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Background: Strain elastography is relatively new ultrasound technique that assess elasticity of the tissue .This technique utilize the relative strain of the tissue of interest to generate color coded image and to measure the strain ratio. It has been extensively investigated and introduced as a promising technique for characterization of breast masses. Aim of study:To assess the relative strain of fat and glandular tissue as well as the strain ratio in relation to different mammographic breast density categories and with respect to the depth of tissue Subjects and Methods: A total of 162 women with normal breast imaging were included and categorized according to the American College of Radiologists (ACR) mammographic breast density categories (A, B , C , D). Elastography examination was performed for a single normal breast and the relative strain of the fatty tissue and glandular tissue at most superficial location as well as at 1-2 cm in depth were recorded . In addition the fat:glandular strain ratio was estimated. The elastographic variables were assessed with each other and with respect to breast density & the tissue depth Results:The subjects were subdivided according to ACR density system. It has been found that the mean relative strain of fat was higher in category A (0.30) in comparison with the other categories with statistically significant difference , however the relative strain of the glandular tissue was rather comparable between category A (0.25), category B (0.24) and category C (0.22) with statistically significant difference with category D(0.20) .The strain ratio was significantly different in category B (1.10) and category C (1.05) in comparison with category A (1.22).Comparing the relative strain in different depth revealed rather similar results where relative strain of fat in ( 1- 2 cm ) below Ref show significant difference in category B(0.21) and category C(0.20) in comparison with category A (0.29) as category group D is extremely dense so no fat tissue below ROI could be measured. Similarly the relative strain of glandular tissue in 1-2 cm below ROI show significant difference between category A (0.23) and category B (0.22) with category C and D (0.19 for both). Conclusion:The fat tissue elasticity is rather different in different breast density categories with higher elasticity of the fat in category A breast density (fatty breasts) as compared to other breast density categories .The glandular tissue strain is rather more comparable in the breasts in category B, C, and D and thus it may more convenient to use the glandular tissue as a reference tissue when assessing the relative elasticity of breast masses. Moreover , the depth of the measurement of the tissue elasticity has a significant influence on the quantitative assessment of the elastographic image

strain elastography --- ACR breast density --- relative strain --- strain ratio.