Stent-induced arterial strain distributions are characterized using an optical surface strain measurement system. This visualization technique is used to gain insights into the impact of stent implantation on the host vessel.
Clinical trials have reported different restenosis rates for various stent designs1. It is speculated that stent-induced strain concentrations on the arterial wall lead to tissue injury, which initiates restenosis2-7. This hypothesis needs further investigations including better quantifications of non-uniform strain distribution on the artery following stent implantation. A non-contact surface strain measurement method for the stented artery is presented in this work. ARAMIS stereo optical surface strain measurement system uses two optical high speed cameras to capture the motion of each reference point, and resolve three dimensional strains over the deforming surface8,9. As a mesh stent is deployed into a latex vessel with a random contrasting pattern sprayed or drawn on its outer surface, the surface strain is recorded at every instant of the deformation. The calculated strain distributions can then be used to understand the local lesion response, validate the computational models, and formulate hypotheses for further in vivo study.
1. Preparation of the Latex Vessel
2. In vitro Test System and Calibration of ARAMIS System
3. Pretest to Avoid Excessive Background Noise
4. Stent Deployment
5. Images Analysis
6. Representative Results
The stent struts expand the vessel wall outwards, strains will generally be higher around stent location. Figure 1 is an example of strain mapping during the recoil process of balloon-expandable stent, as well as major strain history at one specific point. The black dots in Figure 1 are reference points, which were used by the high-speed cameras to capture and track the displacements of these reference points on the conduit. Based on the recorded movement of reference points, the software will then be used to calculate the strains of the conduit or any other targeted object. Major strain, also referred to as the maximum principal strain, is calculated as follows:
It is clear that the implanted stent led to non-uniform strain distribution on the vessel surface. This could be explained by the recoil loading from ends-constrained latex conduit and the mesh structure of stent. This strain field corresponds to the initial stage of stent recoil, as identified by the red cross marker in the bottom image of Figure 1. The major strain-history curve of a specific point 10 demonstrated distinguishable stages of stent implantation. The balloon expansion occurs from approximately 10 to 12 seconds and stent recoil following the deflation of the balloon occurs between 12 and 14 seconds.
Figure 1. Experimental setup (top); non-uniform strain distribution on the stented conduit surface (middle); the major strain history at the point 10 (bottom).
The stereo optical surface strain measurement system is used to measure the local strains over the deforming surface for both the in- and out-of-plane motions without contacting the specimen. This system uses two high-speed optical cameras to take pictures of a random contrasting pattern putting on the surface to construct accurate measurements of motions of each point, with a high accuracy of resolving surface strains.
It should be noted that the required contrasting pattern need adheres to the surface sufficiently enough to provide accurate measurements. In addition, the targeted sample area need be well lit, without glare, for the cameras to distinguish the movements of the contrasting pattern. Otherwise, the captured glare images will create void data regions. Two light sources, at opposite ends of the latex vessel, angled at approximately at 45 degree angles relative to the tubing is recommended. A flat spray paint rather than a gloss paint for the stochastic pattern will also help to reduce the amount of glare.
Here we present a protocol of surface strain measurements using a mocked vessel, which could be used to test the nonuniform strain mapping on the heterogeneous native vessel. Ex vivo native vessels study will be incubated in physiological solution to maintain the cellular activity. The common black inkjet stylus could be used to stain a real vasculature, which has been used on the femoral artery of rabbit by Squire et al10. This optical surface strain measurement system could then capture the movement of reference points through transparent window. Surface strain measurements using ex vivo native vessels with histological assessments of the vessels will provide more insight on the injury mechanism of stented artery. The three dimensional surface strains demonstrated in this work may also be extended to obtain the strain map anywhere in the heterogeneous test sample including its inner surface as well as across the thickness of the vessel through further numerical analysis.
The presented stereo optical surface strain measurement system is one of the very unique methods that can capture and measure the local strains observed over all the deforming surface without actually contact the specimen and with high accuracy for both the in- and out-of-plane motions of the surface. It was compared with other strain measurement systems such as intravascular ultrasound (IVUS) imaging as well as inflation test11,12. The traditional inflation test is useful for obtaining the averaged strain along the test conduit11; however it cannot provide the three dimensional local strain captured by the optical surface strain measurement system in this work. The IVUS elastography12 could obtain the two dimensional strain map throughout the cross section of the vessel, and hold great potential for clinical application. The optical system demonstrated in this work has its unique advantage by providing three dimensional surface strains and displacements on irregular surfaces, particularly those resulting from irregular shapes or inhomogeneous bodies.
The authors have nothing to disclose.
This study was supported in part by the NASA Nebraska Space Grant and National Science Foundation under grant No. 0926880.
Equipment Used | Company | Catalogue number | Comments |
ARAMIS Camera System | GOM: Optical Measuring Techniques | ||
PALMAZ Genesis TRANSHEPATIC BILIARY STENT | Cordis Corporation | PG5910B | Balloon-expandable stent |
Z-MED Balloon Dilatation Catheter | B. Braun Medical Inc. | PDZ336 | Balloon dilatation catheter |