Measuring Flows in Patient-Specific Aneurysms
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Streamlines in a patient-specific cerebral aneurysm.

Measuring Flows in Patient-Specific Aneurysms

Despite its clinical importance, accurately assessing the progression and risk of rupture of cerebral aneurysms is challenging. Hemodynamic factors play an important role in aneurysm progression, but previous studies have reported contradictory findings, preventing specific mechanisms from being defined.

To date, the vast majority of studies use computational fluid dynamics (CFD) with patient-specific geometries. CFD is inherently limited by modeling assumptions, the uncertainty of geometries and boundary conditions obtained from medical images, and validation methods. Other modalities used include 4D Flow MRI measurements, which are under-resolved, noisy, and have a limited dynamic range, as well as particle image velocimetry (PIV), which has primarily only been done in 2D and maintains experimental and segmentation limitations. These modality-specific assumptions and limitations affect the treatment of near-wall velocities and subsequent hemodynamic analysis, likely contributing to the conflicting results reported. Yet, very few studies have considered these confounding factors.

Graphical Abstract.
Patient-specific basilar tip aneurysm. (Left-Top) Flow pathlines at peak systole. (Left-Bottom) Average error in wall shear stress (WSS), oscillatory shear index (OSI), and relative residence time (RRT) caused by virtual voxel averaging (VA) of PIV and CFD data. (Right) Bland-Altman analysis of WSS, OSI, RRT comparing in vivo 4D flow MRI with all in vitro data.

In this work, we used in vivo 4D Flow MRI to inform pulsatile volumetric PIV and CFD modalities in two patient-specific aneurysms. This study reports the first volumetric PIV experiment using patient-specific geometries and pulsatile flow and, thus, the first multi-modality comparison of its kind. We compare wall shear stress (WSS), oscillatory shear index (OSI), and relative residence time (RRT) across each modality. The results demonstrated that OSI, a non-dimensional parameter, was the most robust and consistent across modalities and spatiotemporal resolution.

A major difficulty of multi-modality studies utilizing in vivo measurements is the lack of an established “ground truth”. Thus, this work provides a framework for improving the universality of risk of rupture metrics. Further, this paper takes a first step towards the goal of optimally synthesizing information by harnessing the strengths and compensating for the weaknesses across modalities.

Flow pathlines throughout the pulsatile cycle computed using velocity fields from in vivo 4D flow MRI, in vitro PIV (using Shake the Box, STB), and CFD for two patient-specific geometries.

Published Paper

Data Repository

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