Marcelo A. Castro

Efficient pipeline for image-based patient-specific analysis of cerebral aneurysm hemodynamics: technique and sensitivity

Hemodynamic factors are thought to be implicated in the progression and rupture of intracranial aneurysms. Current efforts aim to study the possible associations of hemodynamic characteristics such as complexity and stability of intra-aneurysmal flow patterns, size and location of the region of flow impingement with the clinical history of aneurysmal rupture. However, there are no reliable methods for measuring blood flow patterns in vivo. In this paper, an efficient methodology for patient-specific modeling and characterization of the hemodynamics in cerebral aneurysms from medical images is described. A sensitivity analysis of the hemodynamic characteristics with respect to variations of several variables over the expected physiologic range of conditions is also presented. This sensitivity analysis shows that although changes in the velocity fields can be observed, the characterization of the intra-aneurysmal flow patterns is not altered when the mean input flow, the flow division, the viscosity model, or mesh resolution are changed. It was also found that the variable that has the greater impact on the computed flow fields is the geometry of the vascular structures. We conclude that with the proposed modeling pipeline clinical studies involving large numbers cerebral aneurysms are feasible.

Hemodynamic Patterns of Anterior Communicating Artery Aneurysms: A Possible Association with Rupture

BACKGROUND AND PURPOSE: The anterior communicating artery (AcomA) is a predilect location of aneurysms which typically carry higher rupture risks than other locations in the anterior circulation. The purpose of this study was to characterize the different flow types present in AcomA aneurysms and to investigate possible associations with rupture. MATERIALS AND METHODS: Patient-specific computational models of 26 AcomA aneurysms were constructed from 3D rotational angiography images. Bilateral images were acquired in 15 patients who had both A1 segments of the anterior cerebral arteries, and models of the whole anterior circulation were created by fusing the reconstructed left and right arterial trees. Computational fluid dynamics simulations were performed under pulsatile flow conditions measured on a healthy subject. Visualizations of flow velocity, instantaneous streamlines, and wall shear stress (WSS) were performed. These were analyzed for flow patterns, size of the impaction zone, and peak WSS and then correlations were made with prior history of rupture. RESULTS: Aneurysms with small impaction zones were more likely to have ruptured than those with large impaction zones (83% versus 63%). Maximum intra-aneurysmal WSS (MWSS) for the unruptured aneurysms ranged from 10 to 230 dyne/cm2 (mean, 114 dyne/cm2) compared with ruptured aneurysms, which ranged from 35 to 1500 dyne/cm2 (mean, 271 dyne/cm2). This difference in MWSS was statistically significant at 90% confidence levels (P = .10). CONCLUSIONS: Aneurysms with small impaction zones, higher flow rates entering the aneurysm, and elevated MWSS are associated with a clinical history of previous rupture.

Blood-flow models of the circle of Willis from magnetic resonance data

Detailed knowledge of the cerebral hemodynamics is important for a variety of clinical applications. Cerebral perfusion depends not only on the status of the diseased vessels but also on the patency of collateral pathways provided by the circle of Willis. Due to the large anatomical and physiologic variability among individuals, realistic patient-specific models can provide new insights into the cerebral hemodynamics. This paper presents an image-based methodology for constructing patient-specific models of the cerebral circulation. This methodology combines anatomical and physiologic imaging techniques with computer simulation technology. The methodology is illustrated with a finite element model constructed from magnetic resonance image data of a normal volunteer. Several of the remaining challenging problems are identified. This work represents a starting point in the development of realistic models that can be applied to the study of cerebrovascular diseases and their treatment.

CFD analysis incorporating the influence of wall motion: application to intracranial aneurysms

Haemodynamics, and in particular wall shear stress, is thought to play a critical role in the progression and rupture of intracranial aneurysms. A novel method is presented that combines image-based wall motion estimation obtained through non-rigid registration with computational fluid dynamics (CFD) simulations in order to provide realistic intra-aneurysmal flow patterns and understand the effects of deforming walls on the haemodynamic patterns. In contrast to previous approaches, which assume rigid walls or ad hoc elastic parameters to perform the CFD simulations, wall compliance has been included in this study through the imposition of measured wall motions. This circumvents the difficulties in estimating personalized elasticity properties. Although variations in the aneurysmal haemodynamics were observed when incorporating the wall motion, the overall characteristics of the wall shear stress distribution do not seem to change considerably. Further experiments with more cases will be required to establish the clinical significance of the observed variations.

Hemodynamics and rupture of terminal cerebral aneurysms.

RATIONALE AND OBJECTIVES The objective of this study was to investigate the relationship between hemodynamics patterns and aneurysmal rupture in cerebral aneurysms of the same morphology regardless their location. Particularly, terminal aneurysms in both the anterior and posterior circulation were studied. MATERIALS AND METHODS A total of 42 patient-specific vascular models were constructed from three-dimensional rotational angiography images. All patients had terminal aneurysms at different arteries: a) middle cerebral; b) anterior communicating; c) internal carotid (terminus); d) internal carotid-posterior communicating; e) basilar; or f) anterior cerebral. Hemodynamics information (intra-aneurysmal velocity and wall shear stress distributions) was derived from image-based computational fluid dynamics models with realistic patient-specific anatomies. RESULTS The group of aneurysms with an inflow jet that splits in two secondary jets, one of which enters the aneurysm before reaching one of the daughter vessels (type B), had the highest peak wall shear stress (WSS) and the highest rupture rate. The peak WSS averaged over each flow type showed a higher value in the ruptured group. The average peak WSS in the ruptured group (all types) was 188 dyn/cm(2) (compared to 118 dyn/cm(2) for the unruptured). CONCLUSIONS This finding is in agreement with a previous work in which only anterior communicating artery aneurysms were investigated. The significance of these findings is that, if they are statistically confirmed with larger number of cases, flow types could be directly observed during angiographic examinations and linked to WSS categories that may help evaluate which aneurysms are more likely to rupture.

Quantitative hemodynamic analysis of brain aneurysms at different locations.

BACKGROUND AND PURPOSE: Studies have shown that the occurrence of brain aneurysms and risk of rupture vary between locations. However, the reason that aneurysms at different branches of the cerebral arteries have different clinical presentations is not clear. Because research has indicated that aneurysm hemodynamics may be one of the important factors related to aneurysm growth and rupture, our aim was to analyze and compare the flow parameters in aneurysms at different locations. MATERIALS AND METHODS: A total of 24 patient-specific aneurysm models were constructed by using 3D rotational angiographic data for the hemodynamic simulation. Previously developed computational fluid dynamics software was applied to each aneurysm to simulate the blood-flow properties. Hemodynamic data at peak pulsatile flow were recorded, and wall shear stress (WSS) and flow rate in the aneurysms and parent arteries were quantitatively compared. To validate our method, a comparison with a previously reported technique was also performed. RESULTS: WSS and flow rate in the aneurysms at the peak of the cardiac cycle were found to differ in magnitude between different locations. Multiple comparisons among locations showed higher WSS and flow rate in middle cerebral artery aneurysms and lower WSS and flow rate in basilar artery and anterior communicating artery aneurysms. CONCLUSIONS: We observed changes in hemodynamic values that may be related to aneurysm location. Further study of aneurysm locations with a large number of cases is needed to test this hypothesis.

Patient-specific hemodynamic analysis of small internal carotid artery-ophthalmic artery aneurysms

BACKGROUND Prophylactic treatment of unruptured small brain aneurysms is still controversial due to the low risk of rupture. Distinguishing which small aneurysms are at risk for rupture has become important for treatment. Previous studies have indicated a variety of hemodynamic properties that may influence aneurysm rupture. This study uses hemodynamic principles to evaluate these in the context of ruptured and unruptured small aneurysms in a single location. METHODS Eight small internal carotid artery-ophthalmic artery (ICA-Oph) aneurysms (<10 mm) were selected from the University of California, Los Angeles, database. We analyzed rupture-related hemodynamic characteristics including flow patterns, wall shear stress (WSS), and flow impingement using previously developed patient-specific computational fluid dynamics software. RESULTS Most ruptured aneurysms had complicated flow patterns in the aneurysm domes, but all of the unruptured cases showed a simple vortex. A reduction in flow velocity between the parent artery and the aneurysm sac was found in all the cases. Inside the aneurysms, the highest flow velocities were found either at the apex or neck. We also observed a trend of higher and more inhomogeneous WSS distribution within ruptured aneurysms (10.66 +/- 5.99 Pa) in comparison with the unruptured ones (6.31 +/- 6.47 Pa) (P < .01). CONCLUSION A comparison of hemodynamic properties between ruptured and unruptured small ICA-Oph aneurysms found that some hemodynamic properties vary between small aneurysms although they are similar in size and share the same anatomical location. In particular, WSS may be a useful hemodynamic factor for studying small aneurysm rupture.

Pilot clinical study of aneurysm rupture using image-based computational fluid dynamics models

Although the natural history of cerebral aneurysms remains unknown, hemodynamics is thought to play an important role in their initiation, growth and rupture. This paper describes a pilot clinical study of the association between intraaneurysmal hemodynamic characteristics and the rupture of cerebral aneurysms. A total of 62 patient-specific models of cerebral aneurysms were constructed from 3D angiography images. Computational fluid dynamics simulations were performed under pulsatile flow conditions. The aneurysms were classified into different categories depending on the complexity and stability of the flow pattern, the location and size of the flow impingement region, and the size of the inflow jet. These features were analyzed for associations with history of rupture. A large variety of flow patterns was observed. It was found that 72% of ruptured aneurysms had complex or unstable flow patterns, 80% had small impingement regions and 76% had small jet sizes. Conversely, unruptured aneurysms accounted for 73%, 82% and 75% of aneurysms with simple stable flow patterns, large impingement regions and large jet sizes, respectively.

Combined clinical and computational information in complex cerebral aneurysms: application to mirror cerebral aneurysms

Although the incidence of ruptured cerebral aneurysms is relatively small, when rupture occurs, morbidity and mortality are exceptionally high. The understanding of the pathological and physiological forces driving aneurysmal pathogenesis and progression is crucial. In this paper we analyze the occurrence of mirror cerebral aneurysms in 8 patients and speculate on the effect of haemodynamics on the localization and course of the disease. By mirror cerebral aneurysms we indicate two aneurysms in the same patient and at the same location in the cerebral vasculature but symmetrically with respect to a sagittal plane. In particular we focus on cases of mirror cerebral aneurysms where only one of the two aneurysms presented subarachnoid hemorrhage (SAH). Anatomical information is extracted from 3D rotational angiography (3DRA) images and haemodynamic information is obtained through blood flow simulation in patientspecific anatomical models. The distribution of Wall Shear Stress (WSS) and the flow patterns through the vessels and inside the aneurysms are reported. By combining clinical observations on asymmetry of the cerebral vasculature and aneurysmal shape and size with computed information on blood flow patterns we explore the causes behind a specific localization and a different outcome of disease progression.

Computational modeling of cerebral aneurysms in arterial networks reconstructed from multiple 3D rotational angiography images

Previous patient-specific computational fluid dynamics (CFD) models of cerebral aneurysms constructed from 3D rotational angiography have been limited to aneurysms with a single route of blood flow. However, there are numerous aneurysms that accept blood flow from more than one avenue of flow such as aneurysms in the anterior communicating artery. Although the anatomy of these aneurysms could be visualized with other modalities such as CTA and MRA, cerebral rotational angiography has the highest resolution, and is therefore the preferred modality for vascular CFD modeling. The purpose of this paper is to present a novel methodology to construct personalized CFD models of cerebral aneurysms with multiple feeding vessels from multiple rotational angiography images. The methodology is illustrated with two examples: a model of an anterior communicating artery aneurysm constructed from bilateral rotational angiography images, and a model of the complete circle of Willis of a patient with five cerebral aneurysms. In addition, a sensitivity analysis of the intraaneurysmal flow patterns with respect to mean flow balance in the feeding vessels was performed. It was found that the flow patterns strongly depend on the geometry of the aneurysms and the connected vessels, but less on the changes in the flow balance. These types of models are important for studying the hemodynamics of cerebral aneurysms and further our understanding of the disease progression and rupture, as well as for simulating the effect of surgical and endovascular interventions.