## 4 Discussion
Additionally, the mean HU value of the sample with a 5.5 cm diameter (\(-\)582.4 HU) was found to be significantly lower than the mean HU values of the other two samples (\(-\)589.1 HU and \(-\)587.4 HU). Although statistically significant, this difference corresponds to a relative difference of 1.1% and does not represent a critical change. The STD\({}_{\rm z}\), on the other hand, was observed to change arbitrarily among the samples, suggesting that it does not depend on the dimensions of the print job. The reproducibility of the print job was analyzed, and similar HU values were consistently obtained for the repeated printing jobs at 210 \({}^{\circ}\)C, 230 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C, as illustrated in Table 5. The most significant absolute difference occurred between the first and second printing jobs performed at 230 \({}^{\circ}\)C with a 60% flow rate, corresponding to a relative difference of 4%. The majority of samples exhibited HU values remaining within one standard deviation of the average HU, recorded as \(\pm\)5.9 at most. These findings indicate that the printing jobs are reproducible, as supported by strong correlations (R\({}^{2}\) > 0.99) between the flow rate and mean HU at all printing temperatures, as depicted in Figure 6.  
The HU range achieved in this study suggests that the LW-PLA filament is capable of replicating the radiological properties of most tissues and organs, as illustrated in Figure 7. This excludes only compact bones and low-density portions of the lungs, which have HU values beyond +350 HU and around \(-\)900 HU, respectively [30; 31]. Nevertheless, our findings suggest that low-density lung-equivalent samples could be successfully achieved via the further adjustment of the printing parameters, especially the print speed. The selection of a layer thickness higher than 0.15 mm can also contribute to a further reduction in the density, as reported in the literature [12]. Finally, a slight reduction in the infill percentage could be considered to achieve lower HU values while ensuring that the homogeneity (i.e., STD) of the sample remains at an acceptable level. Importantly, this study demonstrated a significantly improved homogeneity in the lower Hounsfield Unit (HU) range compared to studies where a similar HU range is achieved by controlling the infill percentage. For instance, [PERSON] et al. used a high-impact polystyrene (HIPS) filament in their study to 3D print low- and high-density lung inserts with reduced infill percentages. Following CT imaging with a resolution of 0.5 mm \(\times\) 0.5 mm \(\times\) 0.4 mm, they reported mean HU values comparable to those of a commercial phantom, recorded as \(-\)826 HU and \(-\)483 HU. However, the STD\({}_{\rm xy}\) value was found to be \(\pm\)120 HU for the 3D-printed lung low insert, which is far beyond the \(\pm\)17 HU observed in the commercial phantom [19]. Other studies also reported similar limitations due to a reduced infill percentage [21; 22; 23; 24; 29; 18]. [PERSON] et al., on the other hand, conducted a similar study with a variety of filaments, including regular PLA, photoluminescence PLA, and ABS, for printing soft-tissue-equivalent inserts as an alternative to a commercial Gam-mx phantom. Differently from former studies, they conducted a CT scan with a relatively lower spatial resolution of 1.37 mm \(\times\) 1.37 mm \(\times\) 2 mm and reported STD\({}_{\rm xy}\) values between \(\pm\)11 HU and \(\pm\)14 HU for 3D-printed inserts with mean HU values between \(-\)580 HU and \(-\)900 HU [14]. [PERSON] et al. designed and printed gyroid structures with different geometric parameters and obtained \(-\)874 HU samples with an STD\({}_{\rm xy}\) of \(\pm\)23 HU following a CT scan at 140 kVp with a spatial resolution of 1.17 mm \(\times\) 1.17 mm \(\times\) 3 mm [32]. These results highlight the impact of different CT imaging protocols, especially on the STD\({}_{\rm xy}\) values of the scanned objects. It is known that image noise can be significantly reduced by selecting lower spatial resolution settings.