## 3 Results and Discussion
### \({}^{159}\)Gd vs. \({}^{99m}\)Tc Scintigraphic Imaging  
The GATE MC simulation for \({}^{99m}\)Tc and different photopeaks of \({}^{159}\)Gd were conducted separately, the output of the simulation (file format planar.mhd) was imported into the software VV Image Viewer version 1.4 to obtain the planar images, as illustrated in Figure 10.  
Figures 11 and 12 demonstrate the gamma spectra of \({}^{159}\)Gd and \({}^{99m}\)Tc detected from the root file of the GATE MC simulation; ROOT software version 6.26/10 was used for this process. Figure 10: Planar image acquired from GATE MC simulation for \({}^{159}\)Gd and \({}^{99m}\)Tc utilizing VV image viewer: (**a**) \({}^{159}\)Gd (58 + 363.54 keV), (**b**) \({}^{159}\)Gd (58 keV), (**c**) \({}^{159}\)Gd (363.54 keV), and (**d**) \({}^{99m}\)Tc (140 keV). The quality assessment of planar images obtained using ImageJ software for \({}^{99m}\)Tc and different photopeaks of \({}^{159}\)Gd were determined using Equations (2) and (3) to compute SBR and CV, as demonstrated in Figures 13 and 14. Figure 11: Gamma spectrum for \({}^{159}\)Gd detected from output GATE MC simulation root file, using ROOT software. Figure 12: Gamma spectrum for \({}^{99m}\)Tc detected from output GATE MC simulation root file, using ROOT software. Figure 13: Signal to background ratio for the image acquired from \({}^{99m}\)Tc and different photopeaks of \({}^{159}\)Gd using ImageJ software. The beta-emitting characteristics of \({}^{159}\)Gd ensure that it is an efficacious radioisotope for cancer oncotherapeutic applications [38; 39; 40; 41]. Radioembolization procedures of liver tumours necessitate quantitative imaging following therapy [42]. Currently, there are no studies regarding the use of \({}^{159}\)Gd as a treanostic radioisotope in this context. We used GATE MC simulation in this study to investigate the scintigraphic imaging possibility of \({}^{159}\)Gd for hepatic radioembolization. The \({}^{159}\)Gd photopeak selection was 58 vs. 363.54 vs. 58 + 363.54 keV. Figure 13 shows that the image quality using both photopeaks combined (58 + 363.54 keV) is better with the highest SBR; a higher SBR indicates that the images can provide superior spatial information. Figure 14 shows that the lowest CV is obtained using both photopeaks combined (58 + 363.54 keV); a lower CV indicates minor variation (better estimation). This observation can be explained by combining two photopeaks resulting in the highest count statistics. The hexagonal hole pattern of the collimator is visible in the \({}^{159}\)Gd acquired planar image because of \({}^{159}\)Gd emitting high-energy gamma rays, the thicker septa, and the larger hole size of the HEGP collimator used [43; 44] (Figure 10). The 58 keV photopeak of the \({}^{159}\)Gd gamma spectrum appears with significantly higher intensity than expected based on its emission intensities (Figure 11), because low-energy photons are more likely to penetrate the HEGP collimator's larger hole size. In comparison to \({}^{99m}\)Tc, in the \({}^{159}\)Gd planar image acquired using the two photopeaks in combination, SBR for \({}^{99m}\)Tc was 19.7% higher than for \({}^{159}\)Gd (Figure 13); CV was 53% lower than for \({}^{159}\)Gd (Figure 14). [PERSON] et al. (2019) employed the SIMIND Monte Carlo simulation code using \({}^{159}\)Gd to determine the energy windows for the triple energy window (TEW) scatter correction approach. However, it has been observed that 20% of the main energy windows with 3 and 6 keV sub-energy windows were best for the TEW method implementation in \({}^{159}\)Gd [45]. Furthermore, a similar pattern of results was obtained in our study, which found that using two energy windows during acquisition admits better planar image quality results than using a single peak energy window, as illustrated in Figures 13 and 14, respectively. The research findings suggest completing the following study phases, where cell cultures (in vitro) and animal experiments (in vivo) can be investigated to supplement the trials before moving into the clinical phase.