## 4 Discussion  
In the present work, we investigated the variability in texture features extracted from DBT of breast parenchyma in relation to the DBT acquisition angle. Insulating the effect of the acquisition angle was not a simple task. Different manufacturers use different angles, and comparisons are influenced by numerous confounding variables (e.g., different detector technology, different X-ray quality, different dose levels). The Fujifilm Amulet Innovative offers two acquisition modalities (ST and HR) on the same DBT system, which enabled us to eliminate many of these variables. We succeeded in insulating the DBT acquisition angle effect by designing a specific acquisition and data normalization protocol that equalized the image resolution and the dose of the two modalities. Furthermore, our comparison between feature values was performed on a paired dataset. The DBT acquisition angle influenced second-order features (from GLCM). GLCM features have been widely used in DM to build classifiers of malignant breast tissue [5; 7; 21; 26; 27]. Research has also been extended to DBT by other authors using different DBT systems with different acquisition angles [11; 12; 28]. In the present work, we demonstrated that the DBT acquisition angle significantly influences several GLCM features extracted from healthy breast parenchyma. This must be taken into consideration when developing classifiers for malignant tissue in DBT (i.e., the GLCM feature values for healthy breast tissue may differ simply because of the different DBT acquisition angle) and also when comparing texture values  
Figure 4: Boxplots of Mean (**a**) and Variance (**b**) histogram-based features. These features show no significant difference between \(15^{\circ}\) acquisition angle (standard mode) and \(40^{\circ}\) acquisition angle (high resolution mode) DBT images. Figure 5: Scatter plot of Correlation vs. Contrast, the two most relevant features, showing the distribution in 2D space of the two imaging modalities. calculated from different DBT systems, as these may differ on the basis of the acquisition angle. Texture features showing a significant difference may translate to physical considerations in DBT images and in the DBT acquisition process. In particular, the GLCM Contrast feature is a measure of the amount of local variation present in the image [23]. From this viewpoint, our results demonstrate that DBT images show a higher level of local contrast for healthy breast parenchyma (around 50% increase in median value) when DBT reconstruction is performed using projections with a wider acquisition angle (see Table 2). The observed difference can only be attributed to the wider acquisition angle because other variables such as higher dose or higher resolution were ruled out by the chosen acquisition protocol, and the image processing performed before the texture analysis. This result can be considered to be an addition to the existing findings regarding the quantitative comparisons between different DBT acquisition systems [29; 30]. Our finding is coherent with findings of other authors regarding the positive effect of wider DBT acquisition angle on several quantitative parameters such as z-resolution, contrast-to-noise ratio of disk details, and subjective scoring of image quality by multiple readers [15; 21]. In this context, texture analysis applied to DBT images has the potential to be an additional quantitative method for scoring and comparing DBT images. Analogous considerations can be made regarding the difference observed for the Correlation feature, which is a measure of gray levels linear dependency in the image [23]. In our study, DBT images, reconstructed using wider acquisition angle projections, showed pixel gray levels which were less correlated. Histogram-based features were not affected by DBT angular range, which may have been because histogram-based features do not depend on the local distribution of gray levels. Rather, they depend globally on the gray levels which were aggregated in the histogram within the ROI.