Ultrasonics 84 (2018) 172–179

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Ultrasonics
journal homepage: www.elsevier.com/locate/ultras

A method for the design of ultrasonic devices for scanning acoustic
microscopy using impulsive signals
Mototaka Arakawa a,b,⇑, Hiroshi Kanai b,a, Kazuo Ishikawa b, Ryo Nagaoka a, Kazuto Kobayashi c,
Yoshifumi Saijo a,babcGraduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan
Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Division of Research and Development, Honda Electronics, Co. Ltd., Toyohashi 441-3193, Japana r t i c l e

i n f oArticle history:
Received 27 June 2017
Received in revised form 17 September
2017
Accepted 30 October 2017
Available online 2 November 2017
Keywords:
Scanning acoustic microscopy
Ultrasonic device
Loss
Frequency characteristic
Impulsive signala b s t r a c t
Scanning acoustic microscopy (SAM) using impulsive signals is useful for characterization of biologicaltissues and cells. The operating center frequency of an ultrasonic device strongly depends on the performance characteristics of the device if the measurement is conducted by using impulsive signals. In thispaper, a method for the design of ultrasonic devices for SAM using impulsive signals was developed. A
new plane-wave model was introduced to calculate frequency characteristics of loss of ultrasonic devices
by taking into account the conversion loss at the ultrasonic transducer, the transmission loss at the acoustic anti-reflection coating, and the propagation loss in the couplant. Ultrasonic devices were fabricatedwith a ZnO ultrasonic transducer using two acoustic lenses with aperture radii of 1.0 mm and 0.5 mm,
respectively. The frequencies at which measured losses became minima corresponded to the calculation
results by the plane-wave model. This numerical calculation method is useful for designing ultrasonicdevices for acoustic microscopy using impulsive signals.
Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction
Scanning acoustic microscopy (SAM) was developed to observe
the microscopic region beneath the specimen surface of solids andbiological tissues in the 1970 s [1–3]. To obtain higher-resolution
images, higher-frequency ultrasonic devices have been developed[4]. Quantitative measurement methods of acoustic properties,
such as velocity and attenuation coefficient, have also been developed to characterize biological tissues and cells. SAM has been
applied for characterization of tissues or cells by measuring acoustic properties [5–10].
Rf burst signals are used as input signals for an ultrasonic transducer in conventional SAM. Acoustic images can be measured atarbitrary frequencies by setting the carrier frequency. However,
it is impossible to separate the reflected signal from the front surface and that from the back surface for thin-sliced specimens if theround-trip propagation time in the specimen is shorter than thepulse width of the rf burst signal. In addition, it is time consuming
to obtain two-dimensional distributions of acoustic properties
because it is necessary to measure the reflected signals at several⇑ Corresponding author at: Graduate School of Biomedical Engineering, Tohoku
University, Sendai 980-8579, Japan.
E-mail address: arakawa@ecei.tohoku.ac.jp (M. Arakawa).
https://doi.org/10.1016/j.ultras.2017.10.023
0041-624X/Ó 2017 Elsevier B.V. All rights reserved.

different frequencies with changing the carrier frequency of the
rf burst signals at each measurement point [9,10].
On the other hand, SAM using impulsive signals has also beendeveloped [11–16]. By using a high-frequency, broadband ultrasonic device, it is possible to distinguish the reflected signal fromthe front surface and that from the back surface of the thin specimens in the time domain. Thus, it is possible to obtain velocity of a
specimen by measuring the difference of the arrival time of the signals. Saijo et al. developed a sound-speed microscope using impulsive signals for biological tissue and cell specimens [17–19].
However, it is difficult to distinguish two reflected signals for thin
specimens with a thickness of several microns, even if impulsive
signals are used in the measurements. In order to obtain acoustic
properties of such thin specimens, a pulse spectrum method,which is an analysis procedure in the frequency domain of thesuperposed signals, was also developed [20–22]. However, in themeasurements using impulsive signals, the operating center
frequency and the bandwidth strongly depend on the performance
characteristics of the ultrasonic device.
Ultrasonic devices for SAM have been designed based on theoutcome of experimental measurement using rf burst signals. In
the burst mode, measurements can be conducted at a desired
frequency within the bandwidth of the ultrasonic device. Therefore, to obtain as high a resolution as possible, the operating centerM. Arakawa et al. / Ultrasonics 84 (2018) 172–179

frequency of the ultrasonic device is usually set to be higher than
the frequency at which the loss of the device becomes minimum
while taking the signal-to-noise ratio (S/N) into consideration.
Otherwise, the operating center frequency and the bandwidthstrongly depend on the operating characteristics of the ultrasonicdevice in measurements using the impulsive signals. Thus, theoperating center frequency in the impulsive mode might be lower
than the frequency that we expected based on the experience ofthe burst signals.
In this study, based on the results of numerical calculations andexperiments, we evaluated performance characteristics of ultrasonic devices, viz., frequency characteristics of loss. A new procedure for the design and fabrication of ultrasonic devices for SAM
was developed. Two fabricated ultrasonic devices were alternatively attached to a SAM, and two-dimensional distributions ofintensity and velocity for a tissue specimen were measured as ademonstration.
2. A method for designing ultrasonic devices

An acoustic lens or a concave transducer is usually used as thefocusing device for SAM. The acoustic lens is mainly used in thefrequency range above 100 MHz. Fig. 1 shows a cross-section of
the ultrasonic device. An ultrasonic transducer is constructed on
the plane surface of the rod having an acoustic lens. In order toobtain a narrow focused beam, material with high acoustic velocity
should be selected for the rod. A Z-cut sapphire single crystal rod isnormally used in frequency ranges higher than 100 MHz.
The focal length F of the acoustic lens is obtained by the following equation [23,24]:

R;
1  VVCS

ð1Þ

where R is the curvature radius, and VC and VS are the velocities ofliquid coupler and buffer rod, respectively. F is 1.15R when the coupler is water and the rod is a Z-cut sapphire.
The transmission coefficient from the sapphire rod to the wateris very small because of the large difference of their acoustic impedances. Therefore, an acoustic anti-reflection coating (AARC) with athickness of k /4 (k: wavelength) is normally formed on the aperture surface. All ultrasonic energies propagate from sapphire towater when the acoustic impedance of AARC ZAARC has the following relationship:

Z AARC ¼

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ZS  ZC ;2.2. Factors of loss
As the factors of loss in the measurement using an acoustic lens,conversion loss (CL) from electric energy to mechanical energy at
the ultrasonic transducer, propagation loss (PL) in the liquid coupler, and transmission loss (TL) at the AARC should be considered.
Strictly speaking, the loss caused by focusing, e.g., aberration,
should also be considered in the measurement using an acoustic
lens. Here, we assumed that all ultrasonic energies emitted fromthe ultrasonic transducer converged at the focal point and returnedto the ultrasonic transducer because two-dimensional scanning is
conducted at the focal point of the ultrasonic device in the measurement of acoustic properties of biological tissues and cells.
The total loss of the ultrasonic device was thus calculated as the
product of CL, TL, and PL by the plane-wave model, which ignoresthe focusing effect. Diffraction loss caused by plane-wave propagation from the ultrasonic transducer with finite size, ultrasonic loss
in the AARC, and electric loss at the electrode were also neglected.
2.3. Calculation method

2.1. Ultrasonic device

F¼

173

ð2Þ

where ZS and ZC are the acoustic impedance of the sapphire and thatof the coupler, respectively.
Curvature radii R were designed as 1.0 mm around 225 MHz
and 0.5 mm around 375 MHz based on experimental experiencesof the measurements using rf tone burst pulses.

CL was calculated by the method described in Ref. [25]. The
velocity, thickness, and acoustic impedance of the piezoelectricmaterial and the electrode, the electromechanical coupling factor
and permittivity of the piezoelectric material, and the acoustic
impedance of the acoustic lens were used in the calculation.
PL was calculated by the following equation.

PL ¼ expð2aC FÞ;

ð3Þ

where aC is attenuation coefficient of the liquid coupler. The focal
length F was obtained by Eq. (1) and aC of the couplant such as
water [26] or physiological saline solution [27] were obtained by
the published data.
Reflection coefficient Rij and transmission coefficient Tij frommedium i to medium j are expressed as follows:

Rij ¼

Zj  Zi;
Zj þ Zi

ð4Þ

T ij ¼

2Z j:
Zj þ Zi

ð5Þ

Here, we assume media 1, 2, and 3 are the buffer rod, AARC, and theliquid coupler, respectively. The transmission coefficient from thebuffer rod to the liquid coupler through AARC T13 and that fromthe liquid coupler to the buffer rod through AARC T31 are expressed
as follows by assuming infinite reflections in AARC:

T 13 ¼ T 12  T 23  expðk2 l2 Þ
þ T 12  R23  R21  T 23  expð3k2 l2 Þ
þ T 12  R223  R221  T 23  expð5k2 l2 Þ þ  
¼ T 12  T 23  expðk2 l2 Þ f1 þ R23  R21  expð2k2 l2 Þ
þ R223  R221  expð4k2 l2 Þ þ  g¼ ð1 þ R12 Þ  ð1 þ R23 Þ  expðk2 l2 Þ
1XnfR23  R21  expð2k2 l2 Þg


ð6Þ

n¼0

¼ ð1 þ R12 Þ  ð1 þ R23 Þ  expðk2 l2 Þ1
1  R23  R21  expð2k2 l2 Þ
ð1 þ R12 Þ  ð1 þ R23 Þ  expðk2 l2 Þ¼;
1 þ R12  R23  expð2k2 l2 Þ
T 31 ¼
Fig. 1. Cross-section of the ultrasonic device for ultrasonic microscopy.

ð1 þ R32 Þ  ð1 þ R21 Þ  expðk2 l2 Þ;
1 þ R32  R21  expð2k2 l2 Þ

ð7Þ

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M. Arakawa et al. / Ultrasonics 84 (2018) 172–179

Table 1
Acoustic parameters used in the calculation.
Material

Velocity[m/s]

Density[kg/m3]

Acoustic impedance
106 [kg/(m2s)]

Attenuation coefficient[s2/m]

Z-cut Sapphire
SiO2 glass
Water

11217 [28]5968 [30]1491 [26]3986 [28]2200 [30]998 [31]

44.7
13.1
1.5

2.1  1018 [29]1.3  1016 [29]2.23  1014 [32]Table 2
Acoustic parameters used in the calculation.
Material

Velocity[m/s]

Acoustic impedance
106 [kg/(m2s)]

Electromechanical coupling factor

Permittivity [F/m]Zinc Oxide
Gold6330 [33]3240 [30]36.0 [33]62.5 [30]0.24 [34]–7.8  1011 [33]–Fig. 2. Calculated result of loss of an ultrasonic device with R = 1.0 mm. The
thickness of the ZnO transducer is 11.3 µm, and that of the SiO2 AARC film is
6.63 µm.

Fig. 3. Calculated result of loss of an ultrasonic device with R = 0.5 mm. The
thickness of the ZnO transducer is 6.73 µm, and that of the SiO2 AARC film is
3.98 µm.

where k2 and l2 are the wavenumber and length of AARC, respectively. TL was calculated by the following equation.

TL ¼ T 13  T 31 :

ð8Þ

2.4. Calculation
In the calculation, a Z-cut sapphire single crystal, SiO2 glass, andwater were assumed to be materials of the acoustic lens, the AARC,
and the liquid coupler, respectively. We also assumed the following conditions. The ultrasonic transducer was composed of a ZnO
thin film and gold electrodes. The aperture half angle was 60°.
The size of the ultrasonic transducer was same as that of theaperture, and the diameter of the ultrasonic transducer was
1.73R. The parameters at room temperature used in the calculation
are summarized in Tables 1 and 2.
At first, we calculated frequency characteristics of an ultrasonicdevice used around 225 MHz in the measurements using rf burst
signals. Typical parameters are as follows: R is 1.0 mm, the thickness of ZnO is 11.3 µm, the thickness of gold electrode is 0.3 µm,
and the thickness of SiO2 AARC film is 6.63 µm. The frequency fAARC
at which TL became minimum was 225 MHz. The calculation
results of loss are shown in Fig. 2. The dotted line, dashed line,
and dashed-dotted line exhibit CL, PL, and TL, respectively. We
calculated round-trip losses because the measurements were conducted in the reflection mode. The total loss (LOSS) was obtained as
the product of CL, PL, and TL. CL was minimum at 186 MHz, smaller
than 280 MHz of k /2 resonance frequency of the ultrasonic transducer because of the high acoustic impedance of sapphire and thecomplex resonance of the ultrasonic transducer consisting of a ZnO

Fig. 4. Relationship between curvature radius and frequency in the case that the
propagation loss in the water coupler is 15 dB.

transducer and gold electrodes. PL is the main factor of LOSS in thehigher-frequency region because it is proportional to the square of
frequency. The minimum LOSS was 36.3 dB at 173 MHz, and thefrequency was 52 MHz less than the frequency at which TL wasminimum. The calculated results suggested that the operating center frequency in the measurements using impulsive signals became
smaller than the frequency at which TL was minimum because CL
and PL also affect the total loss.

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M. Arakawa et al. / Ultrasonics 84 (2018) 172–179

Table 4
Minimum losses of the point-focus-beam ultrasonic devices. The frequencies in theparenthesis are the frequencies at which the losses became minimum.
DeviceMeasured

Calculated

No. 1

43.8 dB(159 MHz)45.6 dB(268 MHz)35.7 dB(166 MHz)38.3 dB(267 MHz)

No. 2

Fig. 5. Calculated result of loss of an ultrasonic device with R = 0.125 mm. The
thickness of the ZnO transducer is 3.33 µm, and that of the SiO2 AARC film is
2.98 µm.

Table 3
Parameters of ultrasonic devices.

Lens

Transducer
Electrode

AARC

No. 1

No. 2

Material
Diameter of rod
Length of rod
Radius of aperture

Z-cut Sapphire
6 mm
5.5 mm1 mm

Z-cut Sapphire
6 mm
3 mm
0.5 mmHalf angle of aperture
Material
Thickness

60°
ZnO
12.66 µm60°
ZnO
6.33 µm

Material
DiameterThickness
Material
Thickness

Au-Cr
1.73 mm
0.3 µmSiO2 glass
7.46 µm

Au-Cr
0.87 mm
0.3 µmSiO2 glass
4.97 µm

Fig. 7. Comparisons of measured and calculated results of losses of the plane-wave
ultrasonic devices. Solid lines: measured. Dotted lines: calculated. (a) Device 1P. (b)Device 2P.

Next, we calculated the frequency characteristics of an ultrasonic device used around 375 MHz in measurements using rf burst
signals. Typical parameters are as follows: R is 0.5 mm, the thickness of ZnO is 6.73 µm, the thickness of gold electrode is 0.3 µm,
and the thickness of SiO2 AARC film is 3.98 µm (fAARC = 375 MHz).
The calculation results are shown in Fig. 3. CL was minimum at
303 MHz, and minimum LOSS was 41.1 dB at 261 MHz. The operating center frequency was 114 MHz less than desired. From Figs. 2
and 3, the operating center frequencies were lower than the frequency at which TL was minimum we set based on the experimental experiences in the burst mode.
2.5. Design methodology

Fig. 6. Comparisons between measured and calculated losses of the PFB ultrasonicdevices. Solid lines: measured. Dotted lines: calculated. (a) Device 1 (R = 1.0 mm).
(b) Device 2 (R = 0.5 mm).

The frequencies at which LOSS became minimum were less than
the frequency at which CL and TL became minimum in Figs. 2 and3. These results were mainly caused by the large propagation loss
in the liquid coupler (water). In Figs. 2 and 3, propagation losses
were 13.3 dB and 15.2 dB at the frequencies at which LOSS became
minimum, respectively. Meanwhile, they were 22.6 dB and 31.3 dB
at the frequencies at which TL became minimum for devices No. 1
and No. 2, respectively. Therefore, the curvature radius of theacoustic lens should be determined so that propagation loss inthe coupler is around 15 dB at the operating center frequency.

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M. Arakawa et al. / Ultrasonics 84 (2018) 172–179

Fig. 9. Reflected signals from the surface of a slide glass. (a) Device 1. (b) Device 2.

Fig. 8. Comparisons of measured and calculated results of the reflection losses ofthe plane-wave ultrasonic devices. Solid lines: measured. Dotted lines: calculated.
(a) Device 1P. (b) Device 2P.

Table 5
Minimum losses and maximum reflection losses of the plane-wave ultrasonic devices.
The frequencies in the parenthesis are the frequencies at which the losses became
minimum or the reflection losses became maximum.
DeviceNo. 1P

Minimum loss
Maximum reflection loss

No. 2P

Minimum loss
Maximum reflection loss

Measured

Calculated

17.7 dB(191 MHz)
7.0 dB(203 MHz)

16.7 dB(172 MHz)
7.1 dB(200 MHz)14.2 dB(418 MHz)
6.0 dB(302 MHz)

16.2 dB(316 MHz)
7.1 dB(300 MHz)

Fig. 4 indicates the relationship between the curvature radius of
the acoustic lens and the frequency at which PL calculated by
Eqs. (1) and (3) became 15 dB. From Fig. 4, the frequencies were
calculated as 183 MHz and 259 MHz for R = 1 mm and 0.5 mm,
respectively.
Next, we demonstrated the procedure to design ultrasonicdevices by calculating LOSS of a device with an operating center
frequency of 500 MHz. First, the curvature radius was obtained
as 0.125 mm from Fig. 4. The diameter of the ultrasonic transducer
became 0.22 mm from the aperture half angle of 60°. The focal
length F was calculated by Eq. (1), and AL was calculated by Eq.
(4). The thickness of SiO2 AARC was determined to be 2.98 µm sothat fAARC became 500 MHz. TL was calculated by Eq. (8). The thickness of ZnO was determined to minimize LOSS at the operating
center frequency by calculating CL with changing the thickness.
Fig. 5 shows the calculated loss in the case that the thickness of
ZnO was 3.33 µm and that of the gold electrode was 0.2 µm. LOSS
was a minimum of 42.0 dB at 483 MHz. So, ultrasonic devices withthe desired operating center frequency could be designed by using
this method.

Fig. 10. Frequency characteristics of the reflected signals shown in Fig. 9 obtainedby FFT analysis. (a) Device 1. (b) Device 2.

3. Experiments and discussion
Point-focus-beam (PFB) ultrasonic devices No. 1 and No. 2 were
fabricated using acoustic lenses with R = 1.0 mm and R = 0.5 mm,
respectively. Parameters of the devices are shown in Table 3.
Taking the numerical calculation results in Sec. 2 into account,the thicknesses of SiO2 AARC were determined to be 7.46 µm

M. Arakawa et al. / Ultrasonics 84 (2018) 172–179(fAARC = 200 MHz) for device No. 1 and 4.97 µm (fAARC = 300 MHz)for device No. 2. Thicknesses of ZnO film were 12.66 µm for No. 1
and 6.33 µm for No.2, which were optimized to minimize minimum LOSS. To investigate the factors of loss experimentally,
plane-wave (PW) ultrasonic devices No. 1P and No. 2P were alsofabricated with the same parameters as those of the PFB devices
No. 1 and No. 2, respectively, by using cylindrical rods with a diameter of 6 mm and a length of 6 mm instead of acoustic lenses.
The fabrication procedure of the ultrasonic devices was as follows. First, the acoustic lens was cleaned with organic solvents
by ultrasonic bath. Then, SiO2 AARC film was deposited on the lens177

surface by rf sputtering. Next, Au-Cr film was deposited on theplane surface by vacuum evaporation, ZnO film was deposited by
dc sputtering [34], and Au-Cr film was again deposited. Then, thelens was inserted into a stainless holder, an SMA connector was
attached to the holder, and copper wires were used to connect
the electrodes and the connector.
Frequency characteristics of loss of the devices were measured
by the pulse mode measurement system [35]. The input
peak-to-peak voltage to the ultrasonic transducer was less than
2.8 V. The pulse widths of the rf tone burst pulses were 350 ns
and 250 ns for the devices 1 and 2, respectively. The pulse

Fig. 11. Measured intensities and velocities for tendon tissue of the left leg of an S-D rat. (a) Intensity by 80-MHz device. (b) Velocity for 80-MHz device. (c) Intensity by
device No. 1. (d) Velocity by device No. 1. (e) Intensity by device No. 2. (f) Velocity by device No. 2.

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M. Arakawa et al. / Ultrasonics 84 (2018) 172–179

repetition frequency was set to 1 kHz taking into account reverberations in the acoustic lens. Water and a synthetic silica (SiO2) glass
substrate were used as the coupler and the reflector, respectively.
The glass surface was set at the focal point of the PFB ultrasonicdevice. The input power to the ultrasonic device and the output
power of the reflected signal from the glass surface were measured,and loss was obtained by the ratio of these signals. The results areshown in Fig. 6. The reflection loss from the coupler to the reflector
was calculated as 2.0 dB from their acoustic impedances, and it wasadded to the calculation results. Minimum values of losses and thefrequencies at which the losses became minimum are summarizedin Table 4. The frequencies at which the losses became minimum inthe measured loss almost coincided with the calculated values forboth devices. These frequencies were also close to the frequencies
of 183 MHz and 259 MHz at which PL became 15 dB. 6-dB bandwidths were 116 MHz (73%) for No. 1 and 170 MHz (63%) for No.
2, and it was possible to obtain a sufficient bandwidth for the pulse
drive.
Next, we experimentally discuss factors of loss of the PFB
ultrasonic device. We also measured conversion loss at the ultrasonic transducer and reflection loss at the AARC for PW devices
No. 1P and No. 2P. Conversion loss was obtained by measuring
the input power to ultrasonic transducers and the output powerof a signal reflected from the opposite end of the buffer rod.
Reflection loss at the AARC was obtained by measuring the output
powers of the signals reflected from the opposite end of the buffer rod without and with water [36]. The results are shown in
Figs. 7 and 8. Minimum values of the conversion loss and the frequencies at which the conversion losses became minimum, andmaximum values of the reflection loss and the frequencies atwhich the reflection losses became maximum are shown in
Table 5. The measured conversion loss and reflection loss almost
corresponded to the calculated values for devices No. 1P and No.
2P. Therefore, CL and TL of PFB ultrasonic devices No. 1 and No. 2
should have almost coincided with the calculated values. The
attenuation coefficient of water was obtained from publisheddata [32]. Therefore, the differences between the measured andcalculated losses of the PFB ultrasonic devices No. 1 and No. 2
were considered to be caused by the effect of the focused beam.
Particularly, aberration of the acoustic lens was considered to be
the main factor. Transmission coefficient at the AARC is different
from the position of lens because of the thickness distribution(l2cos h, h : incident angle) caused by the fabrication process,and it was also considered to affect the frequency characteristics.
However, the frequencies at which the measured losses became
minimum corresponded to the calculated values. Thus, thecalculation method using the plane-wave model is indicated to
be useful for designing PFB ultrasonic devices with a desired
operating center frequency for the pulse signal.

Next, a Sprague-Dawley (SD) rat was taken as a specimen. The
tendon tissue of left leg was sliced and mounted on a slide glass.
Two-dimensional distributions of intensity and velocity were measured with PFB ultrasonic devices No. 1 and No. 2, and a concave
transducer with a center frequency of 80 MHz [18]. The results
are shown in Fig. 11. First, two-dimensional measurements wereconducted in 8-µm steps for an area of 2.4 mm  2.4 mm by using
a 80-MHz concave transducer. Then, two-dimensional measurements were conducted in 4-µm steps with an area of 1.2 mm 
1.2 mm indicated by the red dashed lines in Fig. 11(a) and (b) byusing devices No. 1 and No. 2. Similar results were obtained by
all devices because of sufficient S/N. As the center frequency of
the ultrasonic device was higher, the spatial resolution in acoustic
image became higher. Similar results were obtained in velocity distributions for each device. The fabricated ultrasonic devices had
sufficient amplitude and bandwidth for measurement of biologicaltissues.
5. Conclusions
In this study, we examined frequency characteristics of ultrasonic devices for SAM using impulsive signals by numerical calculations and experiments. We developed a new method to calculate
the loss of ultrasonic devices by the plane-wave model, taking into
consideration the conversion loss at the ultrasonic transducer, thepropagation loss in the liquid coupler, and the transmission loss at
the AARC. We first calculated frequency characteristics of the ultrasonic device used in the measurements in the burst mode. We clarified that the operating center frequency of the ultrasonic device in
the impulsive mode was lower than the expected value if thedevice was designed by the experimental experiences in the burst
mode. From the results, we developed a procedure to design theultrasonic device, taking the propagation loss in the coupler atthe operating center frequency into consideration. Next, we fabricated ultrasonic devices with a ZnO transducer using acoustic
lenses with R = 1.0 mm and R = 0.5 mm. By comparing the measured losses with the calculated results, the frequencies at which
the loss becomes minimum in the measured loss almost coincidedwith the calculated values for both devices, although the values
were different. Then, the ultrasonic devices were installed in a
medical SAM, and measurements of ultrasonic images and acoustic
properties of tissue specimens were successfully demonstrated.
Therefore, this numerical calculation method is indicated to be
useful for designing ultrasonic devices for SAM using impulsive
signals. Of course, this method is also useful for designing ultrasonic devices for the burst mode.
As a next step, we will fabricate devices with higher operating
center frequencies and apply them to measurement of the acoustic
properties of cells.
References

4. Application
PFB ultrasonic devices No. 1 and No. 2 were attached to a medical SAM (AMS-50SI, Honda Electronics, Co. Ltd.) and driven by
impulsive signals. The focal point of the device was set at the surface of the slide glass. Reflected signals from the front surface ofthe slide glass in the time domain are shown in Fig. 9. The frequency spectra obtained by fast Fourier transform (FFT) of Fig. 9
are shown in Fig. 10. The pulse widths were narrow and the frequency characteristics were wide. The operating center frequencies in Fig. 10 were lower than those in Fig. 6, because theresults in Fig. 10 were not normalized by the frequency characteristics of the input pulse, which has frequency characteristic thatdrops as the frequency increases [18].

[1] A. Korpel, L.W. Kessler, P.R. Palermo, Acoustic microscope operating at 100
MHz, Nature 232 (1971) 110–111.
[2] R.A. Lemons, C.F. Quate, A Scanning acoustic microscope, Proc. 1973, IEEE
Ultrason. Symp (1973) 18–21.
[3] R.A. Lemons, C.F. Quate, Acoustic microscopy: biomedical applications, Science
188 (1975) 905–911.
[4] V. Jipson, C.F. Quate, Acoustic microscopy at optical wavelength, Appl. Phys.
Lett. 32 (1978) 789–791.
[5] J.A. Hildebrand, D. Rugar, Measurement of cellular elastic properties by
acoustic microscopy, J. Microsc. 134 (1984) 245–260.
[6] H. Lüers, K. Hillmann, J. Litniewski, J. Bereiter-Hahn, Acoustic microscopy ofcultured cells Distribution of forces and cytoskeletal elements, Cell Biophys. 18(1991) 279–293.
[7] A. Kinoshita, S. Senda, K. Mizushige, H. Masugata, S. Sakamoto, H. Kiyomoto, H.
Matsuo, Evaluation of acoustic properties of the live human smooth-muscle
cell using scanning acoustic microscopy, Ultrasound Med. Biol. 24 (1998)
1397–1405.

M. Arakawa et al. / Ultrasonics 84 (2018) 172–179[8] T. Kundu, J. Bereiter-Hahn, I. Karl, Cell property determination from theacoustic microscope generated voltage versus frequency curves, Biophys. J. 78(2000) 2270–2279.
[9] Y. Saijo, M. Tanaka, H. Okawai, F. Dunn, The ultrasonic properties of gastriccancer tissues obtained with a scanning acoustic microscope system,
Ultrasound Med. Biol. 17 (1991) 709–714.
[10] Y. Saijo, M. Tanaka, H. Okawai, H. Sasaki, S. Nitta, F. Dunn, Ultrasonic tissue
characterization of infarcted myocardium by scanning acoustic microscopy,
Ultrasound Med. Biol. 23 (1997) 77–85.
[11] G.A.D. Briggs, J. Wang, R. Gundle, Quantitative acoustic microscopy ofindividual living human cells, J. Microsc. 172 (1993) 3–12.
[12] R.M. Lemor, E.C. Weiss, G. Pilarczyk, P.V. Zinin, Mechanical properties of single
cells - measurement possibilities using time-resolved scanning acoustic
microscopy, Proc. 2004, IEEE Int. Ultrasonics, Ferroelectr., Freq. Control(2004) 622–629.
[13] P.V. Zinin, P. Anastasiadis, E.C. Weiss, R.M. Lemor, Variation of the soundattenuation inside HeLa cells during cell division using high-frequency timeresolved acoustic microscope, Proc. 2007, IEEE Int. Ultrasonics Symp (2007)
813–816.
[14] E.C. Weiss, P. Anastasiadis, G. Pilarczyk, R.M. Lemor, P.V. Zinin, Mechanicalproperties of single cells by high-frequency time-resolved acoustic
microscopy, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr 54 (2007) 2257–
2271.
[15] E.M. Strohm, M.C. Kolios, Measuring the mechanical properties of cells using
acoustic microscopy, Proc. 31st Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. (2009)
6042–6045.
[16] E.M. Strohm, G.J. Czarnota, M.C. Kolios, Quantitative measurements ofapoptotic cell properties using acoustic microscopy, IEEE Trans. Ultrason.,Ferroelectr., Freq. Contr 57 (2010) 2293–2304.
[17] Y. Saijo, N. Hozumi, C. Lee, M. Nagao, K. Kobayashi, N. Okada, N. Tanaka, E.S.
Filho, H. Sasaki, M. Tanaka, T. Yambe, Ultrasonic speed microscopy for imaging
of coronary artery, Ultrasonics 44 (2006) e51–e55.
[18] Y. Saijo, E.S. Filho, H. Sasaki, T. Yambe, M. Tanaka, N. Hozumi, K. Kobayashi, N.
Okada, Ultrasonic tissue characterization of atherosclerosis by a speed-ofsound microscanning system, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr 54(2007) 1571–1577.
[19] M. Arakawa, J. Shikama, K. Yoshida, R. Nagaoka, K. Kobayashi, Y. Saijo,
Development of an ultrasonic microscope combined with optical microscopefor multiparametric characterization of a single cell, IEEE. Trans. Ultrason.,Ferroelectr., and Freq. Contr. 62 (2015) 1615–1621.

179[20] N. Hozumi, R. Yamashita, C.-K. Lee, M. Nagao, K. Kobayashi, Y. Saijo, M. Tanaka,
N. Tanaka, S. Ohtsuki, Ultrasonic sound speed microscope for biological tissue
characterization driven by nanosecond pulse, Acoust. Sci. & Tech. 24 (2003)
386–390.
[21] K. Miura, H. Nasu, S. Yamamoto, Scanning acoustic microscopy forcharacterization of neoplastic and inflammatory lesions of lymph nodes, Sci.
Rep. 3 (2013) 1255.
[22] K. Miura, S. Yamamoto, A scanning acoustic microscope discriminates cancer
cells in fluid, Sci. Rep. 5 (2015) 15243.
[23] R.A. Lemons and C.F. Quate, ‘‘Acoustic microscopy,” Physical Acoustics XIV (Ed.
W. P. Mason and R. N. Thurston), Academic Press, London, (1979) 20–21.
[24] G.A.D. Briggs, O.V. Kolosov, Acoustic microscopy, Oxford University Press,
Oxford, 1979, pp. 13–14.
[25] Y. Kikuchi, N. Chubachi, S. Yamamizu, Analysis of ultrasonic multi-layertransducer for UHF and VHF ranges, J. IEICE Trans. A 55-A (1972) 331–338.
[26] W. Kroebel, K.-H. Mahrt, Recent results of absolute sound velocity
measurements in pure water and sea water at atmospheric pressure,
Acustica 35 (1976) 154–164.
[27] J.C. Carman, Classroom measurements of sound speed in fresh/saline water, J.
Acoust. Soc. Am. 131 (2012) 2455–2458.
[28] M. Arakawa, J. Kushibiki, Y. Madani, Ultrasonic micro-spectroscopy ofsynthetic sapphire crystals, Jpn. J. Appl. Phys. 47 (2008) 2285–2287.
[29] B.A. Auld, Acoustic fields and waves in solids, John Wiley & Sons, New York,
1973, p. 96.
[30] W.P. Mason, Physical Acoustics and the Properties of Solids, McGraw-Hill, New
York, 1958, pp. 17–20.
[31] F.E. Jones, G.L. Harris, ITS-90 density of water formulation for volumetricstandards calibration, J. Res. Nat. Inst. Stand. Tech. 97 (1992) 335–340.
[32] Y. Hashimoto, N. Akashi, J. Kushibiki, Measurements of ultrasonic attenuation
coefficients of water in VHF/UHF range, Tech. Rep. IEICE 97 (1997) 37–42.
[33] N.F. Foster, G.A. Coquin, G.A. Rozgonyi, F.A. Vannatta, Cadmium sulphide andzinc oxide thin-film transducers, IEEE Trans. Son. Ultrason. SU- 15 (1968) 28–41.
[34] N. Chubachi, M. Minakata, Y. Kikuchi, Physical structure of DC diode sputteredZnO films and its influence on the effective electromechanical coupling factors,
Jpn. J. Appl. Phys. Suppl. 2 (Pt. 1) (1974) 737–740.
[35] J. Kushibiki, T. Sannomiya, N. Chubachi, A useful acoustic measurement systemfor pulse mode in VHF and UHF ranges, IEEE Trans. Son. Ultrason. SU- 29(1982) 338–342.
[36] J. Kushibiki, H. Maehara, N. Chubachi, Acoustic properties of evaporated
chalcogenide glass films, Electron. Lett. 17 (1981) 322–323.