## 3 Sample, Experimental Devices, and Protocols
### Transducers and Electro-Acoustic Devices  
The three-frequency transducers used for data acquisition were Imasonic (Voray-sur-I'Ognon, France) piezo-composite transducers, with center frequencies (f\({}_{0}\)) of 500 kHz, 1 MHz, and 2.25 MHz (Figure 4). The transducers were positioned on a 28 cm-high linear antenna, and were 98 mm apart from one another. Each transducer was 60 mm high and 56 mm in outer diameter, with a lateral aperture size of 40 mm and an axial aperture size of 30 mm (12 cm\({}^{2}\), the cylindrical focalization being in the plane of the slice at 150 mm), and a slice thickness of 3 mm. It was possible to orient the acoustic beam linearly and rotationally using millimetric adjustment wheels.  
The transducers were powered by two different generators for the two ultrasonic wave transmission configurations (Figure 5). The transducers were driven using a pulse-receiver generator (Sofranel 5077 PR, Olympus, Waltham, MA 02453, USA) for the pulse-mode method, and by a waveform generator (TGA 1241, Thurlby Thandar Instruments Limited, Huntingdon, UK) coupled with a diplexer (RDX-6 Diplexer, Ritec Inc., Warwick, RI, USA) for the WCE-mode method. The maximum voltage possible with the waveform generator was 10 V peak-to-peak. In order to have the same output power in both configurations, a power amplifier (KMP Electronic, model 44702, 150 W, 0.5-15 MHz, Sofia, Bulgaria) was connected between the waveform generator and the diplexer, and the transmitted signal dynamics were balanced between the WCE-mode and pulse-mode configurations.  
The radio-frequency signals (RF-signals) were conveyed from the 12-bit oscilloscope (Lecroy HDO 6104, Teledyne Inc., Thousand Oaks, CA, USA) to a personal computer using a USB interface file transfer, and stored. Each signal contained \(n\) = 65,536 samples, and the sampling frequency was \(F_{e}\) = 500 MHz. The signal and image processing algorithms were implemented using Matlab(r) (The MathWorks, Inc., Natick, MA, USA). The final size of USCT was 2048 \(\times\) 2048 pixels2.  
Figure 4: (Inset on the left) Newborn arm phantom. Cross-sections were chosen in areas 6 mm and 14 mm from the hand at the level of the laser cross. (Left part) Complete fresh chicken drumstick containing skin, fat, muscle, tibia, and fibula. Cross-sections were chosen in the proximal area 25 mm from the upper tibo-fibular joint at the level of the laser cross. (Right part) Three-frequency linear transducer array; \(f_{O}\) = 500 kHz (bottom), 1 MHz (middle), 2.25 MHz (top).  
Transducer calibration (bandwidth, nominal frequency, energy and intensity distribution) was carried out using a 0.5 mm needle hydrophone (Precision Acoustics, Dorchester, UK) mounted on the mechanical ultrasonic scanner (Figure 5, Configuration A).  
A stainless-steel reference plate placed at the center of the bench (distance of \(\sim\)150 mm) was used as a perfect reflection interface to record reference signals for the WCE-mode method. The size of the plate was more than 10 times larger than the transducer's active area (Figure 5, Configuration B).  
A 70 \(\upmu\)m-diameter copper thread was placed on the target holder, and was used to measure the impulse response of the mechanical apparatus (point spread function); i.e., to measure the position errors of the transducers with respect to the object during rotation (Figure 5, Configuration C).