## 3 Results and Discussion  
The PA imaging results from the phantom test are shown in Figure 2a-e, where Figure 2b,c exhibit the recovered maps using the unfocused and focused 10 MHz transducer, respectively. It is evident from Figure 2 that the human hairs embedded in the phantom were clearly identified with submillimeter resolution using our PA imaging system. By estimating the full width at half maximum (FWHM) of the specific profiles for the images shown in Figure 2b,c, we discovered the recovered object size for a single hair was 0.2 mm (Figure 2d) and 0.08 mm (Figure 2e), respectively, for the unfocused and focused case. As expected, the reconstruction results with an unfocused transducer overestimated the target size because the measured size of the human hair size is around 0.1 mm. It is also evident from Figure 2 that, compared to the recoveries from the unfocused transducer, the imaging contrast of the hair to the background media based on the focused transducer was considerably reduced. For the in vivo experiment, the PA imaging results from the different sections of an adult zebrafish are displayed in Figures 4-6 by using the unfocused and focused high-frequency ultrasound transducers. We discovered from the imaging results in Figures 4b, 5b and 6b that the major organs were clearly identified with high contrast using the unfocused ultrasound transducer. However, the detailed regional structures within the different organs were not clearly revealed due to the low resolution of the recovered PA images using the unfocused transducer. By contrast, this is not the case for the PA images in Figures 4c, 5c and 6c, which were reconstructed based on the focused high-frequency transducer. Interestingly, the location, size and detailed regional morphological information of the eyes, and other inner organs were effectively recovered, which is in good agreement with the anatomy of adult zebrafish in Figure 3. In particular, we discovered that the generated PA images from the focused high-frequency transducer enable us to more accurately characterize the sizes, locations, and shapes of the internal organs, compared to those from the unfocused one. The downside of the PA images generated with the focused transducer is that the contrast for the regional structures is worsened, compared to those from the unfocused one. Interestingly, Figure 4 shows the reconstructed PA images from the top section of the zebrafish. The black line in Figure 4a denotes the imaging slice of adult zebrafish, whereas Figure 4b,c display the PA images reconstructed with the unfocused and focused transducer, respectively. Very specifically, the recovered PA images from the unfocused transducer have a higher contrast but a lower resolution, while the recovered PA images from the focused transducer exhibit higher resolution but lower contrast. Figure 3: The anatomy of a typical adult zebrafish. Reproduced with permission from [17], Copyright Nature Publishing Group, 2013. Figure 4: **(a)** The photograph of the adult zebrafish, in which the black line denotes the imaging slice of the zebrafish. The PA images were recovered using a 10 MHz unfocused transducer (**b**) and a focused one (**c**). Here, E represents the eye and B denotes the brain. The gray scale (right) records the PA signals in a relative unit, whereas the color scale (right) illustrates the spatial scale, in mm. Figure 5: **(a)** The photograph of the adult zebrafish, in which the black line denotes the imaging slice of the zebrafish. The PA images were recovered using a 10 MHz unfocused transducer (**b**) and a focused one (**c**). Here, K denotes kidney, P denotes posterior intestine, H denotes heart, and E denotes eyes. The gray scale (right) records the PA signals in a relative unit, whereas the color scale (right) illustrates the spatial scale, in mm. Figure 6: **(a)** The photograph of the adult zebrafish, in which the black line denotes the imaging slice of the zebrafish. The PA images were recovered by using 10 MHz unfocused transducer (**b**) and focused one (**c**).