## 3 Results
### Simulation Results
(**h**) PSMR and (**i**) SMER averaged over the angle between transducers. Figure 3: **(a)** FWHM, **(b)** PSMR amplitude and (c) SMER as function of the imaging depth, angle between arrays (\(\theta\)), and \(\alpha_{\text{max}}\), with 0 mm gap between the arrays and varying \(\theta\) from \(95^{\circ}\) to \(165^{\circ}\). While line corresponds to the selected \(\alpha_{\text{max}}\) for each depth. Results obtained with Tukey apodization on transmit and compounding 3 PWs, \((-\alpha_{\text{max}},0^{\circ},\alpha_{\text{max}})\), per array. between the transducers, which mainly dictates the imaging depth together, to a minor extent, with the gap (Figure 1). At large \(\theta\), PSMR and SMER are more sensitive to changes in the gap between the arrays. Note that, as indicated in the figure captions, Figures 3-5 display results obtained with Tukey apodization on transmit. Similar trends were observed for the case of no apodization (rectangular window), so are not shown here. Thus, the selected configurations do not change with the transmit apodization. From these results and at certain imaging depth, it is possible to identify two extreme configurations, i.e., a first configuration with minimum-side lobe energy, and a second configuration with a minimum main lobe width. These two configurations and their corresponding metrics at 70 mm depth are indicated with a white dot and a white star, respectively, in Figure 5.  
Figure 6 shows an example of the PSF and its lateral cross-section for a numerical point scatterer at 70 mm depth using the two extreme geometries (minimum-side lobe energy (Figure 6a,b) and minimum main lobe width (Figure 6c,d)) and with the different apodization laws used on transmit (rectangular (Figure 6a,c) and Tukey (Figure 6b,d) windows). The corresponding imaging metrics are summarized in Table 1. Minimum-side lobe amplitude results in PSMR up to 16.5 dB lower than minimum main lobe width, but worse resolution (0.49 mm vs. 0.29 mm). The use of apodization on transmit only affects the metrics PSMR and SMER, and no others. Compared to a transmit rectangular apodization, when a Tukey law is used on transmit, the PSMR increases by 3 dB and 1.1 dB in the minimum-side lobe energy and minimum main lobe width configuration, respectively, while SMER increases by 0.9 dB in both configurations. Although the amplitude of the first side lobe is higher on Tukey apodization, the images of the point scatterer look more refined because the second side lobe amplitudes are much smaller (Figure 6e). The B-mode images of the tissue-mimicking phantom using the proposed configurations at 70 mm depth and with Tukey apodization on transmit are shown in Figure 7. The lateral section of the anechoic region (Figure 7c) shows that the mean gray level inside is similar in both cases. Using these configurations, the inclusion is visible with a CR of \(-\)22.7 dB, CNR of 1.65, and gCNR of 0.97 in the minimum-side lobe energy case vs. \(-\)23.4 dB, 1.72 CNR, and 0.97 gCNR in the minimum main lobe width case. Significant differences in the speckle texture can be appreciated between Figure 7a,b, relating to minimum-side lobe energy and minimum main lobe width configurations, respectively. The latter, in agreement with the resolution measured from the PSF, produces a thinner speckle size and better-defined edges, making Figure 7b more residue than Figure 7a. Table 1 shows the imaging metrics for both configurations and transmit apodization laws. Based on calculations in Table 1, the use of apodization on transmit does not affect any of the contrast metrics. Figure 5: (a) FWHM, (b) PSMR and (c) SMER as function of the angle and gap between the arrays. Results obtained by coherently compounding 7 PWs per array with \(\alpha_{max}\) determined according to the imaging depth (Figure 3) and with Tukey apodization on transmit. Selected configurations at 70 mm depth with minimum-side lobe energy (white dot) and minimum main lobe width (white star).