## 3 Results  
### Fluorescence and Optoacoustic Imaging  
Figure 5 demonstrates FL and OA images of the phantom, obtained consequently at the probing wavelength of 690 nm corresponding to the absorption peak of BPD. In the FL modality, the probing power density on the surface was about 30 mW/cm\({}^{2}\) and the diameter of illuminated area was about 25 mm. The exposure times of 10 and 30 ms were used for the water phantom (top row) and agar mixture phantom (bottom row), respectively. In the OA modality, the irradiance on the surface was about 7.5 mJ/cm\({}^{2}\) and the sample interval between B-scans was 30 s. The OA B-scans of the phantom were acquired by mechanical scanning of the OA head with a step of 50 \(\upmu\)m, resulting in a B-scan consisting of 330 A-scans. Figure 5b shows the OA B-scan cross-sections along the Z-direction. In the water phantom, the tube with the BPD solution was easily located and demonstrated a higher signal as compared to the IRDye800 tube. Despite out-of-resonance excitation, the signal from the IRDye800 tube was detected in OA images due to its long tail of absorption spectra in the visible wavelength range (Figure 2). For the agar mixture phantom, the fluorescence image became blurred due to multiple scattering, thus limiting exact localization and detection of the BPD-containing tube, although its presence was evident. The OA signal from both BPD and IRDye800 tubes decayed in this case due to the attenuation of probing radiation in the agar phantom, although both BPD and IRDye800 tubes were still detectable. One can see that the tube cross-sections in the OA images are not represented by uniform circles. This phenomenon is governed by the OA reconstruction artifacts originating from the finite numerical aperture of the focused detector [19]. Complementary information from FL and OA imaging can allow evaluation of the size of the fluorescent object based on both the in-depth information from OA and the fluorescence profile width from FL imaging [19]. Figure 6 demonstrates the FL and OA images of the phantom obtained at the probing wavelength of 785 nm, corresponding to the absorption peak of IRDye800. Similar to the previous case, for each modality the images were obtained for a cuvette filled with water (top), in order to locate the tubes, and with agar mixture (bottom). The signal from the IRDye800 tube was pronounced in both FL and OA images, while there was no signal from both BPD and DMSO tubes, since the probing wavelength was  
Figure 5: Fluorescence images (**a**) and reconstructed optoacoustic B-scans (**b**) of the water phantom (top) and agar mixture phantom (bottom) for the excitation wavelength of 690 nm. Solid green circles represent the areas of averaging for the contrast calculation. Dashed green lines depict the position of tubes filled with dyes. outside the BPD absorption band. Similarly to Figure 5, the presence of agar mixture in the cuvette led to broadening of the fluorescence profile and a decrease in OA signal. However, since the attenuation of probing radiation in the phantom at 785 nm was smaller, the OA signal decay was less than that for the excitation wavelength of 690 nm. Table 2 summarizes the Weber contrast of FL and OA images calculated by the following formula  
\[\centering Contrast=\frac{mean\big{(}I_{Dye}\big{)}-mean(I_{DMSO})}{mean(I_{DMSO})}\@add@centering \tag{1}\]  
where _mean_ (\(I_{Dye}\)) is the intensity of the corresponding (FL or OA) signal averaged over the tube with the dye, and _mean_ (\(I_{DMSO}\)) is the intensity of the signal averaged over the tube with DMSO. Averaging was performed over the green solid circles in the OA images (Figure 5b), and over the whole tube for FL imaging. As seen from Table 2, the fluorescence/absorption contrast in the turbid phantom was significantly reduced as compared to that in water. Generally, IRDye800 provided higher contrast compared to BPD, which proves its potential as an auxiliary contrast agent.