Computed poststenotic flow instabilities correlate phenotypically with vibrations measured using laser Doppler vibrometry : perspectives for a promising in vivo device for early detection of moderate and severe carotid stenosis

Early detection of asymptomatic carotid stenosis is crucial for treatment planning in the prevention of ischemic stroke. Auscultation, the current first-line screening methodology, comes with severe limitations that create urge for novel and robust techniques. Laser Doppler vibrometer (LDV) is a promising tool for inferring carotid stenosis by measuring stenosis-induced vibrations. The goal of the current study was to evaluate the feasibility of LDV for carotid stenosis detection. LDV measurements on a carotid phantom were used to validate our previously verified high-resolution computational fluid dynamics methodology, which was used to evaluate the impact of flowrate, flow split, and stenosis severity on the poststenotic intensity of flow instabilities (IFI). We evaluated sensitivity, specificity, and accuracy of using IFI for stenoses detection. Linear regression analyses showed that computationally derived pressure fluctuations correlated (R2 = 0.98) with LDV measurements of stenosis-induced vibrations. The flowrate of stenosed vessels correlated (R2 = 0.90) with the presence of poststenotic instabilities. Receiver operating characteristic analyses of power spectra revealed that the most relevant frequency bands for the detection of moderate (56–76%) and severe (86–96%) stenoses were 80–200 Hz and 0–40 Hz, respectively. Moderate stenosis was identified with sensitivity and specificity of 90%; values decreased to 70% for severe stenosis. The use of LDV as screening tool for asymptomatic stenosis can potentially provide improved accuracy of current screening methodologies for early detection. The applicability of this promising device for mass screening is currently being evaluated clinically

Prediction of post stenotic flow instabilities in a patient-specific common carotid artery model

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High-frequency fluctuations in post-stenotic patient specific carotid stenosis fluid dynamics : a computational fluid dynamics strategy study

PurposeScreening of asymptomatic carotid stenoses is performed by auscultation of the carotid bruit, but the sensitivity is poor. Instead, it has been suggested to detect carotid bruit as neck's skin vibrations. We here take a first step towards a computational fluid dynamics proof-of-concept study, and investigate the robustness of our numerical approach for capturing high-frequent fluctuations in the post-stenotic flow. The aim was to find an ideal solution strategy from a pragmatic point of view, balancing accuracy with computational cost comparing an under-resolved direct numerical simulation (DNS) approach vs. three common large eddy simulation (LES) models (static/dynamic Smagorinsky and Sigma).MethodWe found a reference solution by performing a spatial and temporal refinement study of a stenosed carotid bifurcation with constant flow rate. The reference solution dwas compared against LES for both a constant and pulsatile flow.ResultsOnly the Sigma and Dynamic Smagorinsky models were able to replicate the flow field of the reference solution for a pulsatile simulation, however the computational cost of the Sigma model was lower. However, none of the sub-grid scale models were able to replicate the high-frequent flow in the peak-systolic constant flow rate simulations, which had a higher mean Reynolds number.ConclusionsThe Sigma model was the best combination between accuracy and cost for simulating the pulsatile post-stenotic flow field, whereas for the constant flow rate, the under-resolved DNS approach was better. These results can be used as a reference for future studies investigating high-frequent flow features

Investigating the Link Between Patient-specific Morphology and Hemodynamics: Implications for Aneurism Initiation?

Brain aneurysms are focalized diseased blood vessels, often shaped like a balloon, which can rupture and cause bleeding in the brain. The presence of aneurysms is correlated with internal carotid artery (ICA) extracranial area variations and wider angles in the intracranial ICA terminus bifurcation. The goal of this thesis was to investigate a plausible hemodynamic stimulus that is statistically correlated with aneurysm presence by altering the morphological features in patient-specific geometries accordingly. The thesis is split into the following three chapters. To trust the numerical results, we first validate the solver for biomedical flows by employing the U.S Food and Drug Administration's benchmark for an idealized medical device [Stewart et al. 2012 Cardiovascular Engineering and Technology]. We obtained different results compared to previous studies, but offering reasonable explanations to the observed discrepancies, we put faith in our numerical solution. In the second chapter, a framework for objective manipulation of morphological features of intracranial arteries is presented and validated. As part of this work, an incremental improvement is made to the method for aneurysm removal presented in [Ford et al. 2009, British journal of radiology]. Finally, in the third chapter, we use the results from the previous two chapters to computationally investigate a plausible stimulus causing aneurysm initiation. Our results indicate that the plausible hemodynamic stimulus that statistically correlated with the presence of aneurysms is instable flow. That being said, in vitro/vivo studies are needed to confirm the mechanistic link between flow instabilities and the presence of aneurysms. Finally, we also present plausible explanations for why the leading theory of aneurysm initiation is an unlikely to occur in vivo

The FDA nozzle benchmark: “In theory there is no difference between theory and practice, but in practice there is”

The utility of flow simulations relies on the robustness of computational fluid dynamics (CFD) solvers and reproducibility of results. The aim of this study was to validate the Oasis CFD solver against in vitro experimental measurements of jet breakdown location from the FDA nozzle benchmark at Reynolds number 3500, which is in the particularly challenging transitional regime. Simulations were performed on meshes consisting of 5, 10, 17, and 28 million (M) tetrahedra, with Δt   =  10−5 seconds. The 5M and 10M simulation jets broke down in reasonable agreement with the experiments. However, the 17M and 28M simulation jets broke down further downstream. But which of our simulations are “correct”? From a theoretical point of view, they are all wrong because the jet should not break down in the absence of disturbances. The geometry is axisymmetric with no geometrical features that can generate angular velocities. A stable flow was supported by linear stability analysis. From a physical point of view, a finite amount of “noise” will always be present in experiments, which lowers transition point. To replicate noise numerically, we prescribed minor random angular velocities (approximately 0.31% ), much smaller than the reported flow asymmetry (approximately 3% ) and model accuracy (approximately 1% ), at the inlet of the 17M simulation, which shifted the jet breakdown location closer to the measurements. Hence, the high‐resolution simulations and “noise” experiment can potentially explain discrepancies in transition between sometimes “sterile” CFD and inherently noisy “ground truth” experiments. Thus, we have shown that numerical simulations can agree with experiments, but for the wrong reasons