Fig. 3.

Pictorial summary of the definition of principal strain. a The deformation of a tissue element from its initial (end-diastolic) to a final (end-systolic) configuration is constituted of longitudinal and circumferential shortening, plus radial expansion (thickening) and 6 angular deformations (shear deformation). When using only three geometry-dependent directions (radial, circumferential and longitudinal), strain obtained in those directions cannot account for shear and, therefore, does not offer a complete description of the strain undergone by the element. b However, the same deformation can be described without shear in terms of principal strain along 3 principal directions, these established through a comparison of the initial and final configurations of the tissue element