Resumen de Estudi i desenvolupament d'un nou sistema d'inspecció de stents basat en tècniques de metrologia òptica
Stent quality control is a critica! process. Coronary stents have to be inspected 100% so no defective stent is implanted into a human body. We have developed an optical non-contact measurement instrument that provides stent inspection in an automated and high speed approach, delivering the user all the necessary information to assess whether a stent is valid or needs to be rejected.
As regads to the instrument design, we have developed and built an optical sensor head that obtains high resolution color images of the stent surfaces at high speed. Acquired images contain not only contour information of the struts, but also depict the edges roundnesses caused by the surface treatment. This has been achieved by the integration of a triple illumination system in combination with a high precision rotational stage and an area sean camera which also acquires in line-scan mode. As a result, a patent filed by Sensofar Medical, S.L. protects this invention, which is also being commercialized by the same com pany. Regarding to the rotational system, different driving configurations have been analyzed, and the motion errors that they produce to the images have been studied. Taking this into account, manufacturing tolerances have been established accordingly to guarantee high quality image acquisition.
As regards to image acquisition and analysis, we have studied and characterized the difference between the image obtained with a conventional, bright field microscope and the proposed system. Moreover, we have introduced a new method to measure sidewalls critica! dimensions, consisting of taking the same, line-scanned images at a certain observation angle. Together with a correction model, 1 micron precision measurements are obtained. To be able to process the measurements in an automated approach, we have developed critica! dimension measurement algorithms for the four stent surfaces: abluminals (inner and outer) and luminal (sidewalls). Additionally, we have studied, developed and implemented defect detection algorithms controlled by a sensitivity parameter adjusted by the user. Oefects are also classified in a supervised classifier approach by means of a previous training with a defects databas e.
The instrument has also been enabled with tridimensional measurement capabilities, based on coherence scanning interferometry to obtain the topography and roughness of any surface. This technique is also used to characterize defects in 30 in order to reduce the stent rejection rate and thus increasing the process efficiency, as the height of the candidates which maybe are not defects can be also as ses sed. Besides, this technique can also measure the transparent film thicknesses of drug eluting stents. In this case, due to the cylindrical shape of the samples, measured thicknesses are altered by the local slope at the measuring point, so we have proposed and evaluated two correction models that provide the thickness 30 map corrected automatically.
All the capabilities described provide a number of advantages in the stent production and quality control stages. Specifically, objectivity and traceability of these proces ses are s ignificantly increased, making stents every day more safe devices, reducing the restenosis rate, cardiac accidents or ilnesses resulting from an implant malfunction.
