Computational fluid dynamics in brain aneurysms

Intracranial bypass for giant aneurysms treatment assessed by computational fluid dynamics (CFD) analysis

Unruptured giant intracranial aneurysms (GIA) are those with diameters of 25 mm or greater. As aneurysm size is correlated with rupture risk, GIA natural history is poor. Parent artery occlusion or trapping plus bypass revascularization should be considered to encourage intra-aneurysmal thrombosis when other treatment options are contraindicated. The mechanistic background of these methods is poorly studied. Thus, we assessed the potential of computational fluid dynamics (CFD) and fluid-structure interaction (FSI) analyses for clinical use in the preoperative stage. A CFD investigation in three patient-specific flexible models of whole arterial brain circulation was performed. A C6 ICA segment GIA model was created based on CT angiography. Two models were then constructed that simulated a virtual bypass in combination with proximal GIA occlusion, but with differing middle cerebral artery (MCA) recipient vessels for the anastomosis. FSI and CFD investigations were performed in three models to assess changes in flow pattern and haemodynamic parameters alternations (wall shear stress (WSS), oscillatory shear index (OSI), maximal time averaged WSS (TAWSS), and pressure). General flow splitting across the entire domain was affected by virtual bypass procedures, and any deficiency was partially compensated by a specific configuration of the circle of Willis. Following the implementation of bypass procedures, a reduction in haemodynamic parameters was observed within the aneurysm in both cases under analysis. In the case of the temporal MCA branch bypass, the decreases in the studied parameters were slightly greater than in the frontal MCA branch bypass. The reduction in the magnitude of the chosen area-averaged parameters (averaged over the aneurysm wall surface) was as follows: WSS 35.7%, OSI 19.0%, TAWSS 94.7%, and pressure 24.2%. FSI CFD investigation based on patient-specific anatomy models with subsequent stimulation of virtual proximal aneurysm occlusion in conjunction with bypass showed that this method creates a pro-thrombotic favourable environment whilst reducing intra-aneurysmal pressure leading to shrinking. MCA branch recipient selection for optimum haemodynamic conditions should be evaluated individually in the preoperative stage.

Intra-abdominal temperature variation during hyperthermic intraperitoneal chemotherapy evaluated via computational fluid dynamics modeling

Background

Hyperthermic intraperitoneal chemotherapy (HIPEC) targets intraperitoneal tumors with heated drug solutions via catheters inserted into the peritoneal space. Although studies have focused on clinical outcomes, the flow dynamics at specific intra-abdominal locations at-risk of harboring malignant cells remain poorly understood but are likely to impact the drug pharmacokinetics. Consequently, optimal protocols remain uncertain, with efficacy critically dependent on drug temperature and flow rate. This study tested the hypothesis that fluid flow dynamics at specific at-risk locations could be evaluated via a computational fluid dynamics (CFD) model of closed HIPEC in a simulated human abdominal cavity, with the goal to enable protocol optimization.

Methods

A computer-aided-design (CAD) model of a human intraperitoneal cavity (30 L) was coupled with computational fluid dynamics analysis. The tested HIPEC cycle parameters included catheter position and flow rates. The cavity was subjected to forward (superior to inferior flow) or reverse flow directions at 800 or 1,120 cc/min through four catheters, two as inlets and two as outlets, placed in upper and lower abdominal positions (net fluid volume: 18.5 L). Probes to measure temperature and flow were simulated between small and large bowels, inferior to small bowel mesentery, next to duodenum, superior to liver, superior to fundus, posterior to stomach, and posterior to liver.

Results

The simulations highlight heterogeneity in temperatures and flow that may occur during HIPEC at particular at-risk locations as a function of chemotherapy flow rate and direction. Temperature and fluid flow over the course of 90 min respectively varied from 0.93 K and <0.001 m/s inferior to small bowel mesentery (800 cc/min forward flow) to 3.6 K and 0.01 m/s next to the duodenum (either 800 or 1,120 cc/min forward flow). The results further suggest that monitoring outflow temperature may be inadequate for assessing HIPEC performance at at-risk locations.

Conclusions

Without intra-abdominal temperature monitoring at at-risk locations, it may be unfeasible to determine whether target temperatures and temperature homogeneity are being achieved during HIPEC. This work demonstrates that computational analysis offers the capability to monitor intra-abdominal locations at-risk of suboptimal heating and fluid flow given specific HIPEC parameters, and represents a first step towards designing efficacious tumor targeting during HIPEC.

Evaluation of predictive models of aneurysm focal growth and bleb development using machine learning techniques

BACKGROUND

The presence of blebs increases the rupture risk of intracranial aneurysms (IAs).

OBJECTIVE

To evaluate whether cross-sectional bleb formation models can identify aneurysms with focalized enlargement in longitudinal series.

METHODS

Hemodynamic, geometric, and anatomical variables derived from computational fluid dynamics models of 2265 IAs from a cross-sectional dataset were used to train machine learning (ML) models for bleb development. ML algorithms, including logistic regression, random forest, bagging method, support vector machine, and K-nearest neighbors, were validated using an independent cross-sectional dataset of 266 IAs. The models' ability to identify aneurysms with focalized enlargement was evaluated using a separate longitudinal dataset of 174 IAs. Model performance was quantified by the area under the receiving operating characteristic curve (AUC), the sensitivity and specificity, positive predictive value, negative predictive value, F1 score, balanced accuracy, and misclassification error.

RESULTS

The final model, with three hemodynamic and four geometrical variables, along with aneurysm location and morphology, identified strong inflow jets, non-uniform wall shear stress with high peaks, larger sizes, and elongated shapes as indicators of a higher risk of focal growth over time. The logistic regression model demonstrated the best performance on the longitudinal series, achieving an AUC of 0.9, sensitivity of 85%, specificity of 75%, balanced accuracy of 80%, and a misclassification error of 21%.

CONCLUSIONS

Models trained with cross-sectional data can identify aneurysms prone to future focalized growth with good accuracy. These models could potentially be used as early indicators of future risk in clinical practice.

The influence of contralateral circulation on computational fluid dynamics of intracranial arteries: simulated versus measured flow velocities

BACKGROUND

This study aimed to retrospectively evaluate the influence of contralateral anterior circulation on computational fluid dynamics (CFD) of intracranial arteries, by comparing the CFD values of flow velocities in unilateral anterior circulation with the measured values from phase-contrast magnetic resonance angiography (PC-MRA).

METHODS

We analyzed 21 unilateral anterior circulation models without proximal stenosis from 15 patients who performed both time-of-flight MRA (TOF-MRA) and PC-MRA. CFD was performed with the inflow boundary condition of a pulsatile flow of the internal carotid artery (ICA) obtained from PC-MRA. The outflow boundary condition was given as atmospheric pressure. Simulated flow velocities of the middle cerebral artery (MCA) and anterior cerebral artery (ACA) from CFD were compared with the measured values from PC-MRA.

RESULTS

The velocities of MCA were shown to be more accurately simulated on CFD than those of ACA (Spearman correlation coefficient 0.773 and 0.282, respectively). In four models with severe stenosis or occlusion of the contralateral ICA, the CFD values of ACA velocities were significantly lower (< 50%) than those measured with PC-MRA. ACA velocities were relatively accurately simulated in the models including similar diameters of both ACAs.

CONCLUSION

It may be necessary to consider the flow condition of the contralateral anterior circulation in CFD of intracranial arteries, especially in the ACA.

RELEVANCE STATEMENT

Incorporating the flow conditions of the contralateral circulation is of clinical importance for an accurate prediction of a rupture risk in Acom aneurysms as the bidirectional flow and accurate velocity of both ACAs can significantly impact the CFD results.

KEY POINTS

• CFD simulations using unilateral vascular models were relatively accurate for MCA. • Contralateral ICA steno-occlusion resulted in an underestimation of CFD velocity in ACA. • Contralateral flow may need to be considered in CFD simulations of ACA.

Computational fluid dynamics-based virtual angiograms for the detection of flow stagnation in intracranial aneurysms

The goal of this study was to test if CFD-based virtual angiograms could be used to automatically discriminate between intracranial aneurysms (IAs) with and without flow stagnation. Time density curves (TDC) were extracted from patient digital subtraction angiography (DSA) image sequences by computing the average gray level intensity inside the aneurysm region and used to define injection profiles for each subject. Subject-specific 3D models were reconstructed from 3D rotational angiography (3DRA) and computational fluid dynamics (CFD) simulations were performed to simulate the blood flow inside IAs. Transport equations were solved numerically to simulate the dynamics of contrast injection into the parent arteries and IAs and then the contrast retention time (RET) was calculated. The importance of gravitational pooling of contrast agent within the aneurysm was evaluated by modeling contrast agent and blood as a mixture of two fluids with different densities and viscosities. Virtual angiograms can reproduce DSA sequences if the correct injection profile is used. RET can identify aneurysms with significant flow stagnation even when the injection profile is not known. Using a small sample of 14 IAs of which seven were previously classified as having flow stagnation, it was found that a threshold RET value of 0.46 s can successfully identify flow stagnation. CFD-based prediction of stagnation was in more than 90% agreement with independent visual DSA assessment of stagnation in a second sample of 34 IAs. While gravitational pooling prolonged contrast retention time it did not affect the predictive capabilities of RET. CFD-based virtual angiograms can detect flow stagnation in IAs and can be used to automatically identify aneurysms with flow stagnation even without including gravitational effects on contrast agents.

Depiction of Cerebral Aneurysm Wall by Computational Fluid Dynamics (CFD) and Preoperative Illustration

Introduction  Preoperative illustration is a part of an important exercise to study the configuration, direction, and presence of any perforations, and is the weakest point in the wall of the cerebral aneurysm. The same illustration is used to study the surrounding brain structures to decide the best and safe surgical approach prior to any surgical procedure. With the evolution of the aneurysm wall study and study of flow dynamic within the involved artery and its aneurysm wall using computational fluid dynamics (CFD), a better surgical plan can be formulated to improve the flow dynamics. As one of the clinical applications of CFD, we propose a study using a composite image that combines preoperative illustration and CFD, which is traditionally widely used in neurosurgery.

Methods and Materials  We study the use of illustrations of the unruptured cerebral aneurysm of internal carotid-posterior communicating (ICPC) artery and anterior communicating artery (AcomA) treated at our hospital. The combinations of both preoperative illustrations and CFD images by using “ipad Pro” were used.

Result and Conclusion  Medical illustration in the preoperative study of unruptured cerebral aneurysm with combinations of CFD and surrounding brain structures is helpful to decide the surgical approaches and successful surgical treatments.

The effect of stiffened diabetic red blood cells on wall shear stress in a reconstructed 3D microaneurysm

Blood flow within the vasculature of the retina has been found to influence the progression of diabetic retinopathy. In this research cell resolved blood flow simulations are used to study the pulsatile flow of whole blood through a segmented retinal microaneurysm. Images were collected using adaptive optics optical coherence tomography of the retina of a patient with diabetic retinopathy, and a sidewall (sacciform) microaneurysm was segmented from the volumetric data. The original microaneurysm neck width was varied to produce two additional aneurysm geometries in order to probe the influence of neck width on the transport of red blood cells and platelets into the aneurysm. Red blood cell membrane stiffness was also increased to resolve the impact of rigid red blood cells, as a result of diabetes, in blood flow. Wall shear stress and wall shear stress gradients were calculated throughout the aneurysm domains, and the quantification of the influence of the red blood cells is presented. Average wall shear stress and wall shear stress gradients increased due to the increase of red blood cell membrane stiffness. Stiffened red blood cells were also found to induce higher local wall shear stress and wall shear stress gradients as they passed through the leading and draining parental vessels. Stiffened red blood cells were found to penetrate the aneurysm sac more than healthy red blood cells, as well as decreasing the margination of platelets to the vessel walls of the parental vessel, which caused a decrease in platelet penetration into the aneurysm sac.

Simulation of intracranial hemodynamics by an efficient and accurate immersed boundary scheme

We use computational fluid dynamics (CFD) to simulate blood flow in intracranial aneurysms (IAs). Despite ongoing improvements in the accuracy and efficiency of body-fitted CFD solvers, generation of a high quality mesh appears as the bottleneck of the flow simulation and strongly affects the accuracy of the numerical solution. To overcome this drawback, we use an immersed boundary method. The proposed approach solves the incompressible Navier-Stokes equations on a rectangular (box) domain discretized using uniform Cartesian grid using the finite element method. The immersed object is represented by a set of points (Lagrangian points) located on the surface of the object. Grid local refinement is applied using an automated algorithm. We verify and validate the proposed method by comparing our numerical findings with published experimental results and analytical solutions. We demonstrate the applicability of the proposed scheme on patient-specific blood flow simulations in IAs.

Numerical simulation flow dynamics of an intracranial aneurysm

BACKGROUND

Computational fluid dynamics provides a new method for the study of the blood flow characteristics of the formation and development of intracranial aneurysms.

OBJECTIVE

To compare blood flow characteristics between the healthy internal carotid artery and normal intracranial aneurysms.

METHODS

The internal carotid arteries were simulated to obtain hemodynamic parameters in one patient.

RESULTS

The internal carotid artery associated with aneurysm presents low wall shear stress, high oscillatory shear index, and high particle retention time compared with the normal internal carotid artery.

CONCLUSIONS

There are differences in blood flow between the normal internal carotid artery and intracranial aneurysm. The vortex of the aneurysm will produce turbulence, indicating that it is unstable, which results in the growth and rupture of the aneurysm.

Cerebrovascular development: mechanisms and experimental approaches

The cerebral vasculature plays a central role in human health and disease and possesses several unique anatomic, functional and molecular characteristics. Despite their importance, the mechanisms that determine cerebrovascular development are less well studied than other vascular territories. This is in part due to limitations of existing models and techniques for visualisation and manipulation of the cerebral vasculature. In this review we summarise the experimental approaches used to study the cerebral vessels and the mechanisms that contribute to their development.

Can biomechanical analysis shed some light on aneurysmal pathophysiology? Preliminary study on ex vivo cerebral arterial walls

BACKGROUND

The pathophysiology of cerebral aneurysm is complex and poorly understood, and it can have the most catastrophic clinical presentation. Flow dynamics is a key player in the initiation and progression of aneurysm. Better understanding the interaction between hemodynamic loading and biomechanical wall responses can help to add the missing piece on aneurysmal pathophysiology. In this laboratory study we aimed to analyze the effect of the application of a mechanical force to cerebral arterial walls.

METHODS

Displacement control tests were performed on five porcine cerebral arteries. The test machine was the T150 Nanotensile. The stiffness variation with the increment of the strain level is modeled as the outcome of an isotropic hyperelastic material model.

FINDINGS

Through the application of an axial force we obtained Stress/Strain curves that showed a marked isotropic hyperelastic behavior, characterized by an increasing of stiffness with the level of strain. This behavior of the cerebral arterial wall is different from the well-established behavior of other arterial vessel (as the aortic vessel) characterized by a marked anisotropic behavior. Additionally, the data scattering observed for higher values of the applied stress are related to different individual packing of collagen fibers that represent the load-bearing mechanics at higher level of the strain.

INTERPRETATION

The data obtained by test in this paper represent a first step in our ongoing research about the mechanics of multi-axial loads on cerebral arterial walls, and in producing more comprehensive patient-specific calculations for potential applications on cerebral aneurysm management.

Focal irregularities in 7-Tesla MRI of unruptured intracranial aneurysms as an indicator for areas of altered blood-flow parameters

OBJECTIVE

In the last several decades, various factors have been studied for a better evaluation of the risk of rupture in incidentally discovered intracranial aneurysms (IAs). With advanced MRI, attempts were made to delineate the wall of IAs to identify weak areas prone to rupture. However, the field strength of the MRI investigations was insufficient for reasonable image resolution in many of these studies. Therefore, the aim of this study was to analyze findings of IAs in ultra-high field MRI at 7 Tesla (7 T).

METHODS

Patients with incidentally found IAs of at least 5 mm in diameter were included in this study and underwent MRI investigations at 7 T. At this field strength a hyperintense intravascular signal can be observed on nonenhanced images with a brighter "rim effect" along the vessel wall. Properties of this rim effect were evaluated and compared with computational fluid dynamics (CFD) analyses.

RESULTS

Overall, 23 aneurysms showed sufficient image quality for further evaluation. In 22 aneurysms focal irregularities were identified within this rim effect. Areas of such irregularities showed significantly higher values in wall shear stress and vorticity compared to areas with a clearly visible rim effect (p = 0.043 in both).

CONCLUSIONS

A hyperintense rim effect along the vessel wall was observed in most cases. Focal irregularities within this rim effect showed higher values of the mean wall shear stress and vorticity when compared by CFD analyses. Therefore, these findings indicate alterations in blood flow in IAs within these areas.

On the optimal choice of a hyperelastic model of ruptured and unruptured cerebral aneurysm

In the last decade, preoperative modelling of the treatment of cerebral aneurysms is being actively developed. Fluid-structure interaction problem is a key point of a such modelling. Hence arises the question about the reasonable choice of the model of the vessel and aneurysm wall material to build the adequate model from the physical point of view. This study covers experimental investigation of 8 tissue samples of cerebral aneurysms and 1 tissue sample of a healthy cerebral artery. Results on statistical significance in ultimate stress for the classification of 2 cohorts of aneurysms: ruptured and unruptured described earlier in the literature were confirmed (p ≤ 0.01). We used the four most common models of hyperelastic material: Yeoh, Neo-Hookean and Mooney-Rivlin (3 and 5 parameter) models to describe the experimental data. In this study for the first time, we obtained a classification of hyperelastic models of cerebral aneurysm tissue, which allows to choose the most appropriate model for the simulation problems requirements depending on the physical interpretation of the considered problem: aneurysm status and range of deformation.

Perspectives on Sharing Models and Related Resources in Computational Biomechanics Research

The role of computational modeling for biomechanics research and related clinical care will be increasingly prominent. The biomechanics community has been developing computational models routinely for exploration of the mechanics and mechanobiology of diverse biological structures. As a result, a large array of models, data, and discipline-specific simulation software has emerged to support endeavors in computational biomechanics. Sharing computational models and related data and simulation software has first become a utilitarian interest, and now, it is a necessity. Exchange of models, in support of knowledge exchange provided by scholarly publishing, has important implications. Specifically, model sharing can facilitate assessment of reproducibility in computational biomechanics and can provide an opportunity for repurposing and reuse, and a venue for medical training. The community's desire to investigate biological and biomechanical phenomena crossing multiple systems, scales, and physical domains, also motivates sharing of modeling resources as blending of models developed by domain experts will be a required step for comprehensive simulation studies as well as the enhancement of their rigor and reproducibility. The goal of this paper is to understand current perspectives in the biomechanics community for the sharing of computational models and related resources. Opinions on opportunities, challenges, and pathways to model sharing, particularly as part of the scholarly publishing workflow, were sought. A group of journal editors and a handful of investigators active in computational biomechanics were approached to collect short opinion pieces as a part of a larger effort of the IEEE EMBS Computational Biology and the Physiome Technical Committee to address model reproducibility through publications. A synthesis of these opinion pieces indicates that the community recognizes the necessity and usefulness of model sharing. There is a strong will to facilitate model sharing, and there are corresponding initiatives by the scientific journals. Outside the publishing enterprise, infrastructure to facilitate model sharing in biomechanics exists, and simulation software developers are interested in accommodating the community's needs for sharing of modeling resources. Encouragement for the use of standardized markups, concerns related to quality assurance, acknowledgement of increased burden, and importance of stewardship of resources are noted. In the short-term, it is advisable that the community builds upon recent strategies and experiments with new pathways for continued demonstration of model sharing, its promotion, and its utility. Nonetheless, the need for a long-term strategy to unify approaches in sharing computational models and related resources is acknowledged. Development of a sustainable platform supported by a culture of open model sharing will likely evolve through continued and inclusive discussions bringing all stakeholders at the table, e.g., by possibly establishing a consortium.

A Retrospective Study in Microsurgical Procedures of Large and Giant Intracranial Aneurysms: An Outcome Analysis

OBJECTIVE

Intracranial aneurysms are considered large if >10 mm and giant if >25 mm. The risk of aneurysmal rupture compounds with increase in size of the aneurysm, thus, warranting appropriate intervention. In this study, we have analyzed the outcome and effectiveness of microsurgical procedures in large and giant aneurysms.

METHODS

A retrospective analysis of all the patients who underwent microsurgical procedures for large and giant cerebral aneurysms from 2014-2018 in our institute was conducted. There were a total of 52 patients, in which direct clipping was performed in 42 (80.7%) patients, proximal trapping in 3 (5.7%) patients, trapping with bypass in 3 (5.7%) patients, suction decompression in 3 (5.7%) patients, and 1 (1.9%) patient underwent surgical reconstruction.

RESULTS

Among the 52 patients, in the postoperative period, 1 (1.9%) patient became comatose, 1 (1.9%) patient developed hemiplegia, 1 (1.9%) patient had a transient hemiparesis, and 1 (1.9%) patient had transient lower cranial nerve palsy. Two (3.8%) patients had chronic subdural hematoma during the 3-month follow-up. There was no mortality in our series.

CONCLUSIONS

There are several treatment strategies available to manage large and giant cerebral aneurysms. In this study, we had minimal morbidity (3.8%), favorable outcome (96.1%), and no mortality. Therefore, we would like to conclude that appropriate microsurgical procedures, in experienced hands, can be considered as first line in the management for large and giant intracranial aneurysms, especially those with complex anatomy, wide neck, mass effect, partial thrombosis, and the presence of critical perforating vessels from the aneurysm wall.

Outcome Analysis of Surgical Clipping for Incidental Internal Carotid Posterior Communicating and Anterior Choroidal Artery Aneurysms

INTRODUCTION

Surgical outcome and ischemic complications of Internal carotid Posterior Communicating (IC PC) and anterior choroidal aneurysms have been questionable due to frequent occlusion of the anterior choroid artery and also due to low incidence of true anterior choroid artery aneurysms. The present series describes the postoperative outcome after clipping of such aneurysms at a single centre.

METHODS

A retrospective analysis of 73 cases with IC PC and Anterior choroidal aneurysms performed at a Fujita Health University, Banbuntane Hotokukai Hospital, Nagoya, Aichi, Japan from 2014 to 2018 have been studied and emphasis is made on the demography and ischemic complications.

RESULTS

A total of 73 patients with IC PC and anterior choroidal aneurysms were studied, out of which 57 patient had a true IC PC aneurysm, 14 patients had aneurysms involving the anterior choroidal artery and only 2 patients had aneurysms which involved both the IC PC and the anterior choroidal arteries. None of the patients had a permanent Anterior Choroidal Artery syndrome, whereas only 2 out of the 73 patients had postoperative complications in the form of transient hemiparesis.

CONCLUSION

Ischemic complications following surgical clipping of IC PC and anterior choroidal aneurysms can be minimised by meticulous micro dissection to identify the anterior choroidal artery thus preserving the patency of the same.

Toward Better Understanding of Flow Diversion in Bifurcation Aneurysms

BACKGROUND AND PURPOSE

Flow diversion is being increasingly used to treat bifurcation aneurysms. Empiric approaches have generally led to encouraging results, and a growing body of animal and ex vivo literature addresses the fate of target aneurysms and covered branches. Our prior investigations highlighted the dynamic nature of metal coverage provided by the Pipeline Embolization Device and suggested strategies for creating optimal single and multidevice constructs. We now address the geometric and hemodynamic aspects of jailing branch vessels and neighboring target aneurysms.

MATERIALS AND METHODS

Fundamental electric and fluid dynamics principles were applied to generate equations describing the relationships between changes in flow and the degree of vessel coverage in settings of variable collateral support to the jailed territory. Given the high complexity of baseline and posttreatment fluid dynamics, in vivo, we studied a simplified hypothetic system with minimum assumptions to generate the most conservative outcomes.

RESULTS

In the acute setting, Pipeline Embolization Devices modify flow in covered branches, principally dependent on the amount of coverage, the efficiency of collateral support, and intrinsic resistance of the covered parenchymal territory. Up to 30% metal coverage of any branch territory is very likely to be well-tolerated regardless of device or artery size or the availability of immediate collateral support, provided, however, that no acute thrombus forms to further reduce jailed territory perfusion.

CONCLUSIONS

Basic hemodynamic principles support the safety of branch coverage during aneurysm treatment with the Pipeline Embolization Device. Rational strategies to build bifurcation constructs are feasible.

Correlation of flow complexity parameter with aneurysm rupture status

Ruptured aneurysms are known to have complex flow patterns and concentrated inflow jet, but a quantifiable measure for the degree of flow complexity in patient-specific geometries has not been established. Previously, we proposed a flow complexity parameter that provides a quantitative description of the complexity of flow patterns through calculated curvature and torsion of the flow field. The purpose of the current study was to provide an analytic solution of the flow complexity parameter and assess a possible correlation with the rupture status of cerebral aneurysms by analyzing the parameter on five ruptured and five unruptured aneurysms from anterior communicating artery. We analyzed the flow complexity parameter in jet and non-jet regions in order to measure the concentration of the jet flow and the complexity of the non-jet flow. We found that on average, in a ruptured case the jet region is significantly less complex (4.5 times) than the jet region in an unruptured case, while the non-jet region is significantly more complex (3.5 times) than the non-jet region in an unruptured case. We also found a strong positive correlation of the non-jet complexity with dome volume in ruptured cases, but no correlation of jet complexity with dome volume. These findings suggest that a ruptured aneurysm has more than 4 times more concentrated inflow jet and more than 3 times more complex flow patterns in non-jet region than an unruptured aneurysm. This newly implemented kinematic parameter provides a measurable degree of complexity of flow patterns in cerebral aneurysms that can better assess aneurysm rupture risk.

Real-World Variability in the Prediction of Intracranial Aneurysm Wall Shear Stress: The 2015 International Aneurysm CFD Challenge

Purpose

Image-based computational fluid dynamics (CFD) is widely used to predict intracranial aneurysm wall shear stress (WSS), particularly with the goal of improving rupture risk assessment. Nevertheless, concern has been expressed over the variability of predicted WSS and inconsistent associations with rupture. Previous challenges, and studies from individual groups, have focused on individual aspects of the image-based CFD pipeline. The aim of this Challenge was to quantify the total variability of the whole pipeline.

Methods

3D rotational angiography image volumes of five middle cerebral artery aneurysms were provided to participants, who were free to choose their segmentation methods, boundary conditions, and CFD solver and settings. Participants were asked to fill out a questionnaire about their solution strategies and experience with aneurysm CFD, and provide surface distributions of WSS magnitude, from which we objectively derived a variety of hemodynamic parameters.

Results

A total of 28 datasets were submitted, from 26 teams with varying levels of self-assessed experience. Wide variability of segmentations, CFD model extents, and inflow rates resulted in interquartile ranges of sac average WSS up to 56%, which reduced to < 30% after normalizing by parent artery WSS. Sac-maximum WSS and low shear area were more variable, while rank-ordering of cases by low or high shear showed only modest consensus among teams. Experience was not a significant predictor of variability.

Conclusions

Wide variability exists in the prediction of intracranial aneurysm WSS. While segmentation and CFD solver techniques may be difficult to standardize across groups, our findings suggest that some of the variability in image-based CFD could be reduced by establishing guidelines for model extents, inflow rates, and blood properties, and by encouraging the reporting of normalized hemodynamic parameters.

NEWTONIAN AND NON-NEWTONIAN BLOOD FLOW AT A 90∘-BIFURCATION OF THE CEREBRAL ARTERY: A COMPARATIVE STUDY OF FLUID VISCOSITY MODELS

The majority of numerical simulations assumes blood as a Newtonian fluid due to an underestimation of the effect of non-Newtonian blood behavior on hemodynamics in the cerebral arteries. In the present study, we evaluated the effect of non-Newtonian blood properties on hemodynamics in the idealized 90[Formula: see text]-bifurcation model, using Newtonian and non-Newtonian fluids and different flow rate ratios between the parent artery and its branch. The proposed Local viscosity model was employed for high-precision representation of blood viscosity changes. The highest velocity differences were observed at zones with slow recirculating flow. During the systolic peak the average difference was 17–22%, whereas at the end of diastole the difference increased to 27–60% depending on the flow rate ratio. The main changes in the viscosity distribution were observed distal to the flow separation point, where the non-Newtonian fluid model produced 2.5 times higher viscosity. A presence of such high viscosity region substantially affected the size of the flow recirculation zone. The observed differences showed that non-Newtonian blood behavior had a significant effect on hemodynamic parameters and should be considered in the future studies of blood flow in cerebral arteries.

Cognitive Imaging

This article, an extended version of ICCI*CC-2017 paper, co-authored by biomedical engineers specializing in brain blood circulation modeling and by experts in meaning-based NLP. This article suggests a cognitive computing technology for medical imaging analysis that removes image artifacts resulting in visual deviations from reality, such as discontinuous blood vessels or two vessels shown merged when they are not. It is implemented by supplying the pertinent knowledge that humans have to the computer and letting it initiate the corrective post-processing. The existing OST resource is centered on the ontology that is made to accommodate the domain with a minor adjustment effort; however, any ontology can be used, as demonstrated in this article. The examples from the ontology demonstrate the disparities between what the image shows and what the human knows. The computer detects them autonomously and can initiate the appropriate post-processing. If and when this cognitive imaging prevails, the post-processed images may replace the current ones as legitimate artifact-free MRIs.

A data-driven approach for addressing the lack of flow waveform data in studies of cerebral arterial flow in older adults

OBJECTIVE

Blood flow waveforms-essential data for hemodynamic modeling-are often in practice unavailable to researchers. The objectives of this work were to assess the variability among the waveforms for a clinically relevant older population, and develop data-based methods for addressing the missing waveform data for hemodynamic studies.

APPROACH

We analyzed 272 flow waveforms from the internal carotid arteries of older patients (73  ±  13 yr) with moderate cardiovascular disease, and used these data to develop methods to guide new approaches for hemodynamic studies.

MAIN RESULTS

Profound variations in waveform parameters were found within the aged population that were not seen in published data for young subjects. Common features in the aged population relative to the young included a larger systole-to-diastole flow rate ratio, increased flow during late systole, and absence of a dicrotic notch. Eight waveforms were identified that collectively represent the range of waveforms in the older population. A relationship between waveform shape and flow rate was obtained that, in conjunction with equations relating flow rate to diameter, can be used to provide individualized waveforms for patient-specific geometries. The dependence of flow rate on diameter was statistically different between male and female patients.

SIGNIFICANCE

It was shown that a single archetypal waveform cannot well-represent the diverse waveforms found within an aged population, although this approach is frequently used in studies of flow in the cerebral vasculature. Motivated by these results, we provided a set of eight waveforms that can be used to assess the hemodynamic uncertainty associated with the lack of patient-specific waveform data. We also provided a methodology for generating individualized waveforms when patient gender, age, and cardiovascular disease state are known. These data-driven approaches can be used to devise more relevant in vitro or in silico intra-cranial hemodynamic studies for older patients.

The Virtual Physiological Human: Ten Years After

Biomedical research and clinical practice are struggling to cope with the growing complexity that the progress of health care involves. The most challenging diseases, those with the largest socioeconomic impact (cardiovascular conditions; musculoskeletal conditions; cancer; metabolic, immunity, and neurodegenerative conditions), are all characterized by a complex genotype-phenotype interaction and by a "systemic" nature that poses a challenge to the traditional reductionist approach. In 2005 a small group of researchers discussed how the vision of computational physiology promoted by the Physiome Project could be translated into clinical practice and formally proposed the term Virtual Physiological Human. Our knowledge about these diseases is fragmentary, as it is associated with molecular and cellular processes on the one hand and with tissue and organ phenotype changes (related to clinical symptoms of disease conditions) on the other. The problem could be solved if we could capture all these fragments of knowledge into predictive models and then compose them into hypermodels that help us tame the complexity that such systemic behavior involves. In 2005 this was simply not possible-the necessary methods and technologies were not available. Now, 10 years later, it seems the right time to reflect on the original vision, the results achieved so far, and what remains to be done.

The Numerical Study of the Hemodynamic Characteristics in the Patient-Specific Intracranial Aneurysms before and after Surgery

The patient-specific pre- and postsurgery cerebral arterial geometries in the study were reconstructed from computed tomography angiography (CTA). Three-dimensional computational fluid dynamics models were used to investigate the hemodynamic phenomena in the cerebral arteries before and after surgery of the aneurysm under realistic conditions. CFD simulations for laminar flow of incompressible Newtonian fluid were conducted by using commercial software, ANSYS v15, with the rigid vascular wall assumption. The study found that the flow patterns with the complex vortical structures inside the aneurysm were similar. We also found that the inflow jet streams were coming strongly in aneurysm sac in the presurgery models, while the flow patterns in postsurgery models were quite different from those in presurgery models. The average wall shear stress after surgery for model 1 was approximately three times greater than that before surgery, while it was about twenty times greater for model 2. The area of low WSS in the daughter saccular aneurysm region in model 2 is associated with aneurysm rupture. Thus the distribution of WSS in aneurysm region provides useful prediction for the risk of aneurysm rupture.

Measurement of flow diverter hydraulic resistance to model flow modification in and around intracranial aneurysms

Flow diverters (FDs) have been successfully applied in the recent decade to the treatment of intracranial aneurysms by impairing the communication between the flows in the parent artery and the aneurysm and, thus, the blood within the aneurysm sac. It would be desirable to have a simple and accurate computational method to follow the changes in the peri- and intraaneurysmal flow caused by the presence of FDs. The detailed flow simulation around the intricate wire structure of the FDs has three disadvantages: need for high amount of computational resources and highly skilled professionals to prepare the computational grid, and also the lack of validation that makes the invested effort questionable. In this paper, we propose a porous layer method to model the hydraulic resistance (HR) of one or several layers of the FDs. The basis of this proposal is twofold: first, from an application point of view, the only interesting parameter regarding the function of the FD is its HR; second, we have developed a method to measure the HR with a simple apparatus. We present the results of these measurements and demonstrate their utility in numerical simulations of patient-specific aneurysm simulations.

Flow-induced ATP release in patient-specific arterial geometries--a comparative study of computational models

The importance of the endothelium in the local regulation of blood flow is reflected by its influence on vascular tone by means of vasodilatory responses to many physiological stimuli. Regulatory pathways are affected by mass transport and wall shear stress (WSS), via mechanotransduction mechanisms. In the present work, we review the most relevant computational models that have been proposed to date, and introduce a general framework for modelling the responses of the endothelium to alteration in the flow, with a view to understanding the biomechanical processes involved in the pathways to endothelial dysfunction. Simulations are performed on two different patient-specific stenosed carotid artery geometries to investigate the influence of WSS and mass transport phenomena upon the agonist coupling response at the endothelium. In particular, results presented for two different models of WSS-dependent adenosine-5'-triphosphate (ATP) release reveal that existing paradigms may not account for the conditions encountered in vivo and may therefore not be adequate to model the kinetics of ATP at the endothelium.

Accurate determination of patient-specific boundary conditions in computational vascular hemodynamics using 3D cine phase-contrast MRI

In the patient-specific vascular CFD, determination of the inlet and outlet boundary conditions (BCs) is an important issue for a valid diagnosis. The 3D cine phase-contrast MRI (4D Flow) velocimetry is promising for this issue; yet, its measured velocities contain relatively large error and are not admissible as the BCs without any correction. This paper proposes a novel correction method for determining the BCs accurately using the 4D Flow velocimetry. First, we reveal that the error of the velocity measured by the 4D Flow at each measurement voxel is large but is distributed symmetrically. Secondly, our method pays attention to the incompressibility of the blood and the fact that the volume flow rate (VFR) in each vessel is constant on any cross sections. We reveal that the average of the cross-sectional VFRs integrated from many measurement voxel in each vessel is accurate despite the large error. Finally, we propose the novel correction method, which applies a smoothing to the measured velocities on each inlet or outlet boundary with a low-pass filter and then corrects them with the VFR. The results of the several phantom studies are presented to validate the accuracy of our method. A demonstrative analysis for an actual aneurysm is also presented to show the feasibility and effectiveness of our method.

Physical factors effecting cerebral aneurysm pathophysiology

Many factors that are either blood-, wall-, or hemodynamics-borne have been associated with the initiation, growth, and rupture of intracranial aneurysms. The distribution of cerebral aneurysms around the bifurcations of the circle of Willis has provided the impetus for numerous studies trying to link hemodynamic factors (flow impingement, pressure, and/or wall shear stress) to aneurysm pathophysiology. The focus of this review is to provide a broad overview of such hemodynamic associations as well as the subsumed aspects of vascular anatomy and wall structure. Hemodynamic factors seem to be correlated to the distribution of aneurysms on the intracranial arterial tree and complex, slow flow patterns seem to be associated with aneurysm growth and rupture. However, both the prevalence of aneurysms in the general population and the incidence of ruptures in the aneurysm population are extremely low. This suggests that hemodynamic factors and purely mechanical explanations by themselves may serve as necessary, but never as necessary and sufficient conditions of this disease's causation. The ultimate cause is not yet known, but it is likely an additive or multiplicative effect of a handful of biochemical and biomechanical factors.

Suggested connections between risk factors of intracranial aneurysms: a review

The purpose of this article is to review studies of aneurysm risk factors and the suggested hypotheses that connect the different risk factors and the underlying mechanisms governing the aneurysm natural history. The result of this work suggests that at the center of aneurysm evolution there is a cycle of wall degeneration and weakening in response to changing hemodynamic loading and biomechanic stress. This progressive wall degradation drives the geometrical evolution of the aneurysm until it stabilizes or ruptures. Risk factors such as location, genetics, smoking, co-morbidities, and hypertension seem to affect different components of this cycle. However, details of these interactions or their relative importance are still not clearly understood.