1J1-3
Relationship between MEMS Mirror Angle and BeamCharacteristics in High-Speed Scanning UltrasoundMicroscopy
Michika Yoshida1‡, Kazuto Kobayashi2, Yuki Kawaguchi2, Takuro Ishii1, 3, Yoichi
Haga1, Yoshifumi Saijo1 (1Grad.School Biomed. Eng., Tohoku Univ.; 2Honda
Electronics Co., LTD. R&D; 3FRIS, Tohoku Univ.)
1. Introduction
Ultrasound microscopy has pioneered thebiomechanicaldiagnosis
ofcancerandatherosclerosis because it can assess the threedimensional distribution of mechanical properties
such as elasticity and viscocity without
histochemical staining1). However, real-time
imaging of ultrasound microscopy is challenging
because the frame rate is limited by mechanicalscanning of the transducer. By using the linear
actuator stage, increase of the scanning speed is noteasy as it often generates greater vibrations of thetransducer and deteriorates the image quality. In theprevious ultrasound microscopes, the imaging speed
to obtain one image was several seconds to several
minutes2). To increase the frame rate with preserving
the image quality, we are developing a new highspeed scanning ultrasound microscope using theMEMS mirror. Since the MEMS mirror can movefaster than the linear scanner without causing any
vibration of the transducer, it has a strong potential
to improve the temporal resolution of ultrasoundmicroscopes3). In this study, the effect of the angle ofthe MEMS mirror on the acoustic beamcharacteristics was investigated for future
development of MEMS-ultrasound microscope.
2. Materials and Methods
2.1 Control of MEMS mirror
This study used the commercial MEMS mirror
unit (S12237-03P, Hamamatsu photonics) to change
the acoustic pulse direction. Fig. 1 shows the control
sequence of a cycle of mirror movement. Div(Frequency division ratio) was determined by thefollowing equation,

Div =𝑓clock−1𝑓set ∙ 𝑁(1)where 𝑓set represents the driving frequency, 𝑓clockis the MEMS mirror driver’s system clock of 100
MHz, and 𝑁 represents the number of sampling
points. Since the MEMS mirror movement wasgiven as a sinusoidal function, the direction of thefirst half cycle of the motion was opposite to that ofyoshida.michika.p1@dc.tohoku.ac.jp

the second half cycle. To unify the scanning direction
for the imaging, the internal trigger was used totransmit and receive acoustic pulses during only thefirst half of the period. PRF of the acoustic pulses
and DAC update rate of the MEMS mirror wererespectively configured so that acoustic transmission
and receiving could complete before the MEMS
mirror angle was changed to the next point. For eachtransmission, the received echo signal was recordedfor 20 s.

Fig.1 Synchronous Signal Timing Chart

2.2 Outline of the experimental system
Controlling the MEMS mirror and acoustic
data acquisition was performed as follows. First, apersonal computer sent the start command to thedriver of the MEMS mirror, which instantly
activated the internal trigger to a function generator(NF Corporation WF1944B). Next, once thefunction generator received the trigger, it output
square pulses to control the timing of the acoustictransmissions. The timing control signal wasreceived by a pulser-receiver (Honda Electronics Co.,LTD.) equipped with a single element transducer
with the central frequency of 20 MHz. After eachtransmission, the echo signal from the target wasreceived by the same transducer and the pulserreceiver. Finally, the received signal was amplified
by 50 dB and sent to an AD converter (SPECTRUMDN2.221-08) to record the data. Note that there
should be nano-scale delay time for each activationof the triggers, but we assumed all the latency wasnegligible for this study.
2.3 Experiments
Using the experimental MEMS-ultrasoundmicroscope, the basic experiments to assess thestability of the echo spectrum in the applied theMEMS mirror angles and the image quality using thesetup shown in Fig. 2.
For the spectrum assessment, the echo signal
from a metal plate placed at 7 mm in depth (thegeometric focus of the transducer) in pure water wasacquired. The MEMS mirror driving range was set to±10 deg in X axis in Fig.2, and the driving frequency
was 10 Hz. PRF of acoustic pulses was 10 kHz, andthe echo data was recorded at the sampling rate of156.25 MHz. For the evaluation, echo signals
acquired at the MEMS mirror angles from 0 to 10
degrees by 1 degree step were extracted, and thespectrum of each echo signal was computed with
Fast Fourier Transform (FFT). The spectrum curves
were used to assess whether the mirror-angulardependency presents in the center frequency and thebandwidth of the echo signals.
In addition, the image quality of the proposedsystem was verified by the full width at half
maximum (FWHM) of the cumulative distributionfunction (CDF). In this experiment, the MEMS
mirror angle was set to 0 degree so that the acoustic
beam direction was converted from Y axis to Z axis,
which is the orthogonal to the surface of the metal
plate placed in the water tank. Then, the metal platewas moved along the X direction using a linear stage
so that the echo signals across the edge of the plate
could be acquired. The edge spread function wascalculated by plotting the maximum amplitude ofeach echo signal and fitting the points with thecumulative distribution function (CDF) of thenormal distribution. Using the estimated parametersof the normal distribution, FWHM was calculated3).
As a comparison, the transducer was placed along
the Z axis to emit pulses to the metal plate without
using the MEMS mirror, and echo signals andFWHM were acquired using the same experimentsteps. Finally, a B-mode image of the metal plate wasgenerated using the proposed system to show its
visualization capability.

Fig. 2 Experimental system

3. Results and Discussion
First, from the spectrum assessment, the meancenter frequency was found to be 14.30 ± 0.01 MHz.
In addition, as shown in Fig. 3(a), the spectrum
curves of the echo signal taken at various MEMS
mirror angles strongly correlated with each other,suggesting the characteristics of the echo signals did
not change with the MEMS mirror angle.
Second, the result of CDF fitting using theecho data of the metal plate’s edge is shown in Fig.
3(b). FWHM from the echo data with the MEMS
mirror was estimated to be 0.686 mm, which wascomparable with 0.791 mm derived from the dataacquired without the MEMS mirror. Thisobservation suggested that the image resolution wasalmost the same regardless of the use of the MEMS
mirror.
Finally, Fig. 3(c) shows the generated B-mode
image. Although the out-of-frame signal from theMEMS mirror was involved in the image, the metalplate could be imaged, indicating that the proposedsystem could be used for B-mode images in thefuture studies.
4. Conclusion
In this study, a prototype of the MEMSultrasound microscope equipped with the MEMS
mirror was developed and the echo signals wereevaluated. The experiments showed that the MEMS
mirror would not affect the spectrum and the beam
shape of the acoustic beam, and hence it will be
useful for improvement of the temporal resolution ofthe acoustic microscopes.
References
1. K. Miura et al.: Sci. Rep. 1255 (2013).
2. K. Kobayashi et al.: JASA 62 (2006).
3. K. Park et al.: Sci. Rep. 13359 (2017).

Fig. 3 (a) Spectrum of echo signals taken at mirror
angles from 0–10 degrees, (b) CDF fitting with theMEMS mirror, (c) B-mode imaging of metal plate