Cases

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 to put it over a balloon catheter (see figure 1). 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.

The focus of IBiTech’s research on stents centers on the fluid and structural mechanical processes caused by 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 different from the original CAD design from 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/microCT images to obtain the stent’s actual crimped geometry. It is in this stage of the research that Mimics proves most helpful. Mimics’ powerful segmentation and 3D reconstruction tools make it possible to transform the nano/microCT images into easily into a very accurate 3D model of the stent (see figure 2).

Mimics’ accuracy and user-friendliness aside, one of the major advantages of using it 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 (see figure 3).

After segmentation and 3D reconstruction, the researchers wanted to perform a precise FEA analysis. In order to do so, they first optimized the 3D model’s mesh by means of Mimics’ highly automated remesh technology (see figure 4). Mimics was also instrumental in providing all necessary dimensions of the 3D structure to create approximate parametric models with in house developed software (pyFormex).

Next the IBiTech team was ready to analyze the stents’ 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 experimental nano/microCT analysis of stent expansion (see figure 5 and 6). IBiTech also used FEA to analyze other stent properties, including flexibility, radial strength and the optimization of balloon length and folding pattern.

Patient-specific data from angio-CT makes it possible to obtain an even more accurate idea about the interaction between the stent and the occluded vessel. After isolating a mask from the target lesion in Mimics, a uniform wall-thickness can be applied to the model in order to evaluate the outcome of a possible procedure (see figure 7). With MR images it is even possible to assign variable thickness and different material properties, based on the grayscale. The effect of stent implantation on blood flow also can be analyzed with CFD (see figure 8).

Figure 7: Virtual stent placement in a narrowed artery that was scanned with a medical scanner

Virtual tests are increasingly important for the design and development of new devices. Currently CFD and FEA are predominantly 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 Mimics 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 the patient’s treatment.