Sensors and Actuators A 297 (2019) 111487

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Sensors and Actuators A: Physicaljournal homepage: www.elsevier.com/locate/sna

Development of an ultrasonic probe to measure both radial arterialpressure and diameter change at the same position for early diagnosisof vascular endothelial function: Preliminary study
Mototaka Arakawa a,b,∗ , Takumi Saito a , Shohei Mori b , Shigeo Ohba b , Kazuto Kobayashi c ,Hiroshi Kanai a,ba

Graduate School of Biomedical Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japanc
Honda Electronics Co. Ltd., Toyohashi 441-3193, Japanba r t i c l e

i n f oArticle history:
Received 20 January 2019
Received in revised form 5 June 2019
Accepted 4 July 2019
Available online 24 July 2019
Keywords:
Radial artery
Blood pressure
Diameter
Piezoelectric effectViscoelasticity
Ultrasonic probea b s t r a c t
We have evaluated the viscoelastic properties of arterial walls by measuring the relationship from radial
arterial pressure to the change in arterial diameter to evaluate vascular endothelial functions. In ourprevious study, these parameters were measured at different positions, which caused timing errors. In
the present study, a novel probe was developed to measure both radial arterial pressure and the change
in arterial diameter at the same position. The central piezoelectric element of the linear array probe
was disconnected from the ultrasonic diagnostic equipment and used to measure the arterial pressure.
To obtain the blood pressure waveform, the output was integrated and calibrated using the systolic anddiastolic pressures measured by a conventional sphygmomanometer. The arterial diameter was measured
using the other 191 elements of the ultrasonic diagnostic apparatus via the phased-tracking method. The
hysteresis loop, which is the relationship between the change in diameter and instantaneous pressureduring a heartbeat, and thus reﬂects the viscoelasticity of the arterial wall, was measured successfully.
The reproducibility for successive two heartbeats was conﬁrmed for two subjects.
© 2019 Published by Elsevier B.V.

1. Introduction
Arteriosclerosis is the main cause of cardiovascular diseases.
Vascular endothelial function decreases in the early stages of arteriosclerosis, followed by thickening of the intima-media complex,
plaque formation, coarctation, and occlusion as arteriosclerosis
advances.
Intravascular ultrasound and X-ray imaging have been used todiagnose arteriosclerosis; however, these techniques are invasive
and unsuitable for repeated diagnoses. Noninvasive methods suchas the pulse wave velocity (PWV) [1,2] and the ankle branchialindex (ABI) [3] have been proposed. However, these indices yield
only average values over a wide range, which may cause late decisions in the case of locally advancing lesions. Intima-media complex
thickness (IMT) of the common carotid artery has been used for
local diagnosis of arteriosclerosis. However, a conventional B-mode∗ Corresponding author at: Graduate School of Biomedical Engineering, Tohoku
University, Sendai, Miyagi 980-8579, Japan.
E-mail address: arakawa@ecei.tohoku.ac.jp (M. Arakawa).
https://doi.org/10.1016/j.sna.2019.07.011
0924-4247/© 2019 Published by Elsevier B.V.

image provides information on only the shape; it cannot be used toevaluate the elasticity.
In the early stage of arteriosclerosis, lesions appear on vascularendothelial functions and the generation of vasodepressor material decreases. It is important to evaluate these functions as earlyas possible because the lesions are still reversible [4]. The conventional way to evaluate vascular endothelial functions is to measure
the change in the inner diameter of the brachial artery using the
ﬂow-mediated dilation (FMD) method [5]. However, it is difﬁcult
to evaluate the vascular endothelial function accurately using the
FMD method because the rate of change in diameter is only approximately 6% in healthy subjects [6,7].
We have proposed evaluating the viscoelastic properties of theblood vessel wall by measuring the arterial pressure and change inarterial wall thickness [8–13] or the arterial pressure and change inarterial diameter [14]. To measure the arterial pressure and change
in diameter, an ultrasonic probe was placed between two pressure
sensors, and the time delay between the ultrasonic probe and thepressure sensors was corrected using the PWV estimated by the twopressure sensors. However, it is difﬁcult to accurately correct thetime delay because the PWV, caused by changes in blood pressure,varies during a heartbeat.

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M. Arakawa et al. / Sensors and Actuators A 297 (2019) 111487

We believed that measuring the arterial pressure and change
in diameter at the same position could be realized if blood pressure was measured using the piezoelectric element of an ultrasonicprobe. However, the main frequency components of blood pressure
waveforms exist below 12 Hz [15], which is much lower than theseveral to 10-MHz frequency range of the ultrasonic probe. Thus,
we investigated a method for measuring blood pressure via thepiezoelectric effect using the piezoelectric element of an ultrasonicprobe [16]. Blood pressure waveforms with a high enough signalto-noise ratio (S/N) were obtained by integrating the output fromthe piezoelectric element.
In the present study, we developed an ultrasonic probe formeasuring both radial arterial pressure and the change in arterial
diameter at the same position simultaneously, which were used
to estimate the hysteresis characteristic of the arterial wall. We
investigated the reproducibility of the hysteresis characteristic for
successive heartbeats. In addition, viscoelastic parameters were
estimated on the basis of the Voigt model.
2. Principle
The main frequency components of a blood pressure waveform are below approximately 12 Hz [15], much lower than the
bandwidth (usually several to 10 MHz) of the ultrasonic probes.
When a piezoelectric element is used to measure blood pressure,the measured waveform is derivation of the blood pressure waveform because the electric charges generated by the piezoelectriceffect are extracted as current by the external circuit. Therefore,
the blood pressure waveform is obtained by integrating the voltage waveform at the load resistance in the output circuit. For leadzirconate titanate (PZT) ceramics, which belong to the class 6 mm ofthe hexagonal crystal system, the force f(t) applied to the directionof spontaneous polarization is expressed as follows [16]:f (t) =

E
c33

Re33

 t
V () d,(1)0

E and ewhere c33
33 are the elastic constant under a constant electricﬁeld and the piezoelectric constant, respectively, and V() is thevoltage at the load resistance R. The force f(t) is the product of thearea of the piezoelectric element and the pressure applied to it, andthe voltage generated increases as the area of the piezoelectric element increases. The force waveform f(t) is obtained by integrating
the voltage waveform V() from ␶ = 0 to t.
To obtain the blood pressure waveform p̂ (t), f(t) in Eq. (1) iscalibrated using the systolic blood pressure psys and the diastolicblood pressure pdia , measured using a conventional manometer, as
follows:p̂ (t) =
psys − pdia f (t) − fmin + pdia ,fmax − fmin(2)

where fmax and fmin are the maximum and minimum values of f(t).
We investigated the voltage output from a piezoelectric element (area = 0.8 mm2 , thickness = 3.2 mm) of the linear probe, theE = 1.28 ×
operating frequency of which was 7.5 MHz, where c33

1011 N/m2 , e33 = 25.9 C/m2 , relative permittivity εS33 /ε0 = 2, 021,
and density  = 7, 500 kg/m3 , and the input resistance R of the output circuit was 1 M [16]. The voltage was estimated as 0.03 mV at
5 Hz via a pressure difference of 50 mmHg. Furthermore, we experimentally conﬁrmed that the voltage was on the same order as thetheoretical voltage and the integrated waveform p̂ (t) resembled
the typical waveform measured by a tonometer in the radial artery[16].
In the present study, we developed an ultrasonic probe for measuring both radial arterial pressure and arterial diameter at theFig. 1. Schematic view of the experimental setup.

same position simultaneously to estimate the hysteresis characteristic of the arterial wall.
3. Method
The schematic view of the experimental setup is shown in Fig. 1.
A commercial linear array probe (Honda Electronics) with a center
frequency of 7.5 MHz was prepared. The probe has 192 channels
and each piezoelectric element was 0.2 × 4 × 3.2 mm3 . The probe
was customized by cutting the connection of the one center piezoelectric element from the transmitting and receiving unit of theultrasonic diagnostic apparatus. The center piezoelectric element
was used to detect the blood pressure and the other 191 channels were used to measure the diameter by ultrasound. The output
voltage increases with the area of the piezoelectric element, that is,as the number of elements used for blood pressure measurement
increases. Thus, a larger area is preferred to measure only the bloodpressure. However, the accuracy of the diameter change could
decrease if the number of elements for the diameter-change measurement is decreased by increasing the number of elements for the
blood pressure measurement. Therefore, lower number of elements
for blood pressure measurement is desirable if the S/N of the bloodpressure waveform is sufﬁcient. The customized probe was connected to an ultrasonic diagnostic apparatus (ProSound F75, Hitachi
Aloka). The phased-tracking method was used to determine theboundary of the artery to estimate the change in its diameter [17].
In this method, the average velocity of the blood wall during thepulse transmission interval was obtained. The position of the wall
was estimated from the velocity. The method is described in detailin Ref. [17]. The diameter was measured at a sampling frequency of
40 MHz and a frame rate of 252 Hz. An electrocardiogram was alsomeasured with three leads by the ultrasonic diagnostic apparatus.
To measure the blood pressure, the output voltage from thecenter piezoelectric element went through an ampliﬁer with anampliﬁcation factor of 100, and a low-pass ﬁlter with a cutoff
frequency of 30 Hz, and then to the external pulse input port of
the ultrasonic diagnostic apparatus. The cutoff frequency of the
low-pass ﬁlter was determined to suppress the commercial power
supply component with a frequency of 50 Hz, and to get throughthe blood pressure component with a frequency below 12 Hz [15].
Therefore, the diameter and the blood pressure were measured
simultaneously. The blood pressure waveform was obtained byM. Arakawa et al. / Sensors and Actuators A 297 (2019) 111487

3

Fig. 2. B-mode image of the left radial artery for subject A.
Fig. 4. Relationship between arterial pressure and diameter for two successive
heartbeats for subject A.

Fig. 5. Relationship between arterial pressure and diameter for two successive
heartbeats for subject B.

Fig. 3. (a) Waveforms of arterial pressure and diameter for subject A. (b) Relationship between arterial pressure and diameter for subject A.

integrating the measured waveform. To obtain absolute blood pressure values, the voltage measured by the piezoelectric element
was calibrated using Eq. (2) with the systolic and diastolic bloodpressures measured with a conventional sphygmomanometer.
We used our probe to measure the blood pressure and arterial diameter simultaneously in the left radial artery of two males(subjects A and B) in their twenties. The output voltage fromthe piezoelectric element strongly depends on the position of theblood vessel. Upon departing from the upper position, the output
decreases. The probe enables conﬁrmation of the position of theblood pressure measurement via the B-mode image in the ultrasonic diagnostic apparatus. Therefore, the position of the center
element can be easily adjusted to be just above the blood vessel.
4. Results and discussion
Fig. 2 shows the B-mode image of the left radial artery for subject
A. Any effect caused by removing the center piezoelectric element
was not observed on the B-mode image, which was produced using
the data from the other 191 elements.
Fig. 3(a) shows the electrocardiogram, measured blood pressure
waveform p̂ (t) of Eq. (2), and the change in diameter for subject A.
The blood pressure waveform p̂ (t) obtained by the single piezoelectric element had a sufﬁcient signal-to-noise ratio. The increasein both arterial pressure and the change in arterial diameter duringsystole was almost simultaneous. psys and pdia were measured as
123 and 61 mmHg, respectively. Both blood pressure and diameter
returned to their initial values after a heartbeat. The effect of ultrasound signals with a frequency of 7.5 MHz was not observed for the
blood pressure waveform because the main frequency of the bloodpressure waveform is much lower than the ultrasound signals anda low-pass ﬁlter with cut-off frequency of 30 Hz was used to obtain
the blood pressure waveform.
Fig. 3(b) shows the relationship between blood pressure andarterial diameter between the two R-waves on the ECG shown in
Fig. 3(a). The times labeled A, B, C, and D are the same in Fig. 3(a)and (b). The diameter expanded linearly with an increase in bloodpressure and then gradually returned to its initial size with viscosity as the blood pressure decreased. The results were similar to
those obtained using pressure sensors and an ultrasonic probe [14].
Therefore, we were successful in showing the relationship between
blood pressure and the change in diameter using only an ultrasonicprobe.
Next, we measured arterial pressure and diameter for two
successive heartbeats. The waveforms showing the relationshipbetween arterial pressure and diameter for subject A are shown in
Fig. 4. The similarity between the two waveforms conﬁrms reproducibility.
Finally, the viscoelastic parameters pressure-strain elastic modulus Ep and pressure-strain viscosity ␩p were estimated using theleast-squares method based on the Voigt model, which is the mostfundamental viscoelastic model of biological tissues [14,18,19]. The
estimation method is described in detail in Ref. 14. We found that
Ep was 1.43 × 102 and 1.47 × 102 kPa and ␩p was 0.65 and 0.62 kPa·s
for the ﬁrst and second heartbeats, respectively. The reproducibil-

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M. Arakawa et al. / Sensors and Actuators A 297 (2019) 111487

ity of results was also conﬁrmed on the basis of the estimatedviscoelastic parameters.
To conﬁrm the reproducibility of the measurements, we applied
the method to subject B. The relationships between arterial pressure and diameter are shown in Fig. 5. Ep was 1.83 × 102 and1.97 × 102 kPa while ␩p was 1.36 and 0.82 kPa·s for the ﬁrst andsecond heartbeats, respectively. Similar hysteresis properties werealso obtained.
In the present study, the linear array probe was customized.
Using other types of probes, such as a sector probe, is also possible
because the blood pressure can be obtained by the piezoelectriceffect.
5. Conclusions
In this paper, we presented our newly developed ultrasonicprobe that simultaneously measures radial blood pressure andchange in arterial diameter. The relationship between blood pressure and change in diameter during a heartbeat was measured
and the hysteresis characteristics caused by the viscoelasticityof the blood wall were conﬁrmed. The reproducibility of results
using the probe was conﬁrmed from the measured waveforms andthe estimated parameters. In a future study, we intend to checkthe reproducibility using multiple subjects and evaluate vascularendothelial functions by measuring the changes in viscoelasticity
caused by an FMD reaction as shown in our previous paper [14].
Acknowledgments
This work was supported in part by JSPS KAKENHI17H02105
and the Murata Science Foundation.
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Biographies
Mototaka Arakawa received the B.E., M.E., and Ph.D. degrees in electrical engineering from Tohoku University, Sendai, Japan, in 1993, 1995, and 2000, respectively. He
is currently an associate professor at the Graduate School of Biomedical Engineering,Tohoku University. His research interests include developments of the ultrasonictransducer and the acoustic microscopy for tissue and cell characterization and theapplication of the line-focus-beam ultrasonic material characterization system.
Takumi Saito received the B.E. degree in electronic engineering from Tohoku University, Sendai, Japan, in 2018. He is currently studying toward the M.E. degree
at Tohoku University. His research interests include the evaluation of vascularendothelial functions using the ultrasonic probe developed in the present paper.
Shohei Mori received the B.E., M.E., and Ph.D. degrees in Mechanical and Control
Engineering from Tokyo Institute of Technology, in 2013, 2015, and 2017, respectively. He is a currently an assistant professor at the Graduate School of Engineering,Tohoku University. His research interests include the ultrasonic tissue characterization of liver disease and the measurement of arterial wall movement.
Shigeo Ohba graduated from Shiroishi technical high school, in 1969, and hereceived the Ph.D. degree in Electronic Engineering from Tohoku University, in 2002.
From 1969, he has been with the Tohoku University, Sendai, Japan, where he is currently a technical expert in the experimental laboratory for students on Department
of Electrical, Information and Physics Engineering, Tohoku University. His researchinterest includes biomedical electrodes and instrumentation for the physiologicalsignal measurement.
Kazuto Kobayashi received B.E. degree in electrical engineering from Shibaura
Institute of Technology, in 1976. He received Ph.D. degrees in 2011 from Tohoku
University. He is currently a director of Department of Research and Development at
Honda Electronics Co. Ltd. in Toyohashi, Japan. His research activities and interests
include medical ultrasound imaging, signal processing and high frequency ultrasound transducers.
Hiroshi Kanai received a B.E. degree from Tohoku University, Sendai, Japan, in
1981, and M.E. and the Ph. D. degrees, also from Tohoku University, in 1983 and in
1986, both in electrical engineering. He is currently a professor in the Department
of Electronic Engineering, Graduate School of Engineering, Tohoku University. Hispresent interests are in transcutaneous measurement of the heart wall vibrations
and myocardial response to propagation of electrical potential and cross-sectional
imaging of elasticity around atherosclerotic plaque with transcutaneous ultrasoundfor tissue characterization of the arterial wall.