From arteriographies to computational flow in saccular aneurisms: the INRIA experience

Blood flow in the cerebral venous system: modeling and simulation

The development of a software platform incorporating all aspects, from medical imaging data, through three-dimensional reconstruction and suitable meshing, up to simulation of blood flow in patient-specific geometries, is a crucial challenge in biomedical engineering. In the present study, a fully three-dimensional blood flow simulation is carried out through a complete rigid macrovascular circuit, namely the intracranial venous network, instead of a reduced order simulation and partial vascular network. The biomechanical modeling step is carefully analyzed and leads to the description of the flow governed by the dimensionless Navier-Stokes equations for an incompressible viscous fluid. The equations are then numerically solved with a free finite element software using five meshes of a realistic geometry obtained from medical images to prove the feasibility of the pipeline. Some features of the intracranial venous circuit in the supine position such as asymmetric behavior in merging regions are discussed.

Hemodynamic impact of cerebral aneurysm endovascular treatment devices: coils and flow diverters

Coils and flow diverters or stents are devices successfully used to treat cerebral aneurysms. Treatment aims to reduce intra-aneurysmal flow, thereby separating the aneurysmal sac from the blood circulation. The focus and this manuscript combining literature review and our original research is an analysis of changes in aneurysmal hemodynamics caused by endovascular treatment devices. Knowledge of post-treatment hemodynamics is a path to successful long-term treatment. Summarizing findings on hemodynamic impact of treatment devices, we conclude: coiling and stenting do not affect post-treatment intra-aneurysmal pressure, but significantly alter aneurysmal hemodynamics through flow reduction and a change in flow structure. The impact of treatment devices on aneurysmal flow depends, however, on a set of parameters including device geometry, course of placement, parent vessel and aneurysm geometry.

Suggested connections between risk factors of intracranial aneurysms: a review

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

Statistical wall shear stress maps of ruptured and unruptured middle cerebral artery aneurysms

Haemodynamics and morphology play an important role in the genesis, growth and rupture of cerebral aneurysms. The goal of this study was to generate and analyse statistical wall shear stress (WSS) distributions and shapes in middle cerebral artery (MCA) saccular aneurysms. Unsteady flow was simulated in seven ruptured and 15 unruptured MCA aneurysms. In order to compare these results, all geometries must be brought in a uniform coordinate system. For this, aneurysms with corresponding WSS data were transformed into a uniform spherical shape; then, all geometries were uniformly aligned in three-dimensional space. Subsequently, we compared statistical WSS maps and surfaces of ruptured and unruptured aneurysms. No significant (p > 0.05) differences exist between ruptured and unruptured aneurysms regarding radius and mean WSS. In unruptured aneurysms, statistical WSS map relates regions with high (greater than 3 Pa) WSS to the neck region. In ruptured aneurysms, additional areas with high WSS contiguous to regions of low (less than 1 Pa) WSS are found in the dome region. In ruptured aneurysms, we found significantly lower WSS. The averaged aneurysm surface of unruptured aneurysms is round shaped, whereas the averaged surface of ruptured cases is multi-lobular. Our results confirm the hypothesis of low WSS and irregular shape as the essential rupture risk parameters.

Computational simulation of breast compression based on segmented breast and fibroglandular tissues on magnetic resonance images

This study presents a finite element-based computational model to simulate the three-dimensional deformation of a breast and fibroglandular tissues under compression. The simulation was based on 3D MR images of the breast, and craniocaudal and mediolateral oblique compression, as used in mammography, was applied. The geometry of the whole breast and the segmented fibroglandular tissues within the breast were reconstructed using triangular meshes by using the Avizo 6.0 software package. Due to the large deformation in breast compression, a finite element model was used to simulate the nonlinear elastic tissue deformation under compression, using the MSC.Marc software package. The model was tested in four cases. The results showed a higher displacement along the compression direction compared to the other two directions. The compressed breast thickness in these four cases at a compression ratio of 60% was in the range of 5-7 cm, which is a typical range of thickness in mammography. The projection of the fibroglandular tissue mesh at a compression ratio of 60% was compared to the corresponding mammograms of two women, and they demonstrated spatially matched distributions. However, since the compression was based on magnetic resonance imaging (MRI), which has much coarser spatial resolution than the in-plane resolution of mammography, this method is unlikely to generate a synthetic mammogram close to the clinical quality. Whether this model may be used to understand the technical factors that may impact the variations in breast density needs further investigation. Since this method can be applied to simulate compression of the breast at different views and different compression levels, another possible application is to provide a tool for comparing breast images acquired using different imaging modalities--such as MRI, mammography, whole breast ultrasound and molecular imaging--that are performed using different body positions and under different compression conditions.

Blood flow imaging: an angle-independent ultrasound modality for intraoperative assessment of flow dynamics in neurovascular surgery

OBJECTIVE

The objective of this study was to investigate the clinical applicability of navigated blood flow imaging (BFI) in neurovascular applications. BFI is a new 2-dimensional ultrasound modality that offers angle-independent visualization of flow. When integrated with 3-dimensional (3D) navigation technology, BFI can be considered as a first step toward the ideal tool for surgical needs: a real-time, high-resolution, 3D visualization that properly portrays both vessel geometry and flow direction.

METHODS

A 3D model of the vascular tree was extracted from preoperative magnetic resonance angiographic data and used as a reference for intraoperative any-plane guided ultrasound acquisitions. A high-end ultrasound scanner was interconnected, and synchronized recordings of BFI and 3D navigation scenes were acquired. The potential of BFI as an intraoperative tool for flow visualization was evaluated in 3 cerebral aneurysms and 3 arteriovenous malformations.

RESULTS

The neurovascular flow direction was properly visualized in all cases using BFI. Navigation technology allowed for identification of the vessels of interest, despite the presence of brain shift. The surgeon found BFI to be very intuitive compared with conventional color Doppler methods. BFI allowed for quality control of sufficient flow in all distal arteries during aneurysm surgery and made it easier to discern between feeding arteries and draining veins during surgery for arteriovenous malformations.

CONCLUSION

BFI seems to be a promising modality for neurovascular flow visualization that may provide the neurosurgeon with a valuable tool for safer surgical interventions. However, further work is needed to establish the clinical usefulness of the proposed imaging setup.

Risk of aneurysmal rupture: the importance of neck orifice positioning-assessment using computational flow simulation

OBJECTIVE

The aim of the present study was to clarify the risk of rupture in terminal-type intracranial aneurysms using computational flow simulation analysis.

METHODS

First, idealized three-dimensional aneurysmal models were built from a solid voxel on the computer. We focused on round terminal-type aneurysms with the positioning of the neck orifice set according to the following three patterns in relationship to the axis of the parent artery: the Type-A neck orifice was positioned directly in line with the flow of the parent artery; the Type-B neck orifice was shifted 1.5 mm offline toward the unilateral branch; and the Type-C neck orifice was shifted 3 mm offline. Computational flow simulations were applied with Fujitsu alpha-Flow software (Fujitusu, Tokyo, Japan). We analyzed flow patterns using modified patient-specific models. We also investigated actual clinical situations to evaluate the differences in neck-orifice positioning between 20 ruptured aneurysms and 26 unruptured ones using three-dimensional angiograms.

RESULTS

The Type-A neck orifice showed completely symmetrical stream lines in the aneurysm, whereas the Type-C orifice showed a clear round circulation. The Type-B neck orifice, on the other hand, exhibited intra-aneurysmal flow separation. The clinical research demonstrated that Type-B aneurysms were more likely to be found in the ruptured group (P < 0.05).

CONCLUSION

Flow separation, recognized as one of the causes of intimal injury, could be observed only in Type-B aneurysms, a result that corresponded well with our clinical experience. From the flow-dynamics point of view, this positioning of the neck orifice may be one of the risk factors most likely to induce the rupture of unruptured aneurysms.