LDA measurements in a non-stenosed and a stenosed model of the carotid artery bifurcation

In order to gain quantitative information of the velocity fields in non-stenosed and stenosed models of the carotid artery bifurcation. Laser Doppler Anemometer (LDA) experiments have been performed. For this purpose a two component backscatter LDA system has been used. The experiments have been conducted in a 1:2.5 enlarged plexiglass model of the carotid artery bifurcation. Both axial and secondary velocities were measured as a function of time at locations of interest. The data were ensemble averaged and analyzed in the frequency domain in order to find characteristic flow phenomena. For the frequency analyses, the transfer functions between velocities at specific sites in the bifurcation and the input flow signal have been calculated for both the non-stenosed and the stenosed bifurcation. Both from the results of the velocity fields and the transfer functions, it can be concluded that the main differences between the flow fields in the non-stenosed and the stenosed bifurcation can be found in an area with high velocity and in a shear layer, which is located at the border between a region with low shear rates at the non-divider wall and a region with high shear rates at the divider wall. The values of the transfer function at these locations seem to be useful for the characterization of the influence of the stenosis

Experimental and numerical analysis of the flow in stenosed carotid artery bifurcation models

In order to gain quantitative information of the variety fields in non-stenosed and stenosed models of the carotid artery bifurcation, Laser Doppler Anemometer (LDA) experiments and finite element computations have been performed. The experiments have been conducted in a 1:2.5 enlarged plexiglass model of the carotid artery bifurcation. Both axial and secondary velocities were measured as a function of time at locations of interest. The date were ensemble averaged and analysed in the frequency domain in order to find characteristic flow phenomena. The numerical computations have been performed by means of a finite element method. Both the experiments and the computations show that a shear layer, which is located at the border between a region with low shear rates at the non-divider wall and a region with high shear rates at the divider wall, is present in the carotid sinus. In this shear layer, flow disturbances are present at the onset of diastole. The main differences between the flow fields in the non-stenosed and the stenosed bifurcation are not present downstream of the stenosis but can be found in the shear layer and the area with high velocit

Analysis of the flow in stenosed carotid artery bifurcation models : hydrogen-bubble visualisation

This paper deals with the effect of geometric changes of mild stenoses on large-scale flow disturbances in the carotid artery bifurcation. Hydrogen-bubble visualisation experiments have been performed in Plexiglas models of a non-stenosed and a 25% stenosed carotid artery bifurcation. The flow conditions approximate physiological flow. The experiments show that shortly after the onset of the diastolic phase vortex formation occurs in the plane of symmetry. This vortex formation is found in a shear layer, which is formed in the carotid sinus. The shear layer is located between a region with low shear rates at the non-divider wall and a region with high shear rates at the divider wall. In order to gain insight into the parameters that are important with respect to the stability of the shear layer, experiments have been performed in which the influence of the shape of the flow pulse, the Reynolds number (Re), the Womersley parameter (a) and the flow division ratio (¿) on the flow phenomena is studied. From these experiments it appears that the flow phenomena in the carotid artery bifurcation are significantly influenced by Re, a the systolic acceleration (sa) and deceleration (sd) and the duration of the peak-systolic flow (Tmax). With these results a simplified flow pulse is chosen, with which the experiments in the non-stenosed and the 25% stenosed bifurcation are performed. Comparison of the hydrogen-bubble profiles in the 0 and 25% stenosed models with similar flow conditions shows that the geometric change of the 25% stenosis only slightly influences the flow phenomena. The most striking influences are found in the stability of the shear layer. Quantitative experiments by means of laser Doppler anemometry measurements and numerical computations are needed to analyse the influence of the stenosis of the flow field more accurately

Diagnostic minor stenoses in carotid artery bifurcation models using the disturbed velocity field

This paper presents a study on the diagnoses of minor stenoses at the non-divider side in the internal carotid artery of the carotid bifurcation using information provided by local disturbances of the flow field. The three-dimensional time-dependent flow fields in stenosed and non-stenosed bifurcation models were analyzed by means of both computational and experimental methods. The computational study indicates that in a model with a 50% area reducing stenosis, the global flow field is significantly disturbed as compared to that in a non-stenosed model. The main characteristic of the stenosed flow field is found in a peak systolic jet-flow distal to the stenosis, resulting in a significant diastolic post-stenotic flow separation area. In a 25% stenosed model, the induced disturbances are much less pronounced since the stenosis is located in the low shear area, which is well known to exist in the internal of non-stenosed artery bifurcations. A detailed experimental study was performed, applying laser Doppler velocity measurements in Plexiglas bifurcation models, in order to analyze the flow field (and its disturbances induced by minor [25%] stenoses) in either spatial, temporal or frequency domains. Analysis of the experimental data revealed that significant differences between stenosed and non-stenosed flow characteristics can be found. However, they probably cannot be used in clinical practice because of their sensitivity to inter-individual differences in bifurcation geometry and shape of the flow pulse

Analysis of the axial flow in stenosed carotid artery bifurcation models : LDA experiments

Laser Doppler anemometer (LDA) experiments were performed to gain quantitative information on the differences between the large-scale flow phenomena in a non-stenosed and a stenosed model of the carotid artery bifurcation. The influence of the presence of the stenosis was compared to the effect of flow pulse variation to evaluate the feasibility of early detection of stenosis in clinical practice. Three-dimensional Plexiglass models of a non-stenosed and a 25% stenosed carotid artery bifurcation were perfused with a Newtonian fluid. The flow conditions approximated physiological flow. The results of the velocity measurements in the non-stenosed model agreed with the results from previous hydrogen-bubble visualization. A shear layer separated the low-velocity area near the non-divider wall from the high-velocity area near the divider wall. In this shear layer, vortex formation occurred during the deceleration phase of the flow pulse. The instability of this shear layer dictated the flow disturbances. The influences of the mild stenosis, located at the non-divider wall, was mainly limited to the stability of the shear layer. No disturbances were found downstream of the stenosis near the non-divider wall. Using a pulse wave with an increased systolic deceleration time, the velocity distribution showed an extended region with reversed flow, a more pronounced shear layer and increased vortex strength. From these measurements it is obvious that the influence of the presence of a mild stenosis, mainly limited to the stability of the shear layer, can hardly be distinguished from the effects of a variation of the flow pulse. From this it can be concluded that methods for detection of mild stenosis, using solely the large-scale flow phenomena, as can be measured by ultrasound or MRI techniques, will hardly have any clinical relevance