## 1 Introduction
Newly established US and elastography techniques have been studied as an additional method to analyze tissue stiffness in order to improve US examination specificity in the characterization of focal breast masses [8; 9; 10]. The European Federation of Ultrasound in Medicine and Biology (EFUMB) and the World Federation of Ultrasound in Medicine and Biology (WFUMB) described US elastography and published their guidelines about physical principles, clinical indications, and limitations in several fields, including breast imaging [10]. Strain elastography (SE) and shear wave elastography (SWE) are the two main methods used in breast imaging. SE is a qualitative and semi-quantitative technique based on compression applied by an external source (mechanical compression performed by the transducer) with the transducer kept still and perpendicular to the body surface. In SE, the absolute value cannot be calculated since the force degree applied during the compression is not known, and the deformation of the lesion can be represented as a relationship with a reference tissue (fat tissue in the breast) and visualized by a color or a gray scale. The stiffness scale has color values ranging from red (soft tissue) to blue (stiff areas), passing through green (intermediate stiffness). For a semi-quantitative as well as for a qualitative evaluation (color map), the strain ratio (SR) may be calculated considering the deformability ratio between the tumor and the breast fat [9; 10]. SWE is a different quantitative method in which the force of acoustic radiation generates shear waves in the tissue. This kind of elastography provides quantitative information (measured in kPa or m/s) shown also as a color map of the lesion and of the neighboring tissues in real time. SWE can be further divided into 2D-SWE (sound touch elastography (STE)) and point shear-wave elastography (pSWE, sound touch quantification (STQ)), in which tissue stiffness is respectively represented as the mean elasticity of the region of interest (ROI) or as a measurement of each pixel included in the field of view. Multiple studies evaluating the possible role of SWE and SE have reported that breast elastography is useful for differentiating benign from malignant lesions and could potentially reduce unnecessary biopsies by improving the accuracy of breast US [10; 11; 12]. However, no specific recommendations have been provided by guidelines regarding the superiority of one method over the other [13]. Recent studies have evaluated the potential role of computer-aided diagnosis (CAD) and artificial intelligence systems in supporting breast imaging in mammography, ultrasound, and MRI, respectively, emphasizing the variability of the results of different papers and the need for large-scale cohort studies [14]. \"SmartBreast\" is a new semi-automatic software able to carry out a multiplanar analysis of breast lesions and provide an intelligent classification and reporting tool, according to the BI-RADS lexicon. The purpose of our study was to compare the diagnostic performance of these new ultrasound techniques in the differentiation of benign and malignant breast lesions, in accordance with the BI-RADS classification, evaluating the agreement between \"SmartBreast\" and the radiologist, and the diagnostic accuracy of elastography performed with strain ratio (SE, SR), pSWE, and 2D SWE.