# An Experimental Ultrasound Database for
## 1 Introduction  
Ultrasound imaging is one of the most adopted imaging modalities for non-destructive testing and diagnostics [1]. Nevertheless, conventional approaches suffer from some limitations, such as the operator-dependent feature and the presence of speckles which degrades the quality of the images, thus requiring the use of proper filtering strategies [2; 3; 4]. In this framework and to provide a quantitative, operator-independent imaging modality, ultrasound tomography has achieved significant interest.  
Ultrasound tomography (UST) represents an imaging modality that exploits ultrasound waves to retrieve information about the mechanical properties of objects of interest (OIs). In UST, the pressure fields are measured by ultrasound (US) transducers located outside the OIs to locate and eventually characterize them in terms of density, compressibility and attenuation [5; 6; 7]. Due to the fact that the data acquisition measures the received voltage at the port of one or more of the receiving transducers to infer the pressure field impinging directly on the sensor, a proper calibration procedure that takes into account the radiation factor, implicitly or explicitly, is required to perform the imaging.  
More in detail, the idea of arranging ultrasound transducers around the imaged objects in a fixed setup goes back to the early 1980s [8]. Since then, several scientists have been working on UST [9; 10; 11; 12; 13], and most of this work is focused on noninvasive medical imaging, especially for breast cancer applications [14; 15; 16; 17], in which field considerable tumor detection and characterization improvements can be appreciated if UST is applied in combination with other imaging approaches [18; 19; 20; 21].  
To perform the data acquisition in UST, two main options are available: the first one exploits the use of a single transmitter-receiver pair that mechanically moves around the investigation region, while the second option carries out the data collection via adoptingmore transmitters and receivers physically located around the area of interest. This latter option, which is also known in communication technologies as the multiple-input-multiple-output (MIMO) configuration, represents the case considered in the proposed manuscript. The aim of the work is to face the common issue in the inverse scattering community of the lack of experimental US tomographic scattering data. The quest for experimental data was already partially satisfied in different contexts [22; 23; 24; 25], in which cases the angular diversity information is collected via mechanical movements of the bistatic pair of transmitter and receiver around the OIs. However, there are several applications for which scattering data are better collected with mechanically static systems, such as biomedical imaging and nondestructive testing.  
Despite the experimental data sets available for ultrasound systems, most of them focus on complex scenarios more suitable for denoising and image processing purposes rather than for testing tomographic coherent imaging approaches in relatively simple laboratory-controlled conditions.  
Thus, with the goal of stimulating the research for both imaging and calibration issues, this work presents a tomographic database of ultrasound scattering measurements. The acquisitions were performed by adopting an in-house imaging system which is described in Section 2. In the following, different scenarios are considered with an increased level of complexity to allow the performance testing of both image formation and detection algorithms.  
The remainder of the paper is the following. Section 2 describes the proposed experimental US system, Section 3 illustrates the acquisition protocol, and Section 4 focuses on the data set description. Some conclusions close the paper.