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8th Dutch Bio-Medical Engineering Conference
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13:40   Vascular - III
Chair: Jolanda Wentzel
13:40
15 mins
Hemodynamic comparison of AFX stent-graft and CERAB configuration for treatment of aortoiliac occlusive disease
Albert Chong, Hadi Mirgolbabaee, Zhonghua Sun, Lennart van de Velde, Shirley Jansen, Barry Doyle, Michel Versluis, Michel M. P. J. Reijnen, Erik Groot Jebbink
Abstract: Aorto-iliac occlusive disease (AIOD) is mostly treated using endovascular techniques. For severe lesions the use of stents is often indicated to maintain patency. The covered endovascular reconstruction of the aortic bifurcation (CERAB) technique is a relatively new endovascular approach in treating extensive AIOD extending into the distal aorta. Compared to the traditional kissing stent procedure, the CERAB configuration reduces radial mismatch, defined as the discrepancy between the stented lumen and the vessel lumen after stent placement, leading to more favourable flow conditions [1]. Recently, it was suggested that the AFX unibody endograft could have advantages over CERAB for this indication. It preserves the aortic bifurcation and thus allows for future cross-over endovascular interventions and avoids limb competition in the distal aorta [2]. The aim of this study is to test the hypothesis that the AFX endograft is also related to superior flow conditions, when compared to CERAB. In this research, laser particle imaging velocimetry (PIV) was used to quantify blood flow dynamics inside two anatomically identical and realistic aorto-iliac phantoms. Phantoms were constructed using Polydimethylsiloxane (PDMS) polymer and inserted with a customized AFX endograft and the CERAB configuration using Avanta V12 stents with transparent covers, enabling visualization of the flow fields and quantification of time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI) and relative residence time (RRT). Undisturbed flow patterns, higher TAWSS or lower OSI and RRT are considered as superior hemodynamic outcomes. This study revealed disturbed flow patterns and re-circulations, due to vortical flow, at the inflow and bifurcation of the AFX, especially at the end systolic velocity (ESV) time-point. In contrast, unidirectional flow was observed at the CERAB inflow and bifurcation. These observations were confirmed by the hemodynamic results, where mean TAWSS was significantly lower in AFX (0.078 [Pa]) compared with CERAB (0.229 [Pa]). Moreover, mean OSI and RRT in AFX (0.318 and 180 [Pa-1]) was higher than CERAB (0.252 and 88 [Pa-1]). In conclusion, although the AFX has a geometrical advantage over CERAB, it corresponds to less favorable hemodynamic parameters, which may predispose to thrombosis, potentially making it less desirable as compared to CERAB configuration.
13:55
15 mins
Computational study of the effect produced by stent location and coronary tree features on in-stent restenosis progression
Pavel Zun, Andrey Svitenkov, Lourens Veen, Frank Gijsen, Alfons Hoekstra
Abstract: Introduction Wall remodelling and in-stent restenosis (ISR) progression is affected through wall shear stress (WSS) [1], with high WSS suppressing the inward remodelling. WSS directly depends on the local flow pattern, which in turn depends on the flow dynamics in the whole coronary arterial tree [2]. Models of coronary vessel remodelling incorporating WSS usually make assumptions about the effects of a local narrowing on the flow through the narrowed vessel, and don’t directly account for the flow changes in the surrounding vessels. Here, we couple a 2D model of ISR to a 1D model of flow in the whole coronary tree. We test the effects of the increased resistance on local flow dynamics, the predictions of an ISR model, and the feedback between tissue growth and flow changes. Methods A 2D model of ISR, described in detail in [3], is coupled to a 1D model of coronary blood flow [4]. The flow from 1D model is used to set up the inlet BC of the 2D model, and the pressure drop in the 2D model is used to update the 1D flow. Two different assumptions are tested. First, that the vasculature is always able to adapt, and the flow through the narrowed vessel is kept constant. Second, that the vasculature does not adapt at all, and the hydrodynamic resistance of all vessels unaffected by the stenosis, as well as aortic and venous blood pressure, stay the same throughout the whole process. Results The two studied assumptions do not significantly affect the growth dynamics for most locations in the coronary tree. Downstream from large bifurcations, where a strong competing flow is present (e.g. the proximal part of the left marginal artery (LMA)), assumption of the constant flow leads to a prediction of a lower endpoint stenosis diameter. The difference between stent location also significantly affects the outcome: smaller vessels develop a higher degree of restenosis for similar injury scores. This is consistent with the clinical observation of small vessel size being a risk factor for restenosis. Conclusions These results suggest that assumption of constant flow is a good approximation for ISR models dealing with the typical progression of ISR in the most often stented locations such as the proximal parts of LAD and LCX. One of the locations that shown significant difference for the two assumptions (proximal part of LMA) is clinically relevant, since in some patients this artery can be a major supplier of blood for the left ventricle. References [1] Iqbal J, Serruys PW, Taggart DP. Nat Rev Cardiol 2013;10:635–47. [2] van de Vosse FN, Stergiopulos N. Annu Rev Fluid Mech 2011;43:467–99. [3] Tahir H, Bona-Casas C, Narracott AJ, Iqbal J, Gunn JP, Lawford P V., et al. J R Soc Interface 2014;11:20140022. [4] Svitenkov A, Pavlov I, Chivilikhin SA. Procedia Comput Sci 2018;136:416–24.
14:10
15 mins
Aortic strain imaging using bistatic coherent dual-transducer ultrasound
Vera van Hal, Hein de Hoop, Jan-Willem Muller, Marc van Sambeek, Hans-Martin Schwab, Richard Lopata
Abstract: Abdominal aortic aneurysms (AAAs) are local dilations of the abdominal aortic wall with a high risk of rupture. Ultrasound (US) imaging is used in the clinic to monitor the aortic diameter as a threshold for surgical intervention. However, knowledge of the entire aneurysm geometry and local mechanical wall parameters is important to correctly assess rupture risk. Aortic strain imaging using conventional US is limited by the lumen-wall contrast, lateral resolution and frame rate. Previous research in our group introduced ultrafast multi-perspective (MP) strain imaging [1] and bistatic imaging, where both probes receive simultaneously on each transmit event. The former improves strain estimates compared to conventional single probe techniques, the latter image contrast and resolution. This study introduces the use of coherent bistatic US imaging to improve aortic strain imaging. The advantage of such bistatic US imaging was investigated by comparing the single-perspective (SP) monostatic, MP monostatic, and MP bistatic imaging configurations. Experimental strain imaging was performed in US simulations and in a phantom study involving ex-vivo porcine aortas. Different compounding strategies were tested to retrieve the most useful information from each received US signal, which were also compared individually. Finally, apart from the conventional sector grid in US imaging, a polar grid with respect to the vessel's local coordinate system is introduced. This new reconstruction method demonstrated an improvement in 2D displacement estimation in aortic US. It reduced the mean motion tracking error from 1.1 mm to 0.25 mm in SP monostatic images, and improved strain estimation precision in MP images. Regarding the differences between MP imaging configurations, the US simulations showed improvements in strain estimation accuracy for MP bistatic imaging compared to MP monostatic imaging, with reductions in the average relative error between 55-94% in vessel wall regions between transducers. MP bistatic imaging was able to reconstruct side-scatter on the aortic wall, which improved contrast-to-noise ratio (CNR) by 4 dB compared to MP monostatic imaging in the experimental results. Finally, the elastographic signal-to-noise ratio (SNRe) was substantially increased in vessel wall regions between transducers by 10 to 15 dB in radial strain and about 6 dB in circumferential strain.
14:25
15 mins
A tissue engineered fibrous cap model to elucidate vulnerable plaque rupture
Tamar Wissing, Sheila Serra, Kim van der Heiden, Anthal Smits, Carlijn Bouten, Frank Gijsen
Abstract: In the Netherlands, approximately 29.000 people suffer from a stroke annually. Stroke can be caused by rupture of the shoulder or mid-cap region of the fibrous cap overlying an atherosclerotic plaque in the carotid artery. The risk of rupture is determined by the balance between blood pressure-induced stress and cap strength. The stress distribution is highly influenced by cap thickness and plaque composition, and the strength of the cap is predominantly affected by the properties of the collagenous matrix and the inflammatory state of the cap. However, how stress and strength are influenced is largely unknown. We propose to tissue engineer collagen-rich constructs, with controllable composition, to systematically investigate how the different components alter plaque mechanics and affect rupture. To create these constructs, human vena saphena cells (HVSCs) were seeded in fibrin-based gels and treated according to established (dynamic) cell culture protocols to stimulate collagen matrix deposition and alignment.[1] After 7 days of static culture, a soft inclusion was created in the middle of each construct and filled with fibrin to mimic lipid core mechanical properties. The constructs were statically cultured for another week, whereafter they were exposed to an intermittent (4% strain for 1 hr followed by 3 hrs or rest) or continuous (4% strain) straining protocol up till 21 days using the Flexcell FX-40001 (Flexcell Int, McKeesport, PA). Statically cultured samples were included as controls. Construct mechanics were determined via uniaxial tensile tests and collagen characteristics (e.g. organization, quantity & type) were assessed via imaging and immunohistochemistry (IHC) at day 21. Preliminary data demonstrates that we can create collagenous tissues in which we can induce rupture, with variable collagen organization, that mimic the bulk mechanical properties of real plaques. All statically cultured samples exhibited an isotropic collagen organization, irrespective of the locus analysed. In contrast, both the samples that obtained an intermittent as well as the continuous loading treatment demonstrated congruous anisotropic collagen configurations in the shoulder and mid-cap region surrounding the soft inclusion. At present, these tissues are used to grasp fibrous cap rupture to ultimately identify new biomarkers for vulnerable plaque detection. [1]Jonge, N. et al. (2013), Annals of Biomedical Engineering


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