## 3 Results and Discussion  
### A Comparison of the Three Different Illumination Methods  
The results discussed in this section focus on analyzing the PAT images and PA signal amplitudes from progressively deeper phantom cross-sections using the discussed illumination methodologies. Figure 4 shows the UST and PAT images for the first and third cross-sections, which are separated by 60 mm. The PAT images are masked based on the region of interest (ROI) as determined by the UST image. For each illumination method, the PA amplitude is normalized to the highest value for the method across all cross-sections. For example, for the full-ring illumination method, images were normalized to the highest amplitude of PA detected within the three slices, which are separated by 30 mm each. This allows for visualization of the effect of depth on the PA signal amplitudes for each illumination method. As can be seen in the PAT images, the graphite absorber is visible in the first and third layers (i.e., larger vertical depth) of the full-ring illumination method, which is not the case for the diffuse and point-source methods. Figure 3: (**a**) Graph illustrates the polyvinyl chloride (PVC) phantom and graphite inclusions and their dimensions. (**b**) A photograph of the experimental setup including the ring US transducer. A motorized translational stage was used to adjust the position of the phantom to acquired images at multiple cross-sections. To further quantify the results shown in Figure 4, the PA signal amplitude across the graphite absorber for each illumination method, and for each cross-section, is plotted in Figure 5. For the full-ring illumination method, the PA values are nearly constant across the three cross-sections (Figure 5a). However, the peak amplitude value of the PA signal decreases by 25 times for point-source illumination  
Figure 4: Ultrasound tomography (UST) and normalized PAT images of the PVC phantom with the graphite absorber using the three different illumination techniques for cross-sections 1 and 3.  
and 15 times for the diffused-beam illumination between the first and third cross-sections. As seen by the uniformity in the amplitudes between three cross-sections for the full-ring illumination method, this imaging technique can provide a consistent image regardless of vertical depth. On the other hand, the diffused and point illumination methodologies show variance in PA amplitude signals. This finding demonstrates that the full-ring illumination is capable of providing sufficient fluences at lower vertical depths, which results in detectable and reliable PAT images across cross-sections. To further compare the performance of the three illumination strategies, the SNR and contrast-to-noise ratio (CNR) of the PAT images were measured. In the SNR, the value is calculated by:  
\[\text{SNR}=20\times log10\left(\frac{M_{S}}{M_{B}}\right)\]  
where \(M_{S}\) refers to the mean of the PA signal, as marked by the US image region of interest, while \(M_{B}\) refers to the mean of the phantom background. The CNR is determined by:  
\[\text{CNR}=20\times log10\left(\frac{M_{S}-M_{B}}{\sigma_{B}}\right)\]  
where \(M_{S}\) refers to the mean of the PA signal, \(M_{B}\) refers to the mean of the phantom background, and \(\sigma_{B}\) is the standard deviation of the phantom background. For the full-ring illumination method, the used PA amplitude values are from irradiating the target cross-section 15 mm below the cross-section of interest. As can be seen in Figure 6a,b, the SNR and CNR are nearly constant for the full-ring illumination across the three cross-sections, which is not the case for the diffuse and point illuminations. Figure 6c also plots the amplitudes across the graphite object at the third cross-section for all illumination methods.