Department of Mechanical Engineering, Babol Noshirvani University of Technology, Iran (Islamic Republic of)
Corresponding author details:
Department of Mechanical engineeringTechnology Development
Babol Noshirvani University of Technology
Iran (Islamic Republic of)
Copyright: © 2020 Sadeghian M. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 international License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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;
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 . Around 50% of patients with aneurysmal will suffer
serious disability or die as a result of the initial hemorrhage . 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  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 , 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 . 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 .
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  (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 , 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  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  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 , with the eventual possibility of determining hemodynamic risk factors implicated in a high possibility of rupture .
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  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 .
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 .
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
Copyright © 2020 Boffin Access Limited.