Virtual tests are increasingly important for the design and development of medical devices, such as stents, since they shorten the time to market. The possibilities of computational fluid dynamics (CFD) and finite element analysis (FEA), combined with contemporary imaging techniques, greatly facilitate stent research. In order to create realistic computational models to perform this research, the geometry of the stent in question and its delivery system should be very accurate. This case describes how IBiTech obtains accurate stent geometry by segmenting nano/micro-CT images and reconstructing them in 3D using the Mimics Innovation Suite. This software proved to be the perfect tool to get this very precise geometry ready for virtual testing.
 

Accurate Reconstruction of a Stent

A stent mounted on a balloonStents are metallic, tube-like structures that are deployed in stenotic arteries to restore blood flow. The main purpose of a stent is to increase the diameter of a blood vessel by propping open the conduit. Stents are often used to alleviate diminished blood flow to organs and extremities beyond an obstruction, maintaining adequate delivery of oxygenated blood. Prior to the procedure, the stent is collapsed to a small diameter and mounted on a balloon catheter. It is then moved into the obstructed area. Subsequently, the balloon is inflated, the stent expands, locks into place and forms a scaffold which keeps the vessel open.
 
3D reconstruction in Mimics from micro-CT data of the folded balloon
The focus of IBiTech’s research on stents centers on the fluid-structure interaction after the implantation of these endovascular devices. In order to examine these processes effectively, IBiTech needs very detailed computer models. Unfortunately, the actual stent geometry is often different from the original CAD design of the manufacturer, due to various processes that take place after laser cutting the stents from a tube (i.e. electropolishing and crimping). The researchers at IBiTech use nano/micro-CT imaging to obtain the stent’s actual geometry. It is in this stage of the research that the Mimics Innovation Suite proves most helpful: its powerful segmentation and 3D reconstruction tools make it easy to transform the nano/micro-CT images into a very accurate 3D model of the stent. This is essential to gain a profound insight in stent expansion and the interaction with the balloon and the blood vessels.
 
Meshed (a) and remeshed (b) stent segment
Its accuracy and user-friendliness aside, one of the major advantages of using the Mimics Innovation Suite for such segmentation is the possibility of combining its automatic and manual features to touch up the mask. The user can, for instance, create a separation where components touch each other or select only a specific section of the mask. From these images one can also reconstruct the balloon and its fold pattern in order to incorporate it into numerical models. After segmentation and 3D reconstruction, the researchers wanted to perform a precise FE analysis. In order to do so, they first needed to optimize the 3D model’s mesh by means of the Mimics Innovation Suite’s highly automated remesh technology. It is also instrumental in providing all necessary dimensions of the 3D structure to create approximate parametric models with IBitech’s in-house developed software pyFormex.
 

Gaining Insight in the Stent’s Properties

Virtual expansion of a three-folded balloon that was scanned with micro-CT
Next, the IBiTech team was ready to analyze the stent’s characteristics. One of the most important properties of a stent is expansion, which can be modeled accurately with FEA by applying appropriate loading conditions and material properties. The virtual expansion obtained in FEA can be verified in great detail by comparing it to 3D reconstructions of nano/micro-CT imaging of stent expansion. IBiTech also uses FEA to analyze other stent properties, including flexibility, radial strength and to optimize balloon length and folding pattern.
 
 
Comparing experimental (left) with virtual (right) stent expansion
Combining the acquired stent geometry with patient-specific artery data from angio-CT makes it possible to obtain a better insight into the interaction between the stent and an occluded vessel. After isolating a mask from the target lesion in the Mimics Innovation Suite, a wall-thickness can be applied to the model in order to evaluate the outcome of a possible stenting procedure. The effect of stent implantation on blood flow can also be analyzed with CFD.
 
Virtual stent placement in a narrowed artery
Virtual tests are increasingly important for the design and development of new and the optimization of existing devices. Currently, CFD and FEA are predominantly used as research tools. In the future, these techniques might lead to image-based, patient-specific analysis and diagnosis, as well as subsequent pre-operative planning of stent choice and placement. Mimics helped the researchers to gain an accurate insight in stent expansion and the interaction with the balloon and the blood vessels. The researchers at IBiTech believe that the Mimics Innovation Suite will also play a significant role in the future development of location-specific stenting. This procedure would take into account pressure, technique, stent type and shape when optimizing the blood flow for an individual patient’s lesion, thus improving this person’s treatment.
CFD analysis of blood flow through a stented artery
 
 

 

 
 
 
 
The standard in 'Engineering on Anatomy'

The Mimics Innovation Suite turns 3D image data into high quality digital models in an accurate and efficient way. Starting from CT, MRI or 3D Ultrasound images, the Mimics Innovation Suite offers the most advanced image segmentation, the broadest anatomical measurement options, powerful CAD tools for Engineering on Anatomy and 3D Printing, and accurate model preparation for FEA and CFD.

The authors used the Mimics Innovation Suite to study and publish virtual stent deployment using following steps:

  • Transform micro-CT data into 3D reconstruction of a real-life stent
  • Remesh the stent structure to a high quality surface mesh
  • Export and add material properties and loading conditions to prepare for FEA solving
  • Combine the stent FEA model with patient-specific anatomy from medical image data for virtual implantation

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