Fig. 1. Magnetron mode microwave source.(a) Schematic of the magnetron modulation mode; (b) typical magnetron modulation mode microwave generator^[78]; (c) top view and side view of the miniaturized microwave generator^[115]; (d) schematic of a small MTAI system^115]

Fig. 2. High voltage discharge mode microwave source.(a) Schematic of high voltage discharge mode; (b) schematic of USMP experimental device^[74]

Fig. 3. Continuous modulation mode microwave source.(a) Schematic of continuous modulation mode; (b) schematic of experimental apparatus for observing TA effect and TAR effect induced by monopulse and multi-pulse microwave sources^[90]

Fig. 4. Experimental diagram of unit ring scanning endoscopy imaging system and resolution^[118]

Fig. 5. Structure diagram and physical diagram of ring array acquisition system^[77]

Fig. 6. Multivariate data acquisition system. (a) MTAI system composed of concave array detectors; (b) flexible array detector^[121]

Fig. 7. FP thermoacoustic detector. (a) Structure of FP thermoacoustic detector^[122]; (b) lateral resolution of FP thermoacoustic detector^[123]; (c) axial resolution of FP thermoacoustic detector^[123]

Fig. 8. Image reconstruction process of thermoacoustic imaging

Fig. 9. Schematic of different imaging algorithms. (a) Schematic of BP algorithm; (b) schematic of HDCS-MTAI algorithm^[124]; (c) schematic of 3D CS-MTAI algorithm^[125]

Fig. 10. MTAI of mammary gland. (a) Schematic of a breast thermoacoustic scanner^[26]; (b) thermoacoustic imaging of a series of coronal and sagittal sections of the female breast, arrows show a large lobulated enhanced mass^[26]; (c) thermoacoustic imaging of the left breast and right breast of human^[26]; (d) in vitro thermoacoustic imaging of tumor-bearing ewe breast^[121]

Fig. 11. MTAI technique for brain imaging of rats in vivo^[129]. (a) Thermoacoustic imaging system of rat brain structure; (b) thermoacoustic imaging results of rat brain structure

Fig. 12. MTAI technique for cerebral hemorrhage imaging^[130]. (a) MTAI imaging system for GMH in mice; (b)-(e) thermoacoustic imaging of GMH in mice; (f)-(i) histological sections of mouse brain; (k)-(n) MTAI images after injection of 5 μL blood into the left ventricle; (j) indication of the location of blood injection; (o) histological images of Fig.12 (j); (p) TA signal curve along the dotted line shown in Fig.12 (l)

Fig. 13. MTAI system and microwave illumination methods^[131]. (a) Schematic of the MTAI system; (b) pyramidal horn antenna; (c) parallel in-phase microwave illumination; (d) parallel anti-phase microwave illumination

Fig. 14. Thermoacoustic imaging of finger arthritis^[116-117]

Fig. 15. Schematic of different thermoacoustic probes. (a) Schematic of anti-Gall-Fe₃O₄ nanoparticles to enhance MTAI in nude mouse model with pancreatic cancer^[132]; (b) schematic of TA signal generation mechanism in BSA-GO nanoparticles^[141]; (c) schematic of TA signal and shock wave generation mechanism in defect-rich TiN NPs^[142]; (d) UHF-RF-acoustic contrast preparation and in vitro imaging^[143]