1J3-2
High-speed Photoacoustic Microscopy Using MEMS
Mirror with a Correction Method for Distortion Causedby the Mirror Scanning
ミラー走査由来の歪みに対する補正手法を適用した MEMS ミ
ラー駆動式高速光音響顕微鏡(1 blank line)

Ryo Shintate1‡, Chulhong Kim2, and Yoshifumi Saijo1(1Grad. School of Biomed. Eng., Tohoku Univ.; 2Pohang University of Science andTechnology (POSTECH)
新楯 諒 1†，Chulhong Kim2 , 西條 芳文 1 (1 東北大学大学院 医工学研究科, 2 Pohang University ofScience and Technology (POSTECH)

1. Introduction
Photoacoustic microscopy (PAM) is animplementation for high resolution in photoacoustic
imaging, and allows for selective and functional
imaging of micro-tissues and -circulations1).
However, conventional PAM with mechanicalscanning stages is very time-consuming to imaging.
In particular, optical resolution PAM (OR-PAM)
requires a lot of scanning point because of its highlateral resolution, consequently it takes ~30 minutes
for an imaging in 2 mm x 2 mm2). This makes not
only the motion artifact but also the difficulty of thebiological interpretation of the imaging result. Thus,a fast-scanning PAM is required for its biologicalapplications, especially in OR-PAM.
Recently, MEMS mirror is expected as asolution for high-speed PAM. It scans optical andultrasonic beams by controlling the angle of themirror with a sinusoidal driving voltage, providing
much faster than the mechanical scanning3).
However, when the mirror is scanned by sinusoidal,
the nonlinear behavior of scan angle is happened. To
correct the distortion, in the previous study, thefollowing two parameters had to be verified in detailin advance.: 1)The characteristics of the MEMS
mirror such as the voltage-angle relationship.
2)Working distance from the mirror to the focal point.
These verifications are very complicated and timeconsuming. Therefore, a simple method to correct
the distortion is needed for ease of the applications.
In this study, we develop an OR-PAM using
MEMS mirror (MEMS-OR-PAM), and propose anovel method to simply correct the distortion causedby the nonlinear angle scanning of the MEMS mirror.
2. Materials and Methods
2.1 Experimental Setup
Fig. 1 shows the developed MEMS-OR-PAM
system. This system employed a Nd:YAG 532 nm

Fig. 1 Schematic of MEMS-OR-PAM systemlaser (10 ns pulse width; 10 kHz pulse repetition rate)
as the laser source. The emitted laser beam is coupledthrough the objective lens (OL1) into a multimodefiber (FC, 10 µm core diameter). The laser beamfrom the fiber output was shaped by a collimatinglens (CL) and then split into two beams by a beam
splitter (BS). The One was directed to thephotodetector (PD) as a trigger signal for dataacquisition timing, and the other beam was tightly
focused by an Achromatic Doublet lens (OL2, 60mm Focal Length; 0.2 NA). The focused laser beam
was gone through a Opto-Acoustic Beam Combiner(BC) to coaxial and confocal with an acoustic focus
consisting of an unfocused ultrasound transducer(UT, 50 MHz center frequency) and an acoustic lens(AL, 0.25 NA). The Opto-Acoustic Beam is reflectedby the 1-Axis-Waterproof-MEMS (1A-WP-MEMS)
mirror (AL coating) and irradiated to the sample. The
photoacoustic signals are detected by an ultrasoundtransducer, amplified by two series connected
amplifiers (AMP, 46dB Gain), and then recorded bya high-speed data acquisition card (500 MS/s
Sampling Frequency).
For 3D volumetric data acquisition of thesignals, angular scanning in the X direction with a1A-WP-MEMS mirror and mechanical scanning in
the Y direction with a mechanical stage wereperformed. The driving voltage for the angularscanning of the 1A-WP-MEMS mirror was driven bya sinusoidal wave for fast scanning.
2.2 Correction for distortion caused by MEMS scanFig. 2 shows the linear/nonlinear behavior ofangle scanning by MEMS mirror. Since PA signals
are acquired at the timing of laser PRF, the acquired
PA lines assumed to transition at a linear angle
before considering the non-linearity, as shown inFig. 2(a). To correct it to the actual nonlinear
behavior as shown in Fig. 2(b), the following
equation is introduced,
𝜃
𝜃
(1)
𝜃
=
sin𝜋 ,2𝜃
where 𝜃is the nonlinear scanning angle,
𝜃is the range of scan angle, and 𝜃
is thelinear scanning angle.
Here, in order to apply Eq. (1) and convert thepolar coordinates to the cartesian coordinates, priorknowledge of 𝜃and r (length from the mirror,
consisting of working distance (WD) and acquired
PA lines) is required. In this study, we introduced anovel method to calculate them by imaging a ruler,and simply correct the nonlinear distortion. Thismethod is performed by following 6 steps.
1) PA imaging of ruler in the interest region (2 x 2mm2 in this study) by adjust the driving voltage.
2) Set the value of WD virtually and calculate r.
3) Calculate 𝜃from the WD and the imaging
range in the X direction (2 mm in this study).
4) Calculate 𝜃by using Eq. (1).
5) Cartesian coordinates transformation, andimage reconstruction.
6) Repeat 2) - 5) while varying the WD to get theimage completely corrected the distortion.
After completing the above steps, the effectivenessof this method was verified by comparing C-modeand B-mode images with Raw Data / Linear
Distortion Corrected / Nonlinear Distortion
Corrected (proposed).

Fig. 2 Distortion caused by MEMS scanning,(a) Linear Distortion, (b) Nonlinear Distortion.

Distortion is simply and completely corrected asshown in Fig. 3(e)(f), under the robust value settingof WD=7 mm and 𝜃~16°.
Even in the in-vivo imaging with developed,by performing the proposed method once in advance,
the distortion can be easily corrected.
4. Conclusion
In this paper, we developed MEMS-OR-PAM
for high-speed imaging. Also, we proposed a methodthat easily considers the non-linearity of the scanning
angle of the mirror, and successfully removed thenon-linear distortion of the photoacoustic image.
References
1. J. Yao et al.: Nat. Methods, 13(8), (2016).
2. J. Yao et al.: Laser Photonics Rev., 7, (2013)758.
3. J. Y. Kim, et al.: Sci. Rep., 5, (2015)7932.

3. Results and Discussions
Fig. 3 shows the result of the comparison. In
the result of Raw Data, both C / B-mode images are
distorted as shown in Fig. 3(a)(b). When only thelinear scan angle characteristics (Linear Distortion)
are corrected by polar transformation, the image isstill distorted as shown in Fig. 3(c)(d). On the other
hand, by using the introduced method, Nonlinear
ryo.shintate.r4@dc.tohoku.ac.jp

Fig. 3 C-mode (left) and B-mode (right, used the centerline of C-mode) images of ruler, (a, b) Raw Data, (c, d)Linear Distortion Correction, (e, f) Nonlinear Distortion
Correction.