CFD following fEVAR

Overview

Fenestrated Endovascular Aneurysm Repair (fEVAR) is one option for treatment of abdominal aortic aneurysms with inadequate neck length for standard EVAR. The Zenith fEVAR device (Cook Medical Inc.) employs a modular design with at least 3 components including a proximal body with fenestrations for the visceral vessels, a bificated body "tromboned" inside the proximal body and a limb extension to achieve distal seal on the contralateral side. The proximal body has stainless steel bare metal stents with barbs which add to the radial fixation force of the sealing stents and oppose downwards distraction forces. This is further reinforced by deploying small covered stents through the fenestrations into the visceral vessels. In contrast the other 2 components are held in place by radial force only. Our aim was to develop a method to model the insitu fEVAR to allow computational fluid dynamic (CFD) assessment of distraction forces acting upon each component.

Characteristics:

  • DICOM import
  • Segmentation
  • 3D editing
  • Manual removal of metal artefacts
  • Morpholigical and Boolean operations

Thanks to

Liverpool Vascular and Endovascular Service, Royal Liverpool Hospital
S.M. Jones • R.K. Fisher

University of Liverpool, UK
R.J. Poole • T.V. How • R.L. Williams

Image Processing

Arterial phase post operative CT scans were cropped and imported with multiple window and level settings to distinguish the lumen and the stent.

The segmentation was performed with a mix of threshold, floodfill, paint and smoothing techniques. To model the outlets temporary rectangular masks were added with 3D editing to the end of the vessels.

The vessels were then dilated to encompass the true vessel region, and the excess vessel (past the outlets) was removed.

Meshing

A CFD model was created using the lumen mask as the fluid domain. The graft fabric territories and outflow masks were set as non-exportable parts and as walls and outflows respectively. Contact between the lumen mask and the proximal extent of the image was defined as the velocity inlet.

Meshing was performed using greyscales with no further pre-smoothing. The FE-Free algorithm was employed with compound coarseness set to -20 and further mesh improvement achieved by setting tetrahedral skew to 0.9.

Simulations

The CFD model was exported as a msh file into FLUENT (ANSYS Inc.) and a steady state flow simulation was performed using boundary conditions representative of peak systole in a hypertensive patient at rest.

Distraction force (DF) in Newtons in x, y and z directions were obtained for each component and for the complete stent-graft.

Models had on average 850,000 tetrahedral elements. Simulations were repeated with more complex meshes created with compound coarseness 0 and no significant differences in distraction force were observed.

Prior validation of CFD simulations was performed using a simple bificated lumen created in +CAD in comparison with a 1 dimensional theoretical model.