The mechanism of the PAI system was previously described2- 3 and is identical to the system presented by Song et al. 1 A tunable OPO laser (Surelite OPO PLUS; Continuum, Santa Clara, California) pumped by a Q-switched Nd:YAG laser (Surelite; Continuum, Santa Clara, California) is used to generate the laser pulse with a repetition rate of 10 Hz and a laser wavelength of 680 nm. The laser light, directed by a prism group, forms a ring-shaped illumination on an optical condenser and is then refocused by a conical lens on the targeted area. The produced photoacoustic signals are collected by a 5-MHz single-element focused ultrasonic transducer (SU-108-013, Sonic Concepts, Bothell, Washington), amplified by a pre-amplifier (5072PR, Olympus-NDT, Waltham, Massachusetts), and finally collected by a personal computer through an A/D Scope Card (CS21G8-256MS, Gage, Lockport, Illinois) with a 125-MHz sampling rate to reconstruct 2-D images. The 5-MHz ultrasonic transducer (fractional bandwidth of 60%, 35-mm focal length, and 33-mm aperture size) is mounted in the center of the condenser and driven by a step motor to scan over the targeted region. It takes ca. 20 min to scan a 20 mm × 20 mm area. Previous work2- 3 on this system has shown that it has a lateral resolution of 270 μm and an axial resolution of 220 μm.