Marrocco, G.

Occhiuzzi, Cecilia

Archivio istituzionale della ricerca - Università di Palermo

Experimental Characterization of the RFIDSTENTag for Passive Vascular MonitoringCecilia OcchiuzziDISP, University of Roma Tor Vergata,Via del Politecnico,100133, Roma (ITALY)Email: occhiuzzi@disp.uniroma2.itGaetano MarroccoDISP, University of Roma Tor Vergata,Via del Politecnico,100133, Roma (ITALY)Email: marrocco@disp.uniroma2.itAbstract—The possibility to monitor biological process inevolution and in particular the state of implanted devices suchas vascular stent, has been recently proposed as one of themost challenging and fascinating application of the RFID passivetechnology. However, several issues are still in question and needto be analyzed in order to evaluate the real medical and sensingeffectiveness of the proposed implantable radio sensor. The aimof this work is to experimentally evaluate the robustness of theRFID platform, its accuracy and its reproducibility as well as theeffectiveness of the proposed sensing approach in term of humanvariability and pathology evolution.I. INTRODUCTIONThe possibility to monitor biological process in evolutionand in particular the state of implanted devices such asvascular stent, has been recently proposed as one of themost challenging and fascinating application of the RFID pas-sive technology. Theoretical and experimental investigationsdemonstrated the feasibility of the idea by means of a ﬁrstprototype, named STENTag [1]. However, several issues arestill in question and need to be analyzed in order to evaluatethe real medical and sensing effectiveness of the proposedimplantable radio sensor.The ﬁrst topic to consider is the robustness of the RFIDsensor in terms of measurement accuracy and reproducibility.The sensing metrics in [1] are based on data directly availablefrom commercial readers and could thus suffer from poorresolution and uncertainty, potentially able to invalidate theirsensing meaning. Furthermore, being the STENTag designedto be used in non-stationary scenarios, i.e. to be interrogatedby means of mobile and personal devices, the effects of theuncertainties in the measurement set-up must be also takeninto account.The second challenging issue is the human variability, e.gthe range of possible values characterizing the body shapeand size as well as the evolution of the biological process. TheSTENTag must be able to properly sense the in-stent restenosis(ISR) for what concerns its patterns and timescale and withequal effectiveness and accuracy regardless the patient whohosts it.Here, all these critical topics are experimentally addressedby means of an articulated measurement campaign over tworealistic phantoms of the neck suitable to dynamically repro-duce a localized ISR in the carotid and the typical humanvariability. The repeatability and the accuracy of the mea-surement are investigated by the help of statistic operators.A multi-frequency analysis is ﬁnally adopted to improvethe measurement reliability and to further corroborate thereasonableness of the idea.II. MEASUREMENT CAMPAIGNAn optimized prototype of the STENTag has been designedand fabricated according to the guidelines in [1], and ex-perimented in vitro by means of equivalent liquid phantoms(Fig.1).Figure 1. Top) STENTag prototype with a details of the integration of RFIDIC and of the inductor. Bottom) Cylindrical model of the neck, the carotidand the ISR (side view and top view). The STENTag is inserted in the middleof the smaller. The ISR is modeled has a localized (shadowed) volume withvariable permittivity extending beyond the margins of the stent. Size in [mm].The neck phantom (Fig.1) is a cylinder of diameter D=9cmand height h=20cm ﬁlled by a homogeneous dielectric havingthe weighted average permittivity of all the main tissues com-posing the neck, (neck = 41.4− j14.8), while a smaller off-centered 1cm-diameter cylinder emulates the human carotidand it is ﬁlled by blood-like dielectric (vessel). The STENTagis inserted in the middle of the small cylinder, tightly ﬁttingits internal surface, within a localized volume simulatingthe diffuse proliferative ISR (one of most severe forms of6th European Conference on Antennas and Propagation (EUCAP)978-1-4577-0919-7/12/$26.00 ©2011 IEEE 3669Table IRE-STENOSIS: PROPERTIES AT 870MHZ OF THE VESSEL’S DIELECTRICVessel’s ﬁller Theoretical ComplexPermittivityMeasured ComplexPermittivity1- HealthyvesselISR = 0%1 = 61.45− j31.5 1m = 57.8− j33.02-NeointimalproliferationISR = 50%2 = 58.22− j25.3 2m = 56.17− j27.63-NeointimalproliferationISR = 100%3 = 55− j19.2 3m = 51.65− j22.74-Plaqueproliferationgrade 14 = 48.2− j17.0 4m = 46.7− j20.195-Plaqueproliferationgrade 25 = 41.4− j14.8 5m = 41.8− j17.66-Plaqueproliferationgrade 36 = 27.5− j10.95 6m = 27.5− j10.957-PlaqueproliferationISR = 100%7 = 14.13− j12.15 7m = 14.13− j12.15restenosis characterized by wide lesions (> 10mm long)extending beyond the margins of the stent [2]. Such a district,of height S=8cm, is homogeneously ﬁlled with a ﬂuid ofvariable permittivity (stent) suitable to reproduce the variousdegrees of the ISR (Tab.I), and ﬁlled/emptied by means of aproper hydraulic circuit.The equivalent dielectrics have been produced according tothe recipes in [4], e.g mixtures of water, sugar and salt. Themeasured permittivity of the liquid phantoms (achieved bymeans of a open-ended coaxial dielectric probe) , averagedover 6 samples, are indicated in Tab.I and compared withthe expected values from the recipes. All the measurementshave been performed within the UHF band (840-960 MHz)by means of a UHF Thing-Magic M5e reader, connected toa 5dB gain linear polarized patch antenna (Fig. 2) which isplaced at 20cm from the neck phantom, as in the simulations.The reader permits to control the input power by 0.5dB stepsand the receiving module performs a 8-bit digital conversionof the signals backscattered by the tag.Fig. 3 shows the curves of the measured turn-on power inthe UHF band, versus the change of permittivity of the liquidﬁlling the STENTag. The curves have similar behavior butare clearly distinguishable. The power required to turn on thechip reduces as the stenosis’s grade gets worse. A nearly 3dBdynamic range is observed at each frequency when movingfrom the healthy condition to the complete plaque occlusionof the duct.A more quantitative representation including the error anal-ysis and the comparison with the computer simulations at870MHz is given in Fig.4 for what concerns the turn-onpower P toin(Ψ). The mean value and the standard deviation (thegray region) have been calculated starting from nine measure-ments at different angular offset (within ±30°) between thereader’s antenna and the tag, so that real-life uncertainty ofFigure 2. Experimental set-up comprising the liquid phantom resemblingthe neck where the STENTag has been implanted , and the interrogation 5dBpatch antenna connected to the Thing-Magic M5e reader (not shown in theﬁgure). The darker area in the central part of the phantom corresponds to theISR. Such a volume is ﬁlled/emptied by means of a proper hydraulic circuit.Figure 3. Measured turn-on power in the UHF (World- Wide) Band for theSTENTag prototype, versus the change of the liquid permittivity ﬁlling thecarotid-like pipe.the measurement set-up are properly reproduced.The sensitivity to the tag’s status is remarkable even inthe early modiﬁcation of the liquid, corresponding to anISR = 50% (from ¯1 to ¯2) with a monotonic behavior in theentire considered permittivity range. The turn-on measurementis almost invariant with the observation angle with less than0.5dB uncertainty, mainly related to the poor resolution of thereader.In order to analyze the sensibility of the STENTag to thehuman variability, a second measurement campaign has beendevoted to a thicker neck phantom, resembling a stout subject.The results (Fig. 4) are similar to those of the previous caseexcept for a 3dB scaling since the increased size of thephantom and the deeper position of the radio-sensor producea rise of the losses and consequently of the power required toestablish the communication.The uncertainty in the measured data may be reducedby using all the frequency-domain data available from the3670Figure 4. Measured (mean and shadowed uncertainty region) and simulatedturn on power for the STENTag prototype at 870MHz The measured dataare sorted according to the real part of the considered complex permittivity.measurement and introducing an averaging operator over fre-quency, e.g the normalized scale factor:γξ =< P toin (Ψ)− P toin (Ψ0) >< P toin (Ψ0) >(1)where the “<>” brackets indicate the average over fre-quency samples and Ψ0 the initial stage of the process. Fig.5shows the averaged γpS for the two phantoms. Since γ isa relative parameter, the proﬁles of the two phantoms arealmost overlapped, demonstrating the repeatability of the sens-ing measurement regardless the human variability. Moreoverthe use of an averaging indicator sensibly reduces the dataﬂuctuation when compared to Fig.4. In the worst case theuncertainty is the 3% of the mean value, while in the earlygrade of the restenosis (ISR <50%) it drops to less than 1.5%.Figure 5. Normalized scale factor γ of the turn on power for the twophantoms, on varying the liquid composition inside the carotid-like pipe.Finally, by a linear ﬁtting of the curves in Fig. 4 and 5, it ispossible to estimate the percentage variation of all the previouspower indicators referred to the healthy state 1 (Tab.II). Inparticular, a 11% of variation of the STENTag response maybe achieved in the early restenosis (1 → 2) by monitoringTable IIMEASURED RELATIVE CHANGES IN THE STENTAG RESPONSESΔP into (%) ΔγP into(%)¯1 → ¯2 11% 6%¯1 → ¯7 56% 42%the changes of the turn-on power and more than 50% changesat the atherosclerotic restenosis (1 → 7).III. CONCLUSIONSIn vitro experimentation seems to further corroborate thepossibility to sense some inner biological processes by meansof implanted passive RFID tags. These phenomena in evolu-tion produce speciﬁc and macroscopic effects on the electro-magnetic response of the radio-sensor, which are detectableby the available low-cost technology and with limited powerbudgets (maximum radiated power less than 1W EIRP),making feasible to embed the required reader capability insidean hand-held device and even into a smart-phone.In addition to the turn-on power, the RFID interrogationprovides another independent measurement, the backscatteredpower which, if observed over the frequency, may providereally robust and effective metrics, rather immune to thespeciﬁcity of patients and to the measurement modality.Beside the promising results, this class of implanted RFIDsensor tags is however still prone to considerable improve-ments concerning the interrogation modalities and instruments.Actual commercial UHF readers are designed to item labelingand not to sensing, and therefore signiﬁcantly better perfor-mances are expected with dedicated receivers having a ﬁnerresolution in the power tuning and in the analog to digitalconverter.ACKNOWLEDGMENTWork developed under project PRIN-2008: MULTI-TAGfunded by Italian Ministry of University. The authors wish tothank prof. Lorenzo De Medici of the Radiology Departmentof San Camillo Hospital, Roma, for inspiration and discus-sions.REFERENCES[1] Occhiuzzi, C.; Contri, G.; Marrocco, G.; , "RFID STENTagfor passive vascular monitoring," Antennas and Propagation(EUCAP), Proceedings of the 5th European Conference on ,vol., no., pp.3476-3478, 11-15 April 2011[2] Brajesh K. Lal, Elias A. Kaperonis, Salvador Cuadra, In-dravadan Kapadia, Robert W. Hobson II, “Patterns of in-stentrestenosis after carotid artery stenting: Classiﬁcation and impli-cations for long-term outcome”, Journal of Vascular Surgery,Volume 46, Issue 5, November 2007, Pages 833-840[3] C. Gabriel, S. Gabriel, “Compilation of theDielectric Properties of Body Tissues at RF andMicrowave Frequencies", Internet document; URL:http://niremf.ifac.cnr.it/docs/DIELECTRIC/home.html[4] G. Hartsgrove, A. Kraszewsky, A. Surowiec, “Simulated biolog-ical materials for electromagnetic radiation absorption studies”,Bioelectromagnetics, vol. 8, n. 4, pp. 29-36, 19873671

Experimental characterization of the RFID STENTag for passive vascular monitoring

Cecilia Occhiuzzi

Gaetano Marrocco

Crossref

https://iris.unipa.it/retrieve/handle/10447/100738/135888/38-STG_EUCAP_2012.pdf

Experimental characterization of the RFID STENTag for passive vascular monitoring

Abstract

Similar works

Full text

Available Versions

Archivio istituzionale della ricerca - Università di Palermo

Crossref