## 4 Discussion  
The use of an advanced robot in this study to move various ultrasonic probes in a versatile manner in any geometry enables good 3D imaging results. All of the probes used for the purpose of this work allowed us to obtain the reconstructed structure with visible inclusions placed inside the phantom. Subjective contrast and edge blur are connected with the ability to recognize a different group of pixels' intensity on the histogram. When two different groups can be simply distinguished in a histogram, the different structures are recognizable in the reconstructed image. However, even with a wide enough distribution of the intensity to hide another structure in a histogram, it can still be recognized in the ROI. Considering the calculated CNR, the mean value of pixels outside of the structure, and the standard deviation in terms of the position of probes, it can be seen that the best values are usually not in the position of the probe moved 3 cm from the center and being placed perpendicular to the tank bottom. It is especially visible for the mean value of the pixels outside of the structure, where the reflection from the breast phantom surface may cause significant artifacts, which can be even more pronounced in a water tank with the surface being close enough to generate secondary reflection. Furthermore, this artifact may also be present inside of the breast tissue at the border parallel to the skin surface at the examination position. This suggests the need for an angled probe mounting with respect to the vertical axis of the system. In terms of the probe used, the convex array probe does not use any software beam steering. The linear array uses beam steering to expand the area of the image. The phased array is a small probe that utilizes complicated beamforming algorithms to create the desired imaging area. The phased array probe performed better in terms of standard deviation analysis and gave slightly better CNR results than the linear array. Nevertheless, it performs significantly worse in terms of artifacts arising from reflection. The convex array generally outperforms the other probes used in the study. Additionally, it enables a wide visualization angle, a large CI area, and a long range. The inclination of the active surface of the probe relative to the horizontal plane is also important. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Position** & **C1** & **L1** & **P1** & **C2** & **L2** & **P2** \\ \hline \((3,0)\) & 6.3 & 12.8 & 9.0 & 9.7 & 21.2 & 13.1 \\ \((4,10)\) & 9.5 & 12.5 & 8.2 & 9.9 & 21.0 & 13.0 \\ \((5,20)\) & 9.7 & 11.7 & 9.6 & 10.2 & 22.0 & 13.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Standard deviation of the pixels of the inclusion backgrounds. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Position** & **C1** & **L1** & **P1** & **C2** & **L2** & **P2** \\ \hline \((3,0)\) & 17.7 & 17.5 & 12.6 & 21.1 & 27.8 & 21.8 \\ \((4,10)\) & 11.8 & 21.4 & 11.8 & 25.1 & 30.3 & 20.3 \\ \((5,20)\) & 7.8 & 22.3 & 12.4 & 18.2 & 24.3 & 20.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Standard deviation of the pixels of inclusions. In principle, there are no additional limitations beyond those that exist for conventional ultrasound B-mode scanners and other 3D ultrasound imaging systems built on HHUS (see Introduction). However, it should be noted that the resolution of 3D images obtained with the MACUI method additionally depends on the number of scanning angles during the rotation of the ultrasound probe. If we assume that the 3D scan of the entire breast volume is performed in time, for example 60 s, and the number of acquired HHUS image frames is 10 fps (e.g., due to additional methods and scanner options incorporated in order to improve the imaging quality), it can be calculated that the maximum angular resolution of MACUI may then be 0.6 degrees per frame of the image. At an imaging rate of 30 fps, this would, in turn, be 0.2 degrees per frame, allowing for a high spatial resolution.