## 4 Discussion
### Phantom Equivalence  
The lung tissue, nodules, and diaphragm in this study achieved radiodensities equivalent to those of the patient. Other studies have often struggled to replicate the attenuation properties of air-filled lung tissue in their phantoms due to the challenge of creating a tissue  
Figure 14: Sample CT slices of phantom. Top row images depict largest nodule. Bottom row images depict smallest lung nodule. (**A**) The lowest dose protocol of 80 kVp, 30 mAs, with slight mottle evident. (**B**) Highest dose protocol of 100 kVp, 352 mAs. Despite increase in noise and hence reduced SNR and CNR, radiologists deemed these images to be diagnostic for detection of lung nodules (arrows). scaffold to support lung structures and for structural integrity [19; 22]. However, this study addressed this issue by developing a lung shell containing air and pulmonary vessels, allowing for attenuation values of aerated lung tissue comparable to those of the patient. Despite this advancement, the design limited the flexibility of placing lung nodules within the airspace, as they had to be attached to either the vessels or lung walls. This constraint reduced the replicability of nodule positioning compared to the actual patient anatomy. In contrast, the skin, subcutaneous fat, muscle, heart, blood vessels, and bone had lower radiodensities, with 82%, 157%, 328%, 331%, 237%, and 71% differences in HU compared to their patient counterparts, respectively, whilst the pulmonary vessels and airway achieved 154% and 8% higher relative radiodensities. This represents a maximum HU error of \(-\)196 HU. Similarly, [PERSON] et al. [19] 3D printed a chest phantom using PLA at varied infill densities to replicate bone (100% infill), fat (40%), muscle (55% infill), lung (10% infill), vessels (70% infill), and nodules (50-60%), achieving marked differences in attenuations between the patient, with a maximum error of up to 505 HU. In contrast, in their chest phantom, [PERSON] et al. [49] utilised ABS to accurately replicate adipose tissue and PLA for muscle (95% infill) and lung (30% infill), with tissue-equivalent radiodensities. [PERSON] et al. [50] corroborated these findings, supporting the application of PLA for mimicking water or soft tissue equivalent materials and ABS for lower density tissues such as fat. A potential reason for such discrepancies is because these materials originate from different manufacturers who may incorporate different ingredients, additives, and colours, creating products with different properties [51]. Alternatively, this may be due to the methodological error and inaccuracies of the preliminary material investigations and selection of suitable infill percentages. For example, a noted limitation of this study is the assessment of material properties using different CT scanner and exposure parameters to those used in the study itself. Since material attenuation coefficients vary according to the energy of the incident beam [50], this may explain the lack of tissue equivalence observed in this study. Moreover, one of the major barriers limiting the applications of 3D printing in medical imaging phantoms is the lack of tissue equivalence of thermoplastic filaments across the full spectrum of CT energies. For example, a material may exhibit tissue equivalence at a specific kVp and mAs but may not at another set of parameters/energies [50]. This may be attributed to the filaments having a lower effective atomic number (Z\({}_{\text{eff}}\)) compared to real soft tissue. As higher Z\({}_{\text{eff}}\) increases photoelectric absorption, the attenuation and HU increase with lower energies, or in the case of soft tissue composed primarily of water, remain constant [52]. However, the lower Z\({}_{\text{eff}}\) of thermoplastic materials causes the opposite effect, such that reduced kVp leads to reduced HU [53]. Thus, future investigations should aim to keep the scanner and parameters consistent within their studies.