3J4-2
Effect of Coherence Factor Weighting for Improving theImage Quality of an Annular Array Photoacoustic
Microscope
アニュラアレイ光音響顕微鏡の画質向上に資するコヒーレン
スファクタの検討
Riku Suzuki1‡, Ryo Shintate1, Takuro Ishii1,2, and Yoshifumi Saijo1 (1Graduate Schoolof Biomedical Engineering, Tohoku Univ.; 2Frontier Research Institute forInterdisciplinary Sciences, Tohoku Univ.)

鈴木 陸 1‡，新楯 諒 1，石井 琢郎 1,2，西條 芳文 1 (1 東北大学大学院 医工学研究科, 2 東北大学 学際科学フロンティア研究所）

1. Introduction
Morphological observation of the peripheral
microvascular network is known as a promising
diagnostic method for detecting subtle changes insystemic
hemodynamics
andevaluating
inflammation in skin and joints1, 2). Microvessels inthe dermis have a diameter of several tens ofmicrometers, and in order to visualize thesesuperficial microvessels, it is necessary to establish
imaging techniques with high spatial resolution andselective extraction of blood vessels. On this topic,photoacoustic (PA) imaging has gathered muchattention as it is capable of selectively visualizing
those micro-scale blood vessels3).
Our research group has developed a novel
photoacoustic imaging system equipped with a fourchannels annular array transducer to achieve
visualization of superficial microvascular networks
with an increased depth-of-field (DoF). The annulararray transducer consists of four ring-shaped
concentric concave elements so that it can improve
the DoF by applying the Delay-and-Sum (DAS)
method. Nevertheless, the DAS method sometimes
results in blurred images when signals from off-axis
cause the phase shift of the received signals4).
As a solution, the coherence factor (CF)weighting may be used in addition to the DAS
method as it has effectively suppressed such off-axissignals in the adaptive beamforming in the signal
processing of ultrasound imaging systems4). In thispaper, we applied the CF weighing to the PA dataacquired with the annular array transducer andevaluated the efficacy of the method for improving
the image quality.
2. Material and Methods
2.1 Experiment setup and data acquisition
A PA imaging system (HadatomoTM ZWEL5200 prototype developed in a joint effort by
Tohoku University and Advantest Corporation) was

utilized in this study. The system can emit pulsed
laser beams (wavelength: 532 nm) and acquire PAsignals from the superficial vessels with the 4-channular array transducer (center frequency: 60 MHz).
In this study, the laser fiber and the transducer wereset to be mechanically moved to scan an area of 9 x
9 mm at a scan step of 30 x 60 µm.
Cutaneous microvessels in the inner side of theforearm of a healthy male in his 20s were measured
with the system. In order to avoid artifacts from themelanin in the epidermis, only the PA signals thatcame from the dermis layer (approximately 1.2–1.8mm) were acquired at a sampling frequency of 500
MHz.
2.2 Processing of the received PA signals
For the photoacoustic signals received by thefour elements, the DAS algorithm was applied bycorrecting the difference in the arrival time of thesignals to each element and summing those correctedsignals together5). Furthermore, the CF weighting
coefficients were calculated and multiplied to thesignals after the DAS processing5). The range of CF
was from 0 to 1 depending on the phase coherencyof the signals among the channels. It is noted that raw
RF data was generated as a control by adding thereceived signals without any phase corrections.
2.3 Visualization of skin microvessels
To visualize the processed PA data, themaximum amplitude projection (MAP) method wasapplied and three C-mode images (Raw, DAS andDAS+CF) were generated. Each image wasnormalized by its maximum amplitude value. To
evaluate the image quality of each condition, the fullwidth at half maximum (FWHM) and peak signal tonoise ratio (PSNR) were calculated. All signal andimage processing and data analysis were carried outin MATLAB software (MATLAB 2020b,Mathworks).

Fig. 1. The microvessels C-mode images: (a) Raw image, (b) Image with the DAS method, and (c) Image with
the DAS and CF weighting. Profiles in white and yellow line correspond to curves in (d) and (e), respectively.
Table 1. Values of full width at half maximum andPSNR for each data.

method because it suppressed signal intensity in thenoise region as well as those in the vessel regions.
4. Conclusion

3. Results and discussion
The micro-scale blood vessels weresuccessfully visualized with our PA imaging system(Fig. 1). The image generated using the raw PA
signals (Fig. 1(a)) was blurred and the vascular
structure was not clear. On the other hand, the image
generated applying the DAS and CF methods (Fig.
1(b, c)) showed that those methods could effectively
sharpen the microvascular structure.
The sharpness of the vessels was evaluated ata large and a small blood vessels marked byrespectively white and yellow lines in Fig. 1(a–c).
Each profile of the amplitude under the lines and its
fitted curve using a gaussian mixture model wereshown in Fig. 1(d, e) and their FWHM values wereshown in Table 1. The FWHM of DAS+CF wasfound to be the smallest among the conditions, andthe visualized diameter of the vessels was sharpened
by 45–71 % compared to the raw image.
In addition, we calculated PSNR using a noise
region marked as blue boxes in Fig. 1(a–c). As
shown in Table 1, PSNR was decreased in DAS+CF
riku.suzuki.t2@dc.tohoku.ac.jp

In this paper, we assessed the effect of the CF
weighting method, in addition to the DAS method,
for the annular array PA imaging system byevaluating the imaging quality of the cutaneous
microvasculature. The experiment showed that theCF weighting was effective to sharpen microvessels,
while it might excessively suppress the entire signal
intensity. To overcome the drawback, we will further
study the appropriate way to utilize the CF weighting
method by considering the effect of the imaging
depth for the CF coefficients.
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