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

In-vivo metallic implant to generate 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. Objective In-vivo metallic implant to generate acoustic waves. Non-contact imaging areas inside or near the implant. Remotely harvest acoustic waves.

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

Experimental Setup 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. 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: 𝜁= 𝐵 𝑚 𝐵 0 2𝜋 𝜇 0 𝑑 0 𝑉 𝑠 × 1 1+ 𝜔 0 2 𝛿 2 2 𝑉 𝑠 2 Parameter Description 𝐵 𝑚 RF electromagnetic field 𝐵 0 Static magnetic field 𝜇 0 Permeability of the vacuum 𝑑 0 Bulk density of the metal 𝑉 𝑠 Acoustic wave velocity 𝛿 Skin depth Fig. 4 Amplitude of the acoustic wave generated using EMAT for different metals

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

Measurement Results 3 mm implanting depth Variable magnet height (d) 20 mm water depth 5 mm RF-coil height 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. Fig. 6. Measured signal power when the height of the magnet is varied and the implantation depth is set to 3mm.

Measurement Results 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. Fig. 7 System frequency response measured using the EMAT setup

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 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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