## 4 Conclusions  
The fusion visualization (FV) technique can be used as an effective numerical treatment to improve computed tomography (CT) imaging based on the nuclear resonance fluorescence (NRF) transmission method. We obtained two three-dimensional (3D) CT images for the distribution of an enriched lead isotope (\({}^{\text{208}}\)Pb) inserted into a cylindrical holder together with rods of various materials (another enriched lead isotope of \({}^{\text{206}}\)Pb, iron, and aluminum). One of the images was the isotope-selective 3D CT image based on the NRF transmission method, which had a relatively low resolution. The other was a significantly higher-resolution 3D gamma-CT image. The two images were measured at the beamline BL1U in the ultraviolet synchrotron orbital radiation-III (UVSOR-III) synchrotron radiation facility at the Institute of Molecular Science at the National Institutes of Natural Sciences in Japan. We generated a laser Compton scattering (LCS) gamma-ray beam with a maximum energy of 5528 MeV, which was able to excite the \(J^{\pi}\) = 1\({}^{-}\) state at 5512 MeV in \({}^{\text{208}}\)Pb. We used two LCS beams with different intensities and beam diameters for two experiments. First, we generated an LCS gamma-ray beam with a diameter of 1 mm and a flux of 0.7 photons/s/eV to obtain a 3D gamma-CT image with a resolution of 1 mm/pixel. Next, we generated an LCS gamma-ray beam with a diameter of 2 mm and a flux of 10 photons/s/eV to measure the 3D NRF-CT image with a resolution of 4 and 8 mm/pixel in the horizontal and vertical planes, respectively. We applied the FV technique using the post-multiply FV method between the two primary images to create a 3D isotope-selective fused CT image based on the NRF transmission method (3D FV  
Figure 11: (**a**) 2D NRF-CT images obtained via the post-multiply FV method, (**b**) the post-multiply FV method (without 2D gamma-CT image segmentation), and (**c**) the post-sum FV method for layer L4. NRF-CT) for the distribution of \({}^{208}\)Pb in the CT sample. The background noise in the fused 3D image was removed and the quantity of the isotope of interest was conserved. Although we also examined the direct overlapping method and the post-sum FV method, we concluded that the post-multiply FV method was the most suitable for isotope-selective CT imaging to be used in nuclear engineering and various nuclear applications. The following are available online at [[https://www.mdpi.com/article/10.3390/app112411866/s1](https://www.mdpi.com/article/10.3390/app112411866/s1)]([https://www.mdpi.com/article/10.3390/app112411866/s1](https://www.mdpi.com/article/10.3390/app112411866/s1)), Movie S1: Three-dimensional visualization of the atomic attenuation (3D gamma-CT image), Movie S2: Three-dimensional visualization of the fused CT image of the NRF attenuation caused by the distribution of \({}^{208}\)Pb (3D FV NRF-CT image). Conceptualization, H.O. ; data curation, K.A. and H.Z. ; formal analysis, K.A. ; funding acquisition, H.O., T.H. and T.S. ; investigation, K.A., H.O. and H.Z. ; methodology, K.A., H.O., T.H. and T.S. ; project administration, H.O. ; resources, H.O., T.H., T.S. and M.K. ; software, K.A. and H.Z. ; supervision, H.O. ; validation, K.A., H.Z., H.O., T.K., T.H., T.S., M.K., Y.T., M.F. and H.T. ; visualization, K.A., H.Z. and H.O. ; writing--original draft, K.A. ; writing--review and editing, K.A., H.Z., H.O., T.K., T.H., T.S., M.K., Y.T., M.F. and H.T. All authors have read and agreed to the published version of the manuscript. This research was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI under Grants 18H01916, 18H03715, and 17K05482. Not applicable. This work was performed at the beamline BL1U of the UVSOR-III Synchrotron Radiation Facility with the approval of the Institute for Molecular Science (IMS), National Institute of Natural Science (NINS), Okazaki 444-8585, Japan (Proposal No. 19-503 and 20-703). The authors declare no conflict of interest.