## 4 Discussion
### Phantom Equivalence
Moreover, a larger variety of thermoplastic filaments, with different higher atomic additives or more similar molecular composition, need also be researched to better mimic the energy dependence of tissue attenuations [53; 54; 55]. Furthermore, simple block materials may present with different attenuations due to lack of external attenuating material as would normally surround the anatomy within the human body. Thus, photons incident on the sample blocks likely exhibit different X-ray energies due to reduced beam hardening and attenuation that would occur from the beam interacting with the surrounding tissue. Future studies are encouraged to improve this methodology by incorporating the sample blocks into simulating tissue environment such as a water bath, for example, which may deliver more reliable and transferrable HU measurements [53]. Additionally, thickness of a material influences the attenuation of a beam [56]; however, the sample blocks did not have the same size and thickness of the tissues they represented. Thus, sample material thickness should additionally be controlled for the improved selection of the tissue-equivalent materials. Manipulating infill percentages is a useful method that researchers employ to control HU in FDM printing due to the linear relationship between infill percentage and HU (Figure 2) [57; 51]. However, this method has been criticised for creating unrealistic texture materials with microscopic or macroscopic voids that lead to high deviations in attenuations, which may underlie the variations in HU reported for the same 3D printing filaments between publications [53]. Consequently, the air gaps present may contribute to the low Hounsfield Units achieved for some of the tissues measured in this study and the large variations of HU relative to the patient (Figure 8) [54]. Modifying infill percentages also creates solid shell artifacts visible at the boundaries of the materials [55], which possibly enhanced the contrast and visibility of the lung nodules in this study (Figure 6). The struggle to replicate the high attenuations of bone is well reported throughout the literature [22]. Although this study employed Glow-PLA to replicate bone, similar to [PERSON] et al. 's [32] phantom experiment, the HU was too low at 77 HU, compared to in their study. This can be attributed to the lower infill percentage used. Alternatively, higher density additives such as PLA doped with 50% iron (Fe-PLA) [58] or granite-PLA [50] have shown to produce HU in the higher ranges (-700-2000 HU), due to the increased occurrence of the photo-electric effect [59]. The phantom's fidelity to real patient tissues is reduced by the notable discrepancies in HU observed between its tissues and those of the patient. Inaccurate HU may contribute to varied ability to visualise the lung nodules relative to what may be in the actual patient scan due to altered signal and contrast-to-noise ratios. This may reduce the radiologist's confidence in the application of such a phantom to dictate appropriate protocols for the clinical context. Furthermore, if the phantom were applied clinically, the discrepant HU values could result in suboptimal exposure settings, lowering the sensitivity of lung cancer screening and increasing the risk of missing lung nodules. Additionally, this study did not consider other material properties such as tensile strength and elasticity to better mimic the true tissue characteristics of a real patient. Table 5 demonstrates the disparity between materials applied in this study and human tissues in terms of these mechanical properties as reported in the literature.