3P5-6
Characteristic Analysis of PA Signals Depending on
Size of Optical Absorber Using an AR-PAM System
Riku Suzuki1‡, Takuro Ishii1,2, and Yoshifumi Saijo1(1Grad. School of Biomed. Eng., Tohoku Univ.; 2FRIS, Tohoku Univ.)
1. Introduction
Photoacoustic (PA) imaging is an imaging
technique that is based on the PA effect, which is thethermal expansion of a light-absorbing material irradiated with a short pulse of light, generating a pressure wave. To achieve high-definition images of microvessels with diameters ranging from 10 to 200 μm,which are reported to be grown by angiogenesis in
inflammatory diseases, our research group has developed a high-frequency annular array transducer andreported an algorithm for enhancing image quality
based on signal phase information between the transducer elements1). For further high-precision functional imaging, identification of the morphologicalstructure of microvessels is required.
Recently, “PA spectral analysis (PASA)” hasreported that the PA signal's power spectrum contains tissue microstructural information2,3). PASA isbased on the fact that the frequency components of
the PA signal are closely correlated with morphological properties such as the size, shape, and density ofthe optically absorbing objects in the tissue2). For example, small objects have a broader power spectrumwith more high-frequency components than PA signals from larger objects.
However, previous studies on PASA havemostly focused on the evaluation of tissue3) or microsphere absorber2), and there are fewer reports onspectral analysis in line structures such as blood vessels or on spectral analysis of PA signals received byannular array transducer.
In this study, to investigate whether PASA
with a high-frequency annular array transducer canidentify morphological information of microvessels,
we analyzed the PA spectrum from several tungsten
wires with different diameters and investigatedwhether there is a relationship between the line structure source and frequency. Six tungsten wires of different diameters (20, 30, 50, 100, 150, and 200 μm)
were measured, and the slope and intercept of the regression line obtained by spectral analysis of the PA
signal received at the central element of the annulararray were calculated.
2. Material and Methods
2.1 Experiment setup and data acquisition
The PA micro-imaging system used in thisstudy has been reported in a previous paper1). The

annular array transducer had a concave surface witha geometric focus of 6 mm from the surface of its anda center hole (I 1 mm) for the laser outlet. The concave geometry was divided into four ring-shaped
PZT elements (a center frequency of 60 MHz and a
detection bandwidth of 24 MHz) with a minimum diameter of 1 mm and a maximum diameter of 6 mm.
Our previous report calculated the theoretical axial
and lateral resolutions as 56.1 and 36.2 μm, respectively.
The Nd:YAG laser was selected to irradiate
532 nm pulsed beams at a repetition rate of 1 kHz
and a pulse duration of 1.2 ns. The PA transmitter/receiver unit was raster-scanned with an XY stage
while repeatedly emitting a laser pulse and receiving
PA signals. In each receiving event, the PA waves
generated at the object by the laser irradiation werereceived by each element of the annular array transducer and recorded at a sampling frequency of 500
MHz. All signal processing and visualization wereperformed on MATLAB (Ver. 2023a, Mathworks).
2.2 PA Spectrum Analysis
Tungsten wires of six different diameters (20,30, 50, 100, 150, and 200 μm) were measured 10
times at the geometric focal point and averaged toreduce noise. The obtained signals were visualized in
C-mode by applying the maximum amplitude projection (MAP) method1), and the A-lines at points
above -6 dB were obtained. For each A-line, only
signals between 15 μm shallow and 120 μm deeper
than the peak signal were extracted and processedwith a 1.42 μs hamming slide window with 30%
overlapping3). The power spectral density (PSD) in
dB of the obtained signalswas calculated as definedby the following equation, ൌ ͳͲ ଵ଴ȁ‫ܨ‬ሺɘሻȁଶ߂݂where ‫ܨ‬ሺɘሻ represents the spectrum obtained by
fast Fourier transform of the acquired signal, and ߂݂is the bandwidth to be analyzed, ߂݂ ൌͷͲͲሺ ሻΤͳͲʹͶ  ሺሻ  ؒͲǤͶͺͺሺ ሻ. These processes were performed for
50 locations above -6 dB in C-mode image, and thePSDs were averaged. In the acquired spectrum, considering that photoacoustic waves are mainly lowfrequency and broadband, and the bandwidth of theannular array transducer, the regression line of thePSD for frequencies from 1 to 72 MHz was calculated, and the slope and intercept of the regression
line equation were obtained.

3. Results and discussion
Fig. 1 shows the results of the spectrum analysis. As shown in Fig. 1(a), the larger the diameter of
the tungsten wire, the stronger the low-frequencycomponent, and the smaller the diameter, thestronger the high-frequency component around 60
MHz. As a result of calculating a regression line foreach spectrum, as shown in Fig. 1(b), the smaller thediameter of the wire, the larger the slope value. As
shown in Fig. 1(c), the intercept value of the regression line increased with wire diameter.
The reason why the intercept value at 20 μm
was calculated larger than that at 30 μm is that themeasured wire diameter was less than half the resolution of the system as described in section 2.1 andthe PSD was calculated larger around 60 MHz,
which is the center frequency of the annular arraytransducer. This suggests that when performing spectral analysis using a high-frequency transducer, theslope and intercept values for optical absorbers
smaller than half the resolution may not vary uniformly with diameter as well as those for diameterslarger than the system resolution.
In summary, these results suggested the possibility of obtaining morphological information on line
optical absorbers by spectral analysis in wires measured with a high-frequency annular array transducer.
4. Conclusion
In this study, we analyzed the PA spectrum
from several tungsten wires with different diameters
to investigate whether line structure sources are related to the frequency and whether it is possible toidentify morphological information of optical absorbers using a high-frequency annular array transducer. As a result, spectral analysis of wires measured with a high-frequency annular array transducer
indicated a relationship between the diameter andfrequency of the line absorber, suggesting the possibility of obtaining morphological information on theline optical absorber by using information on theslope and intercept of the regression line. In the future, we will examine the effectiveness of thismethod in actual human skin microvessels.
Acknowledgment
This work was supported by Grant-in-Aid for JSPS
Fellows, Grant Number JP23KJ0175.

Fig. 1 (a) Results of spectral analysis. (b) (c) The boxplot showing the slope and the intercept value of theregression line for each spectrum, respectively. The
red line in each box is the median of the sample; theupper and lower ends of each box indicate the upper
quartile and lower quartile; The '+' mark indicates outliers.
E-mail: ‡riku.suzuki.t2@dc.tohoku.ac.jp

References
1) R. Suzuki, R. Shintate, T. Ishii, and Y. Saijo, Jpn.
J. Appl. Phys., 61, SG1047 (2022).
2) G. Xu, I. A. Dar, C. Tao, X. Liu, C. X. Deng andX. Wang, Appl. Phys. Lett 101, 221102 (2012).
3) R. E. Kumon, C. X. Deng and X. Wang, Ultrasound Med. Biol., 37, 834 (2011).