By Rob Woolhouse, Fluent Europe Ltd.

View the pdf of this article

The use of CFD in new and diverse application areas is becoming more widespread. In one exciting new field, magnetic resonance imaging (MRI) technology is being combined with computer modeling to simulate the flow in complex physiological channels within the human body. One advantage of using CFD for this purpose is that it allows multiple tests and experiments to be carried out to minimize clinical research. Another is that virtual simulations on an individual prior to surgery can make the operation proceed more smoothly and result in a more successful outcome.

The pulmonary artery, imaged using the FLUENT 6 mesh

As an example of this promising new capability, flow through a pulmonary artery has recently been studied. These results are being used to highlight areas that can result in clotting sites or aneurysms. Clots are formed in low and stagnant flow regions where low fluid shear and high residence times are observed. Conditions such as high surface pressure, shear stress, or strong gradients can result in an aneurysm, where the vessel wall bulges outward, forming a pocket. The repair of aneurysms is normally done in a surgical procedure in which a stent is inserted to stabilize the vessel. By using CFD, the stent location and design can be modeled prior to the operation to determine the optimum size and orientation of the device, reducing the risk of unintentional damage and the time required for the procedure.

The actual process of converting patient data into a suitable CFD geometry is not trivial. Many steps are required, and for the pulmonary artery project, several of these involved the collaboration of Materialise (based in Leuven, Belgium) and their proprietary software, Mimics and Magics. Mimics converts MRI slices into a 3D solid model, and exports in a variety of CAD compatible file formats, including stereolithography (STL). Magics is a dedicated STL editor with a comprehensive set of surface repair tools.

Mimics software from Materialise is used to create a geometry of the chest cavity, and then bones, lungs and unconnected vessels are removed

Pathlines inside the artery and branches colored by velocity

For the pulmonary artery project, an MRI scan of a chest cavity was obtained from the Sheffield University MRI Unit. MRI scan slices are typically produced in a greyscale pixelated DICOM format, and these were joined together to create a 3D solid model. Vessels not connected to those of interest, as well as bones and other tissue, were removed. The 3D solid model was then exported as a 3D surface in STL format, and further edited to remove all additional unwanted features, leaving the pulmonary artery and its primary branches for the CFD model. The resulting smooth geometry of the artery was then read into GAMBIT, where a tetrahedral surface mesh was created. Due to the complexity of the model, further surface mesh adjustment and volume meshing were performed in TGrid.

The physics of blood flow through the body has been the focus of a number of studies over the years, and many of the findings were incorporated into the current model. For example, fluid structure interaction can be neglected for the pulmonary artery, since the thick vessel wall is designed to carry large quantities of blood under (relatively) high pressure, directly away from the heart. Initial checks also confirmed that the flow regime was laminar. Because blood is a non-Newtonian fluid, the shear effect on viscosity needed to be considered. The Carreau-Yasuda model was implemented through a user-defined function (UDF). A velocity boundary was applied to the single large inlet, with a transient, periodic profile that reflects the flow supplied by the heart. Pressure outlet boundary conditions (of equal pressure) were used for the multiple outlets in the model, and the flow split was determined by the vein geometry.

Plots of velocity vectors indicate that there are no recirculation regions or dead zones within the artery or it's primary branches, which was expected, since the scans were taken from a healthy adult. Surface contours of wall shear stress show an increase near some of the constrictions in the vessels. However, it is unlikely that these sites would result in the formation of an aneurysm, since the flow in these regions is not directed toward the surface.

Wall shear stress on the surface of the artery and branches

Overall, this emerging technology shows promise for medical procedures in the future, since it can provide important information specific to an individual using non-invasive tools.

The author thanks the Sheffield University MRI Unit for their assistance with this project.

FluentNEWS