Session A04: Aneurysms

Determination of the Risk of Rupture of Intracranial Aneurysms Through Numerical Simulation and Data Classification

Determining the risk of rupture of intracranial aneurysms is a great challenge. In recent years, hemodynamics parameters, such as shear stress and oscillatory shear index, have gained attention to predict aneurysm rupture. Numerical flow simulations in complex biological structures have also gained considerable attention. However, the primary challenge is the validation of the numerical procedures due to the complexity of measuring (velocity and shear stress) and testing the computed results. The main goal of the present work is to classify geometrical and hemodynamic parameters to predict the possible rupture of the aneurysm.

Numerical simulations were performed using realistic geometries. Aneurysms 3D models were reconstructed from CT scans, using the 3D-Slicer software to perform the numerical simulations and to 3D print the models for validation purposes. The blood was modelled as a Newtonian fluid with constant properties. As a first approximation, the arterial walls were rigid, and no-slip boundary condition was taken into account. 

The 1R machine learning algorithm was used for classifying the geometrical and hemodynamic parameters. The analysis used the following hemodynamic parameters: wall pressure and wall shear stress, oscillatory shear index (OSI), residence time, gradient oscillatory number (GON), vorticity, q criterion, stokes number for particles and enstrophy to classify the aneurysm in combination with the traditional parameters.

The performed numerical simulations showed good agreement with the experiments (qualitatively). The first classification presents a reliability of 82.8%. This is the first achievement of the project. 

    

Impact of body movement on the hemodynamics of cerebral aneurysm, an ex-vivo experimental study

Research on exercise hemodynamics has studied how body movement caused an increased frequency of pulsatile changes in pressure and flows, vessel stiffness, and blood flow shear stress. However, body movement can affect hemodynamics due to the physical motion of the vessel wall shaking the blood. This is a largely ignored yet crucial phenomenon that needs to be quantitatively investigated. In the study, we aim to study how human body movement may impact the hemodynamics of cerebral aneurysms, beyond typical physiological responses. We first employ a motion collection system to capture the human head movement. We then implement a six-degree-of-freedom motion simulation platform to replay the human head motion. Lastly, and most importantly, we perform an ex-vivo hemodynamics measurement in a cerebral aneurysm phantom model, using a high-speed PIV system on this motion simulation platform. Both simplified and patient-specific cerebral aneurysm models are utilized, and different inlet blood flow conditions are tested during this study. In the control group, we measure hemodynamics when the motion simulation platform is at rest. In the treatment group, the hemodynamics measurement and body movement replaying are simultaneously conducted, with identical inlet flow conditions. We evidenced clear differences in hemodynamics in these two groups. This study suggests that, besides physiological response, body movement also physically contributes to the intense hemodynamics changes that need to be considered during any drug or biomedical device design and verification.   

Evaluating the effect of varying inflow waveform conditions on hemodynamics in a patient specific cerebral aneurysm using 4D PIV

Accurately accessing the risk and rupture of a cerebral aneurysm is critical in order to evaluate the treatment options for patients harboring an intracranial aneurysm (IA). Hemodynamic factors such as wall shear stress (WSS) and oscillatory shear stress (OSS) are known to contribute to the progression of IAs. However, current studies, across all modalities, assume fixed inflow conditions and fail to consider the inherently transient nature of the flow in the aneurysm as the patient moves or exercises. In this work, we consider how varying in-flow conditions, including flow magnitude and heart rate, affects the hemodynamics in and around an IA. We use a polydimethylsiloxane (PDMS) model of a patient specific spherical right middle cerebral artery (R MCA) bifurcation aneurysm in a volumetric particle image velocimetry (PIV) study. Moreover, we evaluate the sensitivity of key factors including WSS and OSI to the transient inflow conditions.   

A new heterogenous porous model to simulate effects of intracranial aneurysm coiling for clinical risk prediction, validated using coil-resolved synchrotron scans.

We study intracranial aneurysms treated with endovascular coils deployed in the aneurysm dome to reduce flow, thereby allowing the aneurysm to heal. Standard model for the coils is a homogenous, isotropic porous medium. However, we have found that the distribution of coils produces a highly heterogeneous porosity field. Exploiting the observed pattern of larger porosity values near the wall and lower, more homogenous mass towards the aneurysmal core, a bi-linear model is proposed. This model is resolvable from information known a priori through standard treatment protocols: target porosity and aneurysm volume/shape. To validate this model and gain further insight into effects of the coil mass on intra-aneurysmal flow and treatment effectiveness, simulations of fully resolved coiled aneurysms are performed and compared against the novel model results. The data shows that the bi-linear model is a significant improvement over the standard model for prediction of hemodynamics. This model is being implemented in a large cohort study to predict coil treatment outcomes.   

Recruitment of collateral pathways in the Circle of Willis during vasospasm. A small cohort patient-specific computational fluid dynamics study.

Vasospasm – involuntary constriction of blood vessels – afflicts 50-90% of patients after subarachnoid hemorrhage in the brain. This reduces perfusion to the brain, with some patients exhibiting neurological deficits. In stroke patients, the collateral pathways in the Circle of Willis (CoW) direct flow to the site of the occlusion to try to maintain normal perfusion; however, the role of collateral pathways in vasospasm patients is an active area of research. Only 40% of the population exhibits a complete, balanced CoW, limiting the ability of collateral pathways to be recruited in stroke or vasospasm. Three different patients with distinct CoW variants are compared using the magnitudes and directions of collateral flow as well as loss metrics such as vascular resistance and dissipation. Patient-specific computational fluid dynamics simulations are created by applying transcranial Doppler ultrasound measurements of the velocity in the main CoW vessels on a segmentation of the cerebral vasculature from contrast tomography. The pre-vasospasm simulations are benchmarked against literature values of diameter, velocity, and flow rate to validate the model. Virtual angiograms are compared to clinical angiograms to confirm flow reversal. The influence of CoW morphology and patient characteristics are analyzed in vasospasm.   

Intra-aneurysmal pulsatile flow hemodynamics in low-aspect ratio Abdominal Aortic Aneurysms (AAA)

Abdominal aortic aneurysm (AAA) is a pathological condition associated with irreversible dilation of aortic wall. Transient blood flow, originating at the heart heart [VG1] in the vicinity of a dilation generates abnormal flow hemodynamics that contributes to further wall weakening and sudden arterial ruptures. The present study numerically explores the pulsatile flow characteristics (Re_peak = 700 – 2200; f = 1.2 – 2.4 Hz) within low aspect ratio aneurysms (L/d = 1.5). Results reveal that for Re_peak = 700 - 1200, the aneurysmal flow is analogous to vortex mode i.e. vortex detaches from the free shear layer at the proximal wall and travels downstream towards the distal end. However, at Re_peak = 1200 – 2200, cavity mode is manifested i.e. the free shear layers remain confined within the proximal and distal limits and subsequently drive the primary vortex, that in turn stimulate the secondary and tertiary vortices. Furthermore, wall loading owing to spatio-temporal evolution of vortex structures is addressed through signatures of wall shear stress (WSS) and oscillatory shear index (OSI), respectively. Additionally, proper orthogonal decomposition (POD) is employed to exploit the energetic modes of vortex structures responsible for disease progression in the biomedical context.   

Fluid-Structure-Acoustic Interaction Study on Abdominal Aortic Aneurysms: Application in Phonoangiography

Computational Fluid-Structure-Acoustic Interaction (FSAI) study is presented for effect of Abdominal Aortic Aneurysm (AAA) width and height on hemodynamic parameters and cut-off frequency of sound spectrum on skin surface. An in-house FSAI solver, developed using Arbitrary Lagrangian-Eulerian (ALE) framework with Finite-Volume (Flow and Structure) and Finite-Difference (Acoustics) discretisation, is employed to perform axisymmetric blood flow simulations in artery with fusiform aneurysm. The velocity fluctuation, which is usually sensed by a digital stethoscope is calculated on the skin surface corresponding to pulsatile inlet flow at Womersley number (Wo) of 16.5. Vortex shedding from distal end of aneurysm during decelerating systolic stage and their further impingement and dissipation on aneurysm wall leads to pressure fluctuation and thereby characteristic frequencies. The motion of recirculation zones, along narrow aneurysm wall leads to higher Wall Shear Stress; whereas impingement of shorter length and higher energy scale vortices, for aneurysm with larger height, leads to higher frequencies. The current work provides a physical perspective behind Hemoacoustics and Hemodynamics of AAA which can lead to development of non-invasive diagnostic technique ‘phonoangiography’.