Dieter Liepsch

The effect of stents on intra-aneurysmal hemodynamics: in vitro evaluation of a pulsatile sidewall aneurysm using laser Doppler anemometry

IntroductionHemodynamic modification by means of flow diversion is increasingly used for treatment of intracranial aneurysms. Despite of promising results, there is still a paucity of methods to reliably predict long-term success of this technique. Laser Doppler anemometry (LDA) can be used to quantify the influence of stents on intra-aneurysmal flow in vitro.MethodsAll experiments were performed with a pulsatile model of a sidewall aneurysm. A physiologic flow was created with a circulatory experimental setup, and a transparent non-Newtonian glycerol–water solution was used to substitute human blood. Flow velocity was measured with a one-component LDA system, recording flow components parallel and perpendicular to the parent vessel. Three different stents (Solitaire, Silk, Phenox flow diverter) were deployed over the aneurysm neck, respectively.ResultsFlow reduction was 67.59% (inflow zone), 9.65% (dome) and 37.94% (outflow zone) by the Solitaire stent. The Silk stent reduced the flow by 58.15% (inflow zone), 89.06% (dome) and 90.06% (outflow zone). The Phenox flow diverter reduced the flow by 96.76% (inflow zone), 90% (dome) and 90.91% (outflow zone) when positioned with narrow stent struts but increased the velocity of up to seven times compared to the unstented model when placed with loose strut packing in the proximal part of the aneurysm.ConclusionLDA is a feasible method to quantify intra-aneurysmal flow and flow reduction efficacy of stents in vitro. Flow reduction was negligible with a standard self-expanding stent. For dedicated flow diverters, it depended both on stent design and on appropriate positioning.

Experimental analysis of the influence of stenotic geometry on steady flow.

We studied the flow behavior under steady flow conditions in four models of cylindrical stenoses at Reynolds numbers from 150 to 920. The flow upstream of the constrictions was always fully developed. The constriction ratios of the rigid tubes (D) to the stenoses (d) were d/D = 0.273; 0.505; 0.548; 0.786. The pressure drop at various locations in the stenotic models was measured with water manometers. The flow was visualized with a photoelasticity apparatus using an aqueous birefringent solution. We also studied the flow behavior at pulsatile flow in a dog aorta with a constriction of 71%. The flow through stenotic geometries depends on the Reynolds number of the flow generated in the tube and the constriction ratio d/D. At low d/D ratios, (with the increased constriction), the flow separation zones (recirculation zones, so-called reattachment length) and flow disturbances increased with larger Reynolds numbers. At lower values, eddies were generated. At high Re, eddies were observed in the pre-stenotic regions. The pressure drop is a function of the length and internal diameter of the stenosis, respective ratio of stenosis to the main vessel and the Reynolds numbers. At low Re-numbers and low d/D, distinct recirculation zones were found close to the stenosis. The flow is laminar in the distal areas. Further experiments under steady and unsteady flow conditions in a dog aorta model with a constriction of 71% showed similar effects. High velocity fluctuations downstream of the stenosis were found in the dog aorta. A videotape demonstrates these results.

The dynamics of pulsatile flow in distensible model arteries.

Deposits and blockages are often found in the carotid, coronary, renal and femoral arteries. This paper deals with laser-Doppler velocity measurements in models of bifurcations of the human femoral arteries. Several models were prepared for the studies: a simplified 35 degrees glass model, two elastic-silicone-rubber models with a wall thickness of 1 mm and 2 mm, and true-to-scale rigid and elastic models. These measurements give a clearer picture of how hemodynamics influences the formation of atherosclerotic plaques where there is a hardening of the arterial walls and a loss of elasticity. In addition to the effects of elasticity, the influence of the flows pulsatility were studied. The measurements were done in steady and pulsatile flow. From the velocity measurements the shear stresses were calculated.

Coronary stents cause high velocity fluctuation with a flow acceleration and flow reduction in jailed branches: An in vitro study using laser-Doppler anemometry

Flow disturbance and reduced blood flow have been associated with higher restenosis rates and clinical adverse events after coronary interventions. In the present study, we sought to investigate flow alterations that occurred after stent implantation in a coronary model, within and adjacent to the stented segment. Two stents (Carbostent, Tetrastent) with different strut design were deployed in the left anterior descending artery (LAD) of a 1:1 scaled silicon coronary model. The model was mounted into an artificial circulation and showed distensibility and rheologic behavior comparable to human coronaries. Flow profiles were assessed using laser-Doppler anemometry. Both stents induced a transitional flow within the stents, in the jailed branch as well as in the adjacent segments. However, the alterations in flow were less marked using the Carbostent having stents with thinner struts and a larger strut cell area, and thus seem to be more favorable in avoiding bifurcation lesions. This study shows precisely that stent implantation induces flow disturbances in segments known to be prone for restenosis. Investigations using laser-Doppler measurements may enlighten rheologic phenomena inducing restenosis and help in optimizing stent design and deployment techniques.

Evaluation of intra-aneurysmal hemodynamics after flow diverter placement in a patient-specific aneurysm model.

BACKGROUND The growth and rupture of cerebral aneurysms is intrinsically related to the hemodynamics prevailing in the diseased area. Therefore, a better understanding of intra-aneurysmal hemodynamics is essential for developing effective treatment methods. OBJECTIVE The intention of this study was to evaluate the intra-aneurysmal flow and flow reduction induced by flow diverters in a true-to-scale elastic aneurysm model, obtained from real patient data. METHODS Based on the computed tomography angiography (CTA) data of a fusiform aneurysm of a 34 year old patient, an elastic silicon rubber model of the aneurysm was produced. A physiologic pulsatile flow was created with a circulatory experimental set-up, and a non-Newtonian perfusion fluid was used as a substitute for human blood. Hemodynamics were measured by LDA before and after flow diverter implantation. RESULTS Implantation of a flow diverter device resulted in a reduction of intra-aneurysmal maximum flow velocities of 97.8% at the inflow zone, 89.1% in the dome and 89.3% at the outflow zone, when compared to the native model. A significant reduction of 94% in the mean intra-aneurysmal velocity was found. CONCLUSIONS This promising methodology can optimize patient treatment and will correlate with computational simulations to evaluate their reliability.

Experimental and CFD flow studies in an intracranial aneurysm model with Newtonian and non-Newtonian fluids

BACKGROUND According to the clinical data, flow conditions play a major role in the genesis of intracranial aneurysms. The disorder of the flow structure is the cause of damage of the inner layer of the vessel wall, which leads to the development of cerebral aneurysms. Knowledge of the alteration of the flow field in the aneurysm region is important for treatment. OBJECTIVE The aim is to study quantitatively the flow structure in an patient-specific aneurysm model of the internal carotid artery using both experimental and computational fluid dynamics (CFD) methods with Newtonian and non-Newtonian fluids. METHODS A patient-specific geometry of aneurysm of the internal carotid artery was used. Patient data was segmented and smoothed to obtain geometrical model. An elastic true-to-scale silicone model was created with stereolithography. For initial investigation of the blood flow, the flow was visualized by adding particles into the silicone model. The precise flow velocity measurements were done using 1D Laser Doppler Anemometer with a spatial resolution of 50 μ m and a temporal resolution of 1 ms. The local velocity measurements were done at a distance of 4 mm to each other. A fluid with non-Newtonian properties was used in the experiment. The CFD simulations for unsteady-state problem were done using constructed hexahedral mesh for Newtonian and non-Newtonian fluids. RESULTS Using 1D laser Doppler Anemometer the minimum velocity magnitude at the end of systole -0.01 m/s was obtained in the aneurysm dome while the maximum velocity 1 m/s was at the center of the outlet segment. On central cross section of the aneurysm the maximum velocity value is only 20% of the average inlet velocity. The average velocity on the cross-section is only 11% of the inlet axial velocity. Using the CFD simulation the wall shear stresses for Newtonian and non-Newtonian fluid at the end of systolic phase (t= 0.25 s) were computed. The wall shear stress varies from 3.52 mPa (minimum value) to 10.21 Pa (maximum value) for the Newtonian fluid. For the non-Newtonian fluid the wall shear stress minimum is 2.94 mPa; the maximum is 9.14 Pa. The lowest value of the wall shear stress for both fluids was obtained at the dome of the aneurysm while the highest wall shear stress was at the beginning of the outlet segment. The vortex in the aneurysm region is unstable during the cardiac cycle. The clockwise rotation of the streamlines at the inlet segment for Newtonian and non-Newtonian fluid is shown. CONCLUSION The results of the present study are in agreement with the hemodynamics theory of aneurysm genesis. Low value of wall shear stress is observed at the aneurysm dome which can cause a rupture of an aneurysm.

Opened end‐to‐side technique for end‐to‐side anastomosis and analyses by an elastic true‐to‐scale silicone rubber model

The end‐to‐side anastomosis is frequently used in microvascular free flap transfer, but detailed rheological analyses are not available. The purpose of this study was to introduce a new modified end‐to‐side (Opened End‐to‐Side, OES‐) technique and compare the resulting flow pattern to a conventional technique. The new technique was based on a bi‐triangulated preparation of the branching‐vessel end, resulting in a “fish‐mouthed” opening. We performed two different types of end‐to‐side anastomoses in forty pig coronary arteries and produced one elastic, true‐to‐scale silicone rubber model of each anastomosis. Then we installed the transparent models in a circulatory experimental setup that simulated the physiological human blood flow. Flow velocity was measured with the one‐component Laser‐Doppler‐Anemometer system, recording flow axial and perpendicular to the model at four defined cross‐sections for seven heart cycles in each model. Maximal and minimal axial velocities ranged in the conventional model between 0.269 and −0.122 m/s and in the experimental model between 0.313 and −0.153 m/s. A less disturbed flow velocity distribution was seen in the experimental model distal to the anastomosis. The OES‐technique showed superior flow profiles distal to the anastomosis with minor tendencies of flow separation and represents a new alternative for end‐to‐side anastomosis.

An impact of non-Newtonian blood viscosity on hemodynamics in a patient-specific model of a cerebral aneurysm

Despite of the recent achievements in the research of blood rheology, the effect of non-Newtonian blood viscosity on hemodynamic parameters in cerebral aneurysms is not clarified. The purpose of this study is to investigate the role of different rheological models for prediction of hemodynamic parameters in the patient-specific model of a saccular aneurysm. Transient flow simulations were conducted using OpenFOAM CFD Toolbox. The simulations employed different viscosity models: Newtonian, Power Law, Bird-Carreau, Casson and Local viscosity model. The similar flow patterns were observed both for Newtonian and non- Newtonian fluids in the patient-specific model of the aneurysm. The average difference between Newtonian and non-Newtonian models was about 12%. However for some specific points in the region of recirculating flow the difference was significantly higher - from 20% to 63%. The non-Newtonian blood behaviour influenced the streamlines distribution in the aneurysm region, producing a larger recirculating zone in the center of the aneurysm. The results showed that the major differences between Newtonian and non-Newtonian fluids were found in regions of low velocities and recirculating zones.

Model studies of blood flow in basilar artery with 3D laser Doppler anemometer

It is proposed an integrated approach to the study of basilar artery blood flow using 3D laser Doppler anemometer for identifying the causes of the formation and development of cerebral aneurysms. Feature of the work is the combined usage of both mathematical modeling and experimental methods. Described the experimental setup and the method of measurement of basilar artery blood flow, carried out in an interdisciplinary laboratory of Hospital Rechts der Isar of Technical University of Munich. The experimental setup used to simulate the blood flow in the basilar artery and to measure blood flow characteristics using 3D laser Doppler anemometer (3D LDA). Described a method of numerical studies carried out in Tambov State Technical University and the Bakoulev Center for Cardiovascular Surgery. Proposed an approach for sharing experimental and numerical methods of research to identify the causes of the basilar artery aneurysms.

New Studies in the Cardiovascular System: Diagnostic and Therapeutic Applications

Diseases of the heart and circulatory system are among the leading causes of death in the industrial world. At nearly 75%, this ranks statistically significantly higher than cancer. Among these diseases, the most common are heart attack and stroke, disturbances of kidney function and diabetes.