To represent all categories of breast densities in the dataset, it was necessary to model other ratios of tissues than the ones represented in original MRI data, by varying the number of FE that are assigned with mechanical properties of fibroglandular and fat tissues. To avoid a segmentation step, the approach proposed by Del Palomar et al.  was used to simplify the structural complexity of the breast, by assigning a weighted average value of the mechanical properties of each tissue type to all elements of the model. Hence, the ratios defined by the American College of Radiologists (ACR) classification system - the Breast Imaging Reporting and Data System (BI-RADS®)  - can be considered for weighting the material property values described in , and represent several breast densities. This reporting system identifies 4 categories of breast density (A, B, C and D), which are described in Table 1. Following this strategy, the fibroglandular/fat ratios: A—10/90; B—35/65; C—60/40; D—85/15, were used to average material properties of each category, as detailed in Table 2.