Emily Y. Wong

Clinical Doppler ultrasound for the assessment of plaque ulceration in the stenosed carotid bifurcation by detection of distal turbulence intensity: A matched model study

The assessment of flow disturbances due to carotid plaque ulceration may provide added diagnostic information to Doppler ultrasound (DUS) of the carotid stenosis, and indicate whether the associated hemodynamics are a potential thromboembolic source. We evaluated the effect of ulceration in a moderately stenosed carotid bifurcation on distal turbulence intensity (TI) measured using clinical DUS in matched anthropomorphic models. Several physiologically relevant ulcer geometries (hemispherical, mushroom-shaped, and ellipsoidal pointing distally and proximally) and sizes (2-mm, 3-mm and 4-mm diameter hemispheres) were investigated. An offline analysis was performed to determine several velocity-based parameters from ensemble-averaged spectral data, including TI. Significant elevations in TI were observed in the post-stenotic flow field of the stenosed carotid bifurcation by the inclusion of ulceration (P < 0.001) in a region two common carotid artery diameters distal to the site of ulceration during the systolic peak and the diastolic phase of the cardiac cycle. Both the size and shape of ulceration had a significant effect on TI in the distal region (P < 0.001). Due to the use of a clinical system, this method provides the means to evaluate for plaque ulcerations in patients with carotid atherosclerosis using DUS.

Doppler Ultrasound Compatible Plastic Material for Use in Rigid Flow Models

A technique for the rapid but accurate fabrication of multiple flow phantoms with variations in vascular geometry would be desirable in the investigation of carotid atherosclerosis. This study demonstrates the feasibility and efficacy of implementing numerically controlled direct-machining of vascular geometries into Doppler ultrasound (DUS)-compatible plastic for the easy fabrication of DUS flow phantoms. Candidate plastics were tested for longitudinal speed of sound (SoS) and acoustic attenuation at the diagnostic frequency of 5 MHz. Teflon was found to have the most appropriate SoS (1376 +/- 40 m s(-1) compared with 1540 m s(-1) in soft tissue) and thus was selected to construct a carotid bifurcation flow model with moderate eccentric stenosis. The vessel geometry was machined directly into Teflon using a numerically controlled milling technique. Geometric accuracy of the phantom lumen was verified using nondestructive micro-computed tomography. Although Teflon displayed a higher attenuation coefficient than other tested materials, Doppler data acquired in the Teflon flow model indicated that sufficient signal power was delivered throughout the depth of the vessel and provided comparable velocity profiles to that obtained in the tissue-mimicking phantom. Our results indicate that Teflon provides the best combination of machinability and DUS compatibility, making it an appropriate choice for the fabrication of rigid DUS flow models using a direct-machining method.

Characterization of 3-D flow structures in the stenosed carotid bifurcation with plaque ulceration

Carotid plaque ulcerations, or irregularities in plaque surface morphology, have been identified as an independent risk factor for ischemic stroke. Our previous studies using Doppler ultrasound (DUS) have indicated significant flow disturbances distal to ulceration in the atherosclerotic carotid bifurcation, as characterized by parameters such as turbulence intensity (TI). Additional tools are needed to understand the implications of such flow abnormalities on the risk of thrombogenesis and cerebral ischemic events. Numerical simulations using computational fluid dynamics (CFD) can supplement experimental DUS studies, providing higher resolution, time-resolved models of 3-D flow fields. CFD is also able to quantify hemodynamic factors that indicate thromboembolic or plaque rupture potential. We report a CFD analysis of an ellipsoidal ulcer model and a matched non-ulcerated model in a moderately stenosed carotid bifurcation, with the same vessel geometries and flow conditions used in our previous DUS studies. The CFD models used a spatial finite element discretization of over 160,000 quadratic tetrahedral elements to adequately resolve the flow field. Pulsatile flow simulations with boundary conditions and flowrate waveforms matching DUS experimental conditions were iterated for ten cardiac cycles. Turbulence intensity was calculated for the CFD models and compared with DUS experimental results. The CFD models were able to capture differences in flow patterns between cardiac cycles. As observed in the empirical DUS results, the CFD ulcer model displayed higher levels of TI in the post-stenotic region than the CFD non-ulcerated model. The extent and magnitude of TI was comparable to the DUS results, after modeling for the effects of sample volume geometry, intrinsic spectral broadening, and a high pass filter. Furthermore, the CFD results indicate that sampling volume size and location have an effect on DUS quantification of TI in the post-stenotic carotid artery. Small elevations in maximum shear stress, transit time, shear exposure and level of threshold activation were observed in the ulcerated model, as compared to the non-ulcerated model. CFD facilitates the comparison of hemodynamic parameters between ulcer models and may help to demonstrate the risks of embolism or plaque rupture posed by ulcerated atherosclerotic plaques in the carotid bifurcation.

Doppler ultrasound and numerical analysis for the assessment of hemodynamic disturbances in ulcerated carotid arteries

Carotid plaque ulcerations, or irregularities in plaque surface morphology, have been identified as an independent risk factor for ischemic stroke. Our previous studies using Doppler ultrasound (DUS) have indicated significant flow disturbances distal to ulceration in the atherosclerotic carotid bifurcation, as characterized by parameters such as turbulence intensity (TI). Additional tools are needed to understand the implications of such flow abnormalities on the risk of thrombogenesis and cerebral ischemic events. Numerical simulations using computational fluid dynamics (CFD) can supplement experimental DUS studies, providing higher resolution, time-resolved models of 3-D flow fields. CFD is also able to quantify hemodynamic factors that indicate thromboembolic or plaque rupture potential. We report a CFD analysis of an ellipsoidal ulcer model and a matched non-ulcerated model in a moderately stenosed carotid bifurcation, with the same vessel geometries and flow conditions used in our previous DUS studies. The CFD models used a spatial finite element discretization of over 160,000 quadratic tetrahedral elements to adequately resolve the flow field. Pulsatile flow simulations with boundary conditions and flowrate waveforms matching DUS experimental conditions were iterated for five cardiac cycles. Turbulence intensity was calculated for the CFD models and compared with DUS experimental results. The CFD models were able to capture differences in flow patterns between cardiac cycles. As observed in the empirical DUS results, the CFD ulcer model displayed higher levels of TI in the post-stenotic region than the CFD non-ulcerated model. The extent and magnitude of TI was comparable to the DUS results, after modeling for the effects of sample volume geometry and intrinsic spectral broadening, and by a high pass filter. Furthermore, the CFD results indicate that flow post-stenosis is likely transitional, with both disturbed and turbulent flow present. CFD facilitates the comparison of hemodynamic parameters between ulcer models and may help to demonstrate the risks of embolism or plaque rupture posed by ulcerated atherosclerotic plaques in the carotid bifurcation.

Evaluation of cross-beam vector Doppler ultrasound systems for accurate 3-D velocity measurements

Vector Doppler ultrasound (VDUS) systems offer the potential for improved accuracy in mapping of complex flow parameters, such as recirculation, turbulence, and shear stress which are probable risk factors leading to vascular disease and stroke. Cross-beam VDUS systems were evaluated for velocity accuracy to optimize the number of receivers for the inter-beam angle, wall filter, system orientation, and complexity of flow seen in a stenosed carotid artery. Preliminary results for velocity estimation show promise for validation of numerical results.

Numerical design and evaluation of Vector Doppler Ultrasound systems for accurate 3-D velocity measurements

Vector Doppler ultrasound (VDUS) systems offer the potential for improved accuracy in mapping of complex flow parameters, such as recirculation, turbulence and shear stress, which are probable risk factors leading to vascular disease and stroke. VDUS system configurations with 2 to 6 receivers and corresponding reconstructions of a complex velocity field were numerically evaluated.

Numerical analysis of the hemodynamic effect of plaque ulceration in the stenotic carotid artery bifurcation

The presence of ulceration in carotid artery plaque is an independent risk factor for thromboembolic stroke. However, the associated pathophysiological mechanisms - in particular the mechanisms related to the local hemodynamics in the carotid artery bifurcation - are not well understood. We investigated the effect of carotid plaque ulceration on the local time-varying three-dimensional flow field using computational fluid dynamics (CFD) models of a stenosed carotid bifurcation geometry, with and without the presence of ulceration. CFD analysis of each model was performed with a spatial finite element discretization of over 150,000 quadratic tetrahedral elements and a temporal discretization of 4800 timesteps per cardiac cycle, to adequately resolve the flow field and pulsatile flow, respectively. Pulsatile flow simulations were iterated for five cardiac cycles to allow for cycle-to-cycle analysis following the damping of initial transients in the solution. Comparison between models revealed differences in flow patterns induced by flow exiting from the region of the ulcer cavity, in particular, to the shape, orientation and helicity of the high velocity jet through the stenosis. The stenotic jet in both models exhibited oscillatory motion, but produced higher levels of phase-ensembled turbulence intensity in the ulcerated model. In addition, enhanced out-of-plane recirculation and helical flow was observed in the ulcerated model. These preliminary results suggest that local fluid behaviour may contribute to the thrombogenic risk associated with plaque ulcerations in the stenotic carotid artery bifurcation.

Investigation of the effect of geometrical features of carotid artery plaque on turbulence intensity using Doppler ultrasound and particle image velocimetry

Stenosis severity alone is not a sensitive indicator of ischemic stroke risk; however, it remains the primary indicator for clinical decision-making. Carotid endarterectomy is strongly recommended for patients with severe stenosis, while treatment for lesser stenosis severity remains disputed. Thromboembolism is one of the major causes of ischemic stroke and has shown high correlation with hemodynamic factors, such as turbulence. Geometrical factors - such as the degree of stenosis severity, plaque eccentricity, and ulceration - can alter the local hemodynamics of the carotid artery, such as by inducing flow disturbances. The objective of this work was to investigate the impact of these geometrical features on the level of turbulence intensity using DUS and PIV. A family of carotid artery models was examined with geometries ranging from disease-free to severe stenosis, in both eccentric and concentric forms of plaque symmetry, and in the case of moderate stenosis (50%) with and without ulceration. Plaque eccentricity and ulceration were found to enhance the flow disturbances downstream of a stenosis, suggesting that clinical diagnosis should consider plaque shape and roughness in addition to stenosis severity.