Ansys Dicom To 3D Model

Original Article

Hemodynamics model of fluid–solid interaction in internal

Carotid Artery Aneurysms

Xu Bai-Nan & Wang Fu-Yu & Liu Lei & Zhang Xiao-Jun &

Ju Hai-Yue

Received: 19 December 2009 /Revised: 17 May 2010 /Accepted: 5 July 2010 /Published online: 2 September 2010 # The Author(s) 2010. This article is published with open access at Springerlink.com Abstract The objective of this study is to present a relatively simple method to reconstruct cerebral aneurysms as 3D numerical grids. The method accurately duplicates the geometry to provide computer simulations of the blood flow. Initial images were obtained by using CT angiography and 3D digital subtraction angiography in DICOM format.

Figure: System Model & Simulation Flow for Ansys Dicom To 3D Model

The image was processed by using MIMICS software, and the 3D fluid model (blood flow) and 3D solid model (wall) were generated. The subsequent output was exported to the ANSYS workbench software to generate the volumetric mesh for further hemodynamic study. The fluid model was defined and simulated in CFX software while the solid model was calculated in ANSYS software. The force data calculated firstly in the CFX software were transferred to the ANSYS software, and after receiving the force data, total mesh displacement data were calculated in the ANSYS software.

Figure: System Model & Simulation Flow for Ansys Dicom To 3D Model

Then, the mesh displacement data were transferred back to the CFX software. The data exchange was processed in workbench software. The results of simulation could be visualized in CFX-post. Two examples of grid reconstruction and blood flow simulation for patients with internal carotid artery aneurysms were presented. The wall shear stress, wall total pressure, and von Mises stress could be visualized. This method seems to be relatively simple and suitable for direct use by neurosurgeons or neuroradiologists, and maybe a practical tool for planning treatment and follow-up of patients after neurosurgical or endovascular interventions with 3D angiography.

Figure: System Model & Simulation Flow for Ansys Dicom To 3D Model

Keywords Hemodynamics . Fluid–solid interaction (FSI) .

Introduction

The flow dynamics plays an important role in the initiation, growth, and rupture of cerebral aneurysms [12, 17]. Because of technical factors, most works related to the intra-aneurysmal hemodynamics have largely focused on idealized aneurysm geometry or surgically created aneur- ysms in animals. In vitro studies have allowed a very detailed measurement of hemodynamic variables but are of limited value in understanding the hemodynamic forces in an individual clinical case. Current technology cannot provide analogous information in vivo through either invasive or noninvasive measurements. Wall shear stress (WSS) is a flow-induced stress that can be described as the frictional force of viscous blood. According to the mechanics of the materials, the maximum shear stress safety tool is based on the maximum shear stress failure theory for ductile materials. The theory states that a particular combination of principal WSSs causes failure if the maximum shear equals or exceeds a specific shear limit.

Figure: System Model & Simulation Flow for Ansys Dicom To 3D Model

Furthermore, von Mises stress (VMS, also called equivalent stress) is often used in design work because it allows any arbitrary 3D stress state to be represented as a single positive stress value. Equivalent stress is part of the

Z. Xiao-Jun

College of Mechanical Engineering and Applied Electronics

Doi 10.1007/S10143-010-0282-5

maximum equivalent stress failure theory used to predict yielding in a ductile material. The direct stress measurements in cerebral aneurysmal patients are not available. Though in vivo measurements of the 3D velocity field and WSS have become possible with the development of phase-contrast magnetic reso- nance velocimetry for large and simple arteries like the aorta or the carotid bifurcation, phase-contrast magnetic resonance velocimetry cannot be applied in calculating the spatial distribution of the WSS in small and tortuous vessels like the intracranial arteries. The other clinical technique for assessing flow velocities, specifically sonography, is unable to provide access to flow values in the basilar artery or in the vertebral arteries immedi- ately proximal to their junction. Therefore, researchers

Have Turned To Computational Fluid Dynamic (Cfd)

simulations to gain a better understanding of the intra- aneurysmal hemodynamics. Computational-based models provide an attractive meth- od of investigating intra-aneurysmal flow dynamics by providing the ability to theoretically model and study any possible geometry [7, 16]. Until recently, most computa- tional studies have been performed on the specific patient geometry and to connect specific hemodynamic factors to clinical events allowing statistical analysis in a patient population . These analyses have been done on a variety of assumptions used in modeling including laminar flow, Newtonian properties of fluids, and rigid wall. Actually, the wall of vessels and aneurysms were elastic; effect of pulsatile motion of the wall may be important for the accurate determination of intra-aneurysmal flow patterns, and the material failure rates in CFD models incorporating the mechanical properties of walls may also be important for the rupture of aneurysms, so the fluid–solid interaction (FSI) should be considered. In this work, we describe the method to analyze the complex interaction of blood flow and the compliant aneurysmal wall, so the results of intra- aneurysmal hemodynamics were more realistic.

Fluid Model

The blood flow was assumed to be laminar and Newtonian. The incompressible Navier–Stokes equations were used as the governing equations which are suitable for problems with FSI.

Ð1Þ

where ρf is the fluid density, u is the fluid velocity vector and is the moving coordinate velocity, respectively. In the

Arbitrary Lagrangian Eulerian (Ale) Formulation ,

u−ug is the relative velocity of the fluid with respect to the moving coordinate velocity, and τ is the deviatoric stress tensor. Blood is modeled to have a density ρf=1,053 kg/m3.

Solid Model

The governing equation for the solid domain is the

Momentum Conservation Equation Given By Eq. 2. In

contrast to the ALE formulation of the fluid equations, a

Ð2Þ

where ρs is the solid density, σs is the solid stress tensor, and ds is the local acceleration of the solid. The artery wall was assumed to be hyperelastic, isotropic, incompressible, and homogeneous with a density ρs=1,050 kg/m3.

Fluid–Structure Interaction Model

The boundary conditions on the FSI interfaces state that (1) displacements of the fluid and solid domain must be compatible, (2) tractions at these boundaries must be at equilibrium, and (3) fluid obeys the no-slip condition.

Ð3Þ

where d, σ, and bn are displacements, stress tensors, and boundary normals with the subscripts f and s indicating a property of the fluid and solid, respectively. The 3D hyperelastic M-R model was used to describe the material properties of the artery wall .

Ct Angiography

CT angiography (CTA) (Fig. 1) was done by using a helical CT scanner (LightSpeed Plus-U, GE Medical Systems) with multidetector-row capability. The data were obtained by using a section thickness of 0.625 mm and a table speed of 9 mm/s (120 kV, 315 mA). Zero-degree table and gantry tilt were used. Sections in DICOM format were acquired with a 512×512 matrix. Scanning was started at the C1 level and continued cranially parallel to orbitomeatal line to the skull vertex during the intravenous injection of contrast material at a rate of 3.5 mL/s.

Rotational Digital Subtraction Angiography

We obtained 3D datasets (LCV Plus, GE Medical Systems) from rotational series. The images with a matrix size of

512×512 Pixels Were Acquired. All The Images Were

immediately transferred to a workstation (Advantage Unix,

Ge Medical Systems) For Volume Rendering. A 3D

reconstruction algorithm based on the algebraic reconstruc- tion technique was used to digitally produce 3D digital subtraction angiography (DSA) images (Fig. 2) on the workstation, and the 3D project was transformed into DICOM file with a section thickness of 0.2 mm.

Reconstruction Of Carotid Artery Aneurysms

The digital images and the respective files obtained so far could be directly imported by MIMICS software (V10.0, Belgium Materialise Company) to reconstruct a 3D project.

The luminal surface of blood vessels was extracted from them in the format suitable for import by grid generators. The main role of the MIMICS was to define the region of interest (ROI) (Fig. 3) and to extract the luminal vascular surface from the 3D raster gray-scale image obtained at the previous stage. Using the REMESH command in MIMICS, we can make mesh optimization for CFD analysis and then export the file. The procedure was fully automatic. The fluid model (blood flow) and solid model (wall) were generated. The output file could be converted into Initial Graphics Exchange Specification (IGES) file by the

Commercial Finite Element Package Ansys Workbench

(V11.0, ANSYS, Inc., USA) as geometry input format in most modern commercial grid generators.

Grid Generator

The next stage was volume grid generation. The workbench software was used to divide fluid model into tetrahedral cells (tetrahedral grids are widely used in CFD for computational models of complex shape) (Fig. 4a). The elements chosen for the solid model were a four-node linear quadrilateral shell element and a four-node linear triangular shell element (Fig. 4b).

The number of grid nodes was chosen as a reasonable compromise between better accuracy and shorter computa- tional time of subsequent simulations. Mesh independence was performed in order to determine the optimum number of elements. In order to gain confidence in the mesh size of fluid model, the number of elements was incrementally increased and the peak wall stress was computed. The optimum mesh size was determined once the peak stress did not increase by more than 5%.

Solver

The fully coupled fluid and solid models were solved by the ANSYS workbench. The finite element method (FEM) is used to solve the governing equations in which the ALE algorithm was used. The fluid domain employs special flow-condition-based interpolation (FCBI) tetrahedral ele- ments. We have used the formulation with large displace- ments and small strains in the FSI calculation available in ANSYS workbench. As iteration, we used full Newton method with a maximum of 140 iterations in each time step of 0.025 s. A sparse matrix solver based on Gaussian elimination is used for solving the system. The relative tolerance for all degrees of freedom was set to 0.001. The force data calculated firstly in the CFX software were transferred via the workbench software to ANSYS soft- Fig. 2 3D-DSA image of a cervical internal carotid artery aneurysm Fig. 1 CTA of an internal carotid artery paraclinoid aneurysm and an

41

ware, and after receiving the force data, total mesh displacement data were calculated in ANSYS software.

Then, The Mesh Displacement Data From The Ansys

software were transferred via the workbench software to CFX software. When all the iterations were converged, the results could be visualized in CFX-post.

The workstation used is based on an Intel Xeon(TM) dual core 64 bits processor of 3.0 GHz clock speed, 2.0 GB

Ram Memory And Running On Microsoft Windows Xp

operating system. The simulation time for one FSI case based on three consecutive pulsatile flow cycles employing 150 time iterations was approximately 12 Central Process- ing Unit (CPU) hours.

Results

As seen in Fig. 5, the WSS, wall total pressure, blood streamline, VMS and total mesh displacement are presented in the computation. The aneurysm wall served as an obstacle decelerating the entering stream (diminishing its velocity) and deflecting it along the wall. This was accompanied by a persisting local fluid-induced WSS maximum in the area hit by the stream. The distribution of total pressure is similar with WSS. The distribution of VMS (Fig. 5e) is complex and completely different with WSS (Fig. 5b), but the VMS distribution is nearly proportion to the total mesh displace- ment distribution (Fig. 5f).

Figure 6 shows data from a patient with a cervical internal carotid artery aneurysm and illustrates the compu- tational results as Fig. 5.

Discussion

The method for anatomic reconstruction of cerebral vessels

Should Work For A Broad Spectrum Of Medical Image

obtained from all currently available modalities of cerebral Fig. 4 Grid of fluid model (a) and solid model (b) in WORKBENCH software Fig. 3 Region of interest was defined (a CTA; b 3D-DSA) in MIMICS software

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