Systematic Review on Endovascular Access to Intracranial Arteries for Mechanical Thrombectomy in Acute Ischemic Stroke.

PURPOSE: In acute ischemic stroke for large vessel occlusions, delayed or failed access to intracranial occlusions has a negative impact on procedural and clinical outcomes. The aim of this review is to identify and quantify access failures and challenges in mechanical thrombectomy. METHODS: A systematic literature review of PubMed and Scopus databases from January 2014 to October 2020 was performed. Articles reporting consecutive patients were used to calculate a crude failure rate of femoral and alternative accesses. RESULTS: A total of 50 articles met the inclusion criteria, totalling 12,838 interventions. Failure to access the occlusion through transfemoral access occurred in 4.4% of patients, most commonly due to challenging supra-aortic vessel anatomy, decreasing to 3.6% when all alternative access routes were attempted. Failed access from alternative routes (direct carotid, radial and brachial approaches) attempted first-line or after failed femoral attempt were reported in 7.3% of patients. The occurrence rate of potentially challenging features (anatomical, diseases or others) ranged from 4.7% to 47.4%, primarily impacting the access time, procedure time, recanalization and clinical outcomes. CONCLUSION: Failure to access the occlusion is a significant contributor to failed recanalization, regardless of access routes. Challenging, but eventually successful access is also a relevant factor in procedural and clinical outcomes; however challenging access requires a universal definition to enable quantification, so that methods for procedural optimization can be critically assessed.

An in-vitro evaluation of the flow haemodynamic performance of Gore-Tex extracardiac conduits for univentricular circulation.

OBJECTIVE(S): The Fontan procedure is a common palliative intervention for sufferers of single ventricle congenital heart defects that results in an anastomosis of the venous return to the pulmonary arteries called the total cavopulmonary connection (TCPC). In patients with palliated single ventricular heart defects, the Fontan circulation passively directs systemic venous return to the pulmonary circulation in the absence of a functional sub-pulmonary ventricle. Therefore, the Fontan circulation is highly dependent on favourable flow and energetics, and minimal energy loss is of great importance. The majority of in vitro studies, to date, employ a rigid TCPC model. Recently, few studies have incorporated flexible TCPC models, without the inclusion of commercially available conduits used in these surgical scenarios. METHOD: The methodology set out in this study successfully utilizes patient-specific phantoms along with the corresponding flowrate waveforms to characterise the flow haemodynamic performance of extracardiac Gore-Tex conduits. This was achieved by comparing a rigid and flexible TCPC models against a flexible model with an integrated Gore-Tex conduit. RESULTS: The flexible model with the integrated Gore-Tex graft exhibited greater levels of energy losses when compared to the rigid walled model. With this, the flow fields showed greater levels of turbulence in the complaint and Gore-Tex models compared to the rigid model under ultrasound analysis. CONCLUSION: This study shows that vessel compliance along with the incorporation of Gore-Tex extracardiac conduits have significant impact on the flow haemodynamics in a patient-specific surgical scenario.

An in vitro assessment of atrial fibrillation flow types on cardiogenic emboli trajectory paths.

Atrial fibrillation is the most significant contributor to thrombus formation within the heart and is responsible for 45% of all cardio embolic strokes, which account for approximately 15% of acute ischemic strokes cases worldwide. Atrial fibrillation can result in a reduction of normal cardiac output and cycle length of up to 30% and 40%, respectively. A total of 240 embolus analogues were released into a thin-walled, patient-specific aortic arch under normal (60 embolus analogues) and varying atrial fibrillation (180 embolus analogues) pulsatile flow conditions. Under healthy flow conditions (n = 60), the embolus analogues tended to follow the flow rate split through each outlet vessel. There was an increase in clot trajectories along the common carotid arteries under atrial fibrillation flow conditions. A shorter pulse period (0.3 s) displayed the highest percentage of clots travelling to the brain (24%), with a greater percentage of clots travelling through the left common carotid artery (17%). This study provides an experimental insight into the effect varying cardiac output and cycle length can have on the trajectory of a cardiac source blood clots travelling to the cerebral vasculature and possibly causing a stroke.

Investigation of the Hemodynamics Influencing Emboli Trajectories Through a Patient-Specific Aortic Arch Model.

Background and Purpose- Cardiogenic emboli account for 15% to 20% of acute ischemic stroke cases worldwide. However, the chance of such emboli, of varying sizes, causing a stroke under various flow types has not been evaluated. Methods- A patient-specific aortic arch model was fabricated from a medical image dataset of a 77-year-old male case, with atrial fibrillation and distal occlusion of the right M1 vessel. One hundred and eighty mammalian embolus analogs (EAs) were released one by one into the model under normal and atrial fibrillation flow conditions. A further 270 clots were fabricated using varying levels of thrombin (5-20 National Institutes of Health units thrombin). The effect of releasing several clots simultaneously was also examined by grouping EAs into 18 multiples of 5, 4, 3, and 2 clots, resulting in 504 EAs released. Results- EAs with a length of ≤10 mm were the most common geometry to travel through the common carotid arteries (44%); however, longer clots also traveled through these narrow vessels. Twenty two percent of EAs ranged from 10-20mm in length, 27% from 20-30mm and 7% were >30 mm in length. Higher density clots increased the propensity for clots to travel along the cerebral vessels ( P<0.05). Releasing more clots during each test, increased the probability of at least one clot traveling through an aortic arch branching vessel. Conclusions- Embolus trajectory through the branching vessels of the aortic arch is not exclusively dependent on embolus size. EAs tend to travel proportionally with outlet flow rates, with a greater chance of a stroke caused by multiple breakaway emboli.

Validation of the Strain Assessment of a Phantom of Abdominal Aortic Aneurysm: Comparison of Results Obtained From Magnetic Resonance Imaging and Stereovision Measurements.

Predicting aortic aneurysm ruptures is a complex problem that has been investigated by many research teams over several decades. Work on this issue is notably complex and involves both the mechanical behavior of the artery and the blood flow. Magnetic resonance imaging (MRI) can provide measurements concerning the shape of an organ and the blood that flows through it. Measuring local distortion of the artery wall is the first essential factor to evaluate in a ruptured artery. This paper aims to demonstrate the feasibility of this measure using MRI on a phantom of an abdominal aortic aneurysm (AAA) with realistic shape. The aortic geometry is obtained from a series of cine-MR images and reconstructed using Mimics software. From 4D flow and MRI measurements, the field of velocity is determined and introduced into a computational fluid dynamic (CFD) model to determine the mechanical boundaries applied on the wall artery (pressure and ultimately wall shear stress (WSS)). These factors are then converted into a solid model that enables wall deformations to be calculated. This approach was applied to a silicone phantom model of an AAA reconstructed from a patient's computed tomography-scan examination. The calculated deformations were then compared to those obtained in identical conditions by stereovision. The results of both methods were found to be close. Deformations of the studied AAA phantom with complex shape were obtained within a gap of 12% by modeling from MR data.

Systematic Review and Patient-Level Meta-analysis of the Streamliner Multilayer Flow Modulator in the Management of Complex Thoracoabdominal Aortic Pathology.

PURPOSE: To examine the safety and short-term efficacy of the Streamliner Multilayer Flow Modulator (SMFM) in the management of patients with complex thoracoabdominal aortic pathology who are unfit for alternative interventions. METHODS: Biomedical databases were systematically searched for articles published between 2008 and 2015 on the SMFM. A patient-level meta-analysis was used to evaluate aneurysm-related survival. Secondary outcomes were all-cause survival, stroke, spinal cord ischemia, renal impairment, and branch vessel patency. Other considerations were the impact of compliance with the instructions for use (IFU) on clinical outcome. Mean values and Kaplan-Meier estimates are presented with the 95% confidence interval (CI). RESULTS: Fifteen articles (3 multicenter cohort studies, 3 observational cohort studies, and 9 case reports) were included, presenting 171 patients (mean age 68.8±12.3 years; 139 men). The mean aneurysm diameter was 6.7±1.6 cm (95% CI 6.4 to 6.9 cm). Technical success reported in 15 studies was 77.2%. Aneurysm-related survival at 1 year was 78.7% (95% CI 71.7% to 84.4%). One-year all-cause survival was 53.7% (95% CI 46.0% to 61.3%). There were no reported cases of spinal cord ischemia, renal insult, or stroke. CONCLUSION: The SMFM can be safely utilized in some patients with complex thoracoabdominal pathologies provided operators adhere to the IFU. The SMFM is a novel technology with no long-term published data on its sustained effectiveness and a lack of comparative studies. Randomized clinical trials, registries, and continued assessment are essential before this flow-modulating technology can be widely disseminated.

Comparison of the strain field of abdominal aortic aneurysm measured by magnetic resonance imaging and stereovision: a feasibility study for prediction of the risk of rupture of aortic abdominal aneurysm.

The prediction of the risk of rupture of abdominal aortic aneurysm (AAA) is a complex problem. Currently the criteria to predict rupture of abdominal aortic aneurysms are aneurysm diameter and growth rates. It is generally believed that study of the wall strain distribution could be helpful to find a better decision criterion for surgery of aortic aneurysms before their rupture. The wall strain distribution depends on many biological and biomechanical factors such as elastic properties of the aorta, turbulent blood flow, anatomy of the aorta, presence of thrombus or not and so on. Recently, numerical simulations to estimate rupture-potential have received many attentions. However, none of the medical imaging tools for screening and monitoring of AAAs were studied in terms of mechanical behavior and experimentally to demonstrate their capability to measure relevant variables. The aim of this study was to develop a metrological approach for deployment testing of the ability of techniques for measuring local in-vitro deformations based on comparison of stereovision and MRI. In this paper, we present the implementation approach and results of the study based on cylindrical phantoms with or without AAA representing, respectively, healthy and unhealthy artery. Through this study, an experimental device was developed for the behavior study of AAA during a cardiac cycle. The results show that the stereovision techniques used in laboratory is well suited and is qualitatively and quantitatively equivalent with MRI measurements.

The influence of computational assumptions on analysing abdominal aortic aneurysm haemodynamics.

The variation in computational assumptions for analysing abdominal aortic aneurysm haemodynamics can influence the desired output results and computational cost. Such assumptions for abdominal aortic aneurysm modelling include static/transient pressures, steady/transient flows and rigid/compliant walls. Six computational methods and these various assumptions were simulated and compared within a realistic abdominal aortic aneurysm model with and without intraluminal thrombus. A full transient fluid-structure interaction was required to analyse the flow patterns within the compliant abdominal aortic aneurysms models. Rigid wall computational fluid dynamics overestimates the velocity magnitude by as much as 40%-65% and the wall shear stress by 30%-50%. These differences were attributed to the deforming walls which reduced the outlet volumetric flow rate for the transient fluid-structure interaction during the majority of the systolic phase. Static finite element analysis accurately approximates the deformations and von Mises stresses when compared with transient fluid-structure interaction. Simplifying the modelling complexity reduces the computational cost significantly. In conclusion, the deformation and von Mises stress can be approximately found by static finite element analysis, while for compliant models a full transient fluid-structure interaction analysis is required for acquiring the fluid flow phenomenon.

An experimental investigation of the hemodynamic variations due to aplastic vessels within three-dimensional phantom models of the circle of Willis.

A complete circle of Willis (CoW) is found in approximately 30-50% of the population. Anatomical variations, such as absent or surgically clamped vessels, can result in undesirable flow patterns. These can affect the brain's ability to maintain cerebral perfusion and the formation of cerebral aneurysms. An experimental test system was developed to simulate cerebral physiological conditions through three flexible 3D patient-specific models of complete and incomplete CoW geometries. Flow visualizations were performed with isobaric dyes and the mapped dye streamlines were tracked throughout the models. Three to seven flow impact locations were observed for all configurations, corresponding to known sites for aneurysmal formation. Uni and bi-directional cross-flows occurred along the communicating arteries. The greatest shunting of flow occurred for a missing pre-communicating anterior (A1) and posterior (P1) cerebral arteries. The anterior cerebral arteries had the greatest reduction (15-37%) in efferent flow rates for missing either a unilateral A1 or bilateral P1 segments. The bi-directional cross-flows, with multiple afferent flow mixing, observed along the communicating arteries may explain the propensity of aneurysm formation at these sites. Reductions in efferent flow rates due to aplastic vessel configurations may affect normal brain function.

An in vitro assessment of the cerebral hemodynamics through three patient specific circle of Willis geometries.

The Circle of Willis (CoW) is a complex pentagonal network comprised of fourteen cerebral vessels located at the base of the brain. The collateral flow feature within the circle of Willis allows the ability to maintain cerebral perfusion of the brain. Unfortunately, this collateral flow feature can create undesirable flow impact locations due to anatomical variations within the CoW. The interaction between hemodynamic forces and the arterial wall are believed to be involved in the formation of cerebral aneurysms, especially at irregular geometries such as tortuous segments, bends, and bifurcations. The highest propensity of aneurysm formation is known to form at the anterior communicating artery (AcoA) and at the junctions of the internal carotid and posterior communicating arteries (PcoAs). Controversy still remains as to the existence of blood flow paths through the communicating arteries for a normal CoW. This paper experimentally describes the hemodynamic conditions through three thin walled patient specific models of a complete CoW based on medical images. These models were manufactured by a horizontal dip spin coating method and positioned within a custom made cerebral testing system that simulated symmetrical physiological afferent flow conditions through the internal carotid and vertebral arteries. The dip spin coating procedure produced excellent dimensional accuracy. There was an average of less than 4% variation in diameters and wall thicknesses throughout all manufactured CoW models. Our cerebral test facility demonstrated excellent cycle to cycle repeatability, with variations of less than 2% and 1% for the time and cycle averaged flow rates, respectively. The peak systolic flow rates had less than a 4% variation. Our flow visualizations showed four independent flow sources originating from all four inlet arteries impacting at and crossing the AcoA with bidirectional cross flows. The flow paths entering the left and right vertebral arteries dissipated throughout the CoW vasculature from the posterior to anterior sides, exiting through all efferent vessels. Two of the models had five flow impact locations, while the third model had an additional two impact locations within the posterior circulation caused by an additional bidirectional cross flows along the PcoAs during the accelerating and part of the decelerating phases. For a complete CoW, bidirectional cross flows exist within the AcoA and geometrical variations within the CoW geometry can either promote uni- or bidirectional cross flows along the PcoAs.

Hemodynamic variations due to spiral blood flow through four patient-specific bifurcated stent graft configurations for the treatment of abdominal aortic aneurysms.

Endovascular repair is now a recognised procedure for treating abdominal aortic aneurysms. However, post-operative complications such as stent graft migration and thrombus may still occur. To assess these complications numerically, the correct input boundary conditions, which include the full human aorta with associated branching, should be included. Four patient-specific computed tomography scanned bifurcated stent grafts (SGs) were modelled and attached onto a full human aorta, which included the ascending, aortic arch and descending aortas. Two of the SG geometries had a twisted leg configuration, while the other two had conventional nontwisted leg configurations. Computational fluid dynamics was completed for both geometries and the hemodynamics assessed. The complexity of the flow patterns and secondary flows were influenced by the inclusion of the full human aorta at the SG proximal section. During the decelerating phase significant recirculations occurred along the main body of all SG configurations. The inclusion of the full human aorta did not impact the velocity contours within the distal legs and there was no difference in drag forces with the SG containing the full human aorta and those without. A twisted leg configuration further promoted a spiral flow formation along its distal legs.

In vitro evaluation of the effects of intraluminal thrombus on abdominal aortic aneurysm wall dynamics.

The optimum time to treat abdominal aortic aneurysms (AAAs) still remains an uncertain issue. The decision to intervene does not take in account the effects that wall curvature, intraluminal thrombus (ILT) properties and thickness have on rupture. The role of ILT in aneurysm dynamics and rupture has been controversial. In vitro testing of four silicone AAA models incorporating the ILT and aortic bifurcation was studied under physiological conditions. Pressures (P) and diameters (D) were analysed for models with and without ILT at different locations. The diametral strain, compliance and P/D curves were influenced by the presence, elastic stiffness and thickness of the ILT. In this case, the inclusion of ILT reduced the lumen area by 77% that resulted in a 0.5-81% reduction in compliance depending on ILT properties. With an increase in ILT stiffness from 0.05 to 0.2 MPa, the compliance was reduced by 81%. In the region of maximum diameter, there was a reduction of diametral strain and compliance except for the softer ILT which was more compliant throughout the proximal region. The shifting of the maximum diametral strain and compliance to the proximal neck was pronounced by an increase in ILT stiffness, thus creating a possible rupture site.

Effects of flat, parabolic and realistic steady flow inlet profiles on idealised and realistic stent graft fits through Abdominal Aortic Aneurysms (AAA).

Computational fluid dynamics modelling of bifurcated stent grafts positioned across Abdominal Aortic Aneursysms (AAAs) is influenced significantly by the boundary conditions and geometry. To assess these influencing factors, simulations using three different steady inlet profiles and two geometries were run. The steady velocity inlet profiles (flat, parabolic and realistic) were used as input boundary conditions with a constant pressure outlet for each model. The geometry of an out-of-plane realistic stent graft model, fitted to the internal geometry of an AAA (a realistic geometry generated from CT scans) was compared to an idealised geometry of an in-plane model. It was found that the realistic boundary condition when applied to the idealised model had a 5% difference in outlet flow rates when compared to flat and parabolic inlet profiles, while the realistic models had an 11-16% difference. The blood flowed in a parallel streamlined fashion for all the idealised models with slight recirculation for the realistic boundary condition. All realistic models had significant recirculation and flowed in a left-handed helix motion in the proximal end and left leg and in a right-handed helix motion in the right leg. The drag force was unidirectional in the idealised models, while it was found to act in all three directions for the realistic models with a 26% increase over the idealised models. It was concluded that geometry has the greatest influence on the outlet flow rates, flow patterns and drag forces. Input boundary condition cause variations in the skewness and recirculation of the flow throughout the models.