\text{NRF}}\)4, L\({}_{\text{NRF}}\)12, and L\({}_{\text{NRF}}\)18 were overlapped with the 2D segmented gamma-CT images from L\({}_{\text{sg}}\).3 to L\({}_{\text{sg}}\).8, L\({}_{\text{sg}}\).9 to L\({}_{\text{sg}}\).14, and L\({}_{\text{sg}}\).15 to L\({}_{\text{sg}}\).20, respectively, according to the following equation:  
\[\left[\text{2D FV NRF}-\text{CT}\big{(}x_{i},\ z_{j}\big{)}\right]=\left[ \text{2D NRF}^{\sim}-\text{CT}\big{(}x_{i},\ z_{j}\big{)}\right]\times\left[ \text{2D segmented gamma}-\text{CT}\big{(}x_{i},\ z_{j}\big{)}\right] \tag{1}\]  
We labeled the resulting 2D FV NRF-CT images as L\({}_{\text{F}}\)v3 to L\({}_{\text{F}}\)v20. While the \({}^{208}\)Pb rod is clearly visible in the fused images from L\({}_{\text{F}}\)v3 to L\({}_{\text{F}}\)v8, as shown in Figure 9a, the Fe and \({}^{206}\)Pb rods are almost invisible. Figure 9b shows the fused images from L\({}_{\text{F}}\)v9 to L\({}_{\text{F}}\)v14. Only the \({}^{208}\)Pb rod can be seen clearly, but neither the Fe nor the Al rods are visible. Moreover, we can clearly pinpoint the location of the \({}^{208}\)Pb rod in the reconstructed images from L\({}_{\text{F}}\)v15 to L\({}_{\text{F}}\)v20, as shown in Figure 9c. The \({}^{206}\)Pb rod and the vacant area disappeared. Obviously, the noise in all 2D FV NRF-CT image backgrounds caused by using the post-multiply FV method for the primary data sources was totally eliminated from the reconstructed image background. Since the overlapping process of the post-multiply FV method was performed by multiplying the intensity value at each pixel in the 2D NRF\({}^{\sim}-\) CT by a value of 1 within the \({}^{208}\)Pb locations or by a value of 0 in the surrounding regions, the method preserved the locations of the isotopes of interest and completely eliminated the surrounding distortion and background noise. Furthermore, it kept the intensity value at each pixel constant, making this approach useful for isotope quantification. Figure 8: (**a–c**). Cross-sectional slice images of the CT sample in the layers from L3 to L20, in addition to the binary (0,1) 2D T\({}_{\text{r}}\) gamma-CT images. Figure 9: Reconstructed 2D FV NRF-CT images from (**a**) L\({}_{\rm FV}\)3 to L\({}_{\rm FV}\)8, (**b**) L\({}_{\rm FV}\)9 to L\({}_{\rm FV}\)14, and (**c**) L\({}_{\rm FV}\)15 to L\({}_{\rm FV}\)20 for the post-multiply FV method with the CT sample’s geometry for each layer group. (V) Image Visualization in 3D  
The 22 2D FV NRF-CT images were combined together using the MicroAVS data visualization tool to create the 3D FV NRF-CT image. We also adjusted a value within the intensity range of the 2D FV NRF-CT images as a threshold limit, so that only the intensity values greater than the threshold limit (\({}^{208}\)Pb rods) appeared on the 3D surface. Movie S2 shows a visualization of the fused CT-image of the NRF attenuation caused by the \({}^{208}\)Pb isotope rods within the CT sample (pure NRF) in three dimensions. Figure 10 shows a shot of the 3D FV NRF-CT image, which was captured from the visual-ized 3D movie (see Movie S2 in the Supplementary Materials). The visualization clearly shows the locations of the enriched lead isotope (\({}^{208}\)Pb) rods. In contrast, the rods of \({}^{206}\)Pb, Fe, and Al and the empty areas are not visible. We obtained an isotope-selective 3D FV NRF-CT image with a resolution of 1 mm/pixel for the distribution of an enriched lead isotope (\({}^{208}\)Pb) inserted with rods of different materials within a cylindrical sample 20 mm in height and 25 mm in diameter. Since the fused image has the same pixel resolution as the primary gamma-CT image, the image distortion and the background noise vanished, and the locations of the \({}^{208}\)Pb rods were clearly visible.