Regional quantification of myocardial mechanics in rat using 3D cine DENSE cardiovascular magnetic resonance

Rat models have assumed an increasingly important role in cardiac research. However, a detailed profile of regional cardiac mechanics, such as strains and torsion, is lacking for rats. We hypothesized that healthy rat left ventricles (LVs) exhibit regional differences in cardiac mechanics, which are part of normal function. In this study, images of the LV were obtained with 3D cine displacement encoding with stimulated echoes (DENSE) cardiovascular magnetic resonance in 10 healthy rats. To evaluate regional cardiac mechanics, the LV was divided into basal, mid-ventricular, and apical regions. The myocardium at the mid-LV was further partitioned into four wall segments (i.e. septal, inferior, lateral, and anterior) and three transmural layers (i.e. sub-endocardium, mid-myocardium, and sub-epicardium). The six Lagrangian strain components (i.e. E_rr, E_cc, E_ll, E_cl, E_rl, and E_cr) were computed from the 3D displacement field and averaged within each region of interest. Torsion was quantified using the circumferential-longitudinal shear angle. While peak systolic E_cl differed between the mid-ventricle and apex, the other five components of peak systolic strain were similar across the base, mid-ventricle, and apex. In the mid-LV myocardium, E_cc decreased gradually from the sub-endocardial to the sub-epicardial layer. E_ll demonstrated significant differences between the four wall segments, with the largest magnitude in the inferior segment. E_rr was uniform among the four wall segments. E_cl varied along the transmural direction and among wall segments, whereas E_rl differed only among the wall segments. E_rc was not associated with significant variations. Torsion also varied along the transmural direction and among wall segments. These results provide fundamental insights into the regional contractile function of healthy rat hearts, and form the foundation for future studies on regional changes induced by disease or treatments.