## 5 Discussion  
By focusing attention on the linear array probe for the first three scanners, the outcomes of both the automatic methods were mostly compatible with the mean observers' judgment. Moreover, there was not a significant discrepancy between DOP values in the two configuration settings. On the other hand, the results retrieved for the convex array probe showed a slight disagreement between the two automatic methods, despite the agreement between the TTM and the scores provided by the observers. Nevertheless, it can be seen that mean DOP values, assessed with both automatic methods, were usually higher in the set 1 configuration than the corresponding ones in set 2 for all the US scanners. This phenomenon may suggest that in the default preset condition, the scanner image pre- and post-processing seemed to improve the image quality to the detriment of the maximum penetration depth. For the vector array probe both automatic methods were generally compatible with the visually-assessed DOP values (excluding D scanner results). Lastly, a further consideration is that, independently from the scanner considered, among the US probes, the linear array showed the higher compatibility occurrence for the results obtained with the three different protocols. Furthermore, it can be noticed that such occurrence is mainly verified for C scanner. As regards the percentage error for the AdSTM and the TTM, for A and B scanners, the highest and the lowest percentage errors were observed for the linear array (4.3%) and for the convex array (1.1%) probes, respectively. On the other hand, for C and D scanners, considering the AdSTM only, the highest (5.0% and 3.1%) and the lowest (1.1% and 1.0%) percentage errors were always observed for the vector array probe, whereas, by considering the TTM for C scanner only, the highest and the lowest percentage errors were observed for the convex array (3.3%) and for the vector array (1.1%) probes. Furthermore, considering A, B, and C scanners, data confirm the equivalence between the two automatic methods in evaluating DOP uncertainty. In fact, the mean percentage error for the AdSTM was around 2.3%, whereas for the TTM was around 2.6%. Conversely, the mean percentage error for the visual assessment was around 4.5%, therefore confirming the higher dispersion of the naked eye method in DOP assessment with respect to the automatic methods. Nevertheless, it should be noticed that the lowest observers' absolute uncertainty was obtained for the linear array probe, which was likely due to the differences in probe technology and scanning format between the linear and vector/convex array models. Finally, the performances of the four US systems for the AdSTM, the TTM, and naked eye test were compared on the basis of the highest and lowest DOP mean values. For the linear array probe models (Figure 3a) the best performance was provided by the C scanner (set 2) for all the methods, while the worst performance was given by the A scanner (set 1) for the AdSTM and the D scanner (set 2) for the TTM and naked eye method. On the other hand, among the convex array probe models (Figure 3b), the highest DOP mean value was achieved for the A scanner (set 1) applying the automatic methods, whereas according to the observer judgement such value was obtained by the C scanner (set 1). Besides, the lowest DOP mean value could be found for the B scanner (set 2) in the AdSTM and naked eye method, while for the D scanner (set 2) it was the TTM. Moreover, for vector array probe models (Figure 3c) the AdSTM and naked eye method agreed in attributing the best performance to the D scanner (set 2), while for the TTM the C scanner (set 2) provided the highest depth of penetration mean value. Conversely, the worst performance was provided by the A scanner in the AdSTM and naked eye method (sets 1 and 2, respectively), while the D scanner (set 1) this was for the TTM.