Determination of the restenosis degree inside the implanted stent with integrated wireles pulse wave velocity sensor

Patients with implanted stents were often (approx. 30 %) faced with the restenosis. By now no alternative to clinical examination is known to get a quick diagnosis for the actual state of possible and probable in-stentrestenosis. At Fraunhofer IPA in Stuttgart a simple method to measure and to determine the restenosis degree was invented and implemented. The Proof of principle was conducted on the experimental rig on an artery model. The approach is based on an inductive coupling between the external detection unit and implanted sensors. Two passive sensors were integrated in a stent and consist of a capacitive pressure sensor and an air-coil. Connected they form an oscillating circuit, the resonance frequency of which functionally depends on the local pressure. The extra-corporal detection unit generates an alternating magnetic field by 35 MHz. The spreading pulse wave changes the resonance frequency of the passive oscillating circuits inside the vessel. The short resonance inside the sensor circuit crosses the frequency of the externally applied field and shifts the impedance measured at the excitation coil. As the distance between the two sensors is known the PWV can be determined. The stiffer or the narrower a vessel is the higher is the PWV. A model based approach determines out of the PWV signal the restenosis degree inside the implanted stent. All further measurements are referenced to the first initial value done after the stent implantation. This approach is robust and has mean cross dependences because no absolute pressure measurement is required