## 4 Discussion
As the tomographic system is implemented with a programmable ultrasound machine that can record US waves upon the arrival of a trigger pulse, PA tomography sequences with a pulsed laser could be implemented during scans. The interlacing of PA and US acquisitions will be investigated in future studies for simultaneous PA and US 3-D imaging of the same volume with the scanner. Additional strategies such as tissue harmonic coherent compound imaging using advanced US sequences to isolate non-linear echoes [35; 36] and the use of contrast agents could be integrated within the tomographic system in the future to further improve US image contrast and specificity. Because of the scan duration, it is expected that contrast agents that could be retained within biological tissue would work best, such as targeted microbubbles [37] or nanodroplets converted to microbubbles upon optical [38; 39] or ultrasound [40] activation. Feasibility has been shown, for example, to optically activate microbubbles from nanodroplets that are detectable with PA imaging so that, with the choice of appropriate dual-modality contrast agents, PA and US can be used together to characterize agent arrival and delivery [39]. The system was developed with a single-side access for an accessibility to various body parts. The scan duration is also a crucial point for _in vivo_ application, clinical or preclinical. Long acquisitions may cause discomfort and tissue motion that can cause image blurring. As mentioned above, the number of tomographic positions was already reduced by adjusting the spatial sampling. But the acquisition duration is the number of tomographic positions multiplied by the acquisition rate. With the ultrasound machine used in this study, the acquisition rate could be in the kHz range. Currently, we limited the acquisition rate to 50 Hz because of the maximum acceleration of the rotation stage (see Section 2.3.2). The acquisition duration could then be greatly reduced with a faster rotation stage such as one with a magnetic motor. With our worm gear motor, we achieved a scan duration of 10 s for a 9-cm\({}^{3}\) volume. This acquisition time is smaller compared to other rotate-translate scanning schemes that have been applied _in vivo_ in PA tomography [41] and for incoherent spatial compounding of Doppler data [42; 43]. Although these studies map functional information and not only pulse-echo US imaging, they give a reference for _in vivo_ acquisition duration. [PERSON] et al. [41] performed multispectral PA tomographic acquisitions in 12 min. For 3-D Doppler tomography with incoherent compounding of Doppler images, reported acquisition times were of 15-20 min [42; 43]. To avoid blurring due to respiratory motion, acquisition methods such as respiratory gating, i.e., acquisition only during the respiratory pause, can be considered. A weak oversampling can be applied to compensate for missing tomographic positions. With a current acquisition duration of 10 s for a 9-cm\({}^{3}\) imaged volume (in the DSCA) in the steered transmit mode and a reconstruction duration of 95 s (Table 1), the scanning system can currently achieve a volumetric imaging rate of 0.57 volumes/min. The reconstruction duration could be further accelerated with a high-end graphics processing unit (GPU). However, even if the acquisition rate between tomographic positions was set in the kHz range, which is the maximum rate given the ultrasound propagation time and the ultrasound hardware, the full-volume acquisition duration will still be on the order of 1 second in the steered transmit mode. Therefore, real-time volumetric imaging (>5 volume/s [4]) would only be achievable with degraded image quality (rotate-translate scan in the straight transmit mode and with \(\Delta\alpha\geq\Delta\alpha_{2}\) or a translate-only scan). Such an approach could be used to position the volume of interest in the DSCA with a real-time display prior to a higher-quality image acquisition. Finally, the developed scanning method is applicable to a broad range of already-available US arrays.