## 4 Discussion  
The aim of this study was to assess the effects of depth on the variability and accuracy of pSWE measurements at different levels of depth in an elasticity phantom. The results showed that pSWE has low variability at all depths (i.e., three cm, five cm, and seven cm) with CV ranging from 7 to 8.8%. However, our results revealed significant differences in pSWE measurements at depths of seven centimeters when compared to those at depths of three and five centimeters. In addition, pSWE at depths of three and five cm showed more accurate measurements when compared to the value reported by the manufacturers, but the same is not true at a depth of seven cm. This suggests that pSWE at a deep level could significantly differ from pSWE measured at the superficial area and should be considered in future studies. Similar findings to our study, which shows the variability in low pSWE measurements over different depth levels, have been previously reported [20], in which a low variability in measurement with CV of 6.8% was reported. However, in the aforementioned study, the depth level investigated was limited to a range between 1 and 4 cm. On the other hand, studies using more depth ranges of up to 6 cm have shown evidence for depth dependency with measurements [12; 25; 26]. In the previous studies, although the measurements were recorded in phantom settings, the shear wave technology used is different from the one in our study (2D-SWE vs. pSWE). This study also found no significant difference in pSWE at depths levels of three and five cm (\(p\) = 0.44). However, when depth levels increased, a significant difference was observed. pSWE measurements at depths of three cm and seven cm were significantly different (\(p\) = 0.024). Similarly, the variability significantly increased at depths of five cm and seven cm (\(p\) < 0.001). These findings concurred with what has been reported in other studies in terms of the remarkable effect of depth on elasticity measurements [12; 13; 14; 15; 18; 26]. These results would therefore suggest that pSWE measurements should be taken at a depth of no more than 4 cm, as beyond this depth, the variability would significantly increase. This suggestion is also in line with the recent guidelines and recommendations on thyroid SWE, which emphasize that the depth of acquisition should be between 4-5 cm [27]. The accuracy of pSWE measurements over different depth levels was also investigated in this study. pSWE measurements were accurate for depths of three and five cm, in which no significant differences were noted when compared to the manufactures' value. However, the effect of depth on measurements becomes obvious as the depth increases to seven cm, where overestimated measurements were observed. A possible explanation for this discrepancy in measurement over depth is the technical limitation of the transducer in terms of ROIs. The scanner used in this study has a default ROIs shape and size which cannot be adjusted manually. Therefore, ROIs increase simultaneously with depth. The current study has several limitations. pSWE measurements were obtained using a curvilinear transducer with a maximum depth of seven cm and, thus, deeper areas of more than seven cm were not assessed in this study. Likewise, pSWE measurements were taken from the background of phantom rather than spherical lesions; this was due to the poor image resolution with the use of a curvilinear transcoder at a depth of less than three cm in which the superficial lesions are placed (superficial lesions are at a depth of 1.5 cm). Furthermore, our ROIs were limited and constrained by the transducer's capability, which could have a negative effect on the depth measurements. Having another scanner with adjustable ROIs such as axipolere supersonic would overcome this limitation. Lastly, it is known that different factors have been reported to influence the measurements of pSWE.