Photoacoustic and fluorescence imaging systems. (a) Schematic of the PACT system. (b) In PACT, upon laser excitation, the time-domain PA signals recorded by each transducer element are back-projected into the 2D imaging space based on the delay times, and then summed to form an image of the mouse cortex. (c) Schematic of the OR-PAM system, where the capillary-level lateral resolution is provided by the tight optical focusing. UT, ultrasonic transducer; CorL, correction lens; RAP, right-angled prism; RhP, rhomboid prism; SOL, silicone oil layer. (d) Schematic of the fluorescence imaging system.