## 4 Discussion  
Photoacoustic imaging has shown great promise in preclinical studies [11; 15; 24; 61; 62; 63; 64; 65; 66]. It has also shown great potential for clinical translatability. Such translation, especially for neonatal brain and breast imaging, requires an imaging system with a large detection view, fast acquisition and volumetric imaging capability [7; 20; 21; 22; 23; 24; 25]. Towards clinical translation, we have tested the feasibility of a stationary 3D HA-PACT imaging system with a full-field illumination scheme. Through quantitative analysis of simulation results, we have demonstrated that using a greater number of transducers in the HA-PACT system results in higher quality images; this was expected as the greater number of view angles will provide the image reconstruction algorithm with improved spatial averaging, which  
Figure 3: (**a**) Flowchart of the continuous sequential optimization algorithm; (**b**) reconstructed images of four leg phantom (i) before and (ii) after optimization; (**c**) bar chart demonstrating increase in the contrast to noise ratio of the reconstructed images before and after optimization. aids in artifact removal [56]. Therefore, based on the diameter of transducers and the thickness of the designed hemispherical structure, 50 single element transducers are the maximum amount that can be placed on a 12-cm diameter hemispherical dome. Although the physical increase in the number of transducers is limited by the space on the hemisphere, we ran a series of simulations (Figure 4a,b) to show that using a greater number of transducers with smaller sizes improves the CNR as shown in Figure 4c. Such configuration can be implemented using either smaller transducers or the same size transducers with a rotating scheme. We tested two illumination configurations, full-field illumination and single-fiber overhead illumination. Our experimental results (shown in Figure 2) demonstrated that the proposed homogeneous illumination outperforms the traditional overhead illumination. This was mainly because the overhead illumination does not provide uniform deposition of optical energy, especially on the underside of the objects where they are directly in the field of view of the transducer elements. This inadequate illumination induced a deteriorated signal at the propagation angle to the elements and, therefore, worsened the reconstructed image quality. According to the literature [40], the larger the area of illumination, the more light we can couple to the tissue. This justifies the use of the entire area of the hemisphere and placement of fibers in an equidistant fashion on it. Table 3 provides an estimated cost to develop the proposed system. The overall cost of the proposed hemispherical system is approximately $42K. Other PACT systems such as the one developed by [PERSON] et al. cost $15K using the rotation of one single element. Another system based on ring array presented in [67] that costs over $100K. Figure 4: Simulated reconstruction of three-leg phantom using (**a**) 5-mm diameter active area transducer with (i) 50, (ii) 100, and (iii) 200 elements; (**b**) 10 mm diameter active area transducer with (i) 50 and (ii) 100 elements; and (**c**) absolute contrast to noise ratio of different numbers of elements with 5- and 10-mm diameter transducers. Our preliminary design was a proof of concept to investigate the feasibility of 3D imaging using single element transducers, in which we did not use the most optimum transducers in terms of size, type and sensitivity. The laser repetition rate was not optimal either. Our end goal is to develop a real-time 3D imaging system that is capable of hemodynamic imaging. One way to improve the proposed design is to rotate the hemisphere; such design will maintain inexpensive hardware, but virtually increase the number of view angles and hence dramatically improve the quality of the reconstructed image [68; 69].