Wall shear stress distribution of small aneurysms prone to rupture: a case-control study

Hemodynamics in a three-dimensional printed aortic model: a comparison of four-dimensional phase-contrast magnetic resonance and image-based computational fluid dynamics

OBJECTIVE

This study aims to compare an electrocardiogram (ECG)-gated four-dimensional (4D) phase-contrast (PC) magnetic resonance imaging (MRI) technique and computational fluid dynamics (CFD) using variables controlled in a laboratory environment to minimize bias factors.

MATERIALS AND METHODS

Data from 4D PC-MRI were compared with computational fluid dynamics using steady and pulsatile flows at various inlet velocities. Anatomically realistic models for a normal aorta, a penetrating atherosclerotic ulcer, and an abdominal aortic aneurysm were constructed using a three-dimensional printer.

RESULTS

For the normal aorta model, the errors in the peak and the average velocities were within 5%. The peak velocities of the penetrating atherosclerotic ulcer and the abdominal aortic aneurysm models displayed a more extensive range of differences because of the high-speed and vortical fluid flows generated by the shape of the blood vessel. However, the average velocities revealed only relatively minor differences.

CONCLUSIONS

This study compared the characteristics of PC-MRI and CFD through a phantom study that only included controllable experimental parameters. Based on these results, 4D PC-MRI and CFD are powerful tools for analyzing blood flow patterns in vivo. However, there is room for future developments to improve velocity measurement accuracy.

Association Between Aneurysmal Hemodynamics and Rupture Risk of Unruptured Intracranial Aneurysms

Background

Assessing rupture risk in patients with unruptured intracranial aneurysms (UIAs) remains challenging. Hemodynamics plays an important role in the natural history of intracranial aneurysms. This study aimed to compare aneurysmal hemodynamic features between patients with different rupture risk as determined by PHASES score.

Methods

We retrospectively examined 238 patients who harbored a solitary saccular UIA. Patients were stratified by rupture risk into low-, intermediate-, and high-risk groups according to PHASES score. Flow simulations were performed to compare differences in hemodynamics among the groups.

Results

Aneurysmal time-averaged wall shear stress (WSSa) and normalized WSS (WSSn) decreased progressively as PHASES score increased. WSSa and WSSn significantly differed among the low-, intermediate-, and high-risk groups (p < 0.001). WSSa was significantly lower in the high-risk group than the low-risk group (p < 0.001) and the intermediate-risk group (p = 0.004). WSSn was also significantly lower in the high-risk group than the low-risk group (p < 0.001) and the intermediate-risk group (p = 0.001).

Conclusions

Low WSS was significantly associated with higher risk of intracranial aneurysm rupture as determined by PHASES score, indicating that hemodynamics may play an important role in aneurysmal rupture. In the future, a multidimensional rupture risk prediction model that includes hemodynamic parameters should be investigated.

Machine Learning-Based Prediction of Small Intracranial Aneurysm Rupture Status Using CTA-Derived Hemodynamics: A Multicenter Study

BACKGROUND AND PURPOSE

Small intracranial aneurysms are being increasingly detected while the rupture risk is not well-understood. We aimed to develop rupture-risk models of small aneurysms by combining clinical, morphologic, and hemodynamic information based on machine learning techniques and to test the models in external validation datasets.

MATERIALS AND METHODS

From January 2010 to December 2016, five hundred four consecutive patients with only small aneurysms (<5 mm) detected by CTA and invasive cerebral angiography (or surgery) were retrospectively enrolled and randomly split into training (81%) and internal validation (19%) sets to derive and validate the proposed machine learning models (support vector machine, random forest, logistic regression, and multilayer perceptron). Hemodynamic parameters were obtained using computational fluid dynamics simulation. External validation was performed in other hospitals to test the models.

RESULTS

The support vector machine performed the best with areas under the curve of 0.88 (95% CI, 0.85-0.92) and 0.91 (95% CI, 0.74-0.98) in the training and internal validation datasets, respectively. Feature ranks suggested hemodynamic parameters, including stable flow pattern, concentrated inflow streams, and a small (<50%) flow-impingement zone, and the oscillatory shear index coefficient of variation, were the best predictors of aneurysm rupture. The support vector machine showed an area under the curve of 0.82 (95% CI, 0.69-0.94) in the external validation dataset, and no significant difference was found for the areas under the curve between internal and external validation datasets (P = .21).

CONCLUSIONS

This study revealed that machine learning had a good performance in predicting the rupture status of small aneurysms in both internal and external datasets. Aneurysm hemodynamic parameters were regarded as the most important predictors.

From 2D to 4D Phase-Contrast MRI in the Neurovascular System: Will It Be a Quantum Jump or a Fancy Decoration?

Considering the crosstalk between the flow and vessel wall, hemodynamic assessment of the neurovascular system may offer a well-integrated solution for both diagnosis and management by adding prognostic significance to the standard CT/MR angiography. 4D flow MRI or time-resolved 3D velocity-encoded phase-contrast MRI has long been promising for the hemodynamic evaluation of the great vessels, but challenged in clinical studies for assessing intracranial vessels with small diameter due to long scan times and low spatiotemporal resolution. Current accelerated MRI techniques, including parallel imaging with compressed sensing and radial k-space undersampling acquisitions, have decreased scan times dramatically while preserving spatial resolution. 4D flow MRI visualized and measured 3D complex flow of neurovascular diseases such as aneurysm, arteriovenous shunts, and atherosclerotic stenosis using parameters including flow volume, velocity vector, pressure gradients, and wall shear stress. In addition to the noninvasiveness of the phase contrast technique and retrospective flow measurement through the wanted windows of the analysis plane, 4D flow MRI has shown several advantages over Doppler ultrasound or computational fluid dynamics. The evaluation of the flow status and vessel wall can be performed simultaneously in the same imaging modality. This article is an overview of the recent advances in neurovascular 4D flow MRI techniques and their potential clinical applications in neurovascular disease. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 3.

Development and validation of machine learning prediction model based on computed tomography angiography-derived hemodynamics for rupture status of intracranial aneurysms: a Chinese multicenter study

OBJECTIVES

To build models based on conventional logistic regression (LR) and machine learning (ML) algorithms combining clinical, morphological, and hemodynamic information to predict individual rupture status of unruptured intracranial aneurysms (UIAs), afterwards tested in internal and external validation datasets.

METHODS

Patients with intracranial aneurysms diagnosed by computed tomography angiography and confirmed by invasive cerebral angiograph or clipping surgery were included. The prediction models were developed based on clinical, aneurysm morphological, and hemodynamic parameters by conventional LR and ML methods.

RESULTS

The training, internal validation, and external validation cohorts were composed of 807 patients, 200 patients, and 108 patients, respectively. The area under curves (AUCs) of conventional LR models 1 (clinical), 2 (clinical and aneurysm morphological), and 3 (clinical, aneurysm morphological and hemodynamic characteristics) were 0.608, 0.765, and 0.886, respectively (all p < 0.05). The AUCs of ML models using random forest (RF), multilayer perceptron (MLP), and support vector machine (SVM) were 0.871, 0.851, and 0.863, respectively. There were no difference among AUCs of conventional LR, RF, and SVM (all p > 0.05/6), while the AUC of MLP was lower than that of conventional LR (p = 0.0055).

CONCLUSION

Hemodynamic parameters play an important role in the prediction performance of the models. ML methods cannot outperform conventional LR in prediction models for rupture status of UIAs integrating clinical, aneurysm morphological, and hemodynamic parameters.

KEY POINTS

• The addition of hemodynamic parameters can improve prediction performance for rupture status of unruptured intracranial aneurysms. • Machine learning algorithms cannot outperform conventional logistic regression in prediction models for rupture status integrating clinical, aneurysm morphological, and hemodynamic parameters. • Models integrating clinical, aneurysm morphological, and hemodynamic parameters may help choose the optimal management.

Hemodynamic and Morphological Analysis of Mirror Aneurysms Prior to Rupture

OBJECTIVE

Hemodynamic factors are thought to play important roles in the pathogenesis, progression, and rupture of cerebral aneurysms. Previous hemodynamic studies have been based on comparisons between post-ruptured and unruptured aneurysms. Nevertheless, changes of aneurysm morphology after rupture render these results unreliable. Moreover, pressure, age, gender, and the morphology of the parent artery also influence these results. Therefore, in the present study, we identified hemodynamic and morphological characteristics of aneurysms prior to rupture using twelve mirror aneurysms.

MATERIALS AND METHODS

From our database, we retrospectively analyzed twelve mirror aneurysms (MANs) prior to rupture. Each mirror aneurysm was divided into the prior to rupture or the unruptured group. Patient-specific models were reconstructed from three-dimensional (3D) images of all patients. Hemodynamic and morphological factors were analyzed and compared.

RESULTS

Compared with the unruptured side of MANs, aneurysms prior to rupture were significantly larger and significantly more irregular in shape; they also had significantly higher aspect ratio (AR), size ratio (SR), undulation index (UI), ellipticity index (EI), percentage of low wall shear stress area (LSA) and significantly lower normal wall shear stress (NWSS). The oscillatory shear index (OSI) and nonsphericity index (NSI) in the aneurysms prior to rupture were non-significantly higher than those of the unruptured group.

CONCLUSION

MANs prior to rupture may be extremely useful models to assess the risk of aneurysm rupture. Larger size, irregular shape, higher AR, SR, UI, NI, and lower WSS may be associated with aneurysms at risk for rupture.

CEREBRAL ANEURYSM WALL STRESS AFTER COILING DEPENDS ON MORPHOLOGY AND COIL PACKING DENSITY

Endovascular coil embolization is widely used to treat cerebral aneurysms as an alternative to surgical clipping. It involves filling the aneurysmal sac with metallic coils to promote the formation of a coil/thrombus mass (CTM) to protect the aneurysm wall from hemodynamic forces and prevents rupture. A significant number of aneurysms are incompletely coiled leading to aneurysm regrowth and/or recanalization. Porcine blood and platinum coils were used to construct an in-vitro CTM for uniaxial compression testing with coil packing densities (CPDs) of 10%, 20%, and 30%. Mechanical properties for each case were derived and used in finite element simulations of patient specific 3D reconstructions of aneurysms with simple or complex geometries. Reproducible stress/strain curves were obtained from compression testing of CTM and predicted by a polynomial mechanical response function. An exponential increase in the CTM stiffness was observed with increasing CPD. Elevated wall stresses were found throughout the aneurysm dome, neck, and parent artery in simulations of the aneurysms with no filling. Complete, 100% filling of the aneurysms with whole blood clot and CPDs of 10%, 20%, and 30% significantly reduced mean wall stress (MWS) in simple and complex geometry aneurysms. Sequential increases in CPD resulted in significantly greater increases in MWS in simple but not complex geometry aneurysms. These results provide a quantitative measure of the degree to which CPD impacts wall stress and suggest that complex aneurysmal geometries may be more resistant to coil embolization treatment.

Hemodynamics in Ruptured Intracranial Aneurysms with Known Rupture Points

BACKGROUND

Hemodynamics plays an important role in aneurysm rupture. Microsurgical clipping provides the best chance to confirm the rupture point. The aim of this study was to explore the associations between the rupture point and hemodynamics.

METHODS

Computational fluid dynamic simulations were performed on 16 intracranial aneurysms. The rupture point was detected at the time of clipping by 3 independent neurosurgeons. Hemodynamic parameters, including wall shear stress (WSS) and oscillatory shear index (OSI), were calculated at the rupture point and the whole aneurysm sac. Intra-aneurysmal flow patterns and flow impingement were also studied.

RESULTS

The time-averaged WSS was 3.4855 ± 3.8881 Pa at the aneurysm sac, which was significantly larger than that at the rupture point (1.5403 ± 2.3688 Pa, P = 0.002). The OSI at the rupture point (0.0354 ± 0.0459) was larger than at the sac (0.0220 ± 0.0232) without difference. Thirteen aneurysms (81.3%) showed a complex flow pattern in the aneurysm sac; however, more than two thirds of the cases (68.7%) did not show a flow impact at the rupture point. Of these cases with daughter blebs, the rupture points were confirmed at the blebs in 6 cases. Two cases did not show association between blebs and rupture point.

CONCLUSIONS

The hemodynamic characteristics at the rupture point were different from the aneurysm sac, and the WSS was significantly lower at the rupture point. Further study on the rupture risk assessment is still needed with more data and detailed information.

Rupture prediction of intracranial aneurysms: a nationwide matched case-control study of hemodynamics at the time of diagnosis

The authors used computer simulation to investigate the hemodynamics in 36 unruptured aneurysms on the first day the lesions were discovered; 12 of them later ruptured. Knowledge about any differences in hemodynamics at this early stage improves predictions about which patients will get a subarachnoid hemorrhage-a dangerous bleeding in the brain-and helps decide which patients should be treated in advance to avoid the bleeding.

4D Flow MRI in Neuroradiology: Techniques and Applications

Assessment of the intracranial flow is important for the understanding and management of cerebral vascular diseases. From brain aneurysms and arteriovenous malformations lesions to intracranial and cervical stenosis, the appraisal of the blood flow can be crucial and influence positively on patients' management. The determination of the intracranial hemodynamics and the collateral pattern seems to play to a major role in the management of these lesions. 4D flow magnetic resonance imaging is a noninvasive phase contrast derived method that has been developed and applied in neurovascular diseases. It has a great potential if followed by further technical improvements and comprehensive and systematic clinical studies.

Bifurcation Type and Larger Low Shear Area Are Associated with Rupture Status of Very Small Intracranial Aneurysms

Background

Characterization of the risk factors for rupture of very small intracranial aneurysm (VSIA, ≤3 mm) is clinically valuable, since VSIAs are implicated in subarachnoid hemorrhage. The aim of this study was to identify morphological and hemodynamic parameters that independently characterize the rupture status of VSIAs.

Methods

We conducted a retrospective study of consecutive VSIAs between September 2010 and February 2014 in our institute. A series of morphologic and hemodynamic parameters were evaluated using computational fluid dynamics, based on patient-specific three-dimensional geometrical models.

Results

We identified 186 patients with 206 VSIAs (73 ruptured, 133 unruptured). Univariable logistic regression analysis showed that bifurcation type, parent artery diameter, size ratio, time-averaged wall shear stress (WSS), maximum WSS, minimum WSS, and low shear area (LSA) were related to rupture status. Bifurcation type and larger LSA were independently associated with rupture status in multivariable logistic regression (p = 0.002 and p = 0.003, respectively).

Conclusion

Bifurcation type and larger LSA were independently associated with VSIA rupture status. Further studies are needed prospectively on patient-derived geometries prior to rupturing based on large multi-population data to confirm the present findings.

Association of Hemodynamic Factors With Intracranial Aneurysm Formation and Rupture: Systematic Review and Meta-analysis

BACKGROUND

Recent evidence suggests a link between the magnitude and distribution of hemodynamic factors and the formation and rupture of intracranial aneurysms. However, there are many conflicting results.

OBJECTIVE

To quantify the effect of hemodynamic factors on aneurysm formation and their association with ruptured aneurysms.

METHODS

We performed a systematic review and meta-analysis through October 2014. Analysis of the effects of hemodynamic factors on aneurysm formation was performed by pooling the results of studies that compared geometrical models of intracranial aneurysms and "preaneurysm" models where the aneurysm was artificially removed. Furthermore, we calculated pooled standardized mean differences between ruptured and unruptured aneurysms to quantify the association of hemodynamic factors with ruptured aneurysms. Standard PRISMA guidelines were followed.

RESULTS

The hemodynamic factors that showed high positive correlations with location of aneurysm formation were high wall shear stress (WSS) and high gradient oscillatory number, with pooled proportions of 78.8% and 85.7%, respectively. Positive correlations were largely seen in bifurcation aneurysms, whereas negative correlations were seen in sidewall aneurysms. Mean and normalized WSS were significantly lower and low shear area significantly higher in ruptured aneurysms.

CONCLUSION

Pooled analyses of computational fluid dynamics models suggest that an increase in WSS and gradient oscillatory number may contribute to aneurysm formation, whereas low WSS is associated with ruptured aneurysms. The location of the aneurysm at the bifurcation or sidewall may influence the correlation of these hemodynamic factors.

Imaging of cerebrovascular disorders: precision medicine and the collaterome

Imaging of stroke and neurovascular disorders has profoundly enhanced clinical practice and related research during the last 40 years since the introduction of computed tomography (CT) and magnetic resonance imaging (MRI) enabled mapping of the brain. We highlight recent advances in neurovascular imaging. We describe how the convergence of readily available data and new clinical trial paradigms will recast our methods for studying the neurovascular patient. The application of a precision medicine approach to the collaterome, a comprehensive synthesis of neurovascular pathophysiology, will entail novel methods for clinical trial randomization, collection of routine and clinical trial imaging results, data archiving, and analysis.

Hemodynamic characteristics of large unruptured internal carotid artery aneurysms prior to rupture: a case control study

Objective

Post-ruptured intracranial aneurysm geometry models have been widely used in computational fluid dynamic studies to assess hemodynamic parameters associated with aneurysm rupture. However, their results may not be valid due to the morphological changes of the aneurysm after rupture. Our aim was to identify the hemodynamic features of aneurysms prior to rupture in comparison with unruptured aneurysms.

Materials and methods

We retrospectively identified three large unruptured internal carotid artery (ICA) aneurysms (pre-ruptured group) with adequate image quality just before rupture. Matched with the same location and similar size, eight unruptured aneurysms (unruptured group) were selected as controls during the same time period. Flow simulations for these aneurysms were performed to compare differences in hemodynamics.

Results

Compared with unruptured aneurysms, pre-ruptured aneurysms had a significantly more irregular aneurysm shape, a higher aspect ratio, and lower aneurysm averaged wall shear stress (WSS) (p=0.024, p=0.048, and p=0.048, respectively). Although pre-ruptured aneurysms had a lower low WSS area and higher Oscillatory Shear Index, these were not statistically significant.

Conclusions

For large unruptured ICA aneurysms, low WSS, higher aspect ratio, and irregular shape were indicators of fatal rupture. Early treatment for such lesions with flow diverter and coils may be the best therapeutic option.

MR imaging of myeloperoxidase activity in a model of the inflamed aneurysm wall

BACKGROUND AND PURPOSE

Although myeloperoxidase activity in vivo can be visualized by using noninvasive imaging, successful clinical translation requires further optimization of the imaging approach. We report a motion-sensitized driven-equilibrium MR imaging approach for the detection of a myeloperoxidase activity-specific gadolinium-containing imaging agent in experimental aneurysm models, which compensates for irregular blood flow, enabling vascular wall imaging in the aneurysm.

MATERIALS AND METHODS

A phantom was built from rotational angiography of a rabbit elastase aneurysm model and was connected to a cardiac pulse duplicator mimicking rabbit-specific flow conditions. A T1-weighted turbo spin-echo-based motion-sensitized driven-equilibrium pulse sequence was optimized in vitro, including the addition of fat suppression and the selection of the velocity-encoding gradient parameter. The optimized sequence was applied in vivo to rabbit aneurysm models with and without inflammation in the aneurysmal wall. Under each condition, the aneurysms were imaged before and after intravenous administration of the imaging agent. The signal-to-noise ratio of each MR imaging section through the aneurysm was calculated.

RESULTS

The motion-sensitized driven-equilibrium sequence was optimized to reduce flow signal, enabling detection of the myeloperoxidase imaging agent in the phantom. The optimized imaging protocol in the rabbit model of saccular aneurysms revealed a significant increase in the change of SNR from pre- to post-contrast MR imaging in the inflamed aneurysms compared with naïve aneurysms and the adjacent carotid artery (P < .0001).

CONCLUSIONS

A diagnostic MR imaging protocol was optimized for molecular imaging of a myeloperoxidase-specific molecular imaging agent in an animal model of inflamed brain aneurysms.

Morphological and hemodynamic analysis of posterior communicating artery aneurysms prone to rupture: a matched case-control study

OBJECTIVES

We evaluated the correlation between posterior communicating artery (PcomA) aneurysm rupture and morphological and hemodynamic parameters to assess related rupture risk indices.

METHODS

Six patients with PcomA aneurysms that ultimately ruptured (cases) were studied after initially being included in a prospective database including their three-dimensional (3D) imaging before rupture. For each case, four incidental stable unruptured aneurysms (controls) were randomly selected and matched based on clinical factors. The 3D images from all patients were reconstructed to establish the patient-specific model. Six morphologic parameters and three hemodynamic parameters were measured and calculated. A conditional logistic regression analysis was used to assess the individual risk of rupture.

RESULTS

The analysis demonstrated a larger aneurysm size (p=0.001), higher aspect ratio (p=0.018), ellipticity index (p<0.001), undulation index (p=0.005), percentage of low wall shear stress (WSS) area (LSA%) (p=0.010), and a lower normalized WSS (p=0.005) in the case group. The multivariate conditional logistic regression analysis demonstrated that only normalized WSS was significantly associated with the rupture of PcomA aneurysms (OR 0.151; 95% CI 0.025 to 0.914; p=0.040).

CONCLUSIONS

Hemodynamics and morphology are closely associated with aneurysm rupture, and WSS may be a more reliable parameter characterizing the rupture status of PcomA aneurysms.

Intra-aneurysmal flow patterns: illustrative comparison among digital subtraction angiography, optical flow, and computational fluid dynamics

BACKGROUND AND PURPOSE

Digital subtraction angiography is the gold standard vascular imaging and it is used for all endovascular treatment of intracranial anerysms. Optical flow imaging has been described as a potential method to evaluate cerebral hemodynamics through DSA. In this study, we aimed to compare the flow patterns measured during angiography, by using an optical flow method, with those measured by using computational fluid dynamics in intracranial aneurysms.

MATERIALS AND METHODS

A consecutive series of 21 patients harboring unruptured saccular intracranial aneurysms who underwent diagnostic angiography before treatment was considered. High-frame-rate digital subtraction angiography was performed to obtain an intra-aneurysmal velocity field by following the cardiac-modulated contrast wave through the vascular structures by using optical flow principles. Additionally, computational fluid dynamics modeling was performed for every case by using patient-specific inlet-boundary conditions measured with the optical flow method from both DSA and 3D rotational angiography datasets. Three independent observers compared qualitatively both the inflow direction and the apparent recirculation in regular DSA, optical flow images, and computational fluid dynamics flow patterns for each patient; κ statistics were estimated.

RESULTS

We included 21 patients. In 14 of these 21, the flow patterns were conclusive and matching between the optical flow images and computational fluid dynamics within the same projection view (κ = .91). However, in only 8 of these 14 patients the optical flow images were conclusive and matching regular DSA images (observer κ = 0.87). In 7 of the 21 patients, the flow patterns in the optical flow images were inconclusive, possibly due to improper projection angles.

CONCLUSIONS

The DSA-based optical flow technique was considered qualitatively consistent with computational fluid dynamics outcomes in evaluating intra-aneurysmal inflow direction and apparent recirculation. Moreover, the optical flow technique may provide the premises for new solutions for improving the visibility of flow patterns when contrast motion in DSA is not apparent. This technique is a diagnostic method to evaluate intra-aneurysmal flow patterns and could be used in the future for validation and patient evaluation.