Intraoperative neuromonitoring as real-time diagnostic for cerebral ischemia in endovascular treatment of ruptured brain aneurysms

OBJECTIVE

To evaluate the diagnostic accuracy of intraoperative neurophysiological monitoring (IONM) during endovascular treatment (EVT) of ruptured intracranial aneurysms (rIA).

METHODS

IONM and clinical data from 323 patients who underwent EVT for rIA from 2014-2019 were retrospectively reviewed. Significant IONM changes and outcomes were evaluated based on visual review of data and clinical documentation.

RESULTS

Of the 323 patients undergoing EVT, significant IONM changes were noted in 30 patients (9.29%) and 46 (14.24%) experienced postprocedural neurological deficits (PPND). 22 out of 30 (73.33%) patients who had significant IONM changes experienced PPND. Univariable analysis showed changes in somatosensory evoked potential (SSEP) and electroencephalogram (EEG) were associated with PPND (p-values: <0.001 and <0.001, retrospectively). Multivariable analysis showed that IONM changes were significantly associated with PPND (Odd ratio (OR) 20.18 (95%CI:7.40-55.03, p-value: <0.001)). Simultaneous changes in both IONM modalities had specificity of 98.9% (95% CI: 97.1%-99.7%). While sensitivity when either modality had a change was 47.8% (95% CI: 33.9%-62.0%) to predict PPND.

CONCLUSIONS

Significant IONM changes during EVT for rIA are associated with an increased risk of PPND.

SIGNIFICANCE

IONM can be used confidently as a real time neurophysiological diagnostic guide for impending neurological deficits during EVT treatment of rIA.

Characterizing Tissue Remodeling and Mechanical Heterogeneity in Cerebral Aneurysms

Accurately assessing the complex tissue mechanics of cerebral aneurysms (CAs) is critical for elucidating how CAs grow and whether that growth will lead to rupture. The factors that have been implicated in CA progression - blood flow dynamics, immune infiltration, and extracellular matrix remodeling - all occur heterogeneously throughout the CA. Thus, it stands to reason that the mechanical properties of CAs are also spatially heterogeneous. Here, we present a new method for characterizing the mechanical heterogeneity of human CAs using generalized anisotropic inverse mechanics, which uses biaxial stretching experiments and inverse analyses to determine the local Kelvin moduli and principal alignments within the tissue. Using this approach, we find that there is significant mechanical heterogeneity within a single acquired human CA. These results were confirmed using second harmonic generation imaging of the CA's fiber architecture and a correlation was observed. This approach provides a single-step method for determining the complex heterogeneous mechanics of CAs, which has important implications for future identification of metrics that can improve accuracy in prediction risk of rupture.

Nitrosative stress induces downregulation of ribosomal protein genes via MYCT1 in vascular smooth muscle cells

OBJECTIVE

In our previous genomic studies in human intracranial aneurysms, we observed downregulations in the expression of a number of ribosomal protein genes and the c-Myc-related gene MYC target 1 (MYCT1). So far there is no information about the roles of MYCT1 in vascular cells. Our study aims to investigate the functional roles of MYCT1 in vascular smooth muscle cells (SMCs).

MATERIALS AND METHODS

Primary SMCs were isolated from rat thoracic aorta and cultured in vitro. The mRNA and protein expressions were determined by real-time PCR and western blot respectively. Apoptosis was detected by measuring caspase 3/7 activity. Collagen production was determined with ELISA.

RESULTS

Using PCR, we validated our previous genomic data showing that the expressions of MYCT1 and ribosomal protein genes were decreased in human aneurysm tissues. In vascular SMCs, we showed that nitrosative stress downregulated the expression of both MYCT1 and ribosomal proteins. Knockdown of MYCT1 mimicked the effects of nitrosative stress on ribosomal protein expressions, whereas overexpression of MYCT1 blunted the effects of nitrosative stress. MYCT1-dependent downregulation of ribosomal proteins compromised the protein translational capacity of the cells for collagen production. Moreover, the endogenously expressed MYCT1 in vascular SMCs was involved in maintaining normal cellular functions including survival, proliferation and migration.

CONCLUSIONS

MYCT1-dependent gene regulation may, at least partly, explain the downregulated expressions of ribosomal proteins observed in human intracranial aneurysms. It is suggested that MYCT1 may represent a novel molecular target for counteracting the decreased activity of aneurysmal SMCs for tissue repairmen/regeneration.

Roles of Phytoestrogen in the Pathophysiology of Intracranial Aneurysm

[Figure: see text].

Flow Diversion in Acute Sub Arachnoid Haemorrhage: A Single Centre Five Year Experience

OBJECTIVES

The role of flow-diversion in acute sub-arachnoid haemorrhage (SAH) is controversial. Many of the published data warns of high rates of procedure-related complications and aneurysmal rebleed. This study evaluates the safety, efficacy, clinical and angiographic outcomes of acute flow-diversion at our institute.

METHODS

The institutional database from June 2015 to June 2020 was retrospectively reviewed for aneurysmal SAH (aSAH) treated with flow diversion. Clinical presentation, procedural details, complications, anti-platelet usage, rebleeding and aneurysm occlusion rates and outcomes were recorded.

RESULTS

22 (59% females; median age 56 years) consecutive patients were identified. None of them were on regular antiplatelets/anticoagulation in the 15-days preceding the treatment. The mean aneurysm diameter was 5.4 mm and the median delay to flow-diversion was 2 days. Almost 73% (16/22) of patients had adjunctive coiling in the same session. There was no aneurysmal rebleed at a median follow up of 8.5 months and 86.3% (19/22 patients) had good clinical outcomes (3-month MRS 0-2). Adverse events related to the flow diversion procedure were seen in 3 patients; none of them had a medium to long-term clinical consequence. Three patients died from complications of SAH, unrelated to the procedure. Vascular imaging follow-up was available for 20 patients and the complete aneurysm occlusion rate was 95%.

CONCLUSION

Flow-diversion could be a reasonably safe and effective technique for treating ruptured aneurysms in appropriately selected patients when conventional options of surgical clipping and coiling are considered challenging.

Enhancement of cerebrovascular 4D flow MRI velocity fields using machine learning and computational fluid dynamics simulation data

Blood flow metrics obtained with four-dimensional (4D) flow phase contrast (PC) magnetic resonance imaging (MRI) can be of great value in clinical and experimental cerebrovascular analysis. However, limitations in both quantitative and qualitative analyses can result from errors inherent to PC MRI. One method that excels in creating low-error, physics-based, velocity fields is computational fluid dynamics (CFD). Augmentation of cerebral 4D flow MRI data with CFD-informed neural networks may provide a method to produce highly accurate physiological flow fields. In this preliminary study, the potential utility of such a method was demonstrated by using high resolution patient-specific CFD data to train a convolutional neural network, and then using the trained network to enhance MRI-derived velocity fields in cerebral blood vessel data sets. Through testing on simulated images, phantom data, and cerebrovascular 4D flow data from 20 patients, the trained network successfully de-noised flow images, decreased velocity error, and enhanced near-vessel-wall velocity quantification and visualization. Such image enhancement can improve experimental and clinical qualitative and quantitative cerebrovascular PC MRI analysis.

Numerical study on the energy cascade of pulsatile Newtonian and power-law flow models in an ICA bifurcation

The complex physics and biology underlying intracranial hemodynamics are yet to be fully revealed. A fully resolved direct numerical simulation (DNS) study has been performed to identify the intrinsic flow dynamics in an idealized carotid bifurcation model. To shed the light on the significance of considering blood shear-thinning properties, the power-law model is compared to the commonly used Newtonian viscosity hypothesis. We scrutinize the kinetic energy cascade (KEC) rates in the Fourier domain and the vortex structure of both fluid models and examine the impact of the power-law viscosity model. The flow intrinsically contains coherent structures which has frequencies corresponding to the boundary frequency, which could be associated with the regulation of endothelial cells. From the proposed comparative study, it is found that KEC rates and the vortex-identification are significantly influenced by the shear-thinning blood properties. Conclusively, from the obtained results, it is found that neglecting the non-Newtonian behavior could lead to underestimation of the hemodynamic parameters at low Reynolds number and overestimation of the hemodynamic parameters by increasing the Reynolds number. In addition, we provide physical insight and discussion onto the hemodynamics associated with endothelial dysfunction which plays significant role in the pathogenesis of intracranial aneurysms.

In vivo comparison of braided (Accero) and laser-cut intracranial stents (Acclino, Credo): evaluation of vessel responses at subacute and mid-term follow-up in a rabbit model

This study aimed to investigate in vivo two stent technologies, with particular emphasis on thrombogenicity and inflammatory vessel remodeling processes. The micro-stents tested in this study were developed for intracranial aneurysm treatment. In our study twelve, New Zealand white rabbits were divided into two groups: 18 laser-cut stents (LCS) and 18 braided stents (BS) were impanated without admiration of antiplatelet medication. Three stents were implanted into each animal in the common carotid artery, subclavian artery, and abdominal aorta. Digital subtraction angiography was performed before and after stent implantation and at follow-up for the visualization of occurring In-stent thromboembolism or stenosis. The Stents were explanted for histopathological examination at two different timepoints, after 3 and 28 days. Angiographically neither in-stent thrombosis nor stenosis for both groups was seen. There was a progressive increase in the vessel diameter, which was more pronounced for BS than for LCS. We detected a higher number of thrombi adherent to the foreign material on day 3 for BS. On day 3, the neointima was absent, whereas the complete formation observed was on day 28. There was no significant difference between both groups regarding the thickness of the neointima. The in vivo model of our study enabled the evaluation of blood and vessel reactions for two different stent technologies. Differences in vessel dimension and tissue around the stents were observed on day 28. Histological analysis on day 3 enabled the assessment of thrombotic reactions, representing an important complementary result in long-term studies.

Modeling intracranial aneurysm stability and growth: an integrative mechanobiological framework for clinical cases

We present a novel patient-specific fluid-solid-growth framework to model the mechanobiological state of clinically detected intracranial aneurysms (IAs) and their evolution. The artery and IA sac are modeled as thick-walled, non-linear elastic fiber-reinforced composites. We represent the undulation distribution of collagen fibers: the adventitia of the healthy artery is modeled as a protective sheath whereas the aneurysm sac is modeled to bear load within physiological range of pressures. Initially, we assume the detected IA is stable and then consider two flow-related mechanisms to drive enlargement: (1) low wall shear stress; (2) dysfunctional endothelium which is associated with regions of high oscillatory flow. Localized collagen degradation and remodelling gives rise to formation of secondary blebs on the aneurysm dome. Restabilization of blebs is achieved by remodelling of the homeostatic collagen fiber stretch distribution. This integrative mechanobiological modelling workflow provides a step towards a personalized risk-assessment and treatment of clinically detected IAs.

Clinical Importance of the Posterior Inferior Cerebellar Artery: A Review of the Literature

The posterior inferior cerebellar artery (PICA), with its unique anatomical complexity, is of great clinical importance and involved in many diseases including aneurysm, ischemic stroke, neurovascular compression syndrome (NVCS), arteriovenous malformation (AVM), and brain tumor. However, a comprehensive systematic review of the importance of the PICA is currently lacking. In this study, we perform a literature review of PICA by searching all the associated papers in the PUBMED database hoping to provide a better understanding of the artery. The PICA has tortuous and variable course and territory, divided into 5 segments. Various aneurysms involving PICA were not uncommon, of which the treatment is challenging. The PICA infarct typically manifests lateral medullary syndrome (LMS) and is more likely to cause mass effects. The PICA frequently compresses the medulla and the cranial nerves resulting in various neurovascular compression syndromes (NVCS). Arteriovenous malformation (AVM) fed by PICA are associated with aneurysm and dissection which have high risk of rupture and worse outcome. PICA injured by head trauma can cause fatal SAH. VA terminating in PICA probably cause Bow hunter's syndrome (BHS). The PICA supplies many brain tumors and can be used in intracerebellar chemotherapy. The PICA can be exposed and injured during surgeries especially in telovelar approach, and it also plays an important role in bypass surgeries, hinting the surgical importance of PICA. In conclusion, PICA is very important in clinical practice.

A Simple Flow Classification Parameter Can Discriminate Rupture Status in Intracranial Aneurysms

BACKGROUND

A simple dimensionless aneurysm number ($An$), which depends on geometry and flow pulsatility, was previously shown to distinguish the flow mode in intracranial aneurysms (IA): vortex mode with a dynamic vortex formation/evolution if $An > 1$, and cavity mode with a steady shear layer if $An < 1$.

OBJECTIVE

To hypothesize that $An\ > \ 1$ can distinguish rupture status because vortex mode is associated with high oscillatory shear index, which, in turn, is statistically associated with rupture.

METHODS

The above hypothesis is tested on a retrospective, consecutively collected database of 204 patient-specific IAs. The first 119 cases are assigned to training and the remainder to testing dataset. $An$ is calculated based on the pulsatility index (PI) approximated either from the literature or solving an optimization problem (denoted as$\ \widehat {PI}$). Student's t-test and logistic regression (LR) are used for hypothesis testing and data fitting, respectively.

RESULTS

$An$ can significantly discriminate ruptured and unruptured status with 95% confidence level (P < .0001). $An$ (using PI) and $\widehat {An}$ (using $\widehat {PI}$) significantly predict the ruptured IAs (for training dataset $An\!:\ $AUC = 0.85, $\widehat {An}\!:\ $AUC = 0.90, and for testing dataset $An\!:\ $sensitivity = 94%, specificity = 33%, $\widehat {An}\!:\ $sensitivity = 93.1%, specificity = 52.85%).

CONCLUSION

$An > 1$ predicts ruptured status. Unlike traditional hemodynamic parameters such as wall shear stress and oscillatory shear index, $An$ has a physical threshold of one (does not depend on statistical analysis) and does not require time-consuming flow simulations. Therefore, $An$ is a simple, practical discriminator of IA rupture status.

How patient-specific do internal carotid artery inflow rates need to be for computational fluid dynamics of cerebral aneurysms?

BACKGROUND

Computational fluid dynamics (CFD) has become a popular tool for studying 'patient-specific' blood flow dynamics in cerebral aneurysms; however, rarely are the inflow boundary conditions patient-specific. We aimed to test the impact of widespread reliance on generalized inflow rates.

METHODS

Internal carotid artery (ICA) flow rates were measured via 2D cine phase-contrast MRI for 24 patients scheduled for endovascular therapy of an ICA aneurysm. CFD models were constructed from 3D rotational angiography, and pulsatile inflow rates imposed as measured by MRI or estimated using an average older-adult ICA flow waveform shape scaled by a cycle-average flow rate (Q_avg) derived from the patient's ICA cross-sectional area via an assumed inlet velocity.

RESULTS

There was good overall qualitative agreement in the magnitudes and spatial distributions of time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), and spectral power index (SPI) using generalized versus patient-specific inflows. Sac-averaged quantities showed moderate to good correlations: R²=0.54 (TAWSS), 0.80 (OSI), and 0.68 (SPI). Using patient-specific Q_avg to scale the generalized waveform shape resulted in near-perfect agreement for TAWSS, and reduced bias, but not scatter, for SPI. Patient-specific waveform had an impact only on OSI correlations, which improved to R²=0.93.

CONCLUSIONS

Aneurysm CFD demonstrates the ability to stratify cases by nominal hemodynamic 'risk' factors when employing an age- and vascular-territory-specific recipe for generalized inflow rates. Q_avg has a greater influence than waveform shape, suggesting some improvement could be achieved by including measurement of patient-specific Q_avg into aneurysm imaging protocols.

Discrimination of intracranial aneurysm rupture status: patient-specific inflow boundary may not be a must-have condition in hemodynamic simulations

BACKGROUND

Computational fluid dynamics (CFD) are important in evaluating the hemodynamics of intracranial aneurysm rupture, and the setting of inflow boundary conditions is critical. We evaluated intracranial aneurysm hemodynamics based on generalized versus patient-specific inflow boundary conditions to examine the effect of different hemodynamic results on the discrimination of intracranial aneurysm rupture status.

METHODS

We enrolled 148 patients with 156 intracranial aneurysms. For each included aneurysm, we performed CFD simulation once based on patient-specific and once based on generalized inflow boundary conditions. First, we compared the hemodynamics of intracranial aneurysms based on different inflow boundary conditions. Then, we divided the included aneurysms into a ruptured and unruptured group and compared the hemodynamics between the two groups under patient-specific and generalized inflow boundary conditions.

RESULTS

For the hemodynamic parameters using specific inflow boundary conditions, more complex flow (p = 0.002), larger minimum WSS (p = 0.024), lower maximum low WSS area (LSA) (p = 0.038), and oscillatory shear index (p = 0.002) were found. Furthermore, we compared the hemodynamics between ruptured and unruptured groups based on different inflow boundary conditions. We found that the significant hemodynamic parameters associated with rupture status were the same, including the proportion of aneurysms with flow complex and unstable flow and the minimum and maximum of LSA (p = 0.011, p = 0.003, p = 0.001 and p = 0.004, respectively).

CONCLUSION

Patient-specific and generalized inflow boundary conditions of aneurysmal hemodynamics resulted in significant differences. However, the significant parameters associated with rupture status were the same in both conditions, indicating that patient-specific inflow boundary conditions may not be necessary for predicting rupture risk.

A novel virtual flow diverter implantation method with realistic deployment mechanics and validated force response

Virtual flow diverter deployment techniques underwent significant development during the last couple of years. Each existing technique displays advantageous features, as well as significant limitations. One common drawback is the lack of quantitative validation of the mechanics of the device. In the following work, we present a new spring-mass-based method with validated mechanical responses that combines many of the useful capabilities of previous techniques. The structure of the virtual braids naturally incorporates the axial length changes as a function of the local expansion diameter. The force response of the model was calibrated using the measured response of real FDs. The mechanics of the model allows to replicate the expansion process during deployment, including additional effects such as the push-pull technique that is required for the deployment of braided FDs to achieve full opening and proper wall apposition. Furthermore, it is a computationally highly efficient solution that requires little pre-processing and has a run-time of a few seconds on a general laptop and thus allows for exploratory analyses. The model was applied in a patient-specific geometry, where corresponding accurate control measurements in a 3D-printed model were also available. The analysis shows the effects of FD oversizing and push-pull application on the radial expansion, surface density, and on the wall contact pressure.

Surgical Management of Posterior Communicating Artery Aneurysms in the Presence of a Low-Coursing Internal Carotid Artery and Narrowed Retrocarotid Window

BACKGROUND

Anatomical variations of the course of the internal carotid artery (ICA) may complicate surgical clipping of posterior communicating artery (PCoA) aneurysms by narrowing the retrocarotid window. We evaluated the efficacy of the periclinoid surgical maneuvers for expanding the retrocarotid window and analyzed computed tomography angiography (CTA) data from patients with PCoA aneurysms to define parameters for low-coursing ICAs.

METHODS

Using cadaveric specimens, standard pterional craniotomies were fashioned and extradural or intradural periclinoid surgical maneuvers-cutting of the meningo-orbital band, anterior clinoidectomy, and cutting of the distal dural ring (DDR)-were performed, and their relative advantages for expanding the retrocarotid window were assessed. Additionally, preoperative CTA data from 24 patients with PCoA aneurysms used to calculate the angles of the ICA relative to the skull base.

RESULTS

Periclinoid maneuvers, especially the anterior clinoidectomy, provided additional exposure of the retrocarotid space. Cutting of the DDR allowed for partial mobilization of the ICA and widened the retrocarotid surgical window, enhancing maneuverability. The anterior clinoidectomy with cutting of the DDR allowed for enhanced exposure of the medial, middle, and posterolateral aspects of the retrocarotid space. Cutting the anterior petroclinoid fold and mobilizing cranial nerve III provided wide exposure of the lateral aspect of retrocarotid space.

CONCLUSION

When clipping PCoA aneurysms in the presence of normal-coursing ICAs (approximately ≥30° ICA angle), a standard pterional craniotomy with anterior clinoidectomy and cutting of the DDR allows for substantial expansion of the retrocarotid window.

Custom tissue engineered aneurysm models with varying neck size and height for early stage in vitro testing of flow diverters

Endovascular techniques for treating cerebral aneurysms are rapidly advancing and require testing to optimize device configurations. The purpose of this work was to customize tissue-engineered aneurysm "blood vessel mimics" (aBVMs) for early stage in vitro assessment of vascular cell responses to flow diverters and other devices. Aneurysm scaffolds with varying neck size and height were created through solid modeling, mold fabrication, mandrel creation, and electrospinning. Scaffold dimensions and fiber morphology were characterized. aBVMs were created by depositing human smooth muscle and endothelial cells within scaffolds, and cultivating within perfusion bioreactors. These vessels were left untreated or used for flow diverter implantation. Cellular responses to flow diverters were evaluated at 3 days. Custom scaffolds were created with aneurysm neck diameters of 2.3, 3.5, and 5.5 mm and with aneurysm heights of 2, 5, and 8 mm. A set of scaffolds with varying neck size was used for aBVM creation, and dual-sodding of endothelial and smooth muscle cells resulted in consistent and confluent cellular linings. Flow diverters were successfully implanted in a subset of aBVMs, and initial cell coverage over devices was seen in the parent vessel at 3 days. Direct visualization of the device over the neck region was feasible, supporting the future use of these models for evaluating and comparing flow diverter healing. Tissue-engineered aneurysm models can be created with custom neck sizes and heights, and used to evaluate cellular responses to flow diverters and other endovascular devices.

Lattice-Boltzmann interactive blood flow simulation pipeline

PURPOSE

Cerebral aneurysms are one of the prevalent cerebrovascular disorders in adults worldwide and caused by a weakness in the brain artery. The most impressive treatment for a brain aneurysm is interventional radiology treatment, which is extremely dependent on the skill level of the radiologist. Hence, accurate detection and effective therapy for cerebral aneurysms still remain important clinical challenges. In this work, we have introduced a pipeline for cerebral blood flow simulation and real-time visualization incorporating all aspects from medical image acquisition to real-time visualization and steering.

METHODS

We have developed and employed an improved version of HemeLB as the main computational core of the pipeline. HemeLB is a massive parallel lattice-Boltzmann fluid solver optimized for sparse and complex geometries. The visualization component of this pipeline is based on the ray marching method implemented on CUDA capable GPU cores.

RESULTS

The proposed visualization engine is evaluated comprehensively and the reported results demonstrate that it achieves significantly higher scalability and sites updates per second, indicating higher update rate of geometry sites' values, in comparison with the original HemeLB. This proposed engine is more than two times faster and capable of 3D visualization of the results by processing more than 30 frames per second.

CONCLUSION

A reliable modeling and visualizing environment for measuring and displaying blood flow patterns in vivo, which can provide insight into the hemodynamic characteristics of cerebral aneurysms, is presented in this work. This pipeline increases the speed of visualization and maximizes the performance of the processing units to do the tasks by breaking them into smaller tasks and working with GPU to render the images. Hence, the proposed pipeline can be applied as part of clinical routines to provide the clinicians with the real-time cerebral blood flow-related information.

Large Neck and Strong Ostium Inflow as the Potential Causes for Delayed Occlusion of Unruptured Sidewall Intracranial Aneurysms Treated by Flow Diverter

BACKGROUND AND PURPOSE

Flow diverter-induced hemodynamic change plays an important role in the mechanism of intracranial aneurysm occlusion. Our aim was to explore the relationship between aneurysm features and flow-diverter treatment of unruptured sidewall intracranial aneurysms.

MATERIALS AND METHODS

MR imaging, 4D phase-contrast, was prospectively performed before flow diverter implantation in each patient with unruptured intracranial aneurysm. Two postprocedure follow-ups were scheduled at 6 and 12 months. Responses were grouped according to whether the aneurysms were occluded or remnant. Preprocedural aneurysm geometries and ostium hemodynamics in 38 patients were compared between the 2 groups at 6 and 12 months. Receiver operating characteristic curve analyses were performed for significant geometric and hemodynamic continuous parameters.

RESULTS

After the 6-month assessment, 21 of 41 intracranial aneurysms were occluded, and 9 additional aneurysms were occluded at 12 months. Geometrically, the ostium maximum diameter was significantly larger in the remnant group at 6 and 12 months (both P < .001). Hemodynamically, the proximal inflow zone was more frequently observed in the remnant group at 6 months. Several preprocedural ostium hemodynamic parameters were significantly higher in the remnant group. As a prediction for occlusion, the areas under the curve of the ostium maximum diameter (for 6 and 12 months), systolic inflow rate ratio (for 6 months), and systolic inflow area (for 12 months) reached 0.843, 0.883, 0.855, and 0.860, respectively.

CONCLUSIONS

Intracranial aneurysms with a large ostium and strong ostium inflow may need a longer time for occlusion. Preprocedural 4D flow MR imaging can well illustrate ostium hemodynamics and characterize aneurysm treatment responses.

A Eulerian method to analyze wall shear stress fixed points and manifolds in cardiovascular flows

Based upon dynamical systems theory, a fixed point of a vector field such as the wall shear stress (WSS) at the luminal surface of a vessel is a point where the vector field vanishes. Unstable/stable manifolds identify contraction/expansion regions linking fixed points. The significance of such WSS topological features lies in their strong link with "disturbed" flow features like flow stagnation, separation and reversal, deemed responsible for vascular dysfunction initiation and progression. Here, we present a Eulerian method to analyze WSS topological skeleton through the identification and classification of WSS fixed points and manifolds in complex vascular geometries. The method rests on the volume contraction theory and analyzes the WSS topological skeleton through the WSS vector field divergence and Poincar[Formula: see text] index. The method is here applied to computational hemodynamics models of carotid bifurcation and intracranial aneurysm. An in-depth analysis of the time dependence of the WSS topological skeleton along the cardiac cycle is provided, enriching the information obtained from cycle-average WSS. Among the main findings, it emerges that on the carotid bifurcation, instantaneous WSS fixed points co-localize with cycle-average WSS fixed points for a fraction of the cardiac cycle ranging from 0 to [Formula: see text]; a persistent instantaneous WSS fixed point confined on the aneurysm dome does not co-localize with the cycle-average low-WSS region. In conclusion, the here presented approach shows the potential to speed up studies on the physiological significance of WSS topological skeleton in cardiovascular flows, ultimately increasing the chance of finding mechanistic explanations to clinical observations.

Lessons from Vessel Wall Imaging of Intracranial Aneurysms: New Era of Aneurysm Evaluation beyond Morphology

Recent basic studies have clarified that aneurysmal wall inflammation plays an important role in the pathophysiology of intracranial aneurysms. However, it remains an interdisciplinary challenge to visualize aneurysm wall status in vivo. MR-vessel wall imaging (VWI) is a current topic of advanced imaging techniques since it could provide an additional value for unruptured intracranial aneurysms (UIAs) risk stratification. With regard to ruptured intracranial aneurysms, VWI could identify a ruptured aneurysm in patients with multiple intracranial aneurysms. Intraluminal thrombus could be a clue to interpret aneurysm wall enhancement on VWI in ruptured intracranial aneurysms. The interpretation of VWI findings in UIAs would require much caution. Actually aneurysm wall enhancement in VWI was significantly associated with consensus morphologic risk factors. However, aneurysmal wall with contrast enhancement oftentimes associated with atherosclerotic, degenerated and thickened wall structure. It remains ill defined if thin wall without wall enhancement (oftentimes invisible in VWI) could be actually safe or look over wall vulnerability. We reviewed currently available studies, especially focusing on VWI for intracranial aneurysms and discussed the clinical utility of VWI.

Numerical studies of hemodynamic alterations in pre- and post-stenting cerebral aneurysms using a multiscale modeling

The aim of this work was to use a multiscale modeling to study the influence of stent deployment, with generic stents, on flow distributions within the vascular network and the hemodynamic alterations within the cerebral aneurysms pre- and post-stenting. To achieve this goal, two image-based anatomical cerebral aneurysm models were reconstructed along with the respective aneurysms post-stenting models after deploying a 16- or 24-wire stent. The investigation results revealed that the stent may increase the local pressure resistance resulting in flow alterations. The hemodynamic parameters demonstrated stent placement can reduce the intra-aneurysmal pressure, decrease wall shear stress (WSS) at the neck region, and increase blood turnover time for aneurysm case I (sidewall aneurysm). These findings are consistent with the trends of hemodynamic changes reported previously. However, aneurysm case II (bifurcation aneurysm) showed gradually increased intra-aneurysmal pressure and the pressure at the neck region, decreased WSS over the sac surface, and enhanced flow vortices within the aneurysm. When simulating the hemodynamics of pre- and post-stenting aneurysms for a patient using measured flow waveforms, the flow alteration induced by the stent deployment may affect the hemodynamic predictions for the post-stenting aneurysm. Thus, the remeasurement of boundary conditions once the morphology of the aneurysm is deformed is needed in follow-up studies with a focus on aneurysm growth and stent deployment.

Multi-modality cerebral aneurysm haemodynamic analysis: in vivo 4D flow MRI, in vitro volumetric particle velocimetry and in silico computational fluid dynamics

Typical approaches to patient-specific haemodynamic studies of cerebral aneurysms use image-based computational fluid dynamics (CFD) and seek to statistically correlate parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) to risk of growth and rupture. However, such studies have reported contradictory results, emphasizing the need for in-depth multi-modality haemodynamic metric evaluation. In this work, we used in vivo 4D flow MRI data to inform in vitro particle velocimetry and CFD modalities in two patient-specific cerebral aneurysm models (basilar tip and internal carotid artery). Pulsatile volumetric particle velocimetry experiments were conducted, and the particle images were processed using Shake-the-Box, a particle tracking method. Distributions of normalized WSS and relative residence time were shown to be highly yet inconsistently affected by minor flow field and spatial resolution variations across modalities, and specific relationships among these should be explored in future work. Conversely, OSI, a non-dimensional parameter, was shown to be more robust to the varying assumptions, limitations and spatial resolutions of each subject and modality. These results suggest a need for further multi-modality analysis as well as development of non-dimensional haemodynamic parameters and correlation of such metrics to aneurysm risk of growth and rupture.

Multivariate analysis of hemodynamic parameters on intracranial aneurysm initiation of the internal carotid artery

Although fluctuating hemodynamic wall stressors are known to impact intracranial aneurysms (IA) initiation, specificity of those stressors has not been evaluated. In this study, using human IA data, we investigated: (1) specificity of stressors in regions with and without IA eventual IA formation; and (2) how combinations of multiple stressors could improve IA formation prediction. 3D computational vasculatures were constructed based on angiographic images of 18 subjects having multiple closely-spaced IAs in the internal carotid artery. Two models were created: Model A with all IAs computationally removed, Model B which kept keep one IA. Computational fluid dynamics (CFD) simulated flow within models. Based on simulated flow fields, wall shear stress and its gradient (WSS, WSSG), oscillatory shear index (OSI), gradient oscillatory number (GON), aneurysm formation index (AFI), and mean number of swirling flow vortices (MV) were analysed. Multivariate logistic regression determined the accuracy of different combinations of those above-mentioned stressors. Overall, we found that combining hemodynamic stressors improves IA formation prediction over individual indices. Both Model A and Model B's parsimonious model was MV+WSS+GON: AUROC 0.88 and 0.83, respectively. Future studies are planned to understand biological meanings induced by fluctuating stressors.

Fabrication of Low-Cost Patient-Specific Vascular Models for Particle Image Velocimetry

PURPOSE

Particle image velocimetry (PIV), an in vitro experimentation technique that optically measures velocity components to analyze fluid velocity fields, has become increasingly popular to study flow dynamics in various vascular territories. However, it can be difficult and expensive to create patient-specific clear models for PIV due to the importance of refractive index matching of the model and the fluid. We aim to implement and test the use of poly-vinyl alcohol (PVA) in a lost-core casting technique to create low-cost, patient-specific models for PIV.

METHODS

Anonymized patient vascular anatomies were segmented and processed in Mimics/3Matic to create patient-specific cores from 3D digital subtraction angiographies. The cores were 3D-printed with PVA and post-processed with a 80:20 water:glue mixture to smooth the surface. Two silicones, Sylgard 184 and Solaris, were used to encapsulate the model and the PVA core was dissolved using warm water. Computed tomography scans were used to evaluate geometric accuracy using circumferences and surface differences in the model.

RESULTS

Mean geometric differences in circumference along the inlet centerline and the mean surface difference in the aneurysm between the final Silicone Model and the desired STL Print geometry were statistically insignificant (0.6 mm, 95% CI [- 1.4, 2.8] and 0.3 mm 95% CI [- 0.1, 0.7], respectively). Particle illumination within each model was successful. The cost of one 10 cm × 10 cm × 5 cm model was $69.

CONCLUSION

This technique was successful to implement and test the use of PVA in a lost-core casting technique to create low-cost, patient-specific in vitro models for PIV experimentation.

Risk of rupture of the cerebral aneurysm in relation to traumatic brain injury using a patient-specific fluid-structure interaction model

BACKGROUND AND OBJECTIVE

Cerebral aneurysm, which is defined as one of the weakened area in the wall of an artery in the brain, ruptures when wall tension exceeds its mechanical strength. Traumatic brain injury (TBI) by exerting a sudden impact load to the brain can lead to mechanical failure of the cerebral blood vessels followed by an alteration in not only the structure but also the function of the cerebrovascular. TBI also alters the hemodynamics of the blood flow in the cerebrovascular, while it has been shown that hemodynamics has a key asset in the progression and rupture of the cerebral aneurysms. So far, there is a lack of knowledge on the risk of rupture of the cerebral aneurysm in relation to TBI. Therefore, this study aimed to calculate the mechanical stresses and deformations in the arterial wall as well as the pressure and velocity of the blood using a fluid-structure interaction (FSI) model of the cerebral aneurysm located in the anterior circulation region of the circle of Willis.

METHOD

A patient-specific FSI model of the human skull, brain, and cerebral aneurysm, was established using human computed tomography (CT)/ magnetic resonance imaging (MRI) data and subjected to a frontal TBI.

RESULTS

The results revealed considerable increasing of ∼ 8 kPa (60 mmHg) and 0.40 m/s in the pressure and velocity of the blood in the intraluminal of the cerebral artery after TBI. The von Mises stress, shear stress, and deformation of the cerebral aneurysm wall also showed the increasing of 56.03 kPa, 15.66 Pa, and 0.072 mm after TBI, respectively.

CONCLUSIONS

Although the injury to the aneurysm wall after TBI is lower than that of the aneurysm wall mechanical strength, it still can alter the stress pattern in the wall and disrupt the hemodynamics of the blood. These results have implications in understanding the rupture risk of the cerebral aneurysm due to TBI, which may contribute in establishing preventive and/or treatment methods.

Hemodynamic characteristics associated with thinner regions of intracranial aneurysm wall

Aneurysm wall thickness is an important determinant of aneurysm progression and intra-procedural rupture. Several previous studies have evaluated the association between hemodynamic stress and aneurysm wall thickness, but conflicting results were obtained and no consensus has been achieved. According to the intraoperative findings, twenty-eight unruptured middle cerebral artery (MCA) aneurysms presented with thin-walled regions were enrolled in our study. Patient-specific 3D aneurysm models were constructed from preoperative computed tomography angiography (CTA) data and computational fluid dynamics (CFD) analyses were performed under pulsatile-flow conditions. Thin-walled regions of aneurysm dome were recognized by two experienced reviewers based on the intraoperative microscopy findings. Hemodynamic parameters derived from CFD analysis, including normalized wall shear stress (NWSS), normalized pressure (NP), the oscillatory shear index (OSI) and relative residence time (RRT), were compared between thin-walled regions and surrounding normal-thickness areas. Of the included aneurysms, twenty-eight pairs of thin-walled and normal surrounding regions were determined. Compared with surrounding tissues, thin-walled regions of aneurysm wall tended to present with higher pressure (1.232 vs 1.043, p < 0.05) and lower wall shear stress (0.693 vs 0.868, p < 0.05). Multivariate analysis revealed that elevated NP was significantly associated with thinning of the local aneurysm wall. Higher pressure and lower WSS were characteristic hemodynamic features associated with thinner regions of the aneurysm wall, elevated NP was an independent risk factor for local aneurysm wall thinning. CFD seems to be a useful method to estimate the location of thin-walled region, which will be helpful in reducing the risk of intraoperative rupture.

Self-related processing and future thinking: Distinct contributions of ventromedial prefrontal cortex and the medial temporal lobes

Episodic future thinking depends on a core network of regions that involves, in addition to the medial temporal lobes (MTL), the ventromedial prefrontal cortex (vmPFC). Neuroimaging studies suggest that vmPFC is particularly involved when future thinking requires consideration of self-relevant information, but lesion evidence for a special role of vmPFC in constructing self-relevant scenarios is limited. To clarify the involvement of vmPFC in future thinking, eight patients with vmPFC lesions were asked to imagine future events pertaining to the self or to another person, and their performance was contrasted with that of eight patients with MTL lesions. Patients with vmPFC lesions were no more detailed in their description of future events pertaining to the self than of events pertaining to another person. In contrast, like controls, patients with MTL lesions showed a self-benefit, despite impoverished performance overall. These findings accord with evidence from neuroimaging studies and elucidate the distinct contributions of vmPFC and MTL to future thinking.

Image-based modeling of blood flow in cerebral aneurysms treated with intrasaccular flow diverting devices

Modeling the flow dynamics in cerebral aneurysms after the implantation of intrasaccular devices is important for understanding the relationship between flow conditions created immediately posttreatment and the subsequent outcomes. This information, ideally available a priori based on computational modeling prior to implantation, is valuable to identify which aneurysms will occlude immediately and which aneurysms will likely remain patent and would benefit from a different procedure or device. In this report, a methodology for modeling the hemodynamics in intracranial aneurysms treated with intrasaccular flow diverting devices is described. This approach combines an image-guided, virtual device deployment within patient-specific vascular models with an immersed boundary method on adaptive unstructured grids. A partial mesh refinement strategy that reduces the number of mesh elements near the aneurysm dome where the flow conditions are largely stagnant was compared with the full refinement strategy that refines the mesh everywhere around the device wires. The results indicate that using the partial mesh refinement approach is adequate for analyzing the posttreatment hemodynamics, at a reduced computational cost. The results obtained on a series of four cerebral aneurysms treated with different intrasaccular devices were in good qualitative agreement with angiographic observations. Promising results were obtained relating posttreatment flow conditions and outcomes of treatments with intrasaccular devices, which need to be confirmed on larger series.

Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH)-Phase Ib: Effect of morphology on hemodynamics

Background

Image-based blood flow simulations have been increasingly applied to investigate intracranial aneurysm (IA) hemodynamics. However, the acceptance among physicians remains limited due to the high variability in the underlying assumptions and quality of results.

Methods

To evaluate the vessel segmentation as one of the most important sources of error, the international Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH) was announced. 26 research groups from 13 different countries segmented three datasets, which contained five IAs in total. Based on these segmentations, 73 time-dependent blood flow simulations under consistent conditions were carried out. Afterwards, relevant flow and shear parameters (e.g., neck inflow rate, parent vessel flow rate, spatial mean velocity, and wall shear stress) were analyzed both qualitatively and quantitatively.

Results

Regarding the entire vasculature, the variability of the segmented vessel radius is 0.13 mm, consistent and independent of the local vessel radius. However, the centerline velocity shows increased variability in more distal vessels. Focusing on the aneurysms, clear differences in morphological and hemodynamic parameters were observed. The quantification of the segmentation-induced variability showed approximately a 14% difference among the groups for the parent vessel flow rate. Regarding the mean aneurysmal velocity and the neck inflow rate, a variation of 30% and 46% was observed, respectively. Finally, time-averaged wall shear stresses varied between 28% and 51%, depending on the aneurysm in question.

Conclusions

MATCH reveals the effect of state-of-the-art segmentation algorithms on subsequent hemodynamic simulations for IA research. The observed variations may lead to an inappropriate interpretation of the simulation results and thus, can lead to inappropriate conclusions by physicians. Therefore, accurate segmentation of the region of interest is necessary to obtain reliable and clinically helpful flow information.

Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH)-phase II: rupture risk assessment

PURPOSE

Assessing the rupture probability of intracranial aneurysms (IAs) remains challenging. Therefore, hemodynamic simulations are increasingly applied toward supporting physicians during treatment planning. However, due to several assumptions, the clinical acceptance of these methods remains limited.

METHODS

To provide an overview of state-of-the-art blood flow simulation capabilities, the Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH) was conducted. Seventeen research groups from all over the world performed segmentations and hemodynamic simulations to identify the ruptured aneurysm in a patient harboring five IAs. Although simulation setups revealed good similarity, clear differences exist with respect to the analysis of aneurysm shape and blood flow results. Most groups (12/71%) included morphological and hemodynamic parameters in their analysis, with aspect ratio and wall shear stress as the most popular candidates, respectively.

RESULTS

The majority of groups (7/41%) selected the largest aneurysm as being the ruptured one. Four (24%) of the participating groups were able to correctly select the ruptured aneurysm, while three groups (18%) ranked the ruptured aneurysm as the second most probable. Successful selections were based on the integration of clinically relevant information such as the aneurysm site, as well as advanced rupture probability models considering multiple parameters. Additionally, flow characteristics such as the quantification of inflow jets and the identification of multiple vortices led to correct predictions.

CONCLUSIONS

MATCH compares state-of-the-art image-based blood flow simulation approaches to assess the rupture risk of IAs. Furthermore, this challenge highlights the importance of multivariate analyses by combining clinically relevant metadata with advanced morphological and hemodynamic quantification.

Association Between Flow Acceleration in the Carotid Artery and Intracranial Aneurysms

OBJECTIVES

In physiologic pulsatile flow, velocity acceleration is an independent factor determining wall shear stress experienced by the vascular endothelium. The purpose of this study was to evaluate Doppler indices of systolic velocity acceleration in extracranial cerebral vessels and the occurrence of intracranial aneurysms.

METHODS

We reviewed medical records and 3.0-T brain magnetic resonance imaging with 3-dimensional time-of-flight magnetic resonance angiography of 1323 adults who underwent health checkups from June 2006 to November 2011, in whom 53 intracranial aneurysms were identified in 45 patients. Doppler ultrasound parameters of the carotid and vertebral arteries were analyzed in these 45 patients with aneurysms and compared with another 45 control participants matched for age and sex. We defined the maximum systolic acceleration (ACC_max ) as the maximum slope of the early phase of systolic acceleration on the Doppler waveform and the maximum acceleration index (AI_max ) as the ratio of the ACC_max and peak systolic velocity.

RESULTS

The Doppler analysis showed a significantly increased AI_max and ACC_max in the common carotid artery (CCA), internal carotid artery, and vertebral artery in the aneurysm group. A cutoff 13.89 s^-1 for the AI_max of the CCA had sensitivity of 80% with a negative predictive value of 99% for intracranial aneurysms.

CONCLUSIONS

This study suggests that the AI_max of the CCA with a cutoff of 13.89 s^-1 may be an alternative to 3-dimensional time-of-flight magnetic resonance angiography or computed tomographic angiography as a screening tool for intracranial aneurysms. Further prospective studies are needed to validate the diagnostic performance and cost-effectiveness of these indices for screening.

Patient and aneurysm characteristics in familial intracranial aneurysms. A systematic review and meta-analysis

BACKGROUND AND PURPOSE

Patient and aneurysm characteristics have been reported to differ between patients with familial and non-familial intracranial aneurysms (IAs), although results are inconsistent. We systematically reviewed and meta-analyzed the literature to identify and quantify patient- and aneurysm characteristics associated with familial IAs.

METHODS

We searched PubMed and EMBASE for case-control and cohort studies comparing patient- and aneurysm characteristics between familial and non-familial IAs. Two observers independently assessed study eligibility and appraised quality with the Newcastle Ottawa Scale. With univariable weighted linear regression analysis we calculated β-coefficients with corresponding 95% confidence intervals (CIs) for ruptured and unruptured IAs combined and for ruptured IAs only. Heterogeneity was assessed with Higgins I2.

RESULTS

A total of 15 articles were included in the meta-analysis in which 16,346 patients were analyzed with a total of 14,225 IAs. For ruptured and unruptured IAs combined, multiple IAs were more prevalent in familial (28.5%) than in non-familial IAs (20.4%; β = 0.10, 95% CI, 0.04 to 0.16; I2 0%). For ruptured IAs only, in familial patients IAs were more prevalent on the middle cerebral artery (41.1% versus 29.5%; β = 0.12, 95% CI, 0.01 to 0.24; I2 12%) and ruptured at a younger age (46.5 years versus 50.8 years; β = -5.00, 95% CI, -9.31 to -0.69; I2 98%) than in non-familial patients. No significant differences were found for the proportion of women, size of the aneurysm at time of rupture, smoking or hypertension.

CONCLUSION

These results suggest that characteristics of familial and non-familial IAs show considerable overlap, yet differ on specific aspects. However, results for age at rupture showed considerable heterogeneity. These findings should be taken into consideration for future etiological research into IAs.

Variational data assimilation for transient blood flow simulations: Cerebral aneurysms as an illustrative example

Several cardiovascular diseases are caused from localised abnormal blood flow such as in the case of stenosis or aneurysms. Prevailing theories propose that the development is caused by abnormal wall shear stress in focused areas. Computational fluid mechanics have arisen as a promising tool for a more precise and quantitative analysis, in particular because the anatomy is often readily available even by standard imaging techniques such as magnetic resonance and computed tomography angiography. However, computational fluid mechanics rely on accurate initial and boundary conditions, which are difficult to obtain. In this paper, we address the problem of recovering high-resolution information from noisy and low-resolution physical measurements of blood flow (for example, from phase-contrast magnetic resonance imaging [PC-MRI]) using variational data assimilation based on a transient Navier-Stokes model. Numerical experiments are performed in both 3D (2D space and time) and 4D (3D space and time) and with pulsatile flow relevant for physiological flow in cerebral aneurysms. The results demonstrate that, with suitable regularisation, the model accurately reconstructs flow, even in the presence of significant noise.

The Optimality Principle Decreases Hemodynamic Stresses for Aneurysm Initiation at Anterior Cerebral Artery Bifurcations

OBJECTIVE

To investigate hemodynamic stresses on anterior cerebral artery bifurcation apex and possible mechanism of the optimality principle in protecting bifurcation wall from supercharged hemodynamic stresses.

METHODS

Three-dimensional angiographic datasets of 122 patients with anterior communicating artery (Acom) aneurysms, 21 patients with non-Acom aneurysms, and 220 control subjects with no aneurysms were used. Radii of parent (r0) and daughter branches (r1 and r2) were measured, and bifurcations obeying the optimality principle required optimal caliber control of r0n = r1n + r2n, with the junction exponent n approximating 3. Radius ratio = r03/(r13 + r23) and n were compared between aneurysmal and control bifurcations. Blood flow was simulated for analysis of hemodynamic stresses.

RESULTS

Acom bifurcations in subjects without Acom aneurysms displayed optimal caliber radius, with mean radius ratio of 0.99 and n of 3.25, whereas Acom aneurysmal bifurcations had significantly lower radius ratio, 0.62 (P < 0.05), but higher n, 4.23 (P < 0.05). Peak wall shear stress and corresponding total pressure were significantly smaller for bifurcations obeying than disobeying the optimality principle (P < 0.001 and P < 0.05, respectively). Total pressures in the direct impinging center, peak wall shear stress distance, and anterior cerebral artery bifurcation angle all were significantly smaller for bifurcations obeying than disobeying the optimality principle (P < 0.05 and P < 0.001, respectively).

CONCLUSIONS

Normal anterior cerebral artery bifurcations obey the optimality principle whereas bifurcations with Acom aneurysms do not. Disobeying the optimality principle presents significantly enhanced hemodynamic stresses to possibly damage the bifurcation wall for aneurysm initiation.

Enlarged Anterior Cerebral Artery Bifurcation Angles May Induce Abnormally Enhanced Hemodynamic Stresses to Initiate Aneurysms

OBJECTIVE

To investigate the relationship of anterior cerebral artery (ACA) bifurcation angles with hemodynamic stresses for aneurysm initiation.

METHODS

Forty patients with or without anterior communicating artery aneurysms were enrolled, and 3 patients with ACA bifurcation angles of 169.0°, 136.9°, and 73.2°, respectively, were entered into computational fluid dynamics analysis for hemodynamic stresses.

RESULTS

Larger bifurcation angles had a larger direct flow impinging zone and larger peak pressure area. In the direct flow impinging center, the total pressure was the highest, whereas the other stresses were the lowest. As blood flowed distally, the total pressure decreased rapidly, whereas all other parameters increased quickly to their peaks. The hemodynamic peak distance was decreased as the bifurcation angle became narrower. The total pressure summit and the peak hemodynamic stresses all decreased with the decrease of bifurcation angles. The distance between the hemodynamic peaks was the smallest at 73.2° compared with larger angles. A significant (P < 0.01) positive linear correlation existed in the ACA bifurcation angle with the distance between hemodynamic stress peaks or in the ACA branch diameter with the distance from the direct impinging center to the ipsilateral hemodynamic stress peak. The hemodynamic stresses on the aneurysm dome were significantly (P < 0.001) smaller than at the aneurysm initiation site.

CONCLUSIONS

Larger bifurcation angles may lead to abnormally enhanced hemodynamic stresses, enlarged zones of direct flow impingement, and increased distance between hemodynamic stress peaks to damage the vascular wall for aneurysm initiation on the bifurcation apex wall.

Associations of hemodynamics, morphology, and patient characteristics with aneurysm rupture stratified by aneurysm location

PURPOSE

The mechanisms of cerebral aneurysm rupture are not fully understood. We analyzed the associations of hemodynamics, morphology, and patient age and gender with aneurysm rupture stratifying by location.

METHODS

Using image-based models, 20 hemodynamic and 17 morphological parameters were compared in 1931 ruptured and unruptured aneurysms with univariate logistic regression. Rupture rates were compared between males and females as well as younger and older patients and bifurcation versus sidewall aneurysms for different aneurysm locations. Subsequently, associations between hemodynamics and morphology and patient as well as aneurysm characteristics were analyzed for aneurysms at five locations.

RESULTS

Compared to unruptured aneurysms, ruptured aneurysms were characterized by a more irregular shape and were exposed to a more adverse hemodynamic environment described by faster flow, higher wall shear stress, more oscillatory shear, and more unstable and complex flows. These associations with rupture status were consistent for different aneurysm locations. Rupture rates were significantly higher in males at the internal carotid artery (ICA) bifurcation, ophthalmic ICA, and the middle cerebral artery (MCA) bifurcation. At the anterior communicating artery (ACOM) and MCA bifurcation, they were significantly higher for younger patients. Bifurcation aneurysms had significantly larger rupture rates at the MCA and posterior communicating artery (PCOM). In these groups with higher rupture rates, aneurysms were characterized by adverse hemodynamics and more complex shapes.

CONCLUSION

Hemodynamic and morphological differences between ruptured and unruptured aneurysms are consistent across locations. Adverse morphology and hemodynamics are related to rupture as well as younger age, male gender, and bifurcation aneurysms.

Combining data from multiple sources to study mechanisms of aneurysm disease: Tools and techniques

INTRODUCTION

Connecting local hemodynamics, biomechanics, and tissue properties in cerebral aneurysms is important for understanding the processes of wall degeneration and subsequent aneurysm progression and rupture. This challenging problem requires integration of data from multiple sources.

METHODS

This paper describes the tools and techniques developed to integrate data from multiple sources, including clinical information, 3D imaging, intraoperative videos, ex vivo micro-computed tomography (CT), and multiphoton microscopy. Central to this approach is a 3D tissue model constructed from micro-CT images of aneurysm samples resected during neurosurgery. This model is aligned to vascular models constructed from 3D clinical images and is used to map and compare flow, biomechanics, and tissue data.

RESULTS

The approach is illustrated with data of three human intracranial aneurysms. These case studies demonstrated the ability of this approach to study relationships between different factors affecting the aneurysm wall and produced provocative observations that will be further studied with larger series. For instance, "atherosclerotic" and "hyperplastic" looking parts of the aneurysm corresponded to thicker walls and occurred in regions of recirculating flow and low wall shear stress (WSS); thin regions were associated with inflow jets, flow impingement, and high WSS; blebs had walls of varying structures, including calcified, thin, or hyperplastic walls.

CONCLUSIONS

The current approach enables the study of interactions of multiple factors thought to be responsible for the progressive degradation and weakening of the aneurysm wall during its evolution.

Low Wall Shear Stress Is Associated with Local Aneurysm Wall Enhancement on High-Resolution MR Vessel Wall Imaging

BACKGROUND AND PURPOSE

Some retrospective studies have found that the aneurysm wall enhancement on high-resolution MR vessel wall postgadolinium T1WI has the potential to distinguish unstable aneurysms. This study aimed to identify hemodynamic characteristics that differ between the enhanced and nonenhanced areas of the aneurysm wall on high-resolution MR vessel wall postgadolinium T1WI.

MATERIALS AND METHODS

TOF-MRA and high-resolution MR vessel wall T1WI of 25 patients were fused to localize the enhanced area of the aneurysm wall. Using computational fluid dynamics, we studied the aneurysm models. Mean static pressure, mean wall shear stress, and oscillatory shear index were compared between the enhanced and nonenhanced areas.

RESULTS

The aneurysmal enhanced area had lower wall shear stress (P < .05) and a lower oscillatory shear index (P = .021) than the nonenhanced area. In addition, the whole aneurysm had lower wall shear stress (P < .05) and a higher oscillatory shear index (P = .007) than the parent artery.

CONCLUSIONS

This study suggests that there are hemodynamic differences between the enhanced and nonenhanced areas of the aneurysm wall on high-resolution MR vessel wall postgadolinium T1WI.

Boundary extension is attenuated in patients with ventromedial prefrontal cortex damage

The ventromedial prefrontal cortex (vmPFC) and hippocampus have been implicated in the mental construction of scenes and events. However, little is known about their specific contributions to these cognitive functions. Boundary extension (BE) is a robust indicator of fast, automatic, and implicit scene construction. BE occurs when individuals who are viewing scenes automatically imagine what might be beyond the view, and consequently later misremember having seen a greater expanse of the scene. Patients with hippocampal damage show attenuated BE because of their scene construction impairment. In the current study, we administered BE tasks to patients with vmPFC damage, brain-damaged control patients, and healthy control participants. We also contrasted the performance of these patients to the previously-published data from patients with hippocampal lesions (Mullally, Intraub, & Maguire, 2012). We found that vmPFC-damaged patients showed reduced BE compared to brain-damaged and healthy controls. Indeed, BE attenuation was similar following vmPFC or hippocampal damage. Notably, however, whereas hippocampal damage seems to particularly impair the spatial coherence of scenes, vmPFC damage leads to a difficulty constructing scenes in a broader sense, with the prediction of what should be in a scene, and the monitoring or integration of the scene elements being particularly compromised. We conclude that vmPFC and hippocampus play important and complementary roles in scene construction.

A multiscale computational modeling for cerebral blood flow with aneurysms and/or stenoses

A 1-dimensional (1D)-3-dimensional (3D) multiscale model for the human vascular network was proposed by combining a low-fidelity 1D modeling of blood circulation to account for the global hemodynamics with a detailed 3D simulation of a zonal vascular segment. The coupling approach involves a direct exchange of flow and pressure information at interfaces between the 1D and 3D models and thus enables patient-specific morphological models to be inserted into flow network with minimum computational efforts. The proposed method was validated with good agreements against 3 simplified test cases where experimental data and/or full 3D numerical solution were available. The application of the method in aneurysm and stenosis studies indicated that the deformation of the geometry caused by the diseases may change local pressure loss and as a consequence lead to an alteration of flow rate to the vessel segment.

The relationship between intracranial pressure and lactate/pyruvate ratio in patients with subarachnoid haemorrhage

AIM

The aim of this study was to analyse the relationship between intracranial pressure (intracranial pressure monitoring) and lactate pyruvate ratio (cerebral microdialysis) in patients with ruptured intracranial aneurysms.

METHODS

In a group of fifteen patients, intracranial pressure and lactate/pyruvate ratios were measured and logged in hourly intervals. The relationship between these two variables was subsequently analysed in two ways. 1) Intracranial hypertension (ICP > 20 mmHg) in the presence of energy deprivation (L/P ratio > 30) was noted. 2) The dynamics of L/P ratio changes in relation to immediate ICP and CPP values was analysed.

RESULTS

Out of a total of 1873 monitored hours we were able to record lactate/pyruvate ratios higher than 30 in 832 hours (44 %). Of those 832 hours during which lactate/pyruvate ratios were higher than 30, ICP was higher than 20 in 193 hours (23 %). Out of 219 hours of monitoring, in which ICP was higher than 20, a simultaneously increased L/P ratio higher than 30 was recorded in 193 hours (88 %). L/P ratio values above 30 were associated with decreased CPP values (p = 0.04), but not with increased ICP values (p = 0.79).

CONCLUSION

Intracranial hypertension coincides with energetic imbalance in approximately one quarter of cases. This points to the shortcomings of the most common form of neuromonitoring in SAH patients - ICP monitoring. This method may not be reliable enough in detecting hypoxic damage, which is the major cause of morbidity and mortality in SAH patients (Fig. 5, Ref. 11).

Modeling hemodynamics in intracranial aneurysms: Comparing accuracy of CFD solvers based on finite element and finite volume schemes

Image-based computational fluid dynamics (CFD) has shown potential to aid in the clinical management of intracranial aneurysms, but its adoption in the clinical practice has been missing, partially because of lack of accuracy assessment and sensitivity analysis. To numerically solve the flow-governing equations, CFD solvers generally rely on 2 spatial discretization schemes: finite volume (FV) and finite element (FE). Since increasingly accurate numerical solutions are obtained by different means, accuracies and computational costs of FV and FE formulations cannot be compared directly. To this end, in this study, we benchmark 2 representative CFD solvers in simulating flow in a patient-specific intracranial aneurysm model: (1) ANSYS Fluent, a commercial FV-based solver, and (2) VMTKLab multidGetto, a discontinuous Galerkin (dG) FE-based solver. The FV solver's accuracy is improved by increasing the spatial mesh resolution (134k, 1.1m, 8.6m, and 68.5m tetrahedral element meshes). The dGFE solver accuracy is increased by increasing the degree of polynomials (first, second, third, and fourth degree) on the base 134k tetrahedral element mesh. Solutions from best FV and dGFE approximations are used as baseline for error quantification. On average, velocity errors for second-best approximations are approximately 1 cm/s for a [0,125] cm/s velocity magnitude field. Results show that high-order dGFE provides better accuracy per degree of freedom but worse accuracy per Jacobian nonzero entry as compared with FV. Cross-comparison of velocity errors demonstrates asymptotic convergence of both solvers to the same numerical solution. Nevertheless, the discrepancy between underresolved velocity fields suggests that mesh independence is reached following different paths.

Influence of aging-induced flow waveform variation on hemodynamics in aneurysms present at the internal carotid artery: A computational model-based study

The variation of blood flow waveform in the internal carotid artery (ICA) with age is a well-documented hemodynamic phenomenon, but little is known about how such variation affects the characteristics of blood flow in aneurysms present in the region. In the study, hemodynamic simulations were conducted for 26 ICA aneurysms, with flow waveforms measured in the ICAs of young and older adults being used respectively to set the inflow boundary conditions. Obtained results showed that replacing the young-adult flow waveform with the older-adult one led to little changes (<10%) in simulated time-averaged wall shear stress (WSS), transient maximum WSS, relative residence time and trans-aneurysm pressure loss coefficient, but resulted in a marked increase (32.36 ± 17.24%) in oscillatory shear index (OSI). Frequency-domain wave analysis revealed that the progressive enhancement of low-frequency harmonics dominated the observed flow waveform variation with age and was a major factor contributing to the increase in OSI. Cross-sectional comparisons among the aneurysms further revealed that the degree of increase in OSI correlated positively with some specific morphological features of aneurysm, such as aspect ratio and size ratio. In summary, the study demonstrates that the variation in flow waveform with age augments the oscillation of WSS in ICA aneurysms, which underlies the importance of setting patient-specific boundary conditions in hemodynamic studies on cerebral aneurysms, especially those involving long-term patient follow-up or cross-sectional comparison among patients of different ages.

Extraocular Pressure Measurements to Avoid Orbital Compartment Syndrome in Aneurysm Surgery

BACKGROUND

Orbital compartment syndrome (OCS) is a rare but devastating complication following pterional craniotomy. Although the causes of OCS are unclear, external compression of the orbit by a myocutaneous flap is commonly mentioned as a major factor. We evaluated the ocular influence of external compression using an extraocular pressure monitor.

METHODS

We measured extraocular pressure in 86 patients who underwent surgery for cerebral aneurysm via a pterional approach. Clinical information and radiologic parameters, including the area of the medial rectus muscle (MRM) and the craniotomy height from the bottom of the anterior skull base, were collected. As a control group, 117 patients who underwent surgery without pressure monitoring were also evaluated.

RESULTS

Extraocular pressure reached a maximum during craniotomy (mean, 22.0 mm Hg; range, 18.4-51.0 mm Hg) and decreased after myocutaneous flap adjustment (mean, 7.9 mm Hg; range, 5.4-17.5 mm Hg). Pressure before myocutaneous flap manipulation differed between patients with anterior communicating artery (Acomm) aneurysms and other patients (mean, 16.5 mm Hg vs. 9.4 mm Hg; P = 0.003). Among Acomm aneurysm cases, the monitored group showed a significantly lower MRM swelling ratio (postoperative MRM area/preoperative MRM area) compared with the control group (1.03 ± 0.10 vs. 1.17 ± 0.15; P = 0.036).

CONCLUSIONS

Myocutaneous flaps can produce unnoticed overpressure on the orbit, resulting in OCS-related blindness during aneurysm clipping surgery, especially in cases involving mandatory lower craniotomy. The continuous extraocular compressive pressure monitoring technique is a simple and effective approach to prevent such a serious complication.

Prediction of outcome after aneurysmal subarachnoid haemorrhage using data from patient admission

OBJECTIVES

The pathogenesis leading to poor functional outcome after aneurysmal subarachnoid haemorrhage (aSAH) is multifactorial and not fully understood. We evaluated a machine learning approach based on easily determinable clinical and CT perfusion (CTP) features in the course of patient admission to predict the functional outcome 6 months after ictus.

METHODS

Out of 630 consecutive subarachnoid haemorrhage patients (2008-2015), 147 (mean age 54.3, 66.7% women) were retrospectively included (Inclusion: aSAH, admission within 24 h of ictus, CTP within 24 h of admission, documented modified Rankin scale (mRS) grades after 6 months. Exclusion: occlusive therapy before first CTP, previous aSAH, CTP not evaluable). A random forests model with conditional inference trees was optimised and trained on sex, age, World Federation of Neurosurgical Societies (WFNS) and modified Fisher grades, aneurysm in anterior vs. posterior circulation, early external ventricular drainage (EVD), as well as MTT and T_max maximum, mean, standard deviation (SD), range, 75th quartile and interquartile range to predict dichotomised mRS (≤ 2; > 2). Performance was assessed using the balanced accuracy over the training and validation folds using 20 repeats of 10-fold cross-validation.

RESULTS

In the final model, using 200 trees and the synthetic minority oversampling technique, median balanced accuracy was 84.4% (SD 0.7) over the training folds and 70.9% (SD 1.2) over the validation folds. The five most important features were the modified Fisher grade, age, MTT range, WFNS and early EVD.

CONCLUSIONS

A random forests model trained on easily determinable features in the course of patient admission can predict the functional outcome 6 months after aSAH with considerable accuracy.

KEY POINTS

• Features determinable in the course of admission of a patient with aneurysmal subarachnoid haemorrhage (aSAH) can predict the functional outcome 6 months after the occurrence of aSAH. • The top five predictive features were the modified Fisher grade, age, the mean transit time (MTT) range from computed tomography perfusion (CTP), the WFNS grade and the early necessity for an external ventricular drainage (EVD). • The range between the minimum and the maximum MTT may prove to be a valuable biomarker for detrimental functional outcome.

Giant Serpentine Aneurysm of the Middle Cerebral Artery

BACKGROUND

Giant serpentine aneurysms (GSAs) are a subgroup of giant intracranial aneurysms, distinct from saccular and fusiform varieties, that are defined as partially thrombosed giant aneurysms with tortuous internal vascular channel. Clinicopathologic characteristics of middle cerebral artery GSAs have been rarely reported in the literature, with discussion of radiologic characteristics only. We clarify patient clinical and neuroradiologic features and discuss the mechanism of formation and progression.

CASE DESCRIPTION

A 43-year-old woman presented with a GSA arising from the middle cerebral artery. There was a separate inflow and outflow channel of the aneurysm, with the outflow channel feeding the distal branches of the parent artery and supplying normal brain parenchyma. The GSA was treated successfully by aneurysmectomy and superficial temporal artery-middle cerebral artery bypass followed by proximal occlusion and vascular reconstruction. An aneurysm specimen was examined to correlate pathologic findings and morphologic characteristics.

RESULT

Pathologic results showed that thickness of the aneurysmal wall was typically increased and varied, and no internal elastic lamina or endothelial lining could be identified. The sac contained thrombi of various ages with recanalizing vessel formation and chronic inflammation infiltration. Intimal hyperplasia and neoangiogenesis in the wall and hyaline degeneration of the media were observed. Vessels coursing in their adventitia showed mucoid changes, which are responsible for the contrast enhancement of the aneurysmal rim on computed tomography scan.

CONCLUSIONS

GSAs are a specific pathologic entity with unique morphologic and pathologic characteristics that can affect intracranial blood vessels. The pathogenic mechanisms are unclear; this report suggests that GSAs may be associated with degeneration of the vascular wall.

Hemodynamic effect of bypass geometry on intracranial aneurysm: A numerical investigation

BACKGROUND AND OBJECTIVE

Hemodynamic analyzes are used in the clinical investigation and treatment of cardiovascular diseases. In the present study, the effect of bypass geometry on intracranial aneurysm hemodynamics was investigated numerically. Pressure, wall shear stress (WSS) and velocity distribution causing the aneurysm to grow and rupture were investigated and the best conditions were tried to be determined in case of bypassing between basilar (BA) and left/right posterior arteries (LPCA/RPCA) for different values of parameters.

METHODS

The finite volume method was used for numerical solutions and calculations were performed with the ANSYS-Fluent software. The SIMPLE algorithm was used to solve the discretized conservation equations. Second Order Upwind method was preferred for finding intermediate point values in the computational domain. As the blood flow velocity changes with time, the blood viscosity value also changes. For this reason, the Carreu model was used in determining the viscosity depending on the velocity.

RESULTS

Numerical study results showed that when bypassed, pressure and wall shear stresses reduced in the range of 40-70% in the aneurysm. Numerical results obtained are presented in graphs including the variation of pressure, wall shear stress and velocity streamlines in the aneurysm.

CONCLUSION

Considering the numerical results for all parameter values, it is seen that the most important factors affecting the pressure and WSS values in bypassing are the bypass position on the basilar artery (L_b) and the diameter of the bypass vessel (d). Pressure and wall shear stress reduced in the range of 40-70% in the aneurysm in the case of bypass for all parameters. This demonstrates that pressure and WSS values can be greatly reduced in aneurysm treatment by bypassing in cases where clipping or coil embolization methods can not be applied.

[The management of unruptured intracranial aneurysms]

The rupture of an intracranial aneurysm is a sudden, unpredictable and potentially severe event. The responsible aneurysm has to be excluded from the cerebral circulation to avoid recurrence. More and more commonly, intracranial aneurysms are detected by coincidence. How to react to these fortuitous discoveries is unclear, because the risk of rupture is difficult to estimate. We present our approach to patients facing this situation and the decision-making process.

Intra-aneurysmal flow disruption after implantation of the Medina® Embolization Device depends on aneurysm neck coverage

Background and purpose

Flow disruption achieved by braided intrasaccular implants is a novel treatment strategy for cerebrovascular aneurysms. We hypothesized that the degree of intra-aneurysmal flow disruption can be quantified in vitro and is influenced by device position across the aneurysm neck. We tested this hypothesis using the Medina^® Embolization Device (MED).

Methods

Ten different patient-specific elastic vascular models were manufactured. Models were connected to a pulsatile flow circuit, filled with a blood-mimicking fluid and treated by two operators using a single MED. Intra-aneurysmal flow velocity was measured using conventional and high-frequency digital subtraction angiography (HF-DSA) before and after each deployment. Aneurysm neck coverage by the implanted devices was assessed with flat detector computed tomography on a three-point Likert scale.

Results

A total of 80 individual MED deployments were performed by the two operators. The mean intra-aneurysmal flow velocity reduction after MED implantation was 33.6% (27.5–39.7%). No significant differences in neck coverage (p = 0.99) or flow disruption (p = 0.84) were observed between operators. The degree of flow disruption significantly correlated with neck coverage (ρ = 0.42, 95% CI: 0.21–0.59, p = 0.002) as well as with neck area (ρ = -0,35, 95% CI: -0.54 –-0.13, p = 0.024). On multiple regression analysis, both neck coverage and total neck area were independent predictors of flow disruption.

Conclusions

The degree of intra-aneurysmal flow disruption after MED implantation can be quantified in vitro and varies considerably between different aneurysms and different device configurations. Optimal device coverage across the aneurysm neck improves flow disruption and may thus contribute to aneurysm occlusion.