The blood flow through the liver is unique and complex, mainly because the liver receives blood from two inlet vessels. This results in complicated flow patterns, especially at the microcirculation level. At this level, the liver is often schematized as a lattice of hexagonal liver lobules with blood inlets at the corners and a blood outlet in the center. In between, an interconnected network of tortuous tiny vessels is situated, called the sinusoids. Little is known about the blood flow characteristics at this micro scale. Previous studies assume that the permeability is identical in all directions (isotropy). But the IBiTech team wanted to find out if this assumption is correct, since this can play a crucial role in for example preserving donor livers prior to transplantation. To this end, they developed a model of the liver microcirculation based on 3D reconstructions aided by software from Materialise.
 

Modeling Liver Microcirculation with MimicsMagics and 3-matic

The liver is a very complex organ, certainly at micro-level
To begin the study, the team scanned a sample of a human liver vascular replica using a high resolution micro-CT scanner. They then used Materialise’s Mimics software to segment the dataset based on gray values, thereby extracting the features of the liver lobules and sinusoids. This made it possible to calculate a 3D reconsutruction and visualize three liver lobules. Afterwards, in order to create the fluid domain for the computational fluid dynamics (CFD) model, the team used Materialise’s Magics software to virtually dissect a small cube representing a network of sinusoids. To make this geometry easier to work with, it was further improved using 3-matic (Materialise) and TGrid (Ansys).
 
Virtual dissection of a sinusoidal cube out of a liver lobule
For the next step of the process, the team studied the blood flow through the resulting simulation geometry in three orthogonal directions, which allows them to quantify perfusion characteristics such as the permeability. The blood flow simulation, performed with Fluent (ANSYS) provided data on pressure differences, preferential flow pathways and wall shear stresses. Furthermore, post-processing enabled calculating the permeability characteristics. As such, it became clear from the results of the team’s simulations that the permeability of the sample was not equal in all directions, as was assumed by previous researches. More specifically, the sample showed a longitudinal permeability that was almost twice as high as for the other directions.
 

Gaining Insights and Seeing Potential for Future Research

The research team’s data indicate that the human liver microcirculation is characterized by a permeability behavior that is dependent on the flow direction (anisotropy). As such, liver microcirculation models should take into account these sinusoidal anisotropic permeability properties. 
In conclusion, this modeling approach helps to gain more insight into the complex liver microhemodynamics. Towards the future, it is applicable to study the liver microcirculation in several other (un)physiological and pathological conditions (e.g. machine perfusion, portal hypertension, liver cirrhosis, etc.).

Simulation results in r-direction: pressures

Simulation results in r-direction: preferential flow path

Simulation results in r-direction: wall shear stress

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