## 4 Discussion
### The S/N Focused on the Difference in Area or Diameter  
In the optimization process, the scoring of the imaging quality in various groups is calculated according to the area difference between a practical survey and theoretical preset. However, alternatively, it can focus on the difference in diameters. In doing so, the data were rearranged in three trials, and the corresponding results are listed in Table 6. The fish-bone plot is depicted in Figure 4. A significant difference between the area- and diameter-based results can be observed. Based on the diameter difference, only factor D (amplitude gain) performs as a significant factor (24.22%) while the contribution of other factors is 42.72% and the random error is 20.61%, indicating a high uncertainty (42.72 + 20.61 = 63.33%). In contrast, based on the area difference, the same derived value is 32.20%, that of the others is 11.86%, and the random error is 20.34% (refer to Table 4). In addition, a lower contribution from the others or random error also implies that a larger contribution comes from those assigned factors and suppresses the harmful interference of other factors. Therefore, focusing on the area difference is more effective than focusing on the diameter difference for solving this particular problem.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multicolumn{3}{c}{**Diameter Difference (cm)**} & \multirow{2}{*}{**Ave.**} & \multirow{2}{*}{**Sdev**} & \multirow{2}{*}{**S/N**} \\ \cline{2-2} \cline{5-7}  & \#1 & & & & & \\ \hline
1 & \(-\)0.238 & \(-\)0.238 & \(-\)0.195 & \(-\)0.224 & 0.02 & 13.01 \\
2 & \(-\)0.092 & \(-\)0.082 & \(-\)0.044 & \(-\)0.073 & 0.03 & 22.77 \\
3 & \(-\)0.072 & \(-\)0.039 & \(-\)0.039 & \(-\)0.050 & 0.02 & 26.02 \\
4 & \(-\)0.022 & \(-\)0.022 & \(-\)0.021 & \(-\)0.008 & 0.02 & 42.31 \\
5 & \(-\)0.044 & \(-\)0.119 & \(-\)0.119 & \(-\)0.094 & 0.04 & 20.54 \\
6 & \(-\)0.006 & \(0.027\) & \(-\)0.006 & 0.005 & 0.02 & 46.02 \\
7 & \(0.021\) & \(-\)0.065 & \(-\)0.022 & \(-\)0.022 & 0.04 & 33.15 \\
8 & \(-\)0.006 & \(-\)0.044 & \(-\)0.006 & \(-\)0.019 & 0.02 & 34.58 \\
9 & \(-\)0.072 & \(-\)0.138 & \(-\)0.138 & \(-\)0.116 & 0.04 & 18.71 \\
10 & \(-\)0.022 & \(-\)0.065 & \(-\)0.065 & \(-\)0.051 & 0.02 & 25.91 \\
11 & \(-\)0.044 & \(-\)0.082 & \(-\)0.044 & \(-\)0.057 & 0.02 & 24.93 \\
12 & \(-\)0.072 & \(-\)0.039 & \(-\)0.105 & \(-\)0.072 & 0.03 & 22.85 \\
13 & \(-\)0.022 & \(-\)0.109 & \(0.021\) & \(-\)0.037 & 0.07 & 28.71 \\
14 & \(-\)0.006 & \(-\)0.119 & \(-\)0.044 & \(-\)0.056 & 0.06 & 24.98 \\
15 & \(-\)0.204 & \(-\)0.138 & \(-\)0.171 & \(-\)0.171 & 0.03 & 15.34 \\
16 & \(0.064\) & \(0.021\) & \(-\)0.022 & 0.021 & 0.04 & 33.56 \\
17 & \(-\)0.119 & \(-\)0.006 & \(-\)0.044 & \(-\)0.056 & 0.06 & 24.98 \\
18 & \(-\)0.138 & \(-\)0.105 & \(-\)0.105 & \(-\)0.116 & 0.02 & 18.71 \\ \hline \hline \end{tabular}
\end{table}
Table 6: The evaluation results obtained from three independent measurements via Equation (6). The diameter difference is defined as the difference between the practical survey and the theoretical survey (0.8 cm).  
Figure 3: The fish-bone plot of the simplified S/N (cf. Equation (7)). The predominant factors become (D) the amplitude gain or (E) imaging compression ratio, unlike (C) the frame rate dominance, according to Equation (6).