A wavelength tunable OPO system (Surelite OPO plus, Continuum, Santa Clara, CA) pumped by a frequency-doubled pulsed YAG laser (Surelite I-20, Continuum, Santa Clara, CA) delivered 5 ns pulses with a repetition rate of 20 Hz. The wavelength was tuned to 730 nm, corresponding to the peak absorption wavelength of the nanoprobe. The laser irradiated the tube from the top, tilted by an angle of about 45 deg to the vertical line, creating a 15 mm diameter spot with fluence of about , which was estimated from the measured total pulse energy and illumination area. The level was well below maximum permissible exposures26 and made it acceptable for clinical applications. A linear array transducer (AT8L12–5 50 mm, Broadsound, Taiwan) interfaced with an US imaging system (Verasonics, WA) was used for US pulse-echo imaging and PA signal detection. The transducer was positioned above the tube at a distance of 18 mm, and its lateral direction was aligned parallel to the tube so that objects moving along the tube could be monitored. Given an aperture length of 25.6 mm (128 elements), the reconstruction corresponded to a total detection angle of approximately 71 deg. A field-programmable gate array (FPGA) was used to synchronize the laser and Verasonics system to alternatively acquire one PA image and one US image, each at a frame rate of 4 Hz.