Peak systolic or maximum intra-aneurysmal hemodynamic condition? Implications on normalized flow variables

Impact of workflow variability on image-based intracranial aneurysm hemodynamics

The interplay between intracranial aneurysm progression and hemodynamics motivates the application of image-based blood flow quantification, providing potential for the identification of high-risk aneurysms, treatment planning, and implant optimization. However, uncertainties arise throughout the interdisciplinary process, from medical imaging to parameter evaluation. This study systematically analyzes uncertainty globally, at individual workflow steps and for potential interactions. Eight factors affecting hemodynamic simulation accuracy - image reconstruction, lumen segmentation, surface smoothing, rheological modeling, inlet/outlet boundary condition, ostium/parent vessel definition - are varied for four representative patient-specific intracranial aneurysms. A total of 1024 transient simulations are evaluated considering twelve hemodynamic parameters to assess marginal and interaction effects. Global uncertainty analysis reveals median absolute deviations of 20.8-25.9 % for maximum velocity (Vmax), 6.8-19.2 % for inflow concentration index (ICI), 10.8-40.8 % for normalized wall shear stress (WSSnorm) and 2.8-48.9 % for low shear area (LSA). Isolated variation demonstrates the highest median deviations for the reconstruction algorithm (Vmax: 4.0-7.0 %, ICI: 6.8-18.9 %, WSSnorm: 13.3-25.1 %, LSA: 2.4-16.0 %), inlet (Vmax: 41.5-52.4 %, ICI: 1.4-8.6 %, WSSnorm: 14.6-28.5 %, LSA: 5.5-93.5 %) and outlet boundary condition (Vmax: 2.0-36.5 %, ICI: 0.6-39.9 %, WSSnorm: 2.4-83.2 %, LSA: 1.9-53.5 %). Lowest median deviations are found for rheological modeling and surface smoothing. Only minor interaction effects are observed between the reconstruction algorithm and inlet definition, as well as between inlet and outlet definitions. This study identifies pivotal variables essential for consistent hemodynamic quantification of intracranial aneurysms. Minimal interaction effects validate the isolated analysis of influencing factors in the majority of cases.

Virtual endovascular treatment of intracranial aneurysms: models and uncertainty

Virtual endovascular treatment models (VETMs) have been developed with the view to aid interventional neuroradiologists and neurosurgeons to pre-operatively analyze the comparative efficacy and safety of endovascular treatments for intracranial aneurysms. Based on the current state of VETMs in aneurysm rupture risk stratification and in patient-specific prediction of treatment outcomes, we argue there is a need to go beyond personalized biomechanical flow modeling assuming deterministic parameters and error-free measurements. The mechanobiological effects associated with blood clot formation are important factors in therapeutic decision making and models of post-treatment intra-aneurysmal biology and biochemistry should be linked to the purely hemodynamic models to improve the predictive power of current VETMs. The influence of model and parameter uncertainties associated to each component of a VETM is, where feasible, quantified via a random-effects meta-analysis of the literature. This allows estimating the pooled effect size of these uncertainties on aneurysmal wall shear stress. From such meta-analyses, two main sources of uncertainty emerge where research efforts have so far been limited: (1) vascular wall distensibility, and (2) intra/intersubject systemic flow variations. In the future, we suggest that current deterministic computational simulations need to be extended with strategies for uncertainty mitigation, uncertainty exploration, and sensitivity reduction techniques. WIREs Syst Biol Med 2017, 9:e1385. doi: 10.1002/wsbm.1385 For further resources related to this article, please visit the WIREs website.

Does Arterial Flow Rate Affect the Assessment of Flow-Diverter Stent Performance?

BACKGROUND AND PURPOSE

Our aim was to assess the performance of flow-diverter stents. The pre- and end-of-treatment angiographies are commonly compared. However, the arterial flow rate may change between acquisitions; therefore, a better understanding of its influence on the local intra-aneurysmal hemodynamics before and after flow-diverter stent use is required.

MATERIALS AND METHODS

Twenty-five image-based aneurysm models extracted from 3D rotational angiograms were conditioned for computational fluid dynamics simulations. Pulsatile simulations were performed at different arterial flow rates, covering a wide possible range of physiologic flows among 1-5 mL/s. The effect of flow-diverter stents on intra-aneurysmal hemodynamics was numerically simulated with a porous medium model. Spatiotemporal-averaged intra-aneurysmal flow velocity and flow rate were calculated for each case to quantify the hemodynamics after treatment. The short-term flow-diverter stent performance was characterized by the relative velocity reduction inside the aneurysm.

RESULTS

Spatiotemporal-averaged intra-aneurysmal flow velocity before and after flow-diverter stent use is linearly proportional to the mean arterial flow rate (minimum R² > 0.983 of the linear regression models for untreated and stented models). Relative velocity reduction asymptotically decreases with increasing mean arterial flow rate. When the most probable range of arterial flow rate was considered (3-5 mL/s), instead of the wide possible flow range, the mean SD of relative velocity reduction was reduced from 3.6% to 0.48%.

CONCLUSIONS

Both intra-aneurysmal aneurysm velocity and flow-diverter stent performance depend on the arterial flow rate. The performance could be considered independent of the arterial flow rates within the most probable range of physiologic flows.

Numerical Study of Cerebroarterial Hemodynamic Changes Following Carotid Artery Operation: A Comparison Between Multiscale Modeling and Stand-Alone Three-Dimensional Modeling

Free outflow boundary conditions have been widely adopted in hemodynamic model studies, they, however, intrinsically lack the ability to account for the regulatory mechanisms of systemic hemodynamics and hence carry a risk of producing incorrect results when applied to vascular segments with multiple outlets. In the present study, we developed a multiscale model capable of incorporating global cardiovascular properties into the simulation of blood flows in local vascular segments. The multiscale model was constructed by coupling a three-dimensional (3D) model of local arterial segments with a zero-one-dimensional (0-1-D) model of the cardiovascular system. Numerical validation based on an idealized model demonstrated the ability of the multiscale model to preserve reasonable pressure/flow wave transmission among different models. The multiscale model was further calibrated with clinical data to simulate cerebroarterial hemodynamics in a patient undergoing carotid artery operation. The results showed pronounced hemodynamic changes in the cerebral circulation following the operation. Additional numerical experiments revealed that a stand-alone 3D model with free outflow conditions failed to reproduce the results obtained by the multiscale model. These results demonstrated the potential advantage of multiscale modeling over single-scale modeling in patient-specific hemodynamic studies. Due to the fact that the present study was limited to a single patient, studies on more patients would be required to further confirm the findings.