Fig. 1. Schematic of photoacoustic imaging^[1]

Fig. 2. Typical embodiments of PAM^[1,17]. (a) Transmission-mode OR-PAM; (b) reflection-mode OR-PAM; (c) dark-field AR-PAM; (d) schematic of typical OR-PAM

Fig. 3. Super-resolution PAM imaging^[25-26]. (a) Photoimprint-PAM imaging of gold nanoparticles with a diameter of 150 nm^[25]; (b) imaging of mitochondria in fibroblasts by conventional PAM^[26]; (c) imaging of mitochondria in fibroblasts by super-resolution PAM; (d) imaging of mitochondria in fibroblasts by transmissi

Fig. 4. Principle of spatially invariant resolution photoacoustic microscopy^[51]

Fig. 5. Integrated PAM/OCT for multi-modal chorioretinal imaging^[4]. (a) System schematic; (b) fundus photo of retina vessels in New Zealand albino rabbit in vivo; (c) PAM image of retinal vessels; (d) three-dimensional rendered image of retinal vessels; (e) PAM image of choroidal vessels; (f) OCT B-scan of the fundus

Fig. 6. PAM images of a melanoma cell ^[21] and a red blood cell^[103-104]in vitro. (a) PAM image of a melanoma cell^[21]; (b) bright field microscopy image of a melanoma cell^[21]; (c) fluorescence image of the cell nuclei superimposed with the PAM image[

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Fig. 7. Functional PAM imaging. (a) Sequential snapshots of single RBCs releasing oxygen in a mouse brain^[114];(b) label-free multi-parameter PAM imaging in vivo^[113]: total concentration of hemoglobin (C_Hb), s_O2, blood flow, and profile of blood flow speed across the dashed line, respectively; (c) s_O2 distribution in a mouse brain[

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Fig. 8. Cellular imaging using activatable photonic nanoclusters. (a) Fabrication of nanoclusters based on split green fluorescent protein (GFP) fragments; (b) highly-sensitive PAM images of U2OS cells based on nanoclusters

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Category Lateral resolution Axial resolution Penetration depth OR-PAM 0.51λONAO One optical transport mean free path(~1 mm in soft tissue) AR-PAM 0.71λANAA 0.88cB A few optical transport mean free path

Table 1. Performance of OR-PAM and AR-PAM^[6,20]

Ref. Scanning method Co-axial Transducer focusing Scanning speed Field of vew /(mm×mm) Laser pulserepetition rate /kHz [56] Mechanical Yes Spherical 1 Hz/mm >10×10 - [58] Voice-coil Yes Spherical 40 Hz/mm >5×5 4 [60] MEMS Yes Spherical 50 Hz/B-scan 9×4 10 [64] No Spherical 270 Hz/mm 0.15×0.15 100 [67] Galvanometer No Sylindrical 50 Hz/B-scan 40×40 100 [68] Hybrid Yes Spherical 250 Hz/B-scan 4×8 50 [55] Multifocal Yes Linear array ~20 Hz/mm 6×5 1.35 [70] Hexagon mirror Yes Spherical 900 Hz/B-scan >12×10 800 [51] DMD Yes Spherical 0.05 Hz/volumetricscan 0.18×0.18 1

Table 2. Comparison of different scanning methods