Piece-wise Excitation System for theCharacterization of Local RF-Induced Heating of

AIMD during MR Exposure

Earl Zastrow∗†, Myles Capstick∗, Eugenia Cabot∗ and Niels Kuster∗†∗IT’IS Foundation, Zeughausstrass 43, 8004 Zurich, Switzerland. Email: earl.zastrow@ieee.org

†Department of Information Technology and Electrical Engineering, ETH-Zurich, Zurich, Switzerland

Abstract—An experimental system that is capable of charac-terizing the RF-induced heating of active implantable medicaldevices is developed. The system is used to experimentallycharacterize the local RF-induced heating of generic implants andthe obtained experimental results are compared with numericalresults obtained from full-wave computational electromagneticsimulations.

I. INTRODUCTION

Local radiofrequency (RF)-induced heating is one possiblehazard for patients with active implantable medical devices(AIMDs) undergoing an MR scan. When a patient with animplanted medical device is subjected to the strong RF-fieldof an MRI scanner, the local RF-induced heating in the vicinityof the implant can be substantial. Implants with elongatedconductive structures, such as cardiac pacemakers and deep-brain stimulators, have the ability to pick up the RF-energyfrom MR exposure and locally deposit the RF-energy at itstermination points (e.g. electrode poles). Piece-wise excitation(πX) is a robust method that can be used to characterize theRF-induced heating of AIMDs. The method is applicable toany implants with metallic conductors and is also suitable forimplants with an elongated lead structure.

In this study, an experimental system that is capable ofcharacterizing the RF-induced heating is described. The πXsystem is used to characterize the local RF-induced heatingof generic AIMDs. The experimental results determined arecompared with numerical results obtained from full-wavecomputational electromagnetic simulations.

II. PIECE-WISE EXCITATION SYSTEM

A. Method

The πX system can be used to characterize the local RF-induced heating of any AIMD. The characterization is basedon the technique proposed in [1] where a transfer function(from hereon, referred to as the πX model), h(l), is defined asthe relationship between the local induced electric field arounda tip or an electrode pole of an AIMD and an excitation alonglength l of the AIMD.

Fig. 1 shows a schematic of the method. The tangentialcomponent of the local incident electric field, Etan, is coupledwith the AIMD at length l and the induced electric field aroundan electrode of the AIMD (or radiated components) at pointp1 is evaluated. The total induced electric field at the point p1

Fig. 1. Schematic of the piece-wise excitation method.

contributed to Etan coupling along the entire AIMD of lengthL can be calculated from the relation

Einduced,p1 =

∫ L

0

h(l)Etan(l)dl (1)

B. Experimental Prototype

Computational methods may be used to obtain the πXmodel (h(l)) of a simple AIMD. However, for a complexAIMD structure, the computational burden can be substantial,and the computational method may prove to be unfeasible. Toovercome the challenges of the computational implementationassociated with a complex AIMD structure, we have developedan experimental prototype of the πX system. The system iscapable of generating a well-controlled local Etan along anextremely elongated AIMD and acquiring the induced electricfield at the tip or electrode of the AIMD. The system can beused to obtain the πX model of any AIMD, irrespective ofthe complexity of its construction. The πX system prototypeis illustrated in Fig. 2

III. EXPERIMENTAL AND NUMERICAL EVALUATION OFTHE πX MODELS OF GENERIC AIMDS

To evaluate the performance of the πX system, πX model(h(l)) of three generic implants are evaluated with the πX

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(a) (b)

Fig. 2. (a) The πX system analyzer (signal generator and processor). (b)Close-up view of the local excitation source antenna and the electric fieldprobe placed near the tip of a generic AIMD considered in this study.

system experimentally as well as numerically. The characteri-zation, conducted at 64 MHz, corresponds to the RF-exposureof 1.5 T MRI.

A. Description of Generic AIMDs

To enable numerical evaluation, we consider generic im-plants that are simple in construction. The implants are in-sulated 1.5 mm-diameter stainless steel wires, with insulationthickness of 0.5 mm. At one end of the wire, the insulation isremoved over the length of 10 mm from the end of wire. Theselected implants are 400, 600, and 800 mm in lengths. Fig.3 illustrates the implant samples.

(a) (b)

Fig. 3. Generic AIMD samples: Insulated 1.5 mm-diameter stainless steelwires, with insulation thickness of 0.5 mm and insulation removed at one endof the wire. The implants are 400, 600, and 800 mm in length. (a) Close-up ofthe 10 mm length bare tips. (b) Computational representation of the AIMDs.

B. The πX Models of Generic AIMDs

Each generic implant is placed in a phantom filled withtissue-simulating medium (εr = 78 and σ = 0.47 S/m). Theexperimental setup to obtain the πX model is illustrated in Fig.2. The excitation source antenna is translated linearly along theaxis of the AIMD and the induced electric field at the bare tipof the AIMD is evaluated with a loaded co-axial electric fieldprobe located at a fixed spatial location in the vicinity of thetip of the implant.

Similar to the experimental evaluation, the generic implantis placed in a homogeneous medium with dielectric properties

similar to those of the tissue-simulating medium used inthe experimental setup (εr = 78 and σ = 0.47 S/m). Anadditive current source is used to provide the local electricfield excitation, Etan, to the implant. The current source istranslated linearly along the axis of the AIMD and the inducedelectric field is recorded at a spatial location in the vicinity ofthe implant tip. Fig. 4 depicts the local induced electric fieldin the vicinity of the implant tip. The spatial location in thecomputational domain of which the induced field is monitoredis indicated with a white marker.

(a)

(b)

(c)

Fig. 4. Induced electric field concentrated around the AIMD bare tip. Thespecific example illustrated is for the induced field due to a local excitationcentered at l = 300 mm; where l = 0 mm is defined at the bare tip of theimplant. (a) x-component of the electric field. (b) y-component of the electricfield. (c) z-component of the electric field.

IV. RF-INDUCED HEATING OF GENERIC AIMDS UNDERPHASE-REVERSAL INCIDENT ELECTRIC FIELD

In the proceeding section, the local power deposition atthe bare tip of each generic implant was predicted for aset of incident electric field (Etan) with the πX modelsobtained in Section III. The predictions are compared withbenchmark simulations obtained from full-wave computationalelectromagnetic simulation software, SEMCAD-X (SPEAG,Zuerich, Switzerland). To evaluate the robustness of the πXmodels evaluated both experimentally and numerically, weestablished a set of benchmark simulations such that differentsections of the implants are exposed to different Etan.

Each AIMD is exposed to a phase-reversal electric fieldexposure. Fig. 5 illustrates the phase and amplitude of theincident tangential electric field, Etan, along the implant.

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(a)

(b)

Fig. 5. Phase-reversal incident electric field to the generic AIMDs. (a)Diagram of the numerical setup to provide phase-reversal exposure to theAIMD with a z-propagating TEM excitation with x-polarized electric field.(b) Magnitude and phase of the incident electric field, Etan, with a phase-reversal at location l = L along the AIMD.

V. RESULTS

Fig. 6 shows a comparison of the phase and amplitude ofthe πX models obtained experimentally with the πX systemand the πX models obtained numerically. The πX modelsobtained with the πX system exhibit phase-saturation behaviorsimilar to that of the models obtained numerically. Thereexist shifts in the amplitude and phase responses between thenumerical πX and experimental πX models of approximately30, 50, and 75 mm along the length of the 400, 600, and 800mm AIMDs, respectively.

0 100 200 300 400 500 600 700 8000

0.5

1

1.5

distal end <−−− distance along AIMD (mm). −−−> proximal end

πX

am

plit

ude (

a.u

.)

0 100 200 300 400 500 600 700 800

−4

−3

−2

−1

0

distal end <−−− distance along AIMD (mm). −−−> proximal end

πX

phase (

rad)

Fig. 6. Comparison of the πX models obtained numerically (solid) vs.experimentally (dashed) with the πX system, respectively. The πX modelsof the 400, 600, and 800 mm AIMDs are plotted in blue, red, and black,respectively.

The local power deposition at the bare tip of each genericimplant is evaluated for a set of phase-reversal incident electricfield conditions, described in Section IV and for a unit Etanmagnitude (E0 = 1 V/m). Both the πX models obtained ex-perimentally and numerically are used to obtain the prediction.

Fig. 7 compares the predicted local deposition at the baretip of each generic implant. The peak specific absorption rateover a 10 mg mass (pSAR10 mg) is used to illustrate the levelof local deposition. Table I summarizes the local deposition atthe tip of the generic AIMDs evaluated with the numerically-derived πX models, experimentally-derived πX models, andfull-wave numerical benchmark simulations. The errors ofthe predictions are evaluated against the deposition obtainedfrom numerical benchmark simulations. The depositions arenormalized to an Etan magnitude of 1 V/m.

The prediction of local deposition obtained with the nu-merical πX and the experimental πX models also show a shiftconsistent with the discrepancy in the location of the responsesobserved in the πX models

0 100 200 300 400 500 600 700 8000

2

4

6

phase−reversal position from distal end (mm)S

AR

(W

/kg/(

V/m

)2)

400−

mm

AIM

D

0 100 200 300 400 500 600 700 8000

2

4

6

phase−reversal position from distal end (mm)

SA

R (

W/k

g/(

V/m

)2)

600−

mm

AIM

D

0 100 200 300 400 500 600 700 8000

2

4

6

phase−reversal position from distal end (mm)

SA

R (

W/k

g/(

V/m

)2)

800−

mm

AIM

D

Numerical πX Experimental πX Benchmark

Fig. 7. pSAR10 mg at the bare tip of the generic AIMDs evaluated forphase-reversal incident electric field conditions. The prediction obtained withthe πX models obtained numerically and experimentally are indicated by solidand dashed lines, respectively. The deposition obtained from the numericalbenchmark simulations are indicated by ‘o’ markers. The pSAR10 mg ofthe generic AIMDs exposed to iso-electric incident field is indicated by thehorizontal dotted lines included in the plots.

VI. CONCLUSION

We have evaluated the performance of an experimentalprototype πX system. The system is capable of generating aπX model that relates the incident tangential electric fields(Etan) along the AIMD to the local power deposition at anelectrode (or at any radiated components). The predicted depo-sition are evaluated and compared with those obtained via full-wave electromagnetic simulations. The predictions provided bythe experimental πX models are found to be within 3 dB ofthe benchmark values. Additional development to improve thefidelity of the πX models is underway. Further investigationis also needed to resolve the observed discrepancies between

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TABLE I. SUMMARY OF PSAR10 MG (W/KG) AT THE BARE TIP OF THE GENERIC AIMDS. THE DEPOSITIONS OBTAINED WITH THE NUMERICAL πXMODELS AND EXPERIMENTAL πX MODELS ARE COMPARED WITH THOSE OBTAINED FROM THE FULL-WAVE NUMERICAL BENCHMARK SIMULATIONS. THE

DEPOSITIONS ARE NORMALIZED TO AN ETAN MAGNITUDE OF 1 V/M.

AIMD Phase-reversal SAR10 mg (W/kg) Prediction error (dB)position (mm) Benchmark Numerical πX Experimental πX Numerical πX Experimental πX

400 mm100 1.7 1.7 1.1 0.1 -2.0200 0.8 0.4 0.6 -3.3 -1.5300 3.5 3.5 3.7 0.0 0.3

600 mm

100 3.4 3.4 3.4 -0.1 0.0200 3.5 3.1 2.3 -0.5 -1.9300 1.0 1.5 0.9 2.0 -0.2400 0.5 0.7 1.0 1.3 2.9500 1.3 1.5 2.3 0.6 2.5

800 mm

100 2.1 1.3 2.9 -2.2 1.3200 5.5 3.3 4.7 -2.2 -0.7300 4.7 5.0 5.0 0.3 0.3400 3.5 4.8 3.8 1.3 0.3500 2.9 3.0 2.1 0.1 -1.4600 0.5 1.1 1.1 3.4 3.2700 0.3 0.4 1.0 0.8 5.3

the numerical and experimental implementation of the πXmethods.

REFERENCES

[1] S.M. Park, R. Kamondetdacha, J.A. Nyenhuis, “Calculation of MRI-induced heating of an implanted medical lead wire with an electric fieldtransfer function,” J. Mag. Res. Imag., vol. 26, pp. 1278-1285, November2007.

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