Cumulative lateral tissue displacement and shear strain maps. A: B-scan ultrasound image ROI. B: Sum of tissue displacement over time (cumulative displacement) during one flexion cycle of the table within the ultrasound image ROI. Red indicates tissue displacement toward the right (rostral) and blue indicates tissue displacement toward the left (caudal). C: Cumulative shear strain within the ultrasound image ROI. Red and blue indicate positive (toward the right) and negative (toward the left) shear strain respectively. (B) and (C) respectively correspond to cumulative tissue displacement and shear strain at the end of one flexion cycle of the motorized table. D: Diagram illustrating positive and negative shear strains which represent sliding or deformation of an object in different directions. The shear component is obtained by taking the gradient of lateral displacement (Ux) along the positive axial direction (+y). The x-y coordinates are defined corresponding to the ultrasound imaging configuration (see axes in Figure 2).