Validation of CFD simulations of cerebral aneurysms with implication of geometric variations.

Flow dynamics is a key player in the initiation and progression of vascular diseases such as atherosclerosis and cerebral aneurysms [1–4]. Numerous hemodynamic parameters, such as wall shear stress (WSS), pressure, oscillatory shear index, wall shear stress gradient, impingement size on the arterial wall, and residence time of blood, have been postulated to indicate the tendency for initiation or progression of these vascular pathologies and to evaluate the effectiveness of medical devices in the treatments of such diseases [1–4]. However, in vivo measurements of these hemodynamic parameters for patients are difficult and often cost prohibitive. The combination of noninvasive diagnostic tools (e.g. MRI, CT, or ultrasound) and image-based computational fluid dynamics (CFD) techniques provides an alternative to in vivo measurements to estimate these patient-specific hemodynamic parameters [5–12]. Such image-based CFD analysis has the potential to provide key hemodynamic feedback for prospective studies of vascular diseases and to plan for individual therapeutic options [6,11,13]. Although the application of image-based CFD is rapidly advancing, the ever-present uncertainties in CFD analysis need to be evaluated. In particular, CFD outputs such as WSS or WSS gradients are extremely sensitive to the acquired in vivo geometry. The accuracy and reproducibility of the image-based CFD geometries depend on the orientation of the subject, physiological condition of the subject, variation in vascular structure, contrast-media injection techniques, and operator-dependent decisions in converting the medical images to CFD models [14,15]. Such variations produce poor geometric renditions that can generate as much as 60% variation in hemodynamic parameters obtained using CFD, especially in complex flow regions such as bifurcation apexes, anastomoses, flow separation zones, and aneurysm inflow zones [8,15–17]. Smoothing can reduce poor geometric renditions [16], but the process is somewhat subjective, resulting in an unknown deviation from the true in vivo geometry and subsequent in vitro and CFD models. In addition, the validity of CFD results relies heavily on both temporal and spatial boundary conditions [17–19], which are not routinely available in clinical practice. Significant efforts have been made to improve image-based CFD techniques [8,14,16,20–22], but there have been insufficient efforts to validate the CFD results with experiments. In vitro experimental studies have often been employed to investigate the hemodynamics of vascular diseases and mechanical treatment devices, as well as assisting the development of CFD techniques. Rhee et al. and Imbesi et al. explored the influence of aneurysm geometry and intervention through flow visualization techniques [23,24]. Particle image velocimetry (PIV) and particle tracking velocimetry have been utilized to study the influence of intervention on aneurysmal flow [25–29]. Direct in vivo measurements of blood flow through x-ray imaging, MRI, and ultrasound techniques have also been explored [10,30–33]. MRI and ultrasound allow noninvasive measurement of instantaneous velocities in the 3D domain. Based on the field data, velocity gradients and strain rates are obtained to determine shear stresses and shear stress gradients. However, to date, in vitro studies have been limited to 2D flow fields and scaled-up models that provide optical access. These studies experience significant difficulties in reproducing the in vivo geometries [23] and in vivo biological environments. Direct in vivo measurements pose technical shortcomings, such as inadequate resolutions to derive the flow velocity near the wall, uncertainties in wall positions and insufficient velocity vectors to estimate the derivatives in complex flow regions [10,11,30,31,34–36]. One of the clinical applications of CFD is cerebral vascular diseases such as cerebral aneurysms. Subarachnoid hemorrhage following cerebral aneurysm rupture is often catastrophic, resulting in 12.4% of patient death before patients receive medical attention [37]. Recent studies using aneurysm geometry to predict the risks of aneurysm rupture relied heavily on the imaged aneurysm geometry [38–41]. Prospective patient-specific aneurysm studies that combine medical imaging, CFD analysis, and knowledge of biological responses to hemodynamics forces can provide insight into the hemodynamics of cerebral aneurysms, scientific assessments on therapeutic planning and, ultimately, positive clinical outcomes. However, quantitative validation of image-based CFD analysis against experimental and in vivo data has, to date, not been fully addressed. Following a prior CFD study, which focused on the characteristics of the hemodynamics in an aneurysm located on a curved vessel [42], a PIV validation effort was undertaken in our lab, leading to the investigation of flow sensitivity to aneurysm geometric variations. The goals of this study were to validate our CFD results with PIV experiments using an experimental aneurysm phantom and to quantify the influence of geometric variations on the aneurysmal flow dynamics.