Late systolic onset of regional LV relaxation demonstrated in three-dimensional space by MRI tissue tagging

Left ventricular (LV) relaxation entails myocardial deformation that induces LV filling. Yet, the precise mechanisms of the earliest changes in tissue properties that characterize myocardial relaxation remain incompletely understood. Ten healthy volunteers (seven males), 25-43 yr, underwent tagged and cine MRI with high temporal resolution (25-35 ms). Normal strains including radial (E_rr), circumferential (E_cc), and longitudinal (E_ll) strains, shear strains including E_cl (circumferential-longitudinal), E_cr (circumferential-radial), and E_rl (radial-longitudinal), and principal strains (E₁, E₂, and E₃) were calculated using a displacement field-fitting method. Temporal changes in angular strains indicative of shear and torsion release and normal strains were studied during late systole and early relaxation. The onset of individual relaxation strains was heterogeneous relative to LV filling. Shear strains (E_cr, E_rl, and E_cl) and radial thinning were first to develop. Times of onset of E_cr, E_rl, E_cl, and E_rr occurred 108, 93, 67, and 73 ms before aortic valve closure, respectively. E_ll, E_cc, and LV volume change commenced significantly later after the onset of diastolic shear strains and radial thinning. The onset of E _cc, E_ll, and LV volume change was noted 38 ms before aortic valve closure (P < 0.05 relative to the onset of shear strains and E_rr). Myocardial relaxation is characterized by a three-dimensional unfolding deformation that includes release of torsion, shear, and radial thinning beginning before aortic valve closure. This unfolding pattern precedes longitudinal and circumferential elongation and may facilitate early diastolic filling.