In-vitro imaging of femoral artery nitinol stents for deformation analysis.

PURPOSE: Femoral artery stents are prone to fracture, and studying their deformations could lead to a better understanding of the cause of breakage. The present study sought to develop a method of imaging and analyzing stent deformation in vitro with use of a calibrated test device. MATERIALS AND METHODS: High-resolution (approximately 200 µm) volumetric data were obtained with a flat-panel detector-based C-arm computed tomography system. A nitinol stent placed in a testing device was imaged with various loads that caused bending, axial tension, and torsion. Semiautomatic software was developed to calculate the bending, extension, and torsion from the stent images by measuring the changes in the radius of curvature, eccentricity, and angular distortions. RESULTS: For the axial tension case, there was generally good agreement between the physical measurements and the image-based measurements. The bending measurements had better agreement at bend angles lower than 30°. For stent torsion, the hysteresis between the loading and unloading curves were larger for the image-based results compared with physical measurements. CONCLUSIONS: An imaging and analysis framework has been set up for the analysis of stent deformations that shows fairly good agreement between physical and image-based measurements.

In-vivo imaging of femoral artery nitinol stents for deformation analysis.

PURPOSE: The authors have developed a direct method to study femoral artery stent deformations in vivo. A previously described imaging and analysis approach based on a calibrated phantom was used to examine stents in human volunteers treated for atherosclerotic disease. In this pilot study, forces on stents were evaluated under different in-vivo flexion conditions. MATERIALS AND METHODS: The optimized imaging protocol for imaging with a C-arm computed tomography system was first verified in an in-vivo porcine stent model. Human data were obtained by imaging 13 consenting volunteers with stents in femoral vessels. The affected leg was imaged in straight and bent positions to observe stent deformations. Semiautomatic software was used to calculate the changes in bending, extension, and torsion on the stents for the two positions. RESULTS: For the human studies, tension and bending calculation were successful. Bending was found to compress stent lengths by 4% ± 3% (-14.2 to 1.5 mm), increase their average eccentricity by 10% ± 9% (0.12 to -0.16), and change their mean curvature by 27% ± 22% (0 to -0.005 mm(-1)). Stents with the greatest change in eccentricity and curvature were located behind the knee or in the pelvis. Torsion calculations were difficult because the stents were untethered and are symmetric. In addition, multiple locations in each stent underwent torsional deformations. CONCLUSIONS: The imaging and analysis approach developed based on calibrated in vitro measurements was extended to in-vivo data. Bending and tension forces were successfully evaluated in this pilot study.