## 4 Discussion

Table 2: Imaging metrics from Figure 9 (minimum-side lobe energy, \(\theta=147.57^{\circ}\) and gap = 7.3 mm) and Figure 10 (minimum main lobe width, \(\theta=127.79^{\circ}\) and gap = 32.6 mm). Different apodization laws are compared: no apodization either on transmit or receipt (None), Tukey apodization on transmit (Tx), and Tukey apodization on transmit plus weighted compounding (Tx&Rx). image quality, particularly at large imaging depths, compared with a conventional single transducer system. This was demonstrated even in the presence of acoustic clutter caused by tissue layers of varying speed of sound. The use of transmit apodization on CoMTUS reduces cross-talk artifacts (Figures 9 and 10) but, in contrast to previous studies based on single arrays [22; 23], without affecting the spatial resolution and speckle size (Tables 1 and 2). Experimental results show that, despite the predicted improvements in resolution and target detectability, there are practical limitations to the gains made with CoMTUS, and these mostly depend on the spatial configuration of the arrays. While benefits in the common FOV of the arrays are evident in the calibration phantom (Figures 9 and 10), other parts of the images may be degraded by artifacts created by grating lobes and direct transmissions between arrays. These effects are clearly visible in the configuration of minimum main lobe width (Figure 10), which presents the largest gap between arrays (32.6 mm vs. 7.3 mm) and so larger expected grating lobes. Advanced beamforming methods [34; 35; 36] to reduce side lobes and grating lobes [13; 37] will be explored in the future to further improve CoMTUS performance. There is a complex interplay between FOV and imaging performance as arrays are moved relative to one another. The final CoMTUS image will always achieve an extended FOV; however, the resolution is only able to improve in overlapping regions. This improvement will be greatest toward the center, where the overlap includes transmission and reception for both transducers. Thus, the benefits may be of various kinds in different locations. Those spatial configurations that will maximize the overlapping FOV will lead to more uniform images with enhanced performance. However, the different experimental conditions (different areas of the CIRS phantom were imaged) make it difficult to provide a straightforward comparison between the two different configurations. Indeed, the restricted acoustic window of the phantom limits the position of the arrays and the accessible areas of the phantom, making it infeasible to image precisely the same region with both configurations. Likewise, in the future, a direct comparison of the in vitro images with their simulated counterpart could be used to further support these findings. The wavefront aberration caused by the different speed of sound between the water (used for coupling) and the CIRS phantom is evident also in images acquired by a single array (images not shown). In the presence of sound speed variation, the effect of aberration is less pronounced in the common FOV of both arrays and close to the points used for calibration. However, the errors in the applicability of the calculated positions of the arrays and local speed of sound are likely to increase in regions further from the targets. These effects are visible in the targets around 20 mm depth in Figures 9 and 10. More accurate sound speed estimation would improve beamforming [38] and also enable higher order phase aberration correction in the areas away from the calibration targets. In addition, the use of several probes allows multiple interrogations from different angles that may add extra benefits [39]. Finally, if different transmitted beams are used, such as divergent waves [40], there will be additional factors that could affect CoMTUS performance and should be considered [41]. In future studies, the use of apodization on receive to further improve image performance, and TX/RX strategies capable of increasing the achievable frame rate should be investigated.