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 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:
𝐵𝑚 𝐵0
𝜁=
×
2𝜋 𝜇0 𝑑0 𝑉𝑠
Parameter
1
𝜔02 𝛿 2
1+
2𝑉𝑠2
Description
𝐵𝑚
RF electromagnetic field
𝐵0
Static magnetic field
𝜇0
Permeability of the vacuum
𝑑0
Bulk density of the metal
𝑉𝑠
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
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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