## 4 Discussion  
This study investigates CoMTUS performance as the geometry of a coherent dual-array system, the scan sequence and the apodization laws applied in transmit. Both simulations and in vitro experiments are reported. From the PSF simulations, a general sense of the optimum scan sequence at each spatial configuration can be determined. Simulations showed that the spatial configuration of the different arrays determines the overall CoMTUS imaging performance, with different geometries favoring minimization of PSF main lobe width and side lobe energy (Figure 6). In general, resolution worsens with increasing angle between the arrays while the PSMR and SMER worsen because the amplitude of the side lobes rises at larger separations between the arrays (Figures 4 and 5). The PSF results show that, unlike standard compounding PW imaging with a single probe where the steering angle determines the F-number and lateral resolution [20,31], CoMTUS resolution is imposed by the size of the large effective aperture created, rather than the largest PW angle transmitted (Figure 3). Compounding PWs at varying angles may aid in reducing the side lobe energy (Figure 4). This suggests that both the relative location of the individual probes and the PW transmission angles are directly related and either one or both can determine the achievable resolution and contrast in the final image. This presents the opportunity to adaptively change imaging performance by using the relative location of the arrays to select the range of PW angles to use. For most applications, imaging will be performed in the center of the optimization sweep. This region is useful as the imaging depth is already significant and image quality can be maintained. Nevertheless, in practice, the imaging metrics will be affected by a complex combination of probe positions, aperture size, transmit PW angle, apodization law, and imaging depth. The relative location of the multiple arrays represents the main source of possible measurement uncertainties. These can be minimized by verifying and ensuring a perfect alignment between the scan planes by a preliminary assessment of the transmitted acoustic fields. Furthermore, the relative position of the multiple arrays and the scan sequence should be adapted to the specific application. In terms of resolution, the calculations in Table 1 indicate, as expected, that a better resolution is achieved with the minimum main lobe width configuration. However, the significantly thinner main lobe also affects the speckle texture and the final lesion detection in B-mode images (Figure 7), presenting this configuration also better contrast metrics despite the worse PSMR and SMER. Given that the lesion and target detectability is a function of both the contrast and resolution [32,33], overall, the extended aperture size improves lesion detectability, even when the side lobes are significant. Figure 7 shows that a cyst located at the common FOV is better visible in the configuration with higher resolution and worse PSMR and SMER. A narrow main lobe permits fine sampling of high-resolution objects, providing improved boundary detection for clinically relevant targets. Early studies [16] show that, with limited separation between transducers, the extended aperture created by CoMTUS provides benefits in both resolution and contrast that improve \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & \multicolumn{2}{c}{\(\theta=149^{\circ}\), gap = 3 mm} & \multicolumn{2}{c}{\(\theta=124^{\circ}\), gap = 35 mm} \\  & \multicolumn{2}{c}{**(Minimum-Side Lobe Energy)**} & \multicolumn{2}{c}{**(Minimum Main Lobe Width)**} \\ \hline
**Tx Apodization** & None & Tukey & Tukey & None & Tukey & Tukey \\
**Weighted compounding** & No & No & Yes & No & No & Yes \\
**Speckle size [mm]** & 0.40 & 0.40 & 0.37 & 0.28 & 0.30 & 0.26 \\ \hline \hline \end{tabular}
\end{table}