## 4 Results
### Simulation Results
#### 4.1.4 CoMTUS through Layered Media  
Aberration parameters measured from the simulated data are listed in Table 2. ATF and ELF are defined relative to the arrival time and energy profiles expected for an undistorted spherical wavefront. Comparing with the range of rms ATF values from previous studies [33], the aberrating strength introduced by the layered model can be considered from \"weak\" to \"moderate\". Received wavefronts after geometric compensation are shown in Figure 9. Shape distortions, resulting from the propagation of the wavefront through the tissue layers, are apparent in the receiving aperture in all cases. Figure 8: Computed quality metrics as a function of the imaging depth: (**a**) lateral resolution (LR); (**b**) contrast; and (**c**) contrast-to-noise-ratio (CNR). Two different methods are compared: one probe (dashed line) and CoMTUS (solid line). Figure 7: Comparison of simulated images of the non-aberrating medium at different imaging depths acquired by (**a**) one probe and (**b**) CoMTUS. Figure 10 shows the simulated images for the control case (medium only with soft tissue) and for imaging through fat and muscle layers of 35 mm and 8 mm in thickness, respectively. The different methods (i.e., one probe, two probes coherently combined assuming probe locations are known and the SOS is constant and fully optimized CoMTUS) are compared. It can be seen that in the presence of aberration, the PSF and contrast of the 2 probes image significantly degrade when comparing with the control case. Indeed, in the presence of aberration, it is not possible to coherently reconstruct the image using the two separate transducers with fixed parameters (Figure 10e). However, the CoMTUS system retains its performance (Figure 10f). Figure 11 shows an example of the delayed echoes from the point-like target for the 2 probes and CoMTUS cases, corresponding to a propagation medium with a fat layer of 35 mm in thickness. These flat echoes are obtained by coherently adding the four delayed backscattered echoes from the same point-like target (\(T_{1}R_{1},T_{1}R_{2},T_{2}R_{1},T_{2}R_{2}\)) using Equation (3) and the corresponding beamforming parameters. In the 2 probes case, the different echoes do not properly align, creating interference when forming a coherent addition of signals. However, after optimizing the beamforming parameters in CoMTUS, all echoes substantially align and can be coherently added together, minimizing the aberrating consequences. Similar effects are seen in the anechoic lesion. While differences in the background speckle pattern are observed between the different imaging methods, a higher loss of contrast due to aberration can be appreciated only in the 2 probes images. No significant changes in image quality resulting from aberration are appreciated in either the one probe or CoMTUS systems. Although both systems are able to image through aberrating layers, they show clear differences. CoMTUS shows more detailed images than the one probe system. The speckle size is reduced, and the different tissue layers are only visible in the CoMTUS images. \begin{table}
\begin{tabular}{l c c c c} \hline
**Fat + Muscle** & **rms ATF** & **ATF CL** & **rms ELF** & **ELF CL** \\
**Thickness (mm)** & **(ns)** & **(mm)** & **(dB)** & **(mm)** \\ \hline
5 + 8 & 0.94 & 0.35 & 2.48 & 4.86 \\
10 + 8 & 13.74 & 8.17 & 2.49 & 4.84 \\
15 + 8 & 22.09 & 9.75 & 2.48 & 4.84 \\
20 +  
The corresponding imaging metrics as a function of fat layer thickness are shown in Figure 12. As expected, in the absence of aberration, resolution improves with increasing aperture size.