(431d) Patient-Specific CFD Simulations of the Carotid Artery to Predict Stroke

One out of every twenty deaths in the United States are caused by stroke,¹ eighty-seven percent of which are ischemic and a result of atherosclerosis.² The carotid artery is the major blood vessel responsible for supplying blood to the brain. When plaque deposition hinders the flow in this artery, physicians face important clinical decisions regarding the need for treatment. Carotid endarterectomies are invasive, and therefore there is a need for other predictive models and thresholds to aid medical care decisions.³ The carotid artery carries oxygenated blood from the heart to the head. There are two carotid arteries on each side of the neck. Each common carotid artery branches into two divisions: the internal carotid, supplying blood to the brain, and the external carotid that provides blood to the face.⁴ As the carotid ascends towards the head, it branches out and creates a bifurcation. The internal carotid is especially interesting as it contains a bulge, the carotid sinus, with a diameter greater than the common carotid.⁵ This geometry and the resulting increase in adverse pressure gradient create a flow reversal region during the cardiac cycle. Velocity profiles in the carotid sinus may be linked to stroke, as this location is prone to plaque deposition.^6,7 The goal of this project is to investigate velocity profiles and pressure gradients in the carotid artery to understand ability of computational fluid dynamics (CFD) to contribute to the prediction of stroke risk.

Using CFD simulations of the carotid artery, medicine and engineering intersect. CFD uses finite-element analysis to determine flow characteristics while paying specific attention to the geometry, velocity profiles, streamlines, and pressure gradients. This method has an advantage over solely relying on ultrasound data because it provides insight into the unique flow patterns in a patient’s computerized tomography angiography (CTA) scans. Once a mesh, a collection of vertices, edges, and faces that define the shape of an object in computational simulations, has been created, physiological boundary conditions are defined.⁸ These boundary conditions provide important parameters for blood and artery wall density, blood viscosity (Carreau model), outlet pressures, body temperature, and peak systolic (PSV) and end-diastolic (EDV) velocities. Navier-Stokes Equations are solved simultaneously for flow simulations at nodes created in the meshing portion of the application. A solution is generated when residual limits are met.

Carotid artery geometries from patients with different stroke outcomes along with their respective ultrasound data were studied to understand the differences between those patients that experienced a stroke and those that did not. Previous work has qualitatively shown that stroke patients have irregular velocity gradients and more extreme velocities in the carotid sinus.⁷ However, these comparisons of patients with different stroke outcomes require more than a comparison of velocities and geometries in a steady model. A pulsatile flow user-defined function (UDF) for a transient simulation has been developed to consider velocity differences during diastole and systole. In theory, a pulsatile flow model simulates physiological conditions more realistically, as the heart pumps blood in a pulsatile fashion. Validation of the model confirmed that mass conservation is obeyed. Preliminary comparison of multiple patients showed that patients who experienced a stroke have higher ratios of flow reversal. A UDF that instantly extracts velocity and pressure data at the bulge was developed to streamline the process of correlating patient stroke outcomes. Finally, improvements to the model include accounting for the fluid-structure interaction between the artery and the blood, in which the vessel wall is modeled as a homogenous, isotropic, viscoelastic material. This work provides a predictive CFD tool to improve clinical outcomes in patients with risk of stroke.

References

[1] "Stroke | cdc.gov", Centers for Disease Control and Prevention, 2020. [Online]. Available: https://www.cdc.gov/stroke/index.htm.

[2] "Types of Stroke | cdc.gov", Cdc.gov, 2020. [Online]. Available: https://www.cdc.gov/stroke/types_of_stroke.htm#ischemic.

[3] A. Greenstein et al., "Association between minor and major surgical complications after carotid endarterectomy: Results of the New York Carotid Artery Surgery study", Journal of Vascular Surgery, vol. 46, no. 6, pp. 1138-1146, 2007.

[4] N. Meegalla and B. W. Downs, Anatomy, Head and Neck, Facial Arteries. Wake Forest School of Medicine: StatPearls Publishing LLC., 2020.

[5] J. Krejza et al., "Carotid Artery Diameter in Men and Women and the Relation to Body and Neck Size", Stroke, vol. 37, no. 4, pp. 1103-1105, 2006.

[6] Foster, D.; Salerno, D. “A Computational Fluid Dynamics Model of the Carotid Artery”. Presented at the 2017 AIChE Annual Meeting, Minneapolis, MN, October 29-November 3, 2017.

[7] Foster, D.; Weldy, A. “Computational Fluid Dynamics Simulations of the Human Carotid Artery to Predict Strokes in Patients with Carotid Artery Disease”. Presented at the 2019 AIChE Annual Meeting, Chemical Engineering Principles Advancing Medicine Session, Orlando, FL, November 12, 2019.

[8] C. Jiang, "Fluent-3D Bifurcating Artery", Cornell CFD Tutorials, 2020. [Online]. Available: https://confluence.cornell.edu/display/SIMULATION/FLUENT