Fig. 2.

Signal generation and detection in different implementations of PAI with different penetration limits in scattering tissue. These depths values are the maximum depths reported in the literature. (a) Reflection-mode OR-PAM system with an optical–acoustic combiner that transmits light but reflects sound. OR-PAM system with 1.5 mm imaging depth was reported [65]. (b) AR-PAM system where the laser light is only weakly focused, previous work demonstrated 11 mm imaging depth in biological tissue [61]. (c) PAT system with a ring-shaped ultrasonic transducer array (UTA). The laser beam is expanded and homogenized by a diffuser to provide widefield illumination. Recent research has proved 7 cm imaging depth achieved in biological tissue [40].