## 5 Discussions
### Anatomy and Morphometry
The positioning of the calipers in the images depended on the user who was making the measurement, and was therefore imprecise and subjective, but mean relative differences of less than 10% were acceptable, especially since the low resolution of \(\upmu\)CT and USCT, as well as artifacts on USCT, limited the accuracy of some measurements. Indeed, in Figure 13, the dotted arrows highlight the artifacts. These artifacts interfered with the USCT analysis and did not appear in \(\upmu\)CT. They could correspond to interference waves between the artificial bones and the blood vessels. Given the high impedance contrasts between the hard and soft tissues composing the phantom, and the reflection coefficients close to 1, the waves were reflected several times between the different structures, and the reconstruction algorithm did not take these multiple echoes into account. In the H2 image, part of the soft body contour was not visible on the multifrequency and 500 kHz USCT. This contour was slightly apparent on USCT at 1 MHz and 2.25 MHz. The effects of the bone/vessel/body  
Figure 13: Morphometric analysis of the newborn arm phantom (True Phantom Solutions) by comparing USCT and \(\upmu\)CT. Solid arrows indicate areas of blood vessels and bones. Dotted arrows show artifacts. contrast were the cause of this disturbance, creating spurious signals that were difficult to eliminate. Figure 14 shows the morphometric analysis of the ex vivo chicken drumstick, by comparison of USCT and \(\upmu\)CT. The first observation was that USCT allowed visualization, regardless of frequency and mode, of the different morphologies of the chicken thigh. The tibia (1) was very clearly visible, with a medullary part identified. The shape of the tibia was more rounded in USCT than in \(\upmu\)CT. The mean diameter (L) in USCT was greater (11.81 mm) than that measured in \(\upmu\)CT (10.74 mm), a relative difference of about 10%. This was due to the model used for the forward and inverse problem, which assumed that all objects in the path of the waves were fluids. Since the velocity of the waves in the bone was higher than in the surrounding tissues, the waves propagating in the bone cortex, in the marrow, and then returning to the transducer arrived at a time-of-flight that did not correspond to propagation in a fluid. Even if the dimensions of the medullary canal were not realistic, USCT allowed relative measurements and identification of the different parts of the tibia. Some features of the tibia, such as the small upper protrusion (dotted circle), could be visualized. The fibula (2) was also clearly visible. The resolution in USCT was not the same as in \(\upmu\)CT, but it was quite possible to identify the fibula, distinctly from the tibia, which was much larger in the chicken drumstick. The measured diameter (M) was about 1.94 mm in \(\upmu\)CT and 2.43 mm in USCT. The relative difference was high, and equal to about 25%. The distance (N) between the center of the tibia and the center of the fibula was about 6 mm in USCT, compared with 6.1 mm in \(\upmu\)CT. The relative difference was less than 2%, which was small enough so that it could be considered that the two modalities provided close organ visualization. The use of large-aperture and broadband transducers, and the combination of several frequencies, allowed this identification. This point will be discussed. The Peronaeu's longus (3), Tibialis anterior (4), and Flexor II and III (5 and 6) muscles were well identifiable. The measurements (O-P-Q-R) were respectively 24.1 mm, 19.5 mm, 13.41 mm, and 14.07 mm in USCT; and 26.57 mm, 22.41 mm, 13.42 mm, and 13.93 mm in \(\upmu\)CT. Even though, for point R for example, the resolution of USCT did not make it possible to be very precise, the relative differences remained lower than 13%, and for some measurements lower than or equal to 1%, which was encouraging.