## 5 Discussions
### Anatomy and Morphometry  
Regardless of imaging mode and frequency, USCT shows details not visible in \(\upmu\)CT. For example, for the newborn arm phantom, two blood vessels are clearly visible in USCT at points 3 and 4 for H1, and 2 and 3 for H2 (Figure 13), whereas they were less differentiated from the constant reference medium in \(\upmu\)CT with the settings chosen for this experiment. Despite the low acoustic impedance contrasts between the artificial vessels and the soft body, with a low reflection coefficient between 0.0175 and 0.132, USCT allowed visualization of the position of the vessels. Compared with \(\upmu\)CT, the orders of magnitude of the distances between the bones, the external body border, and the vessels were respected, and the tolerances were acceptable. For example, for the height H1 of the newborn arm phantom, at point 3 (respectively point 4) in \(\upmu\)CT, the distance (D and E) measured between the blood vessels and the external boundary of the phantom was 3.93 mm (respectively 5.55 mm); whereas in the multifrequency USCT, this distance was 4.44 mm (respectively 6.91 mm). The relative difference was about 13% (respectively 24.5%). For the height H2, the distance (I) between the external boundary of the phantom and the bone was 9.85 mm in \(\upmu\)CT, and 10.05 mm in the multifrequency USCT with a relative difference of about 2.03%. The measurements were more complicated for the blood vessels because the resolution at points 2 and 3 was not sufficient. For point 2, the size (H) of the blood vessel was 2.16 mm in \(\upmu\)CT and 2.2 mm in multifrequency USCT, with a relative difference of 1.85%. For the bone, the diameter measurement (F) differed relatively between \(\upmu\)CT (6.59 mm) and multifrequency USCT (4.92 mm). The error was higher (\(\approx\)25%). The boundary of the phantom was identified, despite the low contrast between the tank water and the object (\(\approx\)1). The reflection coefficient tended to 1 (\(\approx\)0.997) between the artificial bones and the phantom body, which meant that all the waves were mainly reflected, and little energy was transmitted into the artificial bones. In addition, the material used was attenuating (\(\approx\)21 dB/cm). Therefore, the amount of energy transmitted beyond the artificial bones was low. Only the contours of the bones were visible, and the information inside the structures did not represent the true medium. The interdiaphyseal area was not well \begin{table}
\begin{tabular}{c c c c} \hline \hline \(f_{0}\)**(MHz)** & **Pulse-Mode** & **WCE-Mode** & **Gain (\%)** \\ \hline \multicolumn{4}{c}{Newborn arm phantom (H1)} \\
0.5 & 0.11 (0.1) & 1.6 (1.3) & 1355 \\
1 & 0.083 (0.076) & 0.62 (0.54) & 647 \\
2.25 & 1 (1) & 3.9 (5.6) & 290 \\ D.a.S & 0.16 (0.16) & 0.89 (0.75) & 456 \\  & \multicolumn{2}{c}{Newborn arm phantom (H2)} \\
0.5 & 0.13 (0.1) & 3.5 (2.6) & 2592 \\
1 & 0.069 (0.058) & 1.3 (1.1) & 1784 \\
2.25 & 0.11 (0.093) & 1.5 (0.64) & 1264 \\ D.a.S & 0.12 (0.11) & 2 (0.99) & 1567 \\  & \multicolumn{2}{c}{Ex vivo chicken drumstick} \\
0.5 & 0.3 (0.23) & 2.1 (1.4) & 600 \\
1 & 0.41 (0.31) & 1.1 (1) & 175 \\
2.25 & 0.19 (0.11) & 0.37 (0.19) & 95 \\ D.a.S & 0.22 (0.18) & 0.46 (0.26) & 109 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Contrast-to-noise ratio \(\langle CNR_{0,r}\rangle\) (average and standard deviation) and gain (%) between pulse-mode and WCE-mode methods for USCT of the newborn arm phantom (True Phantom Solutions), and of the ex vivo chicken drumstick. resolved, regardless of the chosen frequency. Nevertheless, the center-to-center distance (C) could be identified and was measured at 4.86 mm in \(\upmu\)CT and 4.5 mm in the multifrequency USCT, for a relative difference of 7.41%.