Feasibility of Non-Contact Ultrasound

Generation using Implanted Metallic Surfaces

as Electromagnetic Acoustic Transducers

By:Yarub Alazzawi, Chunqui Qian, and Shantanu Chakrabartty

alazzawi, shantanu@wustl.edu

Abstract—In this paper we investigate the feasibility of using an in-vivo metallic implant like a stent for generation of acoustic waves which can then be used for imaging areas inside or near the surface of the implant.

The proposed method relies on time-varying eddy-current loops that are excited on the metallic surface of the stent using an external RF coil.

In this paper we have designed a phantom set up to characterize the acoustic wave generation process and we demonstrate that the acoustic waves can be measured, imaged and harvested remotely using a piezoelectric probe.

Stents are routinely used in the surgical treatment of vascular stenosis where the metallic mesh in the stent provides mechanical support to the tissue walls and to facilitate the flow of vital fluids like blood or bile (as shown in Fig.1).

Fig. 1. Application of metallic stents and technologies used for monitoring stent potency [source: google images]

Post-surgery, the implanted stents are routinely monitored for occlusions which could potentially lead to restenosis and hence require surgical intervention.

A popular method for post-operative imaging of stents include x-ray which do not provide insight to the mechanics or growth of occlusions and the procedure neither provides any information regarding the fluid-flow through the stent. Direct magnetic resonance imaging (MRI) of stents are limited by the generation of eddy-currents which shield the spin signals emanating from within the stent.

►In-vivo metallic implant to generate acoustic waves.

Objective

►Non-contact imaging areas inside or near the implant.

►Remotely harvest acoustic waves.

In this paper, we explore an alternate approach where the metallic surface of the

stent could be used for in-vivo generation of acoustic waves, which can then be used

for characterizing stent potency.

At the core of the proposed technique is the use of electromagnetic acoustic transducers (EMAT) which have been

routinely used for non-destructive evaluation (NDE) of conductive structures like aircraft skins and metallic pipes.

The principle of EMAT is shown in Fig. 2 where an RF coil

generates a time-varying electromagnetic (EM) field. When a

metallic structure is exposed to this field, eddy-current loops

are generated on the surface of the structure. The direction

of the current is such that it opposes the change in the EM

field and the result is loss of energy due to Joule heating.

However, when the structure is simultaneously subjected to

a constant magnetic field, the eddy-current loops experience

a Lorentzian force that mechanically excite the metallic

structure. The result is the generation of pressure waves, and

the frequency of the acoustic wave is determined by the

frequency of the EM field and by the mechanical properties

of the structure. Fig. 2. Operational principle of EMAT

• In this paper we have designed a phantom set up, as shown in Fig. 3, to characterize the acoustic wave generation process and we demonstrate that the acoustic waves can be measured, imaged and harvested remotely using a piezoelectric probe.

Experimental Setup

Fig.3 (a)Schematic of the phantom experimental setup; (b)Photograph of the experimental setup

Experimental Setup

►Water tank to emulate in-vivo conditions►Aluminum strip suspended in water to emulate metallic substrate ►1.7 T permanent magnet suspended above water tank using a pulley►RF coil (coated copper wires) connected to (10V, 500mA) HP function

generator►Piezoelectric probe attached to the bottom of the water tank and

connected to the oscilloscope to measure the acoustic signal generated by EMAT

Amplitude of Acoustic Wave

Real part of the amplitude of the acoustic wave:

Parameter Description

RF electromagnetic field

Static magnetic field

Permeability of the vacuum

Bulk density of the metal

Acoustic wave velocity

Skin depth

Fig. 4 Amplitude of the acoustic wave generated using EMAT for different metals

Measurement Results

Fig. 5 Measured signal power when the height of the magnet is varied and the implantation depth is set to 0mm.

0 mm implanting depth (the aluminum substrate was freely floating on the water surface.)

►Variable magnet height (d)►20 mm water depth►5 mm RF-coil height

Fig. 5 shows the result of the experiment and clearly shows an inverse relationship between the recorded signal and distance (d).This can be attributed to the EM losses inside water which results in smaller magnitude of eddy-current loops.

Fig. 6. Measured signal power when the height of the magnet is varied and the implantation depth is set to 3mm.

3 mm implanting depth

►Variable magnet height (d)►20 mm water depth►5 mm RF-coil height

Measurement Results

The power received by the piezoelectric probe is lower for the case when the aluminum substrate was immersed in the water, as shown in Fig. 6.

Measurement Results

Fig. 7 System frequency response measured using the EMAT setup

0 mm implanting depth

►Variable RF analog signal frequency(Hz)

►20 mm water depth►5 mm RF-coil height►10 mm distance between the RF

coil and the Magnet

Fig. 7 clearly shows the existence multiple system poles which leads to different frequency windows where the EMAT method is more effective. These system poles and frequency windows are determined by several mechanical and electrical factors.

The important point is that for both the experiments, the probe was

able to harvest more than 100nW of power (when accounting for

coupling losses) even when it was placed 2cm away from the surface of

the aluminum plate. Thus, the proposed method could in principle be

used for designing an in-vivo acoustic beacon or an ultrasonic telemetry

system that is powered remotely using the EMAT technique.

Conclusions & Future Work

Conclusions

►Feasibility of non-contact ultrasound generation using in-vivo EMAT based approach►Possibility of non-contact imaging and energy harvesting►Electrical and mechanical factors affect the process efficiency.

Future work►Optimizing EMAT technique for in-vivo studies using multi-physics modeling and analysis of EM and

acoustic phenomena in biological tissue

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References