Aneurysms are one of the most catastrophic cerebral emergencies that causes into high mortality. Clinical observations shed the light on aneurysm development which is reported that aneurysm is likely to be related to the hemodynamics condition of the blood vessel in brain. Although the mechanisms of the formation, growth and rupture of aneurysms are not fully discovered but local hemodynamic crucial factors such as dynamic pressure, wall shear stress, blood flow velocity, and residence time are thought to play dominant roles in the growth and initiation of aneurysms, and also the success or failure of aneurysm therapies. Computational Fluid Dynamics (CFD), a numerical simulation aims to provide data, which will help in clinical decision in recognizing the aneurysms at risk of rupture and in the treatment of them.

Aneurysm; CFD; Wall Shear Stress; Blood Flow; Stent; Endovascular Embolization; Treatment 

Cerebral aneurysms are pathologic dilations of the arterial wall that frequently occurred at or near arterial bifurcations. The exact cause of aneurysm formation remains unclear; but there is evidence that high shear stress and blood velocity may provide the hemodynamic environment for degenerative vascular injury.

Flow patterns in cerebral artery bifurcations and bends have been the subject of comprehensive investigation [1-3]. The growth of cerebral aneurysm may compress brain tissues and surrounding nerves which may cause one or more health conditions; including vomiting, headache, nerve paralysis, seizure and body numbness. When the cerebral aneurysm ruptures generally causes internal bleeding, in the end which in several cases causes death. Stroke is one of the prominent reasons of death in the US. Subarachnoid hemorrhage explains about 7% of stroke cases and most of these cases are caused by rupture of a cerebral aneurysm [4]. Around 50% of patients with aneurysmal will suffer serious disability or die as a result of the initial hemorrhage [5]. The flow dynamics of cerebral aneurysms have been studied in numerous experimental models and clinical studies to investigate the role of hemodynamic forces in the initiation, growth, and rupture of cerebral aneurysms (Figure 1).

Figure 1: Cerebral aneurysm

During the initial period of cerebral aneurysm morbidity, antihypertensive drugs are useful to alleviate the impact on the artery inner wall caused by blood. When cerebral aneurysm ruptures, it must be treated by surgery. A stent is made of a braided tubular assembly of thin metallic wires and lodged against the lumen of the parent vessel to serve as a porous barrier disrupting blood flow into the aneurysm. The treatment of aneurysms often involves the implantation of stents which are designed to prevent ruptures by promoting thrombosis. Studies [6,7] have conducted to measure this effect, Use of Computational Fluid Dynamics (CFD) to analyze blood flow in and around stented aneurysms. However, Due to scale difference disparity between the dimensions of the aneurysm and its parent vessel and the stent wire cross-section, in practice it is hard to incorporate both geometries simultaneously into the solution domain of the discrete Navier-Stokes equations. In treatment of aneurysms intravascular stenting [8] is thought as a better option comparing with clipping treatment. The reason is that craniotomy with clipping cannot fully remove an aneurysm with a wide or calcified orifice and generally has the risk of surgical complications. Stenting across the aneurysmal orifice could sufficiently prevent the rupture aneurysm [8-11]. After 7 days of the metal stent placement in dogs to treat internal carotid aneurysm [11], it is also showed an effective occlusion of the aneurysm based on brain angiography (Figure 2,3).

The treatment of intracranial aneurysms has changed dramatically with the introduction of interventional techniques. Endovascular coiling of aneurysms has been shown to be effective and has arguably replaced surgery for the treatment of most aneurysms. One of the treatment methods of cerebral aneurysm is filling the aneurysm by coil mesh, which is known medically by endovascular embolization. This treatment method is considering the least invasive way of treatment. The coils are introduced to the aneurysm through a catheter implanted into a vessel over the knee, which is navigated through the blood vessels to until it reaches the aneurysm. Coils are used to form a blood clot around it and reduce the blood flow to the aneurysm, hence, reducing aneurysm rupture occurrence. Coil embolization is widely used for cerebral aneurysms due to its minimally invasive nature and the least advancements in embolization devices. Surgical clipping reportedly features similar mortality rates as coiling but significantly higher per procedural morbidity rates [12]. However, recanalization and retreatment occur more often in coil embolization [13,14].

A larger aneurysmal dome or neck, minor recurrence noted on cerebral angiograms early after coil embolization, and significant risk factors for recanalization and retreatment is lower coil packing density [15,16]. A recent study showed that blood flow velocity and high Wall Shear Stress (WSS) were consistently observed on Computational Fluid Dynamics (CFD) near the remnant neck of partially embolized aneurysms prone to future recanalization [17]. Since CFD simulates the hemodynamics based on aneurysm geometry using a patient-specific model, coil packing density and the residual aneurysm would affect post-coiling hemodynamics. However, the hemodynamics simulation of coiled aneurysm has been difficult, because the precise geometry of aneurysmsafter coiling could not be achieved by using Digital Subtraction Angiography (DSA) or threedimensional (3D) Computed Tomography (CT) angiography or due to metal artifacts. Instead, hemodynamicsin coiled aneurysms have been simulated by using numerical methods with porous media modeling [18] (Figure 4).

Figure 2: Using stent in vessel

Figure 3: Visual testing of stent

Figure 4: Endovascular treatment of aneurysm

In order to investigate highly complex problems Computational Fluid dynamics (CFD) evolved to asophisticated method to approach the behavior of flow phenomenon and their effects on key parameters. Within the last decades, several advantages were found in contrast to experimental procedures. For instance, numerical methods may be able to consider scales, which cannot be captured experimentally so far. In many cases, high performance computing hard ware is less expensive than complex measuring equipment. Additionally, a better reproducibility can be achieved due to a lower systematic error and to a lower influence of varying process conditions. CFD tools have been used along with medical imaging techniques, such as MRI or CT scans, in order to further analyze the hemodynamics [19-21].

When conducting CFD-based analysis, blood rheology is an important factor to take into consideration, since blood is a suspension-type fluid, this kind of rheology is explained by the fact that blood is composed of solid particles, the blood cells, suspended in the plasma. Due to this nature, the blood presents a shear-thinning behavior [22-24]. Although studies tend to simulate blood as being a Newtonian fluid, since this consideration provides reasonable results [24], it does not follow the real rheological behavior. Thus, this study aims to simulate blood flow in a generic artery with an aneurysm via CFD tools, whilst comparing non-Newtonian rheological models and their impact in hemodynamic factors, such as velocity and wall shear stress. In various studies, blood was assumed to have constant blood viscosity and as a Newtonian fluid. While, blood is a non-Newtonian fluid, at a low shear rate range, the blood viscosity increases, however, it is constant in a high shear rate range. The non-Newtonian properties of blood are expressed using structural formulae, such as the Carreau, Herschel–Bulkley, and Casson–Yasuda models.

The use of CFD in clinical field has grown dramatically because of the late advancements of high-resolution angiography techniques [25] which provided more exact vascular details such as dimension and geometry. The CFD simulation can provide more accurate information on vascular hemodynamics. Different researches have been done in using of CFD to describe the hemodynamics in the cerebral aneurysm [26-29], coronary arteries [30] and in various applications bio-medical. For instance when the transverse diameters of Abdominal Aortic Aneurysms (AAA) reach 5 cm, it is medically highly recommended to get it treated [31,32].

In fact, such CFD method has been used in simulating the blood flow in anatomically realistic aneurysmal and arterial geometries derived in vivo image [33,34].

Recent Advancement in medical imaging and vessel segmentation/ reconstruction algorithms has led to increase the use of CFD modeling of cerebral aneurysm hemodynamics [35-38]. Researchers have recently demonstrated the development of an efficient pipeline for the CFD simulation of large numbers of aneurysms [39], with the eventual possibility of determining hemodynamic risk factors implicated in a high possibility of rupture [40].

CFD has been used to infer the effects of endovascular treatment on hemodynamics in idealized brain aneurysm models [41-44] and it has been suggested as a tool for patient-specific testing and treatment planning of endovascular devices [45] where as such CFD models are compelling in detail and enticing in their potential to improve our understanding the development and treatment of aneurysm.

Recent research has showed that so-called virtual angiography can be used to indirectly validate patient specific CFD models against clinical digital subtraction angiography DSA_ data [46].

A primary study has demonstrated a good agreement between in vitro and CFD-derived particle visualizations of steady flow in an anatomically realistic aneurysm model [47]. 

Although there are various types of surgical or endovascular treatment options to cure these difficult aneurysms, none of the current option is completely reliable. Whereas, it is difficult to recognize which treatment is superior to the others unless we really try it. Finally, the aneurysmal CFD analysis has been considered as evaluation tools for treatment of aneurysm and can provide hemodynamic information for clinicians into making an optimal therapeutic decision.

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