Optimal vortex formation time is not attained in the heart: In vivo analysis by dynamic 3D enstrophy mapping from 4D Flow MRI

We use dynamic 3D velocity field from in vivo 4D Flow MRI to revisit the optimal vortex formation time (OVFT) hypothesis in human left ventricle during diastole as opposed to the previous 2D-based derivation. In total, 34 healthy volunteers (20-68 years) underwent in-vivo 4D Flow MRI. Dynamic 3D LV geometry was segmented over E-filing (~12 time-points). 3D voxel-wise enstrophy maps were computed over the LV. We then constructed a profile of the instantaneous total volumetric enstrophy (Ei) evolution relative to the vortex formation time (Ti). Despite the wide age range of the studied population, 4D Flow MRI-based volumetric results showed that in all studied healthy subjects Ei never attains its asymptotic maximum at the “optimal” time (Ti~4). Instead, Ei consistently presented a decay phase at around Ti=1.23±0.25, stimulated by the inflow interaction with the confining dynamic LV geometry. Our results indicate that the OVFT hypothesis is not applicable in vivo in the healthy LVs studied due to the revealed decay phase that has not been reported in previous 2D derivations. Thus, our results warrant the need for studies revising/extending the previously hypothesized binary classification (optimal or non-optimal vortex formation) for assessing diastolic (dys)function in vivo.