Instrumentation & Measurement Magazine 25-1 - 9

to 475 V and pulse duration
of 10-70 ns. In the pulseecho
technique shown in
Fig. 3b, the WUS operates
as a generator as well as a
receiver of ultrasound. The
electric pulses were applied
to WUS to generate
pulsed ultrasound. Ultrasound
was transmitted into
a subject and reflected from
the tissue internal boundaries
back to the WUS. The
received ultrasound signals
were converted back
to electrical signals by the
same WUS. The received
signal was amplified
within the output range of
±0.5 V by a built-in amplifier
of the pulser-receiver.
A function generator provided
a trigger signal both
to the pulser-receiver and
analog-digital converter
to control the pulse repetition
frequency and to
synchronize the two devices.
The pulse repetition frequency was set to 1 kHz. The
received ultrasound signals were sampled at the maximum
sampling frequency of 125 MHz by a 14-bit analog-to-digital
converter (Model: ATS 9440, Alazartech, Montreal, QC, Canada)
to achieve the highest depth resolution provided by our
acquisition system. The digitized signals were then stored in a
personal computer for analysis. It may be noted that as a part
of future study, sampling at a lower rate and compressive sensing-based
measurement approaches may be considered for
reducing the power requirement of the WUS system.
Fig. 4. Ultrasound signal segments acquired from the CCA, using (a) WUS system with plane-wave ultrasound and
(b) clinical ultrasound imaging system with focused ultrasound. M-mode image of the CCA using the envelope of the
ultrasound signals obtained by (c) WUS system and (d) clinical ultrasound imaging system.
CCA using the WUS and the clinical ultrasound system are
shown in Fig. 4c and Fig. 4d, respectively. The dynamics of the
artery walls associated with the blood pressure waveform appear
over the time duration of the recording in both images.
Ultrasound Signal Quality
For continuous artery monitoring using the proposed PVDF
WUS system presented in Fig. 3, excessive levels of scattering
echo noise in the acquired signals, as discussed in the signal
model given in Fig. 2, complicate the automatic identification
and tracking of the artery wall boundaries and motion
throughout the cardiac cycle. The ultrasound signals obtained
at the CCA of a human subject using the WUS system and the
conventional clinical ultrasound imaging system using a linear
array probe are shown in Fig. 4a and Fig. 4b, respectively.
The echoes corresponding to the artery walls appear at approximately
12 mm (near wall) and 20 mm (far wall) in both
measurements. The clinical system exhibits defined artery
wall boundary echoes, while the artery wall boundary echoes
are obscured by adjacent tissue scattering echoes in the signal
obtained using the WUS. Motion-mode (M-mode) images
composed of successive pulse-echo signals obtained from the
February 2022
Artery Wall Motion Monitoring
Since the proposed WUS system employs plane-wave ultrasound
in which the wavefronts are greater than the artery
diameter, the desired artery wall echoes in the acquired signals
have inherently poor signal-to-noise ratio, as illustrated in
Fig. 4. Previously, artery motion tracking using the WUS system
was demonstrated by manually segmenting the acquired
signals around the artery walls and using quadrature demodulation
[13] for the artery wall motion estimation [14]. While
this method successfully retrieved the artery wall motion, isolation
and automatic identification of the artery wall echo were
not considered.
Recently, progress toward the development of a methodology
that is able to isolate and track the artery wall echo
within the acquired signals was made using a singular value
decomposition [15] and using a matching pursuit signal decomposition
approach [16]. In [15], the initial location of the
arterial echo was identified using the M-mode method. In
[16], based on a mathematical model of the acquired ultrasound
signals, each signal was decomposed into its composing
echoes. Using the decomposed signal representation, the echo
corresponding to the near artery wall was identified and isolated
in each frame of the sequence. Phase tracking using the
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Instrumentation & Measurement Magazine 25-1

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