Y. Jonathan Zhang

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

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).

Hemodynamics at the ostium of cerebral aneurysms with relation to post-treatment changes by a virtual flow diverter: A computational fluid dynamics study

Computational fluid dynamics (CFD) techniques have been refined for modeling the hemodynamics in cerebral aneurysms. Recent interest has focused on understanding hemodynamic changes by treatment with a flow diverter (FD), i.e. a stent with a dense metal mesh which is placed across the ostium to divert the majority of flow away from the aneurysm. Potential complications include remnant inflow jets but, more seriously, aneurysm hemorrhage. For optimization of treatment outcome, a better understanding of the effects caused by the FD would be beneficial. In particular, pressure and velocity distributions at the aneurysm ostium are of interest, as they will be directly affected by the FD which in turn will influence post-treatment hemodynamics inside the aneurysm. Here, we report the results of a CFD study investigating the relationship between pre-treatment and post-treatment velocities, pressures and wall shear stresses (WSS) in the aneurysm with corresponding hemodynamic conditions at the aneurysm ostium prior to treatment. The study was carried out using a dedicated CFD prototype which allows modeling the effects of a virtual FD integrated into patient-specific geometries utilizing Darcys law. Velocities and WSS were reduced in all cases post FD treatment, pressure increased in one case. Heterogeneous distributions of the velocity magnitude were found at the ostium with focal maxima indicating potential risk zones for remnant inflow jets into the aneurysms. Pressures at the ostium correlated with pressure changes inside the aneurysm which could become a pre-treatment indicator for the evaluation of the suitability of a particular aneurysm for FD treatment.

Validation of computational fluid dynamics methods with anatomically exact, 3D printed MRI phantoms and 4D pcMRI

A new concept of rapid 3D prototyping was implemented using cost-effective 3D printing for creating anatomically correct replica of cerebral aneurysms. With a dedicated flow loop set-up in a full body human MRI scanner, flow measurements were performed using 4D phase contrast magnetic resonance imaging to visualize and quantify intra-aneurysmal flow patterns. Ultrashort TE sequences were employed to obtain high-resolution 3D image data to visualize the lumen inside the plastic replica. In-vitro results were compared with retrospectively obtained in-vivo data and results from computational fluid dynamics simulations (CFD). Rapid prototyping of anatomically realistic 3D models may have future impact in treatment planning, design of image acquisition methods for MRI and angiographic systems and for the design and testing of advanced image post-processing technologies.

Tetrahedral and polyhedral mesh evaluation for cerebral hemodynamic simulation — A comparison

Computational fluid dynamic (CFD) based on patient-specific medical imaging data has found widespread use for visualizing and quantifying hemodynamics in cerebrovas-cular disease such as cerebral aneurysms or stenotic vessels. This paper focuses on optimizing mesh parameters for CFD simulation of cerebral aneurysms. Valid blood flow simulations strongly depend on the mesh quality. Meshes with a coarse spatial resolution may lead to an inaccurate flow pattern. Meshes with a large number of elements will result in unnecessarily high computation time which is undesirable should CFD be used for planning in the interventional setting. Most CFD simulations reported for these vascular pathologies have used tetrahedral meshes. We illustrate the use of polyhedral volume elements in comparison to tetrahedral meshing on two different geometries, a sidewall aneurysm of the internal carotid artery and a basilar bifurcation aneurysm. The spatial mesh resolution ranges between 5,119 and 228,118 volume elements. The evaluation of the different meshes was based on the wall shear stress previously identified as a one possible parameter for assessing aneurysm growth. Polyhedral meshes showed better accuracy, lower memory demand, shorter computational speed and faster convergence behavior (on average 369 iterations less).

Mid- and long-term outcomes of carotid-cavernous fistula endovascular management with Onyx and n-BCA: experience of a single tertiary center

Introduction Endovascular therapy is the preferred treatment for most carotid-cavernous fistulas (CCFs). Early reports have documented excellent initial clinical and radiographic outcomes after embolization of CCFs with Onyx or n-butyl cyanoacrylate (n-BCA), but little evidence is available about the long-term durability of this technique. Objective To characterize the long-term durability of CCF liquid embolization. Methods The authors retrospectively reviewed a database of 24 CCFs in 21 consecutive patients who underwent Onyx or n-BCA embolization of a CCF from 2006 to 2013 at our institution. Results A total of 25 Onyx or n-BCA embolization procedures were attempted and 24 successfully completed during the study, resulting in complete or near-complete occlusion by the end of the study in all 24 CCFs (obliteration success, 100%). Attempted embolization in a single CCF failed initially, but was performed successfully at a later date by a different approach. None of the 24 CCFs recanalized, regrew, or required any further treatment subsequent to Onyx or n-BCA embolization throughout a mean 12.4 months of angiographic follow-up (range 1–36 months). Clinically significant complications were seen in three embolization procedures, including cranial nerve palsies (n=1), embolic infarct (n=1), and intraperitoneal hemorrhage (n=1). Conclusions Early evidence has indicated that endovascular embolization with Onyx is relatively safe and effective at achieving an initial angiographic cure for CCFs. Results of our series suggest that angiographic and clinical outcomes of Onyx and n-BCA embolization remain stable at mid- and long-term follow-up.

Magnetic resonance imaging as a tool to assess reliability in simulating hemodynamics in cerebral aneurysms with a dedicated computational fluid dynamics prototype: preliminary results

PURPOSE As an example of enhancing information in clinical image data by computational methods, simulating hemodynamics in cerebral aneurysms by means of computational fluid dynamics (CFD) is currently a topic of active research. Challenges consist in translating this engineering technology into clinical research, validating the simulations and addressing a potential clinical value of the results. In this preliminary study, we demonstrate the use of phase contrast magnetic resonance imaging (pcMRI) for assessing the reliability of CFD results. MATERIALS AND METHODS For six cerebral aneurysms where intra-aneurysmal velocity information was available by 2D pcMRI, steady CFD simulations with constant inflow were performed using a dedicated CFD prototype system. Major features of the velocity patterns derived from pcMRI were compared to those obtained with the CFD. RESULTS Good qualitative agreement between measured (2D pcMRI) and simulated (CFD) features of the intra-aneurysmal velocity patterns were obtained. These findings are discussed in the broader framework of the expectations towards CFD simulations in a clinical research setting. CONCLUSIONS Computational simulations reproduce major features of measured velocity patterns in cerebral aneurysms. Looking forward, these simulations need to be refined towards specific applications in clinical research.

Quantification of velocity reduction after flow diverter placement in intracranial aneurysm: An ex vivo study with 3D printed replicas

Phase contrast MRI (pcMRI) was used to measure flow before and after placement of a flow diverter (n = 3). Decreases from 18% to 31% in flow velocity were seen in the inflow jet of the aneurysms. Flow patterns were also compared. It was observed that the gross aneurysmal flow patterns were maintained after flow diverter placement despite decreased fluid velocities. All measurements were carried out in 3D printed aneurysm replicas.

Delayed Hydrocephalus and Perianeurysmal Cyst Formation After Stent-Assisted Coil Embolization of a Large, Unruptured Basilar Apex Aneurysm: A Case Report and Literature Review.

PurposeEndovascular aneurysm embolization possesses a unique set of infrequently seen complications distinct from those associated with microsurgical clipping, which may arise after an otherwise uncomplicated coil embolization procedure, including postembolization, hydrocephalus, and perianeurysmal cyst formation. DesignThe authors report an illustrative case of 2 rarely seen complications of aneurysm embolization with literature review. MethodsWe present a case of a basilar apex aneurysm that was treated with endovascular coil embolization with multiple Cerecyte (Micrus Endovascular, San Jose, Calif) coils and 2 Enterprise (Codman & Shurtleff, Inc, Raynham, Mass) stents. ResultsPostembolization angiography demonstrated complete aneurysm obliteration without distal branch occlusion or other complication. Twenty-two months after the embolization, however, the patient presented with progressively worsening headaches. Repeat magnetic resonance imaging revealed hydrocephalus and a perianeurysmal cyst measuring 1 × 2 cm adjacent to the previously coiled basilar apex aneurysm. After endoscopic third ventriculostomy, the patient experienced significant relief of her headaches and was discharged to rehabilitation. ConclusionsClinicians should be aware that worsening or new neuro-ophthalmic findings may be the presenting sign of postembolization hydrocephalus or perianeurysmal cyst formation.

Hemodynamic changes in patient-specific models of cerebral aneurysms with and without virtual flow diverters investigated with a dedicated CFD research prototype

Computational fluid dynamics (CFD) techniques have been pioneered to calculate wall shear stresses, dynamics pressures and other hemodynamics parameters for identifying additional risk factors for cerebral aneurysm rupture [1–4].Copyright

Patient-specific modelling of blood flow dynamics in intra-cranial aneurysms: an example of image-based personalised medicine

The visualisation of blood flow dynamics in intracranial aneurysm using computational fluid dynamics is an example of personalised medicine based on biomedical image analysis. A small percentage of these aneurysms rupture. Blood flow dynamics is thought to play an important role in the rupture process by exerting Wall Shear Stresses (WSS) on the endothelial cells. In this study, the effects of instabilities in the velocity waveform in the parent artery of an aneurysm of the Internal Carotid Artery (ICA) and variations of the computational mesh are investigated with respect to WSS at the aneurysm wall.