## 3 Results and Discussion
### Simulated Images with MLT and Short-Lag F-DMAS
As opposed to the CR and CNR trends, the sSNR decreases when the maximum lag increases; however, the total variation is quite small: it settles between 1.3 and 1.7 for lags ranging from 3 to 63, and the highest observable difference between the sSNR at lag 1 and at lag 63 is \(\sim\)0.9 for 12-MLT in scenario 1 (Figure 7 sSNR-a). Overall, for the sSNR trend, it is hard to  
Figure 7: CR (**top** row), CNR (**middle** row), and sSNR (**bottom** row) trends, measured on the cyst-phantom images with varying maximum lag, in the different scenarios of Table 1: (**a**) both the number of TX beams and the angle among them varies (as in SLT or 4/6/8/12-MLT); (**b**) only the number of TX beams varies (\(N_{TX}=1/4/6/8/12\)), while the angular distance among them is fixed to the one of 12-MLT; (**c**) the number of TX beams is set to four, while the angular distance among them is the one of 4/6/8/12-MLT (in this last case, the legend refers to the number of TX beams which would determine the angular distance). clearly notice a dependence on the number of TX beams or their angular spacing, as in all scenarios, the curves are very similar. Finally, to better highlight the short-lag F-DMAS contrast and sSNR performance, an example of images obtained in the classic 4-MLT configuration (i.e., scenario 1, Table 1) is presented in Figure 8. The figure shows an improved image quality achieved by F-DMAS as compared to DAS. Thanks to the higher lateral resolution, the cyst borders are always more clearly visible with F-DMAS, even when the maximum lag is limited to 10 and the CR is lower than that of DAS. The best resolution (~1.68 mm) is that achieved by the short-lag F-DMAS image with a maximum lag of 10 (Figure 8c); in this case, the cyst dimensions are almost equal to the real ones (i.e., 10 mm diameter), while for higher lags, it looks elliptic and smaller. When the maximum lag is 40 or 63 (Figure 8b,d), the lumen becomes increasingly dark as compared to the short-lag case in Figure 8c, and CR increases (about +9.3 dB and +15.5 dB in absolute value, respectively). This is further highlighted by Figure 9, which compares the cross-sections of the anechoic cysts in Figure 8, showing a definitely lower amplitude level for the pixels in the cyst lumen with standard F-DMAS, as compared to the case with maximum lag = 10. The CNR is almost equal in Figure 8b,d (i.e., ~1.4), as well as the sSNR (i.e., ~1.4), and they are ~0.1 and ~0.25 lower than in Figure 8c, respectively. On the whole, the results highlight a dependence of F-DMAS MLT image quality on backscattered signals SC from two points of view. First, we have shown that, in MLT mode, performance varies with the number and angular spacing of simultaneously transmitted beams, which are in turn related to a variation of backscattered signals SC. The plots highlight that image contrast and speckle uniformity are mainly related to the angular distance between the multiple TX beams, and thus also to the trend of SC over lags; on the contrary, increasing the number of TX beams causes a drop of the measured total SC and has a lower  
Figure 8: 4-MLT cyst-phantom images obtained in scenario 1 by applying **(a)** DAS with Tukey apodization in TX/RX, or F-DMAS with **(b)** maximum lag = 63 (full-version), **(c)** maximum lag = 10, **(d)** maximum lag = 40, and Tukey apodization in TX only. Figures are displayed over a 60 dB dynamic range. impact on these parameters. In general, in fact, CR and CNR decrease when the angle between TX beams reduces, while the sSNR does not show an evident dependency on the number of TX beams or angular spacing. As expected, the lateral resolution measured at \(-6\) dB is generally less or not influenced by the implemented MLT configuration. Secondly, the results demonstrate that a relation exists between the analyzed outcome parameters and the maximum lag of received signals that enter the F-DMAS beamforming algorithm.