Fig. 1. Schematic of photoacoustic imaging^[30]

Fig. 2. Basic implementation of PAM. (a) Transmissive OR-PAM^[30,35]; (b) reflective OR-PAM^[30,36]; (c) AR-PAM based on dark field illumination^[30,39]

Fig. 3. Structural and functional imaging of PAM. (a) Multi-parameter measurement of the mouse brain^[57]; (b) multi-parameter measurement of the mouse ear^[41]; (c) imaging of an in vitro melanoma cell and a red blood cell^[35,58]; (d) observation for hemoglobin at different depths^[59]; (e) distribution of the blood oxygen saturation in a mouse brain^[60]

Fig. 4. Portable PAM based on electronically controlled displacement stage. (a) Photograph of fast scanning PAM probe^[64]; (b) cross-sectional photoacoustic patterns of different parts of human skin^[64]; (c) AR-PAM based on fiber bundle illumination, including schematic diagram and physical map of the probe device^[65]; (d) melanoma imaging in a nude mouse^[65]; (e) diagram of OR-PAM device based on laser diode^[66]; (f) imaging of the defective carbon fiber surface layer and deep carbon fiber embedded in fat^[66]

Fig. 5. Portable PAM using GS galvanometer scanning. (a) PAM system schematic with adjustable light focus and the physical picture of the probe^[71]; (b) imaging of subcutaneous blood vessels at the human wrist^[71]; (c) schematic of PAM system with large field-of-view and physical picture of the probe^[72]; (d) imaging of the mouse ear, iris, and brain microvessels^[72]; (e) schematic of PAM system based on the rotational scanning^[73]; (f) monitoring of vascular changes in a mouse tumor^[73]; (g) schematic and photograph of the PAM device with hybrid scanning^[77]; (h) vessels and oxygen saturation images of the mouse ear and back^[77]

Fig. 6. Portable PAM using MEMS galvanometer scanning. (a) Diagram of the compactly designed handheld PAM system^[78]; (b) imaging of human skin vessels^[78]; (c) photograph of a photoacoustic probe with a mass of 162 g and a diameter of 17 mm^[79]; (d) imaging of mouse ear, iris, and brain vessels^[79]; (e) diagram of the probe with a volume of 22 mm$\times$30 mm$\times$13 mm and a mass of 20 g^[37]; (f) imaging of human oral vessels^[37]; (g) diagram of the photoacoustic pen^[82]; (h) imaging of human oral vessels^[82]

Fig. 7. Wearable PAM for the brain. (a) (b) Partial photograph and schematic diagram based on the suspended ball and head fixation device^[103-104]; (c) photograph of the head-mounted cranial window with mass of 2 g and wide field-of-view of 5 mm$\times$7 mm^[106]; (d) device for monitoring neural activity in freely moving rat^[107]; (e) small brain probe with mass of 8 g and diameter of 13 mm^[108]; (f) schematic of the low-cost and miniaturized system^[109]

Fig. 8. Miniaturized PAM based on multi-modality. (a) Schematic of a dual-modality system combining PAM and CFM^[123]; (b) PAM (upper row) and CFM (lower row) imaging of animal bladder tissue^[123]; (c) device diagram of OR-PAM and OCT dual-modality system based on rotary scanning^[132]; (d) OR-PAM and OCT imaging of mouse ear (upper row) and human lower lip (lower row)^[132]; (e) structure and photograph of the OR-PAM and OCT dual-modality probe with a mass of 35.4 g and a volume of 65 mm×30 mm×18 mm^[133]; (f) OR-PAM and OCT imaging of mouse ear (upper row) and human oral lower lip (lower row)^[133]; (g) schematic diagram and photograph of PAM-US-OCT three-modality probe^[144]; (h) PAM (left), US (right), and OCT (medium) three-modality imaging of mouse ear blood vessels^[144]

Table 1. Comparison of the imaging performance of GS mirror and MEMS mirror