Computational Fluid Dynamics in Aneurysm Research: Critical Reflections, Future Directions

Abstract

significant parameter that has yet to be fully evaluated.2 New technologies develop and evolve so as to both optimize their capabilities and expand the applications to which they may be applied. Such it was with x-rays, and such it is with CFD. New technologies are most often overvalued when first described; then, as they become disseminated, they sink below their true value, finally reaching a state of realistic value only when they have been widely tested and optimized. Potential applications of CFD have not been fully explored, optimization of computational techniques for assessing blood flow in and around IAs is ongoing, and the definition of the most meaningful output parameters are not at a stage where there can be any broad consensus. Thus, in our opinion, it is not realistic to make a value judgment regarding the ultimate value of CFDs, either as a means for investigating basic hemodynamic phenomena or as a tool that may be useful in a clinical environment. Next Step and Closing RemarksTo us, it seems implausible to expect that, in isolation, CFD studies may reveal singular keys to important questions about a biologic process such as the initiation, growth, and rupture of IAs. It does, however, seem quite plausible that the results from CFD studies on large populations could provide great help in categorizing aneurysms according any number of hemodynamic parameters. Perhaps, then, these categories, when correlated with other factors known to be important in vascular health-such as collagen mutations, smoking, family history, and so on-and then if further combined with information specific to individual patients-such as age, sex, and perianeurysmal environment-could give insights that might prove useful in predicting the risk of aneurysm rupture. We fully realize that correlations do not represent causation; however, in our experiences, as well as in those of others, they sometimes offer very significant hints. 3,4 As the ability to perform CFD in clinical environments on large numbers of patients increases, as more insight is gained into the regulation of arterial health (homeostasis) and remodeling, as more understanding is gained about the mechanics of the vascular wall, as the ability to image not only the vascular lumen but also the arterial wall increases, this additional information may send computational scientists back to broaden and refine their mathematic models, thereby leading to methods that would allow investigation and integration of other important and potentially clinically relevant parameters, such as collagen turnover, cross-linking, and so on (eg, fluid-structure-growth modeling).Believing in the great potential for the integration of observations and measurements made by clinicians with simulations, calculations, and models made by scientists, we feel that this is a time to be optimistic and proactive in collaborations that unite and optimize our ability to define just what value CFD adds to the ability to mitigate the death and misery currently associated with IAs.

Mind the Gap: Impact of Computational Fluid Dynamics Solution Strategy on Prediction of Intracranial Aneurysm Hemodynamics and Rupture Status Indicators

“…[2][3][4] Although it has been questioned whether these simulations are really patient specific owing to the various modeling assumptions and uncertainties, 5 CFD is, arguably, a promising tool for future clinical use. 6,7 Key factors that determine the accuracy of a particular CFD simulation are the temporal and spatial discretizations specified by the operator. In principle, these are chosen in anticipation of the expected hemodynamics and then must be demonstrated to converge to within some desired error tolerance via methodical refinement studies.…”

CFD: Computational Fluid Dynamics or Confounding Factor Dissemination? The Role of Hemodynamics in Intracranial Aneurysm Rupture Risk Assessment

“…Furthermore, study of the mechanobiology of the IA wall, for example, the interaction of aneurysm biology and wall mechanics, will help researchers better understand the mechanisms of IA growth and rupture that cannot be explained wholly by hemodynamics. 9 It is important to recognize that most aneurysm datasets obtained clinically are composed of cross-sectional data. Bringing computational tools to the bedside can improve our ability to follow and analyze aneurysm hemodynamics and natural history longitudinally.…”

“…[7][8][9][10] Findings from different studies correlating WSS to IA progression are not converging: both high and low WSS are correlated to growth and rupture. To gain a better understanding of these divergent findings, we systematically review the current literature on hemodynamic studies correlating high and low WSS to IA growth and rupture in the following 2 sections.…”

“…This thought-and debate-provoking question reflects wide concerns over the growing number of hemodynamic parameters and conflicting findings reported in the current literature. Indeed, 3 other editorials published in the same journal [8][9][10] appeared shortly after Kallmes's editorial. Cebral and Meng 8 offered the first counterpoint, in which they commented that inconsistencies in hemodynamic parameter definitions and conflicting findings reflect exploratory efforts of isolated groups with small datasets; yet, more importantly, the conflicting findings could originate from the intrinsic complexity of aneurysm growth and rupture mechanisms and our scarce knowledge about them.…”

Biomechanical Impact of Wrong Positioning of a Dedicated Stent for Coronary Bifurcations: A Virtual Bench Testing Study

“…31 On the other hand, it may also reflect the variability in the assumptions and compromises of aneurysm CFD studies, as well as inconsistent definitions of these (e.g., absolute vs. normalized) and other hemodynamic parameters associated with rupture. 5,31,36 Image-based CFD is subject to numerous sources of uncertainty along its pipeline: the clinical modality used to image the aneurysm 4,16,17 ; digital segmentation of the lumen, often requiring subjective decisions about thresholds, filtering, smoothing, etc. 15,34,38 ; truncation of the domain and attendant assumptions about velocity boundary conditions 7,19,30 ; the need to assume flow rates, 21,25,32 since patient-specific measurements are rarely available; the pragmatic assumption of rigid walls 2,12,46 and simple blood rheologies 6,27,48 when, similarly, patient-specific properties are difficult or impossible to obtain; and the choice of mesh and time-step resolutions, as well as other CFD solver settings.…”