## 5 Discussion  
The implications for imaging using the CoMTUS method with two linear arrays are further investigated here with simulations and experiments. This analysis showed that the performance of CoMTUS depends on the relative location of the sub-arrays; the CoMTUS sensitivity increases with the imaging depth; and the resulting extended aperture is preserved in the layered media with different SOS. These findings show that, if the separation between the transducers is limited, the extended effective aperture created by CoMTUS confers benefits in resolution and contrast that improve image quality, particularly at large imaging depths, compared with a single probe standard system, and even in the presence of acoustic clutter imposed by tissue layers of different SOS (Figures 10 and 13). Extra analyses would be needed to quantify the significance of the results in different scenarios and further support these claims. Unlike the improvement achieved in resolution, benefits in contrast are not so significant. Indeed, simulation results suggest that the discontinuous effective aperture may degrade contrast when the gap in the aperture is bigger than a few centimetres. In the probe design, there is a requirement of half wavelength spacing between elements in order to avoid the occurrence of unwanted grating lobes in the array response [34]. Moreover, previous studies indicated that, unlike resolution, contrast does not continue to increase uniformly at larger aperture sizes [7; 35]. Nevertheless, while the contrast may be degraded by big discontinuities in the aperture, the main lobe resolution continues to improve at larger effective apertures. Since the lesion detectability is a function of both the contrast and resolution [36], overall, there are benefits from the extended aperture size, even when the contrast is limited. A narrow main lobe allows fine sampling of high resolution targets, providing improved visibility of the edges of clinically relevant targets. Both numerical and experimental results show how the speckle size significantly reduces when there is an enlarged receiving aperture produced by the coherent combination of multiple probes (Figures 5 and 15). In addition, when imaging at larger depths, an extended aperture has the potential to improve the attenuation limited image quality. In those challenging cases at large imaging depths, CoMTUS shows improvements not only in resolution, but also in contrast, even for effective apertures with significant gaps that raise the sidelobes (Figures 7 and 8). Our results agree with the hypothesis that in the absence of aberration, the aperture size determines resolution [1]. However, previous works suggest that despite predicted gains in resolution, there are practical limitations to the gains made at larger aperture sizes [3]. Inhomogeneities caused changes in the sidelobes and focal distance, limiting the improvement in resolution. The resulting degradation is primarily due to the arrival time variation called phase aberration. The outer elements on a large transducer suffer from severe phase errors due to an aberrating layer of varying thickness, placing limits on the gains to be made from large arrays [37]. The findings presented here agree with these previous studies when there are fixed average parameters (coherent 2 probes case) in the presence of aberration clutter. However, the adaptive optimisation deployed in the CoMTUS method takes into account the average SOS along the propagation path in the medium and shows promise for extending the effective aperture beyond this conventional practical limit imposed by the clutter. More accurate SOS estimation improves beamforming and allows higher order phase aberration correction. However, other challenges imposed by aberration still remain. Recent studies reveal that both phase aberration and reverberation are primary contributors to degraded image quality [38].