The initiation and progression of cerebral aneurysms are degenerative processes of the arterial wall driven by a complex interaction of biological and hemodynamic factors. Endothelial cells on the artery wall respond physiologically to blood-flow patterns. In normal conditions, these responses are associated with nonpathological tissue remodeling and adaptation. The combination of abnormal blood patterns and genetics predisposition could lead to the pathological formation of aneurysms. Here, we review recent progress on the basic mechanisms of aneurysm formation and evolution, with a focus on the role of hemodynamic patterns.

/pdf/hemodynamics-of-cerebral-aneurysms-1qtdoysrrf.pdf

Hemodynamics of Cerebral Aneurysms.

BACKGROUND AND PURPOSE: Blood flow dynamics are thought to play an important role in the pathogenesis and treatment of intracranial aneurysms; however, hemodynamic quantities of interest are difficult to measure in vivo. This study shows that computational fluid dynamics (CFD) combined with computed rotational angiography can provide such hemodynamic information in a patient-specific and prospective manner. METHODS: A 58-year-old woman presented with partial right IIIrd cranial nerve palsy due to a giant carotid-posterior communicating artery aneurysm that was subsequently coiled. Computed rotational angiography provided high resolution volumetric image data from which the lumen geometry was extracted. This and a representative flow rate waveform were provided as boundary conditions for finite element CFD simulation of the 3D pulsatile velocity field. RESULTS: CFD analysis revealed high speed flow entering the aneurysm at the proximal and distal ends of the neck, promoting the formation of both persistent and transient vortices within the aneurysm sac. This produced dynamic patterns of elevated and oscillatory wall shear stresses distal to the neck and along the sidewalls of the aneurysm. These hemodynamic features were consistent with patterns of contrast agent wash-in during cine angiography and with the configuration of coil compaction observed at 6-month follow-up. CONCLUSION: Anatomic realism of lumen geometry and flow pulsatility is essential for elucidating the patient-specific nature of aneurysm hemodynamics. Such image-based CFD analysis may be used to provide key hemodynamic information for prospective studies of aneurysm growth and rupture or to predict the response of an individual aneurysm to therapeutic options.

http://www.ajnr.org/content/ajnr/24/4/559.full.pdf

Image-based computational simulation of flow dynamics in a giant intracranial aneurysm.

Haemodynamic factors, in particular wall shear stresses (WSSs) may have significant impact on growth and rupture of cerebral aneurysms. Without a means to measure WSS reliably in vivo, computational fluid dynamic (CFD) simulations are frequently employed to visualise and quantify blood flow from patient-specific computational models. With increasing interest in integrating these CFD simulations into pretreatment planning, a better understanding of the validity of the calculations in respect to computation parameters such as volume element type, mesh size and mesh composition is needed. In this study, CFD results for the two most common aneurysm types (saccular and terminal) are compared for polyhedral- vs. tetrahedral-based meshes and discussed regarding future clinical applications. For this purpose, a set of models were constructed for each aneurysm with spatially varying surface and volume mesh configurations (mesh size range: 5119-258, 481 volume elements). WSS distribution on the model wall and point-based velocity measurements were compared for each configuration model. Our results indicate a benefit of polyhedral meshes in respect to convergence speed and more homogeneous WSS patterns. Computational variations of WSS values and blood velocities are between 0.84 and 6.3% from the most simple mesh (tetrahedral elements only) and the most advanced mesh design investigated (polyhedral mesh with boundary layer).

/pdf/tetrahedral-vs-polyhedral-mesh-size-evaluation-on-flow-50as4bw5s0.pdf

Tetrahedral vs. polyhedral mesh size evaluation on flow velocity and wall shear stress for cerebral hemodynamic simulation.

The simulation of blood flow past endovascular devices such as coils and stents is a challenging problem due to the complex geometry of the devices. Traditional unstructured grid computational fluid dynamics relies on the generation of finite element grids that conform to the boundary of the computational domain. However, the generation of such grids for patient-specific modeling of cerebral aneurysm treatment with coils or stents is extremely difficult and time consuming. This paper describes the application of an adaptive grid embedding technique previously developed for complex fluid structure interaction problems to the simulation of endovascular devices. A hybrid approach is used: the vessel walls are treated with body conforming grids and the endovascular devices with an adaptive mesh embedding technique. This methodology fits naturally in the framework of image-based computational fluid dynamics and opens the door for exploration of different therapeutic options and personalization of endovascular procedures.

Efficient simulation of blood flow past complex endovascular devices using an adaptive embedding technique

Computational fluid dynamics (CFD) is a numerical technique that is used for studying haemodynamic parameters in cerebral aneurysms. As it is now possible to represent an anatomically accurate intracranial aneurysm in a computational model, we have attempted to simulate its endosaccular occlusion with coils and demonstrate the haemodynamic changes induced. This is the first attempt to use this particular porous medium-based method for coiling simulation in a CFD model, to our knowledge. Datasets from a rotational 3-D digital subtraction angiogram of a recently ruptured anterior communicating aneurysm were converted into a 3-D geometric model and the discretized data were processed using the computational technique developed. Coiling embolisation simulation was achieved by impediment of flow through a porous medium with characteristics following a series of embolisation coils. Haemodynamic parameters studied were: pressure distribution on the vessel wall, blood velocity and blood flow patterns. Significant haemodynamic changes were detected after deployment of the first coil. Similar, but less dramatic changes occurred during subsequent stages of coiling. The blood flow patterns became less vortical in the aneurysm sac as velocity decreased to stagnation and the wall pressure at the fundus was gradually reduced. Furthermore, the haemodynamic characteristics developed at the area of the neck remnant could form the basis for assessing the likelihood of delayed coil compaction and aneurysm regrowth. Appropriate computational techniques show great promise in simulating the haemodynamic behaviour of the various stages in coil embolisation and may be a potentially valuable tool in interventional planning and procedural decision-making.

Haemodynamic simulation of aneurysm coiling in an anatomically accurate computational fluid dynamics model: technical note

The effect of different percentages of coil mesh in a cerebral aneurysm on the pulsatile flow and pressure in the parent vessel and aneurysm lumen was evaluated. Geometric data on a basilar tip aneurysm and vertebrobasilar arteries after subarachnoid hemorrhage was obtained by computer tomographic angiography. Intraarterial pressure was measured at four vertebrobasilar points before and after treatment with detachable coils. Pulsatile flow was documented by transcranial ultrasonography. A three-dimensional computer simulation was created using a commercial fluid dynamics solver for four aneurysm conditions: (1) before intervention; (2) with a 20% filling showing a complete cessation of the inflow through the aneurysm neck; (3) with a 12% filling showing an incomplete deceleration of inflow through the aneurysm neck, with a remaining flow around the embedded platinum coils; and (4) with a 12% filling and simulation of clotted aneurysm dome, which did not inhibit persisting flow phenomena. The relative pressure amplitudes neither increased nor decreased under the different simulated aneurysm filling conditions. Inserted platinum coils can immediately and decisively relieve the influx of pulsating blood and allow for initial clotting. To reach this effect, a volume density of 20% platinum coil mesh in the aneurysm neck is needed.

https://journals.sagepub.com/doi/pdf/10.1097/00004647-200112000-00011

Three-Dimensional Pulsatile Flow Simulation Before and After Endovascular Coil Embolization of a Terminal Cerebral Aneurysm

Operational performance optimization is a key element to reduce both fuel consumption and emissions and meet current and upcoming legislation, e.g., the EEXI, the CII and the European ETS. While technical innovation and improvements are important, the careful use of fuel on board plays a vital role to meet these targets. Here route and speed optimization are decisive. The basis of any approach to operational optimization is a digital twin which allows predicting the behavior of the ship and its energy consuming systems in a reliable and suitably accurate way. Here geometry plays an important role. The German R&D project MariData (maridata.org) addresses (present) shortcomings of onboard energy management systems. One specific element highlighted in this paper focuses on the desired quality of geometric representations for digital twins when used as a basis for performance analysis and improved energy consumption during operation. The paper compares different approaches and necessary levels of modeling fidelity to indicate attainable accuracy, aiming at further elucidating how good hydrodynamics need to be represented.

The Need for Sufficiently Accurate Geometrical Representations of Ship Hull Forms for Digital Twins for Performance Prediction

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Three-Dimensional Pulsatile Flow Simulation Before and After Endovascular Coil Embolization of a Terminal Cerebral Aneurysm

The Need for Sufficiently Accurate Geometrical Representations of Ship Hull Forms for Digital Twins for Performance Prediction