Using Image-based CFD to Investigate the Intracardiac Turbulence

Abstract

A numerical framework designed to compute the blood flow in patientspecific human hearts is presented. The geometry of the heart cavities and associated wall motion are extracted from 4D medical images while the valves of the heart are accounted for thanks to low order geometrical models. The resulting blood flow equations are solved using a fourth-order low-dissipative finite-volume scheme and a mixed Aribtrary Lagrangian-Eulerian / Immersed Boundary framework. On top of retrieving the main fluid flow phenomena commonly observed in the left heart, the methodology allows studying the heart flow dynamics, including the turbulence characteristics and cycle-to-cycle variations.

Image-Based Simulations Show Important Flow Fluctuations in a Normal Left Ventricle: What Could be the Implications?

“…The intrinsic technical limitations of the CMR exams or the numerical strategies generally employed in LV CFD studies are likely to be responsible for the quasi-absence of studies about fluctuations, as they cannot grasp the entire nature of the flow. Fluctuations were reported in our previous studies 9,10 in an abnormal heart and very recently, using a new CMR method to evaluate the intensity and the localization of velocity fluctuations, Zajac et al 53 confirmed the presence of non-negligible in vivo level of "turbulent" kinetic energy in abnormal and normal LVs. As the nature of the flow directly affects the characteristic time scales and local levels of flow stresses, we believe that the analysis of the velocity fluctuations can provide a new paradigm in the assessment of the cardiovascular flow and mechanosensitive feedbacks.…”

“…The stroke volume is 68 mL and the ejection fraction is 62 %, which falls within the normal physiological range 31 . Details are provided in previous studies 9,10 . The aortic valve (AV) and the MV are difficult to extract from the medical images as they are thin and highly moving structures.…”

“…However, as valves have an impact on the flow structure 5 , low-order models were included in the domain. The valve models have been described in detail in a previous work 9,10 and are only briefly recalled here. Annuluses are reconstructed by inspecting the medical images and placing markers manually on the native 3D domain.…”

“…The open area available for the fluid flow is modelled as an ellipse of axis a = 15 mm and b = 11 mm. Knowing the MV leaflets position during the heart cycle, their effect on the blood flow is accounted for by using an immersed boundary method 9 . For this purpose, the leaflets representations are given a thickness so that a few mesh nodes are located within the valves.…”

“…Under this assumption, the flow rate entering the flow domain is related to the time evolution of either the atrium volume or the atrioventricular volume, by using the mass conservation principle. In other words, the boundary y z conditions are derived from the medical images 9 and the associated heart deformations over time. The resulting waveform is applied as a boundary condition at the four pulmonary veins, assuming an equipartition between the four inlets.…”