# An Experimental Ultrasound Database for
## 4 Data Set Description  
The objects which compose the database are homogeneous objects of different sizes (0.7-5.0 cm), shapes (spheres and cubes), and materials (wooden, styrofoam, cork). A picture of the targets adopted for the measurement campaign is shown in Figure 4 with further details reported in Table 1. The reader can refers to [26] for the evaluation of the target strength. The imaging scenario was obtained by placing the objects inside the imaging plane. As support for the targets, some nylon wires were used to hang them in the central area in the same plane the transducers are. The acquisition of the scenes is static, thus the objects are stable and fixed during the measurements. Basically, the data set is composed of different scenarios with different targets and arrangements. More in detail, the scenarios are illustrated in Figures 5-8 and organized according to the following scheme:  
* _Scenario 1: A single object of different shape, size, and material_ In this set of measurements, the acquisition of a single object of various shapes, sizes, and materials is considered, as illustrated in Figure 5. The types of objects are briefly summarised in Table 1, which provides the main details. In all these measurements, for a total of 13 acquisitions, the position of the object remains fixed and only its size and material change, as detailed in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Object** & **Size (Diameter/Side) [cm]** & **Shape** & **Material** \\ \hline \(\bigcirc\) & \{0.7, 0.9, 1.2, 1.6, 2.0, 2.8\} & Sphere & Wood \\ \(\bigcirc\) & \{2.0, 5.0\} & Sphere & Styrofoam \\ \(\blacksquare\) & \{0.5, 2.0\} & Cube & Styrofoam \\ \(\blacksquare\) & \{0.5, 1.0, 2.0\} & Cube & Cork \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of the objects involved in the measurement campaign. Figure 4: Picture of the objects adopted for the measurement campaign. From the top row: circular wooden spheres of different size, styrofoam cubes and spheres, cork cubes. * _Scenario 2: two wooden spheres of equal size_ In this scenario, two wooden spheres of the same size are located closer and closer to the region of interest to test the resolution capabilities. A sketch of the considered acquisition is illustrated in Figure 6, with quantitative details reported in Table 3.
* _Scenario 3: Two objects of different size, shape, and material_ Several acquisitions were carried out to consider various arrangements of targets of different shapes, sizes, and materials, as shown in Figure 7. In all these measurements, for a total of 20 acquisitions, two objects were considered as detailed in Table 4.
* _Scenario 4: three objects_ In this last scenario, three objects of different sizes, shapes, and materials are arranged in the region of interest. All the details about these acquisitions are provided in Figure 8 and Table 5.  
The data are stored in \".mat\" format and named with the following criterion: \"scenario-X-YY.mat\", with the number \"X\" referring to the scenario ID and with the number \"YY\" referring to the acquisition number. Thus, for instance, if the considered file is named \"scenario-2-07.mat\", this file contains the data related to scenario 2 and set ID number 07. Each of these files contains different variables, \"u-obj\" and \"u-emp\" refer to the acquisition performed with target(s) inside the investigation domain and without any object, respectively. Furthermore, \"r\" is the radius of the measurement curve, \"rx-angles\" and \"tx-angles\" represents the angular position of the receivers and the transmitters, respectively, \"f\", contains the values of the operative frequencies. The remaining variables, i.e., \"h\", \"s\" and \"objects-type\", are specific per each scenario (see Figures 5-8 and Tables 2-5). Regarding the data structure, each variable consists of a three-index matrix: the first two dimensions identify the receiver and transmitter ID, respectively, with sorting as reported in Figure 2b, while the last dimension refers to the number of adopted frequencies.