# An Experimental Ultrasound Database for
## 2 Experimental Ultrasound System  
In this section, the proposed UST system, that was designed, built, and tested at the University of Naples Parthenope, is presented. The overall system, which is shown in Figure 1, was intended for the acquisition of a single slice of the investigation domain and consists of three main components:  
* a circular wooden ring hosting twenty-two US transducers (both transmitters and receivers);
* a signal generator (Agilent Technologies, model 33220A) for the transmitters excitation;
* an analog-to-digital converter (National Instrument, 6363 DAQ USB X Series);
* a standard laptop to control the acquisitions and to perform the processing.  
Figure 1: Picture of the tomographic ultrasound system. The transmitters are connected to the signal generator (bottom left), while the receivers are connected to a data acquisition device (bottom right). The whole acquisition is controlled via a laptop with Matlab software.  
The imaging region is surrounded by twenty-two transducers (i.e., four transmitters and eighteen receivers), approximately equally spaced on a wooden circular ring as shown in Figure 2. The circular wooden ring on which the transducers are located is hump by means of three thin metallic bars (Figure 2a). The height of the imaging plane and the presence of an absorbing element on the floor of the system reduce the presence of clutter considerably, allowing good isolation of the back-scattered signal related to the OIs.  
The internal and external radii of the wooden ring are equal to \(r_{int}=17.5\) cm and \(r_{ext}=22.5\) cm, respectively, while the distance of each transducer from the centre of the investigation area is approximately \(r_{tr}=16.5\) cm. The angular distance between two consecutive transducers \(\theta\) is about \(16.36\lx@math@degree\).  
As detailed in Figure 2b, the four transmitters are located at the cardinal points in a slightly asymmetrical position, i.e., two consecutive transmitters might be separated by either 4 or 5 receivers, according to the considered angular sector. The transducers used as transmitters and receivers are, respectively, the 40 LT16 and 40 LR16 both manufactured by SensComp. Both of these types of sensors have a central working frequency of 40 kHz and narrow bandwidth of approximately 2 kHz. At the central frequency of 40 kHz, the corresponding wavelength is approximately 8.5 mm. The bandwidth was experimentally measured and decreases to a \(-8\) dB power-level difference as proved by Figure 3a. In the following, an operational bandwidth of 4 kHz ([38, 42] kHz) will be considered for the acquisition protocol. Regarding the beam angle, it extends for approximately \(60\lx@math@degree\) at \(-3\) dB both in the vertical as well as horizontal planes (Figure 3b).  
The region of interest (ROI), i.e., the scanning area in which the objects are located, coincides with the central area of the circular array.  
The received US waves are converted into electric signals by 40 LR16 transducers and then digitalized by an analog-to-digital converter manufactured by National Instruments. Once acquired, the beat signals are computed to demodulate the signals. Both acquisition and beat-signal extraction steps are computed via a Matlab script (available in the Supplementary Material).  
Figure 2: (**a**) Picture of the US transducers circular array and (**b**) sketch of its arrangement with details about transmitters (in red) and receivers (in grey). The dimensions are: \(r_{int}=17.5\) cm, \(r_{ext}=22.5\) cm, \(r_{tr}=16.5\) cm and \(\theta=16.\overline{36}\lx@math@degree\).