Abstract
In this study, the flow through a generic abdominal aneurysm under realistic pulsating flow conditions is examined with magnetic resonance velocimetry (MRV), laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD). The influence of flow phenomena on the wall shear stress (WSS) is examined. It is seen that a strong vortex ring develops during systole at the proximal end of the aneurysm and subsequently travels downstream and decays. The vortex formation plays a major role in the temporal and spatial distribution of the WSS, which is analyzed in detail. A peak of the WSS is observed for a very limited time and in a very localized region where the vortex ring initially develops. The intrinsic temporal averaging during the acquisition of the MRV data is found to significantly decrease this peak. CFD and LDV results, which are averaged in the same manner, show a similar behavior. This indicates that besides the spatial resolution, the temporal resolution is a crucial factor, which needs to be considered especially in flows where vortex rings are observed. Results from LDV and CFD show excellent agreement for the velocity field obtained by MRV. While the flow is found to be laminar in the undilated diameter, results show laminar–turbulent transitional behavior for specific phases of the cycle within the aneurysm bulk. Although MRV is not capable of measuring instantaneous velocity fluctuations, we show that the periodic increase in turbulence intensity can be observed from image artifacts in the MRV data. These artifacts increase the velocity uncertainty, which correlates well with the velocity fluctuations measured with LDV. Although the flow encounters laminar and transitional conditions as well as multiple vortices and stagnation and reattachment points, the improved instability-sensitive Reynolds stress model, which is used for the numerical simulations of this work, shows very good agreement with the measurements. Significant effort has been expended by numerous research groups in recent years in improving the estimation of WSS from MRV data. However, an assessment of these various post-processing methods is only possible if the true values of the WSS are known. The present study is therefore aimed at providing such ground truth WSS values as well as the corresponding MRV data, allowing also other research groups to validate their WSS estimation methods using the experimental data set presented in this work.Graphic
Highlights
In cardiovascular medicine, abnormal changes of the vessel geometry are of great interest
A prominent example of such a change is the enlargement of the aortic vessel diameter, so-called abdominal aortic aneurysms (Aggarwal et al 2011), whose initiation and growth triggers are subject to current discussion (Kemmerling and Peattie 2018)
The most common practice is a measurement with phase contrast magnetic resonance imaging (PC-MRI), termed magnetic resonance velocimetry (MRV), which has gained much attention in the last few years due to the work of Markl et al (2012)
Summary
Abnormal changes of the vessel geometry are of great interest. A prominent example of such a change is the enlargement of the aortic vessel diameter, so-called abdominal aortic aneurysms (Aggarwal et al 2011), whose initiation and growth triggers are subject to current discussion (Kemmerling and Peattie 2018). Almost all investigations agree that the wall shear stress (WSS) is one of these and of major importance in the emergence of such cardiovascular diseases (van Ooij et al 2017). Different opinions exist about which quantitative values of the wall shear stress initiate which kind of disease. The general assumption at the moment is that very high or low magnitudes of the WSS contribute to aneurysm growth (Boussel et al 2008; Watton et al 2011; Miura et al 2013; Boyd et al 2016), while spatial gradients provoke the initial development (Boussel et al 2008; Meng et al 2007; Dolan et al 2013). MRV measurements are commonly used to measure the WSS, they may be subjected to large measurement errors and uncertainties, caused by the limited spatial resolution
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