Duanduan Chen

A longitudinal study of Type-B aortic dissection and endovascular repair scenarios: computational analyses

Conservative medical treatment is commonly first recommended for patients with uncomplicated Type-B aortic dissection (AD). However, if dissection-related complications occur, endovascular repair or open surgery is performed. Here we establish computational models of AD based on radiological three-dimensional images of a patient at initial presentation and after 4-years of best medical treatment (BMT). Computational fluid dynamics analyses are performed to quantitatively investigate the hemodynamic features of AD. Entry and re-entries (functioning as entries and outlets) are identified in the initial and follow-up models, and obvious variations of the inter-luminal flow exchange are revealed. Computational studies indicate that the reduction of blood pressure in BMT patients lowers pressure and wall shear stress in the thoracic aorta in general, and flattens the pressure distribution on the outer wall of the dissection, potentially reducing the progressive enlargement of the false lumen. Finally, scenario studies of endovascular aortic repair are conducted. The results indicate that, for patients with multiple tears, stent-grafts occluding all re-entries would be required to effectively reduce inter-luminal blood communication and thus induce thrombosis in the false lumen. This implicates that computational flow analyses may identify entries and relevant re-entries between true and false lumen and potentially assist in stent-graft planning.

A patient-specific study of type-B aortic dissection: evaluation of true-false lumen blood exchange

BackgroundAortic dissection is a severe pathological condition in which blood penetrates between layers of the aortic wall and creates a duplicate channel – the false lumen. This considerable change on the aortic morphology alters hemodynamic features dramatically and, in the case of rupture, induces markedly high rates of morbidity and mortality.MethodsIn this study, we establish a patient-specific computational model and simulate the pulsatile blood flow within the dissected aorta. The k-ω SST turbulence model is employed to represent the flow and finite volume method is applied for numerical solutions. Our emphasis is on flow exchange between true and false lumen during the cardiac cycle and on quantifying the flow across specific passages. Loading distributions including pressure and wall shear stress have also been investigated and results of direct simulations are compared with solutions employing appropriate turbulence models.ResultsOur results indicate that (i) high velocities occur at the periphery of the entries; (ii) for the case studied, approximately 40% of the blood flow passes the false lumen during a heartbeat cycle; (iii) higher pressures are found at the outer wall of the dissection, which may induce further dilation of the pseudo-lumen; (iv) highest wall shear stresses occur around the entries, perhaps indicating the vulnerability of this region to further splitting; and (v) laminar simulations with adequately fine mesh resolutions, especially refined near the walls, can capture similar flow patterns to the (coarser mesh) turbulent results, although the absolute magnitudes computed are in general smaller.ConclusionsThe patient-specific model of aortic dissection provides detailed flow information of blood transport within the true and false lumen and quantifies the loading distributions over the aorta and dissection walls. This contributes to evaluating potential thrombotic behavior in the false lumen and is pivotal in guiding endovascular intervention. Moreover, as a computational study, mesh requirements to successfully evaluate the hemodynamic parameters have been proposed.

A Preliminary Study of Fast Virtual Stent-Graft Deployment: Application to Stanford Type B Aortic Dissection

Aortic dissection is the result of blood intruding into the layers of the aortic wall creating a duplicate channel along the aortic course. This considerably changes aortic morphology and thereby alters blood flow, inducing severe pathological conditions. Endovascular stent-graft placement has become an accepted treatment option for complicated Stanford type B aortic dissection. Stent-graft deployment aims to cover the primary entry, preventing most of the inflow to the false lumen, thereby promoting false lumen thrombosis and true lumen expansion. In recent years the application of this treatment has increased continuously. However, a fast and reasonable prediction for the released stent-graft and the resulting aortic remodelling prior to intervention is still lacking. In this paper, we propose a preliminary study on the fast virtual stent-graft deployment algorithm based on contact mechanics, spring analogy and deformable meshes. By virtually releasing a stent-graft in a patient-specific model of an aortic dissection type Stanford B, we simulate the interaction between the expanding stent-graft and the vessel wall (with low computational cost), and estimate the post-interventional configuration of the true lumen. This preliminary study can be finished within minutes and the results present good consistency with the post-interventional computed tomography angiography. It therefore confirms the feasibility and rationality of this algorithm, encouraging further research on this topic, which may provide more accurate results and could assist in medical decision-making.

Comparison of morphological and rheological conditions between conventional and eversion carotid endarterectomy using computational fluid dynamics – a pilot study

Purpose: To compare postoperative morphological and rheological conditions after eversion carotid endarterectomy versus conventional carotid endarterectomy using computational fluid dynamics. Basic methods: Hemodynamic metrics (velocity, wall shear stress, time-averaged wall shear stress and temporal gradient wall shear stress) in the carotid arteries were simulated in one patient after conventional carotid endarterectomy and one patient after eversion carotid endarterectomy by computational fluid dynamics analysis based on patient specific data. Principal findings: Systolic peak of the eversion carotid endarterectomy model showed a gradually decreased pressure along the stream path, the conventional carotid endarterectomy model revealed high pressure (about 180 Pa) at the carotid bulb. Regions of low wall shear stress in the conventional carotid endarterectomy model were much larger than that in the eversion carotid endarterectomy model and with lower time-averaged wall shear stress values (conventional carotid endarterectomy: 0.03–5.46 Pa vs. eversion carotid endarterectomy: 0.12–5.22 Pa). Conclusions: Computational fluid dynamics after conventional carotid endarterectomy and eversion carotid endarterectomy disclosed differences in hemodynamic patterns. Larger studies are necessary to assess whether these differences are consistent and might explain different rates of restenosis in both techniques.

Chemosignalling, mechanotransduction and ciliary behaviour in the embryonic node: Computational evaluation of competing theories

Precise specification of left–right asymmetry is essential for patterning the internal organs of vertebrates. Within the embryonic node, posteriorly polarised cilia rotate, causing a leftward fluid flow (nodal flow) that establishes left–right asymmetry. The mechanism by which an embryo senses nodal flow remains uncertain. Existing hypotheses argue that either nodal flow carries morphogen(s) or lipid-bounded vesicles towards the left, thereby generating an asymmetric signal, and/or that mechano-sensory cilia sense this unidirectional flow, stimulating left-sided intracellular calcium signalling. To date, direct and definitive evidence supporting these hypotheses has been lacking. In this study, we conduct a multi-scale study to simulate the nodal cilia and the fluidic environment, analysing left–right signal transmission. By employing computational simulation techniques and solving the relevant three-dimensional unsteady transport equations, we study the flow pattern produced by the rotation of active cilia. By importing dilute species and particles into the computational domain, we investigate the transport of morphogens and nodal vesicular parcels, respectively. Furthermore, by extending the analysis to include the solid mechanics of passive deformable cilia and the coupling of their structural behaviour with the emerging fluid mechanics, we study the response of passive cilia to the nodal flow. Our results reproduce the unidirectional nodal flow, allowing us to evaluate the plausibility of both chemo- and mechano-sensing hypotheses. The quantitative measurements of the flow rate, the molecular transport and distribution provide guidance regarding the necessary morphogen molecular weights to break signalling symmetry. The passive sensory ciliary deformation gives indications regarding the plausibility of this mechano-signalling mechanism.

Hemodynamic parameters that may predict false-lumen growth in type-B aortic dissection after endovascular repair: A preliminary study on long-term multiple follow-ups

Thoracic endovascular aortic repair (TEVAR) is commonly applied in type-B aortic dissection. For patients with dissection affects descending aorta and extends downward to involve abdominal aorta and possibly iliac arteries, false lumen (FL) expansion might occur post-TEVAR. Predictions of dissection development may assist in medical decision on re-intervention or surgery. In this study, two patients are selected with similar morphological features at initial presentation but with different long-term FL development post-TEVAR (stable and enlarged FL). Patient-specific models are established for each of the follow-ups. Flow boundaries and computational validations are obtained from Doppler ultrasound velocimetry. By analyzing the hemodynamic parameters, the false-to-true luminal pressure difference (PDiff) and particle relative residence time (RRT) are found related to FL remodeling. It is found that (i) the position of the first FL flow entry is the watershed of negative-and-positive PDiff and, in long-term follow-ups, and the position of largest PDiff is consistent with that of the greatest increase of FL width; (ii) high RRT occurs at the FL proximal tip and similar magnitude of RRT is found in both stable and enlarged cases; (iii) comparing to the RRT at 7days post-TEVAR, an increase of RRT afterwards in short-term is found in the stable case while a slight decrease of this parameter is found in the enlarged case, indicating that the variation of RRT in short-term post-TEVAR might be potential to predict long-term FL remodeling.

The Hemodynamic Comparison of Different Carotid Artery Bifurcation Angles Based on Patient-Specific Models

Carotidatherosclerosis is related to cerebrovascular diseases. The anatomic features of the carotid artery may influence the hemodynamics within the vessel and therefore stimulate atherosclerotic process. This paper establishes two three-dimensional patient-specific models of the carotid artery. By changing the bifurcation angle while keeping the physiological morphology of the carotid artery, we investigate the hemodynamic variations and discuss their relationships to atherosclerosis. The results indicate: (i) the volume flow rate ratio of internal-to-external carotid artery decreases while the external-to-common carotid artery angle and the internal-to-common carotid artery angle increase; (ii) the enlargement of the angle (either external-to-common carotid artery angle or internal-to-common carotid artery angle) can reduce the region of low time-averaged wall shear stress (<0.3Pa) on the affected internal or external carotid artery, a positive effect to protect further plaque formations; and (iii) changing of the bifurcation angle presents littleeffects on the velocity, pressure and wall shear stress distributions in the carotid artery.

Recent evolution in use and effectiveness in mainland China of thoracic endovascular aortic repair of type B aortic dissection

A meta-analysis was performed on 175 studies selected among those published in mainland China between 2008 and 2015 on thoracic endovascular aortic repair (TEVAR) for type B aortic dissection (AD). Most TEVAR were performed in Shanghai, Beijing, Hubei and Guangdong in patients with mean age of 53.9 years, and acute (70%) or chronic (30%) type B AD. Procedural success rate was 99.1 ± 0.8%. Major complication rate was 1.7 ± 2.3%, with paraplegia in 0.4 ± 0.0%. Overall in-hospital mortality rate was 1.6 ± 0.9% with AD rupture in 30% (about 40% during first postoperative day); follow-up mortality rate was 2.3 ± 1.1%, with AD rupture in 39.2% (50% within first year). Compared with 2001–2007 data from China, there appeared to be improvement in rates of major complications, paraplegia and in-hospital mortality. Compared with 1999–2004 Western data, rates of procedural success, stroke, and paraplegia appeared similar, while those for major complications, in-hospital mortality, retrograde type A dissection and follow-up mortality appeared lower. Compared with more recent Western data (2006–2013) on acute complicated type B AD, stroke, paraplegia, in-hospital mortality and follow-up mortality appeared lower. Therefore, in mainland China, safety for TEVAR of type B AD appeared better between 2008 and 2015 than in previous periods in China or Western countries.

Successful use of retrograde branched extension limb assembling technique in endovascular repair of pararenal abdominal aortic aneurysm

Surgeon-modified retrograde branched extension limb assembling technique and bridged endografts were successfully used to exclude an asymptomatic pararenal abdominal aortic aneurysm and to reconstruct the superior mesenteric artery and bilateral renal arteries in a case with high-grade celiac artery stenosis, nondilated aorta above the superior mesenteric artery, and large lumen below the renal arteries. In patient-specific models for hemodynamics analysis, enhanced flow diversion to visceral arteries up to 6-month follow-up confirmed treatment feasibility; however, endograft configurations could be improved to avoid sharp corners at bifurcations, thereby ensuring smooth flow transport and possibly reducing risk for endograft narrowing or the development of thrombosis.

Hemodynamics analyses in treated and untreated carotid arteries of the same patient: A preliminary study based on three patient cases

Carotid atherosclerotic disease is highly related to cerebrovascular events. Carotid endarterectomy is the common operation method to treat this disease. In this study, hemodynamics analyses are performed on the carotid arteries in three patients, whose right carotid artery had been treated by carotid endarterectomy and the left carotid artery remained untreated. Flow and loading conditions are compared between these treated and untreated carotid arteries and evaluation of the operative results is discussed. Patient-specific models are reconstructed from MDCT data. Intraoperative ultrasound flow measurements are performed on the treated carotid arteries and the obtained data are used as the boundary conditions of the models and the validations of the computational results. Finite volume method is employed to solve the transport equations and the flow and loading conditions of the models are reported. The results indicate that: (i) in two of the three patients, the internal-to-external flow rate ratio in the untreated carotid artery is larger than that in the treated one, and the average overall flow split ratio by summing up the data of both the left and right carotid arteries is about 2.15; (ii) in the carotid bulb, high wall shear stress occurs at the bifurcation near the external carotid artery in all of the cases without hard plaques; (iii) the operated arteries present low time-averaged wall shear stress at the carotid bulb, especially for the treated arteries with patch technique, indicating the possibility of the recurrence of stenosis; (iv) high temporal gradient of wall shear stress (>35 Pa/s) is shown in the narrowing regions along the vessels; and (v) in the carotid arteries without serious stenosis, the maximum velocity magnitude during mid-diastole is 32~37% of that at systolic peak, however, in the carotid artery with 50% stenosis by hard plaques, this value is nearly doubled (64%). The computational work quantifies flow and loading distributions in the treated and untreated carotid arteries of the same patient, contributing to evaluation of the operative results and indicating the recurrent sites of potential atheromatous plaques.