## 3 Results and Discussion
### The PA Amplitude of the Targeted Cross-Section as a Function of Illumination Position  
The optimum position for the full-ring beam was investigated by evaluating the effect of the distance between the ring-shaped beam and the targeted cross-section. The distance between the targeted cross-section and the full-ring beam was changed within a range of 0-20 mm. Zero millimeters represents the case where the ring beam is illuminating the targeted cross-section at the graphite rod, while the 20 mm case is when the ring beam is 20 mm below the graphite rod (Figure 7a). In this study, a selected cross-section was imaged while changing the illumination location from 0 to 20 mm. The five different positions chosen to illuminate the targeted cross-section of the graphite rod were 0, 5, 10, 15, and 20 mm. A 532 nm laser with 100 mJ per pulse energy was used for this experiment. The location of the full-ring illumination was adjusted by translating the phantom in steps of 5 mm in the vertical direction. Figure 6: The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the PA amplitudes at three different cross-sections are plotted in (**a**,**b**), respectively. For the full-ring illumination, the values were determined based on the illumination at 15 mm below the cross-section of interest. (**c**) Plots the PA amplitude for the top cross-section (cross-section 3) for full-ring, diffuse, and point illumination. In Figure 7b, the PA amplitude was measured by drawing a line across the targeted graphite for all five positions. The overall shape of the PA amplitude is constant while the maximum increases as one moves further below the desired imaging cross-section. This increased visibility could be due to the fact that the incidence angle of the beam illuminates the central part of the object more directly as it moves below the cross-section. A possible reason for the stronger PA signal, when illuminating 20 mm below the targeted cross-section, could the incident angle of the light diffusion within the tissue. In the current prototype, the ring mirror is illuminating the object cross-section at an angle of 39 degrees with respect to the object surface. The optimum illumination will occur if the ring beam falls normal to the surface. The 0 and 5 mm cross-sections use the US image as a mask to crop out the large PA peaks generated at the surface of the phantom. The graphite absorber was embedded within the PVC background with a margin from the surface. Here, we only evaluated the signal arising from the absorber. In cases where the illumination was coincident with the center of the transducer, a strong PA signal from the surface was observed (not shown in these masked images). Hence, imaging below the transducer can help to better visualize more central parts of the object due to elimination of a large PA signal arising from the light-entering surface. The large PA signal at the surface is primarily due to the large fluence at the surface, which can affect the visualization of deeper regions due to a limited dynamic range in PA acquisition. This is not the case where the illumination was adjusted to 10 to 20 mm with respect to the center of the US detectors (i.e., illuminating below the imaged plane). The averaged PA amplitude from all five illumination methods (shown in red in Figure 7b) is similar to the 10 mm illumination results. It is worth mentioning that these results are not necessarily general for all ring illumination systems and are dependent on the incident angle of the beam with respect to the object (39 degrees in our case). The results might vary in other ring illumination systems if the incident angle is changed. These findings are important because they help to define the illumination scenario based on the characteristics and size of the given object. For example, for a large diameter phantom one might need to acquire PAT images using more than one position of full-ring illumination to cover the regions closer to the outer surface and deeper regions (with illumination offset from the imaging plane).