## 3 Results  
### Simulation Results  
The simulated object was reconstructed and the reconstructed images using a different number of equidistant transducers, i.e., 14, 26, 38, and 50, are shown in Figure 1d. The quality of the reconstructed images as a function of number of elements was quantitatively assessed by evaluating the CNR of the images; the results are given in Figure 1e. Additionally, we present a square root function fit to demonstrate the change in CNR with an increasing number of elements. As defined in Section 2.1, CNR was calculated using the square root of the sum of the variances of the object pixels and background pixels. Therefore, it was assumed that CNR was proportional to the square root of the number of elements in the system. Results of Figure 1e promoted the use of the maximum number of elements that can fit on the surface of the 12-cm diameter hemisphere.  
For light illumination, we evaluated two configurations: (i) single-fiber overhead illumination, in which a single large optical fiber was used on top of the sample and illuminated the entire sample, and (ii) full-field illumination using a bundled optical fiber, in which 40 optical fibers were uniformly distributed on the residual space between the transducer holes in the designed hemisphere using the same algorithm described in [49]. This method was designed based on the fact that the larger the area of illumination, the more light we will be able to couple to the tissue [40]. The 3D projection  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Hemisphere radius & 6 cm \\ Transducer active aperture/number & 12.7 mm/50 \\ Optical fiber diameter/number—full-field illumination & 5 mm/40 \\ Optical fiber numerical aperture—full-field illumination & 0.70 \\ Optical fiber diameter/number—overhead illumination & 12 mm/1 \\ Optical fiber numerical aperture—overhead illumination & 0.35 \\ \hline \end{tabular}
\end{table}
Table 2: 3D HA-PACT System specification.  
model of the hemisphere showing the transducer and optical fiber locations is shown in Figure 1b. The fluence map of the two configurations are visualized using 3 DSlicer software (Brigham and Women's Hospital, Harvard Medical School, USA) and presented in Figure 1f. The simulations were performed using Zemax. The parameters considered in the Zemax simulations are as follows: 1,000,000 rays, 10 mJ input energy for both configurations; NA for the full field and overhead illuminations were 0.70 and 0.35 respectively; and diameters for the full-field fibers and overhead fiber were 5 mm and 12 mm respectively. As shown in Figure 1f(i,ii), the full-field illumination configuration provides a more homogenous illumination with higher optical energy compared to single-fiber illumination. We quantitatively compared these fluence maps within a sphere of 2-cm diameter right above the center of the cap, where the imaging target is located at. The average fluence and standard deviation for the full-field illumination within this sphere were 0.038 and 0.003, while those were 0.078 and 0.01 for overhead illumination, which suggests a less homogeneous illumination by the overhead configuration and consequently lower quality images.