Photoacoustic microscopy (PAM) is the microscopic form of photoacoustic imaging, where the laser-induced tissue photoacoustic phenomenon is probed by ultrasound transducer for image reconstruction (Yao and Wang, 2013). Relying on acoustic detection of tissue optical absorption contrast, PAM can reach an imaging depth of up to several millimeters in tissue over the optical diffusion limit (Wang, 2008). In PAM, the excitation light and the ultrasonic detection are cofocused on the sample to achieve one-dimensional depth-resolved A-scan, which is used for 3-D imaging based on mechanical scanning of the excitation and detection beams (Wang and Gao,
2014). The lateral resolution and imaging depth of PAM are adjustable based on the selective tightening between the optical and acoustic foci, while the axial resolution of PAM is dependent on the bandwidth of the photoacoustic signal that is detected by the ultrasonic detector (Yao and Wang, 2013). With a high-frequency transducer (e.g., 50 MHz), PAM can achieve an axial resolution of ~15 pm (Yao and Wang, 2013). The scalability and unique optical absorption contrast of PAM provides a useful tool to characterize 3-D tissue-engineered samples, specifically, scaffolds, cells, and blood vessels (Cai et al., 2013b).