WO2012119767A1

WO2012119767A1 - High-frequency resonant stents for non-invasive restenosis monitoring

Info

Publication number: WO2012119767A1
Application number: PCT/EP2012/001012
Authority: WO
Inventors: Frank Daschner; Carsten RICKERS; Reinhard Knöchel; Michael Jerosch-Herold
Original assignee: Christian-Albrechts-Universität Zu Kiel; Universitätsklinikum Schleswig-Holstein
Priority date: 2011-03-07
Filing date: 2012-03-07
Publication date: 2012-09-13
Also published as: DE102011013308A1; DE102011013308B4

Abstract

The invention relates to a high-frequency resonant stent, having at least one modulation element designed as a HF energy store, integrated in the stent wire structure and engaging in the HF resonator, wherein said modulation element changes the energy storage capability thereof upon charging with a magnetic field. The invention further relates to a method for evaluating a stenosis in such a stent.

Description

High frequency resonant stents for non-invasive restenosis monitoring

The invention relates to radiofrequency (RF) resonant stents for implantation in living tissue, especially in vessels for repairing a vasoconstriction or vascular occlusion. The invention also relates to a method of measuring data indicative of in-stent restenosis by using the stents as RF-resonant electromagnetic radiation scatterers.

Stents are tubes made of flexible wire mesh that help repair a vasoconstriction

(Stenosis) or a closure are introduced by means of a catheter in the vessels.

They are pressed by balloon dilatation to the vessel wall and stabilize it.

In 20-40% of cases, restenosis even occurs in the vascular support after some time, so it is essential to have a check-up to see if re-intervention is required. But even the re-examination for restenosis requires so far mainly invasive methods.

Non-invasive studies using magnetic resonance tomography (CT) or magnetic resonance imaging (also: magnetic resonance imaging, MRI) have not yet been able to solve the problem of restenosis detection satisfactorily.

For example, from the work of Kivelitz D, Wagner S, Hansel J, Schnorr J, Wetzler R, Busch M, Melzer A, Taupitz M, Hamm B. "The active magnetic resonance imaging stent (AMRIS): initial experimental in vivo results with locally Amplified MR angiography and flow measurements. "Invest Radiol 2001; 36: 625-631 or Melzer, A. et al.," Signal Enhancement of Stents in Magnetic

Resonance Imaging ", Proc. Soc. Mag. Reson. Med., 8 (2000), 1317, metallic stents are known that are shaped by special shaping as inductors and with an additional capacitor to resonant circuits with resonance frequencies at the MRT Larmorfrequenz (eg This makes it possible in principle during MRT by a

Antenna function of the resonant circuit to transmit MRI signals from the lumen of the stent to the outside and thus to detect an in-stent restenosis.

This technique has not yet been put into practical use, partly because it requires an expensive MR tomograph. For a non-invasive examination, inductive coupling of an external MRI must also be possible Coil with the stent exist. However, this is relatively ineffective here, since the inductance of the stent is already small due to its dimensions. Therefore, stents as MRT antennas have so far only been successfully used for direct coupling via a catheter guidewire (see Raman VK, Karmarkar PV, Guttman MA, Dick AJ, Peters DC, Ozturk C, Pessanha BS, Thompson RB, Raval AN, DeSilva R, Aviles RJ, Atalar E, McVeigh ER, Lederman RJ. "Real-time magnetic resonance-guided endovascular repair of experimental abdominal aortic aneurysm in swine." J Am Coli Cardiol. 2005 Jun 21; 45 (12): 2069-77).

In the documents US 2003/100815 AI and WO 03/045236 A2 a stent is considered as a circular waveguide and resonated at microwave frequencies. As a result, the ratio of operating wavelength to stent dimensions is considerably cheaper. Common stent diameters lead to operating frequencies at approx. 1 1 GHz. The scattering behavior of the stent with electromagnetic

Microwave irradiation should read the resonance frequency of the stent. In the course of restenosis in the interior of the stent, its resonance frequency changes, whereupon the stenosis detection is based.

However, there are problems with the identification of the stent scattered signal by the so-called background signal, ie parasitic reflections from the surrounding tissue, which cover the resonance of the implanted stent.

As a remedy, it is proposed to install a semiconductor diode as a frequency multiplier in the stent. This should make it possible to detect a higher harmonic of the original frequency, which should lead to a better signal / interference ratio.

WO 03/045236 A2, in turn, teaches that the irradiated microwave is to be modulated with a signal whose frequency can put the stent into an acoustic resonance. Thus, high frequency energy would be converted into acoustic vibrations (ultrasound), which would then be e.g. catch it with a microphone. From the transmission behavior should then be concluded on the restenosis.

Although the above work describes fundamental possibilities, it has not yet come to a practical practical realization.

In addition, microwave radiation significantly above 1 GHz is increasingly absorbed by living tissue (in the first place water) with increasing frequency, ie the penetration depth of the radiation decreases. For this reason, irradiations in the HF to VHF frequency range (in this case between approximately 100 and 3000 MHz) are more suitable for effecting an easily measurable response to an implanted stent. WO 2010/019773 A1 pursues this approach by irradiating RF radiation ("radio frequency") onto a stent in a blood vessel, for which, inter alia, the cooldown of an electrical oscillation after the end of the irradiation is measured to determine a stenosis As the amount of iron hemoglobin in the vessel increases, the coagulation time increases, and the stent is also seen as an RF resonator in this work whose quality and resonance frequency can be influenced by deposits in the stent.

The US patent application US 2005/0267569 Al describes a device and a method for the non-invasive detection of in-stent restenosis, in which the stent comprises a sensor made of a magnetoelastic material which, when applied magnetic field as a function of the surface load its resonant frequency changes.

The diploma thesis of Hoffmann "Development of a Resonant Stent for the Wireless, Non-Invasive Measurement of Stenosis" (University of Kiel, Germany, 2009, pp. 105-106) summarizes the following:

• Arteriosclerotic plaques can be recognized by a resonant stent based on its resonant frequency because they act as a dielectric in the resonant circuit.

• Plaque is all the more noticeable in a resonance frequency shift, the greater the proportion of the E-field, which runs in the longitudinal direction of the stent.

• Immediately after the implantation of the stent, a reference measurement is necessary, as it does not

avoidable variances in the geometry of the stent after implantation already lead to an altered resonance frequency.

• Furthermore, an electrical insulation of the resonator wires is required, otherwise the ohmic losses attenuate the resonator so much that a resonant behavior is no longer recognizable.

The non-contact measurement of the resonant frequencies of RF resonators by detecting the electromagnetic scattering signature is state of the art in itself and seems adequate to detect any RF resonant implants in the living body without burdening the patient and with comparatively inexpensive measuring devices. In the case of the stent-graft, the resonant frequency carries state information indicating the presence of restenosis. However, their precise detection is severely hampered by the background signal inevitably caused by HF scattering in the living organism. The object of the invention is to propose an RF-resonant stent, which provides a well-identifiable scatter signal in the implanted state, which is easy to distinguish from the background signal.

The object is achieved by a stent having the features of claim 1. The independent claims 8 and 12 are based on a device and a method for detecting a

Resonant frequency of such a stent directed. The dependent claims give advantageous

Embodiments.

An inventive high-frequency resonant stent comprises at least one in the

Stent wire structure integrated and engaging in the stent, designed as a high-frequency energy storage modulation element, which upon application of a magnetic field its

Energy storage capacity changed.

In a preferred embodiment, the stent is designed as a slot resonator, and the at least one modulation element is arranged in the interior of the slot.

Preferably, the at least one modulation element is fixed with a biocompatible adhesive to at least one wire of the stent.

In a preferred embodiment, the at least one modulation element comprises a capacitance diode and an element acting on the thickness of the barrier layer of the capacitance diode by an electrical voltage whose output voltage depends on the applied magnetic field.

The element may in particular be an induction coil. The element may also be a magnetoelectric element.

In a preferred embodiment, the at least one modulation element comprises a YIG sphere.

The invention is also directed to a device for detecting a resonant frequency of a stent comprising: means for applying at least the stent with a time-varying magnetic field having a predetermined magnetic field frequency between 100 kHz and several MHz, preferably between 100 kHz and 20 MHz, and more preferably between 100 kHz and 10 MHz; Means for irradiating and sweeping electromagnetic radiation having a frequency between 100 MHz and 3 GHz in the area exposed to the magnetic field; Funds for Detecting the scattered radiation backscattered from the irradiation region in response to the irradiation and wobble of electromagnetic radiation or the radiation unscrupulously passing the irradiation region as a frequency-dependent scattering signal; Means for identifying that portion of the leakage signal modulated at the magnetic field frequency; Means for determining the resonant frequency of the stent by demodulating the magnetic field frequency modulated leakage signal component; and means for determining a difference of the determined resonant frequency to a predetermined resonant frequency, in particular a stored resonant frequency.

In a preferred embodiment, the means for acting on the stent with the time-varying magnetic field are adapted to the loading of the stent

To change the energy storage capacity of an engaging in the stent, designed as a high-frequency energy storage modulation element.

In a further development, the device is set up to have a channel volume and / or a channel diameter of the stent and / or a layer thickness of a stent formed on the stent

Plaque layer from the difference of the determined resonant frequency to the predetermined

To determine resonance frequency.

Preferably, the device is set up, at least partially from the difference of the determined resonant frequency to the predetermined resonant frequency, a value for a

Channel volume and / or a channel diameter of the stent and / or a value for a

To extrapolate layer thickness of a plaque layer formed on the stent for a selected future time.

The device according to the invention and its developments can serve in particular for the automatic evaluation of a restenosis of an implanted stent. The difference of the determined

Resonance frequency to the predetermined resonant frequency can thereby draw conclusions about a degree of occlusion of the stent or on the still available for the flow cross-section of the stent canal. The device according to the invention can enable an automatic determination of the difference of the determined resonance frequency or the available channel volume and / or channel diameter of the stent or the layer thickness of a plaque layer formed on the stent without the direct participation of medical personnel. Likewise, the device according to the invention can automatically and on the basis of the recorded measurement data and their temporal variations, without a direct involvement medical personnel a value for a channel volume and / or a channel diameter of the stent and / or a value for a layer thickness of a formed on the stent plaque layer for a extrapolate selected future time. For predetermined future times, the device provides the In this way, medical personnel have predicted values for the channel volume of the stent or for the diameter and / or the volume of a formed plaque layer.

In this aspect, therefore, the invention relates to an apparatus for evaluating restenosis of a stent implanted in a living organism, comprising: means for exposing at least the implanted stent-containing region in the living organism to a time-varying magnetic field having a predetermined magnetic field frequency between 100 kHz and a few MHz, preferably between 100 kHz and 20 MHz, and more preferably between 100 kHz and 10 MHz; Means for irradiating and sweeping electromagnetic radiation having a frequency between 100 MHz and 3 GHz in the area exposed to the magnetic field; Means for detecting the scattered radiation backscattered from the irradiation region in response to the irradiation and wobble of electromagnetic radiation or the

Irradiation range unscattered radiation as a frequency-dependent leakage signal; Means for identifying that portion of the leakage signal modulated at the magnetic field frequency; Means for determining the resonant frequency of the stent by demodulating the with the

Magnetic field frequency modulated scattered signal component; and means for inferring the amount of restenosis from the difference in the detected resonant frequency to a resonant frequency determined and stored after implantation of the stent in the absence of restenosis.

The means for applying the region containing the implanted stent to the time-varying magnetic field are preferably configured to change the energy storage capability of a modulation element engaging in the stent and designed as a high-frequency energy store when the stent is acted upon.

To determine resonance frequency.

In a preferred embodiment, the device is set up to extrapolate a value for a channel volume and / or a channel diameter of the stent and / or a value for a layer thickness of a plaque layer formed on the stent for a selected future point in time at least in part from the difference of the determined resonance frequency ,

The invention also relates to a method for detecting a resonant frequency of a stent comprising the following steps: applying at least the stent with a time-varying magnetic field having a predetermined magnetic field frequency between 100 kHz and a few MHz, preferably between 100 kHz and 20 MHz and particularly preferably between 100 kHz and 10 MHz; Irradiation and wobble of electromagnetic radiation having a frequency between 100 MHz and 3 GHz in a region exposed to the magnetic field; Detecting the scattered radiation backscattered from the irradiation region in response to the irradiation and wobble of electromagnetic radiation or the radiation unscrupulously passing the irradiation region as a frequency-dependent scattering signal; Identifying that portion of the leakage signal associated with the

Magnetic field frequency is modulated; Determining the resonant frequency of the stent by demodulating the magnetic field frequency modulated leakage signal component; and determining a difference of the determined resonant frequency to a predetermined resonant frequency, in particular to a stored resonant frequency.

In a preferred embodiment, the method comprises the additional step of changing the energy storage capability of a modulating element engaging in the stent and designed as a high-frequency energy store.

In a development, the method comprises the step of determining a channel volume and / or channel diameter of the stent and / or a layer thickness of a plaque layer formed on the stent from the difference between the determined resonance frequency and the predetermined one

Resonance frequency.

Preferably, the determining comprises comparing the difference of the determined ones

Resonant frequency to the predetermined resonant frequency with a record of empirically determined and / or modeled data.

In a preferred embodiment, the method comprises a step of extrapolating a value for a channel volume and / or a channel diameter of the stent and / or a value for a layer thickness of a plaque layer formed on the stent for a selected future time at least partly from the difference of the determined resonance frequency.

A stent is a structure of metallic wires in which electrical currents can be excited by electromagnetic fields. The stent wire structure has not directly contacted with each other, at least piecewise parallel wires as well as - not always closed - wire loops on. In this respect, a stent can store electric and magnetic field energy to a certain extent and convert it into one another like an electrical resonant circuit. A stent is therefore a resonant circuit with "distributed" E and B field energy storage

In contrast, circuits of electrical engineering usually contain "concentrated" energy stores in the form of capacitive and inductive components. With regard to its resonant properties, the high-frequency resonant stent is also referred to below as the "stent cavity".

According to the invention, a concentrated HF energy store is additionally arranged on the stent, whose capacitance or inductance-and thus the energy storage capacity-can be changed by the application of a magnetic field. The concentrated energy storage is right in the

Stent wire structure integrated and forms either a part of the Stentresonators or is at least inductively or capacitively coupled with this. In any case, the concentrated RF energy storage to engage in the Stentresonator, i. change the resonance frequency of the stent under the influence of a magnetic field.

Even in the implanted state of the additional energy storage of the stent is magnetically controlled, since living tissue is magnetically homogeneous.

If the change in the resonant frequency of the stent at a predetermined frequency (preferably 100 kHz to several MHz) of the externally applied magnetic field, results in a frequency-modulated with simultaneous irradiation of the stent with a frequency in the range of its resonant frequency (preferably 100 MHz to 3 GHz) Scattering signal of the stent according to the invention. It then has a frequency modulation with a known modulation frequency which is not inherent in the background signal.

The person skilled in the art of high-frequency transmission technology knows how to isolate a frequency-modulated signal from a background when the modulation frequency is known to it. The person skilled in the art is also familiar with how to demodulate the frequency-modulated signal in order to obtain the frequency of the carrier wave. This carrier wave frequency is the sought resonance frequency of the stent. It may be evaluated by the physician for possible in-stent restenosis.

For proper interpretation of the resonant frequency, it is necessary to know a comparative value of the resonant frequency of the stent without plaques. Since each stent is widened during implantation and / or otherwise deformed, the comparison value can only be obtained in the already implanted state. There should be a reference measurement of the stent shortly after implantation

at a time when it is safe to rule out the presence of plaques.

The stent's own concentrated energy storage device according to the invention is also referred to below as a modulation element. Such elements for detuning or adjustment of

Resonant circuit frequencies are known as such. The periodic shifting of

Resonant frequencies by means of modulation elements is referred to as effect modulation. The use of modulation elements in RF resonant stents has never been suggested. Essential for the purpose of the invention here is that the control of the modulation elements must be possible via magnetic fields, which easily penetrate into the living body and are harmless to him. The modulation element may be made of any material that also changes its magnetic field in a magnetic field with variation of the magnetic field strength

High-frequency impedance shows. Furthermore, modulation elements can be used which are primarily to be driven by an electrical voltage and therefore for the purpose of the invention also comprise an induction coil or a magnetoelectric (ME) element.

Of course, the stent may not be compromised by the modulation element in its main function as a vascular support. And it is particularly desirable that the stent according to the invention, like any other, be handled by the physician, in particular by implantation. The modulation element is therefore preferably integrated into the stent structure, particularly preferably glued in with a biocompatible adhesive. Suitable and sufficiently small modulation elements that can be easily integrated into a stent structure are already commercially available.

It is impossible to present every imaginable embodiment of the invention in detail. Therefore, two exemplary embodiments will be explained in more detail below, which are to be regarded as particularly advantageous. This explanation is also based on figures. Showing:

1 shows a sketch of an HF-resonant stent with dumbbell slot and an equivalent circuit diagram;

Fig. 2 alternatives to the dumbbell slot resonator;

Fig. 3 stent of Figure 1 with inductively coupled modulation element (YIG ball).

FIG. 4 stent from FIG. 1 with capacitively coupled modulation element (varactor diode).

An ordinary stent can be seen, in a first approximation, as a metallic cylinder, through the mantle of which currents can flow. To a resonator for from outside - i. a. on his coat - irradiated electromagnetic radiation of defined wavelength he will u. a. when in its lateral surface standing electrical and magnetic waves can form with at least half of the irradiated wavelength. This is particularly possible if a slot of appropriate length is introduced into the lateral surface, for example by removing a

Metal strip from the cylinder. The residual cladding then behaves like a closed two-wire line resonator. He is also often referred to as a slot resonator. On the exact shape of the slot it does not matter in principle. Rather, it is important that the forming in the region of the slot E- and B-fields also extend into the interior of the stent and there is thus an interaction with any existing there dielectrics. This is a prerequisite for the sensitivity of the resonator properties with respect to existing plaques or cell layers.

In Fig. 1, a real stent is shown, on the upper (or mantle) surface a dumbbell-shaped surface is marked. To realize the RF resonator function of the stent, provision must now be made to ensure that no currents from the remaining stent structure can flow into the interior of the marked area and vice versa. The marked area is therefore to be separated galvanically from the rest of the stent.

In the simplest embodiment, one could separate the marked area from the stent, but this is likely to affect the mechanical stability of the stent and thus its function. A better alternative is to replace the metallic material in the marked area with a non-conductive material, e.g. Plastic. However, it may already be sufficient to sever the stent structure only in the area of the border of the marked area and after the

Reconnect introducing an insulating material, so that the flow of current is prevented by a barrier material. However, the details of the production of a known RF resonant stent are not the subject of the present invention.

The RF resonant stent may be assigned an equivalent circuit diagram, which is shown in Fig. 1 below. Shown are only inductors and capacitors. Ohmic losses are of course present, but are ignored here. The largest contributions to the capacitance of the stent oscillator circuit are to be seen where the opposite boundaries of the marked area come closest. As can be seen, the dumbbell configuration results in that preferably electric fields are formed along the stent axis, which is favorable for the sensitivity of the plaque detection.

Fig. 2 shows alternative ways of slot design as a meander or as one

Stent axis circumferential spiral. Also for these could be - quite complicated - specify equivalent circuit diagrams. They illustrate the variety of possibilities for the exact

Design of a stent as a slot resonator.

FIG. 3 shows a stent according to the invention, in which a modulation element which can be activated directly by magnetization has been inserted into a wire loop of the stent. This is a small (diameter about 0.1 mm) ball of yttrium iron garnet ("yttrium-iron-garnet", YIG), which itself is a high frequency resonator in the gigahertz range. The resonance frequency of the YIG sphere is largely linear with an external magnetic field and a large one

Frequency range adjustable. The placement in a conductor loop (inductance) of one side of the commercial slot line serves for the inductive coupling between the YIG resonator and the

Slot resonator that forms the stent. Modulating the YIG resonance thus also alters the resonant frequency of the stent. In the lower part of Fig. 3, the equivalent circuit diagram is drawn with the YIG sphere as a variable inductance.

Another embodiment of the stent according to the invention uses at least one varactor diode (also: capacitance diode) and can be seen in FIG. The varactor diode is reverse biased and the thickness of the barrier depends on the applied reverse voltage. The varactor diode is thus used here as a tunable capacitance. In FIG. 4, two varactor diodes are arranged in the stent such that they form the largest part of the capacitance of the slot resonator. Any change in tunable capacitance alters the resonant frequency of the stent. To periodically change the capacitance requires a periodic voltage signal which must be produced by the modulation magnetic field.

For this purpose, a compact, magneto-electric (ME) element is preferably used which translates magnetic fields into voltage signals. A ME element in its simplest form consists of a multilayer system comprising a magnetostrictive layer which responds to a magnetic field along the layer with a change in length, a piezoelectric layer which is bent as a result of the mechanical coupling with the magnetostrictive layer and generates a voltage and a Metal layer (electrode) for picking up the generated voltage. ME elements are common magnetic field sensors.

Alternatively, an induction coil is also considered, in which the variable magnetic field induces the voltage. This can be arranged on the edge of the stent. However, it is also conceivable that the induction coil itself is designed as a vessel-supporting part of the stent.

The tunable inductance (here: YIG sphere) or capacitance (here: varactor diode + ME element or coil) are suitable modulation elements (or concentrated RF energy stores) that influence the RF resonant properties of the stent without significantly disturbing its mechanical properties , For example, they can be inserted relatively easily into a per se known RF-resonant stent structure using biocompatible adhesives approved for medical applications. If these small components are only glued to a single wire of the stent wire mesh, the stent remains as usual flexible and in particular expandable. The RF resonance of the modified stent is now well identifiable under the influence of a controlled modulation magnetic field. The resonance signal (eg backscattered RF radiation or non-scattered radiation in a transmission absorption measurement) can be obtained from

Background signal with known methods of transmission technology as a frequency modulated signal to be separated. The carrier wave of the resonance signal is the actual resonant frequency of the stent at the time of the measurement, which allows conclusions to be drawn on dielectric deposits in the stent interior, in particular on a stenosis, if a reference measurement of the same stent exists in the absence of such deposits for comparison.

The device according to the invention may automatically comprise a comparison of the measured displacement of the stent resonance between the time of the current measurement and an earlier time, for example a reference measurement shortly after the implantation, with empirical or modeled data. The device can then output, for example, the thickness or the volume of a plaque layer formed on the stent and / or the channel volume still available for the flow or the still available channel cross-section of the stent.

Preferably, a gradient and / or an extrapolation value can also be determined and output for a predetermined future time.

Claims

claims

1. high-frequency resonant stent, characterized by at least one in the

Stent wire structure integrated and engaging in the Stentresonator, designed as an RF energy storage modulation element that changes its energy storage capability when exposed to a magnetic field.

2. Stent according to claim 1, characterized in that the stent as a slot resonator

formed and the at least one modulation element is arranged in the interior of the slot.

3. Stent according to one of the preceding claims, characterized in that the

at least one modulation element is fixed with a biocompatible adhesive fixed to at least one wire of the stent.

4. Stent according to one of the preceding claims, characterized in that the

at least one modulation element has a capacitance diode and a thickness of the

Barrier layer of the capacitance diode by an electrical voltage acting element comprises, whose output voltage depends on the applied magnetic field.

5. Stent according to claim 4, characterized in that the element is an induction coil.

6. Stent according to claim 4, characterized in that the element is a magnetoelectric element.

7. Stent according to one of claims 1 to 3, characterized in that the at least one modulation element comprises a YIG ball.

8. Device for detecting a resonance frequency of a stent with:

a. Means for acting on the stent with a time-varying magnetic field having a predetermined magnetic field frequency between 100 kHz and a few MHz; b. Means for irradiation and Wobbein electromagnetic radiation with a

Frequency between 100 MHz and 3 GHz in the area applied with the magnetic field; c. Means for detecting the scattered radiation backscattered from the irradiation area in response to the irradiation and wobble of electromagnetic radiation or the unscratched radiation passing the irradiation area as a frequency-dependent scattering signal;

d. Means for identifying that portion of the leakage signal associated with the

Magnetic field frequency is modulated;

e. Means for determining the resonant frequency of the stent by demodulating the magnetic field frequency modulated leakage signal component; and

f. Means for determining a difference of the determined resonant frequency to a stored resonant frequency.

9. Apparatus according to claim 8, wherein the means for acting on the stent with the time-varying magnetic field are adapted to change the energy storage capability of engaging in the stent, designed as an RF energy storage modulation element in the application of the stent.

10. Apparatus according to claim 8 or 9, which is adapted to a channel volume

and / or to determine a channel diameter of the stent and / or a layer thickness of a plaque layer formed on the stent from the difference between the determined resonant frequency and the stored resonant frequency.

1 1. Device according to one of claims 8 to 10, which is adapted, at least in part from the difference of the determined resonance frequency, a value for a channel volume and / or a channel diameter of the stent and / or a value for a layer thickness of one on the stent extrapolate trained plaque layer for a chosen future date.

12. A method for detecting a resonance frequency of a stent comprising the following steps:

a. Impinging the stent with a time-varying magnetic field at a predetermined magnetic field frequency between 100 kHz and a few MHz; b. Irradiation and wobble of electromagnetic radiation having a frequency between 100 MHz and 3 GHz in a region exposed to the magnetic field; c. Detecting the scattered radiation backscattered from the irradiation region in response to the irradiation and wobble of electromagnetic radiation or the radiation unscrupulously passing the irradiation region as a frequency-dependent scattering signal;

d. Identify that portion of the leakage signal that coincides with the magnetic field frequency

is modulated; e. Determining the resonant frequency of the stent by demodulating the magnetic field frequency modulated leakage signal component; and

f. Determining a difference of the determined resonant frequency to a

stored resonance frequency.

13. The method of claim 10 with the additional step of changing the

Energy storage capability of engaging in the stent, designed as an RF energy storage modulation element.

14. The method of claim 12, further comprising the step of determining a channel volume and / or channel diameter of the stent and / or a layer thickness of a plaque layer formed on the stent from the difference between the determined resonant frequency and the stored resonant frequency; Difference of the determined resonant frequency with a record of empirically determined and / or modeled data includes.

15. The method of claim 12, further comprising the step of extrapolating a value for a channel volume and / or a channel diameter of the stent and / or a value for a layer thickness of a plaque layer formed on the stent for a selected future time, at least in part the difference of the determined resonance frequency.