Ad Augst

Various issues relating to computational fluid dynamics simulations of carotid bifurcation flow based on models reconstructed from three-dimensional ultrasound images.

Abstract Computational fluid dynamics (CFD) flow simulation techniques have the potential to enhance understanding of how haemodynamic factors are involved in atherosclerosis. Recently, three-dimensional ultrasound has emerged as an alternative to other three-dimensional imaging techniques, such as magnetic resonance angiography (MRA). The method can be used to generate accurate vascular geometry suitable for CFD simulations and can be coupled with Doppler ultrasound to provide physiologically realistic flow boundary conditions. However, there are various ways to utilize the flow data acquired, possibly leading to different results regarding both flow and wall shear stress patterns. A disadvantage of three-dimensional ultrasound for imaging the carotid bifurcation has been established as being the scanning limitation of the jawbone position. This may make artificial extensions of the internal and/or external carotid arteries necessary, which in turn may influence the predicted flow patterns. Flow simulations were carried out for three outflow calculation schemes as well as four geometries with different extensions to the carotid daughter vessels. It was found that variation of flow patterns was more strongly influenced by the outflow conditions than by the extensions of the daughter vessels. Consequently, it is recommended that for future CFD simulations of carotid flow using three-dimensional ultrasound data, the outflow boundary conditions should rely on the most accurate measurement available, and flow data recorded in the common and internal carotid are considered more reliable than data from the external carotid. Even though the extended lengths of the daughter vessels have insignificant effects on the predicted haemodynamic parameters, it would be a safer option to extend the internal carotid by approximately three times the diameter of the common carotid artery.

3D ultrasound-based CFD for carotid flow prediction: a reproducibility study

Image-based CID has been a prime technique for studying arteriosclerosis, plaque formation, aneurysm rupture and bypass design in the past decade. The imaging techniques used for vessel geometry acquisition are usually X-ray (CT-scan), intravascular ultrasound or most frequently MRI. It has been shown in previous studies that 3D extravascular ultrasound (3D US) can provide a cost-effective alternative for imaging superficial arteries like the carotid bifurcation or femoral arteries. In this study, the carotid bifurcation of 9 healthy subjects have been scanned twice within two to six weeks. CFD models for each subject and each scan were built using the corresponding anatomical data acquired in vivo. Overall reproducibility was satisfactory. Two main sources of error were identified. (1) Blurred border between vessel lumen and endothelium, causing the operator to overestimate the lumen area; and (2) altered neck angles. It is expected that with further improvement in ultrasound image quality and with standardisation of the imaging protocol, 3D US has a huge potential to become a viable alternative to MRI both for clinical and research uses.