## 1 Introduction  
Nuclear safeguards have a critical role in the non-proliferation of nuclear materials for reprocessing nuclear fuels and the control of nuclear materials. Nuclear resonance fluorescence (NRF) analysis [1] is a promising technology for nuclear safeguards and other nuclear applications [2; 3; 4]. Non-destructive inspection (NDI) for isotopes based upon NRF has recently attracted a lot of attention because of its capacity to identify shielded isotope substances such as fissile material content in spent fuel and commercial cargo [2; 3; 4; 5; 6; 7]. Because the NRF energy is specific to each nuclide, a nuclide of interest can be characterized by measuring the absorption of the incident gamma rays at the NRF energy or of the scattered gamma rays with an energy identical to the NRF energy. Energy-tunable quasi-monochromatic gamma rays generated by laser Compton scattering (LCS) [8; 9] are suitable for the NRF measurements. Furthermore, computed tomography (CT) based on the NRF transmission method [10; 11] with LCS gamma rays has been proposed as a novel technology for imaging an isotope of interest within massive materials [11]. The NRF-CT imaging technique makes it possible to distinguish an isotope of interest from other isotopes within the same element in a sample. Therefore, the isotope selectivity is one of its strengths. [PERSON] et al. [12] experimentally demonstrated the NRF-CT using an LCS gamma-ray beam. They obtained a two-dimensional (2D) NRF-CT image with a resolution of 5 mm/pixel for the \({}^{208}\)Pb isotope in a natural lead rod hidden inside an iron cylinder holder, together with a set of different materials (aluminum, iron, and a vacant area). In our previous study [13], we obtained a 2D isotope-selective CT image based on the NRF transmission method for the distribution of an enriched lead isotope (\({}^{208}\)Pb) and of another enriched lead isotope (\({}^{206}\)Pb) implied inside an aluminum cylinder holder. The 2D NRF-CT image had a resolution of 2 mm/pixel with a data acquisition time of 60 h [13]. As a result, \({}^{208}\)Pb and \({}^{206}\)Pb were clearly distinguished. In a subsequent study [14], we added another one-dimensional scan to obtain a three-dimensional (3D) NRF-CT image with horizontal and vertical resolutions of 4 and 8 mm/pixel, respectively. The data acquisition time was 48 h [14]. It is obvious that improving the resolution of NRF-CT images is crucial for realistic applications. To improve the image resolution while keeping a reasonable data acquisition time, upgrading the detection efficiency of the NRF measurement system and/or increasing the gamma-ray beam intensity are possible approaches. However, in practice, it is difficult to upgrade the detection system and/or the gamma-ray beams system of existing LCS facilities such as 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. In the current study, we proposed an alternative approach that applied a numerical treatment, called the fusion visualization (FV) technique [15; 16; 17; 18], to improve the NRF-CT image quality. In the case of the NDI for the hidden isotopes within an assembly volume, scientific visualization is representing complex raw data as 2D or 3D images to understand the shape of the hidden isotopes that may be overlooked by standard methods alone. The term \"data fusion\" refers to a numerical data processing technique that may give more consistent, accurate, and meaningful information by integrating multiple data sources. In the current study, we applied the FV technique to improve the quality of the 3D NRF-CT image that was obtained in our previous study [14]. The FV technique is based on the integration of various data sources. One of the sources is the primary 3D NRF-CT image [14], which supplies the desired isotope distribution, but its image quality needs improvement.