An in-vivo study on the difference between principal and cardiac strains

Regional myocardial deformation is an important parameter for the assessment of regional myocardial function. As such, ultrasound methods have been proposed to estimate myocardial strain non-invasively in one, two or - more recently - in three dimensions. Although strain is most often reported in a local cardiac coordinate system (radial, longitudinal, circumferential), its calculation implies that these directions are known. As this typically requires (manual) segmentation of the myocardium, authors sometimes report on the principal strains instead as they can simply be obtained through diagonalization of the strain tensor. The assumption made is that the normal cardiac strain components are close to the principal strains but this has not explicitly been tested. The aim of this study was therefore to quantify the difference in strain values obtained in both the principal and cardiac coordinate systems and to define the average position of both coordinate systems with respect to each other in an in-vivo setting.