1J1-1
Performance comparison of cell phantoms developed for
optical-resolution photoacoustic microscopy
Daisuke Nishimae1†, Takuro Ishii2,1, Koetsu Ogasawara3 and Yoshifumi Saijo1*(1Grad.School Biomed. Eng., Tohoku Univ.; 2FRIS, Tohoku Univ., 3IDAC, Tohoku
Univ.)
1. Introduction

2.2 The cell phantom for OR-PAM system

Photoacoustic imaging (PAI) is an imaging
method based on the photoacoustic effect that canselectively image cells and tissue. Among various
PAI modalities1), optical-resolution photoacoustic
microscopy (OR-PAM) has a high lateral resolution
superior to the other PAI modalities and has achieveda sub-micron imaging capability with an optimal
combination of the optical and acoustic elements2).
This spatial resolvability is considered to be usefulfor imaging individual cells and intracellular
structures. Furthermore, using PA contrast agents(e.g., metal nanoparticles) is expected to extend theability to image various cells with low opticalabsorbers. However, there has been a lack ofexperimental tools to simulate and characterizephotoacoustic signals from the cells. Here, we reporta cell phantom for testing a single cellular OR-PAM
system using simple material and fabrication
protocol. In addition, we evaluated the efficacy of thedevised phantom as a cellular phantom for the ORPAM system compared with an agar-gel phantom.

To develop a cell phantom, we considered asimple fabrication protocol utilizing simple and lowcost materials. Fig. 1 shows a schematic of thedeveloped cell phantom. This phantom was made ofecho jelly and cling film. First, an object mimicking
an intracellular optical absorber was placed on thebottom of a glass base dish (35 mm Glass Base Dish:Glass diameter: 12 mm; IWAKI). Later, the opticalabsorber was covered with echo jelly (SONO JELLY,Canon Medtech Supply Corp.) and a cling film. The
echo jelly decreased the attenuation of acoustic
propagation and mimic the intracellular environment.
In addition, the cling film kept an imaging
environment and protected the ultrasound transducer.
When imaging is performed, the coupling medium(i.e. pure water) was filled in the void space of thedish (Fig. 1 (a)). To store the phantom, the coupling

2. Methods
2.1 High-resolution OR-PAM
In this study, we utilized a high-resolution
OR-PAM system3). A green pulse laser (wavelength:532 nm, pulse reputation frequency: 10 kHz, pulsewidth: 6ns) was focused with optical elements andirradiated to imaging targets that induce thephotoacoustic (PA) waves from the targets. The PA
waves were acquired with the ultrasound transducer(Center freq. 50 MHz) at a sampling frequency of 5GS/s and recorded as a three-dimensional (X, Y, andtime) data set. PA “A-line” signal at each pixel wasaveraged over 100 PA wave acquisitions.
The recorded PA signals were transformed
into analytics signal through a bandpass filter(passband: 20-100 MHz) and the Hilberttransformation, and the 2D C-mode images weregenerated with Maximum Intensity Projection (MIP).

†*

daisuke.nishimae.p6@dc.tohoku.ac.jp
saijo@tohoku.ac.jp

Fig. 1 The structure of the cell phantom developedin this study. (a) For imaging, (b) For storing

medium was discarded and the lid was set on the dish(Fig. 1 (b)).
2.3 Comparison with conventional phantom
We compared the developed phantom with thewater and the agar phantom (Fig. 2). we choose blacktape as an imaging target and put it on the glass basedish. Fig. 2 (a) was the water phantom, and thisphantom was filled with pure water. Fig. 2 (b) wasdeveloped cell phantom type. The black tape wascovered with the echo jelly and wrap film. Fig. 2 (c)was the agar phantom4),5). The Agar was prepared bydissolving agar powder in hot water to adeveloped cell phantom can be suitable for cellularimaging OR-PAM.

Fig.2 (a) Water phantom, (b) developed cellphantom, (c) Agar phantom (2% w/v agar)

concentration of 2% w/v and stirring until it became
homogeneous. After that, the black tape was covered
with the agar gel and left at room temperature for 1
hour to solidify.
PA images of each phantom were acquired.
PAI parameters were set as the range of 50 μm x 50
μm at 500 nm step (100,000 A-lines) and a laser
power of 9 nJ/pulse, respectively. After theacquisition the maximum intensity of the signal wascompared. In addition, the averaged A-lines andspectrums of the averaged A-lines were comparedamong the phantoms.
3. Results and discussion
Fig. 3 (a) shows the maximum intensity ofeach phantom. The value of the water phantom washighest because agar and cell phantoms have someboundaries that have acoustic properties changed.
The difference value between the agar phantom andcell phantom 8.0 x 10-5 [a.u.].
Fig. 3 (b) and (c) show the average A-lines andspectrum of the averaged A-lines for each phantom.
The spectrum was normalized with water phantom.
Compared with the water phantom and the cell
phantom, the frequency properties were similar in
the range of the ultrasound transducer’s bandwidth(<100 MHz).
Therefore, we could confirm that the PA signal
characteristic of the developed new style phantom
was comparable to one of the agar phantom. The
developed cell phantom was expected to be asuitable phantom for cellular imaging OR-PAM.
4. Conclusion
In this study, we developed a new cellphantom with the echo jelly and cling wrap film for
OR-PAM system. Experiments showed that theacquired PA signal characteristics were found to be
similar to that of the agar phantom. Therefore, theFig. 3 (a) Maximum intensity of each phantom,(b) average A-line, (c) spectrum of average A-line(Normalized by water phantom criteria)

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