Feasibility of Non-Contact - PowerPoint Presentation

Slide1

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.eduSlide2

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.Slide3

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.Slide4

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

.Slide5

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 EMATSlide6
Slide7

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 setupSlide8

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 Slide9

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

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 metalsSlide10

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.Slide11
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.Slide12

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.Slide13

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.Slide14

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 Slide15

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