Fig. 1. Principle and main modes of PA imaging^[3]. (a) Principle of PA imaging; (b) schematic diagram of PAM; (c) schematic diagram of PACT; (d) schematic diagram of PAE

Fig. 2. Embodiments of different arrayed ultrasound transducers for PACT. (a) PACT system based on planar array ultrasound transducer for PA imaging of mouse brain^[6]; (b) PACT system based on cylindrical array ultrasound transducer for PA imaging of breast^[11]; (c) PACT system based on spherical array ultrasonic transducer for PA imaging of hand^[12]

Fig. 3. Embodiment of PACT based on LED^[16]. (a) Schematic of probe with imaging plane and illumination source; (b) PA and US imaging of skin and vasculature

Fig. 4. Embodiment of deep learning for PA imaging^[34]. (a) Architecture of Y-NET; (b) in-vitro results of chicken breast phantom

Fig. 5. Embodiments of high/super-resolution PAM. (a) Schematic diagram of GR-PAM and GR-PAM imaging of red blood cells^[41]; (b) schematic diagram of ULM-PAM and ULM-PAM imaging of lipids and proteins^[42]

Fig. 6. Embodiments of extended depth-of-field technology in PAM. (a) Diagram of virtual detector concept in SAFT and original and 2D SAFT PA imaging of leaf vein^[45]; (b) numerical simulation and experimental measurement of parameter of Bessel beam and PA imaging of zebrafish based on Gaussian beam and Bessel beam illumination^[47]

Fig. 7. Embodiments of high-speed PAM. (a) High-speed PAM based on MEMS^[54]; (b) high-speed PAM based on GM^[56]; (c) high-speed PAM based on polygon-mirror scanner^[59]; (d) high-speed PAM based on microlens array^[60]

Fig. 8. Embodiments of all-optical PAM. (a) Schematic diagram of BD-AO-PAM system and in vivo PA imaging of microvasculature of mouse ear^[69]; (b) schematic diagram of PARS microscope system and in vivo PA imaging of chorioallantoic membrane from a chicken embryo^[70]

Fig. 9. Embodiments of PAE based on optical scanning. (a) Schematic diagram of PAE probe based on MEMS scanning and PA imaging of resected mouse colon tissue^[76]; (b) schematic diagram of all-optical forward-viewing PAE probe and PA imaging of mouse abdominal skin microvasculature^[77]

Fig. 10. Embodiments of extended depth-of-field technology in PAE. (a) Schematic diagram of imaging probe of AF-PAE and PA imaging of rabbit rectum^[82]; (b) schematic diagram of imaging probe of large-depth-of-field OR-PAE and PA imaging of rabbit rectum^[74]

Fig. 11. Embodiments of dual-modality PAE. (a) Schematic diagram of imaging probe of PA-US endoscope and PA-US imaging of rat rectum; (b) schematic diagram of all-optical PA-OCT intravascular probe and PA-OCT imaging of vascular phantom^[87]; (c) schematic diagram of imaging probe of PA-HSI endscope and PA-HSI imaging of rabbit rectum in vivo^[90]; (d) schematic diagram of imaging probe of PA-PE endscope and US-PA-PE imaging of aorta^[91]

Fig. 12. PA molecular imaging in NIR region. (a) Absorption coefficient spectra of endogenous tissue chromophores at their typical concentrations in human body^[110]; (b) reduced scattering coefficients of different biological tissues and of intralipid scattering tissue phantom as function of wavelength in 400－1700 nm region, which covers visible, NIR-I, and NIR-II windows^[110]; (c) MPE as function of excitation wavelength^[110]; (d) PA imaging of mice tumor with 680 nm and 950 nm^[113]; (e) PA-US imaging of mice tumor with 1064 nm^[112]; (f) PA-US imaging of mice tumor with 1280 nm^[111]

Fig. 13. Embodiment of contrast-enhanced PA molecular imaging^[116]. (a) Synthesis process of prepared AgBr@PLGA NCs and schematic illustration that tumor area is graphically fixed via redox reaction; (b) PA imaging of mice injected AgBr@PLGA＋GSH and graphene

Fig. 14. Embodiment of ratiometric PA molecular imaging^[118]. (a) Design and synthesis of quantitative PA diagnosis of gastric and intestinal diseases; (b) US imaging and PA imaging at 790 nm, PA imaging at 1200 nm, and ratiometric PA molecular imaging of stomach

Fig. 15. Embodiment of highly specific activatable probes^[120]. (a) Schematic diagram of photoconversion process; (b) cartoon illustration of background-suppressed PA molecular imaging; (c) PA molecular imaging of chicken breast tissue

Fig. 16. Embodiment of probe for integration of diagnosis and treatment^[127]. (a) Schematic illustration of NIR-II fluorescence/PA dual-modality imaging guiding tumor targeted combination therapy; (b) FI of Hela-tumor-bearing mice; (c) PA imaging of Hela-tumor-bearing mice; (d) tumor growth profiles of different groups of mice

Table 1. Performance of scanning systems for high-speed PAM