Yiemeng Hoi

Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: The ASME 2012 Summer Bioengineering Conference CFD Challenge

Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.

Characterization of volumetric flow rate waveforms at the carotid bifurcations of older adults

While it is widely appreciated that volumetric blood flow rate (VFR) dynamics change with age, there has been no detailed characterization of the typical shape of carotid bifurcation VFR waveforms of older adults. Toward this end, retrospectively gated phase contrast magnetic resonance imaging was used to measure time-resolved VFR waveforms proximal and distal to the carotid bifurcations of 94 older adults (age 68 +/- 8 years) with little or no carotid artery disease, recruited from the BLSA cohort of the VALIDATE study of factors in vascular aging. Timings and amplitudes of well-defined feature points from these waveforms were extracted automatically and averaged to produce representative common, internal and external carotid artery (CCA, ICA and ECA) waveform shapes. Relative to young adults, waveforms from older adults were found to exhibit a significantly augmented secondary peak during late systole, resulting in significantly higher resistance index (RI) and flow augmentation index (FAI). Cycle-averaged VFR at the CCA, ICA and ECA were 389 +/- 74, 245 +/- 61 and 125 +/- 49 mL min(-1), respectively, reflecting a significant cycle-averaged outflow deficit of 5%, which peaked at around 10% during systole. A small but significant mean delay of 13 ms between arrivals of ICA versus CCA/ECA peak VFR suggested differential compliance of these vessels. Sex and age differences in waveform shape were also noted. The characteristic waveforms presented here may serve as a convenient baseline for studies of VFR waveform dynamics or as suitable boundary conditions for models of blood flow in the carotid arteries of older adults.

An objective approach to digital removal of saccular aneurysms: technique and applications.

Human studies of haemodynamic factors in the pathogenesis of cerebral aneurysms require knowledge of the pre-aneurysmal vasculature. This paper presents an objective and automated technique to digitally remove an aneurysm and reconstruct the parent artery, based on lumen geometries segmented from angiographic images. Relying on robust computational geometry concepts, notably Voronoi diagrams of the digitised lumen surface, the aneurysm attachment region is first defined objectively using lumen centrelines. Centrelines within this region are replaced by smooth interpolations, which then guide the interpolation of Voronoi points within the attachment region. Combined with Voronoi points from outside the attachment region, the parent artery lumen, without the aneurysm, can be reconstructed. Plausible reconstructions were obtained, automatically, for a set of 10 side-wall or terminal aneurysms, of various sizes and shapes, from the ANEURISK project data set. Application of image-based computational fluid dynamics analysis to a five side-wall aneurysm cases data set revealed an association between the recently proposed gradient oscillatory number (GON) and the site of aneurysm formation in four of five cases; however, elevated GON was also evident at non-aneurysmal sites. A potential application to the automated delineation of aneurysms for morphological characterisations is also suggested. The proposed approach may serve as a broad platform for investigating haemodynamic and morphological factors in aneurysm initiation, rupture and therapy in a way amenable to large-scale clinical studies or routine clinical use. Nevertheless, while the parent artery reconstructions are plausible, it remains to be proven that they are faithful representations of the pre-aneurysmal artery.

Correlation Between Local Hemodynamics and Lesion Distribution in a Novel Aortic Regurgitation Murine Model of Atherosclerosis

Following surgical induction of aortic valve regurgitation (AR), extensive atherosclerotic plaque development along the descending thoracic and abdominal aorta of Ldlr−/− mice has been reported, with distinct spatial distributions suggestive of a strong local hemodynamic influence. The objective of this study was to test, using image-based computational fluid dynamics (CFD), whether this is indeed the case. The lumen geometry was reconstructed from micro-CT scanning of a control Ldlr−/− mouse, and CFD simulations were carried out for both AR and control flow conditions derived from Doppler ultrasound measurements and literature data. Maps of time-averaged wall shear stress magnitude (TAWSS), oscillatory shear index (OSI) and relative residence time (RRT) were compared against the spatial distributions of plaque stained with oil red O, previously acquired in a group of AR and control mice. Maps of OSI and RRT were found to be consistent with plaque distributions in the AR mice and the absence of plaque in the control mice. TAWSS was uniformly lower under control vs.xa0AR flow conditions, suggesting that levels (>100xa0dyn/cm2) exceeded those required to alone induce a pro-atherogenic response. Simulations of a straightened CFD model confirmed the importance of anatomical curvature for explaining the spatial distribution of lesions in the AR mice. In summary, oscillatory and retrograde flow induced in the AR mice, without concomitant low shear, may exacerbate or accelerate lesion formation, but the distinct anatomical curvature of the mouse aorta is responsible for the spatial distribution of lesions.

Effect of common carotid artery inlet length on normal carotid bifurcation hemodynamics.

Controversy exists regarding the suitability of fully developed versus measured inlet velocity profiles for image-based computational fluid dynamics (CFD) studies of carotid bifurcation hemodynamics. Here, we attempt to resolve this by investigating the impact of the reconstructed common carotid artery (CCA) inlet length on computed metrics of disturbed flow. Twelve normal carotid bifurcation geometries were reconstructed from contrast-enhanced angiograms acquired as part of the Vascular Aging--The Link That Bridges Age to Atherosclerosis study (VALIDATE). The right carotid artery lumen geometry was reconstructed from its brachiocephalic origin to well above the bifurcation, and the CCA was truncated objectively at locations one, three, five, and seven diameters proximal to where it flares into the bifurcation. Relative to the simulations carried out using the full CCA, models truncated at one CCA diameter strongly overestimated the amount of disturbed flow. Substantial improvement was offered by using three CCA diameters, with only minor further improvement using five CCA diameters. With seven CCA diameters, the amounts of disturbed flow agreed unambiguously with those predicted by the corresponding full-length models. Based on these findings, we recommend that image-based CFD models of the carotid bifurcation should incorporate at least three diameters of CCA length if fully developed velocity profiles are to be imposed at the inlet. The need for imposing measured inlet velocity profiles would seem to be relevant only for those cases where the CCA is severely truncated.

Automatic Neck Plane Detection and 3D Geometric Characterization of Aneurysmal Sacs

Geometric indices defined on intracranial aneurysms have been widely used in rupture risk assessment and surgical planning. However, most indices employed in clinical settings are currently evaluated based on two-dimensional images that inevitably fail to capture the three-dimensional nature of complex aneurysmal shapes. In addition, since measurements are performed manually, they can suffer from poor inter and intra operator repeatability. The purpose of the current work is to introduce objective and robust techniques for the 3D characterization of intracranial aneurysms, while preserving a close connection to the way aneurysms are currently characterized in clinical settings. Techniques for automatically identifying the neck plane, key aneurysm dimensions, shape factors, and orientations relative to the parent vessel are demonstrated in a population of 15 sidewall and 15 terminal aneurysms whose surface has been obtained by two trained operators using both level-set segmentation and thresholding, the latter reflecting typical clinical practice. Automatically-identified neck planes are shown to be in concordance with those manually positioned by an expert neurosurgeon, and automatically-derived geometric indices are shown to be largely insensitive to segmentation method or operator. By capturing the 3D nature of aneurysmal sacs and by minimizing observer variability, our approach allows large retrospective and prospective studies on aneurysm geometric risk factors to be performed using routinely acquired clinical images.

Carotid Bifurcation Hemodynamics in Older Adults: Effect of Measured Versus Assumed Flow Waveform

Recent work has illuminated differences in carotid artery blood flow rate dynamics of older versus young adults. To what degree flow waveform shape, and indeed the use of measured versus assumed flow rates, affects the simulated hemodynamics of older adult carotid bifurcations has not been elucidated. Image-based computational fluid dynamics models of N=9 normal, older adult carotid bifurcations were reconstructed from magnetic resonance angiography. Subject-specific hemodynamics were computed by imposing each individuals inlet and outlet flow rates measured by cine phase-contrast magnetic resonance imaging or by imposing characteristic young and older adult flow waveform shapes adjusted to cycle-averaged flow rates measured or allometrically scaled to the inlet and outlet areas. Despite appreciable differences in the measured versus assumed flow conditions, the locations and extents of low wall shear stress and elevated relative residence time were broadly consistent; however, the extent of elevated oscillatory shear index was substantially underestimated, more by the use of assumed cycle-averaged flow rates than the assumed flow waveform shape. For studies of individual vessels, use of a characteristic flow waveform shape is likely sufficient, with some benefit offered by scaling to measured cycle-averaged flow rates. For larger-scale studies of many vessels, ranking of cases according to presumed hemodynamic or geometric risk is robust to the assumed flow conditions.

On the shape of the common carotid artery with implications for blood velocity profiles

Clinical and engineering studies typically assume that the common carotid artery (CCA) is straight enough to assume fully developed flow, yet recent studies have demonstrated the presence of skewed velocity profiles. Toward elucidating the influence of mild vascular curvatures on blood flow patterns and atherosclerosis, this study aimed to characterize the three-dimensional shape of the human CCA. The left and right carotid arteries of 28 participants (63 ± 12 years) in the VALIDATE (Vascular Aging--The Link that Bridges Age to Atherosclerosis) study were digitally segmented from 3D contrast-enhanced magnetic resonance angiograms, from the aortic arch to the carotid bifurcation. Each CCA was divided into nominal cervical and thoracic segments, for which curvatures were estimated by least-squares fitting of the respective centerlines to planar arcs. The cervical CCA had a mean radius of curvature of 127 mm, corresponding to a mean lumen:curvature radius ratio of 1:50. The thoracic CCA was significantly more curved at 1:16, with the plane of curvature tilted by a mean angle of 25° and rotated close to 90° with respect to that of the cervical CCA. The left CCA was significantly longer and slightly more curved than the right CCA, and there was a weak but significant increase in CCA curvature with age. Computational fluid dynamic simulations carried out for idealized CCA geometries derived from these and other measured geometric parameters demonstrated that mild cervical curvature is sufficient to prevent flow from fully-developing to axisymmetry, independent of the degree of thoracic curvature. These findings reinforce the idea that fully developed flow may be the exception rather than the rule for the CCA, and perhaps other nominally long and straight vessels.

Comparison of carotid plaque ulcer detection using contrast-enhanced and time-of-flight MRA techniques.

BACKGROUND AND PURPOSE: Ulceration in carotid plaque is a risk indicator for ischemic stroke. Our aim was to compare plaque ulcer detection by standard TOF and CE-MRA techniques and to identify factors that influence its detection. MATERIALS AND METHODS: Carotid MR imaging scans were acquired on 2066 participants in the ARIC study. We studied the 600 thickest plaques. TOF-MRA, CE-MRA, and black-blood MR images were analyzed together to define ulcer presence (plaque surface niche ≥2 mm in depth). Sixty ulcerated arteries were detected. These arteries were randomly assigned, along with 40 nonulcerated plaques from the remaining 540, for evaluation of ulcer presence by 2 neuroradiologists. Associations between ulcer detection and ulcer characteristics, including orientation, location, and size, were determined and explored by CFD modeling. RESULTS: One CE-MRA and 3 TOF-MRAs were noninterpretable and excluded. Of 71 ulcers in 56 arteries, readers detected an average of 39 (55%) on both TOF-MRA and CE-MRA, 26.5 (37.5%) only on CE-MRA, and 1 (1.5%) only on TOF-MRA, missing 4.5 (6%) ulcers by both methods. Ulcer detection by TOF-MRA was associated with its orientation (distally pointing versus perpendicular: OR = 5.57 [95% CI, 1.08–28.65]; proximally pointing versus perpendicular: OR = 0.21 [95% CI, 0.14–0.29]); location relative to point of maximum stenosis (distal versus isolevel: OR = 5.17 [95% CI, 2.10–12.70]); and neck-to-depth ratio (OR = 1.96 [95% CI, 1.11–3.45]) after controlling for stenosis and ulcer volume. CONCLUSIONS: CE-MRA detects more ulcers than TOF-MRA in carotid plaques. Missed ulcers on TOF-MRA are influenced by ulcer orientation, location relative to point of maximum stenosis, and neck-to-depth ratio.

Characterization of Common Carotid Artery Curvature and Its Impact on Velocity Profile Shape

Clinical and engineering studies of carotid artery disease typically assume that the common carotid artery (CCA), proximal to the bifurcation, is relatively straight, at least enough to assume the velocity profile is fully-developed (i.e., Poiseuille or Womersley profile) or at least axisymmetric. However, a recent study from our group has shown the surprising presence, in vivo, of strongly skewed velocity profiles in what appeared to be only mildly curved CCAs [1]. A computational fluid dynamics (CFD) study from our group also demonstrated strong velocity profile skewing in an idealized geometry possessing subtle wiggles similar to those observed in the CCA [2].Copyright