# 3D Photoacoustic Tomography System Based on Full-View Illumination and Ultrasound Detection
## 4 Discussion and Conclusions  
A 3D-PACT imaging system based on full-view illumination and detection was proposed and developed in this study. In the design of optical path, the system adopted a mode of circular illumination with the even arrangement of eight laser beams around the water tank to ensure that the imaging target received more uniform laser radiation, which improved the low-efficiency of photoacoustic imaging whose illumination was from the top to the bottom. Second, drawing on the design of photoacoustic coaxial confocal in photoacoustic microscopy imaging technology, a photoacoustic coplanar structure was proposed by means of a quartz bowl with light-transmitting and ultrasound-reflecting properties. The system extended the photoacoustic coupling from a single dimension to a two-dimensional space, increasing the photoacoustic detection range of the imaging section and improving the signal to noise ratio of the photoacoustic signal. In addition, the dual-foci virtual point ultrasonic transducer with a large receiving angle improved the imaging speed of the system via sparse sampling. In the reconstruction method of the photoacoustic image, taking the receiving angle of ultrasound transducer into account, the improved back-projection reconstruction algorithm based on sensitivity factor was achieved, and the photoacoustic image reconstruction using the algorithm took 1.4287 s.  
The practicality of the illumination method and the designed structure enable the system to perform clear and high-quality 3D photoacoustic imaging on targets with complex surfaces or requiring a large imaging range. The feasibility of the system was verified through systematic testing of resolution, imaging range, and photoacoustic coplanar property and 3D photoacoustic experiments on various phantoms. The best compromise was reached between imaging performance and system cost.  
The imaging speed of the system was about 9 s in one cross section (90 sampling points per B-scan, it took about 15 min to scan 100 B-scans), and had not reached the requirements of fast imaging; however, the phantom experiments conducted in this study have demonstrated the characteristics of the system. The next step will be focused on the research of sparse circular array ultrasonic transducer based on virtual point and resolution improvement. In practical applications, the system is expected to be used for corresponding work in the pathological study of an in vitro tumor and the research of targeted photoacoustic probes for breast cancer cells. Moreover, the 3D reconstruction will be further investigated.  
Design and construction of the 3D-PACT, M.S., D.H., and L.M.; Completion of photoacoustic imaging experiments, D.H., W.Z., and Y.Q.; Control system design, M.S., D.H., and L.M.; System debugging and improvement, Y.L., Y.Q., and W.Z.; Algorithm implementation, M.S., D.H., and Y.L.; Paper writing, D.H., W.Z., and M.S.  
This research was funded by the National Key R & D Program of China (Grant No. 2017 YFE0121000 and No. 2018 YFC0114800), National Natural Science Foundation of China (Grant No. 11574064 and No. 11874133), Shandong Provincial Natural Science Foundation, China (Grant No. ZR2017 MF041 and ZR2018 MF026), the Science and Technology Development Plan Project of Shandong Province (Grant No. 2018 GGX103047 and 2016 GGX103032), and the Development Plan of Chinese Academy of Sciences and Wego Group (Grant No. 2017011).  
This work was partly supported by the Institute of Biomedical and Health Engineering at Shen Zhen Institute of Advanced Technology, Chinese Academy of Sciences (SIAT).  
The authors declare no conflict of interest.  
Figure 11: (**a**) Photograph of a knot phantom and (**b**) 3D photoacoustic image.